Effects Of Thermal Residual Stresses Research Articles

Thermal-mechanical Analysis of Satin Weave CFRP Composites with Cracks at Cryogenic Temperatures

This study deals with the thermal-mechanical behavior of satin weave carbon fiber-reinforced polymer (CFRP) laminates with internal and/or edge cracks and temperature-dependent material properties under tension at cryogenic temperatures. Cracks are assumed to be located in the transverse fiber bundles and extend to the interfaces between two fiber bundles. Young's modulus and stress distributions near the crack tip under combined mechanical and thermal loads are predicted by finite element models that assume generalized plane strain. The effects of residual thermal stresses and cracks on the mechanical behavior of CFRP woven laminates at cryogenic temperatures are calculated, and the numerical results are presented in graphical form.

熱残留応力を考慮した層間き裂進展に関するエネルギー解放率：Ｃｒａｃｋ Ｔｉｐ Ｅｌｅｍｅｎｔ法による定式化

Crack tip element (CTE) method is applied to the formulation of energy release rate associated with interfacial crack growth of laminates with arbitrary layups and residual thermal stresses using the Timochenko beam model. Special attention is paid to the energy release rates of mixed mode bending (MMB) test, and mode-I and mode-II energy release rates are formulated including residual thermal stresses. The results derived are verified by the comparison to finite element analysis, and the effect of residual thermal stresses on the mode mixity of the MMB test is discussed.

Effects of inter-fibre spacing on damage evolution in unidirectional (UD) fibre-reinforced composites

A three dimensional (3D) micromechanical study has been performed in order to investigate local damage in UD composite materials under transverse and longitudinal tensile loading. In particular, the influence of non-uniform distribution of fibres in RVEs (representative volume element) with a hexagonal packing array and the effects of thermal residual stresses has been investigated. To examine the effect of inter-fibre spacing and residual stress on failure, a study based on the Maximum Principal Stress failure criterion and a stiffness degradation technique has been used for damage analysis of the unit cell subjected to mechanical loading. Results indicate a strong dependence of damage onset and its evolution from the fibres position within the RVE. Predicted mechanical properties, damage initiation and evolution are also clearly influenced by the presence of residual stress.

Experimental Investigations on the Effects of Thermal Residual Stresses on the Efficiency of Repaired Panels with Glass/Epoxy Composite Patch

The aim of this experimental investigation is to study the effect of various curing temperatures on crack-front shape and crack growth life of centrally cracked aluminium panels in mode-I condition with single-side glass/epoxy composite patches. The aluminium panels are made of Al 2024-T3 with the thicknesses of 2.29 mm. Unidirectional four layers lay-up perpendicular to the initial crack length were used for the patches of all specimens and the adhesive was Araldite LY564. The cyclic remote stress of 118 MPa with the R-ratio of 0.05 was applied for all models. The experiments were performed for different curing temperatures of room temperature, 50oC, 80oC, 100oC and 120oC. It is shown that the fatigue crack growth life of the repaired panels with curing temperatures of 100oC and 120oC is considerably smaller than those obtained for specimens cured at room temperature and 50oC.

Energy methods for fatigue damage modelling of laminates

The initiation and growth of damage in composite materials are phenomena that precede the catastrophic failure event where a material sample or component fragments or separates into two pieces. During fatigue loading, the damage grows stably due to cyclic stressing and leads to a gradual deterioration of mechanical properties and ultimately to failure. For cross-ply laminates, the estimation is necessary of effective stress intensity factors or energy release rates, for statically loaded ply cracks in 90° plies that are bridged by the uncracked 0° plies, particularly when considering the early stages of property degradation. Such relations are used in conjunction with a fatigue crack growth law to predict the progressive development of damage during fatigue. For fatigue loading this paper justifies, on the basis of detailed physical modelling based on energy methods rather than empiricism, the use of the stress range intensity factor as the correlating parameter for fatigue crack growth data, rather than energy release rates or differences of energy release rate. Use is made of an accurate stress transfer model for multiple-ply cross-ply laminates to predict the dependence of energy release rates and stress intensity factors for long bridged ply cracks on the applied stress and ply crack separation. Two methods of analysis are considered. The first uses a method that can be extended to deal with small laminate defects where the energy release rate and stress intensity factor depend on the size of the defect, but the laminate is subject only to a uniaxial load. The second method applies to multi-axial loading, but assumes that the stress intensity factor or energy release rate is independent of the defect size. Both methods are, however, shown to lead to the same energy release rate and stress intensity factor for long ply cracks subject to uniaxial loading. The two methods also take full account of the effects of thermal residual stresses through the use of a crack closure concept. Simplifying assumptions are made when developing a model that predicts the degradation of most of the thermoelastic constants of a fatigue damaged cross-ply laminate as a function of the number of fatigue cycles. Such data are needed to predict the fatigue behaviour of structures having complex stress states using finite element analysis. Preliminary work carried out to validate the fatigue model, based on simplifying assumptions, has led to pessimistic predictions of performance that have the advantage that they can be exploited in the form of conservative design methods.

Elastoplastic damage micromechanics for elliptical fiber composites with progressive partial fiber debonding and thermal residual stresses

Incorporating the interfacial damage and thermal residual stresses, an elasto-plastic damage formulation is proposed to predict the overall transverse mechanical behavior of continuous elliptical-fiber reinforced ductile matrix composites within the framework of micromechanics and homogenization. Based on the concept of equivalent inclusion and taking the progressive interfacial debonding angle into consideration, partially debonded fibers are replaced by equivalent orthotropic, perfectly bonded fibers. Three interfacial damage modes are considered. The Weibull's probabilistic function is adopted to describe the varying probability of progressive partial fiber debonding. The effective elastic moduli of four-phase composites, composed of a ductile matrix and randomly located yet unidirectionally aligned fibers are derived by a micromechanical formulation. Thermal residual stresses are taken into account through the concept of thermal eigenstrain to investigate the effects of the manufacturing processinduced residual stresses. Employing the micromechanical approximation, the overall stress-strain responses and the effective yield function are formulated with the thermal eigenstrain. When comparing with the available experimental data, significant effects of thermal residual stresses are discussed. Moreover, the effects of the interfacial strengths and the cross- sectional shapes of fibers on the mechanical behaviors of composites are systematically investigated.

極低温タンク構造への適用に向けた樹脂フィルム付きＣＦＲＰ積層板の損傷挙動に関する研究

Polymer films are expected to be feasible candidates for the liner of cryogenic composite tank structures. This study investigates the damage behaviors of polymer-lined CFRP laminates under tensile loading at room and cryogenic temperatures. Because matrix cracks in CFRP laminates are considered to be more susceptible to formation than polymer film fracture, damage behaviors of polymer films beyond the onset of matrix cracks in layers adjacent to polymer film are the major concerns herein. Specimens consisting of polymer films and cross-ply laminates were prepared by co-curing and subjected to tension load. Penetration of matrix cracks in composite layers into polymer films was observed in the case of thermoplastic polyimide-lined CFRP laminates, while crack deflection into interfaces between CFRP and polymer films was observed in the case of liquid crystalline polymer and polyimide films. Finally, mechanical properties of polymer films and adhesive properties between CFRP and polymer films were measured in order to discuss the crack penetration/deflection behaviors in polymer-lined CFRP laminates. An energetic criterion on crack penetration/deflection including the effects of residual thermal stress was applied to the present experimental results, and suitable polymer characteristics were clarified.

Influence of Ply Sequence and Thermoelastic Stress Field on Asymmetric Delamination Growth Behavior Emanating From Elliptical Holes in Laminated FRP Composites

The present study encompasses the influence of ply sequence and thermoelastic stress field on asymmetric delamination growth behavior emanating from elliptical holes in laminated fiber reinforced polymeric composites. Results, emphasizing the effect of thermal residual stresses on delamination growth behavior of the composite laminates subjected to two different loading conditions, i.e., in-plane tensile and compressive loadings, are presented. Two sets of full three-dimensional finite element analyses have been performed to calculate the displacements and interlaminar stresses along the delaminated interfaces responsible for the delamination onset and propagation. Modified crack closure integral methods based on the concepts of linear elastic fracture mechanics have been followed to evaluate the individual modes of strain energy release rates along the delamination front. In each case, the delamination is embedded at a different depth along the thickness direction of the laminates. It is observed that the fiber orientation of the plies bounding the delamination front significantly influences the distribution of the local strain energy release rate. Also, the residual thermal stresses have a detrimental effect on the laminates subjected to compressive loading and more so in the case of laminates with delaminations existing closer to the top and bottom surfaces of the laminate.

Micromechanics analysis of progressive failure in cross-ply carbon fiber/epoxy composite under uniaxial loading

Three-dimensional micromechanics models were created for cross-ply carbon fiber/epoxy composite with a layer stacking-sequence arranged in [0/90]s. Elasto-plastic finite element (FE) analysis was performed to study the effects of thermal residual stress and the stress redistribution as individual fiber fractures. The modified Rice and Tracey (RT) void growth model was used to predict the location of transverse matrix crack. The stress amplification factors (SAF) in intact fibers adjacent to a fractured fiber were calculated and compared with the planar array composite. The FE results show that small defects have already formed in curing process, and ply-delamination is likely to occur near the comer of free-edges. The transverse matrix crack was predicted to occur near the fiber fracture location in the models having little inter-fiber spacing.

Correction method for evaluation of interfacial fracture toughness of DCB, ENF and MMB specimens with residual thermal stresses

The energy release rates associated with crack growth in multilayered double cantilever beam (DCB), three-point end notched flexure (ENF), and mixed-mode bending (MMB) specimens, including the effects of residual thermal stresses, are derived as special cases of Nairn’s formulation [Nairn JA. On the calculation of energy release rates for cracked laminates with residual stresses. Int J Fract 2006;139:267–93]. The difference between the true toughness and apparent toughness attributable to the consequence of ignoring residual stresses can be calculated for a given specimen geometry and mechanical properties based on classical lamination theory. The true fracture toughness is obtainable using the apparent fracture toughness and the present correction method. Numerical examples of DCB, ENF, and MMB tests are presented, and the calculated difference in the energy release rates based on the present correction method is compared to those obtained using FEM to verify the correction method.

Torque capacity of tubular adhesive joints with different composite adherends

This study focuses on investigating the effects of adherend materials on the torsional strength of composite tubular adhesive joints. The initial failure index was introduced to consider the effect of residual thermal stresses. Two different composite adherends were considered in this analysis: carbon/epoxy and glass/epoxy. The effects of stacking angle and adhesive thickness on joint strength were also investigated. The results show that the joint with the carbon/epoxy adherend is more affected by the residual thermal stresses. The failure mode and joint strength depend on the stacking angle and adhesive thickness as well as the adherend materials.

Thermal loading of short fibre composites and the induction of residual shear stresses

Abstract A study has been undertaken to identify the effect of thermal residual stresses on the stress transfer between a short fibre and resin using the photoelastic method. As expected, it was observed that fibre fracture occurred at a higher applied load for the fibre embedded in the thermally (80 °C) cured epoxy matrix than in the room-temperature-cured epoxy. Under plane polarised light bright birefringent patterns were observed in the hot-cured epoxy matrix around the fibre-ends prior to loading. These were not present in the room-temperature-cured epoxy, indicating that thermal residual stresses had been induced during thermal-curing. On loading, the birefringent patterns in the hot-cured matrix at the fibre-ends were almost extinguished but at a particular stress reappeared as a bright region, and increased in intensity on further loading. Using a phase-stepping polariscope, four images of the fibre-ends were captured simultaneously so that detailed contour maps of fringe order could be created. To examine the micromechanical response in the matrices at the interfaces the profile of interfacial shear stress at the fibre-ends was calculated. Under a given external load the shear stress at the interface in the hot-cured matrix was significantly lower than that in the cold-cured epoxy matrix. The thermal load which is applied to a resin on cooling from manufacture requires a shear stress at the interface to put the fibre into compression. At the fibre-ends a residual shear stress of opposite sign (to that induced mechanically) leads to extension of birefringent patterns on loading.

Investigating thermal residual stress effect on mechanical behaviors of fiber composites with different fiber arrays

This study aims to investigate the thermal residual stress effect on the constitutive behaviors of fiber composites with three different fiber arrays, i.e., square edge packing, square diagonal packing, and hexagonal packing. The repeating unit cell (RUC) containing fiber and matrix phase was employed to describe the mechanical behaviors of fiber composites. For the fiber phase, it was assumed to be linear elastic, whereas the matrix was a nonlinear material. The generalized method of cell (GMC) micromechanical model originally proposed by Paley and Aboudi [Paley M, Aboudi J. Micromechanical analysis of composites by the generalized cells model. Mech Mater 1992; 14(2):127–39] was extended to include the thermal–mechanical behavior, from which the thermal residual stress within the fiber and matrix phases was calculated. Through numerical iteration, the constitutive relations of the composites in the presence of residual stress were established. Results show that for the composites with square edge packing, the mechanical behaviors are affected appreciably by the thermal residual stress. On the other hand, the composites with hexagonal packing and square diagonal packing are relatively less sensitive to the thermal residual stress.

Shear properties at the PyC/SiC interface of a TRISO-coating

The failure behavior of TRISO-coated fuel particles depends significantly on the shear strength at the interface between the inner pyrolytic carbon (PyC) and silicon carbide (SiC) coatings. In this study, a micro-indentation fiber push-out test was applied to measure the interfacial shear properties of a model TRISO-coated tube. Of particular emphasis is that this study developed a non-linear shear-lag model for a transversely isotropic composite material due to insufficiency in the existing isotropic models as applied to layered TRISO-coating systems. In the model, the effects of thermal residual stresses and the roughness-induced clamping stress were identified as particularly important. The rigorous model proposed in this study provides more reasonable data on two important interfacial parameters: the interfacial debond shear strength and the interfacial friction stress. The modified model coupled with experiments yields an interfacial debond shear strength of 240 ± 40 MPa. This high interfacial strength, though slightly lower than that obtained by the existing isotropic model (∼280 MPa), allows significant loads to be transferred between inner PyC and SiC in application. Additionally, an interfacial friction stress of 120 ± 30 MPa was determined. This high friction stress is attributed primarily to the roughness at the cracked interface rather than clamping effects due to differing coefficients of thermal expansion.

Residual stresses in thermoplastic composites – a study of the literature. Part III: Effects of thermal residual stresses

Abstract This paper is the third part in a series of review papers on residual stresses in thermoplastic composites. The first two parts were dedicated to the formation of thermal residual stresses and experimental techniques to detect these stresses, respectively. This third paper discusses the effects that thermal residual stresses have on the material properties of thermoplastic composites. Composite laminates as well as composite structures were considered. Residual stresses were found to affect the composites’ matrix-dominated, mechanical and durability properties, as well as to inflict damage and dimensional instability. Several mechanisms to relieve thermal residual stresses are proposed.

Strength prediction of tubular composite adhesive joints under torsion

This paper addresses prediction of the strength of tubular adhesive joints with composite adherends by combining thermal and mechanical analyses. A finite element analysis was used to calculate the residual thermal stresses generated by cooling down from the adhesive cure temperature, and a nonlinear analysis incorporating the nonlinear adhesive behavior was performed to accurately estimate the mechanical stresses in the adhesive. Joint failure was estimated by three failure criteria: interfacial failure, adhesive bulk failure, and adherend failure. The distributions of residual thermal stresses were investigated for various stacking angles. The effect of residual thermal stresses on joint strength was also taken into consideration. The results indicate that the residual thermal stresses, depending on the stacking angle, have a significant influence on the failure mode and strength of adhesive joints when a subsequent mechanical load is applied. Good agreement is also obtained between the predicted joint strength and the available experimental data.

Effects of thermal residual stresses on failure of co-cured lap joints with steel and carbon fiber–epoxy composite adherends under static and fatigue tensile loads

Abstract A co-cured joint, which uses excess resin extracted from the composite material (or the polymer material) as the adhesive, has several advantages compared with the adhesively bonded joint. It has no need of the surface treatment of the composite adherend and any adhesive for bonding. In addition, its manufacturing process is very simple because the bonding process is achieved during the curing process of the composite adherend. Thermal residual stresses are important in analyzing failure of the co-cured joint because composite materials are different from metal alloys in thermal and mechanical properties. In general, effects of the thermal residual stresses on failure of the adhesive joint are analyzed through experimental and analytical results. Therefore in this paper, two design parameters, namely the surface roughness of the steel adherend and the stacking sequence of the composite adherend, are considered for static and fatigue tensile tests of the co-cured single and double lap joints. Stress distribution at the interface between the two adherends is used to analyze failure of the co-cured lap joints through the finite element method. Based on the stress distribution, two failure criteria, namely the three-dimensional Tsai–Wu failure criterion and the Ye-delamination failure criterion, are considered to predict the tensile load bearing capacity of the co-cured lap joints. Finally, effects of thermal residual stresses on failure of the co-cured single and double lap joints with the steel and composite adherends under static and fatigue tensile load conditions are presented through comparing the experimental and analytical results.

Avoidance of fabricational thermal residual stresses in co-cure bonded metal-composite hybrid structures

In this work, a smart cure cycle with cooling, polymerization and reheating was devised to nearly completely eliminate thermal residual stresses in the bonding layer of the co-cure bonded hybrid structure. In situ dielectrometry cure monitoring, DSC experiments and rheometric measurements were performed to investigate the physical state and the cure kinetics of the neat epoxy resin in the carbon fiber/epoxy composite materials. From the experimental results, an optimal cooling point in the cure cycle was obtained. Also, process parameters such as cooling rate, polymerization temperature and polymerization time in the curing process were investigated. Then, the thermal residual stresses were estimated by measuring the curvatures of co-cure bonded steel/composite strips and their effects on the static lap-shear strengths of co-cure bonded steel/composite lap joints were measured. Also, the effects of thermal residual stresses on the tensile strength, the interlaminar shear strength and the interlaminar fracture toughness of the composite material itself were measured using tensile, short beam shear and double cantilever beam tests. From these results, it was found that the smart cure cycle with cooling, polymerization and reheating eliminated the thermal residual stresses completely and improved the interfacial strength of the co-cure bonded hybrid structures, as well as the tensile strength of the composite structures.

Design and thermal analysis of plastic injection mould

This paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould. The technique, theory, methods as well as consideration needed in designing of plastic injection mould are presented. Design of mould was carried out using commercial computer aided design software Unigraphics, Version 13.0. The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solved using a commercial finite element analysis software called LUSAS Analyst, Version 13.5. The software provides contour plot of temperature distribution for the model and also temperature variation through the plastic injection molding cycle by plotting time response curves. The results show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions. This uneven cooling effect at different regions of mould contributed to warpage.

Three-dimensional thermoelastic analysis of cracked plain weave glass/epoxy composites at cryogenic temperatures

This paper examines the thermo-mechanical behavior of cracked G-11 woven glass/epoxy laminates with temperature-dependent material properties under tension at cryogenic temperatures. Three-dimensional finite elements are employed to model the architecture of the two-layer woven laminates. It is assumed that the cracks are confined to individual fiber bundles oriented transverse to the tensile load direction and span the thickness of the fiber bundles. The effects of residual thermal stresses caused by differences in the coefficients of thermal expansion of the composite constituents, and cracks on the mechanical behavior of two-layer G-11 woven laminates at cryogenic temperatures are explored. Numerical results for Young's modulus, Poisson's ratio, and stress distributions and concentrations are obtained and discussed in detail.