Effects Of Thermal Residual Stresses Research Articles

Variational analysis of cracking in general composite laminates subject to triaxial and bending loads

A robust variational approach is first developed for predicting stress/displacement fields and effective thermo-elastic constants in a laminate with arbitrary lay-up, containing uniformly spaced ply cracks, subject to general triaxial and bending loads when the effects of thermal residual stresses are also considered. The methodology extends some previously developed variational approaches so that general non-symmetric lay-ups (not only cross-ply) under through-thickness loading conditions can be analyzed. Secondly, an approximate methodology is introduced which enables the approach to predict all effective thermo-elastic constants of laminates having non-uniformly spaced ply cracks, including new types of constant required to account for out-of-plane loading. Thirdly, some novel inter-relationships among effective thermo-elastic constants of cracked and uncracked general laminates are obtained showing that just three macroscopic parameters define the dependence of all relevant laminate constants (thirty five) on the state of cracking. The results are compared with those obtained from finite element methods, experiments and other high accuracy models.

Investigating the effects of cooling rate on distortion of asymmetric composite laminates

Predicting the final shape of composite laminates after the curing cycles is crucial. An experimental and two numerical (with constant and temperature dependent material properties) studies have been presented. The purpose of this study is to investigate the effects of different cooling rates on the distortion of thin asymmetric composite laminates with different lay-ups after the curing cycle. Properties of a UD layer in different temperature have been obtained using TMA, DMA and micromechanical formulation. The samples were subjected to three different cooling stages. Results indicated that the distortion of the asymmetric composite laminate was significantly depending on the cooling rate and lay-up configurations. Accelerating the cooling rate can significantly enhance the out of plane deformation of the asymmetric laminates. In the slowest cooling rate, the effects of thermal residual stresses due to temperature change are minimum and consequently the results of both numerical analyses are closer to experimental observations. The analysis with temperature dependent material can predict the deformation of specimens with slowest cooling rate better than the analysis with constant material properties. Because of the sudden change in temperature, for prediction of distortion after the higher cooling rates, a transient analysis will be needed.

Effect of thermal residual stresses on ductile-to-brittle transition of a bi-material specimen by using the CAFE method

The effect of residual stress on fracture of materials or structures has been widely studied. However, its influence on ductile-to-brittle transition (DBT), a crucial phenomenon of structural materials, has rarely been investigated so far. In the present study, employing the eigenstrain method residual stresses are introduced into a bi-material specimen, where two configurations of crack and interface, e.g., one with interface perpendicular and one parallel to the crack extension, are designed to study the influence of residual stress. The DBT of the bi-material specimen in the presence of residual stresses is numerically studied by using the CAFE method where temperature dependent surface energy is implemented to calculate absorbed energy of Charpy impact testing specimen. It is found that residual stress generated in the two configurations affect the DBT in a similar manner. The DBT curves generally shift to higher temperature due to the decrease of absorbed energy with the increase of residual stress. It is found that the decrease of absorbed energy in both configurations is caused by the additional constraint on the notch root, which is induced by the residual stress and can facilitate the fracture.

Homogenization and Localization of Ratcheting Behavior of Composite Materials and Structures with the Thermal Residual Stress Effect.

In this contribution, the ratcheting behavior and local field distribution of unidirectional metal matrix composites are investigated under cyclic loading. To that end, we extended the finite-volume direct averaging micromechanics (FVDAM) theory by incorporating the rule of nonlinear kinematic hardening. The proposed method enables efficient and accurate simulation of the ratcheting behavior of unidirectional composites. The local satisfaction of equilibrium equations of the FVDAM theory facilitates quick and rapid convergence during the plastic iterations. To verify the proposed theory, a finite-element (FE) based unit cell model is constructed with the same mesh discretization. The remarkable correlation of the transverse response and local field distribution generated by the FVDAM and FE techniques demonstrates the effectiveness and accuracy of the proposed models. The stress discontinuities along the fiber/matrix interface that are generic to the finite-element theory are absent in the FVDAM prediction. The effects of thermal residual stresses induced during the consolidation process, as well as fiber orientations, are revealed. The generated results indicate that the FVDAM is well suited for simulating the elastic-plastic ratcheting behavior of metal matrix composites, which will provide the conventional finite-element based technique with an attractive alternative.

COMSOL Based Analysis on Compensating Thermal Residual Stress Effects in Beam Structured Geometries

COMSOL Based Analysis on Compensating Thermal Residual Stress Effects in Beam Structured Geometries

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.

Effects of adhesive disbond and thermal residual stresses on the fatigue life of cracked 2024-T3 aluminum panels repaired with a composite patch

The technology of bonded composite repair has displayed its effectiveness in increasing the residual fatigue life of cracked aeronautical structures. However, two major phenomena reduce the performance of this technique; the adhesive disbond by the fatigue load and the thermal residual stresses due to the adhesive curing. In this study, the effects of the adhesive disbond and the thermal residual stresses on the fatigue life of repaired cracked aluminum plate were analyzed experimentally and numerically. Fatigue tests were performed on cracked 2024-T3 aluminum alloy specimens repaired using carbon/epoxy patches that have artificial disbonds, to highlight the adhesive disbond effect. Fatigue tests were performed on repaired specimen bonded with elevated temperature cure and ambient temperature cure adhesives to evaluate the effect of the thermal residual stresses. To analyze the effects of the two phenomena, the finite element method was used to compute the stress intensity factor at the front of repaired cracks. The experimental results indicate that fatigue life is significantly reduced by the presence of an initial adhesive disbond and thermal residual stresses. The numerical results are consistent with the experimental observations, and reveal that the stress intensity factor increases with an increase in the disbond width. Moreover, it also increases in the presence of thermal residual stresses.

Effects of thermal residual stresses and thermal-induced geometrically necessary dislocations on size-dependent strengthening of particle-reinforced MMCs

The thermal residual stresses (TRSes) can noticeably increase the yield strength and flow stresses of particle-reinforced metal-matrix composites (PR-MMCs) and show size-dependence due to thermal geometrically necessary dislocations (GNDs) when the size of particles is on the order of micron. This TRSes strengthening is attributed to the conventional thermal-elasto-plastic constituent mismatch (regardless of particles’ size) and the presence of GNDs (size-dependent). A modified Taylor-based nonlocal theory (TNT) of plasticity is proposed to quantify the individual contributions of size-dependent GND strengthening. An axisymmetric unit-cell model of uniform and aligned particle distribution with various particle sizes is built to study thermal-induced non-uniformly distributed GNDs, their evolution and contributions to the stress strengthening under combined thermal and mechanical loading. The results show that the proposed methodology can effectively capture the size dependence of thermal-induced GNDs which keeps a sharp gradient variation in the zone near the matrix-inclusion interface and gradually flattens out in the zone far away from the interface. It is demonstrated that TRSes significantly improve the yield strength and flow stresses. It also revealed that the contribution of thermal-induced GNDs increased as the particle size in the PR-MMCs decreased. The simulation results were validated by experimental results reported in the literature.

The effect of residual thermal stresses on transverse cracking in cross-ply laminates: an application of the coupled criterion of the finite fracture mechanics

A model to predict transverse cracking in cross-ply laminates in the presence of residual thermal stresses is developed here. This model is based on the coupled criterion of the finite fracture mechanics. This criterion has been successfully used for different materials, structures and scales to predict crack initiation. It is based on two main hypotheses: (i) crack initiation occurs as a finite-length crack onset and (ii) the crack onset requires that both stress and energy criteria are fulfilled simultaneously. The present model is developed under the generalized-plane-strain hypotheses combining the results obtained using the laminate theory and a boundary element code. The present analysis shows that the residual thermal stresses affect both the stress and the energy criteria in the form of adding a residual elastic-strain to the strain imposed by external mechanical loads. An explicit expression for this residual elastic-strain is provided. For certain composite materials as carbon/epoxy the value of this residual elastic-strain is shown to be relatively large in comparison with the nominal critical transverse strain of the material. The comparison with experiments shows that considering the residual thermal stresses using the strategy proposed here improves drastically the accuracy of the model predictions.

In-situ neutron diffraction characterization of temperature dependence deformation in α-uranium

In-situ strain neutron diffraction measurements were conducted at temperature on specimens coming from a clock-rolled α-uranium plate, and Elasto-Plastic Self-Consistent (EPSC) modeling was employed to interpret the findings. The modeling revealed that the active slip systems exhibit a thermally activated response, while deformation twinning remains athermal over the temperature ranges explored (25–150 °C). The modeling also allowed assessment of the effects of thermal residual stresses on the mechanical response during compression. These results are consistent with those from a prior study of room-temperature deformation, indicating that the thermal residual stresses strongly influence the internal strain evolution of grain families, as monitored with neutron diffraction, even though accounting for these residual stresses has little effect on the macroscopic flow curve, except in the elasto-plastic transition.

Experimental and numerical evaluation of the mechanical behavior of diagonally reinforced plates subjected to the effect of residual thermal stresses

This paper presents an experimental and numerical study of the effect of residual thermal stresses on the mechanical behavior of diagonally reinforced plates. The study focuses on the analysis of carbon/epoxy square plates onto which diagonal reinforcements were glued. These reinforcements were glued using two different methodologies: the first method was to glue the reinforcements at operating temperature (22 °C), while in the second methodology reinforcements were cured in an autoclave at 177 °C. Mechanical behavior assessment was based on the stiffness, free vibration, and buckling tests. For the study of the stiffness of the plates, an optical technique to determine the transversal displacement caused by the presence of a static load was employed. For the vibration tests, the natural frequencies associated with the first four modes of free vibration were determined by using a dynamic signal analyzer. For the linear and non-linear buckling, compression tests on a universal testing machine were performed, determining the displacements produced during the test using the digital image processing method. All experimental results were compared with results obtained from numerical approximations made with commercial software. The results show the effect of residual thermal stresses caused during the manufacturing process on the mechanical performance of diagonally reinforced plates.

Comparison of Variational and Generalized Plane Strain approaches for matrix cracking in general symmetric laminates

The current research work is aimed at comparing a stress-based variational approach with generalized plane strain (GPS) model for predicting stress transfer in cracked general symmetric laminates, subject to general in-plane loading. For each model, the fundamental assumptions made on stress and displacement fields, and also procedures for defining boundary conditions and finding solutions are detailed. Moreover, a numerical comparison has been made between the results obtained from two models with regard to stresses, displacements, and thermo-elastic constants of an unbalanced symmetric cracked laminate. Finally, the accuracy and quality of both approaches are examined by checking the satisfaction of known inter-relationships among thermo-elastic constants of cracked laminates. The paper extends an existing variational model so that the effects of residual thermal stresses can be considered. It has been also shown how the displacement field can be determined for a stress-based variational approach.

Study on Residual Stresses in Unidirectional Flax Fiber/Vinyl Ester Composites by XRD Technique

The development of flax fiber reinforced polymer composites with improved properties has been increasingly studied by many researchers because of their sustainability. However, the effects of thermal residual stresses on the interfacial properties of flax composites have not been evaluated or very well understood. In this study, the thermal properties of flax, vinyl ester matrices and their composites were accessed by different techniques, including thermomechanical analyzer, dynamic mechanical analyzer, and X-ray diffraction combined with aluminum particles. The effects of thermal properties of flax fibers and vinyl ester resin systems on the mechanical properties of their biocomposites were studied. The theory of modifying the thermal properties of flax and vinyl ester to improve the interfacial adhesion between them has been proven by the study of the thermal residual stresses in their composites.

Analysis of cracked general cross-ply laminates under general bending loads: A variational approach

A novel variational model is developed that can predict the stress transfer between plies in cracked general cross-ply laminates subject to general in-plane (Nxx, Nyy, Nxy) and out-of-plane bending (Mxx, Myy, Mxy) loading. The effects of thermal residual stresses are taken into account. Admissible stress systems, which satisfy equilibrium and all boundary and interface conditions, are constructed and the principle of minimum complementary energy is employed to find the optimal solution. The approach yields rigorous lower bounds for stiffness matrices. A methodology based on Levin's theorem is developed to evaluate the effective thermal expansion coefficients of non-symmetric cracked laminates. A ply-refinement technique is used in order that through-thickness variations of the stress components can be precisely taken into account. It is found that the developed method, when used in conjunction with ply refinement technique, results in stress fields and thermo-mechanical properties comparable in accuracy to refined finite and boundary element solutions.

A micromechanical constitutive modeling of WC hardmetals using finite-element and uniform field models

For constitutive modeling of WC hardmetals we used a full field finite element simulations of accurately reproduced scanning electron microscope images and a simple uniform field two-phase model. Both models combine isotropic Drucker–Prager elastic–plastic behavior of WC grains and isotropic von Mises elastic–plastic behavior of the binder, and include non-linear hardening. We performed simulations for representative volume elements using the generalized 2.5D formulation and demonstrated a good agreement between the two models in terms of the effective mechanical behavior. Effective elastic properties and coefficients of thermal expansion were obtained. Effective yield stresses evaluated at 0.1% of effective plastic strain were also computed for six different loading paths. Probability and joint probability histograms obtained in FE simulations are presented. We also studied the effect of residual thermal stresses, which appear in WC hardmetals due to cooling from sintering temperatures. Finally, we obtained a realistic yield surface for a three-dimensional microstructure using a uniform field model with spherical inclusions. This surface combines a Drucker–Prager region for moderate pressures and a von Mises region for high pressures, with a sharp transition between these two regions. Four different WC hardmetal grades were considered. In total, nine microstructures were reproduced in finite element models with the binder content ranging from 10% to 19%. A sensitivity study on the binder plastic properties was carried out, thus the obtained results for the yield surface are applicable to real hardmetals with different binder materials and various binder content.

Modeling of thermal expansion behavior of densely packed Al/SiC composites

In this paper the approach for finite element (FE) modeling and analysis of thermal expansion behavior of densely packed Al/SiC composites is proposed. The coefficient of linear thermal expansion (CTE) is predicted from 50°C to 500°C by considering temperature dependent linear elastic and elastoplastic matrix material behavior. Three dimensional representative volume elements (RVEs) are generated to model the composite's microstructures. In order to study the effect of thermal residual stresses, first the RVEs are simulated for cooling process from processing temperature to room temperature and thermal residual stresses are estimated by considering different mechanical behaviors of Al matrix. Next, the thermal expansion behavior is examined with existing residual stresses. The presence of thermal residual stresses is observed to influence the effective CTE in the initial temperature range (25 °C–100 °C) of heating. The contact state of reinforced particles significantly influences the thermal expansion behavior. It is observed that the effective CTE of composites with non-interpenetrating microstructure is higher than that of the composites having interpenetrating microstructure. The effects of voids on the thermal expansion behavior are analyzed further.

Finite element modeling of effective thermomechanical properties of Al–B4C metal matrix composites

The present work aims to investigate the influences of thermal residual stresses and material properties on the thermomechanical deformation behavior of Al–B4C composites. Boron carbide-reinforced aluminum matrix composites having 4, 8, and 12 vol% boron carbide were fabricated using squeeze liquid stir casting method for experimental characterization of their microstructure, effective elastic moduli and effective CTEs at room temperature as well as elevated temperatures. Next, the thermomechanical behavior of fabricated composites was investigated using finite element modeling. The effects of thermal residual stresses on the effective material properties were examined by simulating the cooling process of MMCs from processing temperature to room temperature. The effective elastic moduli and the effective CTEs were predicted considering linear elastic as well as elastoplastic deformation of aluminum matrix, and the results obtained were compared with the experimental values. The effects of voids on effective material behavior are studied by simulating the void growth and nucleation using Gurson–Tvergaard–Needleman model.

Influence of residual thermal stresses on fatigue crack growth life of discontinuous reinforcements in metal matrix composites

AbstractThe fatigue life prediction model based on crack propagation from micro‐structural features is derived and presented for planar and randomly oriented Discontinuous Reinforced Metal Matrix Composites (DRMMCs). The model contains the influence of micro‐structural properties such as aspect ratio, volume fraction of particle/fibre and constraint between particle and the matrix. The effect of residual thermal stresses generated within the matrix during development of composite is considered. The particle/fibre plays a dominant role in the development of the cyclic plastic zone size ahead of the crack tip; moreover, it enhances the cyclic plastic deformation characteristics of DRMMC. The theoretical model‐based evaluations for low cycle fatigue in DRMMCs are within the proximity of the experimental results.

Micromechanical Modeling of Viscoplastic Behavior of Laminated Polymer Composites With Thermal Residual Stress Effect

In this paper, a multiscale approach has been developed for investigating the rate-dependent viscoplastic behavior of polymer matrix composites (PMCs) with thermal residual stress effect. The finite-volume direct averaging micromechanics (FVDAM), which effectively predicts nonlinear response of unidirectional fiber reinforced composites, is incorporated with improved Bodner–Partom model to describe the viscoplastic behavior of PMCs. The new micromechanical model is then implemented into the classical laminate theory, enabling efficient and accurate analysis of multidirectional PMCs. The proposed multiscale theory not only predicts effective thermomechanical viscoplastic response of PMCs but also provides local fluctuations of fields within composite microstructures. The deformation behaviors of several unidirectional and multidirectional PMCs with various fiber configurations are extensively simulated at different strain rates, which show a good agreement with the experimental data found from the literature. Influence of thermal residual stress on the viscoplastic behavior of PMCs is closely related to fiber orientation. In addition, the thermal residual stress effect cannot be neglected in order to accurately describe the rate-dependent viscoplastic behavior of PMCs.

Effect of Thermal Residual Stresses on Buckling and Post-Buckling Properties of Laminated Composites Perimetrally Reinforced

Several mechanical properties can be affected by the occurrence of residual stresses due to curing, and their effects are more pronounced in laminated composites. The effect of the thermal residual stresses on the buckling and post-buckling properties of perimetrally reinforced laminated composites is experimentally characterized. Carbon/epoxy laminates were prepared using two different techniques. One group of laminates was prepared by co-curing the reinforcement at 177 °C. The second group of laminates was prepared by secondary bonding of the reinforcement to the laminate at room temperature (22°C). Topogrammetry equipment was utilized in order to determine the buckling and post-buckling properties of these two groups of laminates. A numerical model was achieved using commercial software for comparison with the experimental results. Suitable accuracy was observed when comparing experimental and numerical results. The co-cured laminates developed considerably higher critical load for buckling values than those obtained for laminates produced with secondary bonding at room temperature. The experimental and numerical results of this study demonstrate the importance of curing-induced thermal residual stresses on the mechanical behavior of laminate composites.