Showing papers on "Residual stress published in 2004"

The effect of machined topography and integrity on fatigue life

Abstract: The paper reviews published data which address the effect of machining (conventional and non-conventional processes) and the resulting workpiece surface topography/integrity on fatigue performance, for a variety of workpiece materials. The effect of post-machining surface treatments, such as shot peening, are also detailed. The influence of amplitude height parameters (Ra, Rt), amplitude distribution (Rsk) and shape (Rku) parameters, as well as spatial (Std, Sal) and hybrid (Ssc) measures, are considered. There is some disagreement in the literature about the correlation between workpiece surface roughness and fatigue life. In most cases, it has been reported that lower roughness results in longer fatigue life, but that for roughness values in the range 2.5–5 μm Ra it is primarily dependent on workpiece residual stress and surface microstructure, rather than roughness. In the absence of residual stress, machined surface roughness in excess of 0.1 μm Ra has a strong influence on fatigue life. Temperatures above 400 °C reduce the effects of both residual stress and surface roughness on fatigue, due to stress relieving and the change in crack initiation from the surfaces to internal sites. The presence of inclusions an order of magnitude larger than the machined surface roughness generally overrides the effect of surface topography.

Assessing residual stress development and stress relaxation in restrained concrete ring specimens

Abstract: Recently, the American Association of State Highway and Transportation Officials implemented a provisional standard that uses the `ring test' to help quantify a materials' propensity for cracking. While this test may provide qualitative information that enables different mixtures to be compared, it does not provide quantitative information to describe how close a specimen is to failure. This paper will describe how the ring test may be used to provide quantitative information about stress development that may be used to assess the potential for cracking in concrete. An analytical stress formulation is presented to compute the actual residual stress level in the concrete using only the measured strain from the steel ring. The theoretical elastic stress is computed using the free shrinkage, ring deformation, and elastic modulus of the concrete. A comparison of the residual and theoretical elastic stress levels provides information about the extent of stress relaxation in a material. Continuously monitoring the strain that develops in the steel ring from the time of casting enables the effects of autogenous shrinkage to be determined as well as the effects of drying shrinkage.

Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats

Abstract: Thermal spray offers a variety of sub-sets of processing approaches to produce coatings. The various processes are classified based on the thermal spray source (from low velocity combustion spray to high temperature plasma jets) and method of material injection (in the form of powder, wire or rod). However, it is this intrinsic versatility which sets-up variations in characteristics of the applied coatings. Properties of thermally sprayed coatings, including process induced residual stress, are controlled by various parameters of the spraying process. This study examines three thermal spraying techniques with significantly different particle temperatures and velocities. They are air plasma spraying (APS), twin wire-arc spraying (TWA) and high velocity oxy-fuel (HVOF) spraying. For comparison purposes the recently developed cold spray processed materials were included in the study. For each method, in-flight particle diagnostics was performed; Ni–5 wt.%Al splats and deposits were fabricated and analyzed. Porosity, elastic modulus and thermal conductivity of the deposits were evaluated and correlated to the process variables. Using indentation at different loads and analysis of the indented region, stress–strain relationships for these coatings were obtained. Surprising differences in the properties were observed and were explained based on the fundamental variations in microstructure development. Through-thickness residual stress profiles in Ni–5 wt.%Al coatings on steel substrates were determined non-destructively by neutron diffraction. The stresses range from highly tensile in the APS coating to compressive in the HVOF coating. Various stress generation mechanisms—splat quenching, peening and thermal mismatch—are discussed with respect to process parameters and material properties.

Residual stress and surface roughness when facing age hardened Inconel 718 with CBN and ceramic cutting tools

Abstract: The demand for increasing productivity when machining heat resistant super alloys has resulted in the use of advanced cutting tools such as ceramics and cubic boron nitride (CBN). However, the effects of these tools on the surface integrity, especially the residual stresses created, in the high speed facing operation of Inconel 718 has not been dealt with. In this paper, the residual stresses and the surface roughness when facing age hardened Inconel 718 using CBN and mixed ceramic cutting tools at their respective optimum performance based on productivity has been investigated. The residual stress and surface finish generated during facing with CBN cutting tools have been investigated as a function of speed, depth of cut, coolant, tool geometry and nature of the tool coating. In addition, mixed ceramic cutting tools have been investigated for comparison. The results show that mixed ceramic cutting tools induce tensile residual stresses with a much higher magnitude than CBN cutting tools. The residual stresses and the surface roughness generated by CBN cutting tools are more sensitive to cutting speeds than depth of cut. The use of coolant results in either compressive residual stresses or lowers the magnitude of the tensile residual stresses, whereas dry cutting always resulted in tensile residual stresses. From this investigation, it is suggested that round CBN cutting tools should be used at slow cutting speeds (150 m/min) and small depths of cut (0.05 mm) and with the use of coolant to achieve compressive or minimal tensile residual stresses and good surface finish.

Surface integrity when machining age hardened Inconel 718 with coated carbide cutting tools

Abstract: Considerable attention has been given to the use of ceramic cutting tools for improving productivity in the machining of heat resistant super alloys (HRSA). However, because of their negative influence on the surface integrity, ceramic tools are generally avoided particularly for finishing applications. As a result the main high end manufacturers are more or less dependent on carbide cutting tools for finishing operations. Still the improper use of carbide cutting tools can also result in poor surface integrity. The objective of this investigation is to develop a set of guidelines, which will assist the selection of the appropriate cutting tools and conditions for generating favorable compressive residual stresses. This paper specifically deals with residual stresses and surface finish components of surface integrity when machining (facing) age hardened Inconel 718 using two grades of coated carbide cutting tools specifically developed for machining HRSAs. The cutting conditions were obtained from investigations based on optimum tool performance. The effect of insert shape, cutting edge preparation, type and nose radius on both residual stresses and surface finish was studied at this optimum cutting condition. This investigation, suggested that coated carbide cutting tool inserts of round shape, chamfered cutting edge preparation, negative type and small nose radius (0.8 mm) and coolant will generate primarily compressive residual stresses.

Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy

Abstract: Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 1.2 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto a water-immersed type aluminum samples. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the higher the pulse density the larger the zone size with compressive residual stress. Densities of 900, 1350 and 2500 pulses/cm 2 with infrared (1064 nm) radiation are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. Fatigue crack growth rate is compared in specimens with and without LSP process. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness in the 6061-T6 aluminum alloy.

Developments in martensitic and bainitic steels: role of the shape deformation

Abstract: Steels can be designed to exploit the coordinated motion of atoms during the bainite and martensite reactions In this way, it has been possible to make exceptionally strong and tough alloys in bulk form, and at an affordable price Crystal sizes can be reduced to between 20 and 180 nm, by annealing heterogeneously deformed martensite, or by forming bainite under conditions where there is no atomic mobility Engineering failures are dominated by fatigue, the effects of which are exacerbated by welding and the associated residual stresses Alloys can now be designed such that the deformation caused by transformation eliminates residual stresses, with extraordinary improvements in the fatigue life of constrained assemblies of metal These important achievements are described here, within a framework which relates the atomic mechanism to the engineering scale

Prediction of shape distortions Part I. FE-implementation of a path dependent constitutive model

Abstract: There is a great interest, especially from the aircraft industry, to increase the ability to understand and predict development of shape distortions and residual stresses during manufacture of polymer composite components. An increased ability to predict shape distortions will result in more cost efficient development, improved performance and optimised manufacturing of composites. To be able to predict residual stresses and shape distortions a model is needed that accounts for all important mechanisms involved. In a previous work by the authors, it was demonstrated that such—models must account for thermal expansion (different in glassy and rubbery state), chemical shrinkage due to the crosslinking reaction and finally frozen-in deformations. The present paper presents a simple mechanical constitutive model that accounts for the mechanisms mentioned above. The model is a limiting case of linear visco-elasticity that permits us to replace the rate dependence by a path dependence on the state variables: strain, degree of cure and temperature. This means significant savings in computational time, memory requirements and costs for material characterisation as compared to conventional visco-elastic models. This is the first of two papers, the second paper deals with experimental validation and analysis of mechanical boundary conditions during prediction of shape distortion.

Laser surface-contouring and spline data-smoothing for residual stress measurement

Abstract: We describe non-contact scanning with a confocal laser probe to measure surface contours for application to residual stress measurement. (In the recently introduced contour method, a part is cut in two with a flat cut, and the part deforms by relaxation of the residual stresses. A cross-sectional map of residual stresses is then determined from measurement of the contours of the cut surfaces.) The contour method using laser scanning is validated by comparing measurements on a ferritic steel (BS 4360 grade 50D) weldment with neutron diffraction measurements on an identical specimen. Compared to lower resolution touch probe techniques, laser surface-contouring allows more accurate measurement of residual stresses and/or measurement of smaller parts or parts with lower stress levels. Furthermore, to take full advantage of improved spatial resolution of the laser measurements, a method to smooth the surface contour data using bivariate splines is developed. In contrast to previous methods, the spline method objectively selects the amount of smoothing and estimates the uncertainties in the calculated residual stress map.

Residual stress in friction stir-welded Al sheets

Abstract: Friction stir welding (FSW) is a relatively new joining technique particularly for aluminum alloys that are difficult to fusion weld. A potential field of application is aircraft structures where cost and weight can be reduced by using new joining techniques instead of riveting. Efforts are currently being made to qualify FSW for this purpose. In this study, the influence of a coolant applied during welding of Al sheets on the residual stress state of the FSW joint was investigated. Liquid CO2 coolant was applied near the weld seam for rapid cooling of the weld zone. The residual stresses across the weld were measured by neutron diffraction. Three sheets were produced, one without cooling, and two with cooling, where two different distances of the coolant nozzles from the FSW tool were chosen. The results show that, by applying a coolant, the magnitude of the tensile stress in the center of the weld can be reduced significantly.

Fabrication and structural characterization of self-supporting electrolyte membranes for a micro solid-oxide fuel cell

Abstract: Micromachined fuel cells are among a class of microscale devices being explored for portable power generation In this paper, we report processing and geometric design criteria for the fabrication of free-standing electrolyte membranes for microscale solid-oxide fuel cells Submicron, dense, nanocrystalline yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) films were deposited onto silicon nitride membranes using electron-beam evaporation and sputter deposition Selective silicon nitride removal leads to free-standing, square, electrolyte membranes with side dimensions as large as 1025 μm for YSZ and 525 μm for GDC, with high processing yields for YSZ Residual stresses are tensile (+85 to +235 MPa) and compressive (–865 to -155 MPa) in as-deposited evaporated and sputtered films, respectively Tensile evaporated films faul via brittle fracture during annealing at temperatures below 773 K; thermal limitations are dependent on the film thickness to membrane size aspect ratio Sputtered films with compressive residual stresses show superior mechanical and thermal stability than evaporated films Sputtered 1025-μm membranes survive annealing at 773 K, which leads to the generation of tensile stresses and brittle fracture at elevated temperatures (923 K)

Reduction and re-oxidation of anodes for solid oxide fuel cells (SOFC)

Abstract: The influence of reduction and re-oxidation on microstructure and residual stress has been studied for anode supported planar SOFCs. The transition in oxidation state shows a non-reversible behaviour. Shrinkage caused by the reduction of NiO to Ni is over-compensated in the re-oxidation step. Microstructural SEM observations reveal that after re-oxidation the NiO has a higher porosity and requires a larger volume than in the initial, oxidised state. The residual stress development has been analysed via monitoring of the curvature changes of unconstrained half-cells. Re-oxidation is a dynamic process that starts at the free surface of the anode and proceeds towards the interface with the electrolyte. Accordingly the residual stresses in the anode and electrolyte change. Ultimately, the tensile residual stress in the electrolyte exceeds tensile strength, resulting in cracks which lead to deterioration of the entire cell fraction.

Residual stress in HVOF thermally sprayed thick deposits

Abstract: Due to the nature of the high velocity oxy-fuel thermal spray process, residual stress build up in thick deposits is a significant and a limiting problem. The residual stress-state that evolves in a deposit is largely dependent on the thermal conditions to which the system has been subjected, and is a combination of quenching stresses, which arise during deposition, and cooling stresses, post-deposition. It follows that precise control of these phenomena is essential, if a thick deposit is to be thermally sprayed. This paper applies an analytical technique to calculate residual stress in thermally sprayed deposits based on geometric properties. Residual stress results for WC–Co (tungsten carbide–cobalt) samples are compared to experimental results (X-ray diffraction and hole-drilling method). A change in deposit stress-state from tensile to compressive stress with deposit thickness is analysed in terms of quenching and cooling stresses.

Measuring stress intensity factors during fatigue crack growth using thermoelasticity

Abstract: Thermoelastic stress analysis has been developed in recent years as a direct method of investigating the crack tip stresses in a structure under cyclic loading. This is a consequence of the fact that stress intensity factors obtained from thermoelastic experiments are determined from the cyclic stress field ahead of a fatigue crack, rather than inferred from measurement of the crack length and load range. In the present paper the results of fatigue crack growth tests performed on welded ferritic steel plates are reported. From the results it can be observed that the technique is sensitive to the effects of crack closure and the presence of tensile and compressive residual stresses due to welding.

Microscale Laser Shock Peening of Thin Films, Part 1: Experiment, Modeling and Simulation

Abstract: Microscale Laser Shock Peening (LSP), also known as Laser Shock Processing, is a technique that can be potentially applied to manipulate residual stress distributions in metal film structures and thus improve the fatigue performances of micro-devices made of such films. In this study, microscale LSP of copper films on single crystal silicon substrate is investigated. Before and after-process curvature measurement verifies that sizable compressive residual stress can be induced in copper thin films using microscale LSP. Improved modeling work of shock pressure is summarized and the computed shock pressure is used as loading in 3D stress/strain analysis of the layered film structure. Simulation shows that the stress/strain distribution in the metal film is close to equi-biaxial and is coupled into the silicon substrate.