Showing papers on "Residual stress published in 1985"

Failure Mechanisms in Ceramic‐Fiber/Ceramic‐Matrix Composites

Abstract: Mechanisms of failure in a unidirectional Sic-fibe/glass-ceramic composite are investigated using in situ observations during tensile and flexural loading. These experiments show that failure in tension occurs in several stages (similar to certain other brittle fiber composites): multiple matrix cracking, followed by fiber fracture and pullout. In flexural loading the failure process is more complex. Consequently, the flexural test cannot be used for measurement of tensile strength (although it can be used for measurement of the stress for matrix cracking). The application of conventional fracture mechanics to describe tensile failure is discussed. The in situ observations provide direct indication of the importance of frictional bonding between the matrix and fibers. Some novel methods for measuring the frictional forces and residual stresses are investigated, and the influence of surface damage on strength is assessed.

Characterization of granular material

Abstract: Evaluation of the resilient modulus of granular material as used in design and structural evaluation of flexible pavements is addressed. It is shown that the well-known equation relating the modulus to the sum of principal stresses does not properly describe granular behavior: the predicted response is not compatible with laboratory test results that show a strong dependence of the modulus on the stress ratio from which the equation is derived. A general law that includes the effects of shear strains is shown to be in good agreement with test results. The response of nonlinear materials is sensitive to their state of stress during loading. A literature review covers case histories of full-scale retaining wall models with compacted backfill, where residual stresses induced by compaction were measured. A proposed theory based on limit equilibrium is reported to give good estimates of residual stresses. It is found that granular base and subbase materials, compacted with heavy rollers, may develop relatively high residual horizontal stresses. The general law for characterizing granular materials is used with different postulated residual stresses in pavement analyses. The results obtained appear to be in good agreement with all aspects of granular material behavior, provided that a residual stress of the order of 1 to 2 psi is assumed to be induced by compaction.

Neutron diffraction methods for the study of residual stress fields

Abstract: A technique is described for the measurement of the residual stress tensor averaged over a specified volume within a component. The method involves measurement of small changes in lattice spacing using high resolution neutron diffraction. The stress is inferred from these measurements of the strain, and the theory of the relationship between the two quantities is described, including the effects of crystalline anisotropy. The various types of high resolution neutron diffractometer suitable for the work are described. Experimental results validating the method are given for a simple bent bar of mild steel of known strain, a plastically strained mild steel bar, and a mild steel tube of known torsional strain. Examples of the method in practical use are given by a cracked fatigue test specimen, a double-V test weld and a weld joining a tube to a plate. A more detailed example is the anisotropic response of a polycrystalline sample under elastic and plastic strain; this is illustrated by measurements...

Neutron diffraction methods for the study of residual stress fields

Abstract: A technique is described for the measurement of the residual stress tensor averaged over a specified volume within a component. The method involves measurement of small changes in lattice spacing using high resolution neutron diffraction. The stress is inferred from these measurements of the strain, and the theory of the relationship between the two quantities is described, including the effects of crystalline anisotropy. The various types of high resolution neutron diffractometer suitable for the work are described. Experimental results validating the method are given for a simple bent bar of mild steel of known strain, a plastically strained mild steel bar, and a mild steel tube of known torsional strain. Examples of the method in practical use are given by a cracked fatigue test specimen, a double-V test weld and a weld joining a tube to a plate. A more detailed example is the anisotropic response of a polycrystalline sample under elastic and plastic strain; this is illustrated by measurements...

Residual Stresses in Meta/Ceramic Bonded Strips

Abstract: Residual stresses that develop during cooling of a metal/ ceramic strip are calculated analytically. It is shown that the metal may behave elastically or plastically (with full or partial plasticity) depending on the mechanical properties, the thickness of the two constituents, and the mismatch in thermal expansion. Residual stresses are also calculated for a sequence consisting of constrained undercooling, removal of the constraint, and reheating. It is demonstrated that reheating, which results in elastic stress relaxation, may be used to eliminate the residual stress. The optimum undercooling and reheating conditions needed to produce a stress-free strip, at the operational temperature, are calculated, and specific results are presented for the system Cu/A2, O2.

Stress–phase-transformation interactions – basic principles, modelling, and calculation of internal stresses

Abstract: The two main effects of stress on phase transformation, kinetics modification and transformation plasticity, are reviewed for both diffusional and non-diffusional transformations. Results for these interactions during the pearlitic and martensitic transformation of steels under uniaxial tensile stress are analysed from a metallurgical point of view. These results are used to produce a model for a triaxial stress state, and in a finite element program for calculating internal stresses during quenching. Transformation plasticity is introduced in the calculation of internal stresses as an additional strain related to the stress state and to the progress of transformation, and the kinetics of martensitic transformation are also related to the stress state. The calculated results show that these phenomena have important consequences on the stress and plastic strain histories during quenching.MST/9

Matrix solidification and the resulting residual thermal stresses in composites

Abstract: 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.

Thermoelastic analysis of residual stresses in unidirectional, high‐performance composites

Abstract: We present a thermoelastic analysis of the composite cylinder model for a undirectional composite including anisotropic fibers and an interphase region. We have found the magnitude of the residual thermal stresses on the micromechanics level induced by differential shrinkage between the anisotropic fibers and the matrix. For typical composites the largest residual stress is tension along the fiber direction, and a simple lower bound expression for this stress is given. Prediction of the magnitude of the thermal stresses requires knowledge of the thermal and physical properties of the matrix. The relevant properties for epoxy and thermoplastic matrices are discussed. The magnitude of the residual stresses can be reduced by tailoring the interphase region, but only if the interphase region serves to reduce the temperature for the onset of stress buildup. The volume fraction dependence of the longitudinal and transverse thermal expansion coefficients of the composite is compared to analogous expressions in the literature which do not include anisotropy of the fibers.

Measurement of residual stresses by neutron diffraction

Abstract: A neutron diffraction method has been applied to measure residual stresses in rings sliced from as-recieved and autofrettaged thick-walled steel tubing. The principles of neutron diffraction are outlined and the method is described. A comparison is made with results obtained using the more traditional Sachs boring technique. It is shown that satisfactory agreement is achieved between the two procedures and that the neutron diffraction method can be used with acceptable accuracy to measure residual stress non-destructively in bulk samples of steel several centimetres thick.

Residual Stresses and Cracking in Metal/Ceramic Systems for Microelectronics Packaging

Abstract: Residual stresses that develop in a metalkeramic system due to thermal expansion mismatch were calculated for a workhardening metal. The calculations were conducted for a cylindrical configuration, pertinent to certain microelectronics packaging systems. Experimental measurements of the stress have also been made on a Cukordierite ceramic system, using an indentation technique. It is shown that porosity in the metal can plastically expand and provide a mode of dilatational relaxation. Porosity in the metal thus emerges as an important stress-relaxing mechanism.

The effect of binder thickness and residual stresses on the fracture toughness of cemented carbides

Abstract: Much of the data on WC-Co cermets show that the fracture toughness,KIc, increases with increasing tungsten carbide grain size at fixed volume fraction of the cobalt binder phase. It is shown that the origin of this effect can be explained on the basis of the plane stress fracture of constrained cobalt phase and the periodic internal stresses arising due to differential thermal contraction of the two phases. Quantitative models have been derived which take these two effects into account. The effect of macroscopic residual stresses, such as those generated by milling WC-Co drilling inserts, on the apparent toughness has also been analysed. It is shown that for the chevron-notched type specimen the macroscopic residual stress affects not only the maximum load but also the length of the crack at which the maximum occurs. A graphical method is presented which permits the evaluation of the true K‡Ic.

Analytical and graphical aids for the fatigue design of weldments

Abstract: There are four important attributes of weldments which, together with the magnitude of the fluctuating stresses applied, determine their resistance to metallic fatigue: the ratio of the applied or self-induced axial and bending stresses; the severity of the discontinuity or notch which is an inherent property of the geometry of the joint; the notch-root residual stresses which result from fabrication and subsequent use of the weldment; and the mechanical properties of the notch-root material in which fatigue crack initiation and growth takes place. A simple version of a more elaborate analytical model of the fatigue behavior of weldments [1] is presented here which provides analytical and graphical design aids for design.

Measurement of residual-stress distribution by the incremental hole-drilling method

Abstract: The hole-drilling method is widely used to measure residual stresses in mechanical components. Recent developments have shown that strains measured on the surface during an incremental drilling can be related to residual-stress distribution. Researchers throughout the world have proposed different calibration methods which lead to more or less accurate results. The present paper discusses different approaches used. A new calibration method is proposed. We also show how finite-element analysis can be used to determine the correlation coefficients. The variation of the strains measured on the surface for each increment is due to, first, the residual stresses in the layer and, second, the change of the hole geometry. Most authors do not consider the latter aspect. Our results show that this causes a significant error in the experimental data. The finite-element method has been used to compute the coefficients for all types of strain-gage rosettes when the hole diameter is predetermined. Another problem of the hole-drilling method is the selection of the drilling tool. Two systems have been studied: ultra-high-speed air turbine and conventional milling machine. The method has been applied on both shot-peened and water-quenched test specimens. The results are successfully compared with the bending-deflection and the X-ray method.

Dependence of acoustoelastic birefringence on plastic strains in a beam

Abstract: Acoustoelastic birefringence is measured by transmitting ultrasonic shear waves at two perpendicular polarizations through the thickness of an aluminum beam. From the data obtained before bending, after bending, and after the beam has been sectioned to relieve the residual stresses, the acoustoelastic birefringence induced by residual stress is separated from that induced by texture change. Supplemented with strain‐gauge data at all these states, the texture‐induced anisotropy is shown to be related to plastic (permanent) strains in the beam. A nearly linear relationship is observed in compressional deformation of 6061‐T6 aluminum up to −0.6% plastic strain in the beam, whereas the birefringence is almost independent of tensile plastic strain.

Improvements in grinding steels by cryogenic cooling

Abstract: The intensive temperatures in high-speed machining and grinding not only limit the tool life but also impair the machined surface by inducing tensile residual stresses, micro-cracks and thermal damage. This problem becomes acute when the components are made of hard and strong materials and used in dynamic loading. In such cases drastic cooling is essential during machining. The authors have employed liquid nitrogen in the form of a jet in the surface grinding of a number of steels and found appreciable improvement, though to varying degrees, in their surface quality without significant change in specific energy requirement.

Analysis of residual stress and distortion resulting from quenching in large low-alloy steel shafts

Abstract: In order to predict residual stresses and distortion caused by quenching in a large low-alloy steel shaft, a computer program based on the finite element method was developed, and uniaxial restraint testing of the transient strain during cooling was carried out. Using the program, the transient stresses occurring during quenching are calculated by a step-by-step procedure, and the effects of transformational behaviour on residual stresses and distortion discussed. The results show that the transient stresses and the transformation affect each other, and that residual stresses and distortion are strongly related to the transformational behaviour.MST/8

Morphology, Composition, and Residual Stresses of Compound Layers of Nitrocarburized Iron and Steels

Abstract: Specimens of pure iron, an iron-carbon (1 wt-%C) alloy, and steels Ck 45 and En 40B (=24CrMo13) were salt bath nitrocarburized. M etallographic examination was performed after Murakami etching. This etchant provides a sensitive discrimination between pure nitrides and carbon containing nitrides ( carbonitrides). Thus the two-phase (e and γ' ) constitution, as well as the carbon concentration profile within the compound layer, was revealed. Nitrogen, carbon, oxygen, and sulphur concentration profiles were quantitatively determined by electron microprobe analysis. Residual stress measurement and phase analysis were carried out by X-ray diffraction methods. Both macros trains (by analysis of line shift using the sin2 Ψ method) and microstrains (by analysis of line broadening using a Voigt single-line method) were determined. In view of a possible interaction between residual stresses in the compound layer and the diffusion zone, residual stresses were also determined in the diffusion zone beneath the...

Effect of Contact Residual Stress Relaxation on Fracture Strength of Indented Soda‐Lime Glass

Abstract: The strength of Vickers-indented soda-lime glass measured in air at room temperature steadily increases with time after indentation, whereas optical retardation steadily decreases in the same interval. Annealing after indentation causes further strength increase and retardation decrease. The results are consistent with Marshall and Lawn's treatment of the slow crack growth of indentation flaws driven by the combined influence of residual contact stress and applied stress. Post-indentation strengthening of indentation flaws can be explained without recourse to flaw blunting.