A. Steuwer

Methods for obtaining the strain-free lattice parameter when using diffraction to determine residual stress

The determination of residual stress by diffraction depends on the correct measurement of the strain-free lattice spacing d(hkl)(0), or alternatively the enforcement of some assumption about the state of strain or stress within the body. It often represents the largest uncertainty in residual stress measurements since there are many ways in which the strain-free lattice spacing can vary in ways that are unrelated to stress. Since reducing this uncertainty is critical to improving the reliability of stress measurements, this aspect needs to be addressed, but it is often inadequately considered by experimenters. Many different practical strategies for the determining of d(hkl)(0) or d(ref) have been developed, some well known, others less so. These are brought together here and are critically reviewed. In practice, the best method will vary depending on the particular application under consideration. Consequently, situations for which each method are appropriate are identified with reference to practical examples.

A high energy synchrotron x-ray study of crystallographic texture and lattice strain in soft lead zirconate titanate ceramics

The lattice parameters and crystallographic texture in La, Sr-doped soft lead zirconate titanate ceramics were examined using high-energy synchrotron x-ray diffraction. The preferred orientations in poled tetragonal and rhombohedral ceramics near the morphotropic phase boundary, caused by ferroelectric domain switching, were determined by monitoring the (002)∕(200) and (111)∕(111¯) intensity ratios, respectively. The lattice strains were also monitored using the {111} and {200} plane spacings in tetragonal and rhombohedral ceramics, respectively. The diffraction experiments were carried out in transmission, enabling the true “bulk” state to be characterized. It was observed that for the tetragonal phase both the lattice spacing d{111} and the intensity ratio R{200} varied linearly as a function of sin2Ψ,Ψ being the angle between the plane normal and the macroscopic polar axis. Similar observations were made for d{200} and R{111} in rhombohedral ceramics. The results are interpreted in terms of the remanen...

High-resolution strain mapping in bulk samples using full-profile analysis of energy-dispersive synchrotron X-ray diffraction data

The feasibility of both high spatial and strain resolution is demonstrated using high-energy X-rays between 100 and 300u2005keV on beamline ID15A at the ESRF. The data analysis was performed using a multiple-peak Pawley-type refinement on the recorded spectra. An asymmetric peak profile was necessary in order to obtain a point-to-point uncertainty of 10−5. The measurements have been validated with complementary techniques or reference data.

Residual stress engineering in friction stir welds by roller tensioning

Abstract The authors investigate the efficacy of applying rolling pressure along the weld line in thin butt welds produced using friction stir welding (FSW) as a means of controlling the welding residual stresses. Two cases are examined and in each case, comparison is made against the as welded condition. First, for FSW of AA 2024 aluminium alloy, roller tensioning was applied during welding using two rollers placed behind and either side of the FSW tool. Very little effect was seen for the down forces applied (0, 50, 75 kN). Second, for FSW AA 2199 aluminium alloy, post-weld roller tensioning was applied using a single roller placed directly on the FS weld line. In this case, significant effects were observed with increased loading, causing a marked reduction in the longitudinal tensile residual stress. Indeed, a load of just 20 kN was sufficient to reverse the sign of the weld line residual stress. Only slight differences in Vickers hardness were observed between the different applied loads. Furthermore, unlike some methods, this method is cheap, versatile and easy to apply.

Dissimilar friction stir welds in AA5083-AA6082. Part II: Process parameter effects on microstructure

The aim of this study is to explore the bounds of the so-called processing window, within which good-quality welds can be produced, for the friction stir welding of AA5083 to AA6082 using a systematic set of rotation and traverse speeds. The first paper in this series examined the thermal and macroscopic aspects. In this paper, several microstructurally related characteristics, including hardness, grain size, and precipitate distribution, have been examined in greater detail. The observed variations are correlated and contrasted with the observed and predicted thermal distributions. In addition, the thermal model developed in part I has been coupled to hardness models based on classical isothermal aging studies for each alloy to predict the hardness variations across the welds.

Comparison of residual stresses in Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo linear friction welds

Abstract In this paper, the levels of residual stress in the vicinity of linear friction welds in Ti–6Al–4V (Ti-64), a conventional α–β titanium alloy, and Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near α titanium alloy with higher temperature capability, are mapped and contrasted. The alloys have significantly different high temperature properties and the aim of this work was to investigate how this might affect their propensity to accumulate weld residual stresses and their response to post-weld heat treatment. Measurements are reported using high energy synchrotron X-ray diffraction and the results are compared to those made destructively using the contour method. The strain free lattice plane d 0 variation across the weld has been evaluated using the biaxial sin2Ψ technique with laboratory X-rays. It was found that failure to account for the d 0 variation across the weld line would have led to large errors in the peak tensile stresses. Contour method measurements show fairly good correlation with the diffraction results, although the stresses are underestimated. Possible reasons for the discrepancy are discussed. The peak tensile residual stresses introduced by the welding process were found to be greater for Ti-6242 (∼750 MPa) than for Ti-64 (∼650 MPa). Consistent with the higher temperature capability of the alloy, higher temperature post-weld heat treatments have been found to be necessary to relieve the stresses in the near α titanium alloy compared to the α+β titanium alloy.

Using pulsed neutron transmission for crystalline phase imaging and analysis

The total scattering cross section of polycrystalline materials in the thermal neutron region contains valuable information about the scattering processes that neutrons undergo as they pass through the sample. In particular, it displays characteristic discontinuities or Bragg edges of selected families of lattice planes. We have developed a pixelated time-of-flight transmission detector able to record these features and in this paper we examine the potential for quantitative phase analysis and crystalline phase imaging through the examination of a simple two-phase test object. Two strategies for evaluation of the absolute phase volumes (path lengths) are examined. The first approach is based on the evaluation of the Bragg edge amplitude using basic profile information. The second approach focuses on the information content of certain regions of the spectrum using a Rietveld-type fit after first identifying the phases via the characteristic edges. The phase distribution is determined and the coarse chemical species radiographic image reconstructed. The accuracy of this method is assessed.

In situ determination of stresses from time-of-flight neutron transmission spectra

The pulsed neutron transmission diffraction technique exploits the sharp steps in intensity (Bragg edges) appearing in the transmitted spectra of thermal neutrons through polycrystalline materials. In this paper the positions of these edges acquired by the time-of-flight (TOF) technique are used to measure accurately the interplanar lattice distances to a resolution of Δd/d ~10^-4 of specimens subjected to in situ uniaxial tensile loading. The sensitivity of the method is assessed for elastically isotropic (b.c.c. ferritic) and anisotropic (f.c.c. austenitic) polycrystalline specimens of negligible and moderately textured steels. For the more anisotropic austenitic steel, the elastic anisotropy is studied with regard to a Pawley refinement, and compared with previous results from conventional neutron diffraction experiments on the same material. It is shown that the method can be used to determine anisotropic strains, diffraction elastic constants, the residual and applied stress state as well as the unstrained lattice parameter by recording transmission spectra at different specimen inclinations, by complete analogy with the sin2ψ technique frequently used in X-ray diffraction. The technique is shown to deliver reliable measures of strain even in the case of moderate texture and elastic anisotropy.

Elastic strains around cracked cold-expanded fastener holes measured using the synchrotron X-ray diffraction technique

In this work, the capability of the synchrotron X-ray diffraction technique to measure changes in the residual strains caused by fatigue crack growth from cold-expanded holes in an aluminium alloy plate has been investigated. The measurement technique was validated by comparing the synchrotron measurements around uncracked holes with those obtained on similar specimens using the neutron diffraction and Sachs techniques. Measurements were performed with and without superimposed tensile mechanical loading and the results compared with analytical results. The improved spatial resolution of the synchrotron technique relative to the other methods has allowed the strain fields around a fatigue crack growing from a cold-expanded hole to be evaluated in the unloaded state and at two loading levels. Only at the higher load does the crack appear to open along its full length.

Mapping of unstressed lattice parameters using pulsed neutron transmission diffraction

Stress measurement by neutron diffraction depends critically on knowledge of the unstressed lattice parameter (a0) of the specimen under study. As a result, measurement of stress profiles in components where a0 is not homogeneous throughout the sample, such as welds or carburized surfaces, can be particularly difficult. An efficient solution to this problem is proposed based on the pulsed neutron transmission diffraction technique. This technique exploits the sharp steps in intensity, the so-called Bragg edges, appearing in the transmitted neutron spectra of polycrystalline materials, such steps being produced by coherent scattering from lattice planes. The position of these Bragg edges as defined by the time-of-flight technique is used to determine precisely local interplanar distances. In this work it is shown that the unstressed lattice parameter of thin specimens subjected to plane stress fields can be defined by recording transmission spectra at different sample inclinations, in complete analogy with the sin2ψ technique used in X-ray diffraction. Moreover, by using an array of detectors it is possible to produce a radiographic `image of a0 for plane specimens or thin sections out of three-dimensional ones. The capability of the technique is exemplified by mapping the changes in a0 for a ferritic weld that was used as a round robin sample in an international program for standardization of stress measurements by neutron diffraction.