Micromechanical Failure Research Articles

Effect of the orientation on the fatigue crack growth of polyamide 12 manufactured by selective laser sintering

Purpose Polyamide 12 (PA12) properties meet specific requirements for various applications in the automotive and aerospace industries. Bulk specimens made of PA12 and processed via the additive manufacturing technique such as selective laser sintering (SLS) present a layered structure. In case of structural applications, the fatigue performance of SLS PA12 parts is of vital importance and fatigue response studies in these type of materials are still scarce. Therefore, the purpose of this paper is to analyse the effect of the applied load orientation on the fatigue crack propagation behaviour of the layered structure of SLS PA12. Design/methodology/approach With the aim of understanding the effect of the applied load with respect to the layer orientation on the fatigue crack growth of SLS PA12, fatigue crack growth tests were carried out at two orientations. The specimens called PARA were orientated in such a way that the applied force direction belongs to the layer plane while in the group called PERP, the tensile force direction is coincident with the build direction, that is, perpendicular to the slice. Besides, special attention has been paid to the analysis of the fracture surfaces of the specimens, linking the micromechanisms of failure with the microstructure of the material. Findings The SLS PA12 specimens tested with the load applied parallel to the layered structure show a little better fatigue response than those tested at perpendicular orientation. The fracture surfaces of the specimens tested at perpendicular orientation are slightly smoother than those tested at parallel orientation. Crazes are the main micromechanism of failure with a crater size of 50 microns, which coincide with the spherulite size. This indicates that the void nucleation of the crazes takes places between lamellae inside the spherulites, and consequently, the craze growth and rupture occurs mainly in a transspherulitic mode. Originality/value PA12 parts manufactured via SLS are becoming more valuable in structural elements in the automative and aeronatical fields. In such applications, fatigue performance is vital for design. Fatigue studies are scarce in literature and even more when dealing with fatigue crack growth behaviour. The value of this work is the analysis of the fatigue crack growth response of these materials taking into account the anisotropic microstructure and to get a better understanding, this behaviour is explained taking into account the micromechanisms of failure and the microstructure of the material.

A thermodynamics- and mechanism-based framework for constitutive models with evolving thickness of localisation band

Localised failure in geomaterials invalidates the assumption of homogeneous deformation that constitutive models based on continuum mechanics rest on. In such cases, the deformation and nonlinear processes inside the localisation zone dominate the inelastic response of the material, while the material outside this zone usually undergoes negligible inelastic or even elastic deformation. As a consequence, internal variables representing micromechanical failure processes should better be defined inside the localisation zone, not averaged over the whole volume element containing it. In this study, we propose a thermodynamics-based framework for constitutive models that take into account the transition from homogenous to localised deformation. Two spatial scales involved in the mechanisms of localised failure, macro scale of the considered volume element and smaller scale of the localisation zone, are included in the formulation and derived constitutive models. This separation of spatial scales is combined with enrichments of the constitutive kinematics for the integration of three constitutive relationships describing the behaviour of the materials inside and outside the localisation zone, and the evolving size of this zone. As a result, the internal variables are associated with their own spatial zones, instead of being averaged over the whole volume element like in classical continuum approaches. The gradual transition from homogenous to localised deformation is represented by the onset and evolution of the thickness of the localisation band, both of which appear naturally in the proposed formulation. The obtained model therefore consists of both size and orientation of the localisation band, and three constitutive relationships connected through the equilibrium condition across the boundary of the localisation zone. They help provide a smooth transition from homogeneous to localised failure. Numerical examples show promising features of the proposed approach in connecting the macro behaviour with the underlying evolution of the localisation zone.

A stochastic model based on fiber breakage and matrix creep for the stress-rupture failure of unidirectional continuous fiber composites

Stress rupture is a time-dependent failure mode occurring in unidirectional fiber composites under sustained tensile loads, resulting in highly variable lifetimes. Stress-rupture is of particular concern in composite overwrapped pressure vessels (COPVs) since it is unpredictable, and has catastrophic consequences. At the micromechanical level, stress rupture begins with the breakdown of individual fibers at random flaws, followed by local load-transfer to intact neighbors through shear stress in the matrix. Over time, the matrix creeps in shear causing lengthening overload zones around fiber breaks, resulting in even more fiber breaks, and eventually, formation of a catastrophically unstable break cluster. Current reliability models are direct extensions of classic stochastic breakdown models for a single fiber, and do not reflect such micromechanical activity. These models are adequate for modeling composite stress rupture under a constant load, however, they may be unrealistic under more complex loading profiles, such as a constant load that follows a brief ‘proof test’ at a load level up to 1.5 times this constant load. For carbon fiber/epoxy COPVs, current models predict a reliability, conditioned on survival of a proof test, that is always higher than the reliability without such a proof test. Concern exists that this is incorrect, and that a proof test may result in reduced reliability over time. While the failure probability during a proof test may be very low, overwrap damage occurs nonetheless in the form of a large number of fibers breaks that would not occur otherwise based on fiber Weibull strength statistics. This phenomenon of increased fiber breakage during a proof test is captured in the model we develop and that specifically builds on the micromechanical failure process described above. For typical proof-test load ratios, the model predicts conditional reliabilities for lifetime that are typically much lower than those calculated in the absence of a proof test.

Damage characterization of carbon/epoxy composites using acoustic emission signals wavelet analysis

ABSTRACTThis paper will investigate wavelet packet transform of acoustic emission (AE) signal processing in order to identify failure mechanisms of unidirectional carbon/epoxy composites. For this purpose, carbon/epoxy composites were tested in tensile test by two different loading conditions. For the first specimen, the load was applied through fiber direction (T0) and for the second specimen, the load was applied perpendicular to the fiber direction (T90), AE signals were collected during experimental test procedure and the signal’s waveforms were decomposed into various wavelet levels that each includes details and approximations so-called. Each data is related to a specific frequency range. The results show that type and percentage of failure mechanisms in various loadings are different. In T0, four micro-failure mechanisms of matrix cracking, fiber/matrix debonding, fiber pull-out and fiber breakage were observed and Fiber pull-out was determined as main inter-phase failure mechanism. Debonding were dominant failure micro mechanisms of T90. The results were verified with scanning electron microscopic observations.

Mechanical behavior of Graphene decorated carbon fiber reinforced polymer composites: An assessment of the influence of functional groups

Decoration of the Carbon fiber (CF) surface using graphene based nano-fillers (GBN) by the electrophoretic deposition (EPD) is possibly one of the most emerging techniques for the enhancement of the mechanical response of carbon fiber reinforced polymer (CFRP) composites. Present investigation is focused on enhancing the flexural and interlaminar properties of a CFRP composite by surface modification of carbon fiber by EPD technique using GBN (graphene (G), graphene oxide (G-O), graphene hydroxyl (G-OH) and graphene carboxyl (G-COOH)). The highest flexural strength and interlaminar shear strength was obtained for the G-COOH modified CFRP composite, which are 9.6% and 22.9% higher than that of control CFRP respectively. Dynamic mechanical thermal analysis (DMTA) was conducted in the temperature range of 30 °C to 180 °C to understand the temperature dependent mechanical behavior of all the composites. In addition, fractographic analysis was carried out using scanning electron microscopy (SEM) to understand various failure micro-mechanisms.

Fracture analysis and mechanical properties of three phased glass/epoxy laminates reinforced with multiwalled carbon nanotubes

Abstract Herein, we report the use of Multi Wall Carbon Nano Tubes (MWCNTs) as nano-compatibilizers based on their astonishing mechanical properties and ease of processing. To fabricate laminate samples, pure MWCNTs were homogeneously dispersed in the fiber-reinforced plastic (FRP) composite with 0, 0.5, 1 and 1.5 wt. % loading. The laminates were prepared with eight plies (4.0 ± 0.1 mm thickness) using the hand layup technique assisted by the compression moulding method. It was found that the tensile, compressive and inter-laminar shear strength (ILSS) increase by 103.81%, 139.78% and 36.06%, respectively corresponding to 1 wt. % loading of MWCNTs as compared to neat GFRP specimen. However, a rapid decrease in strength beyond 1 wt. % loading of MWCNTs has been noted. Interestingly, the maximum of the tensile strength was higher than that of the compressive strength, and the maximum of the tensile modulus was larger than that of the compressive modulus in the case of 1 wt. % loading of MWCNTs. It was observed that after a certain loading, the mechanical properties of such laminates can only reach the best value with an optimum loading of MWCNTs. In addition, the micromechanical failure modes and effect of MWCNTs loading on internal morphologies of the composites were also intensively explored with the help of Field Emission Scanning Electron Microscopic (FESEM) analysis.

Analysis of Open-Hole Compressive CFRP Laminates at Various Temperatures Based on a Multiscale Strategy

In this paper, a multiscale analysis strategy was proposed to analyze the failure behaviors of open-hole compressive (OHC) CFRP laminates. Micro-level intralaminar failure was defined in the constituents (fiber and matrix) with a modified micromechanics failure theory. In the multiscale stress transformation, the effect of thermal residual stress was considered using constant thermal amplification factor. Meanwhile, macro-level interlaminar failure was defined with cohesive elements. Based on the simulated and experimental results, the sub-laminate scaled OHC laminates of the stacking sequence [45/0/−45/90]4s were studied at different temperatures. The established multiscale model showed good precision in the strength and failure mode predictions. Transverse throughout damage at the hole section led to the final failure. As the temperature increased, the damage process began at a lower load level and the strength of the laminates decreased significantly. Stiffness reductions and small load drops were more likely to occur before final failure. The differences in the delamination size among all interfaces tended to be smaller. Besides, matrix failure lagged under shear loading conditions if the thermal residual stress was neglected in the multiscale analysis.

Experimental Study on Shear Failure Mechanism and the Identification of Strength Characteristics of the Soil‐Rock Mixture

Soil‐rock mixture (SRM) is a special geological material that has the unique properties of rock or soil. Studies on the strength characteristics of the SRM have very important theoretical significance and practical value. In this study, the gravel proportions, gravel sizes, gravel shapes, and repetitive results of shear experiments are considered in laboratory experiments and for the identification of strength parameters. To the gravel shapes, from the angle of the composition materials of SRM, the experimental samples include samples with breccia gravels and with subrounded gravels. And, in this study, the laboratory model experiment is used to research the strength characteristics of SRM. In addition, the shear failure mechanism is used to establish the relationship between the microfailure mechanism and the macrostrength parameters identification. Taking gravel proportion, gravel size, gravel shape, and repetitive shear process as the influencing factors of the SRM, the laboratory models have been remolded, and laboratory direct shear experiments have been carried out. The shear deformation laws of the SRM are researched on the basis of the analysis of the curves of shear stress and the horizontal displacement.

Parametrically homogenized continuum damage mechanics (PHCDM) models for composites from micromechanical analysis

This paper develops a parametrically homogenized continuum damage mechanics (PHCDM) model for unidirectional fiber reinforced epoxy composites undergoing progressive damage. The PHCDM models are designed to overcome limitations of prohibitive computational overhead associated with many homogenization methods. They are thermodynamically consistent, reduced order continuum models with explicit representation of microstructural morphology through strategically determined parameters. The PHCDM model is derived from detailed micromechanics simulations of the representative volume element (RVE) using energy equivalence principles. Micromechanical failure is due to fiber–matrix interface debonding and matrix cracking. The macroscopic PHCDM models represent damage anisotropy through a second order damage tensor that contributes to the evolution of a damage surface in the space of damage work conjugate. The damage surface characterizes the initiation and evolution of damage in the composite. The constitutive relation between damage and its work conjugate is represented by an anisotropic fourth order damage surface tensor Pijkl, whose components are expressed as functions of current damage state. These are calibrated and validated from homogenized micromechanics (HMM) simulations. The PHCDM model is incorporated in a commercial finite element code and structural analysis of macroscopic composite components are executed for understanding concurrent damage and failure at multiple scales.

Investigation of maleic anhydride effect on wood plastic composites behavior

The aim of this study is to investigate the effect of maleic anhydride on mechanical properties of wood plastic composites using acoustic emission monitoring and wavelet analysis. Wood plastic composite samples were prepared by combining olive wood sawdust as reinforcing material with volume fraction of 40 wt% up to 60 wt% with polyethylene as matrix phase. Maleic anhydride with weight ratio of 1 to 3% as a coupling agent and stearic acid with weight ratio of 1% as a lubricant agent were used in all the compounds. Wood plastic composite samples were manufactured using extrusion and injection molding process in accordance with ASTM D638. Once specimens were produced, tensile tests were carried out in order to characterize their tensile properties. Results demonstrated that wood plastic composite sample containing 40 wt% sawdust and 3 wt% maleic anhydride possesses highest tensile strength. This experimental work established the criteria for the lower amount of reinforcement material and higher amount of compatibilization agent in improving mechanical properties. Failure micro-mechanisms were characterized by wavelet analysis of acoustic emission data. Ultimately, the experimental tests were proved by the scanning electron microscope observations.

Multiscale Analysis of CFRP Laminates with MMF3 Criterion under Different Off-Axis Loading Conditions.

A multiscale analysis strategy with physical modified-micromechanics of failure (MMF3) criterion was proposed to analyze the failure behaviors of carbon fiber reinforced plastic (CFRP) laminates. The quantitative relationship between the macro- and micro- stresses was determined considering two typical fiber distributions. Thermal residual stress was taken into account in the stress transformation. The failures were defined and the properties of damaged elements were degraded at the constituent level. The back-calculation method based on the iteration algorithm was proposed to determine the micro strength with macro mechanical tests. A series of off-axis loading tests were conducted to verify the established multiscale models. The predicted strength was also compared with the results using micromechanics of failure (MMF) criterion to present accuracy improvements. Thermal residual stress was found to affect the strength by contributing to the matrix damage status. Meanwhile, sensitivity analysis was provided for the matrix-dominant micro strength to investigate its physical meaning. Results suggest that the micro tensile and compressive strength of the matrix influenced the off-axis tensile and compressive strengths respectively, with relative large off-axis angles, while the micro shear strength of the matrix dominated when the off-axis angles were relative small.

Correlation of Structure and Fractographic Characteristics of High-Strength Pipe Steel Welded Joint Metal Failure Micro-Mechanism

The structure and microstructure of impact fracture specimens of the heat-affected zone (HAZ) of low carbon micro-alloyed high-strength pipe steel welded joints are investigated. Correlation of structure parameters and fractographic characteristics of the failure micro-mechanism channel is established, providing information about elementary acts of failure, i.e., the dimensions of ductile and brittle microcracks.

Dynamic In Situ Three-Dimensional Imaging and Digital Volume Correlation Analysis to Quantify Strain Localization and Fracture Coalescence in Sandstone

Advances in triaxial compression deformation apparatus design, dynamic X-ray microtomography imaging, data analysis techniques, and digital volume correlation analysis provide unparalleled access to the in situ four-dimensional distribution of developing strain within rocks. To demonstrate the power of these new techniques and acquire detailed information about the micromechanics of damage evolution, deformation, and failure of porous rocks, we deformed 3-cm-scale cylindrical specimens of low-porosity Fontainebleau sandstone in an X-ray-transparent triaxial compression apparatus, and repeatedly recorded three-dimensional tomograms of the specimens as the differential stress was increased until macroscopic failure occurred. Experiments were performed at room temperature with confining pressure in the range of 10–20 MPa. Distinct grayscale subsets, indicative of density, enabled segmentation of the three-dimensional tomograms into intact rock matrix, pore space, and fractures. Digital volume correlation analysis of pairs of tomograms provided time series of three-dimensional incremental strain tensor fields throughout the experiments. After the yield stress was reached, the samples deformed first by dilatant opening and propagation of microfractures, and then by shear sliding via grain rotation and strain localization along faults. For two samples, damage and dilatancy occurred by grain boundary opening and then a sudden collapse of the granular rock framework at failure. For the third sample, a fault nucleated near the yield point and propagated in the sample through the development of transgranular microfractures. The results confirm findings of previous experimental studies on the same rock and provide new detailed quantifications of: (1) the proportion of shear versus dilatant strain in the sample, (2) the amount of dilatancy due to microfracture opening versus pore opening when a fault develops, and (3) the role of grain boundaries and pore walls in pinning microfracture propagation and slowing down the rate of damage accumulation as failure is approached. Our study demonstrates how the combination of high-resolution in situ dynamic X-ray microtomography imaging and digital volume image correlation analysis can be used to provide additional information to unravel brittle failure processes in rocks under stress conditions relevant to the upper crust.

A data-driven complex systems approach to early prediction of landslides

Landslides are a common natural disaster that claims countless lives and causes huge devastation to infrastructure and the environment. The recent spate of landslides worldwide has prompted renewed calls for better forecasting methods which could boost the performance of early warning systems in real time. Although the variety, volume and precision of monitoring data have steadily increased, methods for analysing such data sets for landslide prediction have not kept pace with the rapid advances in complex systems data analytics and micromechanics of granular failure. Here we help close this gap by developing a new model to analyse kinematic data using complex networks. Like no other, our model incorporates lessons learned from micromechanics experiments on granular systems, with a focus on space-time variations and correlations in motion germane to the precursory dynamics of localised failure. We apply our model to ground-based radar data and predict where failure locates in a rock slope, spanning hundreds of meters, almost two weeks in advance. This is a first step in a broader effort to quantify the probability of a landslide occurring within a specified time based on data on kinematics and common triggers such as precipitation.

Multi-scale modelling of dynamic progressive failure in composite laminates subjected to low velocity impact

This paper aims to investigate the structural mechanical responses and progressive damage behaviors of composite laminates subjected to low velocity impact. First, a three-dimensional multi-scale model based on micromechanics of failure (MMF) criterion with new damage evolution laws is introduced for intralaminar damage, which considers the different mechanical behaviors of the microscopic constituents for fiber and matrix. The macroscopic damage variables of the new damage evolution laws are directly evaluated by the degraded elastic parameters of the microscopic constituents calculated from representative volume elements (RVEs). Second, by using this multi-scale approach, the relationship between macroscopic stress and microscopic stress is established by stress amplification factors (SAFs) through the RVEs using ABAQUS-PYTHON scripting language. The proposed model is implemented by the user-defined material subroutine VUMAT built on ABAQUS/Explicit platform, and the bilinear cohesive model in ABAQUS is employed to capture the onset and progression of interlaminar delamination. Finally, impact numerical simulation for the impact force-time/central displacement curves and energy dissipation is performed on three composite laminates with different layup patterns. Relatively good agreements are achieved between the experimental and numerical results, which validates the effectiveness of the multi-scale model.

Compressive and shear properties of carbon fiber composite square honeycombs with optimized high-modulus hierarchical phases

This paper focuses on strengthening carbon fiber hierarchical composite square honeycomb structures (HCSH) by selecting high-modulus hierarchical phase and optimizing its thickness. HCSH have been manufactured from carbon fiber composite foam sandwich structures by a simple snap-fit and bonding method. The measured compressive and shear strengths are shown to be well predicted by micromechanics failure models of HCSH. Compared to the monolithic composite square honeycomb structures (CSH), the measured specific out-of-plane compressive strength of HCSH improves to approximately 330% and the specific shear strength improves to about 180%. Furthermore, the relationships between hierarchical phase modulus and its optimum thickness are developed to design the optimum HCSH. Carbon fiber HCSH are found to exhibit better mechanical properties than other cellular structures, and thus provide new opportunities for light-weight multifunctional sandwich structures.

Experimental and numerical study on internal pressure load capacity and failure mechanism of CO2 corroded tubing

With the purpose of researching on the influence of CO2 injection tubing with corrosion defects to the safety production of CO2-EOR flooding, experimental tests and finite element simulations were conducted to analyze the failure pressure and stress distribution of P110 pipe with single point corrosion, axial line corrosion and uniform corrosion defects. Metallographic examination was also done to investigate the micro-mechanism of pipe failure. Depending on the experimental results, P110 tubes with corrosion defects mainly ruptured with different sizes of opening. For single point corrosion defects, the injection tubing did not fail when the inner corrosion pressure is 93.9 MPa and the corrosion depth ratio(d/t) is 0.964. For axial line corrosion defects, the higher the failure pressure, the larger the opening was. The result of injection tubing with uniform corrosion defect is contrary to the result of injection tubing with axial line corrosion. In conclusion, the integrity of CO2 injection tubing is crucial for CO2 flooding production, in which the defect depth plays a more important role than defect length. DNV RP-F101 model can be a right model to predict failure pressure of all kinds of corrosion defect except single point corrosion.

Fracture of Elastomeric Materials by Crosslink Failure

If an elastomeric material is subjected to sufficiently large deformations, it eventually fractures. There are two typical micromechanisms of failure in such materials: chain scission and crosslink failure. The chain scission failure mode is mainly observed in polymers with strong covalent crosslinks, while the crosslink failure mode is observed in polymers with weak crosslinks. In two recent papers, we have proposed a theory for progressive damage and rupture of polymers with strong covalent crosslinks. In this paper, we extend our previous framework and formulate a theory for modeling failure of elastomeric materials with weak crosslinks. We first introduce a model for the deformation of a single chain with weak crosslinks at each of its two ends using statistical mechanics arguments, and then upscale the model from a single chain to the continuum level for a polymer network. Finally, we introduce a damage variable to describe the progressive damage and failure of polymer networks. A central feature of our theory is the recognition that the free energy of elastomers is not entirely entropic in nature; there is also an energetic contribution from the deformation of the backbone bonds in a chain and/or the crosslinks. For polymers with weak crosslinks, this energetic contribution is mainly from the deformation of the crosslinks. It is this energetic part of the free energy which is the driving force for progressive damage and fracture of elastomeric materials. Moreover, we show that for elastomeric materials in which fracture occurs by crosslink stretching and scission, the classical Lake–Thomas scaling—that the toughness Gc of an elastomeric material is proportional to 1/G0, with G0=NkBϑ the ground-state shear modulus of the material—does not hold. A new scaling is proposed, and some important consequences of this scaling are remarked upon.

Crystallographically resolved damage initiation in advanced high strength steel

Recently, the third generation of advanced high strength steels (AHSSs) show promising properties for automotive applications. The improvement of macroscopic mechanical performance is not feasible without a deep understanding of the micromechanical behavior and failure micro-mechanisms involved during its response under various loading conditions. In this study, a uniaxial tensile test is conducted on a low silicon bainitic steel with second phase constituents (martensite and carbides). A comprehensive image processing on SEM micrographs is performed in order to quantify the damage evolution as a function of plastic deformation. A new methodology is examined to address the correlation between crystallographic orientation and damage initiation. In this multiphase steel, it appears that orientation dependence of damage initiation is blurred by the presence of different phases and hence there is not an obvious preferential orientation from where damage has initiated.

A synchrotron X-ray and electron backscatter diffraction based investigation on deformation and failure micro-mechanisms of monotonic and cyclic loading in titanium

Synchrotron X-ray diffraction technique has been used to estimate defect structure in terms of dislocation density, crystallite size and micro-strain in commercially pure titanium subjected to tension and cyclic deformation in stress and strain control mode. Statistical analysis of micro-texture data collected from electron backscatter diffraction approximately from the same region as that of synchrotron X-ray has been used to correlate orientation dependent micro-strain and dislocation density with deformation microstructure and micro-texture. Two different orientations, namely, A with prismatic-pyramidal and B with basal orientation along the loading axis has been considered. Weak initial texture yet significant anisotropy in hardening/softening response and failure mode for monotonic tension and cyclic loading paths has been observed. Higher strain hardening response of orientation A during monotonic tensile deformation can be attributed to the evolution of lower micro-strain on basal orientation grains i.e, 〈0002〉ǁND along with extensive multi-variant twinning that also restricts crack propagation and delays failure in stress control mode. On the other hand, in strain control mode, orientation B shows higher fatigue life due to the generation of lower micro-strain in the basal orientation grains and single variant twinning that can undergo detwinning easily is responsible for delayed crack nucleation and subsequent failure.