Showing papers on "Residual stress published in 1975"

Curing Stresses in Composite Laminates

Abstract: Analysis of curing stresses in resin matrix composite laminates involves a temperature range over which variation of elastic moduli is appreciable. A method based on total stress-strain-temperature relations is formulated and applied to determine the curing stresses in boron/epoxy composite laminates. This method is shown to be preferable to the incremental method because the former requires the thermal strains and the stress-strain relations only at the final temperature of interest. The use of incremental constitutive equations is also discussed, and it is shown that proper care must be exercised to insure inclusion of interaction terms that have no counterparts in linear theories.

A New Measuring Method of Residual Stresses with the Aid of Finite Element Method and Reliability of Estimated Values

Abstract: In estimation of residual stresses, the existing methods are mainly based on the idea that variation of strains on the surface of the object is measured by sectioning continuously until there is no variation of the measured strains, which corresponds to the residual stresses. In this kind of method, some definite mathematical relation between the variation of stresses and the released surface force is required. This kind of relation was obtained for the cases where the geometry, boundary condition, and pattern of residual stress distribution are simple. This difficulty is solved when numerical analytical methods, such as the finite element method, etc. are applied.In this paper, a general theory is developed based on the finite element method. With this method, three dimensional residual stresses can be measured. Furthermore, reliability of estimated values, of residual stresses by this method is mathematically studied when error is contained in the measured strains.

Elastic-plastic analysis of an infinite sheet having a circular hole under pressure

Abstract: An exact elastic-plastic solution for the stresses in an infinite sheet having a circular hole subject to pressure is obtained on the basis of J2 deformation theory together with a modified Ramberg-Osgood law. The sheet is orthotropic but isotropic in its plane. The results are assessed on the basis of Budiansky's criterion for the acceptability of J2 deformation theory. By using exact elastic-plastic stresses, the function connecting the pressure at the hole with the radial enlargement is obtained. Upon release of the pressure, residual stresses around the hole are produced.

Inverse piezoelectric effect and electrostrictive effect in polarized poly(vinylidene fluoride) films

Abstract: The piezoelectric stress coefficient and electrostriction coefficient for unpoled and poled films of polyvinylidene fluoride have been determined by applying a sinusoidal electric field and detecting stresses with the same and double frequency as the applied field, respectively. The piezoelectric coefficient shows a hysteresis with the cyclic change of the d.c. bias field. Under the poling field, the dipoles in polymers are preferentially oriented in the direction of the field, thus producing a residual polarization associated with a residual stress. Both coefficients increase with increasing temperature. A phenomenological interpretation for the piezoelectricity in poled polymers is given in terms of electrostriction and residual stress.

Cleavage Fracture in Prestrained Mild Steel

Abstract: Notched bars, subjected to varying amounts of uniform prestrain, have been fractured over a range of low temperatures, to examine the effects of prestrain on cleavage fracture. The parameter studied mainly has been the local tensile stress required to produce cleavage fracture below the notch and it has been. shown that this is increased by prestrain, although the effect of such strain was to increase the transition temperature. Detailed metallographic examination failed to show any features to which the increase in fracture stress could be primarily ascribed and a new interpretation of ‘plastic work’ energy terms is suggested. Delaminations caused by heavy prestrains are related to grain-boundary weakening and to residual stresses.

Full-range analysis of steel plates and stiffened plating under uniaxial compression

Abstract: A finite element formulation is presented for the large deflexion elasto-plastic analysis of thin mild steel plates subject to buckling loads. The method can be used to trace the behaviour of imperfect steel panels over the full load range, including the unloading stage following collapse. A series of simply supported plates under uniaxial compression has been analysed to study different in-plane boundary conditions, levels of geometric imperfection and residual stress. An approximate method is proposed for handling residual stresses and load-shortening curves are derived. These load-shortening relationships are used in a 'column' analysis to predict the behaviour of wide eccentrically stiffened plating subject to uniaxial compression. The computed collapse loads for both the individual plates and the stiffened plating are compared with experimental results. (A) /TRRL/

The inherent strain method for residual stress determination and its application to a long welded joint

Abstract: In this paper, we investigate a relatively new method for determining three-dimensional sub-surface residual stress (Ueda, 1975). This hybrid experimental/analytical method determines the source of residual stress by sectioning and strain measurement, and then uses it to deduce residual stress in the original body. When certain assumptions about the spatial distribution of the source of residual stress can be made, estimates of residual stress can be generated for locations which are remote from strain measurement points. We outline the method and then illustrate its application to measurement of residual stress in a long welded joint. First, background regarding the nature of residual stress and a formulation of the inherent strain approach to residual stress determination is discussed. Then, application of this method to a long continuously welded joint is described. A model problem is developed and solved by application of finite element analysis. Taking the finite element stress results as if they were experimental measurements, the inherent strain method is used to find residual stress. The accuracy of the method is then investigated by comparing the inherent strain and finite element solutions. Finally, the advantages and drawbacks of this relatively new method are discussed.

Interfacial Crack Propagation

Abstract: Interfacial crack propagation studies were conducted on specimens of epoxy bonded to aluminum under both tensile and bending loads. The effect of surface roughness of the aluminum on fracture toughness was measured for four different surface finishes: polished, milled, glass-peened and sandblasted. It was found that increased surface roughness resulted in greater fracture toughness and that the interfacial cracks replicated the surface features of the aluminum. Microscopic studies of the failure surfaces indicated that crack propagation occurs in the epoxy near the interface, and that a residue of epxoy remains bonded to the aluminum. The magnitude of the residual stresses due to casting and curing of the epoxy was determined by photoelastic techniques. Methods were developed for analyzing the birefringent pattern in the epoxy to determine the magnitude of the elastic residual stress and the frozen stress. It was found that the residual stresses contributed 15 to 20% of the strain required for cr...

An Ultrasonic Technique for the Measurement of Residual Stress

Abstract: An ultrasonic shear wave technique to measure stress in metals is described. The technique utilizes a pulse-echo system operating at 7 MHz to measure changes in the time of travel of the ultrasonic shear wave to 1 part per million. Linear changes in the velocity of a shear wave occur with stress and are dependent on the higher order elastic constants of the material. The linear change in velocity with stress allows the definition of a constant called the stress acoustic constant for the material. The amount of birefringence in a specimen is measured, and after accounting for the portion caused by anisotropy of the material, the remainder gives an accurate measure of the residual stress in the material. The amount of birefringence caused by anisotropy is a constant for specimens having the same nominal mechanical and thermal treatments. Measurements using the ultrasonic technique of simulated residual stress, introduced by bending of a 6-ft (1.8-m) section of A-36 steel I-beam, yielded values in the vicinity of those measured using strain gage and O-ring techniques. Application of this technique to the measurement of other simple states of stress is suggested, along with the precautions to be observed for measuring stress.

Residual Thermal Stresses in a Solid Sphere Cast From a Thermosetting Material

Abstract: Expressions are developed for the residual thermal stresses in a solid sphere cast from a chemically hardening thermosetting material in a rigid spherical mold. The description of the heat generation rate and temperature variation is derived from a first-order chemical reaction. Solidification is described by the continuous transformation of the material from an inviscid liquidlike state into an elastic solid, with intermediate properties determined by the degree of chemical reaction. Residual stress components are obtained as functions of the parameters of the hardening process and the properties of the hardening material. Variation of the residual stresses with a nondimensionalized reaction rate parameter and the relative compressibility of the hardened material is discussed in detail.

Lamination residual stresses in fiber composites

Abstract: An experimental investigation was conducted to determine the magnitude of lamination residual stresses in angle-ply composites and to evaluate their effects on composite structural integrity. The materials investigated were boron/epoxy, boron/polyimide, graphite/low modulus epoxy, graphite/high modulus epoxy, graphite/polyimide and s-glass/epoxy. These materials were fully characterized. Static properties of laminates were also determined. Experimental techniques using embedded strain gages were developed and used to measure residual strains during curing. The extent of relaxation of lamination residual stresses was investigated. It was concluded that the degree of such relaxation is low. The behavior of angle-ply laminates subjected to thermal cycling, tensile load cycling, and combined thermal cycling with tensile load was investigated. In most cases these cycling programs did not have any measurable influence on residual strength and stiffness of the laminates. In the tensile load cycling tests, the graphite/polyimide shows the highest endurance with 10 million cycle runouts at loads up to 90 percent of the static strength.

Residual Stress Measurement Using Acoustic Emission

Abstract: An analysis is presented which shows how acoustic emission caused by microcracking is expected to vary during an elastically reversible load cycle. The acoustic emission rate during unloading can be predicted directly from the acoustic emission rates observed during the loading and holding periods of a test cycle. Any discrepancy between the measured and predicted rates during unloading indicates irreversible behavior. A measure of the severity of the residual stress that results from irreversibility can be determined from the measured acoustic emission.

Method and apparatus for coating a glass substrate

Abstract: A continuous glass ribbon advances from a float forming chamber past a first heat source and into a coating chamber where a surface of the glass ribbon is provided with a pyrolytic oxide coating as a second heat source heats the opposed surface of the glass ribbon. Thereafter the glass ribbon is advanced into an annealing lehr to relieve residual stress in the glass ribbon. The first heat source reduces the heat loss of the ribbon as it advances into the coating chamber to provide a durable pyrolytic oxide coating on the glass ribbon. The second heat source heats the ribbon to minimize or eliminate glass ribbon warpage.

Determination of residual stresses in aluminum-alloy sheet material

Abstract: A method has been developed for the determination of residual stresses in thin sheet material. Thin layers are removed by chemical etching, which induces an increasing curvature of the sheet. The initial distribution of the residual stress is derived from curvature measurements. Details of the etching procedures and the equations required for the calculations are presented. Results are given for as-rolled sheet material and for spot-peened material.

Thermal action of high-power laser radiation on the surface of a solid

Abstract: A theoretical analysis is made of the thermal action of high-power laser radiation on the surface of a solid. Temperature and stress fields generated in an irradiated solid are investigated. Dependences of the maximum laser radiation intensities are calculated as a function of the pulse duration. These calculations are made for three states of a reflecting surface corresponding to maximum permissible elastic stress on the surface, plastic flow producing residual stresses, and melting of the surface layer.

Method for processing and densifying metal powder

Abstract: A method of producing a densified compact from a loose metal powder, the method comprising introducing strain into each particle of the loose metal powder to impart a residual stress to increase the potential energy level of the particles above the potential energy which the particles may have acquired during production thereof, the strain being introduced at a temperature below the recrystallization temperature thereof, and thereafter hot consolidating the loose metal powder.

Measurement of residual strains in boron-epoxy and glass-epoxy laminates

Abstract: Embedded-strain-gage techniques were developed and used for measuring strains in composite angle-ply laminates during curing and thermal cycling. The specimens were 2.54 by 22.9 cm eight-ply boron-epoxy and S-glass-epoxy laminates. Unidirectional specimens were used for control purposes. Strain readings were corrected for the purely thermal output of the gages obtained from an instrument quartz reference specimen. The strains measured during the cooling part of the curing cycle were in agreement with those recorded during subsequent thermal cycling, indicating that residual stresses induced during curing are primarily caused by differential thermal expansions of the various plies. Restraint strains were computed for the 0-deg and 45-deg plies of the angle-ply laminates tested, and the residual stresses obtained using the anisotropic constitutive relations and taking into account the temperature dependence of stiffnesses and strains.

Stress Corrosion Susceptibility of Stress-Coined Fastener Holes in Aircraft Structures

Abstract: The cold working process of stress coining is used to provide fatigue improvement of fastener holes in aircraft structures. The cold working produces a radial flow of the metal. The residual stresses resulting from stress coining provide protection against fatigue damage by opposing the applied tensile stresses in service at the edge of the fastener hole. However, it is shown in this paper that in addition to the compressive stresses surrounding the stress coined hole, there are tensile stresses that result from the coining operation and that these residual tensile stresses can have a deleterious effect on the stress corrosion susceptibility of the postcoined structure.

A New Measuring Method of Residual Stresses with the Aid of Finite Element Method and Reliability of Estimated Values

Abstract: In estimation of residual stresses, the existing methods are mainly based on the idea that variation of strains on the surface of the object is measured by sectioning continuously until there is no variation of the measured strains, which corresponds to the residual stresses. In this kind of method, some definite mathematical relation between the variation of stresses and the released surface force is required. This kind of relation was obtained for the cases where the geometry, boundary condition, and pattern of residual stress distribution are simple. This difficulty is solved when numerical analytical methods, such as the finite element method, etc. are applied.In this paper, a general theory is developed based on the finite element method. With this method, three dimensional residual stresses can be measured. Furthermore, reliability of estimated values, of residual stresses by this method is mathematically studied when error is contained in the measured strains.

Ultimate strength of square plates subjected to compression (1st Report)

Abstract: When structures are constructed by welding, structural elements are always accompanied by welding residual stresses and usually also by deformation. Therefore, when the rigidity and strength of the welded structures is considered, it is very important to have sufficient information about the effect of initial deflections and welding residual stresses on them. In this paper, as a fundamental study on this matter, the square plates under compression are dealt with. At first, two series of experiments were conducted, one using the specimens with initial deflection, and the other using the specimens with initial deflection and welding residual stresses. The specimens are 500×500 mm spuare plates of which thicknesses are 4.5 mm, 9.0 mm and 12.7 mm, and are simply supported along the four edges. Secondly, a series of the elastic-plastic large deflection analysis, by the finite element method, was carried out in order to clarify the effects of the shape of initial deflection, the magnitude of initial deflection, and welding residual stresses with initial deflection. It was found that the initial deflection and residual stress reduce the rigidity and ultimate strength and this tendency becomes larger for thicker plates.

Recent x-ray diffraction studies of metal fatigue in Japan:

Abstract: This paper describes results of several recent studies, carried out in Japan, on metal-fatigue problems using X-ray diffraction techniques. The subjects covered are the effect of residual stress on fatigue strength, non-destructive detection of fatigue damage from information supplied by X-ray diffraction, and X-ray microbeam analysis of stress and strain near the tips of fatigue cracks and fracture surfaces. The usefulness of the X-ray approach to fatigue problems is emphasized and possible future developments are suggested.