Microscale Failure Mechanisms Research Articles

Martensite size and morphology influence on strain distribution and micro-damage evolution in dual-phase steels; comparing segregation-neutralised and banded grades

A change in strain partitioning and microscale failure mechanisms in dual-phase (DP) steel was found when both the morphology and distribution of martensite were altered compared to a banded DP steel grade benchmarked against a specific commercial DP grade. To achieve a DP microstructure with equiaxed and well-dispersed martensite, the concept of segregation neutralisation was utilised, where the ratio of Mn to Si elements was decreased (from 7.4 to 0.3) to neutralise the effect of Mn segregation on generating the banded martensite. A combination of micromechanical modelling simulations and in-situ notch tensile testing (within an SEM) was employed to compare the micro strain field and void formation rate between the segregation-neutralised and the benchmark grades. The benchmark grade showed extensive void coalescence along the direction of shear bands in the tensile sample after the average tensile strain of 30%. In contrast, no void coalescence was observed in the segregation-neutralised DP steel even at the average tensile strain of 80% just before failure. As a result, post-uniform elongation in the segregation-neutralised grade increased to 63.3%, compared to 30.1% in the benchmark grade.

Effect of microstructural roughness on the performance and fracture mechanism of multi-type single lap joints

Surface roughness of adherends is a crucial factor in determining the performance and failure mechanism of adhesive joints. However, the research dedicated to the examination of fracture mechanism for adhesive joints influenced by surface roughness at microscale is limited. This work conducts systematic experimental and numerical investigations into the effect of microstructural roughness on the performance and fracture mechanism of multi-type adhesive single lap joints (SLJ). The adherend materials are aluminium alloy (Al) and polyphthalamide (PPA), ground into three roughness grades for the fabrication of SLJs using an epoxy adhesive. The mechanical properties of the adhesive, adherends and SLJs derived from experimental studies are utilized to calibrate the microparameters in Discrete Element Method (DEM) models for a numerical analysis. The newly developed DEM models demonstrate efficacy in predicting the performance and capturing the failure modes of multi-type SLJs realistically with distinct microroughness profiles. Finally, the influencing mechanisms of microstructural roughness on the performance of multi-type SLJs are investigated, including the microscale interfacial bonds and mechanical interlockings. The effects of microstructural roughness on the microscale failure mechanisms of multi-type SLJs are also explored and discussed, including the crack initiation, coalescence, and propagation within the adhesive and interface.

Strain rate-dependent behavior of cold-sprayed additively manufactured Al–Al[formula omitted]O[formula omitted] composites: Micromechanical modeling and experimentation

Metal matrix composites (MMCs) fabricated by cold spray additive manufacturing (CSAM) are increasingly gaining attention as structural materials due to their rapid production and scalability. Herein, the failure behavior of CSAM Al–Al2O3 composites under quasi-static and dynamic compression was studied by an experimentally informed/validated 3D microstructure-based finite element (FE) model. The debonding mechanism was found to grow at a higher rate consequently dampening the particle cracking mechanism when the strain rate rises to dynamic regimes. The stress-bearing capacity of the particles plays a key role in enhancing the flow stress and elongation at failure of the CSAM composite under high strain rates due to the lower propensity of particle cracking. Eventually, the model was exercised to study the microscale failure progression in the material under elevated temperatures. For the first time in the literature, this study informs on the correlation between the microscale failure mechanisms and the mechanical performance of CSAM MMCs at the macro scale across strain rates and temperatures whose outcomes are applicable to the design of next-generation materials with a tailored performance.

A multiscale experimental study of CFRP-UHPFRC interfacial bond behaviour: Failure mechanisms and a fibre volume dependent bond-slip law

Carbon fiber reinforced polymer (CFRP) materials are being increasingly used with ultra high performance fiber reinforced concrete (UHPFRC) to construct composite structural members making use of their unique properties. As the interfacial bond behaviour between CFRP and UHPFRC plays a key role in the design of such members, it was investigated in detail by multiscale experiments of single shear pull-off tests of 26 CFRP-UHPFRC bonded joints with different bond lengths and widths of CFRP laminates and 0.5–4% volume fraction (Vsf) of straight high-strength steel fibers (length = 13 mm, diameter = 0.2 mm). 12 CFRP-normal strength concrete (NSC) bonded joints were also tested for comparison. The surface strain evolution and the interfacial debonding process were captured and analysed by the digital image correlation (DIC). The complex microscale failure mechanisms, such as the mortar cracking, the CFRP-UHPFRC interfacial debonding, and the steel fibers’ slipping and pulling out of the mortar, were visualised and analysed by the 3D micro X-ray Computed Tomography (μXCT) scanning. It was found that the debonded layer of mortar under the laminates was much thinner than that in the CFRP-NSC bonded joints, due to the lack of coarse aggregates in UHPFRC. The peak pulling forces for the specimens with Vsf = 0.5%, 1%, 2%, and 4% were 9%, 21%, 28%, and 29% higher than those without steel fibers, respectively. An empirical bond strength model and an interfacial bond-slip model were proposed based on the test results, accounting for Vsf, the material properties, and the laminate to UHPFRC width ratio. Finally, the proposed bond-slip model was applied in finite element modelling to simulate the flexural behaviour of an FRP-UHPFRC composite deck in the literature, with the interfacial debonding failure and the ultimate loads successfully predicted.

Investigation of the interface stick-slip friction behavior of clay nanoplatelets by molecular dynamics simulations

As lamella and porous materials, the failure of clay soil is closely related to the microscale friction features between nano minerals sheets. This study delves into the microscale interface stick-slip friction behavior of clay minerals, with a particular focus on kaolinite and halloysite models, under varying hydrostatic pressure conditions. The research unveils several significant discoveries. Firstly, under hydrostatic pressure, the mechanical properties of clay minerals perpendicular to the mineral platelet weaken, accompanied by a gradual reduction in the space occupied by interlayer water molecules. As pressure increases, these molecules assemble into "water molecular clusters", resulting in the formation of two or three layers of water molecular films. At 6 GPa, interlayer water nearly reaches a state of stagnation. Secondly, both kaolinite and halloysite exhibit "stick-slip friction behavior" during shearing, with rising hydrostatic pressure intensifying interatomic interactions and augmenting shear stress fluctuations. Lastly, the calculated friction coefficients (μ) for the three systems (kaolinite –7 Å, halloysite –8.6 Å, and halloysite –10 Å) are 0.035, 0.048, and 0.033, respectively, while the friction angles (φ) are 2.270, 2.900, and 2.263. The cohesion values (C) for these systems, obtained through different methods, demonstrate consistency. These findings advance the understanding of clay mineral behavior under varying conditions and contribute valuable insights to the microscale failure mechanism of clay soil.

Micromechanical damage analysis of Al-Al2O3 composites via cold-spray additive manufacturing

3D micro-scale finite element (FE) models were developed to analyze the stress-state-dependent initiation and growth of failure of cold-sprayed additively manufactured (CSAM) Al-Al2O3 composites. Informed by experimental particle size distributions and porosity measurements, representative volume elements (RVEs) were generated by Digimat. Stress-state-dependent constitutive models for the Al matrix and Al2O3 ceramic particles were implemented by VUMAT subroutines in ABAQUS/Explicit FE solver. For the first time in the literature on metal–ceramic composites, the microscale failure mechanisms involving matrix ductile failure, particle cracking, and matrix/particle debonding were all quantified based on the crack volume fraction, the fraction of cracked particles, and the fraction of fully debonded interfacial nodes, respectively. The FE model was quantitatively and qualitatively validated by the experimental data for Al-46 wt% Al2O3 under quasi-static uniaxial compression. The validated model was used to investigate the effect of the particle content and size on the material behavior subjected to different stress states. The results revealed that the failure mechanisms are activated in a specified order across different stress states: I. matrix/particle debonding, II. particle cracking, and III. matrix ductile failure. The material ductility observed under compression/shear vanishes under tension due to the earlier activation of debonding and the faster growth (∼ 10 times) of the particle cracking mechanism. Additionally, it was shown that the particle size minimally affects the material strength and flow stress under shear which is likely related to the low load transfer effect through the interfaces. The novelty of this work stems from the provision of a better understanding of the stress-state-dependent evolution of failure mechanisms through a systematic quantification framework whose outcomes have implications for the design of better-performing Al-Al2O3 composites via control of the evolution of failure mechanisms and developing micromechanism-based constitutive models for CSAM metal–ceramic materials.

Ex-situ micro X-ray computed tomography tests and image-based simulation of UHPFRC beams under bending

Micro-structures of fibre reinforced composites play a pivotal role in their damage, fracture and failure mechanisms. This study first carried out ex-situ micro X-ray Computed Tomography (μXCT) tests of three small ultra-high-performance fibre reinforced concrete (UHPFRC) beams under progressive three-point bending. The first two beams were continuously loaded until failure and their central regions with the main flexural crack were scanned by μXCT before and after the test. The third beam was subjected to five cycles of loading-unloading until failure, and was scanned by μXCT after each unloading. The 3D μXCT images with 30 μm voxel resolution were then processed and analysed, and the spatial distribution, number, orientation and dimension of pores and fibres were statistically quantified. The micro-scale failure mechanisms, such as fibre bridging of cracks, fibre-matrix interfacial slipping and debonding, and matrix failure, were visualised and analysed. The μXCT images of the first two beams were then segmented and directly converted to meso-scale finite element models based on orientation-dependent bond-slip relations for the fibre-matrix interfaces and continuum damage plasticity for the matrix. The simulated load-displacement curves, fracture processes and final crack morphologies were found in excellent agreement with the tests. It was also found that the position, dimension and orientation of fibres and pores have significant effects on the damage and fracture initiation and evolution from both the tests and the simulations.

Prediction of the inter-fiber mechanical properties of composites: Part II Failure criterion based on micromechanics and cross-scale stress calculation

A novel inter-fiber failure criterion of composites based on micro-scale failure mechanism is proposed in this paper. The composites failure is determined by the failure of constituents near interface, where the micro-scale stress is “risk”. A cross-scale stress calculation formula is derived according to inclusion theory, and proved precise by comparing with the micro-scale simulation results. The accuracy of the failure criterion is verified through the experimental results and simulation results of micro-scale model under biaxial stress. The failure criterion considers the micro-scale stress distribution, fiber volume fraction and elastic properties of constituents, and can give precise predictions on the strength and failure angle of fiber reinforced polymer composite.

Grain-scale failure mechanism of porous sandstone: An experimental and numerical FDEM study of the Brazilian Tensile Strength test using CT-Scan microstructure

Many widely used numerical models of rock fracture based on mesoscale laboratory test characterisation of effective ‘intact’ strength parameters neglect microstructure effects. They therefore cannot explain grain boundary and pore effects on crack propagation and consequently are inadequate for models of rock destruction that exploit point and indentation stresses. Understanding deep drilling processes involving drill-bit buttons and/or water-jetting where rock loading is concentrated in domains with fewer mineral grains will therefore require models with microstructure. To investigate microscale failure mechanisms of granular rocks in diverse scenarios, we target a porous sandstone and introduce a novel workflow consisting of a computerized tomography (CT) based microstructure construction approach and a complementary mechanical numerical approach. The construction approach extracts the realistic rock microstructure and transforms the large voxel number CT-scan data into significantly fewer triangular elements. The finite-discrete element method (FDEM) with grain-based model (GBM) is adopted to solve the mechanics. The microscale failure mechanism of sandstone during the Brazilian test was thoroughly analysed using the numerical results together with the post failure CT-scan test data. The build-up of compressive and tensile stress chains, micro-crack nucleation, local relaxation, chain switching and final crack-path development exploiting pores was illustrated, revealing the micro-to-macro failure mechanism in time and space. Fracture paths in the specimens during Brazilian tensile test were dominated by the pores and the inter-grain boundaries. The tensile strength of the inter-grain joints was estimated to be at least 3.67 times the mesoscale specimen's intact tensile strength, while the pores account for 72.76% of the fracture path. The influence of the cementation distribution and microscale discontinuities was investigated with numerical cases.

Experimental investigation of failure mechanisms and evaluation of physical/mechanical properties of unidirectional flax–epoxy composites

Using natural fibers as reinforcement in polymer matrix composites necessitates evaluating the latter under different modes of solicitation. This allows extracting its material properties for engineering design and research purposes. The main objective of the study is preparing a consistent set of material properties for unidirectional flax fiber-reinforced epoxy composite with defined composition and basic configuration. These data are prerequisites for growing researches on flax fiber-reinforced epoxy composites, especially for numerical analysis purposes using the finite element method. In this work, partially green unidirectional-flax fiber-reinforced epoxy composites are tested for physical and mechanical properties and studied for their failure modes. Tension, compression, flexion, and shear properties, as well as physical properties like density, specific heat capacity and thermal diffusivity, are evaluated according to ASTM standard test methods. Flax fibers, which are composites by themselves, come in bundles in the composites and demonstrate a complex behavior. Therefore, a fractographic analysis has been conducted to understand the macro and microscale failure mechanisms to correlate them with the material properties. The results are in good agreement with those of the literature, when available, but they mainly show the specific behavior of unidirectional-flax composites subject to different solicitation modes, especially compression and direct shear modes evaluated this way for the first time for unidirectional-flax fiber-reinforced epoxy composite. They cover most of the data required for engineering design and numerical analysis by methods like finite element method, particularly for simulating the machining process of flax fiber-reinforced epoxy composite in the ongoing works.

Investigation of the microstructural fracture behaviour of asphalt mixtures using the finite element method

To limit and control the degradation of asphalt pavements and to increase the durability of the asphalt layers, a thorough understanding of the mechanical pavement response is necessary. As a contribution, it is imperative to investigate the fracture behaviours of asphalt mixtures. This paper applies two-dimensional (2D) cohesive zone modeling (CZM) techniques to simulate the microscale crack initiation and propagation within the asphalt mixtures. X-ray computed tomography (X-ray CT) scanning and digital image processing (DIP) techniques were applied to reconstruct the microstructure of asphalt specimens. Finite element (FE) simulations were conducted to simulate the quasi-static indirect tensile test (IDT) with the implementation of a 2D bilinear cohesive zone model. The FE models were optimised and validated based on experimental results. The validated model was then used to investigate the effect of temperature and the air voids on the fracture behaviour of asphalt mixtures. The results show that more significant damage is observed in broader areas outside of the main crack trajectory as the temperature is increased in the range considered in this study (−10 °C, 0 °C, +10 °C). The aggregate-mortar interface exhibited more significant damage than the bulk fracture within the mortar. The deformation voids which are defined as those not crossed by the final crack trajectory have no effect on the maximum reaction force at temperatures above 0 °C. Also, deformation voids are found to extend the time to failure. The results contribute to the current research on the fracture behaviour of asphalt mixtures and future investigations are recommended to facilitate the establishment of an asphalt mixture design method and evaluation method based on microscale failure mechanisms.

On the effect of adhesive thickness on mode I fracture energy - an experimental and modelling study using a trapezoidal cohesive zone model

ABSTRACTIn this paper, a combined experimental-numerical method was proposed to study the effect of adhesive thickness on mode I fracture energy of the adhesive layer, using Double Cantilever Beam (DCB) testing and Cohesive Zone Modelling (CZM). A Trapezoidal Cohesive Zone Model (TCZM) was introduced in tabular form into commercial Finite Element (FE) package to simulate the crack propagation behaviour in the adhesive layer and include the ductile characteristics of the selected adhesive. In order to overcome the difficulty in the accurate monitoring and measurement of crack length during loading, the Compliance-Based Beam Method (CBBM) was adopted to deduce the mode I strain energy release rate data. The load–displacement curve, Resistance curve (R curve), and mode I fracture energy data obtained from both experiment and simulation were analysed to validate the established FE model. It was observed that adhesive thickness has a significant influence on the ductile fracture behaviour of adhesive under mode I loading. The mode I fracture energy increases initially, and then decreases with the increase of adhesive layer thickness in the selected thickness range, while obvious transition was also observed in the perspective of micro-scale failure mechanism of the fracture surface through Scanning Electron Microscopy (SEM).

Strain rate-dependent compressive behavior and failure mechanism of cementitious syntactic foams

The present work was focused on studying the strain rate sensitivity of cementitious syntactic foams (CSF), which are particulate composites reinforced with hollow glass microspheres (HMG). Different density CSFs (1.31–1.74 g/cm3) with different volume fractions (20–40%) of HGMs were tested with a split-Hopkinson pressure bar setup. The true particle densities of the HGMs were in the range of 0.38–0.60 g/cm3. In addition, the macro- and micro-scale failure mechanisms were investigated with high-speed camera imaging, micro-CT scanning, and electron microscopy. The results showed that both the CSFs and the baseline material (control sample), which is the cement paste matrix of the CSFs, showed strain rate sensitivity in mechanical properties in the 102–103 s−1 strain rate range. CSFs had relatively lower strain rate sensitivity in comparison to the matrix material. In the same range of strain rate, both the CSFs and the control sample showed significant changes in their macro and micro failure mechanisms depending on their age, composition and loading rate. The level of damage at the peak load for the high strain rate was higher in comparison to the same materials tested under quasi-static loading conditions for CSFs and the cement matrix.

Spatiotemporal characterization of 3D fracture behavior of carbon-fiber-reinforced polymer composites

This study proposed a spatiotemporal algorithm to quantitatively characterize the in-situ 3D fracture behavior of carbon-fiber-reinforced polymer (CFRP) composites at microscale. In-situ micro X-ray computed tomography (µXCT) integrated with a tensile stage was applied to capture the 3D fracture evolution of the CFRP composites, where the initiation and propagation of fracture features (e.g., fiber tip-end crack and fiber/matrix debonding) were identified. After the reconstruction of the 3D material microstructure, the proposed spatiotemporal algorithm thereafter extracted the fracture features by employing multiple image processing techniques for quantitative analysis. A similar distribution of the 3D strain obtained from the volumetric digital image correlation demonstrated the feasibility of the developed spatiotemporal algorithm. Moreover, this algorithm provided in-depth and quantitative analysis of fracture features, which provided insights into the microscale failure mechanism and thus shed light on the improvement of failure criteria for CFRP composites with complex microstructures.

The effect of through-thickness compressive stress on mode II interlaminar crack propagation: A computational micromechanics approach

A micromechanics framework for modelling the mode II interlaminar crack onset and propagation in fibre-reinforced composites is presented with the aim of i) modelling the micro-scale failure mechanisms that underlie interlaminar crack propagation, and ii) determining the effect of the through-thickness pressure on mode II fracture toughness. An algorithm for the generation of the fibre distribution is proposed for the generation of three-dimensional Representative Volume Elements (RVEs). Appropriate constitutive models are used to model the different dissipative effects that occur at crack propagation. Numerical predictions are compared with experiments obtained in previous investigations: it is concluded that the proposed micromechanical model is able to simulate conveniently the interlaminar crack propagation and to take into account the effect of the through-thickness pressure on mode II interlaminar fracture toughness.

Effect of long-term operation on steels of main gas pipeline. Reduction of static fracture toughness

The effect of the long-term operation on changes in the parameters of fracture toughness and a tendency for a delayed fracture of the steel of main gas pipelines is investigated. Based on the method of the full strain diagrams the general regularities of operational degradation influence onto the static fracture toughness and microscale failure mechanisms of 17MnSi steel after long term operation have been established. Generalization of results on the influence of absorbed hydrogen onto the pipeline steel structure and fracture pattern made possible to systemize the data on dispersed damage accumulation. The schematization of fracture mechanisms of 17MnSi steel is offered. The proposed approaches to the study of static fracture toughness of the steel of the main gas pipelines can be used in the evaluation of their technical conditions and planning of their overhaul.

Microscale failure mechanisms leading to internal short circuit in Li-ion batteries under complex loading scenarios

One of the least understood mechanisms of Li-ion batteries is the development of internal short circuits under mechanical loads. In this study, a micro mechanical model is developed and subjected to various loading scenarios to understand the sequence of failure in the multi-layer, multi-material structure of a Li-ion battery jellyroll. The constitutive response of each component of the electrode stack is obtained by comprehensive experimental tests using uniaxial and biaxial tensile and compressive loads. The homogenized response of the model is recovered through the computational homogenization theory. The model is validated by comparing the results of a macroscale simulation against experimental data. The study focuses next on the development of a failure criterion for the electrode stack based on the microstructural observations. Results show distinct failure mechanisms when the loading is predominantly tensile versus when it is compressive or combined tensile/compressive. A failure locus is plotted from the results of the simulations as a criterion to detect the onset of short circuit under complex multi-axial loading scenarios.

Effect of Processing Conditions on Fracture Resistance and Cohesive Laws of Binderfree All-Cellulose Composites

The fracture properties of all-cellulose composites without matrix were studied using Double Cantilever Beam (DCB) sandwich specimens loaded with pure monotonically increasing bending moments, which give stable crack growth. The experiments were conducted in an environmental scanning electron microscope to a) perform accurate measurements of both the fracture energy for crack initiation and the fracture resistance and b) observe the microscale failure mechanisms especially in the the wake of the crack tip. Since the mechanical behaviour of the all-cellulose composites was non-linear, a general method was first developed to obtain fracture resistance values from the DCB specimens taking into account the non-linear material response. The binderfree all-cellulose composites were prepared by a mechanical refinement process that allows the formation of intramolecular bonds between the cellulose molecules during the drying process. Defibrilation of the raw cellulose material is done in wet medium in a paper-like process. Panels with different refining time were tested and it was found than an increase in fibre fibrillation results in a lower fracture resistance.

Compressive behavior of aluminum/copper hybrid foams under high strain rate loading

The accessible interconnected structure of open-cell metal foams offers the opportunity to create hybrid foam materials through electrodeposited metal coatings, which has great potential for the fabrication of functionally-graded foam systems. Nanocopper coated aluminum (Al) foam was created by reinforcing conventional open-cell Al foams with electrodeposited nanocrystalline copper (Cu). The mechanical properties of such Al/Cu hybrid foam under high strain-rate compression were investigated using a split Hopkinson pressure bar and numerical methods were used to gain a further understanding on the micro-scale failure mechanisms. It was found that the stable compressive response of open-cell Al foam can be effectively enhanced by electrodeposited nanocopper coatings. However, such enhancement is limited by the relatively high brittleness of the Al/Cu hybrid foam due to the low ductility level of the electrodeposited nano-coating material. Nonetheless, this study also shows that the overall energy absorption performance of Al/Cu hybrid foams under high strain rate loading can be significantly improved by increasing the ductility level of the electrodeposited copper coating.

Effect of micromechanical parameters of microstructure on compressive and tensile failure process of rock

A discrete element model is proposed to examine rock strength and failure. The model is implemented into UDEC to incorporate a new constitutive law for particle boundary behaviour. This purpose is achieved through establishing a user-defined model by creating a dynamic link library (DLL) and attaching it to the code. Rock material is represented as a collection of irregular-sized deformable particles interacting at their cohesive boundaries. The interface between two adjacent particles is viewed as a flexible contact whose constitutive law controls the material fracture and fragmentation properties. To reproduce different behaviours of rock in compression and tension, an orthotropic cohesive law is developed for contact, which allows the interfacial shear and tensile behaviours to be different from each other. The model is applied to a crystallised igneous and a soft sedimentary rock, and the individual and interactional effects of the microstructure parameters on the rocks compressive and tensile failure response are examined quantitatively and qualitatively. Statistical analysis and analytical solutions are employed to establish a methodical calibration process. It is shown that micro-shear mechanisms control rocks failure in a variety of rock types and loadings except for crystallised rocks under uniaxial compression where failure is mainly dominated by micro-tensile fractures. A practical way using the standard laboratory data is also presented to identify the controlling micro-scale failure mechanism.