Mapping of unstressed lattice parameters using pulsed neutron transmission diffraction

Abstract

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.

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

“…A thin slice cut from a sample will tend to be in a condition of plane stress with the normal component averaging to zero through thickness ( 33 = 0). As a result the sin 2 technique can be applied using either neutron transmission (Santisteban et al, 2002) or synchrotron X-ray diffraction techniques (Steuwer et al, 2007) to map d 0 . Following equation 6a series of measurements are made at different tilts, at each of two angles ' and ' +90 .…”

“…The experimental setup on the ENGIN instrument at ISIS is shown in Fig. 11, where the angles ' and are indicated (Santisteban et al, 2002). Instead of using the Bragg reflections, the Bragg edges are used.…”

Measurement of the residual stresses in a stainless steel pipe girth weld containing long and short repairs

“…As many reflections (and hence many crystal orientations) are involved in the definition of the measured lattice parameter, the macroscopic values of Young's modulus and Poisson's ratio (EZ195.6 GPa, nZ0.294) were adopted in the analysis to derive stresses. The second set of stress free lattice parameter measurements were performed on the ENGIN spectrometer using the Bragg edge transmission technique [20] at the ISIS pulsed neutron source, UK [15]. Measurements were carried out on the parent material and HAZ cubes from DH1 measured at Studsvig, plus specimens produced from the second header parent material and HAZ as described above.…”

Position Dependence of SS304 and Iron Phases in Welded Specimen by Neutron Diffraction and Bragg-Edge Transmission Method

“…Such measurements were carried out at the pulsed neutron source ISIS in the United Kingdom. [4,5] The total neutron cross section evaluated from the transmission spectrum exhibits a steplike decrease, the so-called Bragg edge ( Figure 6 in Reference 4). The Bragg edges occur, because for a given hkl reflection, the Bragg angle increases as the wavelength increases until 2h = 180 deg.…”