Subsequent Mechanical Loading Research Articles

Experimental study of poromechanical behavior of Callovo-Oxfordian claystone in undrained triaxial compression and extension tests

The Callovo-Oxfordian (COx) claystone is selected as the host rock in the French project for geological disposal of radioactive waste. In its initial state, the host rock is fully saturated. Due to the low permeability, the pore fluid pressure can locally evolve in a quasi undrained condition due to subsequent mechanical and thermal loading, and then significantly affects deformation and cracking process of geological barrier. Most previous studies were carried out in quasi drained conditions or with constant pore pressure. Poromechanical behavior of COx claystone in undrained condition has been so far rarely investigated. In the present work, we carry out a new series of laboratory tests. Two representative loading paths are considered: triaxial compression with constant confining pressure and triaxial extension with constant mean stress. The variations of strain and pore pressure are measured during the tests. It is found that the poromechanical behavior of the claystone is clearly affected by loading path. The failure strength is higher in triaxial compression than in extension. The evolution of pore fluid pressure is correlated with volumetric strain. Compared with the results obtained in drained tests, it seems that the Terzaghi effective stress can be used for the description of pore fluid pressure effect on failure strength.

A computational study of the effects of polycrystalline structure on residual stress-strain concentrations and fracture in metal-matrix composites

Deformation and fracture of metal-matrix composites are numerically investigated. Composite microstructures with a single ceramic particle in a homogeneous or polycrystalline matrix are generated by the step-by-step packing method. The thermoelastic-plastic reaction of the matrices is described using isotropic and anisotropic relations, including the associated plastic flow rule and crystal plasticity theory. The thermoelastic response of a ceramic particle is isotropic. In order to describe the crack initiation and propagation in the matrix and particle, two fracture criteria with the limiting values of the equivalent plastic strain and stress are developed. During cooling both pure shear and volumetric tension stress regions are formed in the matrix around the particle. The polycrystalline structure of the matrix prevents through-the-thickness crack formation during subsequent mechanical loading of the composite. Grain boundaries cause multiple stress concentrations and plastic strain localizations all over the matrix material, which delays fracture near the matrix-particle interface.

The Modeling of Laboratory Experiments on Granular MX-80 Bentonite with COMSOL Multiphysics

Radioactive waste disposal, as the final step of the open nuclear fuel cycle, is an important process to protect humans and the environment from harmful effects of ionising radiation. Approaching the construction of the geological repository, the understanding and predictability of the behavior of engineered barrier material becomes more important than ever. Therefore, a number of research studies are being focused on the experimental and numerical analysis of the engineered barrier material state and behavior under repository conditions. Engineered barrier material will be in contact with the host rock and waste packages, and its properties and behavior will be governed by complex and coupled thermo-hydro-mechanical processes. This paper presents the modeling activities of the Lithuanian Energy Institute, performed in the framework of the H2020 project BEACON (Bentonite Mechanical Evolution). The numerical model, developed in COMSOL Multiphysics (Burlington, MA 01803, USA), was applied for the modeling of experiments, performed by Ecole Polytechnique Federale de Lausanne (EPFL, Switzerland), on granular MX-80 bentonite in the odeometer cell. The hydromechanical behavior of a compacted bentonite sample was analyzed under different conditions: hydration with groundwater under confined volume conditions and hydration under free swelling conditions and subsequent mechanical loading. Model outcomes (swelling pressure, saturation, dry density, and void ratio) were compared to the available experimental data. The modeling results were in line with the analyzed experimental data.

Multiscale temperature-dependent ceramic matrix composite damage model with thermal residual stresses and manufacturing-induced damage

This work presents a multiscale thermomechanical simulation framework to capture the temperature-dependent damage behavior of woven ceramic matrix composites (CMCs). The framework consists of cooldown simulations, which capture a realistic material initial state, and subsequent mechanical loading simulations to capture temperature-dependent nonlinear stress–strain behavior. The cooldown simulations result in a realistic material initial state with thermal residual stresses and damage hotspots that occur due to constituent property mismatch and post-manufacturing cooldown. A fracture mechanics-informed thermomechanical progressive damage model is extended to capture the manufacturing-induced damage that occurs because of the high thermal residual stresses and to simulate the mechanical response of two-dimensional (2D) plain weave carbon (C) fiber, silicon carbide (SiC) matrix (C/SiC) CMCs at temperatures ranging from room temperature (RT) to 1200 °C. A combination of temperature-dependent material properties and damage model parameters are included in the model to simulate the effects of temperature on deformation and damage behavior. Model calibration was conducted using quasi-static tensile experimental data from the literature for RT, 700 °C, and 1200 °C and the nonlinear, temperature-dependent predictive capabilities of the reformulated model are demonstrated for 1000 °C. The model is also applied to simulate the temperature-dependent thermomechanical response of a 2D woven five harness satin (5HS) SiC/SiC CMC at RT and 1200 °C and shows excellent agreement with experiments.

Influence of Trapped Gas on Pore Healing under Hot Isostatic Pressing in Nickel-Base Superalloys

Under the typical hot isostatic pressing (HIP) processing conditions, plastic deformation by dislocation slip is considered the primary mechanism for pore shrinkage, according to experimental observations and deformation mechanism maps. In the present work, a crystal plasticity model has been used to investigate the influence of applied pressure and holding time on porosity reduction in a nickel-base single crystal superalloy. The influence of trapped gas on pore shrinkage is modeled by coupling mechanical deformation with pore–gas interaction. In qualitative agreement with experimental investigations, we observe that increasing the applied pressure or the holding time can effectively reduce porosity. Furthermore, the effect of pore shape on the shrinkage is observed to depend on a combination of elastic anisotropy and the complex distribution of stresses around the pore. Simulation results also reveal that, for pores of the same shape, smaller pores (radius < 0.1 μm) have a higher shrinkage rate in comparison to larger pores (radius ≥ 0.1 μm), which is attributed to the increasing pore surface energies with decreasing pore sizes. It is also found that, for smaller initial gas-filled pores (radius < 0.1 μm), HIP can result in very high gas pressures (on the order of GPa). Such high pressures either act as a driving force for argon to diffuse into the surrounding metal during HIP itself, or it can result in pore re-opening during subsequent annealing or mechanical loading. These results demonstrate that the micromechanical model can quantitatively evaluate the individual influences of HIP processing conditions and pore characteristics on pore annihilation, which can help optimize the HIP process parameters in the future.

Temporary materials: comparison of in vivo and in vitro performance

ObjectiveThe aim of this investigation was to compare clinical performance and in vitro wear of temporary CAD/CAM and cartridge crowns. This study is an approach to estimate the influence of in vivo use and laboratory simulation on temporary crowns.Materials and methodsA total of 90 crowns were fabricated from each temporary CAD/CAM or cartridge material. Also, 10 crowns of each material were clinically applied for 14 days, and 80 identical duplicate restorations were investigated in the laboratory after storage in water (14 days; 37 °C) and subsequent thermal cycling and mechanical loading (TCML, 240.000 × 50N ML, 600 × 5°C/55 °C). After in vivo application or in vitro aging, facture force, superficial wear (mean and maximum), surface roughness (Ra, Rz), thermal weight loss (TGA), and heat of reaction (DSC) were determined for all crowns. Statistics: Bonferroni post hoc test; one-way analysis of variance (ANOVA); α = 0.05).ResultsThe fracture resistance of the temporary materials varied between 1196.4 (CAD in vivo) and 1598.3 N (cartridge crown in vitro). Mean (maximum) wear data between 204.7 (386.7 μm; cartridge in vitro) and 353.0 μm (621.8 μm; CAD in vitro) were found. Ra values ranged between 4.4 and 4.9 μm and Rz values between 36.0 and 40.8 μm. DSC and TG analysis revealed small differences between the materials but a strong influence of the aging process.ConclusionsComparison of in vivo and in vitro aging led to no significant differences in fracture force and wear but differences in roughness, DSC, and TGA. SEM evaluation confirmed comparability. Comparison of CAD/CAM and cartridge temporary materials partially showed significant differences.Clinical relevanceIn vitro aging methods might be helpful to estimate materials’ properties before principal clinical application. CAD/CAM and cartridge temporary materials provided comparable good clinical performance.

Effect of Thermomechanical Treatment on Subsequent Deformation Behavior in a Binary Z1 Magnesium Alloy Studied by the Acoustic Emission Technique

Pre‐straining of magnesium alloy modifies the final texture of the material and therefore leads to a different deformation behavior during subsequent loading. At the same time, heat treatment in the context of thermo‐mechanical procedures is used as softening mechanism, as well as for age hardening processing. In mechanical conditions with twin‐dominated deformation in textured magnesium alloys, the twin boundaries can be pinned if existing after a pre‐deformation of samples and therefore influence their mobility. The impact of the ongoing mechanical behavior is discussed. In the present paper, a procedure of pre‐compression and reverse tensile loading of extruded binary magnesium alloy Mg–1 wt% Zn (Z1) is used and intermediate aging at 200 °C is applied in two different routes. The effect of the heat treatment on the flow behavior during subsequent mechanical loading is correlated with the dominating deformation mechanisms revealed by in situ acoustic emission measurements and ex situ scanning electron microscopy. The pinning of twin boundaries can restrict their mobility in experimental environments that favor the recovery of the samples. As a result, new twins easily nucleate if the concurrent re‐orientation of the lattice is favorable for strain accommodation.

Thermo-mechanical behavior assessment of smart wire connected and busbarPV modules during production, transportation, and subsequent field loading stages

Thermo-mechanical loads induce stresses in photovoltaic (PV) modules, leading to crack formation. In this context, the understanding of module's thermo-mechanical behavior is important. To investigate the thermo-mechanical behavior of smart wire connected technology (SWCT) and busbarPV modules throughout their entire life, the present study is conducted that probes the stress distribution and deformation during production, transportation, and subsequent mechanical and thermal loading stages in a consecutive step-by-step manner using finite element modellingapproach. Pre-stresses and non-linearitiesare considered in simulation models. Stresses and displacements experienced by different parts/layers are examined, and crack sensitive regions are identified. In addition, the SWCTand busbarmodules are compared, and it is found that SWCTinterconnection is relatively a less stress inducing process and less susceptible to thermal and dynamic affects. During production stage, stresses of 39.3 MPaand 40.4 MPaare generated in SWCTcells and copper wires respectively; while, stresses of 60 MPaand 87 MPaare generated in busbarcells and busbarrespectively. Similarly, lower stresses are induced in SWCTPV modules during subsequent stages. The comparison results show advantages of SWCTmodule in terms of mechanical stability which can lead to improve the performance and reliability of PV modules.

A three-scale framework for fibre-reinforced-polymer curing part II: Mesoscopic modeling and macroscopic effective properties

Important phenomena in a resin transfer molding (RTM) process for polymeric materials are temperature-dependent visco-elastic effects accompanied by curing. In Part 1 we propose a three-scale model and a comparative study for different meso material properties including bounds and effective material constants. Based on those preliminaries, in this work, Part 2, we present basic relations in a thermodynamic framework for temperature-dependent visco-elastic effects accompanied by curing which is specialized to the scenario of a mechanical-thermal-chemical process. During curing and subsequent mechanical loading, the periodic mesostructure defined by a visco-elastic polymeric matrix and linear-thermo-elastic carbon fibres is taken into account as a representative unit cell (meso-RVE) subjected to thermo-mechanical loading on the mesoscale. Homogenization of the mesoscale by volume averaging is applied to obtain the effective properties for the fully cured composite on the macroscale. In the examples we use DSC-measurements for parameter identification and simulate the curing process as well as mechanical loading of the cured part with the finite-element-method.

Mechanical loading up-regulates early remodeling signals from osteocytes subjected to physical damage

In the mineralized bone matrix, mechanical loading causes micrometer-sized cracks. These cracks trigger targeted remodeling along the micro-crack. Physical damage to osteocytes was shown to be involved in the initiation of this remodeling process. However, the role of subsequent mechanical loading osteocyte response to physical damage is unclear. In this study, we have designed and developed an in vitro cell model to study the impact of mechanical loading on osteocytes with physical damage. Specifically, a system was developed to create sub-cellular physical damage on MLO-Y4 osteocytes in vitro. This model re-created the spatial distribution of non-viable cells and VEGF expression around microdamage as reported in vivo. Using this system, the short term (24h) effects of fluid shear stress in regulation of osteocyte response to physical damage were investigated. We have observed that the mechanical stimuli had an additive effect in terms of COX-2, VEGF mRNA expressions, as well as PGE2, VEGF concentrations in the media. Interestingly, other inflammatory signals such as IL-6 and TNF-α did not change with these stimuli, at this time point. Moreover, fluid shear also had a modulating effect in regulation of osteoclast differentiation by osteocyte with physical damage. These results show that (1) subcellular physical damage upregulates remodeling signals in osteocytes at early time point, (2) mechanical loading substantially upregulates these signals for remodeling in osteocytes with physical damage.

Slip localization and fatigue crack nucleation near a non-metallic inclusion in polycrystalline nickel-based superalloy

Fatigue crack nucleation at a non-metallic agglomerate inclusion has been studied by high spatial resolution Digital Image Correlation (HR-DIC) and high angular resolution Electron Backscatter Diffraction (HR-EBSD). Spatial and temporal characterization and correlation of deformation with underlying microstructures has been performed, with distributions of plastic strain measured from HR-DIC; and residual stress and density of geometrically necessary dislocations (GND) measured from HR-EBSD. Initial residual stress and GND fields, as a consequence of differing thermal expansivities in the metallic and oxide phases, localized around the agglomerate have been quantified using HR-EBSD. The localization of the pre-existing stress and dislocation states appear to lead to early onset of plasticity upon subsequent mechanical loading. Heterogeneous distributions of plastic strain have been observed in the course of the fatigue test by HR-DIC. Crack nucleation via agglomerate/nickel interface decohesion and particle fracture has been demonstrated and this is correlated with the elevation in strain and dislocation density. The measurements of residual stress, strain, and dislocation density provide key information for the mechanisms of fatigue cracking and the development of damage nucleation criteria in these material systems.

Micromechanics of diffusion-induced damage evolution in reinforced polymers

In this work we numerically investigate the nucleation and evolution of micromechanical damage in reinforced glassy polymers under transient hygro-mechanical loading. In particular, we demonstrate the role that fiber distribution plays in the evolution of overall damage due to fiber–matrix interfacial debonding under moisture ingress. The heterogeneity of fiber distribution (clustering) is characterized by the coefficient of variation C v of the center-to-center distances between interacting fibers, determined by identifying a cut-off radius around a typical fiber. The initial moisture diffusion-induced damage provides synergistic conditions for the rapid evolution of debonding under subsequent mechanical loading. The results indicate that microstructural heterogeneity strongly affects the moisture diffusion characteristics that in turn hurt the overall load carrying capacity of a composite due to aggravated damage. The strong dependence of the moisture-induced damage evolution on the fiber arrangement suggests that one should not resort to using simplistic unit cell models that assume regular fiber arrangements in such cases.

Micromechanical model of 3D cross-ply copper matrix composite reinforced with SiC fibres

A 3D cross-ply micromechanical model is used to analyse the thermomechanical behaviour of copper matrix composite reinforced with SiC fibres, when subjected to cyclic loadings at high temperature. The copper matrix composite is reinforced with 45% fibre volume fraction. A cohesive model is employed to capture the influence of the debonding interface in the composite, during the consolidation and subsequent thermal and mechanical loading.

Mechanooptical investigations of polyimide films exposed to electrons, mechanical loads, and temperatures

Joint effect of high-energy electrons, mechanical loads, and temperature on polyimide films of thicknesses in the range 30–130 µm is investigated. The films were preliminary irradiated by electrons in air using an ELU-6 linear accelerator with energy of 2 MeV and doses D = 1, 5, 10, 20, 30, 40, and 100 MGy and then subjected to uniaxial mechanical tension at temperatures (T) from 293 to 593 K. It is established that at T = 293–450 K and D = 20–40 MGy, the mechanical load causes almost the same deformations (el max) of nonirradiated and irradiated samples; at T = 450–550 K, deformations of films sharply increase, and the character of their dependence changes. The el max value of the initial sample increases almost linearly with temperature by a factor of 10, whereas the character of changing el max(T) of the irradiated films is more complex, and its value increases approximately by a factor of 4. For T > 500 K, the deformation reaches limiting values. Irradiation increases the intensity of IR-spectra by 2–6 times and essentially increases the widths of absorption bands at 720, 1380, and 1775 cm−1, which is caused by the formation of hydrogen bonds and cycles with nitrogen as well as by the formation of nitrogen oxides. External loading applied to film rupture causes an increase in the EPR signal amplitude from 3·103 to 5·103, which is connected with an increased concentration of radicals =N-H and-NH 2. The electron irradiation of the polyimide films with their subsequent mechanical loading causes the spectrum lines to displace from 3475.0 to 3512.5 cm−1 with simultaneous reduction of the signal amplitude from 6·103 to 4·103.

Thermoelastic effects on mixed-mode delamination growth emanating from circular holes in laminated FRP composites

Abstract This paper deals with the thermoelastic effects on mixed-mode delamination growth behaviour emanating from circular holes in laminated Fiber-Reinforced Polymeric (FRP) composites. Two sets of full three-dimensional analyses have been performed (one with the thermal residual stresses developed while curing the laminate and the other without thermal residual stresses i.e., with mechanical loading only) to calculate the displacements and interlaminar stresses along the annular shaped delaminated interface responsible for delamination onset and propagation. Modified Crack Closure Integral (MCCI) method based on the concept of linear elastic fracture mechanics has been followed to calculate the three modes of Strain Energy Release Rates (SERR). The strain energy release rate components GI, GII and GIII along the annular shaped delamination front due to thermal residual stresses developed while curing the laminate (i.e., cooling from an elevated temperature of thermosetting to the room temperature) and the subsequent mechanical loading have been obtained by superimposing their respective effects. Numerical calculations are carried out for multi-layered cross-ply and angle-ply glass/epoxy FRP composite laminates and the strain energy release rate plots demonstrate large asymmetries along the delamination front due to the interaction of residual curing stresses and superimposed mechanical loading. It is observed that the residual curing stresses have the effect of enhancing the behaviour of delamination growth. On subsequent mechanical loading this can be a potential source of causing premature failure due to the superimposed thermomechanical effects.

Retained austenite stability investigation in TRIP steel using neutron diffraction

In situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for the retained austenite stability evaluation during subsequent mechanical loading. The progress in the austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of the transformation is extracted from neutron diffraction spectra. The useful information was also obtained on the retained austenite stability in the TRIP steel during mechanical testing. The in situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of the neutron diffraction technique at monitoring the transformation of the retained austenite during room temperature tensile testing. In both experiments the neutron investigation was focused on the volume fraction quantification of the retained austenite as well as on internal stresses rising in structure phases due to the retained austenite transformation.

In Situ Neutron Diffraction Analysis of Phase Transformation Kinetics in TRIP Steel

The precise characterization of the multiphase microstructure of low alloyed TRIP steels is of great importance for the interpretation and optimisation of their mechanical properties. In-situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for retained austenite stability evaluation during subsequent mechanical loading. The progress in austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of transformation is extracted from neutron diffraction spectra. The integrated intensities of austenite and ferrite neutron diffraction profiles over the time of transformation are then assumed as a measure of the volume fractions of both phases in dependence on transformation temperature. Useful information was also obtained on retained austenite stability in TRIP steel during mechanical testing. The in-situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of neutron diffraction technique in monitoring the transformation of retained austenite during room temperature tensile test. In both experiments the neutron investigation was focused on the volume fraction quantification of retained austenite as well as on internal stresses rising in structure phases due to retained austenite transformation.

Mixed-mode analysis of superimposed thermo-elastic effects in fiber-reinforced composites with embedded interface delaminations

This paper deals with the 3D finite element analysis of superimposed thermo-elastic effect on embedded interfacial delamination crack growth characteristics in fiber-reinforced laminated composites. Interlaminar fracture at the delamination front is found to be a mixed-mode phenomenon due to the anisotropy and heterogeneity of thermo-physical properties of composite materials. This leads to the requirement of finite element evaluation of energy release rates, based on the principles of linear elastic fracture mechanics. The strain energy release rate components along the delamination front due to a uniform temperature drop, during the manufacturing stages of composite laminates, to room temperature and subsequent mechanical loading is obtained by superimposing their respective effects based on the assumptions of linear elasticity. Numerical calculations are carried out for multi-layered cross-ply and angle-ply composite laminates and energy release rate plots demonstrate large asymmetries along the delamination front due to the interaction of residual stresses and superimposed transverse loading.

Evolution and influence of residual stresses/strains of fiber reinforced laminates

This paper presents a meso/micromechanical study of fiber reinforced polymeric matrix cross-ply laminates during and after a high temperature curing process. The emphasis is on the generation and evolution of the curing induced residual stress/strain and their influence on the response under subsequent mechanical loading. The cross-ply laminate is modeled by a representative volume element and the constituent materials are described by an elastic model for the fiber and a non-linear viscoelastic model for the polymer matrix. The residual stress/strain fields at the ambient temperature immediately following the cool-down and their evolution with time are determined through the viscoelastic finite element discretization of the RVE with a special consideration given to the fiber/matrix interface region. The results indicate that the residual stresses decrease with time and tend to small asymptotic values after about 34 h. However, the remaining residual strains still have significant influence on the damage evolution of the cross-ply laminate under subsequently superimposed tensile loading.

Viscoelastic Behavior of Osteoarthritic Cartilage

We have studied the incremental stress-strain behavior of human articular cartilage in tension in an attempt to understand the molecular basis for fibrillation and fissure formation in osteo-arthritis. Our results indicate that the elastic spring constant for collagen in the direction perpendicular to the cleavage line pattern is about 1.6 GPa (2.3 GPa after correction for the collagen content) and the collagen fibril length is between 0.558 μm at low strains and 1.24 μm at high strains for normal cartilage. Values for the elastic spring constant and collagen fibril length were both found to decrease in OA. The value of the elastic spring constant for collagen perpendicular to the cleavage line pattern is similar to that calculated based on stress-strain curves reported by Kempson[1].Our results indicate that the elastic spring constant for collagen and the collagen fibril length decrease as the extent of fibrillation and fissure formation increase. Decreases in the elastic spring constant of collagen are consistent with loss of the superficial layer, degradation of proteoglycans and collagen, and subsequent mechanical fatigue. However, changes in the polymer volume fraction are consistent with enzymatic degradation preceding mechanical disruption. It is concluded that osteoarthritic changes to cartilage involve enzymatic degradation of matrix components and fibril fragmentation that is promoted by subsequent mechanical loading.