Repeatability of the Contour Method for Residual Stress Measurement

This paper describes the results of a residual stress measurement repeatability study using the contour method. The test specimen is an aluminum bar (cut from plate), with cross sectional dimensions of 50.8 mm?×?76.2 mm (2 in?×?3 in) with a length of 609.6 mm (24 in). There are two bars, one bar with high residual stresses and one bar with low residual stresses. The high residual stress configuration (±150 MPa) is in a quenched and over-aged condition (Al 7050-T74) and the low residual stress configuration (±20 MPa) is stress relieved by stretching (Al 7050-T7451). Five contour measurements were performed on each aluminum bar at the mid-length of successively smaller pieces. Typical contour method procedures are employed with careful clamping of the specimen, wire electric discharge machining (EDM) for the cut, laser surface profiling of the cut faces, surface profile fitting, and linear elastic stress analysis. The measurement results provide repeatability data for the contour method, and the difference in repeatability when measuring high or low magnitude stresses. The results show similar repeatability standard deviation for both samples, being less than 10 MPa over most of the cross section and somewhat larger, around 20 MPa, near the cross section edges. A comparison with published repeatability data for other residual stress measurement techniques (x-ray diffraction, incremental hole drilling, and slitting) shows that the contour method has a level of repeatability that is similar to, or better than, other techniques.     

Neutron Diffraction Analysis of Complete Residual Stress Tensors in Conventional and Rolled Gas Metal Arc Welds

Mitigation of residual stress in an arc weld by high-pressure rolling of the weld seam has been investigated using neutron diffraction. Rolling was found to greatly improve the residual stress distribution, causing significant compressive stresses at the weld line. A novel aspect of the data presented is that at each measurement location, normal strains in nine separate directions were evaluated, enabling calculation of the complete strain and stress tensors. It is thus confirmed that the principal stress directions generally lay close to the specimen coordinate axes (i.e. that they are well-aligned with the direction of welding and rolling), and that rolling does not cause any significant additional residual stresses which could have detrimental effects. Methods of uncertainty estimation and the applications of full-tensor residual stress measurements are also discussed.     

Measuring Inaccessible Residual Stresses Using Multiple Methods and Superposition

The traditional contour method maps a single component of residual stress by cutting a body carefully in two and measuring the contour of the cut surface. The cut also exposes previously inaccessible regions of the body to residual stress measurement using a variety of other techniques, but the stresses have been changed by the relaxation after cutting. In this paper, it is shown that superposition of stresses measured post-cutting with results from the contour method analysis can determine the original (pre-cut) residual stresses. The general superposition theory using Bueckner’s principle is developed and limitations are discussed. The procedure is experimentally demonstrated by determining the triaxial residual stress state on a cross section plane. The 2024-T351 aluminum alloy test specimen was a disk plastically indented to produce multiaxial residual stresses. After cutting the disk in half, the stresses on the cut surface of one half were determined with X-ray diffraction and with hole drilling on the other half. To determine the original residual stresses, the measured surface stresses were superimposed with the change stress calculated by the contour method. Within uncertainty, the results agreed with neutron diffraction measurements taken on an uncut disk.     

Intergranular strain evolution near fatigue crack tips in polycrystalline metals

The deformation field near a steady fatigue crack includes a plastic zone in front of the crack tip and a plastic wake behind it, and the magnitude, distribution, and history of the residual strain along the crack path depend on the stress multiaxiality, material properties, and history of stress intensity factor and crack growth rate. An in situ, full-field, non-destructive measurement of lattice strain (which relies on the intergranular interactions of the inhomogeneous deformation fields in neighboring grains) by neutron diffraction techniques has been performed for the fatigue test of a Ni-based superalloy compact tension specimen. These microscopic grain level measurements provided unprecedented information on the fatigue growth mechanisms. A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is adopted to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Predictions from perfect grain-boundary simulations exhibit the same lattice strain distributions as in neutron diffraction measurements, except for discrepancies near the crack tip within about one-tenth of the plastic zone size. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find a significantly improved agreement between predicted and measured lattice strains inside the fatigue process zone. Consequently, the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.     

Controlling the Cut in Contour Residual Stress Measurements of Electron Beam Welded Ti-6Al-4V Alloy Plates

The multiple cut contour method is applied to map longitudinal and transverse components of residual stress in two nominally identical 50 mm thick electron beam welded Ti-6Al-4V alloy plates, one in the as-welded condition and a second welded plate in a post weld heat treated (PWHT) condition. The accuracy and resolution of the contour method results are directly linked to the quality of the electro-discharge machining cut made. Two symmetric surface contour artefacts associated with cutting titanium, surface bowing and a flared edge, are identified and their influence on residual stresses calculated by the contour method is quantified. The former artefact is controlled by undertaking a series of cutting trials with reduced power settings to find optimal cutting conditions. The latter is mitigated by attaching 5 mm thick sacrificial plates to the wire exit side of the test specimen. The low level of noise in the measured stress profiles for both the as-welded and PWHT plates demonstrates the importance of controlling the quality of a contour cut and the added value of undertaking cutting trials.     

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

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.     

A study of the generation and creep relaxation of triaxial residual stresses in stainless steel

This paper presents results from a numerical and experimental research programme motivated by the need to predict creep damage generated by multi-axial states of stress in austenitic stainless steels. It has been hypothesized 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. Two prerequisites to test this hypothesis are the provision of test specimens containing a highly triaxial residual stress field and an accurate knowledge of how this residual stress field relaxes due to creep. Creep damage predictions may then be made for these specimens and compared to damage observed in experiments. This paper provides solutions to both of these prerequisites. Cylindrical and spherical test specimens made from type 316H stainless steel are heated to 850 °C and then quenched in water. Finite element predictions of the residual stress state, validated by extensive neutron diffraction measurements, are presented which confirm the high level of triaxiality present in the specimens. The specimens are then thermally aged at 550 °C and numerical predictions of the residual stress relaxation are given, again validated by extensive neutron diffraction measurements. The results confirm the validity of the creep relaxation models employed. In addition, the results show the influence of specimen size and permit comparisons to be made between three different types of neutron diffractometers.     

Measurement of Torsionally Induced Shear Stresses in Shrink-Fit Assemblies

Shear stresses along the shaft/hub interface in shrink-fit components, generated by torsional loads, can drive premature failure through fretting mechanisms. It is difficult to numerically predict these shear stresses, and the associated circumferential slip along the shaft/hub interface, due to uncertainties in frictional behaviour and the presence of steep stress gradients which can cause meshing problems. This paper attempts to provide validation of a numerical modelling methodology, based on finite element analysis, so the procedure may be used with confidence in fitness-for-purpose cases. Very few experimental techniques offer the potential to make measurements of stress and residual stress interior to metallic components. Even fewer techniques provide the possibility of measuring shear stresses. This paper reports the results of neutron diffraction measurements of shear stress and residual shear stress in a bespoke test specimen containing a shrink-fit. One set of measurements was made with a torsional load ‘locked-in’. A second set of measurements was made to determine the residual shear stress when the torsional load had been applied and removed. Overall, measurement results were consistent with numerical models, but the necessity for a small test specimen to allow penetration of the neutron beam to the measurement locations meant the magnitude of shear stresses was at the limits of what could be measured experimentally.     

CTOD试验焊缝表面开缺口试样裂纹前沿平直度研究

对于裂纹尖端张开位移(CTOD)试验,焊缝试样中预制疲劳裂纹前沿平直度直接影响了试样制备的合格率,是试验的关键难题之一．试验中常对试样进行预处理以提高裂纹前沿平直度,但由此也使试验结果与实际情况产生一定差异．本文深入研究规范BS7448: Part2中对焊缝试样取样方向的规定,对表面开缺口试样裂纹尖端焊接残余应力进行分析,运用大型有限元软件ANSYS模拟90mm厚钢板焊接过程,求解了横向残余应力沿板厚方向及焊缝方向的分布规律,研究出横向残余应力分布是影响焊缝试样预制裂纹前沿平直度的主要原因,并通过试验进行验证．试验结果表明,表面开缺口试样可不经过局部韧带压缩等预处理而得到合格的裂纹前沿平直度,试验不改变原焊缝残余应力,可测得更加接近焊缝实际情况的CTOD韧度值,给CTOD试验中合理选取焊缝试样取样方向提供了新思路．     

Neutron Diffraction Residual Strain Measurements of Molybdenum Carbide-Based Solid Oxide Fuel Cell Anode Layers with Metal Oxides on Hastelloy X

Thermal spray deposition processes impart residual stress in layered Solid Oxide Fuel Cells (SOFC) materials and hence influence the durability and efficiency of the cell. The current study which is the first of its kind in published literature, reports results on using a neutron diffraction technique, to non-destructively evaluate the through thickness strain measurement in plasma sprayed (as-sprayed) anode layer coatings on a Hastelloy®X substrate. Through thickness neutron diffraction residual strain measurements were done on three different anode coatings (Mo-Mo₂C/Al₂O₃, Mo-Mo₂C/ZrO₂ and Mo-Mo₂C/TiO₂) using the vertical scan mode. The three anode coatings (developed through optimised process parameters) investigated had porosities as high as 20%, with thicknesses between 200 μm to 300 μm deposited on 4.76 mm thick Hastelloy®X substrate discs of 20 mm diameter. The results showed that while the through thickness residual strain in all three anodes was dissimilar for the investigated crystallographic planes, on average it was tensile. Other measurements include X-ray diffraction, nanoindentation and SEM microscopy. As the anode layer microstructures are complex (includes bi-layer alternate phases), non-destructive characterisation of residual strain, e.g. using neutron diffraction, provides a useful measure of through thickness strain profile without altering the stress field in the SOFC electrode assembly.     

Residual stresses near a rail end

Stress fields near a cut end of a rail containing longitudinal residual stress typical of roller-straightened rail were studied using analysis and a finite element model. For a self-equilibrating residual stress distribution with equal maximum and minimum stresses, the distance to reach 95% of the mid-rail residual stress field is from 0.7 to 1.8 times the rail height, with the finite element model predicting a length of 1.1 times the rail height. This gives a measure of the accuracy of the simpler analytical models. At the rail end, the longitudinal residual stress goes to zero, and the vertical residual stress near mid-web reaches a maximum of approximately 27 ksi (186 MPa) (1.35 times the maximum mid-rail longitudinal residual stress of 20 ksi, or 138 MPa). The maximum shear stresses are 6 ksi (41 MPa) and −8 ksi (−55 MPa) near the head-web and web-base intersections, respectively, approximately 2 in. (51 mm) from the end of a 7.3 in. (185 mm) high rail. The shear stress is zero at the cut end and in mid-rail. The worst possible end-crack is a horizontal web crack in the vertical residual stress field at the rail end. The stress intensity K_I on such a crack is estimated to reach 20 ksi√in. (22 MPa√m) for cracks 0.5 in. (13mm) long. This is already 0.4 to 0.8 times K_I for carbon and alloy rails, and about 0.5 times K_Ic for a long crack.     

Residual-stress measurements on multi-pass weldments of stainless-steel piping

This paper presents measurements of the bulk residual stresses in 100-mm (4-in.) and 250-mm (10-in.) diam Schedule 80 piping weldments using strain-relief techniques. Both laboratory-welded specimens and field-welded specimens from reactors in service were studied. Axial bulk residualstress distributions were obtained at 45-deg intervals around the circumference. At each azimuthal position, the residual stresses were measured at seven axial positions: on the weld center line and 13, 20 and 25 mm to either side of the weld center line, on both the inside and outside surface. The specimens were parted out using a wire-feed electric-discharge machine, and the resulting strain relief was measured with electrical-resistance strain gages (120-deg rosettes). The bulk residual stresses obtained on the inside surface of the 100-mm weldments exhibit an oscillatory distribution with peak values above 275 MPa (40 ksi) and stress gradients normal to the weld on the order of 35 MPa/mm (127 ksi/in.).     

Mapping Multiple Components of the Residual Stress Tensor in a Large P91 Steel Pipe Girth Weld Using a Single Contour Cut

The contour method is applied in an innovative manner to measure the distribution of hoop residual stress in a large martensitic-ferritic steel pipe containing a multi-pass girth weld. First, a novel one-step wire electro-discharge machining cut is conducted to divide the pipe lengthways into two halves. The deformation of the cut halves is then measured and analysed in a way that simultaneously gives maps of hoop stress across the wall thickness on both sides of the pipe and automatically accounts for through-thickness hoop bending effects and how they may vary along the pipe. Finally the contour method results are combined with X-ray diffraction residual stress measurements using the principle of superposition to determine the distribution of the axial and radial residual stresses in the pipe. It is thereby demonstrated how the distribution of three direct components of the residual stress tensor in a welded pipe can be readily determined using a “hybrid” contour measurement approach.     

Experimental determination of stress intensity factors due to residual stresses

An experimental method is presented that enables stress intensity factors due to residual stress to be determined directly, without prior determination of the residual stress. The method is based on the crack compliance method, where a narrow cut is introduced progressively into the considered component, and the resulting strain change is measured by a strain gage. The required mathematical relations to determine stress intensity factors from strain measurements are established by means of some basic relations of linear elastic fracture mechanics. They are derived explicitly for two exemplary geometrical systems, which allowed for analytical treatment. Experimental data obtained in the case of a steel roller are presented and discussed.     

Application of microline/grid methods to temperature dependent microscopic deformation and damage in CFRP laminates

Tensile tests of CFRP symmetric cross-ply laminates are carried out in a scanning electron microscope (SEM), and microscopic interlaminar deformation and damage near the transverse crack tip are visualized by microlines or microgrids printed on the specimen edge surface. The local deformation around the transverse crack tip is observed at 20°C, 80°C, 120°C and 160°C to evaluate the effect of thermal residual stress on the microscopic deformation and damage in the interlaminar region near the transverse crack tip. Temperature dependence of the axial crack opening displacement (COD) is also measured. The displacement field of the specimen edge surface obtained from these experimental results is compared with the theoretical model proposed by Lee, Allen and Harris. The analysis is modified to consider the temperature effect, and a good agreement is obtained between the modified theoretical predictions and experimental results.     

Non-Destructive Characterization of Subsurface Residual Stress Fields and Correlation with Microstructural Conditions in a Shot-Peened Inconel Component

Shot-peening is an important surface treatment used in a preventative way to guard against fatigue failures. The residual stress state imparted by shot-peening deters the formation and propagation of surface cracks. In this paper, we describe the measurement of residual stresses in an Inconel, IN100, sample using lattice strains measured using High Energy X-ray Diffraction (HEXD) and a Bi-Scale Optimization Method (BSOM). HEXD enabled rapid, non-destructive lattice strain measurements over a large region of the sample. Subsurface strains were obtained using a conical slit setup. The BSOM utilizes a macroscale representation of the sample and a spherical harmonic-based crystal scale representation of crystal orientation space at each experimental point (diffraction volume). A roughly biaxial stress state was predicted with a von Mises equivalent stress between 300 MPa and 400 MPa near the surface. The layer of material with high residual stress induced by shot-peening was found to be approximately 1 mm thick. Diffraction peak width, EBSD, and microhardness measurements were also made on the same sample, which rendered more qualitative measures of the plasticity-related effects of the shot-peening induced residual stress field. All of these measurements show a dimishing shot-peening plasticity with the increasing depth.     

Measurement and Prediction of Machining Induced Redistribution of Residual Stress in the Aluminium Alloy 7449

The residual stress distributions in two 7449 aluminium alloy rectilinear blocks have been determined using neutron diffraction. Heat treatment included cold water immersion quenching and a period of precipitation hardening. Quenching induced very high magnitude residual stresses into the two blocks. One block was measured in this condition while the other was incrementally machined by milling to half thickness. Neutron diffraction measurements were made on the milled half thickness block at equivalent locations to the unmachined block. This permitted through thickness measurements from both blocks to be compared, revealing the redistribution of residual stresses induced by machining. A square cross section post in the centre of the machined face was left to act as a stress free reference sample. The distortions arising on the face opposite to that being milled were measured using a co-ordinate measuring machine. The residual stresses and distortion arising in the blocks have been compared to finite element analysis prediction and found to generally agree. Material removal only caused distortion and the residual stresses to redistribute; there was no stress relaxation evident.     

Intralaboratory Repeatability of Residual Stress Determined by the Slitting Method

This paper presents repeated slitting method measurements of the residual stress versus depth profile through the thickness of identically prepared samples, which were made to assess repeatability of the method. Measurements were made in five 17.8 mm thick blocks cut from a single plate of 316L stainless steel which had been uniformly laser peened to induce a deep residual stress field. Typical slitting method techniques were employed with a single metallic foil strain gage on the back face of the coupon and incremental cutting by wire EDM. Measured residual stress profiles were analyzed to assess variability of residual stress as a function of depth from the surface. The average depth profile had a maximum magnitude of −668 MPa at the peened surface. The maximum variability also occurred at the surface and had a standard deviation of 15 MPa and an absolute maximum deviation of 26 MPa. Since measured residual stress exceeded yield strength of the untreated plate, microhardness versus depth profiling and elastic–plastic finite element analysis were combined to bound measurement error from inelastic deformation.     

Assessment of Primary Slice Release Residual Stress Mapping in a Range of Specimen Types

This paper further explores the primary slice removal technique for planar mapping of multiple components of residual stress and describes application to specimens with a range of alloys, geometries, and stress distributions. Primary slice release (PSR) mapping is a combination of contour and slitting measurements that relies on decomposing the stress in a specimen into the stress remaining in a thin slice and the stress released when the slice is removed from a larger body. An initial contour method measurement determines a map of the out-of-plane stress on a plane of interest. Subsequently, removal of thin slices and a series of slitting measurements determines a map of one or both in-plane stress components. Four PSR biaxial mapping measurements were performed using an aluminum T-section, a stainless steel plate with a dissimilar metal slot-filled weld, a titanium plate with an electron beam slot-filled weld, and a nickel disk forging. Each PSR mapping measurement described herein has one (or more) complementary validation measurement to confirm the technique. Uncertainty estimates are included for both the PSR mapping measurements and the validation measurements. Agreement was found between the PSR mapping measurements and validation measurements showing that PSR mapping is a viable technique for measuring residual stress fields.     

Stress intensity factors and crack initiation directions for ceramic/metal joint

Four points bending tests for Si₃N₄/Cu/S45C joint specimen showed that the bending strengths depend on the residual stresses that originated from joining process. The residual thermal stresses caused an edge sub-interface crack to initiate in the ceramic. The stress intensity factors (SIFs) of the edge sub-interface crack located at distance h from the interface with or without interlayer metal were calculated by the Green's function obtained from a finite element analysis. The crack path at the joint specimen under four points bending loading with the influence of residual stresses was also evaluated by the maximum tensile stress criterion. Finally the effect of residual stress on the crack path was found numerically; the interlayer metal decreases the deflection angle of crack from interface by reducing the residual stress.