Matrix solidification and the resulting residual thermal stresses in composites

The disparate thermal expansion properties of the fibres and matrices in high-performance composites lead to an inevitable build up of residual thermal stresses during fabrication. We first discuss the thermal expansion behaviour of thermoplastic and thermoset polymers that may be used as high-performance composite matrices. The three classes of polymers considered are epoxies, amorphous thermoplastics, and semicrystalline thermoplastics. The relevant thermal expansion data for prediction of the magnitude of the residual stresses in composites is the zero (atmospheric)-pressure thermal expansion data; these data are plotted for a range of thermoplastics and a typical epoxy. Using the technique of photoelasticity, we have measured the magnitude of the residual stresses in unidirectional graphite composites with an amorphous thermoplastic matrix (polysulfone) and with an epoxy matrix (BP907). The temperature dependence of the residual stress build up and the resulting magnitude of the residual stresses correlate well with the thermal and physical properties of the matrix resin.     

Residual stresses formation during the manufacturing process of epoxy matrix composites: resin yield stress and anisotropic chemical shrinkage

A study of residual stresses during the curing process of thermosetting resin composites is presented. A methodology is proposed for predicting the formation and the development of the manufacturing residual stresses, this approach is based on the determination of the resin yield stress. A self-consistent model is used to determine the cure-dependent effective mechanical properties, thermal expansion and chemical shrinkage coefficients of the composite material. This model allows considering for the composite material behaviour an anisotropic chemical shrinkage, which is not represented by a classical linear model. Finally, a one dimensional cure simulation and a modelling of residual stresses formation in composite plate are made by using a finite elements code. The effects of the cure-dependent material properties and of the resin yield stress on the residual stress formation are studied.     

The formation of residual stresses in laminated thermoplastic composites

The means by which residual stresses are generated in continuous carbon-fibre-reinforced thermoplastic composites are reviewed, with specific attention to the macroscopic stresses which form on a ply-to-ply level. Different means for their estimation are assessed, and approximate stress levels are defined for a variety of materials. The capacity for stress relaxation to occur during processing is examined, and it is shown that little useful stress-relieving ability can be promoted in such systems.     

Residual stress measurements of computer aided design/computer aided manufacturing (CAD/CAM) machined dental ceramics

Residual stress analysis is becoming more important in terms of understanding the strength and fatigue behaviour of ceramic materials. The residual stresses after computer aided design/computer aided manufacturing (CAD/CAM) machining according to dental practice were analysed for two different kinds of dental ceramics, a feldspathic porcelain and a glass-ceramic. A mechanical strain gauge element was used to measure the deformation of dental test inlays during material removal by etching the surface of the sample. From these data the residual stress depth profile could be calculated for crystalline as well as amorphous materials. The strain gauge results were compared to X-ray diffraction data. The depth profile of the residual stress for both ceramics showed compressive stress at the surface of the machined ceramics, changing towards tensile stress at a depth of 10 to 15 μm from the surface. Ceramics with pronounced plastic deformation behaviour in CAD/CAM machining revealed higher residual stresses as well as a more distinct stress anisotropy in terms of grinding direction. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal residual stresses in metal matrix composites: A review

Recently, metal matrix composites (MMCs) have generated a considerable interest in the materials field because of their attractive physical and mechanical properties. However, during the fabrication of MMCs, thermal residual stresses are reportedly developed in the matrix as a result of the mismatch of the thermal expansion coefficients between the reinforcement and the matrix. It is well established that these residual stresses have a significant effect on the composite properties. For example, due to the presence of thermal residual stresses, it is almost never possible to achieve the maximum elastic response of the composites. In addition, yield stress and fracture toughness of the composites are significantly affected by thermal residual stresses. In this paper, a critical review of the published literature on thermal residual stresses in MMCs and their effect on composite properties are presented. Also, experimental and numerical techniques that are currently available to measure and estimate thermal residual stresses are reviewed and discussed.     

Stability of shot peening induced residual stresses and their influence on fatigue lifetime

Mechanical surface treatment methods such as shot peening may improve the fatigue strength of materials. In this study, the effect of shot peening on strain controlled constant amplitude fatigue loading of a near pearlitic microalloyed steel was investigated. The stress amplitudes throughout the whole lifetime were followed, in addition to detailed recording of stress-strain hysteresis loops, particularly at small cycle numbers. The detailed relaxation of residual stresses and the changes in full width of half maximum (FWHM) of the X-ray peak at the surface and in depth as function of the number of cycles and plastic strain were recorded. By these techniques, the onset as well as the rate of relaxation of residual stresses could be followed at different strain amplitudes. Pronounced increase in lifetime of the shot peened specimens tested at total strain amplitude smaller than 0.3% (corresponding to 0.034% plastic strain amplitude) was achieved. This coincides with reasonably stable residual stresses at the surface and in depth.     

Residual stress analysis of thin films and coatings through XRD experiments

Residual stresses are one of the crucial parameters determining the performances of structural as well as functional materials. In the case of coatings and films, the substrate and the deposition process may determine very high residual stress fields which can affect both performances and surface integrity, since adhesion or cracking resistance can be strongly altered. The accurate and reliable assessment of residual stress is thus mandatory for the evaluation of these materials. In this paper we applied a new approach to evaluate the residual stress by means of the analysis of a single 2D-diffraction image collected by a laboratory X-ray microdiffractometer equipped with an image plate detector. The residual stress in thin films of LaCoO₃ was calculated and correlated to cubic-to-rhombohedral phase transformation.     

Bistable composite laminates: Effects of laminate composition on cured shape and response to thermal load

This paper develops a finite element (FE) approach using commercial ANSYS V11.0 software to accurately predict the cured shape of bistable composites by including the influence of manufacturing imperfections, such as resin rich areas and ply-thickness variations. Laminate composition was characterised by optical microscopy and their cured shapes measured using a Peak Motus motion analysis system. The FE model accurately predicts observed differences between laminate curvature in the two stable states. Localised reversal of curvature resulting from through-thickness shear stress is also predicted. Structural response to thermal loading was experimentally characterised showing a temperature dependent deflection rate and a residual curvature caused by non-reversible residual stresses. FE-predictions show good agreement with experiment over the range 20–110 °C. The presented data highlights the importance of manufacturing processes and materials selection in the design of thermally stressed multi-stable composite structures.     

Modelling the viscoelastic stress relaxation of glass fibre reinforcements under constant compaction strain during composites manufacturing

Viscoelastic stress relaxation of glass fibre reinforcements is commonly encountered in the manufacture of glass fibre reinforced polymer composites. A better understanding of the phenomenon, coupled with an ability to predict this behaviour, will aid improved manufacturing process control and tooling design. Finished product quality may also be bettered by virtue of increased knowledge of stresses acting within the composite product. This paper presents a simple Maxwell element-based model to both simulate and help explain the viscoelastic stress relaxation of glass fibre reinforcements under compressive strain compaction of layers during composites manufacturing. The model was validated against experimental data for reinforcement materials of different architecture, and good-to-reasonable predictions of the stress relaxation response were obtained.     

Thermal residual stresses in FSS layer-embedded composite plates in co-cure bonding

The residual stresses occur in the frequency selective surface (FSS)-embedded composite structures after co-curing due to mismatch between the coefficient of thermal expansions between the FSS and composite materials. It is well known that residual stress has a great influence on the strength and fatigue life of the FSS-embedded composite structures. Numerous researches have been reported about residual stresses in the composite structures. However, studies on the residual stresses in the FSS-embedded composite structure have not been widely investigated. Therefore, in this study, the thermal residual stresses of FSS-embedded composite structures were analyzed using finite element analysis, with considering the effect of FSS pattern and size. Various FSS patterns, such as square loop, gridded square loop, and double square loop, are considered in the analysis. The effects of dimensional change of design parameters of FSS pattern on the residual stresses in the hybrid composites were also investigated.     

Influence of residual stresses in the stress relaxation of cold drawn wires

This study demonstrates the influence of residual stresses in steel wires on the stress relaxation losses. Standard stress relaxation tests were performed on four types of wire, all with the same mechanical properties but with different residual stresses. Surface residual stresses were measured by X-ray diffraction. The experimental results show that stress relaxation losses decrease as the value of surface tensile residual stresses decrease. The role, sometimes controversial, of initial pre-stretching and heat treatments on stress relaxation losses can also be understood in the light of the residual stresses induced during cold-drawing.     

抗菌剂用于牙科复合树脂的研究进展

龋病是常见的口腔疾病,填充修复是治疗的重要手段.复合树脂凭借色泽美观和理化性能优异等特点,广泛应用于牙科修复材料中.继发龋是临床应用中导致修复失败的主要原因之一.为了抑制继发龋、延长树脂使用寿命,研发具有长期抗菌功能的牙科复合树脂具有重要意义.目前,主要通过向复合树脂体系中添加抗菌型无机粒子或抗菌型有机单体赋予材料抗菌...     

Residual stresses in unidirectional graphite fiber/polyimide composites as a function of aging

In this research, the magnitude and distribution of residual aging stresses in unidirectional graphite fiber/polyimide composites were investigated. First, a viscoelastic aging material model was developed by characterizing the isothermal aging strain and stress relaxation of neat PMR-15 polyimide. Then, the aging material model was applied to numerically evaluate the effect of matrix aging and cooling on the residual stress state of a unidirectional graphite fiber (T-650)/polyimide (PMR-15) composite. It has been shown in this work that residual stresses in the composite are significantly affected by high temperature aging. Most importantly, the magnitude of the residual stress caused by aging in N₂ at 316 °C for one month was found to be comparable to the cooling stress in the composite.     

Influence of the elastic stress relaxation on the microstructures and mechanical properties of metal–matrix composites

Role of the residual stresses on the mechanical properties of metal–matrix composites is studied. It is shown that the stress relaxation can be responsible for the morphologies and spatial distribution of precipitates. Direct measurements of the residual stress is also emphasized and the influence of dislocations in the accommodation process and during interface crossing is exemplified.     

Water sorption and water-induced molecular mobility in dental composite resins

Water sorption in two resin composites, Kulzer's Solitaire (S) and SDI's Wave (W), and in a polyacid-modified composite resin, 3M's F2000 (compomer F), was investigated by means of equilibrium sorption isotherms (ESI) and of dynamic sorption (DS) measurements. Molecular mobility in these materials was studied by means of dielectric relaxation spectroscopy (DRS) and of thermally stimulated depolarization currents (TSDC) measurements. The results of ESI measurements show that at equilibrium, water is molecularly distributed in the materials and the effects of hydrophilic sites and clustering are negligible. Hysteresis effects in sorption–desorption cycles are larger in the resin composites than in the compomer. Equilibrium water uptakes in both ESI and DS conditions are rather low, in the range 1–2%. Diffusion coefficients of water are about 1×10^–8 cm²/s in the resin composites and by a factor of about 2 smaller in the compomer. Molecular mobility increases with hydration, as suggested by preliminary DRS and TSDC measurements. Detailed dielectric measurements may give important information for understanding, at the molecular level, water-induced degradation in dental materials.     

Observations on the nature of micro-cracking in brittle composites

The degree of micro-cracking in BeO-SiC composites due to internal stresses which arise from the mismatch in the coefficients of thermal expansion was monitored by measurements of the thermal diffusivity by the laser-flash technique. The experimental results indicated that micro-cracking was most extensive at approximately 30 and 80 wt% SiC and a minimum at nearly 50 wt% SiC. A theoretical analysis indicated that the magnitude of internal stress increases linearly with SiC content, so that the above observations cannot be attributed to a low internal stress state at ~ 50 wt% SiC. Instead, this effect can be attributed to changes in the statistical variables affecting the brittle fracture as well as the degree of internal stress relaxation. Both these factors are thought to be controlled by the nature of multiaxial stress distribution. At ~ 50 wt% SiC-content, due to anticipated non-hydrostatic triaxial stress distribution, residual stress relaxation is possible in both the components of the composite. However, at low and high fractions of SiC content, such stress relaxation is less likely to occur due to the expected hydrostatic stress distribution in one of the components.     

A procedure for high residual stresses measurement using the ring‐core method

The ring‐core method is a mechanical technique used to determine the residual stresses on the surface of materials by milling a narrow circular groove around the point of interest and monitoring the strain variation. Original stress can be evaluated by using the relaxed strain through the linear elasticity theory. In case of a highly stressed field, the yielding of the material around the groove and its bottom causes an error related to the hypothesis of the basic theory of the method. In this paper, the plasticity effect of calculated residual stresses was considered. For this purpose, the ring‐core development and stress relaxation in a generally stressed body were simulated using a 3D parametric finite element model and the stress calculation errors were obtained by comparing calculation stress with the actual one. According to the results, the error value was affected by some parameters such as ring depth, stress magnitude and state, and material behaviour. In general, for residual stresses above 65% of the local yield strength, the plasticity‐induced errors were significant, as an error close to 35% was found. Based on the result, a correction procedure was proposed for the evaluation of the high level residual stresses in steel materials by the ring‐core method. By the method, true stresses can be obtained with considering the calculated stresses state, material properties, ring geometry, and estimated plasticity‐induced errors.     

Fatigue crack initiation assessment of welded joints accounting for residual stress

The impact of residual stresses on the fatigue crack initiation life of welded joints is evaluated by the finite element method. The residual stresses of nonload‐carrying cruciform joints, induced by welding and ultrasonic impact treatment, are modelled by initial stresses, using the linear superposition principle. An alternative approach of using modified stress‐strain curves in the highly stressed zone is also proposed to account for the residual stress effect on the local stress‐strain history. An evaluation of the fatigue crack initiation life of welded joints based on the local strain approach is carried out. The predicted results show the effect of residual stresses and agree well with published experimental results of as‐welded and ultrasonic impact treated specimens, demonstrating the applicability of both approaches. The proposed approaches may provide effective tools to evaluate the residual stress effect on the fatigue crack initiation life of welded joints.     

Numerical simulation of residual stress relaxation around a cold‐expanded fastener hole under longitudinal cyclic loading using different kinematic hardening models

Cold expansion of fastener holes creates compressive residual stresses around the hole. This well‐known technique improves fatigue life by reducing tensile stress around the holes. However, cyclic loading causes these compressive residual stresses to relax, thus reducing their beneficial effect. Estimation of the fatigue life without considering the residual stress relaxation might lead to inaccurate results. In this research, numerical studies were carried out using 2D finite element (FE) models to determine the initial tangential and radial residual stress distributions generated by cold expansion and their relaxation under cyclic loading. To predict the stress relaxation, four nonlinear kinematic hardening models were applied in simulation of stress/strain path. The results obtained from the FE analysis were compared with available experimental results. A good agreement between the numerical and experimental results was observed.