Delft University of Technology
FACULTY MECHANICAL, MARITIME AND MATERIALS ENGINEERING
Department Maritime and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl
This report consists of 63 pages and 2 appendices. It may only be reproduced literally and as a whole. For
Specialization:
Transport Engineering and Logistics
Report number: 2015.TEL.7975
Title:
Fatigue reliability for cranes
accounting for Bayesian updating
Author:
C.J. Tawjoeram
Title (in Dutch) Actualiseren van hijskraan vermoeiingsbetrouwbaarheid door toepassing van de Bayesiaanse benadering
Assignment: Literature assignment
Confidential: No
Initiator (university): dr.ir. X. Jiang
Supervisor: dr.ir. X. Jiang
Delft University of Technology
FACULTY OF MECHANICAL, MARITIME AND MATERIALS ENGINEERING
Department of Marine and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl
Student: C. Tawjoeram Assignment type: Literature
Supervisor (TUD): Supervisor (TU Delft) Creditpoints (EC): 10 or 12 or 35 or 36 Specialization: TEL/PEL/ME/DE/OE Report number: 20xx.TL.xxxx
Confidential: No / Yes
until Month dd, yyyy
Subject:
fatigue reliability of Cranes accounting for Bayesian updatingFatigue has been recognized as one dominant factor affecting the longevity of general transport equipment. Since both the loadings and resistances are of large uncertainty, it is proper to perform fatigue assessment and prediction in reliability context. In practice, regular inspection and measurement are performed in order to monitor the actual situation of an engineering structure and make a corresponding decision on maintenance. Obviously, the latest measurement data will reveal the actual situation of a structure most approximately. Hence, it is significant to update stochastic modeling of fatigue in order to assess and predict fatigue damage realistically. Bayesian procedure can be deployed to update such a model.
In this literature assignment, the student is demanded to review the development of Bayesian theorem and its application on the fatigue updating of cranes. The following aspects are required to be illustrated in the report:
• Review on methods and standards related to fatigue assessment and prediction of cranes. • Explain the basic procedure and main influential parameters of fatigue analysis on cranes. • Explain the basic theory and development of Bayesian procedure.
• Illustrate the state of the art: application of Bayesian procedure to update fatigue assessment of cranes.
• Discuss and conclude the further research demanded to explore the application of Bayesian procedure for fatigue updating of cranes.
This report should be arranged in such a way that all data is structurally presented in graphs, tables, and lists with belonging descriptions and explanations in text.
The report should comply with the guidelines of the section. Details can be found on the website. If you would like to know more about the assignment, you may contact with Dr. X Jiang through x.jiang@tudelft.nl.
The supervisor, X. Jiang
C. J. Tawjoeram
Delft, February 10, 2016
Fatigue reliability of cranes accounting
for Bayesian updating
Preface
This report is written based on the poor research papers existing on the topic related to Bayesian updating for the fatigue reliability of cranes. In the past some research has been done but based on expert’s judgments which give opportunities for more dedicated research. Crane manufacturers, crane owners, maintenance engineers and crane specialist will have a large benefit in this report, which can be used as information source about the different available fatigue models, the current crane design standard, the Bayesian approach and the current state of the art of the application of the Bayesian approach to fatigue prognosis of cranes. I would like to thank my supervisor Dr. Ir. Jiang for the critical support and guidance during this interesting literature assignment, which had a large benefit on the updating of my knowledge of cranes, fatigue, EN13001 standard, the Bayesian approach and an improvement in my writing skills.
English Summary
Crane fatigue is a common problem is practice. According to an article from offshore-mag (2015) "The boom structure is the component, in about 95% of the cases, of the crane structure which is the most vulnerable to fatigue". Although crane manufacturers use their own state of the art techniques to prevent it, fatigue still occurs in practice. This means it a challenge to monitor the structure vulnerable fatigue locations and continuously make proper accurate initial fatigue life and remaining fatigue life estimates based on updated data. This updating process could be established using the Bayesian updating approach.
However, up to the present moment poor systematical literature reviews of the fatigue reliability of cranes accounting for Bayesian updating have been identified. The main research question is: “How to incorporate the Bayesian approach to update the fatigue reliability of cranes?”.
Until now a large amount of studies have been done related to fatigue. Xu et al. (2014) gives a classification of different fatigue approaches used, still the most common and widely used are the S-N approach, the Fracture Mechanics approach and the Palmgren-Miner rule. These are used to estimate fatigue life, crack growth rate, critical crack length and fatigue damage. To comply with governmental legislation crane standards are designed. The current valid European crane design standard EN13001. The main influential parameters which have a great impact on the fatigue reliability of cranes are: loading, number of cycles, weld classification and material property. The EN13001 standards has compared to the old standard an increase in accurate fatigue calculation, because it is more capable of dealing with variable load spectrum using Rainflow counting method. Also crane owner is
demanded to give input about the crane usage profile. It is not only important to make fatigue assessment in the initial design phase but also when the crane is in operation. Cai et al., (2014) describes an interesting fatigue assessment method. He proposed a stress spectra acquisition method based on measured & simulated & compared & statistics integrated strategy of K-type welded joints for a 25t and 18m lattice boom crane which has been in service for 8 years. In his method he used strain gauges to get actual measurement data, a joint FEM & Multibody dynamics simulation and a non linear form of the Palmgren-Miner rule. Based on all the resources he made accurate reliable remaining fatigue life prediction. Obtaining reliable measurement data is quite a challenge, because a maintenance engineers could make use of traditional NDI or intelligent Structural Health Monitoring systems. The latter one has been widely used in automated damage diagnosis by combining damage detection algorithms with structural monitoring systems (Loh et al., 2006; 38(2):91–128). Intelligent structural monitoring system were used in the work of Kopnov,(1999), Qi et al(2012), Xu et al., (2015) and Tingjun et al., (2015) to measure stress and strain at critical locations in the structure. Xie et al., (2013) and Huang et al., (2013) developed a method for optimal placement of gauges or sensors at the critical locations based on the crane's FEM model. So if inspection data is obtained risk based inspection planning could be performed. Straub and Faber(2005) developed a generic updating RBI planning framework to risk based inspection planning for steel structures which could also be applied for analyzing fatigue risk of crane components. The updating process is based on the Bayesian approach. The Bayesian approach is a different way in looking at problems where a decision has to be made, where there is a large uncertainty. It is based on conditional probability. The founder of this
approach is Tomas Bayes and known for the theorem of Bayes: ( ) 0, ( ) 0 ) ( ) ( ) | ( ) | ( = alsPA> PB> B P A P A B P B A P It uses
two types of information that are of interest: the subjective initial belief, in terms of historical data, experience or prior and the evidence. This information are combined trough the equation resulting in an objective most recent data which is the posterior distribution, expressing what currently is known about the parameters after seeing the data. The approach assume that all parameters are random, having their own probability distribution The main goal in performing a Bayesian analysis and making inferences in terms of probability statements for a specific purpose are according to Kruschke (2014), estimating parameter values, predicting data values and comparing models. When performing a Bayesian analysis, parameter values are only defined in the context of a model which is explicitly defined according to Baye’s rule, because it is the particular model which gives meaning to a certain parameter and all the probabilities of the parameter are defined by the model (Kruschke, 2014). In the literature Kruschke (2014) en Lin (2014) give a brief description how to perform a Bayesian analysis. Some literature mention 3, some 5 and some 11 steps. The Bayesian approach has been widely used to study fatigue related problems in the field of engineering. The works of Press, (2003), Caticha and Giffin, (2006) and Peng et al., (2013) give an overview of the deployment of Bayes theorem for prognosis in various engineering applications. Wickham and Frieze (1990) demonstrated the potential benefits of this approach in terms of inspection scheduling, based on the review of the available Bayesian techniques for performing Bayesian updating of the fatigue reliability of welded joints.
In the field of cranes, not much research has been done on the application of the Bayesian approach to asses and update crane fatigue reliability. Different applications of the approach have been reviewed and it is concluded that currently there is no state of the art framework in respect to the deployment of the Bayesian approach to update crane fatigue reliability. It is noticed from the literature that the Maritime industry has a lead in the application of the approach in updating marine and maritime structural fatigue problems and is currently still in development. The development of an FPSO fatigue damage updating model, according to Radaelli (2011). The framework was developed to get better predictions of the fatigue damage for FPSO vessels, known as the Monitoring and Advisory System (MONITAS. The system gave insight why actual fatigue data differed from design data with respect to fatigue, which lead to opportunities for vessel draft improvements. Chen et al., (2011) proposed a Palmgren–Miner’s rule and fracture mechanics (FM) based approach and coupled these with Bayesian updating in order to establish inspection plans for marine structures. For cranes this updating framework from the maritime industry can be adopted and improved. The only drawback is that advanced Bayesian statistical knowledge is required to develop such framework.
Dutch Summary
Vermoeiing van hijskraan constructies is een veelvoorkomend probleem. Volgens offshore-mag (2015) is de giek constructie in 95% van de gevallen het meeste vatbaar voor vermoeiing en het is een essentiële uitdaging om dit fenomeen goed in de gaten te houden met bestaande inspectie methodieken. Echter blijkt dat er een groot contrast is tussen de oorspronkelijke vermoeiings
voorspellingen die gemaakt zijn tijdens het ontwerpproces en de voorspellingen die gedaan worden na inspecties over de resterende levensduur van de constructie. De Bayesiaanse benadering biedt
hiervoor vele mogelijkheden om vermoeiingsvoorspellingen zo actueel mogelijk te blijven houden voor de hijskraanconstructie.
Tot heden zijn er vele studies gedaan naar vermoeiing, Xu et al. (2014) geeft een opsomming van alle vermoeiingsbenaderingen die ontwikkeld zijn. Toch zijn de stress-amplitude (S-N) benadering, de breukmechanica (FM) benadering en de regel van Palgren-Miner de meest gebruikte. Die worden gebruikt om de totale en de resterende levensduur, de scheurgroei, de kritische scheur lengte en breuk te bepalen.
Om het vermoeiingsmechanisme van kranen beter te begrijpen en het materieel hierop eenduidig te ontwerpen, zijn hijskraan normen ontwikkeld door het Nederlands Normalisatie-instituut. De belasting, het aantal belastingscycli, lasklasse en materiaal blijkt uit onderzoek de meest invloedrijke
vermoeiings paramters voor hijskranen te zijn. De meest recente hijskraannorm is de EN13001 norm die vanaf 2010 voor alle EU-lidstaten geldt. De norm geeft een gedetailleerde beschrijving over het kraanontwerp proces en is accurater dan vorige in termen van vermoeiings calculaties. Het grootste verschil tussen de actuele en de oude normen is dat de hijskraan eigenaar verplicht wordt gesteldt om het gebruik te specificeren b.v. welke soort lading en onder welke condities gewerkt zal worden. Ook is het belangerijk om hijskranen constructucties die reeds operationeel zijn te beoordelen op
vermoeiing. Cai et al., (2014) beschrijft een interessante methodiek toegepast op een giekkraan op rupsen die al 8 jaar in werking was. Hij maakte tijdens de beoordeling gebruik van
hijskraanstandaard, gekoppelde eindige elementen model en multibody dynamics simulatie,
rekstrookjes en een niet lineaire vorm van de regel van Palgren-Miner. Uit al deze informatiebronnen maakte hij een voorspelling van de resterende hijskraan levensduur. Tijdens de beoordeling ontdekte hij scheur lengtes in de giek die hij als kritisch beschouwde. Uit onderzoek blijkt dat er praktisch geen kritische scheur lengte bestaat en in de normen wordt ook niets hierover vermeld. Inspectie data verzameling t.b.v. de vermeoiings beoordeling vormt een essentiele schakel in de betrouwbaarheid van de voorspelling. In de praktijk kan gebruik worden gemaakt van conventionele handmatige meet en inspectie methoden of intelligente constructie monitorings systemen. Xu et al., (2015) maakte in zijn onderzoek gebruik van draadloze rekstrookjes die geplaats waren op de meest vermoeiingskritieke lokaties op de hijskraan. Op basis van de verkregen informatie kunnen dan gerichte inspectie en onderhoudsplannen worden opgesteld. Straub and Faber ( 2005) ontwikkelden een algemene risico gebaseerde benadering hiervoor.
Bayesiaanse benadering is gebaseerd op de regel van Bayes: 0 ) ( , 0 ) ( ) ( ) ( ) | ( ) | ( = alsP A > P B > B P A P A B P B A
P , waarbij de kans wordt geïnterpreteerd als de subjectieve
graad van geloof dat wordt gehecht aan een gebeurtenis. Het is een benaderingsvorm van statistisch onderzoek doen waar onzekerheid over het optreden van een bepaald fenomeen of onderwerp een grote rol speelt. Het belangrijkste kenmerk van deze benadering is dat voorafgaande aan
dataverzameling, een subjectieve inschatting geformuleerd wordt op basis van persoonlijke zienswijze over het optreden van een willekeurige gebeurtenis. Ook biedt de benadering een strategische oplossing hoe alle aanwezige kennis en inzichten zoals individuele ervaring, oordeel van een expert, resultaten van reeds uitgevoerde simulaties, experimenten of onderzoeken, ontwerp standaarden etc., te bundelen met actuele verkregen informatie en te integreren tot een subjectieve actuele conclusie gebaseerd op axioma’s van de kansrekening. De Bayesiaanse benadering is dus een techniek die leert van data, omdat actuele verworven kennis weer gebruikt kan worden als historische data voor vervolg onderzoek. Volgens Flohr (2013) komt op deze manier het verwerven van voortschrijdend inzicht en het accumulatief karakter van kenniswerving tot uitdrukking, die als basis kunnen dienen voor acties of maatregelen. Kruschke (2014) en Lin (2014) beschrijven inde literatuur uitgebreide methoden om de Bayesiaanse benadering te gebruiken. Tot heden zijn vele onderzoeken verricht naar de toepassing van de benadering voor vermoeiiings problemen in de techniek.
Uit het oogpunt van hijskraan vermoeiingsbetrouwbaarheid gaat het zoals eerder vermeld om het accuraat voorspellen van vermoeiing dat vooral optreed bij laswerk en de monitoring van dit phenomeen zo actueel mogelijk te houden. De Bayesiaanse benadering biedt hier praktische en strategische oplossing voor, om op basis van verkregen inspectie resultaten, effectief en accurate vermoeiingsvoorspellingen te doen.
Geconcludeerd kan worden dat de Bayesiaanse benadering geschikter is dan de Frequentistische benadering voor de actualisatie van de hijskraan vermoeiingsbetrouwbaarheid en levert duidelijk een meerwaarde op voor hijskraan bouwers, monteurs en eigenaren.
Helaas is er geen state of the art Bayesiaanse hijskraan vermoeiings voorspellingsmodel ontwikkeld die de betrouwbaarheid van de voorspelling continu actualiseerd na een verichtte meting. De maritieme industrie heeft een voorsprong hierin. De ontwikkeling van een MONITAS model voor een FPSO door Radaelli (2011), Chen et al., (2011) die een model ontwikkelde gebasseerd op de regel van Palgren Miner en de breukmechanica benadering en een uitgebreider model van Dong and Frangopol (2016) ook gebasseerd op die van Chen et al., (2011).
Er zal nog veel onderzoek verricht moeten worden voordat de Bayesiaanse benadering werkelijk geïmplementeerd kan worden als Bayesiaanse hijskraan vermoeiings voorspellingsmodel, waarbij vooral samengewerkt zal moeten worden met statistici.
List of abbreviations
RBI = Risk Based InspectionFM= Fracture Mechanics S-N = Stress-Number of cycles
LEFM = Linear Elastic Fracture Mechanics EPFM = Elastic Plastic Fracture Mechanics MBD = Multi Body Dynamics
FEA = Finite Element Analysis FEM = Finite Element Method SHM = Structural Health Monitoring RBI = Risk Based Inspection planning
ADAMS = Automatic Dynamic Analysis of Mechanical Systems MCMC = Markov Chain Monte Carlo
FPSO = Floating Storage and Offloading MONITAS = Monitoring and Advisory System
List of figures
Figure 1: Schematic of different cyclic loading (nde-ed, 2015) ... 13
Figure 2: Fatigue influencing parameters for top slewing tower crane structure (Bucas et al., 2013) ... 14
Figure 3: Wöhler curve/ S-N curve (Janssen, M. 2015) ... 14
Figure 4: S-N curve for steel and aluminum (recreationalflying, 2016) ... 16
Figure 5: Use of fracture mechanics in practice (Janssen, 2015)... 17
Figure 6: Boom fatigue crack (offshore-mag, 2015) ... 19
Figure 7: European crane standards before 2010 (Mel, 2009)... 20
Figure 8: EN13001design flowchart (Mel, 2009) ... 21
Figure 9: Different position spectra (Mel, 2009) ... 22
Figure 10: Limit state method flow chart (NEN-EN 13001-1, 2009) ... 24
Figure 11: Allowable stress method flow chart (NEN-EN 13001-1, 2009) ... 24
Figure 12 : Limit design stress range philosophy according to EN13001 ... 27
Figure 13: Fatigue calculations in NEN, FEM and EN13001 (Mel, 2009: 90) ... 28
Figure 14: Max. crack length on welded K-joint (Cai et al., 2014) ... 29
Figure 15: General Crane fatigue life assessment approach ... 29
Figure 16: Fatigue life assessment flowchart (Cai et al., 2014) ... 30
Figure 17: manually inspected Crane (Spiegel, 2015) ... 31
Figure 18: cantilever beam strain measurement point (Qi & Wang, 2013) ... 31
Figure 19: FEM model optimal gauge placement flowchart (Huang et al., 2013) ... 31
Figure 20: Wireless strain gauges deployed at critical crane structure locations (Xu et al., 2015) ... 32
Figure 21: Classical RBI decision tree (Straub and Faber, 2005) ... 32
Figure 22: Generic RBI approach (Straub and Faber, 2005) ... 33
Figure 23: Offshore crane MBD model (Krukowski and Maczynski, 2013) ... 34
Figure 24: Joint Simulation of Trolley Vehicle-Frame Structure Coupled Vibration Using ADAMS and ANSYS for Container Crane ... 36
Figure 25: Thomas Baye's (Wikipedia, 2015) ... 37
Figure 26: Venn diagram (oscarbonilla, 2016) ... 38
Figure 27: Information Synthesis by Bayes' theorem (Stevens, 2009) ... 40
Figure 28: Bayesian triplot (Stevens, 2009) ... 40
Figure 29: Multi source Bayesian fusion framework (Rabiei, 2011) ... 44
Figure 30: Failure prognosis updating framework (Cross and Makeev, 2009) ... 47
Figure 31: Bayesian framework (Rabiei, 2011) ... 49
Figure 32: Bayesian updating Framework (Peng et al., 2014). ... 50
Figure 33: FPSO fatigue damage updating model (Radaelli, 2011) ... 54
List of tables
Table 1: Set of crane standards (EN 13001-1, 2009) ... 20Table 2: Individual loads (EN 13001-2, 2014) ... 23
Table 3: Amplification factors (Mel, 2009) ... 23
Contents
Preface ... 4 English Summary ... 5 English Summary ... 5 Dutch Summary ... 7 List of abbreviations ... 9 List of figures ... 10 List of tables ... 10 1 Introduction ... 122 Crane fatigue analysis ... 13
2.1 Fatigue mechanism ... 13
2.2 Fatigue models ... 15
2.2.1 S-N curve approach ... 16
2.2.2 The Fracture Mechanics Approach ... 17
2.3 Main influential parameters of crane fatigue analysis... 18
2.4 Crane fatigue failures ... 19
2.5 Container crane design ... 20
2.5.1 Crane classification... 21
2.5.2 Individual loads ... 23
2.5.3 Load combinations ... 24
2.5.4 Static calculations ... 24
2.6 Fatigue design according to EN13001 ... 26
2.7 Critical structural crane crack length ... 29
2.8 Crane Fatigue assessment ... 29
2.9 Manual inspection Vs intelligent structural monitoring system ... 31
2.9.1 Fatigue risk based inspection planning ... 32
2.10 Joint FEM and multibody dynamics simulation for crane fatigue analysis ... 34
3 Bayesian approach ... 37
3.1 General introduction ... 37
3.2 Mathematical formulation of Baye’s Theorem ... 38
3.3 Steps to perform a Bayesian analysis ... 41
3.4 Application of Bayesian approach ... 47
4 Results, conclusion and expectation ... 52
References ... 58
1
Introduction
From ancient history to modern society of today cranes play a big role in the trade in ports and the economic development of countries. They are designed in different shapes with their specific
application concerning the transport of loads from one place to the other. Nowadays society demands are increasing in larger crane capacities and thus increasing loads varying in magnitude in time. This leads to a crane structure subjected to an increasing variation of cyclic loads. These cyclic loads shorten the life of cranes, because already during the fabrication of these large structures cracks and other flaws are introduced. So therefore it is necessary to assume initial cracks in the design which will grow during the crane's operation and making the welded joints prone to fatigue.
These joints are also known as fatigue details and are characterized by progressive strength degradation which could lead to both economical and human catastrophic. This makes the need to predict crane fatigue damage as accurately as possible very important and requires making reliable risk based inspection (RBI) maintenance planning decisions. This requires a constant monitoring of the degradation progress. The fatigue data obtained by monitoring is thus kept up to date, giving better insight in the fatigue damage progress and prediction. This updating process could be established using the Bayesian updating approach.
However, up to the present moment poor systematical literature reviews of the fatigue reliability of cranes accounting for Bayesian updating have been identified. This gives opportunity to review scientific researches related to this topic. The main goal of this literature report is to review the development of the Bayesian approach and its application on the fatigue updating of cranes in reliability context.
So specific, the main research question is: “How to incorporate the Bayesian approach to update the fatigue reliability of cranes?”. The assignment will be covering and reviewing three main topics namely fatigue, cranes and Bayesian approach.
To get a good understanding about fatigue phenomena in general, a review is performed on the existing fatigue models in chapter 2.1 till 2.2.2. Then researched is done on the common fatigue failures occurring at the cranes in chapter 2.4. To design cranes standards are required as guide in the design process, which is generally discussed in chapter 2.5. Also the crane must be designed against this most occurring failure, discussed briefly in 2.6. In theory conservative assumptions can be made to estimate critical crack length, however in practice this seems different, this discussion about this is given in 2.7. To get an understanding and insight in the deterioration of the crane structure fatigue assessment is required which could be performed for both new designs and for crane in operation. This assessment is reviewed in 2.8. To obtain data needed for the assessment, measurement and inspection are to be performed given in 2.9. By then allot of information about crane fatigue is obtained and one will have an idea about the main fatigue influential parameters discussed in 2.3. Finally to perform crane fatigue assessment, the use of simulation software as tool is increasing, especially the coupling of multi body dynamics and FEM is interesting, which is discussed in 2.10.
Figure 1: Schematic of different cyclic loading (nde-ed, 2015) Bayesian analysis is discussed in 3.3 and some applications of the Bayesian approach to different fatigue problems in engineering are illustrated in 3.44.
The results for this literature review about the current state-of-the-art in respect to Bayesian updating of fatigue damage predictions and reliability of cranes are illustrated, conclusions are given and some recommendations for further research are discussed in chapter 4
2
Crane fatigue analysis
This chapter discusses the basic theory behind fatigue mechanism, fatigue models available and a review of the current actual crane standard with the emphasis on the design against fatigue. Also common crane fatigue failures, monitoring, inspection and a general fatigue assessment method is presented. Finally this chapter ends with the discussion about the main parameters effecting and influencing the fatigue mechanism of cranes.
2.1
Fatigue mechanism
Fatigue is the damage mechanism whereby cracks initiate from the surface of a structural component, beginning as shear cracks on crystallographic slip planes. The surface shows the slip planes as intrusions and extrusions, as they grow further in the structure. After a transient period, crack growth takes place in a direction normal to the applied stress, resulting in the occurrence of unstable cracks. Final failure occurs when reduced structural cross section becomes insufficient to carry load
(Pijpers, 2015). In Figure 1 different types of cyclic loading which occur in practice are listed and spectrum loading is typically for cranes. Spectrum loading is a series of fatigue loading events other than uniformly repeated cycles (efatigue, 2015).
Bucas et al., (2013) concluded that crane welds are mainly subjected to fatigue phenomenon which is general a result of:
1. The random loading on the structure; due to the variability of crane use by the owners and its working environment in general.
2. The non-even manual welding process, leading to material and geometry differences in the structure.
3. The large variation of geometries makes it impossible for life prediction to reach the same predictability in every single crane component.
Figure 3: Wöhler curve/ S-N curve (Janssen, M. 2015) Figure 2: Fatigue influencing parameters for top slewing tower crane structure (Bucas et al., 2013) An example of a crane
component subjected to fatigue phenomenon is the jib of a top slewing tower crane structure subjected to load variation which depend on the radius and the
instantaneous hoisted load; see Figure 2.
So this phenomenon must be always be considered in the design because the structure is subjected to fluctuating hoisting loads.
Fatigue was discovered by the German engineer August Wohler (Wohler, 1863). His systematic studies of fatigue of metals were the first to give insight into the design methods to prevent it. Typically fatigue is divided into:
1. low-cycle fatigue; which is characterized by typical cyclic stresses about the yield strength, meaning that the component survives for only a small number of load cycles
2. high-cycle fatigue; where the stresses remain generally in the elastic region and thus below the yield strength, meaning that the component will survive a large number of load cycles before failure occurs
Cranes are large structures which are subjected to the second category according to Shercliff (2014). These are manufactured and fabricated where cracks and other flaws cannot be avoided. Welded joints are the weak locations where cracks are introduced during the welding process. Because a crane is a high-cycle fatigue structure, most of the total fatigue life is spent in crack initiation which can be seen in Figure 3.
2.2
Fatigue models
Mostly it is not possible for a physical model to take most of the fatigue influencing parameters into account. Due to this, there exist a large number of fatigue models which describe the prediction of the fatigue life metal structures. In this chapter general fatigue life prediction models that exist are discussed.
Xu et al. (2014) classified the fatigue models into two groups:
1. Cumulative damage theories, which are the traditional theoretical frameworks for fatigue strength assessment
a. Stress-based approach (S-N curve approach); approach where the fatigue life is related to the applied stress range or stress amplitude.
i. Nominal stress approach is based on far-field stresses due to forces and moments at the potential site of cracking or the stresses not containing any stress increase due to structural details or welds (Brennan et al., 2015). i. Hot-spot-, geometric-, or structural stress approach assesses the
fatigue strength of any structural detail is by making reference to the intensity of the stress or strain field measured in the area where crack initiation will most likely occur. It is often used by the maritime field (Brennan et al., 2015).
ii. Effective notch stress approach represents a very generalized view of welds subjected to fatigue loads where the measured stress is the local maximum stress in linear-elastic weld models where the true notches have been substituted by fictitious notches of radius
b. Strain-based approach shows the elastic-plastic mechanism which takes place around a notch root when the elastic constraint of the surrounding material controls the deformations inside such a zone (Brennan et al., 2015).
c. Energy based approach is based on thermodynamics. The model describes the
effect of a loaded material, where part of the supplied energy is dissipated into heat and the remaining part is stored in the material, consisting of recoverable elastic strain energy and the irrecoverable plastic strain energy per load cycle. The latter is the cause of damage. This approach could be further divided in the hysteresis-, the plastic strain- and the total strain energy approach (Ellyyin, 2012).
d. Continuum damage mechanics approach is used to predict crack initiation life and deals with the mechanical behavior of a deteriorating medium at the continuum scale (Cui, 2002). The literature of Ellyyin (2012) gives a concise overview of the current state of damage mechanics.
Figure 4: S-N curve for steel and aluminum (recreationalflying, 2016)
a. Long crack growth approach or linear elastic fracture mechanics approach
predicts the fatigue crack propagation length at the tip of a long crack subjected to cyclic stress. The foundation of this approach is based on the Paris equation introduced by Paris et al. (1961) and discussed in chapter 2.2.2.
b. Physically small crack growth approach or elastic plastic fracture mechanics
approach, predicts fatigue crack growth of a microstructurally small crack which is usually discontinuous in nature, with accelerations and decelerations in crack growth rate during growth of the small crack (efatigue, 2015)
c. Microstructurally small crack growth approach or Microstructural fracture
mechanics approach is able to predict handle crack propagation at the microcrack level (Cui, 2002).
According to Suresh and Ritchie (1984) the primary distinction made between long and physically small fatigue cracks is the shielding zone behind the crack tip, which gives rise to closure. For welded joint fatigue design, the most applied models are the S-N approach and the linear fracture mechanics approach, but according to Hobbacher (2009) the fatigue model is chosen based the available information about the welded joint.
2.2.1
S-N curve approach
The S-N approach is based on experimental material fatigue life measurements in terms of the stress amplitude and the number of cycles to fatigue failure. The resulting data of these experiments are performed for different material, load levels and specimen geometries and are plotted on a log-log or semi-log paper. Mostly the curve tends to approach a lower horizontal limit called the endurance limit. This is the stress level at which the
material will never show fatigue failure for an indefinitely large number of cycles. This stress level is usually lower than the yield strength. In Figure 4 a typical S-N curve is illustrated for steel and aluminum, showing that steel can withstand higher stress amplitudes compared to aluminum. Each material has a different S-N curve and also each fatigue detail in a crane structure has a different S-N curve. The approach assumes in general linear fatigue damage accumulation independent of previous load cycle.
Figure 5: Use of fracture mechanics in practice (Janssen, 2015)
2.2.2
The Fracture Mechanics Approach
The fracture mechanics approach is based on the assumption of initial existing cracks in a structure which propagate during cyclic loading. According to Sinclair et al. (1953), Virkler et al. (1979) and
Ghome et al. (1987) initiation and crack growth are inherently random processes due to material variability and microstructural irregularities. Initial flaws existing in welded joints of cranes are just under the threshold of detection. Its fatigue life to
failure is predicted based on the required number of cycles to grow the crack to a specific unstable rate of propagation, which is proportional to the stress range in terms of the stress intensity factor. This approach can’t predict how quick a crack grows from an initial to a critical crack length size; it helps in describing the mechanical state and behavior of the material. It determines the life time of a cracked structure for safety purposes and answering questions like, how large the allowable crack length is, service life and safe inspection period, see Figure 5.
The most basic equations used in the fracture mechanic approach are: 1. Paris law, for crack growth:
2. Stress intensity range:
3. Fatigue life or the number of cycles to failure:
4. Crack length at failure:
The main difference between the S-N approach and the Fracture mechanics approach is, that the S-N approach is applicable at the initial stage of a microscopic crack at which most cracks will not be detected with a NDI tool and FM approach used in the phase where an initial crack is detected until the critical crack length. These two approaches should be used as complementary tools when performing fatigue analysis of welded structures (Lassen et. al 2013).
2.3
Main influential parameters of crane fatigue analysis
Because cranes are very large structures it is impossible to fabricate a crack free structure. Structural components are fabricated in a production process with surface imperfections, inclusions and finally welded together to join each component to form the crane structure. Welding introduces local stresses and cracks as a result of local heating of the metal. So based on this it is reasonable to assume existing cracks in the structure.
This chapter gives a brief discussion on the main influential parameters on the fatigue life in the design phase and operation condition.
1. Crane material
Different materials act differently subjected to fatigue loading. In the design phase crane material is specified. The following material parameters influence the fatigue strength according to (Cui, 2002): basic material properties, surface finishing effect, fatigue limit, initiation vs. propagation and crack closure. The mechanical properties of the material are the most of concern in crane fatigue assessment (Bos, 2015)
2. Crane structure
The way the crane structure is joined together has a large influence on how fatigue loads propagate through the structure. Welded joins are the most vulnerable to fatigue. According to Khoshaba and Heinrich (2013), the terminology "high quality weld should be innovated to "right quality" weld to incorporate economical considerations. The following structural factors influence the fatigue behaviour according to (Cui, 2002): geometry, fabrication defects (welds), residual stresses and improvement techniques. The weld classification is the most of concern in crane fatigue assessment (Bos, 2015).
3. Crane operation
The main function of a container crane is to hoist containers of different weight from ship to shore. Due to this fluctuation in hoisting load the fatigue life of the equipment also depends on this factor. The real use needs to be taken into account according to EN13001. The following operational factors influence the fatigue behaviour according to (Cui, 2002): mean stress effect, variable amplitude loading, multi-axial fatigue and frequency effects. The average hoisting weight and number of cycles is of most of concern in crane fatigue assessment (Bos, 2015).
4. Crane environment
Container crane are operational in different environments such as regions with very low or high temperatures and very aggressive corrosion environments at sea where ports are mainly situated. The main environmental factors affecting the fatigue life according to (Cui, 2002): corrosion, temperature and maintenance. Not of most concern in crane fatigue assessment (Bos, 2015).
2.4
Crane fatigue failures
The demand of crane capacity is showing an increasing trend in crane size, hoisting speed, lift capacity and lifting methods like the twin- and tandem lift. Al these parameters demand the need for proper fatigue assessment methods and predictions of crane structures. According to (CORDIS, 2015) the costs related to structural failure accident of a Ship-to-shore
crane in terms of compensation and injury treatment alone equals to €21.1 million.
In the literature various researches have been performed related to fatigue of cranes and crane components. Bradley, S. & Bradley, W. (1995) adopted fracture mechanics to analyze the failure of axle housing of crane truck. Kopnov (1999) applied intrinsic fatigue curves to a fatigue life prediction problem of the metalwork of a travelling gantry crane. Caglayan et al., (2010) generated load spectra based on former crane operation records, and detailed finite element models of the crane runway girders were prepared using shell and beam elements, and then numerical analysis by FEA were performer to evaluate the remaining fatigue life. Torkar and Arzenšek (2002) performed a failure analysis of a broken multi strand wire rope from a crane. Zrnic et al., (2011) identified the causes that led to the failure of the hammerhead tower crane counterjib.
Common fatigue failures occurring are: 1. crane runway girder
2. trolley structures 3. boom girder joints 4. trolley rail
5. crane leg structure 6. gusset plate of forestay 7. main equalizer beam 8. rope sheave supports
If these failures could be predicted at an early stage, it would spare catastrophic situations. According to an article from offshore-mag (2015) "The boom structure is the component, in about 95% of the cases, of the crane structure which is the most vulnerable to fatigue". Figure 6 shows a visual fatigue crack in the boom that initiated from the weld between lattice and chord, which then propagated to the chord.
Figure 7: European crane standards before 2010 (Mel, 2009)
2.5
Container crane design
Large numbers of cranes are designed based on historical data and experience. Because cranes are machines first needed to be properly designed before being manufactured and assembled and finally operated in real world to fulfill its specific task, still there is a degree of trial and error in any new designed crane. And although there is a large number of modern design software available on the market which helps crane manufacturers to optimize their design, still fatigue will occur in real world. This chapter discusses the characteristics of the current state of the art crane standard with the emphasis on the design against fatigue and the differences related to the previous design standard.
To establish the design, manufacture, supply, safe operation, maintenance, care, handling and inspection of cranes within the European Union standards are developed. They enable crane manufacturers to comply
with governmental legislation. According to Mel (2009) each European country used its own standard before 2010 which can be seen in Figure 7, currently the new general European crane design standard EN13001 applies to all EU countries. The main reason for a new standard was that updated crane fatigue knowledge gave new insight and accounted for the real use of the crane owner which was not covered in the old standard.
EN13001-1 (2009) lists a selection of a set of available crane standards for a certain crane application given in Table 1.
This literature review will discuss only the EN13001 standard. This standard consists of different parts: 1. Part 1: General principles and requirements
Explains general introduction about standard principles, limit state method, allowable stress method, crane classification and new fatigue calculation approach
Gives a general list of loads required to be taken into account, multiplication factor descriptions, load combinations, mass distribution classes and partial safety factors 3. Part 3.1: Limit States and proof competence of steel structure
Explains mechanical material characteristics, bolts, pinned and weld static calculations, fatigue-, buckling calculations
4. Part 3.2: Limit states and proof of competence of wire ropes in reeving systems Explains the design calculations for ropes and static and fatigue strength for rope reeving systems
5. Part 3.3 Limit states and proof of competence of wheel/ rail contact Explains static and fatigue design calculations for wheel and rail contacts 6. Part 3.4: Limit states and proof of competence of machinery
Still in development
7. Part 3.5: Limit states and proof of competence of hooks and hook blocks Explains static and fatigue design calculations for hooks and hook blocks
To design a crane according to the standard, the design philosophy flowchart in
Figure 8 can be followed. In this chapter the discussion will be limited to step 1 "Basis
for calculation". The steps performed are:
i. Crane classification; specifying crane design parameters, position
spectrum ii. Individual loads iii. Load combinations iv. Static calculations
These fundamental calculation steps are discusses in the following chapter.
2.5.1
Crane classification
First a specification of crane design parameters is required which are covered in the crane
classification. This is intended to specify, determine and agree the service conditions for components or mechanisms, which are determined by:
1. The total number of working cycles during the specified useful life; sequence of
movements which commences when the crane is ready to hoist the payload, and ends when the crane is ready to hoist the next payload within the same task.
2. The average distances; The average linear (trolley travelling, crane travelling, luffing) or angular (slewing) working movements
Figure 9: Different position spectra (Mel, 2009)
3. Load spectra describing the different net loads to be handled during the working movements
4. The average number of accelerations per movement; number of intended and
additional accelerations of any drive to reach the intended position of the load
5. Position spectrum describes
the method for loading and unloading a specific crane. For example the method how containers are loaded and unloaded from vessels and
loaded and unloaded from the quay crane. This means a client is required to specify the crane use. This must be considered to increase accurate fatigue results for different structural components. Figure 9 gives an overview of the different position spectra assumed in NEN/FEM and EN13001. Because of different working conditions not all components are equally loaded, resulting in different cycles and thus different fatigue life, which can be seen in the 3rd
illustration. The first one assumed equal cycles for every component resulting in an equal fatigue life for the whole Boom for example.
The fatigue life thus depends on the crane classification based on the task it needs to fulfill which could vary from a crane with irregular use with long periods of rest to a full, continuous operation.
2.5.2
Individual loads
Identify and estimate the individual loads acting on a crane, which are divided into the following categories as seen in Table 2. These loads should be increased with the specified amplification factors. The reader is referred to EN 13001-2 (2014) for a detailed overview of the individual load calculations.
Table 2: Individual loads (EN 13001-2, 2014)
Category Description Individual load
1 Regular loads
loads that occur frequently under normal operation
a Hoisting and gravity effects acting on the mass of the crane
b inertial and gravity effects acting vertically on the hoist load c loads caused by travelling on uneven surface
d loads caused by acceleration of all crane drives e loads induced by displacements
2 Occasional loads
Loads that occur infrequently, usually neglected in fatigue assessment
a loads due to in-service wind
b snow and ice loads
c loads due to temperature variation d loads caused by skewing
3 Exceptional loads
loads that occur infrequent and are excluded from fatigue assessment
a Loads caused by hoisting a grounded load under exceptional circumstances
b loads due to out-of-service wind c test loads
d loads due to buffer forces e loads due to tilting forces
f loads caused by emergency cut-out
g loads due to dynamic cut-off by lifting force limiting device h loads due to dynamic cut-off by lifting moment limiting
device
i loads due to unintentional loss of hoist load j loads caused by failure of mechanism or components k loads due to external excitation of crane support l loads caused by erection and dismantling
The loads should be multiplied with the specific amplification factors, see Table 3. Table 3: Amplification factors (Mel, 2009)
Figure 10: Limit state method flow chart (NEN-EN 13001-1, 2009)
Figure 11: Allowable stress method flow chart (NEN-EN 13001-1, 2009)
2.5.3
Load combinations
The individual loads must be multiplied with partial safety factors which are only valid in connection with the limit state method in accordance with EN 13001-1 and superimposed in accordance with specified load combinations given in EN 13001-2. These load combinations are:
1. Load combinations A cover regular loads under normal operations 2. Load combinations B cover regular loads combined with occasional loads
3. Load combinations C cover a selection of regular loads combined with occasional and exceptional loads
The Calculation can be done using: 1. Limit state method is an approach
where individual loads are increased with amplification factors and with the correct partial safety factors. These are combined into load combination, resulting in combined stresses which are compared with limit design stress. Figure 10 is the flow chart illustrating the limit state method for the proof calculation based on stresses.
2. Allowable stress method is an approach where forces are combined with multiplication factors into load combinations. Resulting stresses are then compared with allowable stress, see Figure 11.
According to Mel (2009), the allowable stress method is a special case of the limit state method where all the partial safety factors are equal and combined to form an overall safety factor. The limit state method is applicable without any restriction while the allowable stress method is valid only for specific crane systems where a linear relation exists between load actions and load effects (EN 13001-2, ). The limit state method does take into account the probabilistic nature of the loads, whereas the allowable stress method does not.
2.5.4
Static calculations
The static calculations are discussed in EN 13001-3-1 (2013) and cover the following parts: 1. Structural members
As concluding remarks, the main difference between the old standards and the new one according to Mel (2009) are:
1. EN13001 uses the fatigue strength proof calculation method
2. No use of the mean stress of a cyclic load in EN13001, because it doesn't influence fatigue life 3. In EN13001, results are depending on the partial factor which is a design choice for specific
various weld details
4. Old standards assume that one load cycle results in one stress cycle, while EN13001 assumes more than one stress cycle for each load cycle
5. Old standard assumes one load cycle as an individual cycle, which doesn't take historical and future load cycles into account
The advantages of the new standard are:
1. Increasing fatigue calculation accuracy because different partial factors are applied to different loads
2. More capable of dealing with variable load spectrum using Rainflow counting method
The disadvantages of the new standard are:
1. The classification system lacks single structural component classification; it uses average displacement and average number of acceleration to classify mechanisms.
2. Use of position spectrum results in lower number of stress cycles which results in an increase of the limit design stress range
The following Table 4 gives a brief explanation on the classification differences for the different standards (Bos, 2015).
2.6
Fatigue design according to EN13001
This chapter gives a detailed review of the fatigue calculation, according to the current crane standard EN13001. This is the remaining parts of the design flowchart in
Figure 8: Structural model, Simulation, Stress time-history, Rainflow counting, Stress history
parameter, S-class, characteristic fatigue strength and the limit design stress range. The latter is the main goal of the standard for a specific detail.
In general there are two fatigue design philosophies (Vazifdar, 2002):
1. Safe life design - a design that there is an acceptable structure life reliability, resulting in heavy conservative structure, which is economical where regular inspections are impossible like for satellites floating in space.
2. Damage tolerant design - a design that is economically feasible because regular inspection is required to maintain an acceptable safety level. This is the only economic design philosophy for cranes. So if fatigue cracks were to occur in any given joint in the structure, the remaining structure should be able to safely carry loads until a routine inspection detects these cracks. With this philosophy questions could be answered like e.a. "what an acceptable risk of fatigue failure is"
As an introduction to the design against fatigue according to the standard, Figure 12 gives an overall illustration to estimate for the limit design stress range for a single specific welded joint at a location on the bridge. Because there are different welded joints at this cross section of the bridge and thus different weld details, different weld classes can be found in the weld atlas. Each specific detail subjected to dynamic loading results in different limit design stress ranges.
The old standard discussing crane fatigue design, was DIN 15018. The current new EN13001 standard is intended to "prove theoretically that a crane, taking into account the service conditions agreed between the user, designer and/or manufacturer, as well as the states during erection, dismantling and transport, has been designed in conformance to the safety requirements to prevent mechanical hazards" (NEN-EN 13001-1, 2009).
A proof of fatigue strength is intended to prevent risk of failure due to formation and propagation of critical cracks in structural members or connections under cyclic loading. Stresses in this standard are calculated in accordance with the nominal stress concept. This is a stress in the base material
adjacent to a potential crack location, calculated in accordance with simple elastic strength of materials theory, excluding local stress concentration effects. The standard does not use other methods like Hot Spot Stress Method (NEN-EN 13001-1, 2009).
The fatigue design is continued according to the steps in Figure 8:
1. Step 2 “Structural model”. The steps performed are: a. Create FEM model based on info of step 1 b. Add load combination from step 1 to FEM model
c. Perform static calculation for all crane sections and joints d. Static results are analyzed according to standard
e. Identify critical locations
f. Update critical cross sections until results satisfied static requirements Figure 12 : Limit design stress range philosophy according to EN13001
Figure 13: Fatigue calculations in NEN, FEM and EN13001 (Mel, 2009: 90)
a. Determine raw crane stress-time history based on designer specific input data and results from step 2, using simulation software like excel.
i. Spectrum could also be generated via actual measured test data from a crane in operation.
b) Step 4 “Stress time-history” is generated for one specific detail or for the whole crane. c) Step 5 “Rain flow counting” reduces the time history to a sequence of tensile peaks and
compressive troughs (Bos, 2015). The main steps of this algorithm are: a. Rain flow count simplify graph
b. Rain flow process
c. Making a stress cycle table
A detailed explanation about this method could be found in the literature from (Bos, 2015). d) Step 6 “Stress range histogram”. Via the previous step the stress range histogram for
individual stress ranges is generated, neglecting effects of previous and future load cycles. For each stress range, stress amplitude with accompanied number of cycles is determined
e) Step 7 “Stress history parameter”: during crane lifetime, the stress history is a numerical representation of all stress variations that are significant for fatigue. It is determined using the stress spectrum factor, the relative number of cycles and the total number of cycles. This parameter could be used directly to calculate the limit design stress range or indirectly by determining the S-class first.
f) Step 8 “S-class of stress history parameter”. Each estimated stress history belongs to a specific class S
g) Step 9 “Characteristic fatigue strength” is determined from the specific weld detail which is found in the weld catalogue in EN13001
h) “Specific resistance factor” is determined from the standard. It depends on the
accessibility and other hazardous conditions for maintenance personnel.
i) Step 10 “Limit design stress Range”
Mel, (2009:90) performed a fatigue analysis which showed a differences between the old standards see
Figure 13 with the new standard EN13001, for one specific weld detail with one specific stress-time spectrum. His analysis resulted in 4% difference between lowest and the highest unity check values.
Figure 15: General Crane fatigue life assessment approach
2.7
Critical structural crane crack length
According to Cai et al., (2014) who made an inspection of the critical points for the top boom section of a lattice boom crane which has been in service for eight years, he detected several different degrees of cracks, which in general depend on the application of the structure and its type (Rabiei, 2011:18) see Figure 14. The discovered cracks were in a stable or fast crack growth state. The latter was discovered, meaning cracks approached critical lengths and thus
endangered the structures safety, because that would cause fracture in the structural member, the boom was required to be repaired.
Although no practical standard is available mentioning critical crane crack lengths, Cai et al., (2014) concludes for the lattice boom crane, a maximum length of about 10mm to 15mm for the welded K-joints.
According to Straub and Faber (2005) two common definitions of structural failure are given:
1. Failure occurs when the fatigue crack becomes a trough thickness crack, which is assumed to be the critical crack length in first failure criterion. This is usually chosen to be equal to the structural element thickness.
2. Failure is reached with the initiation of a crack, where the initiated crack is often defined as the visible crack.
2.8
Crane Fatigue assessment
This chapter describes some crane fatigue assessment case studies which have been performed and the fatigue assessment methods and prediction of cranes.
Previous studies on fatigue life assessment were primarily focusing on bridge cranes, gantry cranes, harbour cranes and other cranes with high levels of utilization according to Kopnov (1999), Fan et al., (2011) and Xu et al., (2011). According to the mentioned fatigue life researches in Kopnov, (1999), Xu et al. (2014), Xu et al. (2015) and Tingjun et al. (2015), their crane fatigue assessment was conducted as follows:
1. An analytical analysis for a critical structural component based on design data or FEM analysis
2. Performed experimental analysis on this same component using strain rosettes applied to a critical
fatigue detail, providing the tested stress range over a certain period.
Figure 16: Fatigue life assessment flowchart (Cai et al., 2014) 3. Preprocessed this gathered data by compressing equal points, extracting peak and valley
values and eliminating invalid values.
4. The Rainflow method is used to obtain the number of cycles. These were then revised by Goodmann equation (Dowling, 2004) to achieve precise statistics.
5. The load spectrum of this detail was generated
6. Fatigue data using SN-curve for structural material was selected.
7. The total damage was calculated using Miner's principle to predict the residual fatigue life of the crane.
Cai et al., (2014) proposed a stress spectra acquisition method based on measured & simulated & compared & statistics integrated strategy of K-type welded joints for a 25t and 18m lattice boom crane which has been in service for 8 years. His fatigue life assessment is discussed as follows:
1. Load characteristic analysis. As a result of the variety from light to heavy used working conditions, crane loads change randomly. The weld details are subjected to high stress and variable amplitude multi-axial loading depending on the number of crane working cycles corresponding to a crane utilization class. The latter
represents the frequency of the crane use. When the number of cycles exceeds the utilization level
according to the crane standard, the crane may be in a state of fatigue failure.
2. Critical fatigue location analysis. Based on the different load condition determined in step 1, using FEM software it is capable to determine all critical locations on the crane based on static analysis. Load conditions results are compared and worst case scenario for fatigue critical joints are analyzed
further in detail by inspection to ensure these point match real locations on the crane as calculated by the software. If cracks are detected with critical crack lengths repair is necessary.
3. Stress-time spectra analysis. With multi body dynamic simulation software Adams, the stress-time spectra is obtained from the FEM model (this topic is reviewed in detail in chapter and from actual strain measurements using strain gages placed at the critical fatigue joint, a measured stress-time spectra is obtained. Measurements are performed under the same load conditions and working cycles as in the simulation environment. These spectra could be obtained for short as for long time measurement taking both crane fatigue assessment quality and economical consequences into account. Using the rain-flow counting method for both stress spectra, results in the corresponding stress amplitudes.
4. Non-linear cumulative Fatigue damage analysis. Material properties are required in this method. For the constant amplitude strain-life, the Basquin-Manson- Coffin (BMC) equation is applied
2.9
Manual inspection Vs intelligent structural monitoring
system
During the operational phase inspection, measurement and monitoring are required, prior to spending large amount of money on repair, refurbishment or retrofitting. The main questions to be asked during inspection are what to look for, where to look for and how to properly record data in the context of fatigue reliability.
The conventional and cheap visual inspection method has a very low accuracy level in providing data, because in the phase of crack initiation visual inspection is not possible due to the microscopic size. Because cranes are large structures with crack locations in blocked or inaccessible areas, it is very difficult to be noticed by visual inspection,
see Figure 17. In practice a so called Probability of Detection is used to judge the reliability of a used Non-destructive testing technique (ndt, 2015). Nondestructive inspection technology allows crane parts and material to be inspected and measured without damaging the structure. The currently used standard EN13001 doesn't propose anything about fatigue inspection technique, but only takes a fatigue strength specific resistance factor into account for the accessibility of safe and non- fail-safe components.
This develops the need for intelligent automated structural monitoring systems which could identify cracks from the very beginning at critical joint locations. Structural Health Monitoring provides automated damage diagnosis by combining damage detection algorithms with structural monitoring systems (Loh et al., 2006; 38(2):91–128). Intelligent structural monitoring system were used in the work of Kopnov,(1999), Qi et al(2012), Xu et al.,(2015) and Tingjun et al.,(2015) to measure stress and strain at critical locations in the structure.
Xie et al., (2013) and Huang et al., (2013) developed a method for optimal placement of gauges or sensors at these critical locations for example see Figure 18, based on the crane's FEM model see Figure 19. This results in actual raw data gathering for the different working conditions of the crane. A drawback is that the crane is a large structure with a huge amount of joints which are all potential fatigue locations. This large amount results that not all points can be covered as desired.
Figure 17: manually inspected Crane (Spiegel, 2015)
Figure 18: cantilever beam strain measurement point (Qi & Wang, 2013)
Wireless strain gauges Xu et al., (2015) were adapted to measure strains of interesting points, as illustrated in Figure 20. Of course this system has its
advantages of simple installation, convenient data transmission and short-term measurement and one main reliability problem; signal stability (Niu et al., 2014).
One must realize that the right measurement instrument must be used in agreement with the required data to harvest. From this data, value is created from a risk based inspection planning point of view. This is discussed in the following chapter.
2.9.1
Fatigue risk based inspection planning
To perform reliable fatigue risk based inspection planning, reliable data is required. This chapter will give a brief review about this topic.
According to Straub and Faber (2005) risk based inspection planning (RBI) is a risk prioritizing procedure, concerned with the optimal allocation of deterioration control of risks associated with the failure of components. It is a reliability based analysis procedure which combines the principles of risk and operational experience. RBI is distinguished in qualitative and quantitative procedures. For the application to crane structures
RBI procedures are quantitative, because it accounts for data and inspection quality and also makes use of probabilistic deterioration and inspection models. So in the end, the main objective is to make systematic decisions concerning structural maintenance problems.
Figure 20: Wireless strain gauges deployed at critical crane structure locations (Xu et al., 2015)
The state of the art RBI planning methodology presented by Straub and Faber(2005) is illustrated in Figure 21. The inspection decision maker needs quantitative input to make repair or no repair decisions based on economical and failure safe consequences. Like concluded in chapter 2.7 it is not clear, what the critical crack length for crane
structural failure is. So visible inspection doesn’t give a accurate indication of the physical size of the crack. Based on these uncertainties inspection performance models have been created, which are detailed described in Straub and Faber (2005).
Straub and Faber (2005) presented a generic updating framework to risk based inspection planning for steel structures in
Figure 22, which could also be applied for analyzing fatigue risk of crane components.
The approach is in general an experiment which couples the inputs (model parameters) with outputs (inspection plans) via an empirical relation.
The generic fatigue RBI modeling is established by the following: 1. Input
2. Deterioration models 3. Inspection models
4. Structural reliability analysis/ Simulation methods 5. Decision analysis
6. Output
The reader is referred to Straub and Faber (2005) for detailed explanations for this generic modeling. Jordan (2008) discusses the structural maintenance program of dockside container cranes. He gives an outline how to be able to make proper judgments about what to do when cracks are detected to improve the crane reliability.
Chen and Soares (2011) mentions some recent researches which have been done on the fatigue reliability analysis and inspection planning for marine structures based on fracture mechanics.
2.10
Joint FEM and multibody dynamics simulation for
crane fatigue analysis
Cai et al., (2014) used multi body dynamic simulation software Adams to obtain stress-time spectra from the crane’s FEM model using FEA software, seen in chapter 2.8. This coupling is reviewed in detail in this chapter to see if MBD and FEM have been applied as multidisciplinary design approach to fatigue reliability for cranes.
According to mscsoftware (2016) “a multibody dynamic (MBD) system is one that consists of solid bodies or links that are connected to each other by joints that restrict their relative motion. The study of MBD is the analysis of how mechanism systems move under the influence of forces. Motion analysis is important because product design frequently
requires an understanding of how multiple
moving parts interact with each other and their environment”.
A crane system could be modeled as a multi-body system, because it consists of several components which can be seen as bodies. These bodies are connected with various types of joints and wire ropes, see Figure 23. The MBD analysis and simulation of cranes is primarily about simulating the structure or mechanism’s dynamic characteristics in the working conditions of gantry travel, trolley travel and hoisting. An accurate system dynamics model is the most important in the analysis, improving the computational efficiency on the premise of the simulation accuracy is another problem, which is solved using computational multi-body system dynamics software.
The current most applied commercial software is the machinery dynamics analysis software ADAMS (Automatic Dynamic Analysis of Mechanical Systems). According to Ku and Roh (2014:1), it is a software system that consists of a number of integrated programs that aid an engineer in performing three-dimensional kinematic and dynamic analysis of mechanical systems to obtain vibration
characteristics.
This vibration induces fatigue to the crane components. In relation to the fatigue assessment, ADAMS software could be coupled with a crane’s FEM model to get more detailed insight in the occurring fatigue phenomena. Recent study from Cai et al., (2014) shows the use of a coupled/ joint FEM and MBD simulation to obtain stress-time spectra for fatigue assessment for a crawler crane. He used ADAMS MBD software.
