Effects Of Thermal Residual Stresses Research Articles

Effect of low temperature on interlaminar fracture toughness of multi-directional CFRP and CFRP-steel interfaces

Delamination is the dominant failure type in FMLs, particularly when combining CFRP and steel. Practical applications usually contain multi-directional interfaces. Therefore, delamination of multi-material and multi-directional interfaces is investigated using the DCB and ENF setups. Analysis of such interfaces requires asymmetric layups. Thus, thermal residual stresses (TRS) build up. Effects of TRS are corrected to compare true fracture toughness values, since neglecting TRS can result in mispredictions of up to 200 %. The ply orientation at the delamination interface affects the fracture toughness of monolithic and hybrid interfaces. In general, hybrid interfaces exhibit slightly lower interface properties compared to monolithic interfaces. While mode I behavior appears mostly unaffected by temperature, mode II fracture toughness slightly increases with decreasing temperature.

The Effect of Thermal Residual Stress on the Stress State in a Short-Fiber Reinforced Thermoplastic

Upon their cooling and solidification, significant thermal residual stresses can develop in short-fiber reinforced thermoplastics due to the mismatch in coefficient of thermal expansion between fiber and matrix. In this study we set out to investigate this effect numerically. The build-up of thermal residual stresses is modeled by expanding a well-established constitutive model, the Eindhoven glassy polymer (EGP) model, with thermal expansion. The experimentally measured thermal residual stresses can be described using an effective glass-transition temperature and a constant coefficient of thermal expansion without the need for complex equilibrium kinetics associated with the glass transition itself. Subsequently, the influence of thermal residual stress on the deformation behavior for a short-fiber reinforced thermoplastic is studied employing multi-fiber representative volume elements (RVEs) for different fiber-weight fractions. The micromechanical models are evaluated on the importance of thermal residual stresses on the local and nominal stress state. From these analyses it can be concluded that the thermal residual stresses should be accounted for when assessing the quantitative local stress state and are therefore essential when local mechanisms are studied. In contrast, thermal residual stresses are not required to capture the nominal transient stress–strain response.

Disclosing residual thermal stresses in UHT fibre-reinforced ceramic composites and their effect on mechanical behaviour and damage evolution

The reusability of CMCs with inherently high-temperature capabilities and ablation resistance goes through understanding of the effects of residual thermal stresses (RTS) caused by thermal expansion mismatch between fibre and matrix. In this work, the RTS of ultra-high temperature ZrB2–SiC based ceramic reinforced with carbon fibres was dependent on fibre anisotropy and fibre coating. A detailed microstructural analysis about the state of matrix (i.e. residual porosity and cracks) and fibre (i.e. porosity within the fibre bundles, volumetric content of the fibres and their degree of dispersion) was performed. The different level of RTS, among the composites, was studied by comparing stiffness-displacement curves of the bending test and the cyclic dilatometric curves. The correlation between the level of RTS and thermomechanical behaviour was also evaluated by flexural tests at 1500 °C and after thermal shock. The results showed that RTS not only reduced flexural strength, but also influenced damage evolution and stiffness linearity. As a result, the release of RTS can alter not only the pristine geometry, obtained after sintering or machining, due to the formation of inner freed fibres (IFFs) but also the Young's modulus of the matrix and the stress and strain at failure onset of the investigated CMCs.

Micromechanics-based simulation of B4C-TiB2 composite fracture under tensile load

Micromechanics modeling was performed to study the effects of thermal residual stress, weak interphases, TiB2 volume fraction and particle size on the mechanical responses and fracture behaviors of B4C-TiB2 composites. Experimentally observed fracture behaviors including micro-cracking and crack deflection were successfully captured. The weak interphases at B4C-TiB2 boundaries and the thermal residual stress induced during cooling by the large CTE mismatch between B4C and TiB2 were identified as two major factors to promote micro-cracking that caused the enhanced progressive failure behavior. Micro-cracking was enhanced with higher TiB2 volume fraction due to higher fraction of weak interphase and material affected by thermal residual stress. Meanwhile, micro-cracking behaviors exhibited limited change with varying TiB2 particle sizes. This modeling study successfully captured the main fracture behaviors and their trends by varying micro-structures of B4C-TiB2 composites and can potentially aid microstructure design of tougher B4C-TiB2 composites in the future.

Phase field mechanics of residually stressed ceramic composites

ABSTRACT A phase field theory for crystalline solids accounting for thermoelasticity, fracture, twinning, and limited slip is presented. Residual stresses are incorporated via referencing thermoelastic strain to a reference state that is not always stress-free. Rate dependence, dissipated energy, and residual strain energy (possibly degraded by local fracture) are included in the theory, with physically valid predictions first verified for homogeneous stress states. A variational form of the model is implemented in finite element (FE) calculations of three-dimensional (3-D) polycrystalline aggregates consisting of up to two different crystal constituents and a binding matrix along grain and phase boundaries. Model specifics correspond to constituents of boron carbide-titanium diboride (BC-TiB) ceramic composites. Effects of thermal-residual stresses incurred during processing, as well as other local microstructure properties and physical features, on deformation and failure mechanisms are revealed. Peak aggregate strengths observed for different boundary conditions demonstrate a pressure-dependent failure surface.

Microstructure and phase composition evolution of dual-phase ytterbium silicate coatings plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder

The main challenges of atmospheric plasma spraying (APS) when dealing with rare earth silicate are the amorphous phase formation and silica volatilization in as-sprayed coatings. In this study, the microstructure and phase composition evolution of dual-phase ytterbium silicate environmental barrier coatings (EBCs) plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder are investigated. The unavoidable silica volatilization leads to the coating composition deviate from stoichiometry of disilicate composition and result in the presence of Yb2SiO5 in coatings. By controlling the phase fraction of Yb2SiO5 through modulation of plasma character allows minimizing the effects of thermal residual stress on the coating durability in thermal cycling. The high crystallinity and crack-free coatings were achieved through optimized crystallization heat treatment. The presence of dislocation and deformation twins endow the dual-phase coating with intrinsic quasi plasticity and improvement in reliability. The present study provides useful inspirations on the optimization of spraying parameters and design of advanced EBC systems.

Effect of Residual Stresses on Fatigue Crack Growth: A Numerical Study Based on Cumulative Plastic Strain at the Crack Tip.

Residual stresses affect the fatigue behavior, given that compressive stresses delay the phenomenon, while tensile stresses accelerate it. However, the mechanisms behind the effect of residual stresses are not totally understood. A numerical study is developed here to understand the effect of thermal residual stresses (TRSs) on fatigue crack growth (FCG). The crack driving force was assumed to be the cumulative plastic strain at the crack tip. The heating of a region ahead of the crack tip produced elastic compressive TRS, which were 69% of material’s yield stress. Alternatively, plastic deformation was produced by severe cooling followed by heating to generate compressive residual stresses. The crack propagation in the compressive residual stress field produced a decrease in the FCG rate. On the other hand, without the contact of crack flanks, the TRS showed no effect on FCG. Therefore, the TRSs only affect FCG by changing the crack closure level.

Experimental characterization and micro-modeling of transverse tension behavior for unidirectional glass fibre-reinforced composite

The mechanical behaviour of unidirectional fibre-reinforced polymer composites under transverse tension was studied using micromechanical computation and macroscopic experimentation. The representative volume element (RVE) of a composite with random fibre distribution was built by the proposed algorithm, which can effectively solve the jamming-limit problems for the hard-core-based model. The progressive damage process of the interface and matrix was simulated using the cohesive zone model and the extended Drucker–Prager model. The solutions of the mechanical loading case and the thermal mechanics loading case were compared to illustrate the effect of thermal residual stress. According to parametric studies, the failure models were mainly divided into two categories: interface-dominated and matrix-dominated damage, which rely on the interface strength, interface fracture energy, and matrix plastic deformation.

Effect of thermal residual stress on the tensile properties and damage process of C/SiC composites at high temperatures

Due to the mismatch of the thermal expansion coefficients between the matrix and yarns, thermal residual stress will appear in C/SiC composites. In this paper, a progressive damage model was used to predict the thermal-mechanical behavior of C/SiC composites and reveal the failure mechanism. Firstly, the properties of the composites under tensile load were tested at three different temperatures in vacuum. Then, the elastic-plastic progressive damage constitutive laws were used and implemented by a user-defined subroutine UMAT in ABAQUS. The thermal residual stress evolution in the cooling and heating processes was characterized. Finally, the stress-strain curves of the composites under tensile load at different temperatures were studied. The effects of thermal residual stress on the tensile properties and progressive damage process of C/SiC composites were revealed sequentially. This work can give design guidance for strengthening of C/SiC composites.

Efficient two-scale analysis with thermal residual stresses and strains based on self-consistent clustering analysis

The thermal residual stresses and strains are crucial in the industrial manufacturing process of the metal alloys and other composites. In representative volume element (RVE) of these complex composite structures, how to accurately and efficiently simulate the responses considering thermal residual stresses and strains is also vital, which could be served as a foundation to multiscale analysis. Recently an effective reduced-order method named self-consistent clustering analysis (SCA) has been proposed to solve microscopic responses in RVE. This paper modifies the original SCA method for thermoelastic and thermal elastoplastic problems, and the microscopic responses of the RVE are compared with the results of analytical and Fast Fourier Transform (FFT) methods to validate its accuracy and feasibility. Then, this study demonstrates the influences of different clustering strategies in offline stage, develops the clustering number optimization and discusses the two-scale analysis using the modified SCA method with/without thermal residual stresses and strains in microscale. In two-scale analysis, the macroscopic and microscopic responses are studied to explore the effects of thermal residual stresses and strains. The two-scale analysis framework with modified SCA method proposed in this work is proved to be efficient in simulating composites considering thermal residual strains and stresses.

Effects of residual thermal stress on mechanical behavior of cermets with different grain sizes

The distribution of the residual thermal stress (RTS) in the as-sintered cermets and its interactions with the applied stress were studied by simulations and experiments, using the WC-Co cemented carbides as examples. The influences of grain size, grain morphology and phase configuration on the stress state and mechanical behavior were quantified in the model, which was constructed from real multi-phased microstructures. The results indicate that the volume-averaged RTS is a power function of the mean WC grain size. There is a high level of compressive stress in the WC regions near the triple junctions of adjacent WC grains and Co binder, and a high level of tensile stress inside the Co phase in the vicinity of WC/Co phase boundaries. In the cermets with RTS, the metallic binder exhibits a gradually increasing strain response rate under the external loading. It is found that a smaller WC grain size, a lower proportion of Co thin-layers, and a higher fraction of triple junctions of adjacent WC grains and Co phase, are beneficial to have less stress concentration and simultaneously high strength and fracture toughness of the cermets. This study provides a new approach to characterize stresses in the multi-phased materials and a strategy to adjust stress distribution by tailoring the microstructure for excellent comprehensive mechanical properties.

Effects of heat treatment on the mechanical properties at elevated temperatures of plain-woven SiC/SiC composites

The effects of heat treatment on the mechanical properties of plain-woven SiC/SiC composites at 927 °C and 1200 °C in argon were evaluated through tensile tests at room temperature and at elevated temperature on the as-received and heat-treated plain-woven SiC/SiC composites, respectively. Heat treatment can improve the mechanical properties of composites at room temperature due to the release of thermal residual stress. Although heat treatment can damage the fiber, the effect of this damage on the mechanical properties of composites is generally less than the effect of thermal residual stress. Heat treatment will graphitize the pyrolytic carbon interface and reduce its shear strength. Testing temperature will affect the expansion or contraction of the components in the composites, thereby changing the stress state of the components. This study can provide guidance for the optimization of processing of ceramic matrix composites and the structural design in high-temperature environments.

An innovative digital image correlation technique for in-situ process monitoring of composite structures in large scale additive manufacturing

As additive manufacturing (AM) continues to develop and become a standardized manufacturing method, there will be a continued need to provide in-situ monitoring during the manufacturing of polymer composite printed components. Thermal residual stress is a primary cause of failures such as interlayer disbonds or delamination, micro cracking, and dimensional instability, which can occur during or after the build. This study reports a novel digital image correlation (DIC) adaptation to monitor thermal residual stresses during the entire print process for large-scale AM. In this work, DIC has been investigated (a) by the natural speckle produced by the polymer surface for correlation, (b) to monitor AM build, and (c) to evaluate the effect of thermal residual stress on warpage of the printed component. The natural speckle pattern of the AM material resulted in a respectable 3.57% error compared to the traditional painted speckle pattern of 3.05% error. DIC measured a 190% increase in vertical displacement at the edge of the wall compared to the center, indicating warpage during AM. This work is a step towards a non-intrusive residual stress measuring technique using DIC for large-scale AM.

Multiscale modeling of unidirectional composites with interfacial debonding using molecular dynamics and micromechanics

In this study, a new multiscale micromechanical model based on the finite-volume direct averaging micromechanics (FVDAM) theory and molecular dynamics (MD) is constructed. It describes the interfacial debonding phenomena of unidirectional composites, whereby a solid fracture interface is incorporated with the FVDAM for the first time. To better elucidate the debonding behavior of the solid interface, a six-degrees-of-freedom (6-DoF) cohesive zone model (CZM) is proposed using MD simulation. The damage matrix Dc(k) in the kth subvolume of the interface is obtained using the 6-DoF CZM, and its explicit form is described herein to facilitate its implementation in commercial software packages. To verify the effectiveness of the model, both experimental data and numerical simulations were adopted for comparison. The experimental data of the unidirectional SiCf/Ti6Al4V composite were compared with the simulation results of FVDAM, and a good agreement was established. A finite element method-based unit cell model is constructed using ABAQUS for numerical validation, and homogenized responses and local fields are compared. The excellent correlations prove the effectiveness of the proposed model. Furthermore, the effects of thermal residual stress and fiber orientations are revealed, which may provide some useful information for the design and manufacture of composites.

Effects of Thermal Residual Stresses on Tensile and Interlaminar Shear Behaviors of GLARE Laminates

The residual stresses caused by curing, forming and machining process significantly affect the mechanical behaviors of composite materials, especially for super hybrid structure like GLARE (glass fiber reinforced aluminum laminates). In this work, the curing thermal residual stresses characteristics of GLARE and corresponding effects on tensile and interlaminar shear behaviors were investigated. Firstly, the characteristics of residual stresses were described by a finite element (FE) model and verified by layer removal method. Then, thermal residual stresses were incorporated into FE mechanical behavior prediction models in the form of predefined field to analyze the effects on tensile and interlaminar shear behaviors. The corresponding simulation conclusions were consistent with the experimental observations. The results indicated that there were about 40 MPa tensile residual stress in aluminum layers and 70 MPa compressive residual stress in GFRP (glass fiber reinforced polymer) layers of GLARE. Residual stresses had obvious reduction effect on the yield strength of GLARE, and the prediction accuracy was hence improved by 9%. Moreover, the failure mode of tensile tests was hardly affected by the residual stresses, but the tensile strength and the initial damage loads were reduced slightly. In interlaminar shear tests, the peak load and initial damage load were reduced by the residual stresses in the adhesive layers of GLARE, and the interlaminar shear strength was slightly influenced. An edge failure mode caused by edge effect was observed during the propagation of the delamination cracks, which was verified by interlaminar shear finite element models and SEM (scanning electron microscope) images.

Numerical stress analysis in adhesively bonded joints under thermo-mechanical loading

The objective of this work is to evaluate the effect of residual thermal stresses, arising after assembling a single-lap joint at elevated temperature, on the inelastic thermo-mechanical stress state in the adhesive layer. The numerical analysis (FEM) employing linear and non-linear material models, with geometrical nonlinearity accounted for, is carried out. Simulating the mechanical response, the calculated thermal stresses are assigned as initial conditions to polymeric, composite and metallic joint members to reflect the loading sequence where the mechanical strain is applied to cooled-down structure. It is shown that the sequence of application matters and simulations with simultaneous application of temperature and strain give different result. Two scenarios for adhesive joints with composites are studied: joining by adhesive curing of already cured composite parts (two-step process) and curing the adhesive and the composite simultaneously in one-step (co-curing). Results show that while in-plane stresses in the adhesive are higher, the peaks of out-of-plane shear stress and peel stress (most responsible for the joint failure) at the end of the overlap are reduced due to thermal effects. In joints containing composite parts, the one-step joining scenario is more favorable than the two-step. The ply stacking sequence in the composite has significant effect on stress concentrations as well as on the plateau value of the shear stress in the adhesive.

Numerical stress analysis for single-lap adhesive joint under thermo-mechanical load using non-linear material

A comprehensive stress analysis by means of Finite Element Method (FEM) for single-lap joint subjected to thermal and mechanical loads is presented in this paper. Simulation is used to predict the effect of residual thermal stresses (caused by difference of temperature of use and elevated temperature during the assembly of the joint) on stress distribution within adhesive layer. The residual thermal stresses are assigned to joint members as initial condition before the mechanical load is applied. The FEM model employs linear and nonlinear material model and accounts for geometrical nonlinearity. It is confirmed that the difference between the manufacturing and the ambient temperature results in high residual thermal stresses, especially in axial and lateral directions of the joint. The calculation of total stress as superposition of thermal and mechanical stresses works only for linear materials. Moreover, simultaneous application of temperature and mechanical load (applied strain in case of displacement controlled test) in FEM produces inaccurate results, since in real situation the strain is applied to already thermally loaded structure. It is also found that the residual thermal stresses may reduce the peel and shear stress concentration in the adhesive at the ends of overlap and the shear stress within the overlap.

Matrix cracking initiation, propagation and laminate failure in multiple plies of general symmetric composite laminates

This paper formulates an analytical model based on intrinsic material properties to predict ply cracking initiation and propagation in multiple plies and its impact on laminate strength in general symmetric laminates (possibly made of thin-plies) under combined triaxial normal loads and in-plane and out-of-plane shear loads while the effect of thermal residual stresses and statistical distribution of fracture energies are considered. To do so, a novel complete homogenization technique combined with recently developed accurate stress transfer models is implemented in conjunction with multi-axial energy-based failure criteria, Monte Carlo type of simulation and a strain-based fiber failure criterion. The model predictions show very good agreement with experimental results.

Temperature dependent strengthening mechanisms and yield strength for CNT/metal composites

Strengthening mechanisms in CNT/metal composites are significant guidelines on composites design. However, the strengthening mechanisms at high temperatures have few been reported. In this work, the evolution of four common strengthening mechanisms in CNT/metal composites, including load transfer strengthening, grain refinement strengthening, enhanced dislocation density strengthening and Orowan strengthening, with temperature was investigated by our developed theoretical models. Then by considering those temperature dependent strengthening mechanisms, a temperature dependent yield strength model for CNT/metal composites was established. Further, the effects of residual thermal stress and quicker softening of matrix on the temperature dependent yield strength of composites were also taken into account. Good agreement is obtained between the model predictions and available experimental results. This study contributes a simple method to predict the yield strength for CNT/metal composites over a wide range of temperatures. In addition, the contribution of each strengthening mechanism to the yield strength of composites at different temperatures was extensively discussed. Some useful suggestions for the design of new CNT/metal composites with high yield strength at elevated temperatures were provided.

Glass–Ceramics in Dentistry: A Review

In this review, we first briefly introduce the general knowledge of glass–ceramics, including the discovery and development, the application, the microstructure, and the manufacturing of glass–ceramics. Second, the review presents a detailed description of glass–ceramics in dentistry. In this part, the history, property requirements, and manufacturing techniques of dental glass–ceramics are reviewed. The review provided a brief description of the most prevalent clinically used examples of dental glass–ceramics, namely, mica, leucite, and lithium disilicate glass–ceramics. In addition, we also introduce the newly developed ZrO2–SiO2 nanocrystalline glass–ceramics that show great potential as a new generation of dental glass–ceramics. Traditional strengthening mechanisms of glass–ceramics, including interlocking, ZrO2–reinforced, and thermal residual stress effects, are discussed. Finally, a perspective and outlook for future directions in developing new dental glass–ceramics is provided to offer inspiration to the dental materials community.