A new procedure based on Sachs’ boring for measuring non-axisymmetric residual stresses: experimental application

A new procedure has been developed based on Sachs’ boring to measure non-axisymmetric residual stresses in cylindrical components. The new procedure uses a Fourier series to represent the residual stresses. This paper describes an experimental application of the procedure to measure the residual stresses around a hole in a specimen subjected to a uniaxial overload. Good agreement is found between the measured residual stresses and those predicted using a finite element simulation.     

Effects of micro-stresses from machining and shot-peening processes on fatigue life

An investigation of the effects of tensile machining and compressive shot-peening residual stresses on fatigue life is presented. The paper is focused on fatigue life modelling using the finite element method (FEM). A representative broached notched specimen under three-point bending is modelled as a 2D plane stress geometry using the FEM. Inconel 718 mechanical and fatigue material properties at 600°C are used. Machining and shot-peening residual stresses are mapped to the macro FE models using mathematical algorithms around the notched area of the specimen and their influence on the fatigue life experiencing high cycle fatigue (HCF) and low cycle fatigue (LCF) is investigated. The results show that the compressive shot-peening residual stresses significantly increase the life at HCF compared to the LCF.     

Application of quenching to create highly triaxial residual stresses in type 316H stainless steels

It has been proposed that highly triaxial residual stress fields may be sufficient to promote creep damage in thermally aged components, even in the absence of in-service loads. To test this proposal, it is necessary to create test specimens containing highly triaxial residual stress fields over a significant volume of the specimen. This paper presents results from an experimental and numerical study on the generation of triaxial residual stresses in stainless steel test specimens. Spray water quenching was used to generate residual stress fields in solid cylinders and spheres made from type 316H stainless steel. A series of finite element simulations and measurements were carried out to determine how process conditions and specimen dimensions influenced the resulting residual stress distributions. The results showed that highly compressive residual stresses occurred around the surfaces of the cylinders and spheres and tensile residual stresses occurred near the centre. Surface residual stresses were measured using the incremental centre hole-drilling technique, while internal residual stresses were measured using neutron diffraction. Overall there was good agreement between the predicted and measured residual stresses. The level of triaxiality was found to be very sensitive to the heat transfer coefficient, and could be controlled by adjusting the cooling conditions and changing the dimensions of the steel samples. This differed from other processes, such as welding and shot-peening, where the magnitudes and distributions of residual stresses are ill-defined and the volume of material subjected to a triaxial residual stress state is relatively small.     

Progression of fretting fatigue damage in Ti–6Al–4V

An investigation was conducted to explore the nature of fretting fatigue damage in the stages prior to crack formation. In the unique experimental apparatus employed in this study, where total slip never occurs, several locations on each test specimen exist where cracks can develop due to local contact conditions. Under the test conditions used, not all of the sites had cracks upon test completion. This study evaluated the condition of non-cracked sites on several fretted specimens in an effort to identify differences between these and sites where small cracks were observed.A single test condition of 620 MPa average applied static clamping stress and 250 MPa applied axial fatigue stress for R=0.5 was selected, which corresponds to a fretting fatigue life of 10⁷ cycles based on prior work. For specimens tested to 10⁶ cycles, or 10% of life, several destructive and non-destructive characterization methods were chosen: scanning electron microscopy (SEM), residual stress measurement and transmission electron microscopy (TEM). Each site at which crack nucleation could be expected was inspected in the SEM and was then characterized using surface X-ray diffraction to quantify the residual stresses field near that location. Then TEM foils were cut from one area on a specimen with tiny cracks and dislocation densities were observed. A novel technique was used which permitted TEM samples to be obtained from regions in close proximity on the original specimen.Comparisons were made between as-received (AR) and stress-relief annealed (SRA) specimens, on which the stress-relief was applied prior to fretting fatigue testing. SEM inspection was useful for qualitative analysis of wear debris and identification of cracks as small as 20 μm, but was unable to provide quantitative data on the level of fretting fatigue damage beyond crack size. Although differences were noted in the residual stresses for the SRA versus the AR specimens, no residual stress peaks were noted in the edge of contact regions where cracks would eventually develop. TEM observations in the vicinity of the crack nucleation region showed that the dislocation structure decayed rapidly into the specimen thickness. The cause of the dislocations was attributed to plastic deformation caused by the clamping stresses.     

A case study on the effect of thermal residual stresses on the crack-driving force in linear-elastic bimaterials

The effect of thermal residual stresses in bimaterial structure with initial crack located near a sharp interface is discussed in this paper. Bimaterial compact tension (CT) specimen is used in the analysis, and the residual stresses are introduced by cooling of the specimen. The residual stresses affect the stress and strain fields near the crack tip, and the crack-driving force is different compared with that in the homogeneous material without residual stresses. This difference can be quantitatively expressed through an additional crack-driving force term—the material inhomogeneity term, Cinh. In this paper, it is evaluated using the post-processing procedure based on the concept of configurational forces, following a finite-element analysis. The results indicate that accurate numerical analysis of pre-cracked bimaterials should include the effect of thermally induced residual stresses. This effect cannot be neglected, even for bimaterials with homogeneous mechanical properties and inhomogeneity in thermal properties only (e.g. welded joints of ferritic and austenitic steel). Based on the obtained results, data from this study can be used in engineering practice to improve integrity and work safety of various inhomogeneous structures.     

Investigation of the influence of specimen geometry on quench behaviour of steels by X-ray determination of surface residual stresses

Considerable residual stresses may form during quenching due to the differential cooling and the increase in volume accompanying the phase transformations. The design of a part may be entirely responsible for the formation of residual stresses at a critical level and even cracking during quenching. Furthermore, a certain design may be perfectly safe for one type of steel, or one type cooling conditions, and unsafe for another.In this study, an experimental procedure to investigate the influence of specimen geometry on the evolution residual stresses is proposed. The cylindrical specimens with 30 mm outer diameter were prepared from C60 and 90MnCrV8 steel bars. First, solid cylinders were quenched according to different procedures. Then, the treatment giving the minimum residual stress was applied to the hollow cylinders having various hole diameter and degree of eccentricity. By changing the position of holes in the cross-section of the specimens, a thickness gradient as a function of the eccentricity ratio was obtained. Thus, for a given transformation behaviour and quenching conditions, the effect of shape becomes more discriminating on the eccentrically drilled holes. The tangential residual stresses were determined at the specified points along the circumference of the cylinders by X-ray diffraction and d-sin² Ψ technique. The microstructures of the specimens were determined by metallographic investigation also using hardness values and respective CCT-diagrams. The results were discussed considering the microstructural evaluation of the specimens.     

On analysis and measurement of residual stresses in the bending of sheet metals

A new method for evaluating distribution of residual stresses in bent sheet metals is presented. Due to the non-uniform distribution of strain and stress across thickness, springback on unloading induces residual stress in a bent specimen. The authors have formulated the problem and shown that the springback and residual stresses can be expressed as a function of geometric parameters and material properties of sheet metals, e.g. bending curvature, thickness, Young's modulus, work-hardening index, etc. The layer-removing method was used to determine the residual stresses in the study. By simulating the layer-removing process, analytical measurement of the residual stress was made. The analytical results were compared with the experimental data measured, and a good agreement was found.     

Finite element analysis of the effect of sequential cuts and tool–chip friction on residual stresses in a machined layer

A thermo-elastic–viscoplastic model using explicit finite element code Abaqus was developed to investigate the effect of sequential cuts and tool–chip friction on residual stresses in a machined layer. Chip formation, cutting forces and temperature were also examined in the sequential cuts. The affected layer from the first cut slightly changes the chip thickness, cutting forces, residual strain and temperature of the machined layer, but significantly affects the residual stress distribution produced by the second cut. Residual stress is sensitive to friction condition of the tool–chip interface. Simulation results offer an insight into residual stresses induced in sequential cuts. Based on simulation results, characteristics of residual stress distribution can be controlled by optimizing the second cut.     

Comparison of experimental and theoretical residual stresses in welds: The issue of gauge volume

Welding residual stresses have an effect on many aspects of the integrity of structures but are normally one of the largest unknown stresses. Residual stresses are difficult to measure and to estimate theoretically but are often significant when compared with the service stresses on which they superimpose. High tensile residual stresses can lead to loss of performance in corrosion, fatigue and fracture.In this research, measurement of residual stresses by the neutron diffraction technique is compared to an analysis of a sample geometry by theoretical finite-element procedures. The results indicate good qualitative agreement. One of the key issues in this comparison relates to what is termed “gauge volume” in the measurement technologies and what might be described as a “calculation volume” in theoretical approaches.     

Specific Features of the Behavior of the Coercive Force in Low-Carbon Plastically Deformed Steels

The relationship between the coercive force in low-carbon steels under plastic extension and compression and the values of deformation and actual and residual stresses are studied. This relationship is investigated for both “ slow” loading (when an equilibrium deformation is attained for each load value) and “fast” loading (when such equilibrium is not attained). It is shown that (i) a comparatively small increase in the coercive force in a loaded condition is due only to an increase in the density of dislocations in the process of plastic extension; (ii) a significant steep increase in the coercive force accompanying removal of the load from a plastically stretched specimen is fully due to residual compression stresses; (iii) the values of the coercive force under “slow” and “fast” loading are significantly different in the region of small deformations less than 2.5%; (iv) these values are close to each other in the loaded state for all deformations up to 10%; (v) a relief of the compression stress that creates plastic deformations causes a steep decrease in the coercive force that is as large as its increase following relief of plastic extension; this is explained by the emergence of a significant residual tension stress. The obtained results are of importance for the use of the method based on measuring the coercive force to test steel structures under the conditions when plastic deformations develop.__________Translated from Defektoskopiya, Vol. 41, No. 5, 2005, pp. 24–38.Original Russian Text Copyright © 2005 by Kuleev, Tsar’kova, Nichipuruk.     

A new procedure based on Sachs’ boring for measuring non-axisymmetric residual stresses

Sachs’ boring is an established technique for the measurement of axisymmetric residual stresses in cylindrical components. There are however important cases where non-axisymmetric residual stresses need to be found. In this paper a new procedure is developed to measure non-axisymmetric residual stresses. The essential feature of the new procedure is the use of a Fourier series to represent the residual stresses. The correctness of the new procedure is demonstrated using a finite element simulation where a set of non-axisymmetric residual stresses are determined. Excellent agreement is found between the predicted residual stresses and those derived using a finite element simulation of the new procedure.     

Micro-Raman depth analysis of residual stress in machined germanium

Raman spectroscopy has proved to be a useful nondestructive technique for measuring residual stresses in semicondutors. The Raman microprobe is used to investigate the effects of machine parameters on residual stresses in single point diamond turned germanium (Ge). A profiling technique that provides a method of obtaining the residual stress information as a function of depth with depth resolutions of 10.0 nm is discussed. This method is used to analyze the asymmetrically broadened and shifted spectral features in the machined samples. Residual stresses are sampled across ductile feed cuts in (100) Ge wafers, which were single point diamond turned using various feed rates (12.5, 25 nm/rev), rake angles (0°, −10°, −30°), and clearance angles (6°, 16°). In general a region of plastically deformed material that shows slight compressive stresses exist near the surface of the diamond turned sample. The compressive surface stress increases to a maximum at a depth of ≈ 50 nm beneath the surface at which point the stress rapidly changes sign. The rapid sign change is indicative of the transition from plastic to elastic deformation. Deeper probe regions exhibit increasing tensile stresses, which reach a maximum and then relax to zero at greater depths in the sample. A related study of the stress field occurring around Vicker's hardness indents provides a link between theoretical and experimental stress profiles and demonstrates the accuracy of the micro-Raman profiling technique.     

A Comparative Study of Residual Stress Distribution Induced by Hard Machining Versus Grinding

This study investigates the residual stress distribution induced by hard machining and grinding and compares its impact on fatigue parameters. The residual stress distribution below hard turned and ground surfaces is investigated after a thermally damaged layer is removed. Fatigue parameters are computed based on the residual stress distribution to compare the impact of the residual stress distribution on the fatigue performance. Rolling contact fatigue tests are then performed to substantiate the computations. The effect of residual stresses on crack initiation depth is shown to be significant for the ground specimen. The maximum shear stress at crack initiation depth of the hard turned specimen is smaller than that of the ground specimen. Due to a significant increase in crack initiation life, the predicted rolling contact fatigue life of the hard turned specimen is longer than that of the ground specimen. The overall average in the ratios of predicted life to experimental life for the hard turned specimen is closer to 1 than that for the ground specimen. The results demonstrate that the hard turned specimen shows better rolling contact fatigue performance and better accuracy in the fatigue life prediction.     

Finite element modeling of residual stresses in machining induced by cutting using a tool with finite edge radius

A thermal elastic-viscoplastic finite element model is used to evaluate the residual stresses remaining in a machined component. An improvement in the accuracy of the predicted residual stresses is obtained by: (a) using a modified Johnson–Cook material model that is augmented by a linearly elastic component to describe the material behavior as non-Newtonian fluid; (b) using a remeshing scheme to simulate the material flow in the vicinity of the rounded cutting tool edge without the use of a separation criterion; (c) properly accounting for the unloading path, and (d) considering the thermomechanical coupling effect on deformation. Case studies are performed to study the influence of sequential cuts, cutting conditions, etc., on the residual stresses induced by orthogonal machining.     

Modelling the evolution of residual stresses during tensile testing of elastoplastic wires subjected to a previous bending operation

During coiling operations high residual stresses are frequently developed in steel wire. In this paper the stress distribution in wires during coiling, unwinding and subsequent tensile testing is modelled for numerous bending degrees, assuming perfect Voce plastic deformation and linear elastic behaviour. The influence of such residual stresses on the observed tensile test data can be deduced. It is shown that coiling with spool radii as used today industrially can lead to measurement of wire properties deviating significantly from the “true” properties of a properly coiled wire. Also, a method is proposed to deduce the original flow behaviour of coiled samples from tensile test curves, hence filtering the effect of the residual stresses.     

Simulation of thermal stresses due to grinding

An efficient finite element procedure has been developed to calculate the temperatures and stresses arising due to a moving source of heat. The procedure is applied to calculate the thermal stresses produced in hardened steels during grinding. The thermal load during grinding is modeled as a uniformly or triangularly distributed, 2D heat source moving across the surface of a half-space, which is insulated or subjected to convective cooling. The grinding of elastic and elastic–plastic workpiece materials has been simulated. The calculated transient stresses and temperatures in an elastic solid are found to be in good agreement with prior analytical and numerical results. In an elastic–plastic workpiece material, for which no analytical solution is available for the residual stress distributions, the finite element calculations show that the near surface residual stress is predominantly tensile and that the magnitude of this stress increases with increasing heat flux values. Based on an analysis of the effects of workpiece velocity, heat flux magnitude and convective cooling, on the residual stress distributions in an elastic–plastic solid, it is seen that the calculated thermal stress distributions are consistent with experimentally measured residual stresses on ground surfaces. Furthermore, the results explain often cited observations pertaining to thermally induced grinding stresses in metals.     

喷丸强化残余应力对疲劳性能和变形控制影响研究进展

作为机械表面强化技术之一,喷丸强化使工件表层发生形变硬化,引入较高的残余压应力,减少了疲劳应力作用下微裂纹的萌生并抑制其扩展,从而显著提高零件的抗疲劳断裂和抗应力腐蚀开裂的能力。基于喷丸残余应力解析计算模型,从余弦函数模型、接触应力模型和球腔膨胀模型三个方面介绍喷丸强化残余应力的产生,进而对喷丸残余应力的仿真预测及影响规律进行论述。为了提高试件疲劳强度而引入的残余压应力会带来影响形位精度的变形,基于此阐述了喷丸残余应力对疲劳性能的影响及其在疲劳过程中的演化,同时论述了喷丸残余应力变形预测及控制的研究现状,最后对喷丸残余应力未来的研究内容与发展方向进行展望。     

Three-dimensional finite element analysis of cold expansion of adjacent holes

Three-dimensional elasto-plastic finite element analysis was conducted to evaluate the development and growth of the plastic zone and unloading residual stresses resulting from the cold expansion of two adjacent holes. The contact between the mandrel and the hole was modelled using special contact elements. The work examined the effect of the geometry of two adjacent cold-expanded holes upon the resulting three-dimensional residual stress field. Both simultaneous and sequential expansion of the two holes was considered. It was shown that the compressive residual stress varies across the thickness of the specimen and that two-dimensional finite element models are incapable of accurately predicting the residual stresses resulting from the cold expansion process. It was further shown that sequential expansion reduces dramatically the compressive residual stresses. The validity of the developed finite element model was verified with existing experimental results.     

垫板导热能力对钛合金薄板焊接残余应力的影响

针对钛合金TC4薄板钨极氩弧焊(GTAW)分别在试件背面衬以铜垫板与覆盖石棉的垫板两种情况下所焊的对接试件,采用切条应力释放法测量了其中纵向残余应力和纵向残余塑性应变的分布,比较研究了不同导热能力的垫板对钛合金薄板焊接残余应力及纵向残余塑性应变的影响。测量结果表明:钛合金GTAW焊接过程中垫板不仅提供了对焊缝背面的保护,也影响了焊接纵向残余应力与纵向残余塑性应变的分布与大小。不同导热能力的垫板控制应力与变形的效果不同。铜垫板控制应力与变形的效果好于覆盖石棉的垫板。     

Residual stress field of ballised holes

Ballising, involving pushing a slightly over-sized ball made of hard material through a hole, is a kind of cold working process. Applying ballising process to fastener holes produces compressive residual stress on the edge of the holes, and therefore increases the fatigue life of the components or structures. Quantification of the residual stress field is critical to define and precede the ballising process. In this article, the ballised holes are modeled as cold-expanded holes. Elastic-perfectly plastic theory is employed to analyze the holes with cold expansion process. For theoretical simplification, an axially symmetrical thin plate with a cold expanded hole is assumed. The elasticplastic boundaries and residual stress distribution surrounding the cold expanded hole are derived. With the analysis, the residual stress field can be obtained together with actual cold expansion process in which only the diameters of hole before and after cold expansion need to be measured. As it is a non-destructive method, it provides a convenient way to estimate the elastic-plastic boundaries and residual stresses of cold worked holes. The approach is later extended to the case involving two cold-worked holes. A ballised hole is looked upon as a cold expanded hole and therefore is investigated by the approach. Specimens ballised with different interference levels are investigated. The effects of interference levels and specimen size on residual stresses are studied. The overall residual stresses of plates with two ballised holes are obtained by superposing the residual stresses induced on a single ballised hole. The effects of distance between the centers of the two holes with different interference levels on the residual stress field are revealed.