Showing papers on "Residual stress published in 1976"

A Finite-Element Analysis of Fatigue Crack Closure

Abstract: Experiments have shown that fatigue cracks close at positive loads during constant-amplitude load cycling. The crack-closure phenomenon is caused by residual plastic deformations remaining in the wake of an advancing crack tip. The present paper is concerned with the application of a two-dimensional, nonlinear, finite-element analysis for predicting crack-closure and crack-opening stresses during cyclic loading. A two-dimensional finite-element computer program, which accounts for both elastic-plastic material behavior and changing boundary conditions associated with crack extension and intermittent contact of the crack surfaces under cyclic loading, has been developed. An efficient technique to account for changing boundary conditions was also incorporated into the nonlinear analysis program. This program was subsequently used to study crack extension and crack closure under constant-amplitude and two-level block loading. The calculated crack-closure and crack-opening stresses were qualitatively consistent with experimental observations.

Residual Stresses in Polymer Matrix Composite Laminates

Abstract: Residual stresses in composites are induced during fabrication and by environmental exposure. The theory formulated can describe the shrinkage commonly observed after a thermal expansion test. Comparison between the analysis and experimental data for [02/± 45]s laminates of various material systems indicates that the residual stress-free temperature can be lower than the curing temperature, depending on the curing process. Ef fects of residual stresses on ply failure including the acoustic emission characteristics are discussed.

On the Hygrothermal Response of Laminated Composite Systems

Abstract: The hygroscopic nature of polymeric systems, which find widespread application as matrices in advanced composite materials, requires that dila tations induced by the absorption of moisture be considered m the stress analysis of composite laminates. Considerable attention has recently been focused upon the reduction m both strength and constitutive properties of fiber-reinforced polymeric composites at elevated temperatures when the composite has been subjected to environments which enhance moisture diffusion. This apparent degradation in elevated temperature properties may be magnified even more by residual stresses induced by both the hygroscopic and thermoelastic characteristics of the unidirectional com posite.A unified treatment of the hygrothermal response of the laminated composite plate element is derived. The analysis develops effective mois ture inplane force resultants and bending resultants, which when coupled with mechanical and thermal loadings, yield laminae stresses resulting from the total...

Residual stresses in polymers and their effect on mechanical behavior

Abstract: Plates of bisphenol-A polycarbonate and poly(methyl methacrylate) have been quenched in ice water from temperatures slightly above their glass transition temperatures. Residual stresses are thus created, Measurement of these residual stresses has been accomplished by the “layer removal” method and the stress distributions through the thickness are presented. Compressive stresses, approximately 3000 psi, exist at the surface while tensile stresses-of at least 1000 psi exist in the interior. It is shown that these residual stresses can influence the notched Izod impact strengths for polycarbonates. The mechanism is thought to be suppression of craze initiation in advance of the notch due to the presence of residual compressive stresses for specimens notched prior to quenching. In the case of poly(methyl methacrylate), it is shown that compressive residual stresses at the surface can cause plastic yielding to occur in bending experiments resulting in permanent deformation and greater energy absorption.

The propagation of surface waves in bodies with material boundaries

Abstract: A study is made of the propagation of Love and Rayleigh waves in elastic bodies whose boundaries support surface stresses, such as crystals cleaved in vacuo or bodies upon which are deposited extremely thin films. The surfaces of such bodies are herein modelled as two-dimensional elastic continua that adhere without slipping to their substrates and in which is located residual stress. The corresponding boundary condition gives rise to behaviour which is contrasted with that occurring in the absence of surface stress. The results are also related to previous studies of surface waves in thin strata superposed on elastic half-spaces, from which the present theory differs by taking into account residual stress. The behaviour is shown to be sensitive to the relative values of the residual stress, elastic moduli, and density of the surface. Attention is drawn to the relevance of the study to those signal-processing devices which utilize surface waves in thin films.

Accurate measurement of residual stress on any steel using the centre hole method

Abstract: The centre hole method has been shown to be a simple and reasonably accurate method of measuring residual stress. With a 1.6 mm diameter hole, the method can be applied on operational plant and the equipment required is sufficiently portable for use on site. Its application has, however, been limited to use on low-strength steels, due to problems associated with forming the hole. This paper describes the development of an air-abrasive technique for forming the hole, which allows the centre hole method to be used on any steel with a measurement accuracy of better than±8%.

A method of determining the elastic properties of alloys in selected crystallographic directions for x-ray diffraction residual stress measurement

Abstract: A technique and apparatus are described for obtaining the elastic constant E/(1 + ν) in selected crystallographic directions for the purpose of calibrating x-ray diffraction residual stress measurement methods. The preparation of a simple rectangular beam specimen with two active electrical resistance strain gages applied to the test surface is described. Samples are clamped in a diffractometer fixture designed to minimize displacement errors, and loaded in four-point bending to several stress levels below the proportional limit. A method is described for calculating E/(1 + ν) and an estimate of the experimental error. Values of E/(1 + ν) obtained for several alloy-(hkl) combinations are presented. The results indicate that several alloys of current commercial interest exhibit significant elastic anisotropy.

Residual stress in silicon nitride films

Abstract: The mechanical stress caused by Si3N4 films on (111) oriented Si wafers was studied as a function of the Si3N4 film thickness, deposition rate, deposition temperature and film composition. The Si3N4 films were prepared by the reaction of gaseous SiH4 and NH3 in the temperature range 700–1000°C. The curvature of the Si substrates caused by the Si3N4. films is related to the film stress; the substrate curvature was measured by an optical interference technique. The measured Si3N4. film stress was found to be highly tensile with a magnitude of about 1010 dynes/cm2. For the thickness range of 2000–5000A, there was no change in the measured stress. The total film stress was observed to decrease for decreasing deposition rate and increasing deposition temperature. A large change in film stress was observed for films containing excess Si; the stress decreased with increasing Si content. Based on published values for the thermal expansion coefficients for Si and Si3N4, a published value for Young’s Modulus for Si3N4, and the measured total stress values, a consistent argument is developed in which the total stress consists of a compressive component due to thermal expansion coefficient mismatch and a larger tensile intrinsic stress component. Both the thermal and intrinsic stress components vary with film deposition temperature in directions which decrease the total room temperature stress for higher deposition temperatures.

Impact strength of polymers: 1. The effect of thermal treatment and residual stress

Abstract: The effect of thermal annealing and quenching on the notched Izod impact strength of several polymers has been studied. Primary emphasis was placed on polycarbonate, but ABS, PVC, polysulfone, and polymethylmethacrylate were also studied. It was determined that residual stresses created by thermal quenching from above the glass transition temperature can have a great effect on impact strength for the polycarbonate, PVC, and polysulfone polymers studied. In fact, it is shown that the thickness transition observed in impact strength for polycarbonates is governed by the residual stresses and not by thickness. In polycarbonates, quenched sheets up to 3/8 in. in thickness have shown impact strengths of 18 ft-lb/in. whereas sheets 1/8 in. in thickness can be embrittled by annealing, showing an impact strength of 2 ft-lb/in. However, it has been shown that this embrittlement results from the absence of residual stress. Residual stresses having maximum values up to 3000 psi (in Compression) have been determined at the polycarbonate sheet surface using birefringence measurement techniques. The existence of these compressive stresses is postulated to restrict the extent of craze growth at the notch tip, and the impact specimen can yield rather than fail in a brittle manner if the stress state is sufficient.

Effects of Residual Stresses on Fracture from Controlled Surface Flaws

Abstract: RFracture tests were conducted on surface flawed Si/sub 3/N/sub 4/ bend specimens as function of surface removal Results show that residual stresses at the Knoop microhardness indentation affect fracture of the controlled surface flaw beneath the indentation Annealing was found to heal surface flaws and because of this, the relative contribution of residual stresses to the effects of annealing on the critical stress intensity factor in Si/sub 3/N/sub 4/ could not be assessed (JRD)

Stress measurement in polymeric materials by X‐ray diffraction

Abstract: Experiments are described which show that stresses in amorphous polymers can be measured by diffracting X-rays at high Bragg angles from a filler consisting of crystalline particle or powders. The method is applicable, when calibrated, to both applied and residual stresses, to stress relaxation studies, to both tensile and compressive stresses, to both interior and surface positions in an object, and to composites of various types as well as to polymers that are substantially homogeneous. In the lower (Hookian) range, strains and stresses in metallic embedded particles increase linearly with applied stresses and strains in the matrix. When applied stresses exceed an apparent yield point, which correlates with the yield strength of the metallic filler, the elastic strains in the particles increase only slightly or even decrease as the matrix strains are increased, and with constant applied strain, the particles reveal changes due to relaxation. Strains in the particles are found to be smaller, and stresses higher, than in the matrix. Tests of graphite-fiber epoxy composites with embedded filings of silver or aluminum alloy show that compressive residual stresses from curing at 350°C and cooling to room temperature are registered in the particles as well as stresses externally applied to the cured composite. Precision of the order of 1000 psi (0.7 kg/mm2 or 6.9 M Pa) was obtained with measurements made and evaluated rapidly and simply; with more refined techniques, much higher precision would be possible.

Calculation of Residual Thermal Stresses in Injection Molded Amorphous Polymers by the Finite Element Method

Abstract: The finite element method (FEM) is used to calculate the distribution of residual internal stresses in an injection molded plate-shaped polystyrene specimen. The first step of the calculating procedure is the determination of the temperature distributions in the plate and its variation with cooling time. The temperature distribution data are then used for the determination of the corresponding stress distribution. Also this latter step is accomplished using the FEM-technique. The residual stress distribution is obtained when the sample has been cooled to the temperature of the mold. The main result of the calculations is the finding that the surface layer of the plate is subject to compressive stresses, while the interior accommodates stresses of tensile type. The calculations relate to a polystyrene specimen. The results are shown to agree well with earlier data concerning internal stresses in injection molded objects. The possible influence of anisotropy and visco-elastic relaxation has been ne...

Spectrum Fatigue Crack Growth in Lugs

Abstract: Analytical and experimental investigations were conducted to determine the crack growth behavior in lug joints subjected to a randomized flight-by-flight spectrum. Specific objectives were to evaluate capabilities for analyzing spectrum crack growth in lugs and to assess the effects on crack growth of K I c and cold working the material surrounding the lug holes. Stress intensity solutions were developed for cracks radiating from lug holes using Bueckner's weight function approach for various stress distributions surrounding the lug hole. The stress distributions were determined using an elastic/plastic finite element analysis. Included in these analyses were residual stress distributions following either large lug loads or mandrel hole enlargement. Spectrum crack growth calculations were made using the resulting stress intensity values and the Wheeler retardation crack growth model. Lug specimens were fabricated from two heats of 6Al-4V mill annealed titanium representative of material having low and high fracture toughness levels. Basic mechanical properties including ultimate and yield strength, percent elongation, and reduction in area and fracture characteristics, including K I c and constant-amplitude crack growth rates, were determined for each heat of material. EDM cuts in the lug specimens were used as points of stress concentration where precracks were developed by low-level constant-amplitude fatigue loading. The lug holes of selected specimens were cold worked to determine the effect of compressive residual stresses on the crack growth rate. Plots of crack growth versus spectrum hours were obtained for each specimen by post-failure examination of the fracture surfaces using the scanning electron microscope.

A modified hole-drilling technique for determining residual stresses in thin plates

Abstract: This paper presents a modified hole-drilling technique for measuring residual stresses in sheet and thin-plate materials. The primary advantage of the modification is that it eliminates the necessity for calibration of each experimental hole-gage assembly. The relaxation coefficients are calculated from theory, and the strain components which are extraneous to the true relaxation strains are determined and separated from the measured relaxation strains. Experiments were conducted on 0.050-in. (1.27-mm) and 0.125-in. (3.175-mm)-thickness aluminum-alloy specimens. Sources of extraneous strain components are analyzed and values for these strain components resulting from machining residual stresses and localized plastic yielding are determined. Finally, the recommended range of the nondimensional ratio of hole diameter to distance between hole center and strain-gage center is determined by the maximum permissible error in residual-stress estimates. The modified technique appears to be accurate within ±5 percent or better and is, therefore, comparable in precision with the X-ray technique.

The indentation behaviour of hard metals

Abstract: The indentation behaviour of hard materials which deform plastically and also exhibit surface fracture is discussed. It is shown that both radial and circumferential fracture can develop around the indentation and that this can be explained in terms of a transition from elastic/plastic behaviour. In general the circumferential cracks form at lower loads but remain short and shallow, and their initiation is affected by the grain size of the material. Experiments with a hard spherical indenter showed that radial cracks in water-quenched EN44B steel develop at loads above approximately 40 F*, where F* is the applied load to cause the first plastic deformation. Residual stresses in the surface layers can cause these cracks to continue growing after load removal. Finally, an equation relating contact area and pressure is developed from earlier work of K L Johnson (1970) and it is shown that this gives good agreement with experiment.

Surface cracking in proton-irradiated glass

Abstract: It is well known that radiation (both particle and photon) can cause substantial density changes in certain solids. In cases where the radiation damage is non-uniformly distributed, these changes are manifested as gradients of residual stress. With ion bombardment, where penetrations under accelerating voltages of several hundred thousand volts are typically on the micrometre scale, one may reasonably describe the mechanical damage in terms of a lateral “surface stress.” Now if the irradiated material is brittle, the possibility exists of such stresses causing incipient surface flaws (so-called Griffith flaws, present in abundance on all typical brittle surfaces)1 to grow into dangerous large-scale cracks. Measurements on a number of brittle solids2–6 reveal a general tendency for the level of radiation-induced surface stress to increase steadily with fluence up to a maximum, beyond which saturation (or even decline) sets in. Of the solids investigated, fused is unique in that the surface stress ...

Residual Stress in Damaged SrTiO 3 Single Crystals

Abstract: The residual stress introduced in SrTiO3 crystals by lapping, grinding and cutting is examined using a polarizing microscope. The parallelepiped specimens with {100} faces or {100} and {110} faces are damaged on one {100} or {110} face. The extinction directions in the damaged face and in faces normal to its face coincide with the directions along the edges of the specimen. This indicates that the resultant residual stress has its normal components only and no shear components with respect to the faces of the specimen. The analysis leads to the conclusion that lapping induces a weak tensile stress along the damaged face and a strong compressive stress normal to its face, while grinding or cutting work induces only a strong tensile stress along its face and normal to its face. The observed residual stress is of the order of 107 to 108 dyn/cm2 in magnitude at a depth of 100 µm from the damaged surface.

Buckling of Welded Hybrid Steel I-Beams

Abstract: Allowance is made in the analysis for the effects of the residual stresses produced by welding and sections fabricated from both Universal Mill (UM) plates and flame cut (FC) plates are considered. Early yielding of the beam's web is shown to have little effects upon lateral stability, the stiffness of the compression flange being the most important factor. For a constant weld size, hybrid sections and sections fabricated from the flange material throughout are found to have very similar lateral buckling strengths. Plotting the results on an equivalent lateral-torsional slenderness ratio basis actually causes the hybrid sections' curve to be the highest. Use of this technique also minimizes the effect of variations in cross-sectional shape although the level of residual stresses in the sections remains an important parameter. Sections fabricated from FC plates are found to possess slightly differently shaped lateral buckling curves to sections fabricated from UM plates.

Controlled Surface Flaw-Initiated Fracture in Reaction-Densified Silicon Carbide

Abstract: Controlled surface flaws were produced in commercial reaction-densified SiC by Knoop microhardness indentation. The flaws themselves could not be observed easily, thus an etching technique was used to delineate their semielliptical shape, thereby enabling calculation of the critical stress- intensity factor KIC at room temperature. Room-temperature fracture was insensitive to annealing environment (air or vacuum), flaw “healing” being observed at ≫1000°C. The variation in fracture stress of indented specimens with temperature showed 3 distinct regions of behavior which were interpreted in terms of residual stress relief, flaw healing, and Si-SiC bond weakening.

Measurement of elastic-plastic stresses by scattered-light photomechanics

Abstract: Several birefringent materials were studied for their suitability for use in three-dimensional photoplasticity. This study resulted in the selection of cellulose propionate as model material. Its close match in index of refraction with ordinary mineral oil makes cellulose propionate suitable for scattered-light photomechanics. Viscoelastic behavior of the material is used to simulate elastic-plastic behavior of metals. The stress, strain and optical behavior of the material has been studied under slow loading in finite steps. A successful solution of an elastic-plastic torsion problem was obtained, demonstrating the applicability of the techniques of scattered-light photoplasticity to three-dimensional problems. This experimental method does not require the unloading and slicing of the model, avoiding any errors that might be introduced by residual stresses due to unloading. A major advantage of the method is the use of live loading, which allows the investigation of several load levels with the same model. Stress- and strain-concentration factors for the grooved shaft in torsion showed excellent agreement with Neuber's analytical results. Distributions of shear stress and shear strain across the minimum section of the model were compared to elastic theory. Integration of the shear-stress distributions showed good agreement with the measured values of applied torque.

Solution treatment method for pipe

Abstract: PURPOSE:To carry out the solution treatment while generating the compressive residual stress at the inner surface of the stainless steel pipe and to prevent the occurrence of the stress corrosion cracking, by heating the area sensitized by welding of the austenitic stainless steel pipe to the solution treatment temperature and then by quenching the inner surface of the pipe.