Presence Of Thermal Residual Stresses Research Articles

Modeling of progressive high-cycle fatigue in composite laminates accounting for local stress ratios

A numerical framework for simulating progressive failure under high-cycle fatigue loading is validated against experiments of composite quasi-isotropic open-hole laminates. Transverse matrix cracking and delamination are modeled with a mixed-mode fatigue cohesive zone model, covering crack initiation and propagation. Furthermore, XFEM is used for simulating transverse matrix cracks and splits at arbitrary locations. An adaptive cycle jump approach is employed for efficiently simulating high-cycle fatigue while accounting for local stress ratio variations in the presence of thermal residual stresses. The cycle jump scheme is integrated in the XFEM framework, where the local stress ratio is used to determine the insertion of cracks and to propagate fatigue damage. The fatigue cohesive zone model is based on S-N curves and requires static material properties and only a few fatigue parameters, calibrated on simple fracture testing specimens. The simulations demonstrate a good correspondence with experiments in terms of fatigue life and damage evolution.

Ultimate Strength Prediction of Fibrous Composites only upon Independently Measured Constituent Properties

This study presents an accurate and easy-to-use micromechanical model to predict the ultimate strength of unidirectional polymer composites under an arbitrary load condition only upon independently measured constituent properties. In this model, the micromechanical method based on generalized model of cells (GMC), which effectively predict the nonlinear deformation of unidirectional composites, is used to analyze the repeating unit cells of composites. At the same time, a unified plastic theory (i.e. modified Ramaswamy-Stouffer model) is incorporated into the GMC's analytical framework to describe viscoplastic behaviors of matrix phase. Additionally, because of the stress concentration effect which causes the difference between the matrix in-situ and original strength behaviors, a stress concentration factor is introduced in order to utilize the measured constituent properties directly. The prediction results with SCF match better with the experimental results than the prediction results without SCF. In addition, the prediction results show that the presence of thermal residual stress and material plastic effects generally has important influence on the strength prediction of a composite.

Argon-helium mixtures as Laser-Powder Bed Fusion atmospheres: Towards increased build rate of Ti-6Al-4V

The effect of the gas thermal properties and density on the laser powder bed fusion (L-PBF) process was investigated by using inert argon, characterized by high density, and helium, characterized by high thermal conductivity and heat capacity, and their gas mixtures. The results highlighted that for L-PBF of Ti-6Al-4V, the effect of residual impurities such as oxygen and nitrogen on the process stability and defect generation is prevailing the type of the process gas. However, by monitoring the residual oxygen level in the process atmosphere, the results showed that using the argon-helium mixtures allows to increase the produced material density upon higher build rates. High density, greater than 99.98 % is indeed achieved using a mixture of 50 % argon and 50 % helium, allowing for a build rate increase of 44 % in comparison to the standard build rate. The analysis of the produced material revealed the presence of thermal residual stresses attributed to an enhanced energy input when using the gas mixtures. The latter offer a positive balance of density and thermal properties, and in turn, probably reduce the accumulation of process by-products at the melt pool that interfere with and attenuate the laser radiation. The possible detrimental effect of the introduced residual stresses is efficiently eliminated by the conventional stress relieving treatment leading to the decomposition of α´martensite into a fine (α + β) microstructure. This study opens the perspective on the development of the gas recipes for improved process stability and increased productivity of L-PBF process.

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.

The role of thermal residual stress on the yielding behavior of carbon nanotube–aluminum nanocomposites

The initial yield envelopes of aluminum (Al) nanocomposites reinforced with carbon nanotubes (CNTs) subjected to biaxial loading are predicted in the presence of thermal residual stress (TRS) arising from the manufacturing process. Micromechanical model based on the unit cell method is presented to generate the yielding surfaces. The formation of the interphase caused by the interfacial reaction between the CNT and Al matrix is taken into account in the analysis. The effects of several important parameters, i.e. the change of temperature, CNT volume fraction, interphase thickness and Al material properties on the yielding onset of the CNT/Al nanocomposite are explored extensively. The results clearly reveal that the initial yield surfaces of nanocomposite are dependent on the TRS. Also, the interphase has a significant influence on the yielding behavior of Al nanocomposite in the presence of TRS. The results demonstrate that the size of initial yield surfaces become minimum with considering the coupled effects of TRS and interphase. With increasing the temperature variation, interphase thickness, elastic modulus and coefficient of thermal expansion of Al matrix, the size of initial yield surfaces reduces. The present study is consequential for understanding the key role of TRS on the initial damage of CNT/Al nanocomposites.

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.

Steady state creep behavior of thermally graded isotropic rotating disc of composite taking into account the thermal residual stress

The present paper investigates the steady-state creep behavior of thermally graded isotropic disc rotating at elevated temperature. The composite discs are made of aluminum matrix reinforced with silicon-carbide particulate. The creep analysis is carried out using isotropic Hoffman yield criterion. The stress and strain rate distributions have been calculated for the discs. The creep parameters vary along the radius of the disc and have been estimated by regression fit of the available experimental data. Investigations for disc operating under linearly decreasing temperature from inner to outer radii has been done taking into account the phase-specific thermal residual stress. Further work has been done for discs operating under linearly increasing, parabolically decreasing and parabolically increasing temperatures. The results are displayed and compared graphically in designer friendly format for the above said temperature profiles. It is observed that there is a significant change in the stress distribution due to the presence of thermal residual stress. The radial strain rate is compressive for the discs operating at elevated temperature in the absence of residual stress, but due to the presence of residual stress the strain rate becomes tensile as one gradually moves along the radial distance and again becomes compressive near the outer radius. However, the presence of residual stress led to an increase in the tangential strain rate in the discs as compared to the discs without residual stress. Thus it is concluded that there is a need to extend the domain of thermal gradation in designing rotating discs.

Investigation of the effect of thermal residual stresses on deformation of α-uranium through neutron diffraction measurements and crystal plasticity modeling

The strong thermoelastic–plastic anisotropy of single crystal, orthorhombic α-uranium leads to the generation of significant thermal residual stresses (TRSs) after cooling from the processing temperatures of polycrystals. The present study shows the effect that these TRSs have upon subsequent room temperature deformation after aging. In situ neutron diffraction strain experiments performed on specimens machined from a clock-rolled plate, combined with ex situ texture measurements and elastoplastic self-consistent polycrystalline plasticity modeling, provides the necessary information to quantify the relative strengths and activities of the various deformation mechanisms known to operate within α-uranium. The results demonstrate that the activation of hard slip modes is necessary in order to generate macroscopic flow at the stress levels observed experimentally. Although the thermal residual stresses do not drastically affect the “bulk” response, inclusion of the TRSs drastically improve the agreement between predicted and experimentally measured internal strain evolution and strongly alter the predicted slip and twinning mode activities. As such, modeling efforts to discern the roles of the various mechanisms must account for the presence of thermal residual stress. In agreement with prior crystal plasticity modeling efforts, the crystallographic texture and polarity of twinning mechanisms explain the tension–compression strength asymmetry observed in the material (i.e. significantly more of the soft {130} twinning mode is observed to occur during compression along the prior plate RD than tension along the same direction).

Analysis of Steady State Creep in Whisker Reinforced Functionally Graded Thick Cylinder Subjected to Internal Pressure by Considering Residual Stress

The steady state creep behavior of a functionally graded thick composite cylinder subjected to internal pressure has been analyzed in the presence of residual stress. To study the effect of residual stress, the yielding of composite material is described by Hoffman's yield criterion. The results obtained are compared with a similar cylinder but yielding according to von Mises yield criterion, which ignores the presence of residual stress. The cylinder is made of functionally graded composite containing a linearly varying amount of silicon carbide whisker in a matrix of 6061Al. It is revealed that the presence of residual stress in the FG cylinder having linearly varying whisker content in the radial direction, the radial stress decreases slightly over the entire radius of the cylinder, whereas the tangential and axial stresses increase considerably near the inner radius but show a significant decrease towards the outer radius. In the presence of residual stress, the effective stress also increases in the cylinder, except for some decrease observed near the inner radius. The presence of thermal residual stress in the cylinder leads to increase the tangential as well as radial strain rate throughout the cylinder. The increase observed in strain rate increases with increasing radial distance.

Microstructure, mechanical and thermal properties of in situ toughened boron carbide-based ceramic composites co-doped with tungsten carbide and pyrolytic carbon

Abstract Boron carbide (B 4 C)-based ceramics were pressureless sintered to a relative density of 96.1% at 2150 °C, with the co-incorporation of tungsten carbide and pyrolytic carbon. The as-batched boron carbide power was 7.89 m 2 g −1 in surface area. A level of fracture toughness as high as 5.80 ± 0.12 MPa m 1/2 was achieved in the BW-6C composite. Sintering aids of carbon and tungsten boride were formed by an in situ reaction. The toughness improvement was attributed to the presence of thermal residual stress as well as the W 2 B 5 platelets. The thermal conductivity and thermal expansivity of the BW-6C composite as a function of temperature are also reported in this work. Our current study demonstrated that the B 4 C–W 2 B 5 composites could be potential candidate materials for structural applications.

EFFECT OF RESIDUAL STRESS AND REINFORCEMENT GEOMETRY IN AN ANISOTROPIC COMPOSITE ROTATING DISC HAVING VARYING THICKNESS

The influence of the thermal residual stress and reinforcement geometry on the creep behavior of a composite disc has been analyzed in this paper. The creep analysis in a rotating disc made of Al-SiC (particle/whisker) composite having hyperbolically varying thickness has been carried out using anisotropic Hoffman yield criterion and results obtained are compared with those using Hill's criterion ignoring difference in yield stresses. The steady state creep behavior has been described by Sherby's creep law. The creep parameters characterizing difference in yield stresses have been used from the available experimental results in literature. It is observed that the stresses are not much affected by the presence of thermal residual stress, while thermal residual stress introduces significant change in the strain rates in an anisotropic rotating disc. Secondly, it is noticed that the steady state creep rates in whisker reinforced disc with/without residual stress are observed to be significantly lower than those observed in particle reinforced disc with/without residual stress. It is concluded that the presence of residual stress in an anisotropic disc with varying thickness needs attention for designing a disc.

A failure study of condenser tube manufactured from low nickel stainless steels

A failure analysis of condenser tubes manufactured from Type 204 stainless steel was conducted to find the root cause of failures. The failures were observed in the parent materials of the tube rather than from welding or heat-affected zones. A detailed metallography, corrosion test and stress analysis study has been carried out to find out the root cause of failure, and it revealed that the parent materials of the tube were highly sensitized, and post-annealing treatment had been not performed by the manufacturers. The condenser tube failed prematurely due to sensitization and the presence of thermal residual stresses exacerbates this. This study emphasizes the metallurgical analysis performed to identify the root cause of failure of condenser tubes manufactured from Type 204 stainless steel.

Effect of thermal residual stresses on matrix failure under transverse tension at micromechanical level: A numerical and experimental analysis

In the present work the influence at micromechanical scale of thermal residual stresses, originated in the cooling down associated to the curing process of fibrous composites, on inter-fibre failure under transverse tension is studied. In particular, the effect of the presence of thermal residual stresses on the appearance of the first debonds is discussed analytically, whereas later steps of the mechanism of damage, i.e. the growth of interface cracks and their kinking towards the matrix, are analysed by means of a single fibre model and making use of the Boundary Element Method (BEM). The results are evaluated applying Interfacial Fracture Mechanics concepts. The conclusions obtained predict, at least in the case of dilute fibre packing, a protective effect of thermal residual stresses against failure initiation, the morphology of the damage not being significantly affected in comparison with the case in which these stresses are not considered. Experimental tests are carried out, the results agreeing with the conclusions of the numerical analysis.

Micromechanical modelling of the transverse damage behaviour in fibre reinforced composites

A micromechanics damage model is presented which examines the effect of fibre–matrix debonding and thermal residual stress on the transverse damage behaviour of a unidirectional carbon fibre reinforced epoxy composite. It is found that for a weak fibre–matrix interface, the presence of thermal residual stress can induce damage prior to mechanical loading. However, for a strong fibre–matrix interface the presence of thermal residual stress is effective in suppressing fibre–matrix debonding and improving overall transverse strength by approximately 7%. The micromechanical model is subjected to a multiple loading cycle (i.e. tension–compression–tension), where it is shown to provide novel insight into the microscopic damage accumulation that forms prior to ultimate failure, clearly highlighting the different roles that fibre–matrix debonding and matrix plasticity play in forming the macroscopic response of the composite. Such information is vital to the development of accurate continuum damage models, which often smear these effects using non-physical material parameters.

Size-dependent Probabilistic Micromechanical Damage Mechanics for Particle-reinforced Metal Matrix Composites

A size-dependent micromechanical framework is proposed to predict the deformation responses of particle-reinforced metal matrix composites by incorporating the essential features of the dislocation plasticity. Within the framework of probabilistic micromechanical formulation, the damage caused by the manufacturing process and by the external mechanical loading in the presence of thermal residual stresses is considered. The effective elastic moduli of four-phase composites, consisting of a ductile matrix and randomly located spherical intact or damaged particles are derived. Subsequently, the size-dependent plastic deformation behavior of particle-reinforced metal matrix composites is predicted with a dislocation theory. Specifically, the density of dislocations due to the thermal contraction misfit and the plastic deformation misfit is taken into consideration within the micromechanical methodology to account for the dislocation strengthening. To predict the overall elastoplastic damage behavior of composites, a size-dependent hybrid effective yield function is presented on the basis of the ensemble-volume averaging and the modified matrix yield strength. The comparisons between our predictions and available experimental data illustrate the potential capability of the proposed framework. Numerical simulations are also performed to exhibit the salient features of the proposed methodology.

Al/SiC Carriers for Microwave Integrated Circuits by a New Technique of Pressureless Infiltration

Materials with high thermal conductivity and thermal expansion coefficient matching with that of Si or GaAs are being used for packaging high density microcircuits due to their ability of faster heat dissipation. Al/SiC is gaining wide acceptance as electronic packaging material due to the fact that its thermal expansion coefficient can be tailored to match with that of Si or GaAs by varying the Al:SiC ratio while maintaining the thermal conductivity more or less the same. In the present work, Al/SiC microwave integrated circuit (MIC) carriers have been fabricated by pressureless infiltration of Al-alloy into porous SiC preforms in air. This new technique provides a cheaper alternative to pressure infiltration or pressureless infiltration in nitrogen in producing Al/SiC composites for electronic packaging applications. Al-alloy/65vol% SiC composite exhibited a coefficient of thermal expansion of 7/spl times/10/sup -6/ K/sup -1/ (25/spl deg/C-100/spl deg/C) and a thermal conductivity of 147 Wm/sup -1/K/sup -1/ at 30/spl deg/C. The hysteresis observed in thermal expansion coefficient of the composite in the temperature range 100/spl deg/C-400/spl deg/C has been attributed to the presence of thermal residual stresses in the composite. Thermal diffusivity of the composite measured over the temperature range from 30/spl deg/C to 400/spl deg/C showed a 55% decrease in thermal diffusivity with temperature. Such a large decrease in thermal diffusivity with temperature could be due to the presence of micropores, microcracks, and decohesion of the Al/SiC interfaces in the microstructure (all formed during cooling from the processing temperature). The carrier showed satisfactory performance after integrating it into a MIC.

High toughness in laminated SiC ceramics from aqueous tape casting

Abstract Toughness as high as 9.5 MPa · m 1/2 was achieved in a SiC material prepared from aqueous tape casting, laminating and hot pressing. The increased toughness is related to the formation of second phase at triple pockets, YAlO 3 , which was different from the composition of sintering additives Al 2 O 3 and Y 2 O 3 (Y 3 Al 5 O 12 ). The extra Al 2 O 3 was found at thin amorphous films covering most grain boundaries where Y 2 O 3 was absent. A similar difference between the initial additive composition and the final second phase was found in another SiC material with lower toughness. It is proposed that the presence of thermal residual stress at second phase is responsible for the improvement in fracture toughness.

Elastic–plastic stress transfer in short fibre-reinforced metal–matrix composites

The classical shear lag model was modified to study the elastic–plastic stress transfer in short fibre-reinforced metal–matrix composites. The effect of the matrix plastic deformation on the stress transfer in the elastic range was analysed by introducing a plastic strain. The stress transfer in the overall plastic range was analysed by using the incremental form of the stress and strain relation. The expressions for the fibre stress and the interfacial shear stress in a cylindrical unit cell including the fibre end region were derived with the use of the technique of adding imaginary fibre. The modified model was then used to study the effects of thermal residual stresses on the tensile and compressive stress–strain responses of the composite. It was demonstrated that the stress transfer efficiency in the elastic range is reduced due to the matrix local plastic deformation. In the presence of thermal residual stresses, different variations of the matrix plastic strain result in asymmetric tensile and compressive stress–strain responses. The model predictions exhibited very good agreements with the experimental results in a SiCw/Al–Li composite.

Numerical modelling of the fracture behaviour of a glass matrix composite reinforced with alumina platelets

Abstract In this work the fracture behaviour of a composite material constituted by a borosilicate glass matrix reinforced with Al2O3 platelets is studied by means of a numerical model. This material, which was experimentally investigated in a previous paper, is characterised by the presence of thermal residual stresses that arise upon cooling from the processing temperature due to the thermal expansion coefficients mismatch between the matrix and the reinforcements. A numerical model based on finite element simulations of realistic composite microstructures was adopted for the present material. The crack propagation was studied using finite elements coupled with selected failure criteria (Griffith and Weibull approaches) implemented inside the elements. Computational determination of crack propagation during failure is compared with previously obtained experimental data and microscopy images of platelet–crack interactions, showing that the model provides results in good agreement with the experimental observations.

Micro-Mechanical Modeling of Polymer Based Unidirectional Composites: Use of Bulk Properties of Matrix with the Smeared Crack Model

Traditional approach to micro-mechanical modeling uses in situ properties of ingredients, which are so fitted as to match predictions of the micromechanical analysis with the experimental in-plane shear response of a lamina for a particular value of fiber volume fraction. In situ properties thus derived, cannot, in general, be used with a different fiber volume fraction or a different micromechanical model. The study presented here is based on the premise that micromechanical models should be transparent and rational tools for obtaining composite properties. Therefore, an attempt is made to characterize the composite from the actual properties of the ingredients and accounting for the presence of thermal residual stresses, shear softening and micro cracking in the matrix throughout the loading history. Experimental comparisons show that a micro-mechanical model with a realistic representative volume element (RVE) and diligent accounting of matrix behavior provides good bounds on experimental response with consistent accuracy, when bulk material properties are used.