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 60 pages and 3 appendices. It may only be reproduced literally and as a whole. For commercial purposes only with written authorization of Delft University of Technology. Requests for consult are only taken into consideration under the condition that the applicant denies all legal rights on liabilities concerning the contents of the advice.
Specialization: Transport Engineering and Logistics
Report number: 2018.TEL.8202
Title:
Application of Carbon Fiber
Reinforce Polymer (CFRP) to
reinforce cranes structures
Author:
A.F. Rosan
Title (in Dutch) Toepassing van Carbon Fiber Versterkende Polymeer ter versterking van kraan constructie
Assignment: Literature assignment
Confidential: No
Initiator (university): dr.ir. X. Jiang Supervisor: dr.ir. X. Jiang
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 60 pages and 3 appendices. It may only be reproduced literally and as a whole. For commercial purposes only with written authorization of Delft University of Technology. Requests for consult are only taken into consideration under the condition that the applicant denies all legal rights on liabilities concerning the contents of the advice.
Student: A.F..Rosan Assignment type: Literature
Supervisor1 (TUD): dr.ir. X. Jiang (TU Delft) Creditpoints (EC): 10 Supervisor1 (TUD): Msc Z. Li (TU Delft) Specialization: TEL
Report number: 2018.TL.8202 Confidential: No
Subject: Application of Carbon Fiber Reinforce Polymer (CFRP) to reinforce crane structures
Carbon Fiber Reinforced Polymer (CFRP) has been widely used in advanced manufacturing, aircraft manufacturing, aerospace, concrete reinforcement and giant steel structure reinforcement. CFRP offers several advantages over steel because of the ease and speed of installation, the structural efficiency of the repair, the corrosion resistance of the materials, and the minimal effect that these materials have on structural dimensions aesthetics and versatility. This literature assignment aims to make an overview of the application of CFRP on reinforcement of crane structures. The following aspects are required to be illustrated in the report:
1. The reasons to demand such a reinforcement on crane structures 2. Available reinforcement methods and their respective characters
3. The definition of CFRP, its main material and mechanical property (affecting its applicability for reinforcement)
4. The state of the art - application of CFRP to reinforce crane structures (standards , installation, maintenance, experimental and numerical studies, etc. )
5. The future development of this reinforcement method. To explore the feasible way(s) to improve the performance of CFRP as a kind of reinforcement
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
Application of Carbon Fiber
Reinforced Polymer in crane
structures (CFRP)
A.F.Rosan (1325418)
T ec hnische Universiteit DelftA
PPLICATION OF
C
ARBON
F
IBER
R
EINFORCED
P
OLYMER IN CRANE
STRUCTURES
(CFRP)
by
A.F.Rosan (1325418)
in partial fulfillment of the requirements for the course literature assignment
at the Delft University of Technology, Supervisor: Dr. ir. X. Jiang
A
SSIGNMENT
Subject:Application of Carbon Fiber Reinforced Polymer (CFRP) to reinforce crane structures
Introduction
Carbon Fiber Reinforced Polymer (CFRP) has been widely used in advanced manufacturing, aircraft man-ufacturing, aerospace, concrete reinforcement and giant steel structure reinforcement. This method offers several advantages in the application of steel-structure because of the ease and speed of installation, the structural efficiency of the repair, the corrosion resistance of the materials, and the minimal effect that these materials have on structural dimensions aesthetics and versatility.
This literature assignment aims to make an overview of the application of CFRP on reinforcement of crane structures. The following aspects and sub questions are required to be illustrated in the report:
Main research question
What is the state of the art on reinforcement of crane structures by CFRP?
Sub research questions are:
• What are reasons to demand such a reinforcement on crane structures?
• Which reinforcement methods are available and what are respective characters?
• What is the definition of CFRP, its main material and mechanical property (affecting its applicability for reinforcement)?
• What is the state of the art - application of CFRP to reinforce crane structures (standards , installation, maintenance, experimental and numerical studies, etc. )?
• What are the future developments of this reinforcement method?
• Which are the feasible way(s) to improve the performance of CFRP as a kind of reinforcement?
Boundary conditions
The following boundary conditions were used during this assignment: 1. Reinforcement limit only to the cracks which are formed by fatigue 2. Steel reinforcement of cranes
3. Research limit to steel-plates and -pipes in 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
S
UMMARY
The cranes that are applied in the offshore industry has been operated for almost 20 years and do not full fill the current operation requirements anymore [1]. Operations nowadays are overloaded by the loads that the transportation which leads to fatigue. Examples of this similar problem are cited in [2–4]. Fatigue and aging develops cracks in the crane structure. The most sensitive part of the crane is the boom, because it hoists the loads [5]. Different reinforcement methods are applied to stop the crack propagation in the steel-structure of the crane. Another reason of reinforcement is preventing failures and extending the fatigue life of the steel-construction. In this report the following reinforcement methods are compared with each other:
• Mechanical fastening by bolting. By attaching a steel plate with bolts on the crack-location to be cov-ered and reinforced.
• Welding. The weld-machine with assistance of electrode, convert metal from solid form to the fluid form and closing the crack.
• Reinforcement with Carbon Fiber Reinforced Polymer1. CFRP layers or -sheets are laid down with an adhesive layer on the location of the reinforcement.
These aforementioned reinforcement methods have their own pros and cons. From the comparison it is deduced that CFRP reinforcement has the two best properties:
1. Stiffness 2. Lightweight
The material that is used in CFRP, consists of Carbon Fiber bonded by polymer [6,7]. The effectiveness of the CFRP reinforcement is investigated by the following research methods:
• Analytical. Here theoretical formulas are applied to obtain the added value of the CFRP reinforced steel-structures. For example the fatigue life time is determined by the Paris law formula [4,8–12] .
• Numerical. In this method simulations based Finite Element Method2are applied to study the effec-tiveness of CFRP reinforcement on the steel-structures. Most of the results are near of the results ob-tained by analytical- and experimental research. For example the highest stress locations and displace-ment in the steel-structure around the crack-location are represented.
• Experimental. Here the fatigue lifetime is determined by experiments with specimens reinforced by CFRP.
The conclusion from these research methods is that this reinforcement technique expands the fatigue lifetime of reinforced steel-structure. Study the effectiveness of CFRP the following parameters are examined:
1. The optimal thickness of adhesive -and CFRP layer 2. The number of CFRP layers and -thickness
3. The optimal orientation of CFRP reinforcement compared to the crack From practical view there are some methods proposed which can improve the bond:
• Application of the stapler or pop-nails to join the end of the CFRP layers on the steel-structure
• Applying different shapes of the end of CFRP layers
• Polishing, clean and coating of the crack location above CFRP layers can improve the joint strength
1CFRP 2FEM
vi 0.SUMMARY
• Applying extra load on the crack location during the dry process of the adhesive
The demand for CFRP reinforcement is growing in different sectors. From the life cycle analysis perspective CFRP reinforcement consists of several stages. The installation-, the maintenance- and the end of life are the commonly examined parameters. Unfortunately the standards for the crane CFRP reinforcement application are not developed, so other standards applications are proposed to apply. The reinforcement has to maintain to stay in the best possible shape. For this purpose two different inspection methods can be applied:
• Non Destructive Inspection [13–15]
– Digital image correlation method [14]
– Radio frequency eddy current technology [15]
• Destructive inspection or testing is not suitable for this application
Fatigue life time of the CFRP will gradually come to an end. Then the end stage will start with possibilities to recycle CFRP. Two ways of recycle CFRP are proposed:
1. Re-manufacture 2. Recycle
The crane boom in the future can be designed and built up by hybrid materials as applications of boom structure in offshore sector for example: Design of lightweight FRP T-boom for an offshore gangway [16].
S
AMENVAT TING
Heden ten dage zijn offshore kranen(hijswerktuigen) die 20 jaar terug op de markt zijn gebracht nog opera-tioneel [1]. Tegenwoordig worden zij gebruikt om zwaardere lasten op te tillen en te verplaatsen dan eigen-lijk voor ontworpen en het dageeigen-lijks hijsen van ladingen veroorzaken vermoeiing. Soortgeeigen-lijke voorbeelden met het zelfde probleem worden genoemd in [2–4]. Vermoeiing en veroudering leiden tot scheurvorming in staalconstructies van kranen. Om het probleem van verder scheurverloop te stoppen of andere schade te voorkomen worden de staalconstructies versterkt door verschillende methoden. Hierdoor wordt grote schade veroorzaakt door breken van constructies voorkomen. Elke versterkende methode heeft zijn voor -en nadelen. In dit rapport worden de volgende versterkende methoden met elkaar vergeleken:
1. Mechanische verbindingen 2. Lassen
3. Carbon fiber versterkende polymeer
Uit deze vergelijking kwam naar voren dat Carbon fiber versterkende polymeer zeer goede eigenschappen bezit zoals lichtgewicht en stijfheid. Deze CFRP techniek wordt verder beschreven in dit rapport. In het bij-zonder de toepassing op kraan staalconstructies. CFRP bestaat uit Carbon Fiber verbinding met Polymeer. De effectiviteit van CFRPversterking kan door verschillende methoden worden gemeten. De onderzoeksmetho-den die de effectiviteit van de CFRP versterkingstechniek onderzoeken:
1. Analytische methode 2. Numerieke methode 3. Experimentele methode
Deze onderzoeksmethoden hebben de volgende aandachtspunten naar voren gehaald die een sterke verbind-ing kunnen bewerkstelligen:
1. De optimale dikte van de lijm laag
2. Het optimale aantal lagen en - dikte van CFRP
3. De beste orientatie van de versterking CFRP ten op zichtte van de scheur
Vanuit een praktisch oogpunt kunnen de volgende methoden voorgedragen worden om de verbinding te versterken:
1. Toepassing van nieten of nagels bij de uiteinden van de CFRP lagen 2. Toepassing van verschillende vormen van uiteinden van de CFRP lagen
3. Schoongemaakte, gepolijste en gelakte van de scheur vergroot de sterkte van individuele CFRP lagen De vraag naar CFRP zal in de naaste toekomst alleen maar toenemen in verschillende sectoren. Vanuit de life cycle analysis zijn er verschillende periode in het leven van de versterking te onderscheiden die ook bespro-ken worden in dit rapport. De installatie-, onderhouds en levenseindfase worden behandeld. Eerst worden de voorbereidingen getroffen op de locatie, hierna wordt de installatie verricht. Deze processen worden con-form de standaarden uitgevoerd. Helaas zijn er CFRP versterking op kraantoepassen standaarden weinig tot helemaal niet ontwikkeld. Na de installatie moet er onderhoud plaats vinden om de CFRP versterking in de juiste conditie te behouden. Hiervoor zijn er verschillende non destructieve methoden aangedragen:
• Non destructieve inspectiemethoden. Hier wordt het materiaal zorgvuldig geinspecteerd om de nodige fouten op te sporen.
viii 0.SAMENVATTING
– Digital image correlation method
– Radio frequency eddy current technology
• Destructieve inspectie methoden kunnen hier niet van toepassing zijn. Omdat het materiaal dan totaal wordt vernietigd.
Door de toename van de vraag naar versterking met CFRP worden de mogelijkheden voor hergebruik onder-zocht. Twee methodes in het hergebruik van CFRP zijn:
• Remanufacture
• Recycle
Ook zullen bepaalde methoden en onderzoek punten aangedragen worden om een sterke verbinding te be-werkstelligen. Een nader onderzoek naar de kwaliteit van lijm en CFRP met name de mechanische eigen-schappen in het bijzonder de trek sterkte zou er toe bijdragen dat verbinding met CFRP sterker word.
P
REFACE
The crane structures built in the past are required to be replaced or maintained after a certain period. Steel structures built in crane, transport or civil engineering applications are subjected to various loads. In these applications cracks can occur due to fatigue in the steel structure. The cracks can lead to fracture with fatal effects. In order to prevent the crack growth reinforcement is applied. The reinforcement technique applica-tion of Carbon Fiber Reinforcement Polymer3will extend the life time of the steel structure. This technique consists of several advantages compared to other reinforcement techniques. The behavior- and performance of CFRP is investigated in this report. The standards which CFRP reinforcement has to fulfill on the differ-ent applications are also discussed. The durability of the CFRP reinforcemdiffer-ent is measured by performance indicators. The 3-main research techniques which are applied to measure the performance indicators ,they are: experiments, numerical-and analytical methods. A summary of the results of these methods are com-pared with each other. This literature assignment is a compulsory part of the study master Transport Engi-neering Logistics. The subject of the literature assignment is: Application of Carbon Fiber Reinforced
Poly-mer(CFRP) to reinforce crane structure. In the first chapter, the different reasons behind reinforcement are
investigated. In the second chapter the different reinforcement techniques are discussed. Hereafter the ma-terial CFRP itself with the properties are explained. Then the fourth chapter discusses the standards of CFRP, installation of CFRP and the research methods to measure the performance of CFRP. Hereafter the latest de-velopment about CFRP is discussed as a reinforcement method with a final conclusion. Herein also topics which need attention for future research are included. I would like to thank Mr. Msc Z. Li for supervising me during my literature study. The feedback was helpful and steered me into the right direction. Last but not least Dr.Ir. Jiang for her trust and the positive input and guidance which helped to achieve this final goal.
A.F.Rosan (1325418) Delft, January 2018
3CFRP
LISTOFABBREVATIONS
R
OMAN
S
YMBOLS
Symbol Description S.I. Units
A Area (m2) L Characteristic length (m) P Pressure (P aormN−2) g Gravitational acceleration (m s−2) h Height (m) l Length (m) n Surface normal (-) ˆ n Unit normal (-) p Pressure (k g m−1s−2)
K Stress Intensity Factor (N
m2)
n Surface normal (-)
E a Elastic modulus of the adhesive between the steel and the FRP composite
Ec Elastic modulus of the FRP composite consisting of FRP fiber and matched adhesive E f Elastic modulus of the FRP fiber
E s Elastic modulus of steel (-)
N f or Nor d N Number of cycles of the fatigue life (cycles)
M pa MegaPascal
G pa GigaPascal
σ Stress
G
REEK
S
YMBOLS
Symbol Description S.I. Units
ε Normal Strain (-) δ thickness (mm) θ Angle (◦) τ Shear stress (P a) σ Stress
S
UBSCRIPTS
Symbol Description max MaximumS
UPERSCRIPTS
Symbol Description ∗ Dimensionless quantity ˘ Dimensionless quantity xiA
BBREVIATIONS
Abbreviation Description
CFRP Carbon Fiber Reinforcement Polymer SIF Stress Intensity Factor
FEM Finite Element Method
SHS Square Hollow Section
LEFM Linear Elastic Fracture Mechanics FRP Fiber Reinforce Polymer
MVCCT Modified Virtual Crack Closure Technique MTS Maximum Tangential Stress criterion
SED Strain Energy Density
SERR Strain Energy Release Rate CTOD Crack Tip Opening Displacement MTS Maximum Tangential Stress criterion VCCT Virtual Crack Closure Technique
FCG Fatigue Crack Growth
EIFS Equivalent Initial Flaw Size
2D Two-dimensional
3D Three-dimensional
D
IMENSIONLESS GROUPS
Symbol Description Definition
E-Modulus Elasticity-Modulus F lo
C
ONTENTS
Assignment iii Summary v Samenvatting vii List of symbols xi 1 Introduction 1 1.1 Reasons of reinforcement. . . 1 2 Reinforcement methods 5 2.1 Mechanical fastening. . . 5 2.1.1 Bolting. . . 5 2.1.2 Welding . . . 8 2.2 CFRP technique. . . 9 2.3 Conclusion . . . 11 3 CFRP reinforcement technology 13 3.1 Introduction/main material. . . 13 3.1.1 Steel . . . 16 3.1.2 Adhesive. . . 16 3.1.3 CFRP. . . 17 3.2 Crack propagation . . . 21 3.3 Failure modes. . . 23 3.4 Conclusion . . . 254 Steel structures reinforcement with CFRP 27 4.1 Introduction . . . 27 4.2 Standards. . . 29 4.3 Installation . . . 32 4.4 Maintenance CFRP reinforcement . . . 33 4.5 Analytical research . . . 34 4.6 Experimental . . . 35 4.7 Numerical research. . . 38
4.8 End of life stage. . . 39
4.9 Conclusion . . . 41
5 CFRP reinforcement on circular shape 43 5.1 Experimental Research . . . 43
5.2 Numerical Research. . . 44
5.3 Analytical Research. . . 44
5.4 Conclusion . . . 45
6 Future Development of CFRP reinforcement on cranes 47 6.1 Introduction . . . 47
6.2 Feasible ways to improve the performance of CFRP. . . 47
6.3 Future trends . . . 48 6.4 Conclusions. . . 48 List of Tables 51 List of Figures 53 Bibliography 55 xiii
1
I
NTRODUCTION
1.1.
R
EASONS OF REINFORCEMENT
This chapter of the report describes the reasons why reinforcement has to be executed on crane structures. Then the applications of CFRP reinforcement in other fields are described. At the end a conclusion is drawn. Cranes in general are applied to hoist loads from place A to B. The following cranes types were examined in this research:
1. Crawler crane [17] 2. Drag-line [8] 3. Container crane [18] 4. Overhead crane [4]
The reason that those type of cranes is taken as example are that their boom-structure consist of plate- or pipe-shape. For example a crawler crane and drag lines are applied to hoist construction segments in the construction industry. Overhead cranes are built in several industry plants to move and hoist different loads. Hoisting after a number of years different loads continuously cause fatigue in the crane structure. Fatigue is the result of being subjected to varying and repeated loads [22] or as explained in [13] fatigue, or metal fatigue, is the failure of a component as a result of cyclic stress. Every crane has a lifetime from the manu-facturer. Crane steel structures are built up of parts which are connected to each other and are characterized with durability and strength. These two are parameters of the designed lifetime of the crane. The reasons of reinforcement are:
• Fatigue and aging reduce the strength and the durability [23]. For example container cranes in the harbor and on ships hoist different loads during the operation hours. Hoisting different loads lead after a while to fatigue in the crane structure.
• Hoisting heavier loads than designed purpose [8]. Loads which can lift and move depend on the crane design itself, the hoisting load -and carrying capacity. An example of the trend of lifting capacity is depicted in table1.1.
• Aggressive environmental conditions [24,25]. These locations are exposed to aggressive environmental conditions[24,25]. For example on sea on the offshore platforms and seaports.
• Damage and collision [3]. On every crane there could be collision and damage to cranes.
• Economical advantage [26]. Purchase of a new crane is multiple times more compared to the reinforce-ment costs for a crane.
In this report the scope will be limited to cracks caused through fatigue. In 95% circumstances fatigue occurs in the boom-structure of the crane, which is the most vulnerable part subjected to fatigue quoted in [5,17]. Fatigue mostly occurs in steel-members and connections, such as crane-steel beams, welded connections of
2 1.INTRODUCTION
Figure 1.1: Crawler crane and dragline [19,20]
beams and columns under cyclic tensile loading [10]. In other words aging of cranes is an important factor to consider [2,8] which can causes fatigue. Fatigue results into crack-forming that has to reinforce to prolong the durability of the structure. The strength related to the fatigue is determined by taking into account the following parameters [27]:
1. The number of cycles of a crane loading and unloading (operation) (N) 2. The stress spectrum in the crack
3. The material of the crane and crane structure
4. The working/operation environment of the crane [24,25]
The first one represents the fatigue lifetime and the second one is related to the stress which occurs in the steel-structure. These parameters will be discussed further in chapter four. In table1.1the load increase development of Container cranes from the brand Kone and Liebherr are compared. The crane type shows a substantial increase compared to the first method which is developed. Applications of steel structures in different working fields, for instance are:
• Civil engineering has a lot of applications of steel structures in bridges, buildings, road signboards, cellular antennas [29], side protectors of highways and cranes [8,30]. A more specific application of
1.1.REASONS OF REINFORCEMENT 3
Figure 1.2: Container crane and overhead crane [18,21]
fatigue in steel-bridge are vehicles passing over a bridge, for example the Erasmus-bridge consist of load-variations. These load-amplitudes and load variations lead to fatigue.
• Maritime- or offshore engineering. The applications in the offshore - or maritime sector are the drill platforms, container cranes and pipelines. Drill platforms operate on the sea to obtain oil in aggressive environmental conditions. The basement of the drill platform has to deal with all kinds of loading conditions.
• Mechanical and process engineering. The pipelines which transport gas or fluid cause internal stresses due to pressure and temperature. Stresses induce cracks into the wall of the pipelines. The environ-mental conditions of the pipeline location has an effect on the formation of cracks in particular outside the pipeline.
• Aerospace engineering Air-crafts are built up by CFRP applications.
Some other common situations where the steel structure needs strengthening during its lifespan [25] are: 1. Upgraded loading requirements
4 1.INTRODUCTION
Crane type Loads by Konecranes Loads by Liebherr Panamax 30-40 tons/m 30-45 tons/m Post Panamax 35-55 tons/m 45-55 tons/m Super Post Panamax 46-90 tons/wheel 60-80 tons/m
Table 1.1: The load increase of container cranes [28]
3. Initial design flaws
Other causes of fatigue are the defects of the steel structure which occur as a result of lack of fusion during the welding process (welding seams), including volume defects (thinning, pits), or corrosion [31,32]. The reinforcement techniques proposed in the reviewed scientific papers are:
1. Bolting [3,4,30,33] 2. Welding plates [3,4,33–35]
3. Application of Carbon Fiber Reinforce Polymer [3,4,12,30,33–41]
These reinforcement techniques will be discussed in the next chapter. By reinforcement of steel structures the life span is expanded. Every piece of equipment which contains a steel structure possesses a fatigue life. This fatigue life is declared by the manufacturer and it differ for each type of unit or machine. For cranes the life expectancy is 20-25 years [42] compared to the operating time of existing steel pipelines that is 20-30 years [41]. Manufactures provide their cranes with an operating life time and the maintenance intervals to acquire the best performance of the crane. To get an insight into the fatigue with working loads on the structure of the crane, a calculation is made below. For example the container crane operation in the harbor of Rotterdam lifts a container every 45 seconds see table1.2[43]. In one hour this crane make= 3600/45= 80 cycles. Consider that this crane is working 24 hours in one day then the total number of cycles= 1920 cycles see table ??. Per year 42048000 ton weight is carried by the crane. The effects of fatigue are apparent because the growth of
Cycles /hour Cycle per day Yearly operation 3600/45= 80 cycles 80 *24 =1920 cycles 700800 cycles Average hoist per year 60 tons/cycle 365 * 1920 * 60= 42048000ton weight
Table 1.2: The normal operations of cranes on harbor
the cracks can cause failures. Therefore it is necessary to take measures in order to extend the fatigue life of steel-structure. Another cause of crack formation is the aggressive environmental condition. Damage and collision are also causes to reinforce the steel structures. So fatigue is caused by increase of weight loads and normal lifting or hoisting,aging of the crane-structures, aggressive environmental conditions,damage and collision. In the other application fields CFRP reinforcement has already shown its extended value. The main things which a reinforcement achieve are that the crane can operate again, the failure in the crane-structure is prevented for a period. Last but not least the financial benefits resulted by postpone the purchase of a brand-new crane. In the following chapter the aforementioned three reinforcement techniques are presented. In chapter three steel-structure reinforcement technique with CFRP is highlighted in particular the material and its properties. Then chapter four gives the state of the art of the reinforcement technique with CFRP applied on beams or plates. Chapter five the reinforcement of CFRP on steel-structures with the shape of pipes is worked out. Finally chapter six the future development of the CFRP application as reinforcement technique and conclude with a conclusion part.
2
R
EINFORCEMENT METHODS
Reinforcement of crane structures are executed with different methods. In this chapter the following rein-forcement techniques are described:
1. Bolting [3,4,30,33,44,45] 2. Welding [3,4,33–35]
3. Reinforcement with CFRP [3,4,12,30,33–41]
In the further part of this chapter the first two reinforcement techniques bolting and welding are described. Thereafter the CFRP technique is investigated.
2.1.
M
ECHANICAL FASTENING
In this part mechanical fastening is meant by bolting and welding. First of both the application method is discussed then these two reinforcement techniques are discussed with their corresponding standards and properties.
2.1.1.
B
OLTINGBolting is done by thin steel plates and bolts on the thicker steel element see figure2.1. The corresponding standards applied in bolt joints are:
• EC3 [22,23]. EC3 stands for Eurocode 3, it implies the bolt-positions: the minimal- and maximal dis-tances of the edge-and between the bolts.
• An important standard often applied is NEN –EN-ISO 898-1:2009 [46]. This represents bolts strength, other mechanical- and physical properties for parts of joints.
• Another important standard of the bolt is the strength class ,see table2.1.
Class Yielding stress
4.6 240 GPA
8.8 640GPA
10.8 900GPA
Table 2.1: Bolt class [22,23]
The installation process first starts with drilling the holes for the bolts into the steel-plates and in the location of the steel-structure. The steel plates can be attached on existing steel structure in order to close the crack or gap. Here after the steel-plates are fastened on crack location with bolts. The preparation in the workshop can reduce the required job-time on the structure. Examples for bolt joints are mentioned in:
6 2.REINFORCEMENT METHODS
Figure 2.1: Forces on the bolting reinforcement [44]
1. One bolt reinforcement joint see figure2.2[44] 2. Two bolt reinforcement joint see figure2.3[30]
In figure2.2an example is shown which consist of only one bolt-connection. The plates are situated into the center of the crack to cover the whole crack see figure2.2. Therein is a center initial crack depicted in figure
2.2b. The setup of double bolt-reinforcement with double steel-plates or cover-joints is depicted in figure
2.3. In between the steel-plates and the CFRP lamina, an adhesive layer is placed between the plates. The
Figure 2.2: Single bolt joint and hybrid joint [44]
different loads in a bolt connection which can occur [23] see figure2.1: 1. Tension forces (compression or strain)
2.1.MECHANICAL FASTENING 7
2. Shear forces in the bolt joint
Figure 2.3: Bolting reinforcement [30]
For reinforcement with little applications in steel-structure where the weight is not of vital importance the bolt method is convenient and fast. But locations in a narrow side to execute reinforcement is very difficult. A summary with the pros and cons of the bolt technique is described in table2.2. The weight of the steel-plates
Sources Cons Pros
[3] Increase self- weight Fast method for little applications [3] Introduce additional stress concentration zones
[3] Time consuming
[3] Costly
Table 2.2: Pros and cons of bolting
is additional which increases the total weight of the whole structure see table2.2. The extra joints on steel-structure create or introduce additional stress concentration zone at location where the bolts are fastened. This method requires a lot of preparations on the structure which translates itself in a lot of time. The cost for this method depend on the number of labor hours required, the material and the equipment which are high.
8 2.REINFORCEMENT METHODS
2.1.2.
W
ELDINGWelding is a process where a welding electrode is melting due to short-circuit and this is caused by the elec-trode and the steel [23]. The environment where a weld process is executed influences the mechanical quality of the weld. Reinforcement in this case means that the metals and electrode are melted down into the crack. Standard EN ISO 4063 establishes a nomenclature for weld and allied processes [23].
The weld processes are classified into 6 groups [47]: 1. Arc welding
2. Electric resistant welding 3. Autogenous welding 4. Tension welding 5. Bundle welding 6. Remain group
The first group, arc welding is the most applied group, this can further classify into: 1. MIG/MAG welding [47]
2. TIG welding [47] 3. Powder welding [47] 4. BMBE welding [47]
MIG/MAG welding
The only difference between the MIG and MAG welding is that the gas in the MAG is the active and in the MIG is the inert. The above mentioned are mostly automated processes and the TIG a hand-process, here mullein consist a tungsten to achieve the weld arc. Compare to the previous one this is a slow process. The applications are stainless steel and aluminum.
Powder welding
Powder supports the powder welding process by protecting the atmosphere [47]. Standard ISO 3834-2 guar-antees the quality of the weld from order acceptation till handing over of the product [47]. Welding technique to reinforce cracks is done by welding-machine apply with electricity and gases [48]. An example of the weld-ing process is shown in figure2.4. The disadvantages of the welding process are worked out here under. The welding process results into the increase of temperature of the steel-structure and afterwards the cooling down ensue crack-forming. This phenomenon creates residual stresses which also affect the steel-structure. Figure2.5depicted the reinforcement of longitudinal crack at the back. This shown the thickness of the weld on the corner of 2 plates. Application of welding reinforcement is also done in Evans [50], Bhowmick and Grondin [51]. In [51] the flanges of the existing steel columns are reinforced with steel plates. A summary of the cons and pros of welding are described in table2.3.
The disadvantages are worked out here under. The welding procedure promotes stress- and weight-increase
Sources Cons Pros
Extra residual stresses [7,51] Faster joint process [52] Initial imperfections [51] Robust and cheap [52] Affects a reduction of local strength [7] No filler material required [52] Structures where fire risks must be minimized [7] Accurate process [52] Table 2.3: Cons and pros of welding
in the steel-structure. Stress on the steel-structure and the plates around them results into displacement of the different layers. And due to displacement the stress increase in the layers which can lead to crack prop-agation. By welding the risk for fire is increased. In particular petrol refineries, hospitals and other related locations there the fire risk should absolutely avoided. Overall the welding process has it pros and cons com-pared to the CFRP reinforcement technique.
2.2.CFRPTECHNIQUE 9
Figure 2.4: Arc welding [49]
Figure 2.5: Weld crack reinforcement [50]
2.2.
CFRP
TECHNIQUE
In the structural engineering this technique is already applied for almost 50 years [6]. The rest of the report the CFRP technique is fundamental. The choice for this technique is here under argued with the pros and cons see table2.4. As seen in figure2.6one beam is reinforced with one layer and the other is reinforced with 4 layers. Beside the main reinforcement function the application of CFRP technique consist more functions [3,4,32,33,35,51,54]:
• Delays propagation of the crack [32,55]
• CFRP layers also protect the crack from outside influences [55]
• The stiffness is enhanced [54] by applying this technique
• Last but not least it extends the fatigue life [55] of the steel structure.
The application of reinforcement technique consists of the following three materials see figure3.14: 1. The existing steel-structure
10 2.REINFORCEMENT METHODS
Figure 2.6: CFRP crack reinforcement [53]
2. The adhesive
3. The Carbon Fiber Reinforcement Polymer1
These three materials will explore in the next chapter. Table2.4shows the choice for CFRP reinforcement
Sources Properties Bolting Welding CFRP
[3,4,6,7,25,30,36–38,56] High strength + + +
[3,4,6,7,25,36,37,56] Light weight - - +
[3,4,7,36,37,56] No Corrosion - - +
[3,4,30,36–38] High fatigue resistance or performance - - +
[52] Economical advantage - - +
[52] Joint is homogeneous stress distribution - - +
[52] Fast method + + +
Table 2.4: Comparison of the reinforcement techniques with their properties
technique with their properties. The overall structure-weight will not increase due to the application of CFRP and adhesive [3,4,6,7,25,36,37,56] compare to bolting and welding. It is proved by [25] that reinforcement with steel require an amount of 175 kg and CFRP reinforcement require only 6.2 kg of CFRP see table2.3. In table2.3the cost-analysis and advantages of CFRP application are wrote out compare to steel. The weight reduction with a factor of 28 results into an advantage for CFRP in comparison to the cost-price (economical) advantage with factor of 15 for steel. The high fatigue resistance is a plus point of the CFRP application as a
Material type Amount (kg/m) %(+/-) price (USD/kg) Total material costs %(+/-)
CFRP [57] 6.2 2822% 110 682
Steel[58] 175 0.25 43.75 1558%
Table 2.5: Cost analysis
consequence of it fatigue is delayed. The quality of the reinforcement can be improved by the following: 1. The strength of the separate materials which are applied [7,56]
2. Weight saving [6,7,25]
2.3.CONCLUSION 11
2.3.
C
ONCLUSION
Reinforcement of steel-structures using the traditional mechanical fastening reinforcement methods shows a lot of disadvantages. Extra residual stresses or stress concentrations, increase self-weight in the steel-structure pave the way for the technique named CFRP reinforcement . The most important advantages are the high strength and the lightweight after CFRP reinforcement. Another advantage is the ease of application in particular narrow locations for instance beneath a big column or with another obstacle in front or clearly perceptible, this delays the reinforcement installation progress. This causes a lot of difficulties to drill a hole for bolting or weld for reinforcement purposes. The comparison of total material costs exhibits that reinforce-ment with steel is cheaper. But as already reinforce-mentioned in the other properties, labor and installation costs put CFRP in a favor position. In the following chapters the improvement on fatigue life is distinctly shown. The cost aspect, the savings of labor, easy installation [56] aspects and corrosion resistance are the main pillars to choose for the CFRP reinforcement method. The next chapter works out the whole technique with the concerned materials and their properties.
3
CFRP
REINFORCEMENT TECHNOLOGY
The materials applied in the CFRP reinforcement technique and their related properties are described in this chapter. CFRP reinforcement on steel-structures consists of three materials which are mentioned in the previous chapter. This technique is already applied into the automotive- [59,60], aerospace - [61,62] and civil industry [36]. First a description is given about steel material itself which requires to reinforce. Here after the materials adhesive and CFRP are worked out. Then an introduction into the failure modes of the joint is given followed by a conclusion.
3.1.
I
NTRODUCTION
/
MAIN MATERIAL
Before the introduction of the main material CFRP itself, first a brief summary of the most frequent applied properties is clarified. The steel structure is built up by parts. The parts consist of different shapes: bars, beams or pipes. These parts are joined on each other by welding or bolting. Due to fatigue cracks occur in the steel-structure. In this part of the report, different material properties of will explained. The fatigue which is already mentioned is caused by load cases. Load cases are for instance:
• Compression
• Tension
• The wind, storm or other weather influences which work as forces on the crane
The relationship between load and surface area is characterized by stress, see equation3.1. Here the distribution and the location of the peak stress and crack are depicted of bare steel plate and the stress-distribution of a reinforced steel plate with CFRP. It is obvious that the peak stress in the crack of the bare steel-plate is higher compared to the one of the reinforced crack with CFRP see3.1. The stresses results into
Figure 3.1: Stress distribution before and after reinforcement with CFRP [26]
expansion or increment between two small line segments of the structure. This is called elongation or normal strain (ε) of the small segments. Shear strain is on the other way around the change in angle which occurs between two small line segments that are originally perpendicular to each other [64]. Due to the changes in the segments of the structure, the alteration can be temporarily or non-temporal. The strain is determined by the relationship betweenδl= the increase of length or extension of the length and L= original length (3.3). The stiffness and the strength are the basic subjects on which a structure is based on. Resistance against
14 3.CFRPREINFORCEMENT TECHNOLOGY
Figure 3.2: Strain [63]
Figure 3.3: Shear strain [65]
Figure 3.4: Shear stress and shear strain [66]
reversible (temporal) deformation is stiffness and the irreversible (non-temporal) deformation case is called strength.
Plastic deformation in the plastic zone is irreversible and elastic deformation in the elastic zone is reversible see figure3.5. The stress concentration occurs at sections where the cross-sectional area suddenly changes.
3.1.INTRODUCTION/MAIN MATERIAL 15
In case of a crack this creates larger stress concentration. The stress is determined in [64] by: σ =F or ce(F (kN ))
Ar ea((A(m2))= E ∗ ε (3.1)
The formula3.1represents the calculation of stress (N/m2) which is the relationship between loads (Newton) working on the cross-section area (m2). This formula represents also the relationship between the E-modulus multiply by strainε. The determination of E- modulus is done by the formula:
E = F L
δL ∗ A (3.2)
The formula3.2of E-modulus (N/m2) is built up by the F represents force (kN) multiplied by original length (L) which is divided by the extension of the length (δL) multiplied by cross section area (A). This E modulus is a measure for the stiffness of the material and represents the linear elastic mechanical behavior. To get a better view of all different material mechanical properties of the steel-structure, the CFRP and the adhesive are compared see tables3.1,3.3and3.4. The highest CFRP E-modulus in [67] have the value of 640 MPa, which is the strongest in this research. In all the other reviewed papers the values of the CFRP E-Modulus vary between 100 till 400 Mpa.
ε =δl
L (3.3)
As every material, steel consist of an elastic zone and a plastic zone see figure3.5. These values in figure3.5
Figure 3.5: Stress (f )-Strainε diagram for steel in tension [68]
are obtained from standard tensile stress experiments. The tensile stress notifies the ultimate tensile force with a steel-structure per area. This figure3.5is called a stress-strain diagram. Begin of the elastic zone the stress and the strain increase linear till the yield point (Fy). This is also the border of the elastic zone and the plastic zone where the Fy (Yield Point) occur. The strain- hardening zone is between the plastic zone and the failure zone. That zone, here the ultimate tensile strength crosses the two aforementioned zones. The failure zone stops at the fracture point see figure3.5. Two properties which involve in the further of the report are:
16 3.CFRPREINFORCEMENT TECHNOLOGY
• Ductility is a measure of the material’s ability to undergo significant plastic deformation before rupture, which may be expressed as percentage of elongation or percentage of area reduction from a tensile test
• Brittle on the other way around when it is subjected to stress, the material breaks without significant plastic deformation
3.1.1.
S
TEELIn this paragraph the properties of steel will be described. Some of them are already explained in the part here before. Actually, steel is specified with code Sxxx, here by xxx characterizes the value of the yielding stress. In table3.1the frequently mentioned mechanical properties namely are :
1. Yielding stress 2. Elastic modulus 3. Tensile stress
The values of the E modulus differ from 200 till 210 GPA, from this can be concluded that there is not a big discrepancy between them. And as known stiffness is related to E-modulus so the stiffness stays stable. Ten-sile stress on the other hand differs much bigger from 563 till 320 MPA. The standard mechanical properties
E-Modulus Tensile stress Yield stress
Q345B steel [4] 206 GPA 563(Ultimate)MPa 408 MPa
S355[37] 210GPA 500MPa 355 MPa
JIS SM400A [34] 204.5GPA 453 MPa 293 MPa Wrought iron [36] 200GPA 320MPA 220MPA
EN 10025 [3] 200GPA 320MPA 220MPA
Table 3.1: Material mechanical properties of steel
of steel pipes are collected from the steel companies which provide these products [69].
3.1.2.
A
DHESIVEIn this part the function and mechanical properties which affect the effectiveness of the adhesive are dis-cussed. The function of the adhesive is to join CFRP or FRP on the steel structure see fig3.14. Another vital function is: "To transfer energy- and stress of the crack to the composite or CFRP material [32,54]". On the top of the crack area in the steel structures the CFRP is glued with the adhesive layer. This can be up- or below the crack location or position of reinforcement on bottom-flange in [4] see figure4.4. Or as applied in [30] here a double reinforcement underneath and upside of the fatigue crack see figure2.3. In the ideal situation fatigue crack surface require no preparation, cures rapidly and maintains high bond strength under all the operating condition [70]. But in practice this is regrettably not the case. The requirement is to clean the steel-surface before the adhesive-layer is lay down on the steel-structure. From table3.3the conclusion can be drawn that the E- modulus of adhesive material ranges between 2.5 and 8 GPa. The tensile strength ranges from 24.8 to 40 MPA. This concludes that there is not a big scatter between maximum- and minimum value. Due to failure after the CFRP reinforcement still the possibility exists that a failure can take place in the adhesive part (see figure3.14). The optimal performance of the bond depends on the adhesive-thickness. If the adhesive - thickness is very thin or too thick than the adhesive strength is not strong enough as it should be. In Yue et al. [4] quoted that the optimal thickness of the adhesive layer is 0.2 mm for plate shapes. For circular steel shapes in [41] the proposed optimal adhesive thickness between 0.1-.014 mm give the best strength. Greater than the thickness of 0.14 mm there is no significant improvement. A sufficiently low viscosity encourages the flow of the adhesive in the steel surface and fill the pores [7]. These conditions affect the effectiveness of the adhesive joint:
1. High temperature [3] 2. Water [3]
3.1.INTRODUCTION/MAIN MATERIAL 17 Test results of bolted joints reinforcement
B1 B2
Calculated values
Yield load[kN] (un-reinforced specimen) 114.4 114.4 Ultimate load[kN] (un-reinforced specimen) 101.3 101.3
Experimental results
Composite failure load [kN] 84.1 86.4
Ultimate load [kN] (un-reinforced specimen) 117.1 107.1 Test results of double side reinforcements
R1 R2 H1
Yield load[kN] (un-reinforced specimen) 114.4 114.4 -Experimental results
Yield load[kN] (reinforced specimen) 112.1 120.1 111.1 Elongation at yield load[mm] (reinforced specimen) 1.57 1.93 1.78
Ultimate load[kN] 118.1 120.6 117.2
Test results of double lap joints
J1 J2 J3
Yieldload[kN] (un-reinforced specimen) 114.4 114.4 114.4 Experimental results
Yield load[kN] (reinforced specimen) 112.6 115.6 Elongation at yield load[mm] (reinforced specimen) 1.79 1.65
Ultimate load [kN] 117.6 120.4 104.9
Table 3.2: Test results of bolted joints reinforcement/double side and double lap joints [30]
E-Modulus (GPa) Tensile strength (MPa)
Yue et al. [4] 2.5 40.96
Sikadur[3,30] 30 4.5 24.8
330 3.8 30
(Sikadur-31 Normal) [71] 8 29.7
Epoxy resin adhesive [34] 1.5 30
Table 3.3: Material mechanical properties of adhesive
4. Salt spray conditions [72]
Further the brittle- and the ductile-behavior are exhibited by the following adhesive Sikadur 30 and Sikadur 330 [30]. The most fragile part of the joint is the steel-adhesive interface [3] herein there is still room for research.
3.1.3.
CFRP
In this part the material Carbon Fiber Reinforcement Polymer1(see figure3.8) is introduced. First the ma-terial mechanical properties are depicted in table3.4. Then the advantages of CFRP are stated. The overall functions of CFRP [10] are:
1. Reduction of the effective stress range around the crack tip 2. Reduction of the crack opening displacement
3. Promotion of crack closure
18 3.CFRPREINFORCEMENT TECHNOLOGY
Carbon Fiber Reinforcement Polymer(chapter2.2) consist as the basis material the carbon fibers embedded non-metallic composite material made of polymer resin [37]. Composite is a mixture of different materials which behave as one material. Other sorts of composites consist with fiber are:
1. Fiber [Carbon, Glass] 2. Resin [Polymer] [73] 3. 3= (1+2) = CFRP
An impression of common different fiber-forms and -structures are mentioned here and depicted in figure
3.7:
1. Short Fibers figure3.7
2. Chopped Fibers figure3.7
3. Long Fibers figure3.7
4. Woven Fibers figure3.7
In other composites for example are GFRP, the Carbon of CFRP is substituted by glass [9] see figure3.6.
Fig-Figure 3.6: Comparison of Stress-strain relationship of CFRP and Mild steel [74]
ure3.6justifies that the CFRP strength is far ahead compared to the strength of mild steel and -Glass Fiber Reinforced Polymer2. Here the relationship between strain and stress is depicted, CFRP has highest value of 2500 MPa (stress) corresponds with 1.5 % (strain). The improvements which can contribute to a better and stronger joint are discussed in the conclusion part. The content of Carbon Fiber Polymer consists at least 90% and up to 100% weight of carbon [37]. By applying the reinforcement of CFRP, the disadvantages of weld-ing and boltweld-ing disappear. For example, the initial imperfections are prevented because the CFRP layer has a fixed thickness compare to welding and bolting. Another advantage is that large preparations are not required before adding the adhesive and CFRP on the steel-structure. In the next chapter this will be discussed fur-ther. Extension of fatigue lifetime it is required that the strength of the bond enough to suffer number of load cycles. The mechanical properties possess an essential contribution to achieve this requirement. Here under a table3.4with the most applied CFRP material is depicted. The different CFRP sheets which are applied are:
3.1.INTRODUCTION/MAIN MATERIAL 19
Figure 3.7: Different fiber forms [73]
Figure 3.8: CFRP materials Jiao [33]
Material type E Modulus (GPa) Tensile strength (MPa) Thickness
Carbon Fiber [37] Standard 230 3530
High strength 294 7060 High modulus 441 3450 CFRP [3] 195 2800 CFRRP strips[4] 245 4306 CFRRP fiber sheets [75] CF130 240 3800 CF530 640 2650 CFRP [67] normal 100<CFRP<250 high modulus 640 CFRP plates[36] 150/2000 167.2 2.710MPA 1.2 mm CFRRP fiber sheets[30] M614 200 2.8 CFRRP laminates sheets[56] 430 2.8
Table 3.4: CFRP mechanical properties
sheets [26,33,38], pultruded plates [33], strips [3,34], plates [30,39,76], pre stressed plates [36,77], laminates [78],patches [11]. [33] proved that reinforcement applying CFRP plate performs better than CFRP sheets. It
20 3.CFRPREINFORCEMENT TECHNOLOGY
is recommended by the reviewed literature to apply pre-stress CFRP layers or sheets [3,25,79]. The CFRP sheets with pre-stress level are recommended in the reviewed literature because of the following advantages:
1. The pre-stress level avoids or delaying the fatigue crack [3] 2. Leads to reduction of the mean stress level [3,25]
3. Stiffer behavior [25]
4. Improving serviceability and durability [25]
Effectiveness of CFRP bond depend on the following parameters are:
• Thickness of the CFRP layer [3,4,34,41]
• Number of CFRP layers [3,4,53,54]. Number of layers maximum applied in [4] are longitude CFRP five layers with the one layer-thickness of 0.167 mm. This means the overall thickness of the CFRP is 0.835 mm. It gave the best results confirmed by experiments in [4]. That the thickness of one fiber was 0.131 mm, four layers = 0.524 mm and of the adhesive was 0.869 mm.
• Orientation of the reinforcement layer compare to the crack (orientation) [9,10,56,80]. The orientation of the layers contributes to a stronger bond between the CFRP and the steel-structure [56].
• Stress concentration around the crack due to load conditions [3,4,30,33]
The first highlighted paper [9] showed that the ninety 90 degree orientation of the CFRP patch layer laid on a crack steel-plate. The other two orientations of the directions [10,35,56] are highlighted:
• Transverse means perpendicular to the direction of the crack.
• Longitudinal means parallel to the direction of the crack. In [10] the longitudinal fiber direction is signed with the vector for plate applications.
Another example of orientation applied in the CFRP reinforcement is [80] namely: 1. One layer placed transversely with one layer longitudinal called 1T1L 2. Two layers places transversely with two layers longitudinal called 2T2L
Here the reference of orientation is that the direction of the axial load. The stress concentration in crane cause due to high-cycle of fatigue [27]. High-cycle fatigue represents the stresses remain generally in the elastic region [27]. The orientation 2T2L in figure3.9shows extra ability to bear greater force and -displacement
Figure 3.9: Force-displacement relationship of different orientation [80]
3.2.CRACK PROPAGATION 21 Shape Bond type Source Adhesive type
Steel plate layer [4] Steel beam strips [3]
Pipe Wrap [41]
Circular Tube Patching [8]
Table 3.5: Results of the shear and normal stress concentrations in different locations [39]
Figure 3.10: Comparison of Stress-strain relationship of different FRP [81]
3.2.
C
RACK PROPAGATION
In this part the crack-propagation is discussed. The crack propagation is the process of the crack after it is initiate. This process is a reaction due subject to the loads to the CFRP reinforced steel-structure. The stages of the crack propagation process are depicted in figure3.11. Crack due to fatigue go through different stages
22 3.CFRPREINFORCEMENT TECHNOLOGY
is depicted in the figure3.11. In the first crack initiation period the initiation of the crack at micro-scale starts and can be considered as the longest period. From there it develops into the macro scale. The final two stages can be considered as the fracture event where the fracture occurs. Fatigue promotes reduction of durability and strength leads to crack forming see figure3.12. This figure3.12depicted a plate with the thickness of 2h
Figure 3.12: Crack forming due to subjected load [82]
is subjected to force in the vertical direction that result in enforced displacement with crack forming. First form of the crack is a little notch. The direction of the crack propagation also shown here. In this figure3.13
Figure 3.13: Crack propagation at crack tip [10]
the behavior of the crack tip is represented with the related stresses. The crack propagation starts at the crack tip. The two stressesσ1work on both side increase toσ2. Here due to the stress the distance or crack-length is growing in the vertical side in the direction where the stress work. But on the other side the crack propagate into the horizontal direction as well both in two dimensional. The stress than increase fromσ2toσ3then the curve widens in the vertical direction and also the crack length in the horizontal direction increases. In the final stage, the maximum stress represented byσ(max) till the maximum both in the vertical direction and in the horizontal direction. In the horizontal direction it reaches the maximum and goes through the thickness of the beam. These behavior, the relationship between increase of fatigue crack length and the stress. It shows that the difference of the crack length relates to the time in stress. But stress one is bigger than stress two and than stress three. The crack propagates through the thickness of wall of material. The propagation of the fatigue crack can cause failure of the whole structure. In [83] the classification of the crack mode is:
• Passive, in this period the crack growth can considered as passive not growing
• Semi-active in this period the crack starts to grow and propagate with a little speed
3.3.FAILURE MODES 23
3.3.
F
AILURE MODES
In this part the failure modes which can occur in the CFRP reinforcement. Due to fatigue the possibility is still available that the crack can propagate in every part of the joint. This is depicted in figure3.14. A failure in
Figure 3.14: Schematic view of failure modes [67].
the CFRP reinforcement of the steel-structure means that this can propagate itself through the structure and can lead to a destruction. The various possible failure modes [7,30,56,67] in a CFRP bonded steel system subjected to a tensile force are:
• Steel and adhesive interface debonding [30,67] this is the debonding of the combination from the steel layer and adhesive
• Adhesive layer failure [56,67] occurs in the adhesive layer. The adhesion failure mostly occurs in the steel/adhesive interface instead of the FRP/adhesive interface [3]. Thin adhesive layer can be a cause for this failure.
• FRP and adhesive interface debonding [56,67] where the failure take place the combination of the frp -and adhesive layer.
• FRP delamination [67] is a failure into the horizontal direction of the FRP layer see figure3.14. FRP delamination is one of the most common failure modes for composite structures and this works per-pendicular direction of the FRP rupture [84]. Delamination results into a separation of adjacent layers at locations sensitive to transverse effects [85]. At the moment carbon fibers come in contact with the steel surface a galvanic cell is obtained which results in to galvanic corrosion.
• FRP rupture [56,67] can consider as a cleavage of the CFRP material in the vertical direction see figure
3.14the opposite direction of the delamination
• Steel yielding [67] is a failure occur in the steel-structure Failure modes([67]) depend on properties of the materials:
• The elastic modulus of CFRP sheets (230-640GPA)
• Types of adhesive
24 3.CFRPREINFORCEMENT TECHNOLOGY
For exact determination, when and where the failure occurs and the effectiveness of the FRP, the inter-facial behavior between steel and FRP is investigated [7]. The weakest link is the bonding between CFRP plate and the metallic substrate [71]. The experiments executed in [30] confirm the failure mode of steel and adhesive interface debonding and take place after yielding of the steel-plates. Debonding of the CFRP layer [7,10,39,
40,86] is the phenomenon in the reinforcement, where the CFRP layer is pulling out from the adhesive layer on the steel-structure. This phenomenon is reducing the quality of the CFRP bound. The debonding failures has in common the highly stress and can occur as:
• Intermediate debonding [7] Intermediate debonding mostly starts at the crack or a location of concen-trated plasticity where FRP is highly stressed [7]
• Plate end debonding [10]. In [7] plate end debonding occurs due to high localized inter-facial shear stress and peeling stresses in the vicinity of the plate end.
Figure 3.15: Debonding [10]
Phases of debonding are shown in figure3.15shows that in period one that no debonding occurs. Point A indicates that the micro debonding start to the SIF3. This point A corresponds with the period one. Next point B indicates the transition period half debonding length reaches the half crack length. Here it goes from micro debonding to macro debonding. And in the third state elliptical debonding occurred around the crack tip. Opening and ovalization of the crack tip. In [10] conducts the debonding and crack-propagation for CFRP using FEM at the top of the crack. The research in [40] conducts crack growth rate depend on:
1. The change of shape of the debonded region 2. The increase of the size of the debonded region
These two parameters have influence on the debonding region. The moment the debonding of the CFRP strips starts at the joint, the strength of this becomes weaker and causes failure. Another paper [39] stated that debonding failure, is caused by stress concentration at the notch location. Complete debonding of the CFRP strips also take place in the experiments with the CFRP strips results into specimen failure in [30]. The debonding in relation to the SIF are conduct in [10]. Here also a formula is proposed to determine the debonding area. The debonding area is assumed to be an ellipse where bd (debonding) = the crack length see figure3.15. The relationship between half crack length and debonding and the fatigue cycles is depicted here3.15. From this can derive the fatigue life is related to the debonding process. The transition of the first
3.4.CONCLUSION 25
two processes i and ii the crack length stays stable. In the third process the crack length starts to increase and then the debonding area starts to increase as well. Due to these two parameters increase the fatigue cycles increase so the fatigue life is getting to its end. Complete debonding comes here along. The crack- and the debonding propagation in figure3.15has the same parallel curve.
3.4.
C
ONCLUSION
The elastic modulus, the tensile - and the yield stress are the most influential properties because of their contribution to the strength- and stiffness of the CFRP reinforcement. The other influential properties of CFRP are:
• Weight
• No corrosion forming
• Stiffness
The separate materials of the CFRP reinforcement consist of their own behavior. This create a separate be-havior for the one material itself. Depend on the stress concentration on the crack or the debonding location and how it fluctuate and where the failure first occur in the CFRP reinforcement. Behavior of a crack itself after reinforcement not accurate predictable. The different CFRP joint failures and the weakest part of it are highlighted. Improvement of the adhesive layer can help the fatigue life to extend after CFRP reinforcement. To attack the debonding problems:
• Extra strong adhesive on the crack/notch location
• Reduction of SIF of the debonding locations
Another point of interest where future research can focus on is the interface debonding between the adhesive and the steel-structure because of the micro vacuum which occur in the crack. The debonding increase so fully debonding occur. As the fatigue cycles increase, the stresses create the debonding increase. The crack behavior in particular the propagation in normal steel structure is different compare to reinforced steel-structure. But this depend on the following factors:
1. Number /thickness of CFRP layer 2. Thickness of the adhesive layer
3. Sort or type of CFRP layer for example pre stress
Debonding occurred at locations where the highest stress concentration occur. So the relationship between these two is the higher the stress concentration, the bigger the area of debonding. It leads to the reductions of the CFRP strength and further propagation of the crack.
4
S
TEEL STRUCTURES REINFORCEMENT WITH
CFRP
In this chapter the state of the art of CFRP reinforced steel structure with a shape of plate and beams are investigated. The most important added value of the CFRP reinforcement is how long it can bear to be in operation without failure, by the fatigue life time. It relates to the strength and the durability of the bond. To measure the strength and the durability of this reinforcement joint first the standards has to be fulfilled. The type of adhesive, CFRP and the procedure of bonding both CFRP and steel elements are the main limitations of a successful strengthening method [56]. The first paragraph gives an introduction the CFRP reinforcement process. Then the standards apply in CFRP reinforcement are discussed. Here after the CFRP installation on steel-structures is described. Then numerical-, experimental- and analytical researches are applied to measure the effectiveness of the reinforcement. At the end a conclusion is worked out.4.1.
I
NTRODUCTION
Steel structures are nowadays reinforced by welding and bolting (see chapter 2). CFRP reinforcement of the crane booms in particular is highlighted very poor. That is why the available research literature from other sector applications can guide for this application. As already mentioned the fatigue in the crane-boom results into the reinforcement. The most existing shapes of the crane structure boom are:
1. Plate shape [17]
2. Circular/round shape [8]
The first one is discussed in this chapter and the second one in the next chapter. Fatigue lifetime of steel CFRP reinforcement on steel-structure goes through different stages. The stages of lifetime are depicted here in fig-ure4.1. The stages of the fatigue life time will be explored further in this chapter. For a correct representation of the fatigue life it has to model. The models of fatigue are classified [27] into:
• S-N (Stress based approach).These S- N based approach represented the fatigue life in the relationship between stress amplitude and the number of cycles to fatigue failure. This is based on experimental data.
• Fracture mechanics approach. Fracture Mechanics is based on the assumption of initial existing cracks in a structure which propagate during cyclic loading.
The main difference between the S-N approach and the Fracture Mechanics approach is, that the S-N ap-proach is applicable at the initial stage of a microscopic crack at which most cracks will not be detected with a NDI1tool and FM2approach used in the phase where an initial crack is detected until the critical crack length [27]. The most applied formulas of FM are:
1Non Destructive Inspection 2Fracture Mechanics
28 4.STEEL STRUCTURES REINFORCEMENT WITHCFRP
Figure 4.1: Stages of the life cycle
• Paris law
d a
d N = C (∆K )
m (4.1)
This formula consists the following variables:
– dA represents the size of the crack length
– dN the number of cycles together load force. The example in chapter one gives a clear indication of it.
– K is the Stress Intensity Factor3. The K , Stress Intensity Factor4[4,9–11,32,40,54] characterizes the elastic stress field intensity of the crack front (or crack tip) applied in fracture mechanics.
– C and m are material constants and standard values for steel [76].
The basic assumption of Linear elastic fracture method (LEFM) is that the growth of a crack is controlled by the stress field at the crack tip, it follows that the crack growth will be characterized by SIF [38]. In this formula the relationship between crack-length and the number of cycles are related to the SIF.
• Stress intensity range K in fracture mechanics for plate shapes in [10,38] is determined by:
K = f σpπa[10,12,38] (4.2)
Here thefis the geometrical factor,σ stress, π
• Fatigue life of the number of cycles to failure (for circle applications) N (a) = Z a ai d a C (∆Ke f f)m [8] (4.3)
• Crack length at failure
Besides of the Paris law, the VCCT formula can apply to determine the fatigue life. The assumption here is that the amount of energy require to separate a surface is the amount of energy require to close the same surface [87]. Fatigue lifetime here is determined by: VCCT5. This technique determines the energy require to close the crack [9,12,41,84,88]. The assumption here is that the life time can relate as well to the VCCT
3SIF 4SIF
