Composite residual stress

Several experimental procedures can be used to measure the residual stresses. The three preferred methods involve diffraction (X-ray or neutron), beam deflection, and permanent strain determination. X-ray diffraction measurements have the limitation that the penetration depth is small, such that only near-surface information is obtained. Moreover, in composites, residual stresses are redistributed near surfaces.47 Consequently, a full stress analysis is needed to relate the measured strains to either q or a. ... 

Laminated Composite, Residual stress, transverse cracking, fracture mechanics... 

By means of the combined approach of experiment and simulation, the influence of several parameters, such as membrane layer composition, residual stress, thickness, and particle-related properties, were quantified. An empirical model was deduced which allows design rules to be developed for membranes with increased time to failure under dynamic mechanical loads. 

As the resin cures, specifically, in a geometrically constrained environment such as between plates, or within the spaces present between fibers in the case of composites, residual stresses are developed within the resin. These stresses may exceed the intrinsic strength of the resin which depends on its degree of conversion ... 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Another consideration is the difference in thermal expansion between the matrix and the reinforcement. Composites are usually manufactured at high temperatures. On cooling any mismatch in the thermal expansion between the reinforcement and the matrix results in residual mismatch stresses in the composite. These stresses can be either beneficial or detrimental if they are tensile, they can aid debonding of the interface if they are compressive, they can retard debonding, which can then lead to bridge failure (25). 

Attack at welds due to bacteria is possible, but it is not nearly so common as is often supposed. Because of residual stresses, microstruc-tural irregularities, compositional variation, and surface irregularities, welds show a predisposition to corrode preferentially by most corrosion mechanisms. Attack is common along incompletely closed weld seams such as at butt welds in light-gauge stainless steel tubing (Fig. 6.9A and B). Attack at carbon steel welds may occur. Figure 6.10 shows a severely corroded carbon steel pipe from a service water sys-... 

The rise times of the elastic wave may be quite narrow in elastic single crystals, but in polycrystalline solids the times can be significant due to heterogeneities in physical and chemical composition and residual stresses. In materials such as fused quartz, negative curvature of the stress-volume relation can lead to dispersive waves with slowly rising profiles. 

Many workershave investigated the residual stresses introduced by different working processes in brasses of various compositions and the... 

Applications The general applications of XRD comprise routine phase identification, quantitative analysis, compositional studies of crystalline solid compounds, texture and residual stress analysis, high-and low-temperature studies, low-angle analysis, films, etc. Single-crystal X-ray diffraction has been used for detailed structural analysis of many pure polymer additives (antioxidants, flame retardants, plasticisers, fillers, pigments and dyes, etc.) and for conformational analysis. A variety of analytical techniques are used to identify and classify different crystal polymorphs, notably XRD, microscopy, DSC, FTIR and NIRS. A comprehensive review of the analytical techniques employed for the analysis of polymorphs has been compiled [324]. The Rietveld method has been used to model a mineral-filled PPS compound [325]. 

Filiou, C., Galiotis, C. and Batchelder, D.N. (1992). Residual stress distribution in carbon fiber/ thermoplastic matrix pre-impregnated composite tapes. Composites, 23, 29-38. 

Chen, E.J.H. and Croman R.B. (1993). Microdebonding investigation on the effects of thermal residual stress on the bond strength of a graphite/polyimide composites. Composites Sci. Technol. 48, 173-179. 

Sigl, L.S. and Evans, A.G. (1989). Effects of residual stress and frictional sliding on cracking and pull-out in brittle matrix composites. Mech. Mater. 8, 1-12. 

Another important mechanical property of a coating layer is the coefficient of thermal expansion (CTE). Residual stresses generated due to the differential thermal contraction between the composite constituents are extremely detrimental to the... 

Arnold, S.M., Arya, V.K. and Melis, M.E. (1990). Elastic/plastic analysis of advanced composite investigating the use of the compliant layer concept in reducing residual stresses resulting from processing, NASA TM-103204. 

Previous studies of the interphase/interlayer have mainly focused on the coefficient of thermal expansion (CTE) and residual thermal stresses. The importance of residual thermal stresses cannot be overemphasized in composites technology because the combination of dissimilar materials in a composite creates inevitably an interphase across which residual stresses are generated during fabrication and in service due to the difference in thermo-mechanical characteristics. The importance of an interlayer is clearly realized through its effects in altering the residual stress fields within the composite constituents. 

E, V and a are the Young s modulus, Poisson ratio and CTE, respectively, and the subscript i refers to the interlayer or coating. The residual stress at the fiber/matrix interface, (Tai, for the composite without an interlayer can be obtained for , =... 

The micro-residual stresses arise from the differential CTE of the fiber and matrix, and the temperature difference. Table 7.4 gives the linear CTE values for various types of reinforcing fibers and matrix materials used widely for composite fabrication. The CTEs of most fibers and ceramic matrices are relatively lower... 

Three-dimensional distributions of the micro-residual stresses are very complicated, and are affected by the elastic properties, local geometry and distribution of the composite constituents within a ply. Many analytical (Daniel and Durelli, 1962 Schapery, 1968 Harris, 1978 Chapman et ah, 1990 Bowles and Griffin, 1991a, b Sideridis, 1994) and experimental (Marloff and Daniel, 1969 Koufopoulos and Theocaris, 1969 Barnes et ah, 1991 Barnes and Byerly, 1994) studies have been performed on residual thermal stresses, A two-dimensional photoelastic study identified that the sign and level of the residual stresses are not uniform within the composite, but are largely dependent on the location (Koufopoulos and Theocaris,... 

---