Peak Compressive Residual Stress Research Articles

Residual Stress Enhancement by Laser Shock Treatment in Chromium-Alloyed Steam Turbine Blades.

In-service turbine blade failures remain a source of concern and research interest for engineers and industry professionals with attendant safety and economic implications. Very high-pressure shock impacts from laser shots represent an evolving technique currently gaining traction for surface improvement and failure mitigation in engineering components. However, the physical characteristics and effects of parameter variations on a wide range of materials are still not fully understood and adequately researched, especially from a computational point of view. Using the commercial finite element code ABAQUS©, this paper explores the application of laser shock peening (LSP) in the enhancement of residual stresses in Chromium-based steel alloyed turbine blade material. Results of the numerically developed and experimentally validated LSP model show that peak compressive residual stresses (CRS) of up to 700 MPa can be induced on the surface and sub-surface layers, while the informed varying of input parameters can be used to achieve an increase in the magnitude of CRS imparted in the peened material. Analysis of the hierarchy of influence of the five input parameters under investigation on residual stress enhancement reveals the laser shock intensity as the most influential, followed in descending order of influence by the exposure time, shot size, degree of overlaps, and the angle of shot impact.

Material properties and membrane residual stresses of S690 high strength steel welded I-sections after exposure to elevated temperatures

This paper reports a thorough experimental investigation into the material properties and membrane residual stresses of S690 high strength steel welded I-sections after exposure to seven levels of elevated temperatures ranging from 30 °C (room temperature) to 950 °C. The experimental programme included heating, soaking and cooling of S690 high strength steel coupons and welded I-section specimens as well as post-fire material tensile coupon tests and membrane residual stress measurements, with the experimental rigs, procedures and results fully reported. The key post-fire material properties were then carefully analysed together with the test data collected from the existing literature, and a new set of retention factor curves of simple multi-linear shapes was proposed and shown to result in accurate predictions of post-fire yield and ultimate stresses for S690 high strength steel after exposed to elevated temperatures. Regarding post-fire membrane residual stresses, the measured distribution pattern and peak amplitudes in S690 high strength steel welded I-section after exposed to an elevated temperature of 300 °C generally remained unchanged in comparison with those in S690 high strength steel welded I-section at room temperature. However, for higher elevated temperatures ranging from 600 °C to 950 °C, the peak values of both compressive and tensile membrane residual stresses dramatically decreased, and moreover the discrepancy between the peak compressive and tensile membrane residual stress values became smaller and the transition regions (where the peak tensile residual stresses are changed to the peak compressive residual stresses) became narrower; this can be attributed to the fact that prominent elastic strain redistribution and residual stress relaxation of steel starts from around 500 °C–600 °C. A membrane residual stress predictive model was proposed for S690 high strength steel welded I-sections after exposed to elevated temperatures, and shown to well represent the measured membrane residual stress patterns and amplitudes over the full temperature range from 30 °C to 950 °C.

Effect of process temperature on mechanical properties of Ti6Al4V titanium alloy with warm laser shock peening

Warm laser shock peening (WLSP) is a thermomechanical strengthening technique with tempering treatment during laser shock peening (LSP) to optimize the mechanical properties of a metallic material. In WLSP, processing temperature plays a key role in regulating the final product’s mechanical properties. In this work, Ti6Al4V titanium alloy is used to evaluate effect of temperature on mechanical properties during warm laser shock peening. The compressive residual stress (CRS) generated by WLSP at different temperature was measured by X-ray diffraction (XRD) and the surface hardness was investigated to reveal the surface strength features under different heat treatments as well. It was found that regardless of which temperature applied during the WLSP process, the treated specimens all displayed high-amplitude CRS on the surface or in depth, but the surface peak CRS decreases gradually as temperature increases. Specially, when the WLP temperature exceeded 250°C, the decreasing slope raised. The CRS affected width increased first, and then decreased with the increases of temperature. The maximum CRS affected width was about 6.25 mm, appearing at a temperature of 250 °C. For the affected depth of CRS, firstly, as the temperature increases from 20 to 250 °C, the thickness of affected layer rises with temperature. However, when the temperature exceeds 300°C, the affected depth of CRS sharply declines. In addition, the WLSP technology could improve the hardness of Ti6Al4V titanium alloy from 389 HV at room temperature to 418 HV at 350 °C

Experimental study on grinding-induced residual stress in C-250 maraging steel

Residual stress plays a significant role in the performance of a part, while the residual stress in the ground maraging steel, which is often used in the manufacture of precision parts, is rarely mentioned. In order to understand the variations of residual stress in ground maraging steel and provide insight into the controlled-stress grinding process of the steel, the surface and subsurface residual stress distributions in ground C-250 maraging steel (3J33) were studied. The results show that the mechanical effects dominate the thermal effects in the dry grinding process, indicated by only compressive residual stress generated in the ground workpiece. Furthermore, more insights into the residual stress distribution were provided by proposing four residual stress distribution parameters including surface residual stress, peak compressive residual stress, the depth of peak compressive residual stress, and residual stress penetration depth. The variations of these parameters were comprehensively studied. Results show that the surface residual stress and peak compressive residual stress depend greatly on the grinding speed and higher grinding speed generates larger compressive residual stress, while the depth of peak compressive residual stress varies slightly with the grinding parameters. The residual stress penetration depth increases with the increase of the grinding speed and grinding depth, and decreases with the increase of the workpiece speed. The results in this study can be used to assist in controlled-stress grinding applications for high performance critical parts of maraging steel.

Effect of alloy temper on surface modification of aluminium 2624 by laser shock peening

We have investigated the hardening response, residual stress generation and microstructural changes in aluminium alloy 2624 owing to laser shock peening. The alloy was studied in two heat treatment conditions, T351 and T39, that have 20% difference in yield strength: hence the effects of laser power density and multiple peen impacts on materials with nominally identical physical properties but with different hardening responses has been studied. Hardness was characterised by nanoindentation, and residual stresses were measured by incremental hole drilling.The magnitude and the depth of the peak compressive residual stresses increase with increasing power densities as well as the number of laser impacts, before reaching a saturation point above which loss of surface compression occurs. Maximum compressive residual stresses were around −350 MPa, and maximum hardness increase was around 22%. The treatment has a noticeable effect in changing the microstructures of the T351 temper while the T39 remained almost unchanged.

Simulation and experimental validation of residual stress and surface roughness of high manganese steel after shot peening

Shot peening(SP) is one of the most effective surface strengthening treatment technologies in which compressive residual stress(CRS) is induced beneath the specimen surface. In this study, residual stress distribution and surface roughness introduced by SP in the deformed surface layer of high manganese steel were investigated by means of three-dimensional (3D) finite element dynamic simulation and experimental validation. The effects of SP time, SP coverage rate and velocity of shot balls on the residual stress distribution and surface roughness were studied. The surface residual stresses of high manganese steel after SP were measured using X-ray diffraction (XRD) method. And the results of CRS simulation were consistent with the results of experiment, which verified the accuracy of the 3D finite element dynamic analysis. It is shown from simulation that both the peak value and the depth of compressive residual stresses increased with the increase of velocity of shot balls, and they gradually increase with the growth of SP time before they became stable, but the depth of the peak CRS was almost constant. Surface roughness also increased with the growth of SP time and velocity of shot balls.

Characterization of the microstructure, mechanical properties, and shot peening response of an industrially processed Al–Zn–Mg–Cu PM alloy

The objective of this study was to characterize the mechanical/physical properties of an industrially processed Al–Zn–Mg–Cu powder metallurgy (PM) alloy and assess the subsequent effects of shot peening. The research involved a number of experimental techniques, including density measurements, tensile testing, Rockwell hardness measurements, nano-indentation, and optical and scanning electron microscopy. Residual stress measurements were completed using a combination of X-ray diffraction (XRD) and neutron diffraction (ND). Industrially produced specimens attained near full theoretical density and exhibited a nominal yield strength on the order of 460MPa in the T6 condition. It was discovered that zinc had preferentially evaporated from the surface of the components during sintering. The depleted region persisted to a depth of ≈3mm and resulted in reduced nano-hardness of 1.65GPa at the surface versus 2.50GPa in the bulk. Shot peening increased the surface hardness of the alloy and resulted in a peak compressive residual stress of 232MPa at the treated surface.

Multiobjective Optimization of Experimental and Simulated Residual Stresses in Turning of Nickel-Alloy IN100

In this study, physics-based simulations are utilized to predict the forces and residual stresses induced during machining, and the results were validated using the experimental measurements. Physics-based simulations also involve uncertainty in the predicted values that can be represented as expected value and variance of the predictions. These predictions are inputted to a multiobjective optimization methodology to select the optimal machining parameters where competing or conflicting objectives constitute hurdles in the decision-making of the manufacturing plans. The objectives are chosen as related to residual stress measurements and predictions. Multiobjective particle swarm optimization (PSO) procedure is employed in optimizing process parameters. Objectives are solved for minimizing tensile residual stresses on the surface, maximizing peak compressive residual stresses, and minimizing the variance of these variables in order to increase certainty in the predictions. The optimum machining parameters corresponding to this multiobjective optimization are represented in both objective function and decision variable spaces.

Influence of material microstructure changes on surface integrity in hard machining of AISI 52100 steel

Residual stresses are a consequence of thermo-mechanical and microstructural phenomena generated during the machining operation. Therefore, for improving product performance in machined hardened steels, material microstructure changes (commonly referred to as white and dark layers) must be taken into account. This paper presents a finite element model for white and dark layers formation in orthogonal machining of hardened AISI 52100 steel. In particular, a hardness-based flow stress and empirical models for describing the white and dark layers formation were developed and implemented in the finite element code. A series of experiments was carried out in order to validate the proposed simulation strategy and to investigate the influence of material microstructure changes on residual stresses. As main results, it was firstly demonstrated by surface topography analysis as both the white and dark layer are the result of microstructural alterations mainly due to rapid heating and quenching. Furthermore, it was found as both the presence of white and dark layers influence the residual stress profile. Particularly, the former significant impacts on the magnitude of maximum residual stress and on the location of the peak compressive residual stress; the latter reduces the compressive area.

Modeling of white layer formation under thermally dominant conditions in orthogonal machining of hardened AISI 52100 steel

This paper presents a finite element model for white layer formation in orthogonal machining of hardened AISI 52100 steel under thermally dominant cutting conditions that promote martensitic phase transformations. The model explicitly accounts for the effects of stress and strain, transformation plasticity and the effect of volume expansion accompanying phase transformation on the transformation temperature. Model predictions of white layer depth are found to be in agreement with experimental values. The paper also analyzes the effect of white layer formation on residual stress evolution in orthogonal cutting of AISI 52100 hardened steel. Model simulations show that white layer formation does have a significant impact on the magnitude of surface residual stress and on the location of the peak compressive residual stress.

Lift-off effect in high-frequency eddy current conductivity spectroscopy

Precision eddy current measurements have been shown to be capable of characterizing the near-surface residual stress and cold work profiles in surface-treated components. To capture the peak compressive residual stress in moderately shot-peened (Almen 4–8 A) nickel-base superalloys, the eddy current inspection frequency has to be as high as 50–80 MHz. Unfortunately, spurious self- and stray-capacitance effects render the complex eddy current coil impedance variation with lift-off, the so-called lift-off curve, highly nonlinear, which makes it difficult to achieve accurate eddy current conductivity measurements beyond 25 MHz in the presence of even the slightest lift-off uncertainties. As opposed to the well-known inductive lift-off effect that decreases with increasing probe size, the capacitive lift-off effect increases with probe size. Both effects increase with frequency with the inductive effect being initially stronger, but then taken over at high frequencies by the faster growing capacitive effect. Since the two effects produce opposite curvature in the lift-off curve, in the frequency range where they are approximately equal to the lift-off curve becomes essentially linear and fairly accurate, conductivity measurements can be conducted even in the presence of lift-off variations.

Thermal Residual Stress Relaxation in Powder Metal IN100 Superalloy

Abstract Relaxation of shot peen induced compressive residual stresses due to thermal exposure was measured using X-ray diffraction. The material used in this study was a hot isostatically pressed (HIP) powder metal (PM) IN100 nickel base superalloy. A total of 14 IN100 samples were shot peened to an Almen intensity of 6A using MI-170-R shot with 125 % coverage. The sample dimensions were nominally 16×13×4-mm thick with an irradiated X-ray region of 8×5 mm. Residual stress measurements were made at the surface and at nominal depths of 12, 25, 50, 75, 125, 175, 250, and 350 microns. The shot peened samples were thermal exposed at two temperatures (650, 704°C) and a range of exposure times (0.5–300 h). Residual stress measurements on shot peened samples without thermal exposure were used as a basis for comparison. The relaxation of shot peened compressive residual stresses under purely thermal loading was examined. The residual stresses exhibited an initial rapid decrease on the surface and in the depth at both temperatures. However, continued thermal exposure produced little or no change in surface residual stresses while peak compressive stresses in the depth continued to relax with time at both temperatures. In all cases of this study the retained peak compressive residual stress after thermal exposure was greater than 50 % of the baseline value.

Residual stress modification by post-weld treatment and its beneficial effect on fatigue strength of welded structures

High tensile weld residual stress is one important factor contributing to fatigue crack development even under reversal or compressive cyclic loadings. A compressive stress induced by post-weld treatment is beneficial by eliminating the tensile residual stresses and generating compressive residual stresses, which improves fatigue strength of welded structures. A study is underway to characterize the magnitude and subsurface distribution of residual stresses produced by post-weld treatments, particularly by Ultrasonic Impact Treatment (UIT), and to establish the post-weld treatment effect on fatigue resistance. Two post-weld treatments, UIT and shot peening, are involved in the present study. Internal stresses were investigated on three base metal samples treated by UIT and shot peening and on one welded sample treated by UIT through neutron diffraction and X-ray diffraction techniques. This paper presents the experimental conditions and the results of these stress measurements. It is revealed that the peak compressive residual stress induced by both treatments exceeded the yield stress of the base material near the sample surface. The depth of compressive stress layer in base metal was 1.5–1.7 mm for UIT and ~0.8 mm for shot peening. The induced compressive residual stresses benefit to increase the threshold value of stress intensity factor range, Δ K th, for fatigue crack initiation and early propagation. The effect of residual stresses modification on fatigue strength is discussed based on fracture mechanics model and verified by fatigue test results of large-scale welded beams.

Hard turning versus grinding—the effect of process-induced residual stress on rolling contact

Compared with grinding, hard turning may induce a relatively deep compressive residual stress while still achieving an equivalent surface finish. Although it is generally agreed that deep compressive residual stresses are beneficial to component life in rolling contact, the interactions between the residual stress profile, applied load, and surface material are poorly understood at present. With the input of process-induced residual stress profile instead of one point “surface” residual stress, a simulation model of rolling contact for hard-machined components has been developed by directly modeling the interaction between the roller and the machined surface, rather than moving the assumed Hertzian pressure and tangential surface traction across the surface of the traditional method. The equivalent plastic strain could characterize relative fatigue damage under the influence of process-induced residual stress. The slope of a compressive residual stress profile as well as its depth were identified as key factors for rolling contact fatigue damage. Rolling contact tend to reduce the initial compressive residual stress. The predicted residual stress pattern and magnitude agree with the test data in general. The increase of friction dramatically increases the magnitude of peak compressive residual stress but not its location. The normal load shifts the location of peak compressive residual stress much deeper below the surface.