Residual Microstresses Research Articles

Effect of residual microstresses at crystalline multigrain junctions on the toughness of silicon nitride

Two Si 3N 4 materials with either tensile or compressive residual microstresses localized at triple-grain junctions have been analyzed with respect to their fracture behavior and microstructural characteristics. Residual tensile stresses at triple pockets were obtained, upon addition of Sc 2O 3, by initiating the crystallization of Sc 2Si 2O 7 with a negative volume change. On the other hand, compressive microstresses localized at triple-grain junctions were induced by adding fine ZrO 2 particles to the Si 3N 4 material, which underwent martensitic transformation upon cooling with a positive volume change. The presence of these highly localized stress fields has been shown to actually be the critical factor in determining the fracture mode of Si 3N 4 materials and, accordingly, their respective fracture toughness. In particular, tensile stress fields at triple-grain pockets can trigger debonding and splitting of the crack tip at the interface and, therefore, may provide a precursor effect for elastic bridging in the crack wake.

Strengthening contribution arising from residual stresses in Al 2O 3/ZrO 2 composites: a piezo-Spectroscopy investigation

Piezo-spectroscopy measurements have been performed in order to obtain information about the internal stresses in a series of (2 mol% Y 2O 3-stabilized) ZrO 2 ceramics added with fine Al 2O 3 dispersoids. These composites were previously reported to fracture at remarkably high strength levels. Residual stresses arising from the thermal mismatch between the ZrO 2 matrix and the Al 2O 3 dispersoids were found to be of the order of several hundreds MPa, in agreement with the predictions of an elastic stochastic stress analysis. Taking into consideration the results of the present piezo-spectroscopic analysis, a micromechanism of microcrack shielding by residual microstresses has been proposed, which overlaps with the effect of phase transformation on strength. A theoretical assessment is given which enables one to explain the presence of a maximum of strength at 0·2–0·4 volume fraction of Al 2O 3 dispersoid.

The effects of residual macrostresses and microstresses on fatigue crack propagation

The effects of residual microstresses and tensile residual macrostresses on fatigue crack propagation (FCP) are examined in a high-carbon steel. Phase-specific diffraction measurements show that uniaxial deformation and radial cold expansion produce predominantly microstress and tensile macrostress fields, respectively. Microstresses are found to have little effect on FCP rates, while tensile macrostresses increase crack growth rates in a manner that depends systematically on ΔK. The increases are partly attributed to crack closure, which was found to be appreciable near the surface of control samples but absent in the presence of tensile residual stresses. Both the ΔK dependence and absence of microstress effects were explored by X-ray microbeam measurements around propagating fatigue cracks and found to stem from fading and/or redistribution of residual macrostresses and microstresses during fatigue crack growth.

Thermal Residual Micro-Stress Concentration and Strength of SiC-Alumina Nanocomposites

Thermal Residual Micro-Stress Concentration and Strength of SiC-Alumina Nanocomposites

Polypropylene matrix composites reinforced with pre‐stressed glass fibers

AbstractThermoplastic matrix composites have recently emerged as promising engineering materials because of their desirable properties such as high service temperatures, high impact resistance, and processing advantages. However, residual stresses in composites introduced during fabrication are cited as one of the most significant problems in the processing of composites. In some instances these stresses have been shown to significantly degrade the strength of the material, resulting in matrix cracking, debonding, reduced fracture toughness, and delamination. In this work, studies have been carried out on glass fiber reinforced polypropylene composites formed by compression molding process from co‐mingled fabrics. The fibers were pre‐stressed during the process to produce high performance composite products with low residual microstresses, which are harmful to the properties of the composite. Mechanical tests showed that pre‐stress can increase the tensile, flexural and interlaminar shear properties of the composites, and there exists an optimum pre‐stress level to gain best properties for each external loading condition.

Analysis of microresidual stresses in 6H-SiC particles within Al 2O 3-SiC-(Al,Si) CMC using raman spectroscopy

Residual stresses can be generated in composites under two conditions (1): (a) processing of the composite can lead to ‘microresidual stress’ and (b) machining operations, like grinding can lead to ‘macroresidual stress’ (2). The genesis of microresidual stress is the mismatch in the thermal expansion coefficient between the matrix and the reinforcing phase. The term ‘microresidual stress’ is used to denote stress which varies in magnitude over dimensions that are of the order of the microstructural scale. ‘Macrostresses’ generally arise near the composite surface because of the differences in processing between the surface and the bulk. Both these stresses are well known to affect the structure and the behaviour of composites (3,4). The influence of these stresses on the properties of composites may result in strong positive or negative effects depending upon their sign and magnitude. For example, local radial tension in the matrix near a particle-matrix interface can lead to crack deflection (5). Hence, information on the sign and magnitude of the residual stress help to tailor the structure and properties of the material for better performance.

Investigation of thermal residual stresses in layered composite using the finite element method and X-ray diffraction

A SiC particulate-reinforced 2024Al matrix composite was fabricated using a spray atomization and codeposition process to produce a layered macrostructure. The thermal macro- and microresidual stresses that develop in this layered 2024Al/SiC composite during cooling from the codeposition temperature to ambient temperature were studied using thermoelastoplastic finite element analysis. The calculated residual stresses from finite element method were compared with those obtained from X-ray diffraction (XRD), and good agreement was observed between them. The macroresidual stress distribution was very distinct for the Al and the SiC-rich layers. The macroradial stress was tensile in the Al layers and compressive in the SiC-rich layers. The macroaxial stress was found to be compressive in the vicinity of the center region of the spray-deposited material and to have mostly a continuous distribution in the Al and the SiC-rich layers. The magnitude of the macroaxial stress was noted to decrease with increasing deposition thickness from the bottom. In addition, the spray-deposited material exhibited the highest macro-von Mises’ stress around the outer edge of the deposited material. The microradial stress was in a compressive state in the SiC particulate and in the Al matrix in the vicinity of the SiC particulate. The SiC particulate exhibited compressive microhoop stress, whereas the Al matrix exhibited tensile microhoop stress. The micro-von Mises’ stress was of the highest value at the interface between the SiC particulate and the Al matrix.

Mechanical properties of composites fabricated by pressureless infiltration technique

Abstract Two-phase interpenetrating systems with residual microstresses have been shown to exhibit high fracture toughness and strength. In this work, the microstress-induced strengthening concept was applied to Al 2 O 3 Si composites. The composites were fabricated by pressureless infiltration of porous Al2O3 preforms with a molten Si. The materials obtained exhibited superior fracture toughness, strength and hardness when compared with technical aluminas having the same volume fraction of Al2O3 but containing aluminosilicate glass phases instead of silicon. For example, an Al 2 O 3 Si composite with 30 vol% Si had K1c ≈ 4.8 MPa √m and a bending strength of ~ 320 MPa compared to ~ 3.5 MPa √m and ~ 230 MPa, respectively for the technical alumina AD-85 (‘Coors’). It is believed that the strengthening effect is caused by compressive residual microstresses in an inherently weak Si phase generated as a result of the solidification-related expansion of silicon.

Measurement of Micro-Residual Stresses in Fiber-Prestressed Composites

In the present work, micro-residual stresses are measured in fiber prestressed composites. This is carried out by evaluation of residual strains in the fibers subsequent to the curing process. The force-strain curve for the glass fiber before, during and after setting of the epoxy resin shows that some part of the fibers' strain, caused by pretension, is not recovered when the tension is removed at the end of the curing process. The unrecovered strain in the fibers is used to assess the micro-residual stresses in the composite. Experimental results indicate that the micro-residual stresses in the fibers are linear functions of the applied pretension on the fibers during cure.

Microstructural dependence of thermal residual microstresses in PMMCs

During cooling of particle reinforced metal matrix composites (PMMCs), residual microstresses emerge due to differences in the properties of the metal matrix and the ceramic hard phases. The residual microstresses strongly depend on microstructural parameters. In order to investigate this experimentally, neutron diffraction residual stress analysis was applied to Fe- and Ni-base alloys containing NbC and WC hard phases.

Thermal residual microstresses in steel-NbC particulate composites studied by X-ray and neutron diffraction

During cooling of particle reinforced metal matrix composites (PMMCs) residual microstresses emerge owing to the different physical and mechanical properties of the metal matrix and the coarse quasi-ceramic hard phases. The thermal residual microstress state at the sample surface and in the bulk are analysed using X-ray diffraction and neutron diffraction methods, respectively. While the X-ray diffraction results reveal a strong influence of the sample preparation on the surface residual stress state the results obtained by neutron diffraction are in good agreement with theoretical considerations.

Effects of Grain Size and Humidity on Fretting Wear in Fine‐Grained Alumina, Al2O3/TiC, and Zirconia

Friction and wear of sintered alumina with grain sizes between 0.4 and 3 μm were measured in comparison with Al2O3/TiC composites and with tetragonal ZrO2(3 mol% Y2O3). The dependence on the grain boundary toughness and residual microstresses is investigated, and a hierarchical order of influencing parameters is observed. In air, reduced alumina grain sizes improve the micromechanical stability of the grain boundaries and the hardness, and reduced wear is governed by microplastic deformation, with few pullout events. Humidity and water slightly reduce the friction of all of the investigated ceramics. In water, this effect reduces the wear of coarser alumina microstructures. The wear of aluminas and of the Al2O3/TiC composite is similar; it is lower than observed in zirconia, where extended surface cracking occurs at grain sizes as small as 0.3 μm.

Thermal residual stress distribution in WC-Ni composites

The thermal residual stress distributions in a series of WC-Ni composites with WC volume fractions of 0.11, 0.21, 0.41, and 0.60 were measured using neutron diffraction and modeled using finite element (FE) analysis. The FE procedure employed enables the local variation of the residual microstress in a composite microstructure to be studied. The mean (compressive) stresses in WC decrease and the mean (tensile) stresses in Ni increase with increasing WC content. The stress distributions are sufficiently extensive to create regions of tension in the WC and compression in the Ni. The maximum tensile stresses in WC increase in magnitude with WC content. They are located in corners and, for higher WC content, normal to WC/Ni interfaces. High compression occurs in narrow portions of the WC skeleton. Regions of compression in Ni occur at WC/Ni interfaces for low WC content and, for composites of higher WC content, in narrow bands between chains of WC particles. It was also determined that models of actual composite microstructures, rather than geometric arrays, are required in order to properly obtain local residual stress distributions using the finite element analysis procedure.

Residual stress in WC-Co measured by neutron diffraction

Large thermal residual microstresses (TRS) can develop in WC-Co composites owing to the difference of the coefficients of thermal expansion (CTE) of the constituents. The variation with temperature of average stresses in a WC-11 wt.% Co sample were studied between room temperature and 1273 K by measuring the cell parameters of cobalt and WC using neutron diffraction. WC powder was also measured to provide stress free reference standards. At room temperature, a hydrostatic compressive stress of about 500 MPa was measured in the WC. The evolution of TRS shows two temperature domains. The low temperature domain (300 < T < 1000K) is characterised by a decrease of residual stress magnitude as the temperature is increased. The high temperature domain ( T > 1000 K) is characterized by an increase of residual stress in WC, a rapid increase of Co lattice parameter, and a hysteresis between the heating and cooling cycles. A model, based on Eshelby's equivalent inclusion method, predicts the observed behavior in both domains. In the low temperature domain, the CTE mismatch between WC and Co accounts for the decrease of TRS upon heating. In the high temperature domain, the system is modelled by the solution of a layer of WC in the Co, which increases the Co lattice parameter and leads to an increase of compressive stress in WC. The model indicates that there is 2.09 at.% W in solution in the cobalt. The hysteresis is attributed to a difference in the heating and cooling kinetics of solution-precipation of W from WC and WCo 3. The results are compared with the mechanical properties of WC-Co.

Grain size dependent residual microstresses in submicron A1 2O 3 and ZrO 2

X-ray measurements of lattice spacings are used to determine residual microstresses present in sintered alumina and in tetragonal zirconia polycrystals due to thermal expansion anisotropy (TEA). In A1 2O 3 with grain sizes larger than 1 μm the microstresses are 30–100 MPa, in submicrometer samples the grain size influence becomes small and residual stresses range between 20 and 30 MPa. For the grain sizes between 0.3 and 9 μm there is no indication of a change in the high-temperature relaxation mechanism. In ZrO 2 with grain sizes of 0.5–1 μm the residual stresses are similar as observed in A1 2O 3 (20–60 MPa), they decrease further at grain sizes 0.2–0.4 μm. The results are independent of technological approaches like powder processing or sol/gel used to produce the sintered bodies.

Effect of pressure rolling on the residual stress state of a particulate-reinforced metal matrix composite

The large difference between the coefficients of thermal expansion of the matrix alloy and the particle in a metal matrix composite gives rise to residual stresses in the material. In the present work, the effect of pressure rolling on the residual stress state of a silicon carbide particle-reinforced 2014 aluminium alloy has been investigated. The three-dimensional stress state measured in both phases-matrix and reinforcement-has been determined using an X-ray diffraction technique. A twofold effect of pressure rolling on the residual stresses was observed. On the one hand, compressive macrostresses as large as - 250 MPa were induced. On the other hand a significant reduction in pseudo-macrostresses was measured where the plastic deformation reaches a maximum. A modified Eshelby model was used to predict quantitatively and qualitatively the residual microstresses after heat treatment and pressure rolling respectively.

Physical Limitations of the Inherent Toughness and Strength in Ceramic-Ceramic and Ceramic-Metal Nanocomposites

Toughening and strengthening effects, addressed as intrinsic material properties, are discussed in the case of ceramic-ceramic and ceramic-metal nanocomposites according to fracture mechanics argument. The theoretical approach followed is conceptually the same as that usually adopted for explaining the mechanical behavior of brittle metals, polymers or other monolithic ceramics. The theory cannot substantiate any tangible synergistic effect by nanosized ceramic dispersoids, it is worked out either in terms of toughness or in terms of strength. It is suggested that the improved mechanical properties, as reported in some ceramic-ceramic nanocomposites, may be related to residual microstresses stored into the material after the sintering process or even be affected by extrinsic factors related to the specimen preparation and/or to the testing procedure. Addition of metallic nanodispersoids seems to be a potentially suitable way to obtain a 30-40% increase in the inherent material strength, although a much larger dispersoid size may be required for gaining better toughness. The present theoretical treatment, although simplified in order to obtain solutions in close form, may provide some general and physically sound directives for the development of ceramicmatrix composites. This study intends to provide a theoretical counterpart to recent empirical practices in the field which are lacking in scientific logic.

Thermal residual microstress generation during the processing of unidirectional carbon fibre/epoxy resin composites: random fibre arrays

Finite element models are developed to predict the thermal residual stresses generated during the processing of carbon fibre/epoxy resin composites. A previous analysis that assumed regular arrays of fibres is extended to include random fibre arrays, which are thought to be more appropriate to the actual fibre distributions. The stresses are local to single fibres under conditions of plane strain. Initially a hypothetical distribution is assumed which is intermediate between a hexagonal and square array. This model consists of a representative selection of the unit cells used in the analysis of the regular fibre arrays. The fibre volume fraction of the model is 27% and the fibre separation ranges from 1.0 to 12.0 μm. Subsequently a computer program is used to generate random fibre arrays and a region local to a single fibre in a 50% fibre volume fraction array is meshed for finite element analysis. The results are compared with those obtained assuming regular fibre arrays. Observations are made also of actual fibre arrays under an optical microscope; these are compared with the fibre arrays assumed in the modelling. The change from a regular to a random arrangement generally increases the maximum tensile stress produced in the epoxy resin. This tends to occur at positions within the smallest gap between two fibres.

Thermal residual microstress generation during the processing of unidirectional carbon fibre/epoxy resin composites: regular fibre arrays

The thermal residual microstresses generated during the processing of unidirectional carbon fibre/epoxy resin composites are predicted, assuming regular fibre arrays. Stresses are determined in unit cells covering a range of fibre coordinations, fibre diameters, minimum interfibre distances and fibre volume fractions. The method of calculation involves the finite element method. Elastic material behaviour with temperature-dependent epoxy resin properties are assumed, together with transversely isotropic carbon fibres. Of the parameters studied, the greatest effect on the maximum principal stress was produced by the minimum thickness of epoxy resin between the fibres and the ratio of this thickness to the fibre radius. Values of the maximum principal stress were found in some cases to exceed the tensile strength of the epoxy resin. However, there was little experimental evidence to support this prediction. Cracks occurred only to a limited extent and occurred around the fibre/epoxy interface, rather than between the fibres, as predicted by the model. Reasons suggested for this discrepancy include relatively weak fibre/epoxy resin bonds and limitations on the accuracy of the stress generation model. Methods by which the model may be improved are discussed.

Residual stresses in a SiC whisker-reinforced alumina composite by high-temperature X-ray diffraction

Residual strains and microstresses are evaluated for both phase of a hot-pressed, fine-grained α-alumina reinforced with 25 wt% (29 vol%) single-crystal silicon carbide whiskers at temperatures from 25 to 1000 °C. The sample was maintained in a nonoxidizing environment while measurements of the interplaner spacing of alumina (146) and SiC (511 + 333) were made using X-ray diffraction methods. The residual strains were profiled at temperature increments of 250 °C from which the corresponding microstresses were calculated. Linear extrapolation of the SiC ε33 profile indicates that the strains are completely relaxed at a temperature of approximately 1470 °C. These residual stress relaxation results suggest that elevated temperature toughness and fracture strength of this composite may result from cooperative mechanisms.