DETECTION OF DAMAGE INITIATION IN COMPOSITE STRUCTURES SUBJECTED TO SELF-HEATING BASED ON ACOUSTIC EMISSION

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MODELOWANIE INŻYNIERSKIE 2017 nr 64, ISSN 1896-771X

DETECTION OF DAMAGE INITIATION

IN COMPOSITE STRUCTURES SUBJECTED TO SELF-HEATING

BASED ON ACOUSTIC EMISSION

Angelika Wronkowicz

^1a

, Andrzej Katunin

^1b

1Institute of Fundamentals of Machinery Design, Silesian University of Technology

aangelika.wronkowicz@polsl.pl, ^bandrzej.katunin@polsl.pl

Summary

Structural integrity is one of the crucial properties of designed composite elements. However, during their operation they are subjected to various types of loading, and thus, subjected to degradation. Intensity of their degradation is driven by many factors with different degree of influence. One of such degradation mechanisms occurs during cyclic loading, when heat is released on the surface of a polymeric composite structures due to the mechanical energy dissipation, which has a great influence on degradation acceleration. The paper deals with determination of a criticality of the self-heating effect, i.e. temperature value at which damage initiation occurs during cyclic loading of composite structures. For this purpose, polymeric composite specimens were subjected to fatigue tests and resulting surface temperature and acoustic emission were measured and analyzed. The obtained results indicated that analysis of acoustic emission features enables returning information about the two critical moments of the degradation process. From a linear peak amplitude one can assess a critical moment between the two first phases of the three-phase degradation model, when propagation of micro-cracks and initiation of a macrocrack occurs.

Analysis of the amplitude, together with the energy ratio and the total energy of hit-cascade, can also accurately indicate a moment of transition from the second to the third phase, when a macro-crack propagates rapidly, which finally results in a failure of the structure.

Keywords: self-heating effect, acoustic emission, composite structures, damage initiation

DETEKCJA INICJACJI USZKODZEŃ W STRUKTURACH KOMPOZYTOWYCH PODDANYCH SAMOROZGRZANIU NA PODSTAWIE EMISJI AKUSTYCZNEJ

Streszczenie

Integralność strukturalna jest jedną z kluczowych właściwości projektowanych elementów kompozytowych. Jednak podczas eksploatacji są one narażone na różnego rodzaju obciążenia i dlatego ulegają degradacji. Intensywność ich degradacji jest uwarunkowana przez wiele czynników z różnym poziomem istotności. Jeden z takich mechanizmów degradacji występuje podczas obciążenia cyklicznego, gdy ciepło jest uwalniane na powierzchni polimerowej struktury kompozytowej ze względu na dyssypację energii mechanicznej, co ma istotny wpływ na przyspieszenie degradacji. W artykule omówiony został proces określenia krytyczności efektu samorozgrzania, tj. wartości temperatury przy której następuje inicjacja uszkodzeń w strukturach kompozytowych. W tym celu próbki wykonane z kompo- zytu polimerowego zostały poddane badaniom zmęczeniowym oraz ich wynikowa temperatura powierzchni oraz emisja akustyczna została zmierzona i przeanalizowana. Otrzymane wyniki wykazały, że cechy uzyskane w opar- ciu o emisję akustyczną pozwalają na pozyskanie informacji o dwóch krytycznych momentach procesu degradacji.

Na podstawie liniowej amplitudy szczytowej można określić krytyczny moment pomiędzy dwiema pierwszymi fa- zami w trójfazowym modelu degradacji, gdy następuje propagacja mikropęknięć i inicjacja makropęknięcia. Anali- za amplitudy razem z wartościami energii oraz całkowitą energią kaskady impulsów pozawala także dokładnie

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wskazać moment przejścia z drugiej do trzeciej fazy, gdzie makropęknięcie szybko propaguje, co ostatecznie d prowadza do zniszczenia struktury.

Słowa kluczowe: efekt samorozgrzania,

1. INTRODUCTION

Composite structures have found wide application due to their unique properties, such as high strength and stiffness with simultaneous weight savings when co pared to traditional metallic materials. Examples of application are the automotive and aircraft industry, where composites are used for manufacturing of respo sible elements, such us shafts, turbine blades, helicopter propellers, rotors, etc.

A fatigue process of elements made of polymeric comp sites is a very complex phenomenon due to heterogeneity and anisotropy of properties of such materials. Thus, a structural degradation is a consequence of many physical phenomena occurring during fatigue process. Many elements made of polymeric composites are subjected t cyclic loading during operation. A part of mechanical loading energy per cycle is not returned mechanically but is dissipated, i.e. transformed into heat. This is mainly due to a viscoelastic nature of polymeric matrices of composites, matrix plastic deformation, and non reversible processes such as micro-cracks onset and their propagation, internal friction and fiber breakage [ These processes are the cause of out-of-phase oscillations between stress and strain magnitudes. The energy release leads to a self-heating effect, which causes rapid heating and significant intensification of fatigue proces es. Considering low thermal conductivity of a majority of polymers used for manufacturing of composites a generated heat is accumulated inside a struct

causes fast temperature growth. Therefore, the self heating effect is a very dangerous phenomenon since it leads to sudden structural degradation and a failure of a composite structure [3]. Obviously, the process of diss pation of generated energy is an irreversible process [5], which may significantly intensify structural degradation of polymeric composites.

During fatigue of composite structures induced by self heating effect three typical phases can be observed in the case of domination of self-heating effect (see an exemplary temperature evolution in Fig. 1). In the first phase an exponential temperature increase occurs, in the second phase the self-heating temperature grows linearly and in the third phase a rapid temperature growth appears until a failure. The first phase can be associated with initiation of micro-cracks, debonding at weak fibers/matrix interface, and breakage of fibers with low strength. During the second phase, micro

gation and initiation of a macrocrack can b

In the third phase the macro-crack propagates rapidly

kazać moment przejścia z drugiej do trzeciej fazy, gdzie makropęknięcie szybko propaguje, co ostatecznie d

efekt samorozgrzania, emisja akustyczna, struktury kompozytowe, inicjacja uszkodzeń

Composite structures have found wide application due to their unique properties, such as high strength and stiffness with simultaneous weight savings when com- pared to traditional metallic materials. Examples of

lication are the automotive and aircraft industry, where composites are used for manufacturing of respon- sible elements, such us shafts, turbine blades, helicopter

A fatigue process of elements made of polymeric compo- complex phenomenon due to heterogeneity and anisotropy of properties of such materials. Thus, a structural degradation is a consequence of many physical phenomena occurring during fatigue process. Many elements made of polymeric composites are subjected to cyclic loading during operation. A part of mechanical loading energy per cycle is not returned mechanically but is dissipated, i.e. transformed into heat. This is mainly due to a viscoelastic nature of polymeric matrices ormation, and non- cracks onset and their propagation, internal friction and fiber breakage [2,7,11].

phase oscillations between stress and strain magnitudes. The energy heating effect, which causes rapid heating and significant intensification of fatigue processes. Considering low thermal conductivity of a majority of polymers used for manufacturing of composites a generated heat is accumulated inside a structure, which causes fast temperature growth. Therefore, the self- heating effect is a very dangerous phenomenon since it leads to sudden structural degradation and a failure of a ]. Obviously, the process of dissi-

gy is an irreversible process [5], which may significantly intensify structural degradation

During fatigue of composite structures induced by self- heating effect three typical phases can be observed in

heating effect (see an exemplary temperature evolution in Fig. 1). In the first phase an exponential temperature increase occurs, in the heating temperature grows linearly and in the third phase a rapid temperature growth until a failure. The first phase can be associated cracks, debonding at weak fibers/matrix interface, and breakage of fibers with low strength. During the second phase, micro-cracks propagation and initiation of a macrocrack can be observed.

crack propagates rapidly

and finally a failure occurs due to fiber breakage [ Note that temperature-driven fatigue occurs only when loading magnitudes and the resulting stresses are high enough to initiate intensification of propagation of initially occurred damage at the micro level. These microscopic damage sites transform into macroscopic damage and finally cause structural failure. In case when loading is below a certain critical value only two of the mentioned phases occur and self

stabilizes at a certain (usually low) value. The influence of heating in this case has a negligible impact on fatigue of a loaded structure.

The moment of initiation of the third phase can be considered as a critical one since a rapid temperature growth (see Fig. 1) can be observed. This temperature growth is related to development of a macro

additional surfaces resulted from cracking, which are subjected to friction and additional heating

structure. Considering the above-

critical self-heating temperature was previously dete mined from approximation of a self

history curve [4], and the moment of discrepancy b tween approximation curve and measured tem data (at the beginning of the third phase) was consi ered as critical.

Fig. 1. Typical temperature evolution during thermal fatigue In this study, the authors decided to analyze an a plicability of an alternative approach to evaluation of critical self-heating temperature by application of acou tic emission (AE)-based methods due to their high sensitivity to micro-cracking phenomena [

presented study is a part of ongoing research connected with determination of a criticality of the sel

effect in fatigue process of polymeric composites, i.e. a temperature value at which damage initiation occurs.

During performing fatigue tests of polymeric composite kazać moment przejścia z drugiej do trzeciej fazy, gdzie makropęknięcie szybko propaguje, co ostatecznie do-

emisja akustyczna, struktury kompozytowe, inicjacja uszkodzeń

and finally a failure occurs due to fiber breakage [8,9].

driven fatigue occurs only when loading magnitudes and the resulting stresses are high nsification of propagation of initially occurred damage at the micro level. These microscopic damage sites transform into macroscopic damage and finally cause structural failure. In case when loading is below a certain critical value only two of the ned phases occur and self-heating temperature stabilizes at a certain (usually low) value. The influence of heating in this case has a negligible impact on fatigue

The moment of initiation of the third phase can be itical one since a rapid temperature growth (see Fig. 1) can be observed. This temperature growth is related to development of a macro-crack and additional surfaces resulted from cracking, which are subjected to friction and additional heating-up of a -presented criterion, the heating temperature was previously deter- mined from approximation of a self-heating temperature history curve [4], and the moment of discrepancy between approximation curve and measured temperature data (at the beginning of the third phase) was consid-

Fig. 1. Typical temperature evolution during thermal fatigue In this study, the authors decided to analyze an ap- plicability of an alternative approach to evaluation of

heating temperature by application of acous- based methods due to their high

cracking phenomena [6,10]. The presented study is a part of ongoing research connected with determination of a criticality of the self-heating effect in fatigue process of polymeric composites, i.e. a temperature value at which damage initiation occurs.

During performing fatigue tests of polymeric composite

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DETECTION OF DAMAGE INITIATION IN COMPOSITE STRUCTURES

specimens, the surface temperature and simultaneously AE were measured and analyzed. Determining the critical self-heating temperature value can be very helpful during design of composite elements subjected to high-magnitude cyclic loading or vibrations.

2. SPECIMENS AND TESTING PROCEDURE

Specimens used for fatigue tests were manufactured from an epoxy reinforced by a plain weave glass fabric, which were supplied by Izo-Erg S.A. (Gliwice, Poland) in a form of a laminated composite sheet. It was cut to obtain the specimens with dimensions: 1

mm × 2.5 mm (length × width × thickness). These specimens were subjected to fatigue loading on an own designed test rig (see Fig. 2a) in a constant deflection mode with a frequency of 30 Hz and initial loading force of 90 N. The specimens were clamped in such a way that a working length that was subjected to deflection was equal to 50 mm. During the tests a surface temper ture distribution was monitored by an infrared camera and AE was measured by sensors glued to a non deflected part of a specimen (see Fig. 2b).

Fig. 2. Experimental setup (a) and AE sensor tested specimen (b)

Additionally, loading parameters (loading force, accele ation and velocity of vibrations) were monitored by an accelerometer and laser Doppler vibrometer, r

ly. The specimens were loaded until reaching a maximal defined value of a self-heating temperature of 100°C.

specimens, the surface temperature and simultaneously zed. Determining the heating temperature value can be very helpful during design of composite elements subjected to

magnitude cyclic loading or vibrations.

TESTING

Specimens used for fatigue tests were manufactured from an epoxy reinforced by a plain weave glass fabric, which Erg S.A. (Gliwice, Poland) in a form of a laminated composite sheet. It was cut to obtain the specimens with dimensions: 100 mm × 10 mm × 2.5 mm (length × width × thickness). These specimens were subjected to fatigue loading on an own-

in a constant deflection and initial loading force lamped in such a way that a working length that was subjected to deflection was equal to 50 mm. During the tests a surface temperature distribution was monitored by an infrared camera and AE was measured by sensors glued to a non-

en (see Fig. 2b).

Fig. 2. Experimental setup (a) and AE sensor glued to the

Additionally, loading parameters (loading force, acceleration and velocity of vibrations) were monitored by an accelerometer and laser Doppler vibrometer, respectively. The specimens were loaded until reaching a maximal

heating temperature of 100°C.

3. AE FEATURES MEASUREMENT

AE refers to transient elastic waves that result from a sudden energy release from a local source within a material under stress. In case of polymer

sites, sources of AE result from microstructural changes such as matrix micro-cracking, delamination, fiber cracking and breakage, fiber pull

interface debonding [1,13]. Following t be an excellent tool for investigation of real

mechanism formation in a composite structure subjected to cyclic loading and correlation of observed AE events with self-heating temperature values.

The energy of AE is usually proportional to the strain energy released from newly created crack surfaces. AE waves propagate according to the sound velocity of a material, and can be detected and located with an AE sensor array. Other factors that influence stress waves propagation include attenuation, re

and geometry of a tested material. A classical method of identification of AE sources is based on analysis of conventional AE features (e.g. peak amplitude, counts, duration and energy of a signal). A basic approach AE analysis is based on correlating the traditional AE signal features with the speci

mechanisms occurring in composites. A schematic repr sentation of basic AE features of a single acoustic hit is presented in Fig. 3.

Fig. 3. AE features describing a hit (based on [12

In this study, the AE signal was measured by means of the system Vallen^® AMSY-5. A series of features of hit data sets were measured. The term hit means the AE signal of one event received in a channel. Hit data ar generated by each AE channel independently and d scribe usually one or a series of discrete AE signals with a clear beginning and end. The beginning of a hit is defined by its first threshold crossing and the end by absence of threshold crossings in a sp

time. A hit-cascade is a series of discrete hits. In order DETECTION OF DAMAGE INITIATION IN COMPOSITE STRUCTURES (...)

E FEATURES MEASUREMENT

AE refers to transient elastic waves that result from a sudden energy release from a local source within a material under stress. In case of polymer-matrix composites, sources of AE result from microstructural changes

cracking, delamination, fiber cracking and breakage, fiber pull-out, or fiber/matrix ]. Following this, AE seems to be an excellent tool for investigation of real-time failure mechanism formation in a composite structure subjected to cyclic loading and correlation of observed AE events

heating temperature values.

oportional to the strain energy released from newly created crack surfaces. AE waves propagate according to the sound velocity of a material, and can be detected and located with an AE sensor array. Other factors that influence stress waves lude attenuation, reflection, refraction and geometry of a tested material. A classical method of identification of AE sources is based on analysis of conventional AE features (e.g. peak amplitude, counts, duration and energy of a signal). A basic approach of AE analysis is based on correlating the traditional AE signal features with the specific fracture micro- mechanisms occurring in composites. A schematic repre- sentation of basic AE features of a single acoustic hit is

s describing a hit (based on [12])

In this study, the AE signal was measured by means of 5. A series of features of hit measured. The term hit means the AE a channel. Hit data are generated by each AE channel independently and de- scribe usually one or a series of discrete AE signals with a clear beginning and end. The beginning of a hit is defined by its first threshold crossing and the end by absence of threshold crossings in a specified period of cascade is a series of discrete hits. In order

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to investigate sensitivity of various AE features to resulting micro-cracks appeared during fatigue induced by self-heating effect the following features were selected during performed measurements of loaded specimens:

• linear peak amplitude in QV (ALIN) a measured peak amplitude of a hit in dB,

• energy ratio (ETE) – energy is calculated by squa ing the digitized AE signal and integrat

sult during the hit. The energy is given in Energy Units (eu) and 1 eu corresponds to 10

the sensor output (preamplifier input),

• total energy of hit-cascade (CENY) energy is the sum of the energies of the all subsequent hits in a hit-cascade,

• threshold crossings (CCNT) – cascaded counts are the total number of positive threshold crossings during a hit-cascade. Range is 1 to 216

cascade), cascaded hits (CHIT) – hits in a hit-cascade.

In the next section, the signals obtained during the tests are presented and analyzed regarding to their sensitivity to micro-cracking of a matrix of the tested composites subjected to fatigue loading with the self

occurrence.

4. RESULTS AND DISCUSSION

The obtained AE measurements during cycling loading of the exemplary specimen subjected to cyclic loading with appearance of the self-heating effect are presented in Fig. 4.

to investigate sensitivity of various AE features to cracks appeared during fatigue induced heating effect the following features were selected

ormed measurements of loaded specimens:

QV (ALIN) – derived from a measured peak amplitude of a hit in dB,

energy is calculated by squar- ing the digitized AE signal and integrating the re- during the hit. The energy is given in Energy Units (eu) and 1 eu corresponds to 10−14 V²sec at

fier input),

cascade (CENY) – cascaded energy is the sum of the energies of the first-hit and

cascaded counts are the total number of positive threshold crossings cascade. Range is 1 to 216-1 (per hit-

total number of

obtained during the tests are presented and analyzed regarding to their sensitivity cracking of a matrix of the tested composites subjected to fatigue loading with the self-heating effect

RESULTS AND DISCUSSION

The obtained AE measurements during cycling loading of the exemplary specimen subjected to cyclic loading heating effect are presented

Fig. 4. Measurements of various AE features during cyclic loading of the exemplary specimen

In all of these figures one can notice a sharp increase of a given parameter near the end of data series, which may indicate rapid propagation of a macro

thus initiation of damage, which corresponds to the moment of transition from the second phase to the third phase of the self-heating temperature evolution.

From a zoomed view of the AE data (Fig.

noticed that the following features: ALIN, ETE and CENY reveal that rapidly increasing trend of acoustic activity starts in the ca. 225th second of loading, wher as CCNT and CHIT indicates the ca. 233rd second. The self-heating temperature values occurred in these two moments of time are respectively 84.11°C and 86.1°C, based on the acquired series of thermograms.

. Measurements of various AE features during cyclic

In all of these figures one can notice a sharp increase of a given parameter near the end of data series, which may indicate rapid propagation of a macro-crack and thus initiation of damage, which corresponds to the from the second phase to the third heating temperature evolution.

From a zoomed view of the AE data (Fig. 5) it can be noticed that the following features: ALIN, ETE and CENY reveal that rapidly increasing trend of acoustic ts in the ca. 225th second of loading, where- as CCNT and CHIT indicates the ca. 233rd second. The

heating temperature values occurred in these two moments of time are respectively 84.11°C and 86.1°C, based on the acquired series of thermograms.

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Fig. 5. Envelopes of the measurements of AE features (zoomed view of selected data)

A history curve of a self-heating temperature during cycling loading of the exemplary specimen is presented in Fig. 6. The 225th second of loading, i.e. when the first rapid increase of the acoustic events was observed, was highlighted in this figure. Analyzing this figure it can be concluded that this moment is convergent with the moment of transition from the second to the third phase in the three-phase degradation model.

Fig. 6. Self-heating temperature curve during cyclic loading of exemplary specimen.

The results presented in this paper have been obtained within the framework of research grant no.

2015/17/D/ST8/01294 financed by the National Science Centre, Poland.

References

1. Deneshmehr A., Asa A., Abazary S.: A study on the failure mechanisms of composite laminates using acoustic emission monitoring. “International Journal of Current Engineering and Technology” 2012, Vol. 2, p. 409 2. Kahirdeh A., Khonsari M.M.: Energy dissipation in the course of the fatigue degradation: Mathematical deriv

tion and experimental quantification, “International Journal of Solids and Structures” 2015, Vol. 77, p. 74 3. Katunin A., Fidali M.: Fatigue and thermal failure of polymeric

vanced Composites Letters” 2012, Vol. 21, p. 64

. Envelopes of the measurements of AE features (zoomed

heating temperature during cycling loading of the exemplary specimen is presented . The 225th second of loading, i.e. when the first increase of the acoustic events was observed, was highlighted in this figure. Analyzing this figure it can be concluded that this moment is convergent with the moment of transition from the second to the third phase

heating temperature curve during cyclic loading of

Analyzing the AE data for earlier moments during cyclic loading (before the described rapid growth of temper ture), only ALIN reveals a significant increase in ca.

50th second, which may indicate the transition from the second to the third phase of degradation process. A reached temperature value at this moment was 57°C.

Analysis of the rest of AE features returned less evident results and a critical moment between the tw phases of the three-phase degradation model could not be clearly noticed and identified.

5. CONCLUSIONS

In the presented study, an alternative approach for evaluation of critical self-heating temperature by means of acoustic emission was presented.

various AE features collected during experimental fatigue studies of composite structures with appearance of the self-heating effect were analyzed for evaluation of their sensitivity to critical degradation events that accompany a fatigue process. The obtained results indicated that the linear peak amplitude feature is the most sensitive to micro-cracking events occurred during fatigue. Moreover, the preformed analysis shows that there exists a relation between both phase transitions in the investigated fatigue processes: an increase of magn tude of this feature was related with a transition from the first to the second phase as well as with a transition from the second to the third phase, which indicates criticality of the self-heating effect. Further studies will be focused on application of advanced signal processing methods to analyze collected data in more detail.

2015/17/D/ST8/01294 financed by the National Science Centre, Poland.

Deneshmehr A., Asa A., Abazary S.: A study on the failure mechanisms of composite laminates using acoustic

“International Journal of Current Engineering and Technology” 2012, Vol. 2, p. 409 Energy dissipation in the course of the fatigue degradation: Mathematical deriv tion and experimental quantification, “International Journal of Solids and Structures” 2015, Vol. 77, p. 74 Katunin A., Fidali M.: Fatigue and thermal failure of polymeric composites subjected to cyclic loading vanced Composites Letters” 2012, Vol. 21, p. 64-69.

DETECTION OF DAMAGE INITIATION IN COMPOSITE STRUCTURES (...)

Analyzing the AE data for earlier moments during cyclic loading (before the described rapid growth of temperature), only ALIN reveals a significant increase in ca.

second, which may indicate the transition from the second to the third phase of degradation process. A reached temperature value at this moment was 57°C.

Analysis of the rest of AE features returned less evident results and a critical moment between the two first phase degradation model could not

In the presented study, an alternative approach for heating temperature by means of acoustic emission was presented. For this purpose, various AE features collected during experimental fatigue studies of composite structures with appearance heating effect were analyzed for evaluation of their sensitivity to critical degradation events that e process. The obtained results indicated that the linear peak amplitude feature is the cracking events occurred during fatigue. Moreover, the preformed analysis shows that there exists a relation between both phase transitions in he investigated fatigue processes: an increase of magnitude of this feature was related with a transition from the first to the second phase as well as with a transition from the second to the third phase, which indicates fect. Further studies will be focused on application of advanced signal processing methods to analyze collected data in more detail.

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“International Journal of Current Engineering and Technology” 2012, Vol. 2, p. 409-412.

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unified crack initiation and propagation in

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mission for the analysis of material behavior.