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Digital image correlation and acoustic emission for damage analysis during tensile loading of open-hole flax laminates
Journal Pre-proofs Digital image correlation and acoustic emission for damage analysis during tensile loading of open-hole flax laminates Mohamed Habibi, Luc Laperrière PII: DOI: Reference:
S0013-7944(19)31325-6 https://doi.org/10.1016/j.engfracmech.2020.106921 EFM 106921
To appear in:
Engineering Fracture Mechanics
Received Date: Revised Date: Accepted Date:
24 October 2019 28 January 2020 30 January 2020
Please cite this article as: Habibi, M., Laperrière, L., Digital image correlation and acoustic emission for damage analysis during tensile loading of open-hole flax laminates, Engineering Fracture Mechanics (2020), doi: https:// doi.org/10.1016/j.engfracmech.2020.106921
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Digital image correlation and acoustic emission for damage analysis during tensile loading of open-hole flax laminates. Mohamed Habibi1, 2*, Luc Laperrière1, 1 Department
of mechanical engineering, Université du Québec à Trois-Rivières, Quebec, Canada. of Industrial Efficiency and Environment, Centre de recherche industrielle du Québec, Québec City, Canada. *Corresponding author: Mohamed Habibi E-mail address: [email protected] 2Department
Abstract The present paper is dedicated to the assessment of the damage taking place in open-hole flax bio-epoxy laminates with different stacking sequences. Tensile test results have shown significant effects of the open hole on the performance of the laminate. Moreover, strain maps have allowed to detect the onset of cracks and to discuss their contribution to the laminates failure. For unidirectional laminate, splitting was observed as the main damage at the hole boundary along the fiber orientation and parallel to the loading axis. However, the [0 90]6s laminate failure has caused by the onset of subsurface cracks in the 90° plies. Even if the tensile strength was decreased, the strain fields result has shown a less hole sensitive behavior of the [0 +45 90 -45]3s laminate. Finally, damage monitoring using acoustic emission has shown a significant increase of the fiber-matrix debonding and fiber breakage contribution to the laminate failure. Keywords: Biocomposite; Mechanical properties; Acoustic emission; Damage 1. Introduction Natural Fiber Reinforced Composite (NFRC) laminates are attractive and promising structural materials for automotive and construction structure application due to their high specific stiffness and strength [1-12]. However, the presence of holes and notches in composite laminates are still a critical design issue. Depending on the hole/notches size, the lay-up, quality of machining and laminate thickness, mechanical properties of notched laminates are significantly affected [13-15]. In particular, damage at the edge of a notch can developed due to the effect of stress concentrations [13, 16, 17]. Under loading, relative notching damage expands differently
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from the global behavior of the laminates. Therefore, the increase of damage resistance of laminates containing circular holes is a critical issue for the application of NFRCs. The behavior of notched laminates has been the subject of considerable research works, along with the growth in composites usage [18-22]. To better understand the effect of the presence of holes and notches, it is indispensable to understand the mechanisms by which sub-critical damage develops at loads below ultimate failure. The effect of laminate lay-up and hole size on the open hole tensile properties of flax/polypropylene laminates were studied by Kannan et al. [23]. The cross-ply laminates [0/90/0]S have shown less notch sensitivity to the open hole profile, compared to unidirectional flax/PP laminates. Salleh et al. [24] have investigated the effect of open hole and hybridization on the residual tensile stress of kenaf composites and kenaf/fiberglass reinforced polyester composites. Their results have shown more notch sensitivity of the long kenaf composites than long kenaf/woven glass hybrid composite, even if the damage progression mechanisms in the two materials were very similar. Kandare and coworkers [25] have performed a comprehensive study on the damage progression and failure characteristics of open-hole flax fiber aluminum laminate. Their results in tensile loading have shown a decrease of the tensile strengths of 66%, 55%, and 47% with increasing hole diameter to 4 mm, 7 mm and 10mm, respectively. Further, the fatigue cycles to failure decreased with the increase of the applied fatigue stress and hole diameter. Camanho et al. [26] have studied the effect of hole sizes, ranging from 2 mm to 10 mm, on [90/0/45/-45]3s carbon epoxy laminates and have shown that residual strengths decrease with increasing the hole size. The full-field measurements have been used in many studies in the literature to examine the mechanical response of notched laminates. However, a limited number of studies have used Digital Image Correlation (DIC) techniques [14, 16, 17, 27, 28]. Caminero et al. [17] have used DIC to experiment the presence of open hole on the full-field surface strain in carbon-fiber/epoxy composite with different stacking sequences. Their results have shown very high localized strains near the hole, and at directions following the fiber orientation (0, 90, +45/ − 45). Wang et al. [29] have measured the strain distributions in carbon fiber laminates with an open hole using the DIC technique. Then, the measured strain fields and the strain distributions in the strain-concentrated regions have shown a good agreement 2
with results from a three-dimension elastic–plastic Finite Element Modeling (FEM) simulation. Among all current techniques for composites health monitoring in the context of crack detection and location, the Acoustic Emission (AE) technique presents is of the most used tools. It can monitor the evolution of the different damage types, their actual state and where the failure of the composite is to be expected. For damage feature extraction from AE signals, many researchers have used various signal processing and pattern recognition techniques [30, 31]. Multivariable analysis was done through AE using variables such as counts, amplitude, duration, rise time and energy. Different classification approaches have been considered, such as k-means [32-34], Fuzzy c-means [35, 36], Principal Components Analysis (PCA) [33, 35], Artificial Neural Network (ANN) [36-38], either individually or in combination. Arthur et al. [39] summarized some works that have studied the AE events classification based on the amplitude of the event. Various amplitude ranges of AE events were assigned to matrix cracking (from 35-42 dB to 45-60 dB), fiber-matrix debonding (from 45-60 dB to 60-70 dB), fiber pull-out (60-80 dB) and fiber breakage (from 60-85 dB to 95-100 dB). These differences in the ranges clearly demonstrate that an absolute characteristic of a specific damage mode could not be considered. The specimen geometry, material properties, and equipment setting have a significant influence on the amplitude range definition [40]. In this work the performance and the assessment of the damage in flax bio-epoxy laminates with an open-hole under tensile loading are examined. First, the performance of open-hole specimens has been studied and compared to unnotched laminates. DIC technique was used to investigate the tensile strain field in laminates with different stacking sequences. In the second part of the work, acoustic emission was used to monitor the evolution of different damages mechanism. Recorded acoustic activity was correlated with the DIC strain field results. Finally, the effect of the open hole on the evolution of different damage mechanisms and their contribution to the failure of laminates was discussed.
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2. Materials and Experiments The specimens were fabricated from Lingrove prepreg (Ekoatape P-UD 3.2). The prepreg was made of unidirectional flax fiber impregnated with biobased CORAL resins (220 g/m2 with 50% bio-epoxy). The composite plates were manufactured by hand lay-up of 12 prepreg layers in 400 mm wide by 400 mm long with different stacking sequences: [0]12, [0 90]6s and [0 +45 90 -45]3s laminates with a total thickness of 2.25 mm and a resulting fiber content (Vf) of 40%±1.57. The CORAL resins recommended cure cycle was used. A heat ramp of 1-2º/min with dwell at 80ºC for 30 minutes and an additional dwell at the activation temperature of the CORAL resins (120ºC) for 30 minutes, followed by a cooling rate of 2-4ºC /min, was used in a press with 15 bars pressure. The open hole specimens were 250 mm long x 30 mm wide with a hole diameter of 6 mm. ASTM D3039 was followed for tensile testing of laminates. Tensile tests were performed using a 100 kN MTS universal testing machine, at a fixed loading rate of 2 mm/min. Five experiments were conducted for each laminate. To monitor the damage process in composite laminates, DIC was used for full-field strain measurement. Two digital cameras with a CCD matrix of five million pixels, model Imager M-lite, was used to record the deformation state by imaging a speckle pattern bonded on the front edge of the tested sample. All samples were imaged in a constant imaging frequency of 14Hz during the test. LaVision software was used to determine the material deformation based on the deformation of the speckle pattern. Basic mechanical properties of the studied laminates under tensile loading (without holes), are presented in Table 1. Table 1: Mechanical properties of flax laminates. [0]12 [0 90]6s σ(MPa) 312.51±3.19 182.14±7.26 E (GPa) 30.48±0.32 21.25±0.93 ε(%) 1.27±0.063 1.13±0.028
[0 +45 90 -45]3s 155.54±5.38 17.38±0.57 1.19±0.041
For damage monitoring during tensile tests, two-channel AE system supplied by Vallen with a sampling rate of 5 MHz and a 40-dB pre-amplification was used. The AE events were recorded by the AE software AMSY-6. The threshold of AE signals was adjusted to 4
35 dB. The AE measurements were performed using four KRNm300 sensors. High vacuum silicon grease was used as a coupling between the sensors and the specimen surface. Load, strain and head displacement data was fed to the Vallen system and recorded at a 10-Hz sampling frequency. A pencil lead break test was used to calibrate the data acquisition system and ensure good conductivity between the specimen surface and the sensors. 3. Results and discussion 3.1 Strain field analysis using DIC Typical tensile Stress-Strain curves for the three laminates are shown in Figure 1 (OpH refer to specimens with open hole). All samples failed within the gauge length, except [0]12 laminate that failed near the grip due to the axial splitting that developed at the hole edge. The strain fields obtained by the DIC at maximum applied tensile load are shown in Figures 2-4. In addition to the final failure stress and strain (Figure 1), for the three different lay-ups, results show a significant effect of the open hole on the strain fields. At and near the hole edge, the εyy (loading direction), εxx (transverse direction) and εyx (shear strain) results differ from the strain measurements of unnotched laminates. High strain gradients near the hole and different strain concentration at directions following the fiber orientation (0, 90, +45,− 45) are expected to result from different damage types: resin cracks (0° splits, 45° off-axis cracking and 90° matrix cracking) and/or fiber/matrix interface failure.
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Figure 1: Typical tensile Stress-Strain curves for laminates with and without holes. For [0]OpH 12 laminate, splitting was observed as the main damage at the hole boundary along the orientation of the fibers and parallel to the loading axis, that leads to failure stress and strain of 175 MPa and 0.78%, respectively. At this loading level, axial splitting at the hole edge was observed in DIC results in terms of strain distribution. Splitting was observed from a narrow band pattern distribution of εxx that runs along the fiber direction, Figure 2 (d), with relatively low values (εxx max =0.3%) compared to a high shear strain (εxy max =2%). The εxy strain field in Figure 2 (e) is oriented parallel to the loading direction (splitting direction), where εxy was symmetrically distributed with respect to the loading axis and very localized with relatively high values at the hole edge (εyy max =1.35%).
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Figure 2: Strain fields of [0]12 ((a), (b) and (c)) and [0]OpH 12 ((d), (e) and (f)) laminates. Figure 1 shows the final failure of the [0 90]OpH 6s cross-ply laminate at 78 MPa, with a fracture surface almost perpendicular to the loading axis. At that level, transverse matrix cracking is expected to occur. The DIC results of the strain field (Figure 3) differ from unidirectional laminate, where a lower effect on εxy strain field was observed, except a low compressive strain concentration at the hole edge. The shear strain field with a lower maximum strain value (εxy max =0.75%) drives axial splitting again, even if high localized εyy strain causes transverse cracks in the mid-plane of the hole. The ultimate failure of the cross-ply open hole laminate was mainly driven by cracks initiation and running at high strain concentration region, from the hole edge and perpendicular to the loading axis. 7
Figure 3: Strain fields of [0-90]6s ((a), (b) and (c)) and [0-90]6s ((d), (e) and (f)) [0 90]OpH 6s laminates. The tensile behavior of [0 +45 90 -45]3s in Figure 1 confirms the previously observed effect of the open hole, where the tensile strength was dropped to the half. Also, angled plies (±45) transforms the fracture surface from a flat and confined fracture surface in the case of cross-ply open hole laminate to serrated fracture surface. The strain fields in Figures 4 (d), (e) and (f), show a less hole sensitive behavior of the laminate, where high strain values are localized at the hole edge. Even if the maximum strain value εyy, εxx and εyx are lower than those obtained by unidirectional and cross-ply laminates, the strain distributions near and far from the hole give an insight on the material damaging and cracks position and type. The maximum strain value in the loading direction (εyy max = 0.65%) developed near 8
the hole edge at ±45° and the fracture surface of Figure 1 depicts that angled plies drive off-axis matrix and fiber cracks.
Figure 4: Strain fields of [0 +45 90 -45]3s ((a), (b) and (c)) and [0 + 45 90 ― 45]𝑂𝑝𝐻 3𝑠 ((d), (e) and (f)) laminates. 3.2. Damage monitoring using acoustic emission Acoustic emission was used to monitor the progressive damage during tensile testing of unnotched and open hole laminates, to characterize the different failure mechanisms. Amplitude, duration, rise time, energy and numbers of counts were considered for data clustering to classify the recorded acoustic events. To distinguish the AE signal from noise, the acquisition threshold was determined using the pencil lead breaking technique and fixed to 35 dB before testing laminates. 9
Figure 5 presents an example of the obtained Davies-Bouldin index values for the studied laminates [41]. From the lowest index value, which corresponds to the best clusters number, three clusters were considered and will be noted by M1, M2 and M3 in the classification of the events. Event classification was achieved using the K-Means algorithm. In this study, we focus only on the acoustic signature analysis to identify the contribution of each damage mechanisms and the effect of the open hole on the failure of the different laminates. Event classification methods and theoretical background are widely described in the literature [40, 42].
Figure 5: Optimum clusters number determination using the Davies-Bouldin index.
3.2.1. Identification of damage mechanisms AE data processing was applied with the same approach and with the same classification method for all tensile tests of both unnotched and open hole laminates. The amplitude of events was correlated to the applied stress-strain curves and given in Figure 6.
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Figure 6: Axial loading and acoustic events amplitude versus strain for unnotched and open hole [0]12 [0 90]6s and [0 +45 90 -45]3s laminates, respectively. From the damage onset to the ultimate failure, open hole laminates exhibited shorter time histories than the unnotched specimens. That is to say, damage in the notched [0]12 laminate (Figure 6 (d)) is characterized by higher acoustic activity but survived for higher load and longer time. A remarkable difference was observed in the acoustic activity of the notched [0 90]6s and [0 +45 90 -45]3s laminates. A lower number of acoustic events were recorded due to the shorter loading time and the premature failure of the specimens, which was attributed in 11
the previous section to the stress concentration and a crack initiation near and at the edge of the hole (Figures 3 and 4). For these laminates, the AE signals were a mixture of three different damage modes. Therefore, it is crucial to correlate the AE signals with different damage modes to investigate the effect of the open hole on the laminate failure under tensile loading. As shown and labeled in Figure 6, events in group M1 show damage initiation as soon as the axial stress reaches was applied. The further increase of the load level increases the number of recorded M1 events, especially form a strain of about 0.25% and until the specimen failure. Each acoustic event in this group is characterized by an amplitude range of 35–60 dB. The second group of events M2, with a smaller number of event than M1, succeeds the former with an amplitude range of 35–87 dB. For the unnotched laminates, the number of the second type of events increases with loading, particularly from a strain values more than 0.5% and until the failure of the specimens. However, this intensification for open hole laminates is translated to lower strain values, with a significant concentration at the proximity of the ultimate failure strain. Figure 7 shows that the trend and the rate of changes in the normalized cumulative AE energy are characterized by three main phases, for all laminates. The cumulative values remain low until a significant change after the initiation of a third new damage type. This confirms that the first and the second groups, M1 and M2 events, refers to two damage mechanisms with low AE energy, namely M1 corresponding to matrix cracking, with a high number of events, and M2 corresponding to fiber-matrix debonding and delamination. Later on, the AE energy increases remarkably with a steep rise in the slope of cumulative energy related to high amplitude signals. However, most of the researchers relate the matrix cracking to lowest energy emission, fiber breakage to highest energy and amplitude, and fiber-matrix debonding is located between these two damages mechanisms [40, 42]. Therefore, the acoustic events of the third cluster in Figure 6 are dedicated to the M3 class corresponding to fiber breakage mechanism with an amplitude range of 50–100 dB. Based on the fracture surfaces analysis using SEM, shown in Figure 8, no quantitative analysis could be made, but the accurate association of each class of acoustic events to a 12
damage mechanism can be verified. Matrix cracking (M1), fiber-matrix debonding and delamination (M2), and fiber breakage (M3) were observed for all tested laminates. For open hole [0]12 laminate, Figure 8 (a2) shows that fibers debonding from the matrix are qualitatively the main damage mechanisms. Besides, Figure 8 (a3) confirms the contribution of fiber breakage and matrix cracking on specimen failure.
Figure 7: Axial loading and normalized cumulative of AE energy versus strain for unnotched (a) and open hole (b) [0]12 [0 90]6s and [0 +45 90 -45]3s laminates, respectively.
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Figure 8: SEM pictures of the fracture surfaces of open hole: a) [0]12, b) [0 90]6s and [0 +45 90 -45]3s laminates Regarding the [0 90]6s and [0 +45 90 -45]3s laminates, Figure 8 (b) and Figure 8 (c) confirm the results obtained with acoustic emission. However, delamination was observed, especially for [0 +45 90 -45]3s laminate where the 45° oriented layers were torn off and leave a deep gape in the fracture surface. 3.2.2. Correlation with the strain field analysis In the case of [0]12 laminate, the strain fields (Figure 2) were oriented parallel to the splitting direction and very localized with relatively high values at the hole edge. This observation is in agreement with the normalized cumulative AE energy trends (Figure 7). A significant intensification of the acoustic activity, conducting to a linear increase of the cumulated AE energy, was recorded for low strain values of about 0.35%. This intensification, as is shown in Figure 6 (d), is related to higher fiber-matrix debonding 14
events. Hence, the hole presence has promoted splitting and consequently the fiber-matrix debonding, leading to a fiber breakage and failure of the laminate. The normalized cumulative AE energy trends for the [0 90]6s laminate (Figure 7) shows a clear effect of the open hole on the damage evolution during loading. At a strain of 0.35%, the notched laminate is characterized by a sharp increase of the cumulated energy, even if the recorded acoustic events do not reach a remarkable evolution. Figure 6 (e) shows a fiber breakage event concentration near the failure strain, which allow us to conclude that the increase of the cumulated energy is mainly attributed to fiber-matrix debonding and delamination events. This confirms the strain fields results interpretation in Figure 3, where the failure was assigned to the initiation of a crack and running from the hole edge and perpendicular to the loading axis. However, cracks propagation certainly occurs in the 90° oriented layer between adjacent fibers, which produces fiber-matrix debonding. Even if the damage intensification in the [0 +45 90 -45]3s laminate was noticed in higher strain compared to the other laminates (Figure 7), the normalized cumulative AE energy of unnotched and notched samples keeps the same global trends. However, Figure 6 (e) shows a fiber-matrix debonding and fiber breakage events concentration in a short strain range value of 0.5% to 0.6%. Before that strain range, the acoustic activity was seen to be less sensitive to notch effect, which seems in concordance with the results of the strain fields in Figures 4 and where ±45° angled plies drive off-axis matrix and fiber cracks.
3.2.3. Effect of the open hole on the evolution of different damage mechanisms and their contribution to the failure of laminates The number of events can be used to characterize damages evolution in the laminates. Thus, to compare the behavior of different damage mechanisms and the effect of notching, the cumulative of the AE events of each damage mechanisms were used to calculate their contribution on the failure of the different laminates. The contribution of each damage mechanism on the overall failure of the laminates can be defined using a cumulative damage index (CDi). CDi is calculated by dividing the 15
cumulated AE events of the damage mechanism (Evi) by the total cumulated AE events (Evt) of the test when the failure of the composite is reached: 𝐸𝑣𝑖
𝐶𝐷𝑖 = 𝐸𝑣𝑡 =
𝐸𝑣𝑖 𝑛
∑1𝐸𝑣𝑖
(1)
A typical example of the cumulated damage by different mechanisms and their contribution to the failure of the [0]12 laminate is given by figure 9. The calculated damage index using five data record of each laminate type are presented in Table 2. For all laminates, matrix cracking was the dominant damage mechanism. The cumulative damage index (CDi) of matrix cracking ranges between 60% and 80%, where the fibermatrix debonding and delamination events represent up to 30% of the total cumulated damage. Fiber breakage events remain less important with a maximum cumulative damage index of about 6% to 10%. Fiber breakage events occur as a result of the intensification of the two previous damage mechanisms, which can be also deduced from their concentration near the failure strain (As it can be clearly seen in Figure 6). Table 2: Contribution of different damage mechanisms on the failure of laminates M1 M2 M3 Unnotched lamintes 70,26±2.31 26,28±1.67 3,45±0.85 [0]12 78,96±1.59 19,48±1.91 1,56±0.25 [0 90]6s 77,18±2.25 21,09±0.47 1,73±0.64 [0 +45 90 -45]3s Open hole lamintes OpH 60,26±1.87 34,17±1.92 5,57±1.22 [0]12 𝑂𝑝𝐻 71,11±0.94 25,91±1.58 2,98±0.95 [0 90]6𝑠 𝑂𝑝𝐻 31,19±2.27 7,85±1.14 [0 + 45 90 ― 45]3𝑠 60,96±2.81
In the case of [0]12 laminate, the effect of notching led to a significant increase of the cumulative damage index (CDi) of fiber-matrix debonding and delamination mechanism, with a slight increase of the fiber breakage damage index. Then, this result supports the previous explanation and confirms that notching has promoted splitting by promoting fibermatrix debonding under the effect of transverse crack propagation.
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The same behavior was obtained with the [0 90]6s laminate, where the cracks initiation and running from the hole edge and perpendicular to the loading axis has increased both M2 and M3 cumulative damage index. As mentioned in the previous section, for the [0 +45 90 -45]3s laminate, fiber-matrix debonding and fiber breakage events were concentrated in a short strain range value close to the strain failure. In addition, the result of the strain fields (Figures 4) has shown that ±45° angled plies drive off-axis matrix and fiber cracks. However, the significant decrease of the matrix cracking cumulative damage index confirms that [0 +45 90 -45]3s laminate was less sensitive to notching.
Figure 9: Typical evolution of the cumulated damage by different mechanisms in the case of [0]12 laminate. 4. Conclusion In this paper three different stacking sequences ([0]12, [0 90]6s and [0 +45 90 -45]3s) of flax based laminates, without and with an open hole, were used to assess their damage in tensile loading. Digital Image Correlation (DIC), for full-field strain measurement, and acoustic emission, for real-time damage monitoring, were examined. SEM pictures of the fracture 17
surfaces were used to visually identify the different damage mechanisms contributing to the laminate failure. The main conclusions are as follows: -
The DIC strain field, based on the measurement of εyy (loading direction), εxx (transverse direction) and εyx (shear strain), was successfully applied to measure surface strains and relate to possible damage that could occur in the flax laminates.
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Open holes reduce the final failure stress and strain for the three different lay-ups. For [0]12 laminate, splitting was observed as the main damage at the hole boundary along the fibers orientation and parallel to the loading axis. However, the high localized longitudinal strain has caused transverse cracks in the mid-plane of the hole of the [0 90]6s laminate, conducting to the failure. Even if the tensile strength was dropped to the half, the strain fields result has shown a less hole sensitive behavior of the [0 +45 90 -45]3s laminate, where high strain values are localized at the hole edge.
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Acoustic emission was used to monitor the damage evolution under tensile loading of the studied laminates. The use of Davies-Bouldin method and the unsupervised K-Means method to classify recorded events in matrix cracking, fiber-matrix debonding and delamination, and fiber breakage damage mechanisms have allowed to confirm and to quantify their contribution on the laminate failure.
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Cumulative damage index (CDi), used to calculate the contribution of the different damage mechanisms on the total damage, shows a dominant effect of matrix cracking for all tested laminates. However, the presence of the hole has mainly induced an increase of the fiber-matrix debonding and delamination mechanism damage index.
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Highlights
The effect of open-hole on the mechanical properties of flax laminates was investigated. Strain field around hole was analysed using Digital image correlation. Acoustic emission was used to monitor the progressive damage during tensile testing. Acoustic events were classified to calculate the contribution of different damage mechanisms on the failure of laminates SEM pictures of the fracture surfaces were used to visually identify the different damage mechanisms contributing to the laminate failure.
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S0013-7944(19)31325-6 https://doi.org/10.1016/j.engfracmech.2020.106921 EFM 106921
To appear in:
Engineering Fracture Mechanics
Received Date: Revised Date: Accepted Date:
24 October 2019 28 January 2020 30 January 2020
Please cite this article as: Habibi, M., Laperrière, L., Digital image correlation and acoustic emission for damage analysis during tensile loading of open-hole flax laminates, Engineering Fracture Mechanics (2020), doi: https:// doi.org/10.1016/j.engfracmech.2020.106921
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© 2020 Published by Elsevier Ltd.
Digital image correlation and acoustic emission for damage analysis during tensile loading of open-hole flax laminates. Mohamed Habibi1, 2*, Luc Laperrière1, 1 Department
of mechanical engineering, Université du Québec à Trois-Rivières, Quebec, Canada. of Industrial Efficiency and Environment, Centre de recherche industrielle du Québec, Québec City, Canada. *Corresponding author: Mohamed Habibi E-mail address: [email protected] 2Department
Abstract The present paper is dedicated to the assessment of the damage taking place in open-hole flax bio-epoxy laminates with different stacking sequences. Tensile test results have shown significant effects of the open hole on the performance of the laminate. Moreover, strain maps have allowed to detect the onset of cracks and to discuss their contribution to the laminates failure. For unidirectional laminate, splitting was observed as the main damage at the hole boundary along the fiber orientation and parallel to the loading axis. However, the [0 90]6s laminate failure has caused by the onset of subsurface cracks in the 90° plies. Even if the tensile strength was decreased, the strain fields result has shown a less hole sensitive behavior of the [0 +45 90 -45]3s laminate. Finally, damage monitoring using acoustic emission has shown a significant increase of the fiber-matrix debonding and fiber breakage contribution to the laminate failure. Keywords: Biocomposite; Mechanical properties; Acoustic emission; Damage 1. Introduction Natural Fiber Reinforced Composite (NFRC) laminates are attractive and promising structural materials for automotive and construction structure application due to their high specific stiffness and strength [1-12]. However, the presence of holes and notches in composite laminates are still a critical design issue. Depending on the hole/notches size, the lay-up, quality of machining and laminate thickness, mechanical properties of notched laminates are significantly affected [13-15]. In particular, damage at the edge of a notch can developed due to the effect of stress concentrations [13, 16, 17]. Under loading, relative notching damage expands differently
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from the global behavior of the laminates. Therefore, the increase of damage resistance of laminates containing circular holes is a critical issue for the application of NFRCs. The behavior of notched laminates has been the subject of considerable research works, along with the growth in composites usage [18-22]. To better understand the effect of the presence of holes and notches, it is indispensable to understand the mechanisms by which sub-critical damage develops at loads below ultimate failure. The effect of laminate lay-up and hole size on the open hole tensile properties of flax/polypropylene laminates were studied by Kannan et al. [23]. The cross-ply laminates [0/90/0]S have shown less notch sensitivity to the open hole profile, compared to unidirectional flax/PP laminates. Salleh et al. [24] have investigated the effect of open hole and hybridization on the residual tensile stress of kenaf composites and kenaf/fiberglass reinforced polyester composites. Their results have shown more notch sensitivity of the long kenaf composites than long kenaf/woven glass hybrid composite, even if the damage progression mechanisms in the two materials were very similar. Kandare and coworkers [25] have performed a comprehensive study on the damage progression and failure characteristics of open-hole flax fiber aluminum laminate. Their results in tensile loading have shown a decrease of the tensile strengths of 66%, 55%, and 47% with increasing hole diameter to 4 mm, 7 mm and 10mm, respectively. Further, the fatigue cycles to failure decreased with the increase of the applied fatigue stress and hole diameter. Camanho et al. [26] have studied the effect of hole sizes, ranging from 2 mm to 10 mm, on [90/0/45/-45]3s carbon epoxy laminates and have shown that residual strengths decrease with increasing the hole size. The full-field measurements have been used in many studies in the literature to examine the mechanical response of notched laminates. However, a limited number of studies have used Digital Image Correlation (DIC) techniques [14, 16, 17, 27, 28]. Caminero et al. [17] have used DIC to experiment the presence of open hole on the full-field surface strain in carbon-fiber/epoxy composite with different stacking sequences. Their results have shown very high localized strains near the hole, and at directions following the fiber orientation (0, 90, +45/ − 45). Wang et al. [29] have measured the strain distributions in carbon fiber laminates with an open hole using the DIC technique. Then, the measured strain fields and the strain distributions in the strain-concentrated regions have shown a good agreement 2
with results from a three-dimension elastic–plastic Finite Element Modeling (FEM) simulation. Among all current techniques for composites health monitoring in the context of crack detection and location, the Acoustic Emission (AE) technique presents is of the most used tools. It can monitor the evolution of the different damage types, their actual state and where the failure of the composite is to be expected. For damage feature extraction from AE signals, many researchers have used various signal processing and pattern recognition techniques [30, 31]. Multivariable analysis was done through AE using variables such as counts, amplitude, duration, rise time and energy. Different classification approaches have been considered, such as k-means [32-34], Fuzzy c-means [35, 36], Principal Components Analysis (PCA) [33, 35], Artificial Neural Network (ANN) [36-38], either individually or in combination. Arthur et al. [39] summarized some works that have studied the AE events classification based on the amplitude of the event. Various amplitude ranges of AE events were assigned to matrix cracking (from 35-42 dB to 45-60 dB), fiber-matrix debonding (from 45-60 dB to 60-70 dB), fiber pull-out (60-80 dB) and fiber breakage (from 60-85 dB to 95-100 dB). These differences in the ranges clearly demonstrate that an absolute characteristic of a specific damage mode could not be considered. The specimen geometry, material properties, and equipment setting have a significant influence on the amplitude range definition [40]. In this work the performance and the assessment of the damage in flax bio-epoxy laminates with an open-hole under tensile loading are examined. First, the performance of open-hole specimens has been studied and compared to unnotched laminates. DIC technique was used to investigate the tensile strain field in laminates with different stacking sequences. In the second part of the work, acoustic emission was used to monitor the evolution of different damages mechanism. Recorded acoustic activity was correlated with the DIC strain field results. Finally, the effect of the open hole on the evolution of different damage mechanisms and their contribution to the failure of laminates was discussed.
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2. Materials and Experiments The specimens were fabricated from Lingrove prepreg (Ekoatape P-UD 3.2). The prepreg was made of unidirectional flax fiber impregnated with biobased CORAL resins (220 g/m2 with 50% bio-epoxy). The composite plates were manufactured by hand lay-up of 12 prepreg layers in 400 mm wide by 400 mm long with different stacking sequences: [0]12, [0 90]6s and [0 +45 90 -45]3s laminates with a total thickness of 2.25 mm and a resulting fiber content (Vf) of 40%±1.57. The CORAL resins recommended cure cycle was used. A heat ramp of 1-2º/min with dwell at 80ºC for 30 minutes and an additional dwell at the activation temperature of the CORAL resins (120ºC) for 30 minutes, followed by a cooling rate of 2-4ºC /min, was used in a press with 15 bars pressure. The open hole specimens were 250 mm long x 30 mm wide with a hole diameter of 6 mm. ASTM D3039 was followed for tensile testing of laminates. Tensile tests were performed using a 100 kN MTS universal testing machine, at a fixed loading rate of 2 mm/min. Five experiments were conducted for each laminate. To monitor the damage process in composite laminates, DIC was used for full-field strain measurement. Two digital cameras with a CCD matrix of five million pixels, model Imager M-lite, was used to record the deformation state by imaging a speckle pattern bonded on the front edge of the tested sample. All samples were imaged in a constant imaging frequency of 14Hz during the test. LaVision software was used to determine the material deformation based on the deformation of the speckle pattern. Basic mechanical properties of the studied laminates under tensile loading (without holes), are presented in Table 1. Table 1: Mechanical properties of flax laminates. [0]12 [0 90]6s σ(MPa) 312.51±3.19 182.14±7.26 E (GPa) 30.48±0.32 21.25±0.93 ε(%) 1.27±0.063 1.13±0.028
[0 +45 90 -45]3s 155.54±5.38 17.38±0.57 1.19±0.041
For damage monitoring during tensile tests, two-channel AE system supplied by Vallen with a sampling rate of 5 MHz and a 40-dB pre-amplification was used. The AE events were recorded by the AE software AMSY-6. The threshold of AE signals was adjusted to 4
35 dB. The AE measurements were performed using four KRNm300 sensors. High vacuum silicon grease was used as a coupling between the sensors and the specimen surface. Load, strain and head displacement data was fed to the Vallen system and recorded at a 10-Hz sampling frequency. A pencil lead break test was used to calibrate the data acquisition system and ensure good conductivity between the specimen surface and the sensors. 3. Results and discussion 3.1 Strain field analysis using DIC Typical tensile Stress-Strain curves for the three laminates are shown in Figure 1 (OpH refer to specimens with open hole). All samples failed within the gauge length, except [0]12 laminate that failed near the grip due to the axial splitting that developed at the hole edge. The strain fields obtained by the DIC at maximum applied tensile load are shown in Figures 2-4. In addition to the final failure stress and strain (Figure 1), for the three different lay-ups, results show a significant effect of the open hole on the strain fields. At and near the hole edge, the εyy (loading direction), εxx (transverse direction) and εyx (shear strain) results differ from the strain measurements of unnotched laminates. High strain gradients near the hole and different strain concentration at directions following the fiber orientation (0, 90, +45,− 45) are expected to result from different damage types: resin cracks (0° splits, 45° off-axis cracking and 90° matrix cracking) and/or fiber/matrix interface failure.
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Figure 1: Typical tensile Stress-Strain curves for laminates with and without holes. For [0]OpH 12 laminate, splitting was observed as the main damage at the hole boundary along the orientation of the fibers and parallel to the loading axis, that leads to failure stress and strain of 175 MPa and 0.78%, respectively. At this loading level, axial splitting at the hole edge was observed in DIC results in terms of strain distribution. Splitting was observed from a narrow band pattern distribution of εxx that runs along the fiber direction, Figure 2 (d), with relatively low values (εxx max =0.3%) compared to a high shear strain (εxy max =2%). The εxy strain field in Figure 2 (e) is oriented parallel to the loading direction (splitting direction), where εxy was symmetrically distributed with respect to the loading axis and very localized with relatively high values at the hole edge (εyy max =1.35%).
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Figure 2: Strain fields of [0]12 ((a), (b) and (c)) and [0]OpH 12 ((d), (e) and (f)) laminates. Figure 1 shows the final failure of the [0 90]OpH 6s cross-ply laminate at 78 MPa, with a fracture surface almost perpendicular to the loading axis. At that level, transverse matrix cracking is expected to occur. The DIC results of the strain field (Figure 3) differ from unidirectional laminate, where a lower effect on εxy strain field was observed, except a low compressive strain concentration at the hole edge. The shear strain field with a lower maximum strain value (εxy max =0.75%) drives axial splitting again, even if high localized εyy strain causes transverse cracks in the mid-plane of the hole. The ultimate failure of the cross-ply open hole laminate was mainly driven by cracks initiation and running at high strain concentration region, from the hole edge and perpendicular to the loading axis. 7
Figure 3: Strain fields of [0-90]6s ((a), (b) and (c)) and [0-90]6s ((d), (e) and (f)) [0 90]OpH 6s laminates. The tensile behavior of [0 +45 90 -45]3s in Figure 1 confirms the previously observed effect of the open hole, where the tensile strength was dropped to the half. Also, angled plies (±45) transforms the fracture surface from a flat and confined fracture surface in the case of cross-ply open hole laminate to serrated fracture surface. The strain fields in Figures 4 (d), (e) and (f), show a less hole sensitive behavior of the laminate, where high strain values are localized at the hole edge. Even if the maximum strain value εyy, εxx and εyx are lower than those obtained by unidirectional and cross-ply laminates, the strain distributions near and far from the hole give an insight on the material damaging and cracks position and type. The maximum strain value in the loading direction (εyy max = 0.65%) developed near 8
the hole edge at ±45° and the fracture surface of Figure 1 depicts that angled plies drive off-axis matrix and fiber cracks.
Figure 4: Strain fields of [0 +45 90 -45]3s ((a), (b) and (c)) and [0 + 45 90 ― 45]𝑂𝑝𝐻 3𝑠 ((d), (e) and (f)) laminates. 3.2. Damage monitoring using acoustic emission Acoustic emission was used to monitor the progressive damage during tensile testing of unnotched and open hole laminates, to characterize the different failure mechanisms. Amplitude, duration, rise time, energy and numbers of counts were considered for data clustering to classify the recorded acoustic events. To distinguish the AE signal from noise, the acquisition threshold was determined using the pencil lead breaking technique and fixed to 35 dB before testing laminates. 9
Figure 5 presents an example of the obtained Davies-Bouldin index values for the studied laminates [41]. From the lowest index value, which corresponds to the best clusters number, three clusters were considered and will be noted by M1, M2 and M3 in the classification of the events. Event classification was achieved using the K-Means algorithm. In this study, we focus only on the acoustic signature analysis to identify the contribution of each damage mechanisms and the effect of the open hole on the failure of the different laminates. Event classification methods and theoretical background are widely described in the literature [40, 42].
Figure 5: Optimum clusters number determination using the Davies-Bouldin index.
3.2.1. Identification of damage mechanisms AE data processing was applied with the same approach and with the same classification method for all tensile tests of both unnotched and open hole laminates. The amplitude of events was correlated to the applied stress-strain curves and given in Figure 6.
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Figure 6: Axial loading and acoustic events amplitude versus strain for unnotched and open hole [0]12 [0 90]6s and [0 +45 90 -45]3s laminates, respectively. From the damage onset to the ultimate failure, open hole laminates exhibited shorter time histories than the unnotched specimens. That is to say, damage in the notched [0]12 laminate (Figure 6 (d)) is characterized by higher acoustic activity but survived for higher load and longer time. A remarkable difference was observed in the acoustic activity of the notched [0 90]6s and [0 +45 90 -45]3s laminates. A lower number of acoustic events were recorded due to the shorter loading time and the premature failure of the specimens, which was attributed in 11
the previous section to the stress concentration and a crack initiation near and at the edge of the hole (Figures 3 and 4). For these laminates, the AE signals were a mixture of three different damage modes. Therefore, it is crucial to correlate the AE signals with different damage modes to investigate the effect of the open hole on the laminate failure under tensile loading. As shown and labeled in Figure 6, events in group M1 show damage initiation as soon as the axial stress reaches was applied. The further increase of the load level increases the number of recorded M1 events, especially form a strain of about 0.25% and until the specimen failure. Each acoustic event in this group is characterized by an amplitude range of 35–60 dB. The second group of events M2, with a smaller number of event than M1, succeeds the former with an amplitude range of 35–87 dB. For the unnotched laminates, the number of the second type of events increases with loading, particularly from a strain values more than 0.5% and until the failure of the specimens. However, this intensification for open hole laminates is translated to lower strain values, with a significant concentration at the proximity of the ultimate failure strain. Figure 7 shows that the trend and the rate of changes in the normalized cumulative AE energy are characterized by three main phases, for all laminates. The cumulative values remain low until a significant change after the initiation of a third new damage type. This confirms that the first and the second groups, M1 and M2 events, refers to two damage mechanisms with low AE energy, namely M1 corresponding to matrix cracking, with a high number of events, and M2 corresponding to fiber-matrix debonding and delamination. Later on, the AE energy increases remarkably with a steep rise in the slope of cumulative energy related to high amplitude signals. However, most of the researchers relate the matrix cracking to lowest energy emission, fiber breakage to highest energy and amplitude, and fiber-matrix debonding is located between these two damages mechanisms [40, 42]. Therefore, the acoustic events of the third cluster in Figure 6 are dedicated to the M3 class corresponding to fiber breakage mechanism with an amplitude range of 50–100 dB. Based on the fracture surfaces analysis using SEM, shown in Figure 8, no quantitative analysis could be made, but the accurate association of each class of acoustic events to a 12
damage mechanism can be verified. Matrix cracking (M1), fiber-matrix debonding and delamination (M2), and fiber breakage (M3) were observed for all tested laminates. For open hole [0]12 laminate, Figure 8 (a2) shows that fibers debonding from the matrix are qualitatively the main damage mechanisms. Besides, Figure 8 (a3) confirms the contribution of fiber breakage and matrix cracking on specimen failure.
Figure 7: Axial loading and normalized cumulative of AE energy versus strain for unnotched (a) and open hole (b) [0]12 [0 90]6s and [0 +45 90 -45]3s laminates, respectively.
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Figure 8: SEM pictures of the fracture surfaces of open hole: a) [0]12, b) [0 90]6s and [0 +45 90 -45]3s laminates Regarding the [0 90]6s and [0 +45 90 -45]3s laminates, Figure 8 (b) and Figure 8 (c) confirm the results obtained with acoustic emission. However, delamination was observed, especially for [0 +45 90 -45]3s laminate where the 45° oriented layers were torn off and leave a deep gape in the fracture surface. 3.2.2. Correlation with the strain field analysis In the case of [0]12 laminate, the strain fields (Figure 2) were oriented parallel to the splitting direction and very localized with relatively high values at the hole edge. This observation is in agreement with the normalized cumulative AE energy trends (Figure 7). A significant intensification of the acoustic activity, conducting to a linear increase of the cumulated AE energy, was recorded for low strain values of about 0.35%. This intensification, as is shown in Figure 6 (d), is related to higher fiber-matrix debonding 14
events. Hence, the hole presence has promoted splitting and consequently the fiber-matrix debonding, leading to a fiber breakage and failure of the laminate. The normalized cumulative AE energy trends for the [0 90]6s laminate (Figure 7) shows a clear effect of the open hole on the damage evolution during loading. At a strain of 0.35%, the notched laminate is characterized by a sharp increase of the cumulated energy, even if the recorded acoustic events do not reach a remarkable evolution. Figure 6 (e) shows a fiber breakage event concentration near the failure strain, which allow us to conclude that the increase of the cumulated energy is mainly attributed to fiber-matrix debonding and delamination events. This confirms the strain fields results interpretation in Figure 3, where the failure was assigned to the initiation of a crack and running from the hole edge and perpendicular to the loading axis. However, cracks propagation certainly occurs in the 90° oriented layer between adjacent fibers, which produces fiber-matrix debonding. Even if the damage intensification in the [0 +45 90 -45]3s laminate was noticed in higher strain compared to the other laminates (Figure 7), the normalized cumulative AE energy of unnotched and notched samples keeps the same global trends. However, Figure 6 (e) shows a fiber-matrix debonding and fiber breakage events concentration in a short strain range value of 0.5% to 0.6%. Before that strain range, the acoustic activity was seen to be less sensitive to notch effect, which seems in concordance with the results of the strain fields in Figures 4 and where ±45° angled plies drive off-axis matrix and fiber cracks.
3.2.3. Effect of the open hole on the evolution of different damage mechanisms and their contribution to the failure of laminates The number of events can be used to characterize damages evolution in the laminates. Thus, to compare the behavior of different damage mechanisms and the effect of notching, the cumulative of the AE events of each damage mechanisms were used to calculate their contribution on the failure of the different laminates. The contribution of each damage mechanism on the overall failure of the laminates can be defined using a cumulative damage index (CDi). CDi is calculated by dividing the 15
cumulated AE events of the damage mechanism (Evi) by the total cumulated AE events (Evt) of the test when the failure of the composite is reached: 𝐸𝑣𝑖
𝐶𝐷𝑖 = 𝐸𝑣𝑡 =
𝐸𝑣𝑖 𝑛
∑1𝐸𝑣𝑖
(1)
A typical example of the cumulated damage by different mechanisms and their contribution to the failure of the [0]12 laminate is given by figure 9. The calculated damage index using five data record of each laminate type are presented in Table 2. For all laminates, matrix cracking was the dominant damage mechanism. The cumulative damage index (CDi) of matrix cracking ranges between 60% and 80%, where the fibermatrix debonding and delamination events represent up to 30% of the total cumulated damage. Fiber breakage events remain less important with a maximum cumulative damage index of about 6% to 10%. Fiber breakage events occur as a result of the intensification of the two previous damage mechanisms, which can be also deduced from their concentration near the failure strain (As it can be clearly seen in Figure 6). Table 2: Contribution of different damage mechanisms on the failure of laminates M1 M2 M3 Unnotched lamintes 70,26±2.31 26,28±1.67 3,45±0.85 [0]12 78,96±1.59 19,48±1.91 1,56±0.25 [0 90]6s 77,18±2.25 21,09±0.47 1,73±0.64 [0 +45 90 -45]3s Open hole lamintes OpH 60,26±1.87 34,17±1.92 5,57±1.22 [0]12 𝑂𝑝𝐻 71,11±0.94 25,91±1.58 2,98±0.95 [0 90]6𝑠 𝑂𝑝𝐻 31,19±2.27 7,85±1.14 [0 + 45 90 ― 45]3𝑠 60,96±2.81
In the case of [0]12 laminate, the effect of notching led to a significant increase of the cumulative damage index (CDi) of fiber-matrix debonding and delamination mechanism, with a slight increase of the fiber breakage damage index. Then, this result supports the previous explanation and confirms that notching has promoted splitting by promoting fibermatrix debonding under the effect of transverse crack propagation.
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The same behavior was obtained with the [0 90]6s laminate, where the cracks initiation and running from the hole edge and perpendicular to the loading axis has increased both M2 and M3 cumulative damage index. As mentioned in the previous section, for the [0 +45 90 -45]3s laminate, fiber-matrix debonding and fiber breakage events were concentrated in a short strain range value close to the strain failure. In addition, the result of the strain fields (Figures 4) has shown that ±45° angled plies drive off-axis matrix and fiber cracks. However, the significant decrease of the matrix cracking cumulative damage index confirms that [0 +45 90 -45]3s laminate was less sensitive to notching.
Figure 9: Typical evolution of the cumulated damage by different mechanisms in the case of [0]12 laminate. 4. Conclusion In this paper three different stacking sequences ([0]12, [0 90]6s and [0 +45 90 -45]3s) of flax based laminates, without and with an open hole, were used to assess their damage in tensile loading. Digital Image Correlation (DIC), for full-field strain measurement, and acoustic emission, for real-time damage monitoring, were examined. SEM pictures of the fracture 17
surfaces were used to visually identify the different damage mechanisms contributing to the laminate failure. The main conclusions are as follows: -
The DIC strain field, based on the measurement of εyy (loading direction), εxx (transverse direction) and εyx (shear strain), was successfully applied to measure surface strains and relate to possible damage that could occur in the flax laminates.
-
Open holes reduce the final failure stress and strain for the three different lay-ups. For [0]12 laminate, splitting was observed as the main damage at the hole boundary along the fibers orientation and parallel to the loading axis. However, the high localized longitudinal strain has caused transverse cracks in the mid-plane of the hole of the [0 90]6s laminate, conducting to the failure. Even if the tensile strength was dropped to the half, the strain fields result has shown a less hole sensitive behavior of the [0 +45 90 -45]3s laminate, where high strain values are localized at the hole edge.
-
Acoustic emission was used to monitor the damage evolution under tensile loading of the studied laminates. The use of Davies-Bouldin method and the unsupervised K-Means method to classify recorded events in matrix cracking, fiber-matrix debonding and delamination, and fiber breakage damage mechanisms have allowed to confirm and to quantify their contribution on the laminate failure.
-
Cumulative damage index (CDi), used to calculate the contribution of the different damage mechanisms on the total damage, shows a dominant effect of matrix cracking for all tested laminates. However, the presence of the hole has mainly induced an increase of the fiber-matrix debonding and delamination mechanism damage index.
18
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Highlights
The effect of open-hole on the mechanical properties of flax laminates was investigated. Strain field around hole was analysed using Digital image correlation. Acoustic emission was used to monitor the progressive damage during tensile testing. Acoustic events were classified to calculate the contribution of different damage mechanisms on the failure of laminates SEM pictures of the fracture surfaces were used to visually identify the different damage mechanisms contributing to the laminate failure.
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