Showing papers on "Residual stress published in 1996"

Review of low-velocity impact properties of composite materials

Abstract: This paper is a review of low-velocity impact responses of composite materials. First the term ‘low-velocity impact’ is defined and major impact-induced damage modes are described from onset of damage through to final failure. Then, the effects of the composite's constituents on impact properties are discussed and post-impact performance is assessed in terms of residual strength.

Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour

Abstract: Subjecting target metallic samples to a very short pulse (about 20 ns) of intense (GW cm−2) laser light generates, through a surface plasma, a high-pressure stress wave propagating to the first millimetre in depth, which is commonly called laser shock processing (LSP). The purpose of this work was to evaluate the role of this novel process on the cyclic properties of A356, Al12Si and 7075 aluminium alloys. Major contributors to the fatigue performance improvements were investigated in order to determine the optimum shock conditions. These were mainly compressive residual stress (RS) levels for which a large range of incident shock conditions was performed. We showed that stress levels were very sensitive to the laser fluence and the number of local impacts, and experimental RS measurements were found to be in good agreement with analytical modelling results. In comparison, a conventional shot peening (SP) treatment was found to lead to higher surface hardening and RS levels, but with a very detrimental roughening not observed after LSP. High cycle (107) fatigue tests carried out on laser- processed, shot-peened and untreated notched samples illustrated the efficiency of LSP as a new, promising method to improve the fatigue limits σD of structures, especially in comparison with enhancements displayed by SP (+22% vs. +10%). According to crack detection electric measurements, fatigue performance improvements with LSP mainly occurred during the crack initiation stage.

Influences of stress on the measurement of mechanical properties using nanoindentation: Part II. Finite element simulations

Abstract: The finite element method has been used to study the behavior of aluminum alloy 8009 during elastic-plastic indentation to establish how the indentation process is influenced by applied or residual stress. The study was motivated by the experiments of the preceding paper which show that nanoindentation data analysis procedures underestimate indentation contact areas and therefore overestimate hardness and elastic modulus in stressed specimens. The NIKE2D finite element code was used to simulate indentation contact by a rigid, conical indenter in a cylindrical specimen to which biaxial stresses were applied as boundary conditions. Indentation load-displacement curves were generated and analyzed according to standard methods for determining hardness and elastic modulus. The simulations show that the properties measured in this way are inaccurate because pileup is not accounted for in the contact area determination. When the proper contact area is used, the hardness and elastic modulus are not significantly affected by the applied stress.

Residual Stresses in Thermal Spray Coatings and Their Effect on Interfacial Adhesion : A Review of Recent Work

Abstract: An overview is presented of the development of residual stresses in thermal spray coatings and their ef-fects on interfacial debonding. The main experimental techniques for measurement of residual stresse are briefly described, with particular attention given to the method of continuous curvature monitoring. Boundary conditions satisfied by all residual stress distributions are identified and expressions derived for the curvatures and stress distributions arising from a uniform misfit strain between coating and sub-strate.It is noted that stress distributions in thick coatings rarely correspond to the imposition of such a uniform misfit strain, so that recourse to numerical methods becomes essential for quantitative predic-tion of stress distributions. Relationships are presented between residual stresses and corresponding strain energy release rates during interfacial debonding. The effect on this of superimposing stresses from an externally applied load is outlined. The initiation of debonding is then considered, covering edge effects and other geometrical considerations. Finally, some specific case histories are briefly outlined to illustrate how the various theoretical concepts involved relate to industrial practice.

Determining mean and gradient residual stresses in thin films using micromachined cantilevers

Abstract: A technique that provides a first approximation to the mean, , and gradient, , components of residual stress in a thin-film material is discussed. In this method, measurements are made on a single micromachined cantilever, as opposed to an array of structures as used in the related critical-length buckling approach, to find tensile, compressive and gradient stresses. The measured deflection profile of a cantilever is reduced to rotation and curvature components, which are shown to derive independently from and , respectively. Essential to this method is the observation that a micromachined structure with a `nominally-clamped' boundary undergoes subtle rotation at its junction with the portion of the thin film that remains bonded to the substrate but is contiguous with the structure. This boundary rotation effect occurs through in-plane expansion or contraction of the bonded film following relief of residual stress. Thus, the deformation of the micromachined cantilevers considered here and of more general bulk- or surface-micromachined devices, can be strongly influenced by the state of stress in the still-bonded film.

Crack problems in fgm layers under thermal stresses

Abstract: In this study an unconstrained elastic layer under statically self-equilibrating thermal or residual stresses is considered. The layer is assumed to be a functionally graded material (FGM), meaning that its thermo-mechanical properties are assumed to be continuous functions of the thickness coordinate. The layer contains an embedded or a surface crack perpendicular to its boundaries. Using superposition the problem is reduced to a perturbation problem in which the crack surface tractions are the only external forces. The dimensions, geometry, and loading conditions of the original problem are such that the perturbation problem may be approximated by a plane strain mode I crack problem for an infinite layer. After a general discussion of the thermal stress problem, the crack problem in the nonhomogeneous medium is formulated. With the application to graded coatings and interfacial zones in mind, the thickness variation of the thermo-mechanical properties is assumed to be monotonous. Thus, the functions suc...

Measurement of the stress in oxide scales formed by oxidation of alumina-forming alloys

Abstract: The alumina scales on a variety of high-temperature alloys are found to fluoresce when illuminated with light having a frequency greater than 18,000 cm−1. The fluorescence exhibits two narrow lines characteristic of chromium-doped alpha-aluminum oxide. The frequency shift of the two lines from the room-temperature, stress-free values of 14,402 cm−1 (1.786 eV) and 14432 cm−1 (1.789 eV) provides a noncontact measure of the stress in the alumina scales using the piezospectroscopic effect. In addition, the broadening of the lines is a measure of the stress gradient in the scale. The physical basis for the fluorescence technique is described together with its implementation for highspatial-resolution (∼2 μm) measurements. As illustration, room-temperature measurements of the residual stress in scales formed at 1100°C on single-crystal NiAl, polycrystalline Ni3Al, two Fe−Ni−Cr−Al alloys, and two Ni−Al base superalloys are presented.

On the toughness of ductile adhesive joints

Abstract: Crack propagation along one of the interfaces between a thin ductile adhesive layer and the elastic substrates it joins is considered. The layer is taken as being elastic-plastic, and the fracture process of the interface is modeled by a traction-separation law, characterized by the peak separation stress 6 and the work of separation per unit area To. Crack growth resistance curves for mode I loading of the adhesive joint are computed, with emphasis on steady-state toughness, as a function of three extrinsic effects : layer thickness, layer-substrate modulus mismatch, and initial residual stress in the layer. Conditions under which separation first occurs well ahead of the initial crack tip are discussed. 1. SPECIFICATION OF THE MODEL This paper continues the study begun by Tvergaard and Hutchinson (1994) in which an embedded fracture zone model is applied to the mode I fracture of an adhesive joint comprised of a thin elastic-plastic metal layer joining two elastic substrates. The present work employs the model to investigate the influence on joint toughness of both the elastic mismatch between the layer and the substrates and the residual stress in the layer. As in the earlier study, the thickness of the ductile layer is another extrinsic variable which comes into play. The approach adopted was first introduced by Needleman (1987) to study particle debonding in metal matrices and subsequently by Tvergaard and Hutchinson (1992, 1993) to model crack growth resistance in homogeneous solids and along interfaces. A traction-separation law simulating the fracture process is embedded within an elastic-plastic continuum as a boundary condition along the line extending ahead of the crack. In the case of an interface joining dissimilar materials, the separation law necessarily involves both the normal and shear tractions and the two associated relative displacements of the surfaces across the interface.

Bent-beam strain sensors

Abstract: We examine a new class of sensitive and compact passive strain sensors that utilize a pair of narrow bent beams with an apex at their mid-points. The narrow beams amplify and transform deformations caused by residual stress into opposing displacements of the apices, where vernier scales are positioned to quantify the deformation. An analytical method to correlate vernier readings to residual stress is outlined, and its results are corroborated by finite-element modeling. It is shown that tensile and compressive residual stress levels below 10 MPa, corresponding to strains below 6/spl times/10/sup -5/ can be measured in a 1.5-/spl mu/m-thick layer of polysilicon using a pair of beams that are 2 /spl mu/m wide, 200 /spl mu/m long, and bent 0.05 radians (2.86/spl deg/) to the long axis of the device. Experimental data is presented from bent-beam strain sensors that were fabricated from boron-doped single crystal silicon using the dissolved wafer process and from polycrystalline silicon using surface micromachining. Measurements from these devices agree well with those obtained by other methods.

Compatibility and the genesis of residual stress by volumetric growth.

Abstract: The equations of compatibility which are pertinant for growth strain fields are collected and examples are given in simply-connected and multiply-connected regions. Compatibility conditions for infinitesimal strains are well known and the possibilities of Volterra dislocations in multiply-connected regions are enumerated. For finite growth strains in a multiply-connected regions, each case must be examined individually and no generalizations in terms of Volterra dislocations are available. Any incompatible growth strains give rise to residual stresses which are known to occur in many tissues such as the heart, arterial wall, and solid tumors.

Cracking of thin films bonded to elastic-plastic substrates

Abstract: Thin bonded films have many applications In information storage and processing systems, for example, conducting, semiconducting and insulating films are used in integrated circuits, and thin magnetic films are used in disk storage systems In many cases, thin bonded films are in a state of residual tension, which can lead to film cracking Because cracking can alter desired film properties, methods for predicting it are needed The geometry considered in this work is one in which cracks or flaws oriented normal to the film-substrate interface propagate (or “channel”) across the film It is assumed that the film is brittle and the substrate is ductile Plane strain fracture analyses are used to investigate the channel cracking of elastic thin films in residual tension in the presence of yielding in the substrate material Although crack channeling induces yielding in the substrate, channel crack extension in the brittle film occurs under small scale yielding conditions The case of an elastic film bonded to an elastic substrate has been considered in earlier work, and is used as the basis for the current study A numerical model is used to extend the results from the fully elastic problem so that plastic yielding of the substrate is allowed Results are presented for an elastic-perfectly plastic substrate and for substrates exhibiting strain hardening A simple shear lag model of the problem without hardening in the substrate is discussed, which gives reasonable predictions for the dependence of dimensionless fracture quantities on the normalized loading over a wide range of material mismatches In addition, a method is presented by which shear lag modeling can be extended to cases in which the substrate exhibits strain hardening

Practical measurement of the residual stress in coatings

Abstract: The method adopted to measure residual stress depends on the degree of detailed information needed, the size of the part, the coating thickness and the costs which can be incurred. Three of the main approaches used at the present time are discussed here. These are: first, the hole drilling method which returns a value of the macrostress and an indication of any anisotropy in coatings of thickness greater than about 0.3 mm; second, the cantilever beam methods applicable to thinner coatings on thin substrates; third, the X-ray diffraction methods applicable to crystalline coatings of a thickness which can be penetrated by the X-ray beam. This last method can be sub-divided into two groups where, in the first, the standard Bragg-Brentano diffractometer available in most laboratories is used, and, second, the more recent glancing incidence methods in which the stress in thin coatings or the surface of thicker coatings can be studied at depths of as little as 1 μm.

Toughening of Layered Ceramic Composites with Residual Surface Compression

Abstract: Effects of macroscopic residual stresses on fracture toughness of multilayered ceramic laminates were studied analytically and experimentally. Stress intensities for edge cracks in three-layer, single-edge-notch-bend (SENB) specimens with stepwise varying residual stresses in the absence of the crack and superimposed bending were calculated as a function of the crack length by the method of weight function. The selected weight function and the method of calculation were validated by calculating stress intensities for edge cracks in SENB specimens without the residual stresses and obtaining agreement with the stress-intensity equation recommended in ASTM Standard E-399. The stress-intensity calculations for the three-layer laminates with the macroscopic residual stresses were used to define an apparent fracture toughness. The theoretical predictions of the apparent fracture toughness were verified by experiments on three-layer SENB specimens of polycrystalline alumina with 15 vol% of unstabilized zirconia dispersed in the outer layers and 15 vol% of fully stabilized zirconia dispersed in the inner layer. A residual compression of ∼400 MPa developed in the outer layers by the constrained transformation of the unstabilized zirconia from the tetragonal to the monoclinic phase enhanced the apparent fracture toughness to values of 30 MPa.m1/2 in a system where the intrinsic fracture toughness was only 5 to 7 MPa.m1/2.

Early development of fatigue cracking at manual fillet welds

Abstract: — An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking along the wavy toe of manual fillet welds between structural steel plates. Stress relieved and as-welded cruciform joints were tested under R =−1 and R= 0 loading at different stress amplitudes. The depth and the opening level of cracks as small as 10–20 μm were monitored using miniature strain gauges installed along the toe apex, in combination with beach marking. Most of the “initiation life” (25% to 50% of total life), conventionally defined by a crack depth of 0.5 mm, is consumed in short crack propagation. Three types of short crack development for different combinations of local mean stress and stress range are identified and analyzed. Growth rates in as-welded specimens are faster than in stress relieved specimens, which results in shorter “initiation lives”. This is associated with a higher effective stress range, particularly under R = - 1 loading where cracks are open over nearly the full stress range. The V-notch stress intensity factor is a promising parameter to rationalize the crack “initiation life”. It takes into account the thickness effect experimentally observed. Under R = - 1 loading of as-welded joints, using R = 0 data and taking the whole stress range gives a reasonably conservative approximation of the crack “initiation life”.

Cracking of Laminates Subjected to Biaxial Tensile Stresses

Abstract: During the processing of laminar ceramic, biaxial residual stresses can arise due to differential thermal contraction between unlike layers. A tensile stress can cause preexisting flaws to extend across the layer and into the adjacent layers and then tunnel until they meet either another crack or a free surface. A previous analysis has shown that for a given residual stress there is a critical layer thickness, below which no tunnel cracks will exist, regardless of initial flaw size. Here, the previous analysis was modified to take into account the crack extension into adjacent layers. To determine the validity of the analysis, laminates composed of alternating layers of zirconia and alumina/zirconia were fabricated by a sequential centrifugation technique. The composition of the alumina/zirconia layer was varied to change the biaxial, tensile stresses in the zirconia layer. Observations were then made to determine the critical layer thickness for tunnel cracks and their extension into the adjacent layers. These observations were compared to the theoretical predictions.

Röntgenographische Untersuchung von Spannungszuständen in Werkstoffen. Teil III. Fortsetzung von Matwiss. und Werkstofftechn. Heft 3/1995, S. 148–160 und Heft 4/1995, S. 199–216

Abstract: X-Ray Investigation of Stress States in materials X-ray stress analyses on crystalline or partially crystalline materials are based on the determination of elastic lattice strains which are converted to stresses by means of theory of elasticity. The development of the sin2Ψ-method of X-ray stress analysis and of diffractometers substituting film chambers during the 1960s initiated an enormcus progress in X-ray stress analysis during the following three decades both in respect of the knowledge of the underlying principles and in respect of the practical application This report sketches the historical development of X-ray stress analyses and describes the actual state of the art of this important tool for materials science and engineering. Besides some important elements of X-ray physics and theory of elasticity, experimental aspects of practical applications are outlined. Standard measuring procedures and special measuring problems are described and hints for practical solutions are given. In particular, examples of destructive and non-destructive depth profiling of residual stresses, of residual stress analyses in thin coatings, in multilayer structures of thin coatings and in chemically graded coatings, of residual stress analyses in presence of textures, of residual and loading stress analyses in heterogeneous materials, in coarse grained, and in single crystalline materials are presented. The methods established up to now are explained and possible future developments are pointed out.

Development and experimental validation of the deep hole method for residual stress measurement

Abstract: The development of a deep hole method, based on earlier techniques, for measuring the through-thickness distribution of residual stresses is described. The distortion of a reference hole used in the method was interpreted using analytical techniques to determine the residual stresses present. The accuracy of the method was investigated using a 100 mm deep plastically deformed ferritic steel rectangular bar. The stresses in the bar were determined by surface strain gauges. The axial residual stresses through the depth of the bar, measured by the deep hole method, were found to be within ± 35 MPa of the stresses determined from the strain gauges in the central 80 mm of the bar.

Crack Bifurcation in Laminar Ceramic Composites

Abstract: Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al2O3 layers sandwiched between thicker layers of Zr(12Ce)O2. The Al2O3 layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O2 layers were nearly free of residual, tensile stresses because they were much thicker than the Al2O3 layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al2O3 layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al2O3 layer, it bifurcated as it approached the Zr(12Ce)O2/ Al2O3 interface. After the crack bifurcated, it continued to propagate close to the center line of the Al2O3 layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O2 layer, where it bifurcated again as it entered the next Al2O3 layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al2O3 layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.

Approximate Assessment of the Ultimate Longitudinal Strength of the Hull Girder

Abstract: A method is presented to estimate the ultimate moment based on a simplified approach to represent the behavior of stiffened plate columns. The assessment of the strength of a very large crude carrier is performed and compared with the moment at failure in hogging estimated by other methods applied to the same case. The proposed method allows the prediction of the degradation of the strength due to corrosion and residual stresses. It also allows the evaluation of the strength of the hull at several heeling conditions. Finally, an analysis of the efficiency of the high tensile steel is carried out.

Elastic properties and microstructure of LPCVD polysilicon films

Abstract: This paper investigates the influence of film thickness, high-temperature annealing and doping by ion implantation on Young's modulus and the residual stress of LPCVD polysilicon. Films with thicknesses between 100 nm and 800 nm were deposited at with a pressure of 100 mTorr. For annealing investigations, films were annealed in a nitrogen atmosphere for 2 hours at temperatures between 600 and . The implantation doses of boron and phosphorus varied between and . This corresponds to a doping concentration of and . Young's modulus and the residual stress were determined by load-deflection measurements with suspended membranes and by the use of ultrasonic surface waves. The microstructure of the film and grain size was studied by TEM analyses and texture variations were investigated by x-ray deflection. Although Young's modulus was found to be very stable, it showed a small dependence on film thickness and annealing temperature. It varied between 151 GPa and 166 GPa. The residual stress could be strongly influenced by film thickness (-420 MPa to -295 MPa), annealing temperature (-350 MPa to -20 MPa) and ion implantation (-560 MPa to +30 MPa). The as-deposited film always showed compressive stress, a pronounced texture and a grain size of around 55 nm. Strong correlations between the variations of the elastic properties and the variations in the film thickness, annealing temperature, grain size, mass density and refractive index were found. These correlations and the observed microstructure are used to develop a model for the origin of the compressive stress and for the mechanism of stress variation. A theoretical value for Young's modulus of textured polySi was calculated and corresponds well with the measured values.

A study of fatigue crack closure by elastic-plastic finite element analysis for constant-amplitude loading

Abstract: A comprehensive elastic-plastic constitutive model is employed in a finite element analysis of fatigue crack closure. An improved node release scheme is used to simulate crack growth during cyclic loading, which eliminates the associated numerical difficulties. New definitions of crack opening and closing stresses are presented in this paper. Special attention is paid to a discussion of some basic concepts of fatigue crack growth and crack closure behaviour. Residual tensile deformation and residual compressive stress are found to be two major factors in determining the crack opening stress. A comparison of crack tip node release at the maximum or minimum load of each cycle is made and the disadvantage of releasing crack tip node at the minimum load are pointed out.

Residual thermal stresses in injection molded products

Abstract: Nonisothermal flow of a polymer melt in a cold mold cavity introduces stresses that are partly frozen-in during solidification. Flow-induced stresses cause anisotropy of mechanical, thermal, and optical properties, while the residual thermal stresses induce warpage and stress-cracking. In this study, the influence of the holding stage on the residual thermal stress distribution is investigated. Calculations with a linear viscoelastic constitutive law are compared with experimental results obtained with the layer removal method for specimens of polystyrene (PS) and acrylonitrile butadiene-styrene (ABS). In contrast to slabs cooled at ambient pressures, which show the well-known tensile stresses in the core and compressive stresses at the surfaces, during the holding stage in injection molding, when extra molten polymer is added to the mold to compensate for the shrinkage, tensile stresses may develop at the surface, induced by the pressure during solidification.

Calculation of Stresses in Crosslinking Polymers

Abstract: A general constitutive model is presented for calculating the evolution of stresses in crosslinking polymers. The analysis is valid for thermo- and chemorheologically simple polymers and for time-dependent strain histories where the total strain at any time is not large, such that linear viscoelasticity applies. We identify the material parameters required and outline experimental and theoretical methods for their determination. This formalism is then used to calculate the development of cure stresses for an epoxy wherein the glass transition temperature exceeds the cure temperature during the reaction. Finally, we discuss practical methods of stress minimization through control of the cure cycle.