Fiber Microbuckling Research Articles

Effects of interfacial debonding on the stability of finitely strained periodic microstructured elastic composites.

Predicting failure initiation in nonlinear composite materials, often referred to as metamaterials, is a fundamental challenge in nonlinear solid mechanics. Microstructural failure mechanisms encompass fracture, decohesion, cavitation, compression-induced contact and instabilities, affecting their unconventional static and dynamic performances. To fully take advantage of these materials, especially in extreme applications, it is imperative to predict their nonlinear behaviour using reliable, accurate and computationally efficient numerical methodologies. This study presents an innovative nonlinear homogenization-based theoretical framework for characterizing the failure behaviour of periodic reinforced hyperelastic composites induced by reinforcement/matrix decohesion and interaction between contact mechanisms and microscopic instabilities. Debonding and unilateral contact between different phases are incorporated by employing an enhanced cohesive/contact model, which features a special nonlinear interface constitutive law and an accurate contact formulation within the context of finite strain continuum mechanics. The theoretical formulation is demonstrated using periodically layered composites subjected to macroscopic compressive loading conditions along the lamination direction. Numerical results illustrate the ways in which debonding phenomena, in conjunction with fibre microbuckling, may influence the critical loads of the examined composite solid. The sensitivity of the results obtained through the proposed contact-cohesive model at finite strain with respect to its implementation is also explored. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'.

Comparison of single- and hybrid-fiber composite laminates for use in prosthetic sockets

Traditional prosthetic sockets often consist of thermoplastic and wood-based materials. However, these sockets are known to be uncomfortable, heavy, and economically challenging. Natural fibers are quickly adapting and becoming more popular because of their biodegradable, long-lasting, and biocompatible nature. They are replacing synthetic fibers in the design and development of structural parts. This research looked into a new hybrid composite made of natural and synthetic fibers (basalt, jute, and carbon) that are reinforced in a polyester thermoset based matrix. The performance of this hybrid composite was then compared to that of pristine or single fiber-reinforced composites manufactured using woven jute, basalt, and carbon. The samples were manufactured using the vacuum-assisted resin transfer molding technique and cured at room temperature. The samples were then subjected to flexure and impact tests. The hybrid composite had an impact strength of 46.71 kJ/m2, higher than all other single fiber combinations. The flexural strength of the hybrid composite was determined to be 66.25 MPa, matching that of basalt fiber and surpassing that of other conventional single fiber composite laminates. Fractographic analysis has revealed that the main cause of failure in flexure is mostly attributed to delamination and fiber micro-buckling. Although hybrid reinforced composites had superior performance in terms of both impact and flexure strength, basalt fibers demonstrated comparable strength, specifically in flexure. Overall, the hybrid composite possesses exceptional promise for use in prosthetic construction.

Force model of ultrasonic empowered minimum quantity lubrication grinding CFRP

As a weight-reducing material in aerospace applications, carbon fiber reinforced polymer (CFRP) requires precision grinding to ensure the accuracy and assembly of mating positioning surfaces. However, achieving low-damage machining of CFRPs remains challenging. Flood cooling reduces the mechanical properties, while dry grinding deteriorates the surface integrity, making minimum quantity lubrication (MQL) a viable alternative. However, nanolubricants struggle to penetrate the grinding area due to gas barrier obstruction. Consequently, a new CFRP grinding approach using multiangle 2D ultrasonic vibration-empowered nanolubricant MQL was proposed. An analytical force model is crucial for optimizing machining strategies and adjusting parameters. The aim is to reveal grinding mechanics and develop a force model under different ultrasonic and lubrication-cooling conditions. First, a uniform random fiber distribution model and a grinding wheel model were established, and the mechanical properties of the interface were predicted. Then, the intermittent grinding behavior and dynamic stiffnesses of the multiangle 2D ultrasonic vibration-assisted grinding (UVAG) system were investigated based on the geometric kinematics of the grains' trajectory. Furthermore, based on the deformation deflection curve, bending fracture force models for 45°, 90°, and 135° fibers were established with different contact and boundary conditions. The material removal mechanisms of shear and tension-compression buckling in 0° CFRP grinding were revealed, and an elliptical contact normal force model was established. The interlayer shear in 45° CFRPs was analyzed. The longitudinal compressive matrix cracking and fiber shear fracture in the 90° CFRPs were elucidated. Fiber microbuckling and fiber bundle removal of 135° CFRPs were investigated. Finally, grinding force models with different fiber orientation angles (FOAs) were interconnected and experimentally assessed. Experimental assessments demonstrated that the grinding force model has acceptable estimation error and effectively captures the mechanics of CFRPs with different FOAs.

A progressive damage model for 3D woven composites integrating kinking-buckling competition

In this work, a progressive damage model (Kinking-Buckling PDM) considering fiber micro-buckling failure, fiber kinking failure, transverse inter-fiber failure of yarns, matrix failure and interface debonding, is proposed to predict the compressive mechanical behavior of 3D woven composites (3DWC). Fiber kinking and fiber micro-buckling failure are adopted in the form of competition to characterize the longitudinal failure of fiber yarns. A bilinear damage evolution model is established to control the damage accumulation of each constituent. Meanwhile, a material tangent constitutive tensor characterizing the constitutive behavior of 3DWC is derived. The complex failure process of 3DWC under compression is displayed through Kinking-Buckling PDM. As the fiber initial misalignment angle increases, the compressive strength of 3DWC declines significantly, meanwhile, the main failure mode of 3DWC will be transformed from fiber micro-buckling to fiber kinking in warp yarns. The relevant experimental results are used to confirm the accuracy of the PDM.

Modeling Fiber Microbuckling in the Bending of Soft Composites Using Nonlinear Homogenization

A new approach to model the microbuckling of fiber-reinforced soft composites under bending is presented, which takes into account large deformations of the matrix and nonlinear kinematics. It leverages recent advances from the field of nonlinear homogenization of fiber composites to model the response of the matrix under the three-dimensional loading observed during buckling. The model then calculates the strain energy of the system as a function of several parameters, such as the position of the neutral axis or the microbuckling wavelength, whose values are obtained through energy minimization. The moment–curvature relationship predicted by the model agrees with experimental results, particularly in the postbuckling regime. The predictions also capture the buckling wavelength, as well as the fact that microbuckling extends into the tension side for thin laminates, an experimental observation that contradicts kinematic assumptions in previous modeling efforts. A parametric study considering possible variations in the composite laminate shows strong trends for parameters, such as thickness and volume fraction.

Contribution of longitudinal GFRP bars in concrete filled FRP tubular (CFFT) cylinders under monotonic or cyclic axial compression

Most of the current design guidelines/codes ignore the compressive contribution of glass fiber reinforced polymer (GFRP) bars in compression members, due to: (1) the compressive strength of GFRP bars is significantly lower than their tensile strength due to fiber micro-buckling or shear failure of GFRP bars under compression; (2) the peak axial load of an unconfined concrete column is achieved at a relatively low axial strain at which the GFRP bars develop small axial stresses due to their low elastic modulus. However, existing studies have demonstrated that, if adequate transverse bars are provided, the contribution of longitudinal FRP bars to the total load resistance of the column is significant partially due to the enhanced axial strain at peak axial load because of the confinement provided by the transverse bars. In concrete filled FRP tubular (CFFT) columns, which is an emerging and attractive form of columns for new construction, the ultimate axial strain of concrete is significantly enhanced by FRP confinement and thus the contribution of internal longitudinal GFRP bars is also expected to be enhanced. However, the compressive behavior of GFRP bars in CFFT columns has never been investigated. This paper therefore presents the results of an experimental program on CFFT cylinders with internal longitudinal GFRP bars. The test results demonstrated that FRP bars contribute significantly to the total load resistance of CFFT cylinders. The FRP bars in CFFT cylinders experienced crushing failure with load resistance close to that derived based on the compression tests on bare FRP bars. A simple analysis method for the axial load at FRP bar crushing for CFFT cylinders was finally proposed.

Process-induced damage and its effect on strength of composite - aluminum alloy joint by FDS technique

Flow drilling screw (FDS) is a novel joining technique that enables high-speed joining of dissimilar metals with one-shot process of hole machining, female thread forming and fastening from one side. The damage after FDS process and its effect on joint strength and fracture behaviour near the screw by load-bearing tests for FDS joints between aluminum alloy and carbon fibre reinforced thermoplastics (CFRTP) are evaluated. Cross-sectional observation after joining process reveals that delamination in CFRTP is clearly suppressed and matrix resin is filled in the screw thread compared to the case with CFRP using thermosetting resin. In the joint strength test with shear loading and the cross-shaped tensile tests where the screw is pulled out in its axial direction, only the fibre micro-buckling is observed until fracture. These experimental results recommend that CFRTP laminates should be applied as composite side since this technique utilizes frictional heat by the screw that can contribute to plastic flow or deformation of the thermoplastic resin with high fracture toughness. It is also clarified that damage growth is further suppressed by using CFRTP/Al hybrid laminates where aluminum alloy sheets are inserted between CFRTP plies, which leads to improve the joint strength.

Synchronous deployed design concept triggered by carbon fibre reinforced shape memory polymer composites

Synchronous deployments for symmetrical structures such as solar arrays and antennas are desired because they are conducive to subsequent attitude stabilization. In this work, we propose a synchronous deployed mechanism triggered by shape memory polymer composites (SMPCs). Firstly, the material design was developed according to the fibre microbuckling behaviour of unidirectional carbon fibre reinforced laminates undergoing flexural deformation to balance high mechanical requirements and large deploy/fold ratio desirements. Then, two resins with different glass transition temperatures (Tgs) from a similar shape memory epoxy system were used for fabrication. These laminates with different fibre volume fractions, thicknesses, matrices and fibre distributions were tested by three-point bending tests to evaluate their mechanical performance. Recovery tests were carried out under different environmental temperatures. In addition, the recovery stimulated by resistor heaters in vacuum endowed the mechanism with good recovery robustness. Finally, functional validation on an aluminium prototype showed good synchronization. This design concept has the advantage of being lightweight, low-impact and highly reliable. It is expected to provide a reasonable and robust solution for synchronous deployments.

Design of a set-up for measuring the residual compressive strength after high load and high cycle compression fatigue on CFRP

A new testing set-up to measure the compressive strength of continuous fibre composite materials (CFRP) under both static and fatigue loadings is described. A specific symmetric sandwich beam with CFRP face sheets is used with a four-points bending set-up. Its mechanical design is carried out by fine Finite Element Analyses (FEA) to ensure that the failure mode is axial compression by fibre micro-buckling. Precise comparisons between numerical simulations and experimental quasi-static experimental results are found to be sound, validating the FEA model. The latter is used to design the sandwich beam in terms of strength and stability criteria. Finally, a high cycle fatigue campaign (107 cycles) with high modulus CFRP skins demonstrates the performance of the set-up. Repeatable results and acceptable failure modes are obtained.

Effect of hole interaction in the initiation of damage in laminates

In open-hole laminates, when a second hole is made, the stresses and the damage evolution are modified. The main failure mechanisms in laminates subjected to compressive loads are fibre microbuckling and matrix breakage. In this work, the influence of the presence of a second hole in these mechanisms is analysed for two distances between holes. The Serial/Parallel Mixing Theory was used to model the elastic behaviour of the material and a Continuum Damage Model to represent the fibre microbuckling and matrix failure. The equations were implemented in a FEM code developed by the authors. The proposed model has been validated with results from the literature for a laminate with two holes positioned transversally to the applied load. In this problem, it was shown that the growth of the fibre damage is different from that of the matrix.  The laminate is damaged mainly in the zone between the holes. When the distance between holes is reduced, both types of damage around the holes increase.

Compressive and transverse shear behaviour of novel FRP-UHPC hybrid bars

Fibre-reinforced polymer (FRP) bars have become increasingly popular as internal reinforcement in reinforced concrete (RC) structures due to their excellent corrosion resistance. However, the compressive strength of FRP bars is generally much inferior to their tensile strength due to fibre micro-buckling under compression, and their transverse shear performance is much inferior to that of steel bars with the same diameter. To this end, a novel form of steel-free hybrid bars, which consist of an outer FRP confining tube, a central FRP bar and a layer of ultra-high performance concrete (UHPC) (without steel fibres) in the annular space between them (referred to as FRP-UHPC hybrid bars), have been proposed. In this study, compressive and transverse shear behaviour of FRP-UHPC hybrid bars have been investigated via experimentation. The key test variables include fibre winding angles of the FRP tube, fibre types of the FRP tube, the FRP tube thickness and the diameter of the central FRP bar. The test results confirm the validation of the novel hybrid bars: i) the compressive stress-strain curves of hybrid bars exhibit a ductile behaviour with a strain hardening segment, and the compressive behaviour of the central FRP bar in hybrid bars is superior to that of FRP bars in isolation; ii) the stress-strain response of hybrid bars can be designed to meet an elastic-plastic response with a post-yielding strain-hardening response; and iii) the transverse shear performance of hybrid bars is much better than that of FRP bars in isolation due to the contribution of FRP-confined UHPC section.

The indentation responses of composite Y‐type cores sandwich structure under various temperatures

AbstractThe influences of elevated and cryogenic temperature on the quasi‐static indentation responses and damage mechanism for composite Y‐type cores sandwich structure were studied by experiments in present paper. The quasi‐static indentation experiments were performed to explore the indentation performances for composite Y‐type cores sandwich structures under eight different temperature conditions. The influences of the testing environment temperature on the typical indentation force‐deformation curves, damage patterns and indentation failure load were investigated and analyzed. The experimental results demonstrated that the testing environment temperature made an important influence on the deformation mechanism, typical force‐deformation curves and failure load. Fiber fracture of the front panel was found when the testing environment temperature was below the glass transition temperature, while the fiber microbuckling of the front panel was also revealed when the testing environment temperature was above the glass transition temperature. The failure load increased with the decreasing of the testing environment temperature at cryogenic temperature. Nevertheless, at elevated temperature, the failure load decreased with the increasing of the testing environment temperature attributing to the weakening of the fiber‐matrix interface performances.

Fractographic study into the effect of drilling damage on bearing mechanisms and performance in Carbon-Fibre epoxy composites

With the widespread adoption of polymer composites in primary structures, understanding and prediction of the performance of composite to metal hybrid joints is now critical to engineering design of transport structures. This work investigated the damage processes associated with bearing failure of such composite joints, for both pristine holes and holes damaged during drilling. An aerospace grade composite was drilled under three different conditions, tested to failure under quasi-static double bearing loading, and then characterised using fractographic techniques. In the pristine condition, the initial damage process was 0° longitudinal splitting tangential to the lateral extents of the hole which then dictated the extent of the subsequent bearing damage development. Beneath the bearing face of the hole inclined lines of in-plane microbuckled fibres had developed whilst beyond the constraint of the washer there was considerable delamination and massive out-of-plane fibre microbuckling. As the degree of drilling damage increased, 0° longitudinal split development was inhibited, and the local pre-existing damage at the periphery of the hole had extended into the bearing damage zone, directly initiating out-of-plane fibre microbuckling. Consequently, the bearing damage zone exhibited irregular distributions of fibre microbuckles, both across the thickness and depth beneath the bearing face of the hole. The observations in this work provide a means to validate predictive models and offer potential routes to improve bearing performance and the tolerance of laminates with drilling damage when under bearing loads.

Fabrication and compression performance of continuous fiber Octet-truss lattice sandwich structure

Octet-truss is a three-dimensional lattice structure with light configuration and multi-function characteristics. In order to analyze and verify its compression resistance, a method of preparing Octet-truss lattice sandwich panel with continuous fiber was proposed. A set of Octet-truss structure molding tools was designed by this method. Different types of glass fiber and Kevlar fiber specimens were prepared by spatial weaving method and resin curing process, and the axial compression test was carried out. The experimental results show that the mechanical properties of different materials and different sizes of sandwich panels prepared by this method are stable, and the reliability of the preparation process is verified, Combined with the experimental process and electron microscope images, the main failure mode of glass fiber samples is fracture of the struts near the nodes induced by fiber microbuckling, while the Kevlar fiber samples failed by the Euler buckling of the struts.

Compressive damage modeling of fiber-reinforced composite laminates using 2D higher-order layer-wise models

A refined progressive damage analysis of fiber-reinforced laminated composites subjected to compressive loads is presented here. The numerical analysis exploits higher-order theories developed using the Carrera Unified Formulation, specifically 2D plate theories with Lagrange polynomials to enhance the kinematic approximation through each ply's thickness resulting in a layer-wise structural model. The CODAM2 material model, based on continuum damage mechanics, governs the intralaminar composite damage. The Hashin criteria and the crack-band approach provides failure initiation and propagation, respectively. Fiber micro-buckling and kinking is taken into account via the use of nonlinear post-peak softening models. It is shown that linear-brittle stress-strain softening is effective for accurate compressive strength predictions. A series of numerical assessments on coupon-level composite laminates is carried out to verify the proposed numerical framework while its validation is demonstrated by successfully applying the numerical tool to test cases for which experimental data is available from the literature. Various through-the-thickness structural models are evaluated to provide insights for proper modeling. Numerical assessments considered quasi-isotropic laminates, the compressive strength and size-effects under brittle fracture of notched laminates, and progressive damage characteristics due to stable crack growth in compact compression tests. The results show the possibility of using coarser meshes than those used in standard FEM approaches as the accuracy of predictions is preserved through the use of higher-order structural theories.

The three-point bending responses of carbon fiber composite sandwich beams with Y-frame cores at high and low temperatures

An experimental investigation was performed to explore the effects of the temperature on the three-point bending properties and failure mechanisms of carbon fiber composite sandwich beams with Y-frame cores in this paper. The composite sandwich beams with Y-frame cores firstly were fabricated using hot-press method. Subsequently, the three-point bending experiments of composite sandwich beams were tested at eight selected temperatures which ranged from -70 °C to 200 °C. Then the effects of temperature on the bending load–displacement curves, failure mechanism and bending failure load were analyzed and discussed. The experimental results showed that the bending load–displacement curves went down as temperature increased. The fiber fracture occurred for the specimens at temperature below the glass transition temperature, while the fiber microbuckling was observed for the specimens at temperature above the glass transition temperature. In addition to the change of load–displacement curves and failure modes, the bending failure load and the value of PT∕δT decreased as temperature increased. Finally, the analytical model of bending failure load and the value of PT∕δT for the composite sandwich beam was presented. The analytical predictions were calculated and compared with experimental results.

Correction to ‘Fibrous gels modelled as fluid-filled continua with double-well energy landscape’

You have accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Sun Chuanpeng, Chernysh Irina N., Weisel John W. and Purohit Prashant K. 2021Correction to ‘Fibrous gels modelled as fluid-filled continua with double-well energy landscape’Proc. R. Soc. A.4772021002820210028http://doi.org/10.1098/rspa.2021.0028SectionYou have accessCorrectionsCorrection to ‘Fibrous gels modelled as fluid-filled continua with double-well energy landscape’ Chuanpeng Sun Chuanpeng Sun http://orcid.org/0000-0001-7355-1514 Google Scholar Find this author on PubMed Search for more papers by this author , Irina N. Chernysh Irina N. Chernysh Google Scholar Find this author on PubMed Search for more papers by this author , John W. Weisel John W. Weisel Google Scholar Find this author on PubMed Search for more papers by this author and Prashant K. Purohit Prashant K. Purohit Google Scholar Find this author on PubMed Search for more papers by this author Chuanpeng Sun Chuanpeng Sun http://orcid.org/0000-0001-7355-1514 Google Scholar Find this author on PubMed Search for more papers by this author , Irina N. Chernysh Irina N. Chernysh Google Scholar Find this author on PubMed Search for more papers by this author , John W. Weisel John W. Weisel Google Scholar Find this author on PubMed Search for more papers by this author and Prashant K. Purohit Prashant K. Purohit Google Scholar Find this author on PubMed Search for more papers by this author Published:03 February 2021https://doi.org/10.1098/rspa.2021.0028This article corrects the followingResearch ArticleFibrous gels modelled as fluid-filled continua with double-well energy landscapehttps://doi.org/10.1098/rspa.2020.0643 Chuanpeng Sun, Irina N. Chernysh, John W. Weisel and Prashant K. Purohit volume 476issue 2244Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences16 December 2020Proc. R. Soc. A476, 20200643 (Published Online 16 December 2020). (doi:10.1098/rspa.2020.0643)Papers [B,C] should be added to the list of references in the above paperPrior to our own work [6,7], publication [B] was the first to use a continuum phase transition model [8] to explain the hysteretic behavior of fibrous materials via microbuckling of fibers(this should have been mentioned in the introduction, beginning of third paragraph)Publication [C] introduced for the first time a double-well energy function (landscape) in more than one dimension for the mechanical behavior of fibrous networks, derived from microbuckling of single fibers.(this should have been mentioned in the Introduction, top of p. 3, and in section 2 prior to introducing energy function (2.13))Footnotes© 2021 The Author(s)Published by the Royal Society. All rights reserved.References[B]Lakes R, Rosakis P, Ruina A. 1993Microbuckling instability in elastomeric cellular solids. J. Mater. Sci. 28, 4667–4672. (doi:10.1007/BF00414256) Crossref, ISI, Google Scholar[C]Grekas G, Proestaki M, Rosakis P, Notbohm J, Makridakis C, Ravichandran G. 2019Cells exploit a phase transition to mechanically remodel the fibrous extracellular matrix. arXiv preprint arXiv:1905.11246. Google Scholar Previous Article VIEW FULL TEXT DOWNLOAD PDF FiguresRelatedReferencesDetailsRelated articlesFibrous gels modelled as fluid-filled continua with double-well energy landscape16 December 2020Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences This IssueFebruary 2021Volume 477Issue 2246 Article InformationDOI:https://doi.org/10.1098/rspa.2021.0028Published by:Royal SocietyPrint ISSN:1364-5021Online ISSN:1471-2946History: Published online03/02/2021Published in print24/02/2021 License:© 2021 The Author(s)Published by the Royal Society. All rights reserved. Citations and impact Subjectsbiomechanicsmechanics

Micro-buckling of carbon fibers in shape memory polymer composites under bending in the glass transition temperature region

Abstract The influence of a wide temperature range in the glass transition region of a shape memory polymer (SMP) matrix on micro-buckling of the fiber reinforcements in shape memory polymer composites (SMPC) under large bending deformation is described. Analytical expressions to estimate the strain energy, neutral strain surface, critical buckling surface and half wavelength of the buckled fibers in the SMPC are presented based on the minimum energy method. This study considers the reinforced fibers as three-dimensional elastic bodies and the matrix as a temperature stimulated flat plate. A comprehensive study was performed to understand the dynamic temperature behavior of the micro-buckled fibers and corresponding results were validated by previous works in the literature. The effects of fiber volume fraction and thickness of the SMPC plates on the half wavelength are also discussed along with the simultaneous influence of temperature on the parameters computed in the minimum energy analysis.

Experimental characterization of failure behaviour for three-dimensional woven carbon/carbon composites under pin-loading

An experimental approach of three-dimensional woven carbon/carbon composites arranged in various geometrical configuration is presented in this paper. Seven types of samples divided into three groups were tested under pin-loading to determine the effects of width-to-hole diameter ratio (W/D), edge distance -to- hole diameter ratio (E/D) and hole diameter-to- thickness ratio (D/t) on the failure mode. For an advanced understanding of failure propagation, damage mechanism was observed and assessed combining acoustic emission monitoring. The experimental results and observations indicated that net-tension failure and shearing-out failure switched to bearing failure with the increasing ratio of W/D and E/D, respectively. while D/t hardly affected the failure mode. Major features of the damage mechanisms included matrix crashing and cracking, fiber micro-buckling and bending, interface debonding and fiber breakage with different acoustic emission signal ranges.

An experimental study of scaling effects in notched quasi-isotropic carbon/epoxy laminates under compressive loads

The size effects in centre-notched quasi-isotropic carbon/epoxy laminates under compression were investigated. The in-plane dimensions of the baseline specimen were scaled up by a factor of up to 14. The centre-notched specimens were compared to open-hole specimens of the same dimensions. It was found that compressive strength of the small centre-notched specimen is similar to that of the open-hole specimen. However, as the in-plane dimensions increase, the centre notches are weaker than the open holes, and start to follow a Linear Elastic Fracture Mechanics (LEFM) scaling line. These trends were well captured by a Composite Compressive Strength Modeller (CCSM) using the fracture toughness measured in the current study. Fibre micro-buckling was confirmed to be the dominant failure mechanism under compression. The failure mechanisms under compression were also compared against those under tension. Matrix splitting under compression was observed through X-ray Computed Tomography but was less extensive than under tensile loading.