Materials Science Forum Vols. 524-525

Abstract: An engineering diffractometer designed to solve many problems in materials science and engineering including investigations of stresses and crystallographic structures within engineering components is now being developed at J-PARC project. This instrument views a decoupled-poisoned liquid H2 moderator providing neutrons with good symmetrical diffraction profiles in the acceptable wavelength range. The primary flight path and the secondary flight path are 40 m and 2.0 m, respectively, for 90 degree scattering detector banks. A curved supermirror neutron guide will be installed to avoid intensity loss due to the long flight path and to reduce backgrounds from fast neutrons and gamma rays. Therefore, stress measurements with sufficient accuracies in many engineering studies are quite promising. The optimization of this instrument has been performed with a Monte Carlo simulation, and an appropriate resolution of less than 0.2 % in
d/d has been confirmed. A prototyped radial collimator to define a gauge width of 1 mm has been designed and manufactured. From performance tests conducted at the neutron diffractometer for residual stress analysis RESA in JRR-3 of Japan Atomic Energy Agency, the normal distribution with a full width at half maximum of 1 mm was obtained in a good agreement with the simulation.

Abstract: A large distance between the reactor core and the monochromator together with a take-off angle of 90° leads to low flux but high angular resolution of the diffractometer around 90° scattering angle. The sample-table consists of two goniometers for heavy loads. An x-y-z table for heavy loads can be mounted, as well as different eulerian cradles, a mirror furnace or a tensile rig. The detector is a 20x20cm2 position sensitive delay-line detector made by EMBL. A sophisticated primary and secondary slit system together with the recently installed high-resolution camera system allows the application of the high precision instrument alignment and calibration system recently developed at HMI. Here some special features of the instrument are presented and the consequences are outlined.

Abstract: In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. In order to cater for the development of these analytical techniques the new Materials Science Diffractometer STRESS-SPEC at FRM-II is designed to be equally applied to texture and residual stress analyses by virtue of its flexible configuration. The system compromises a highly flexible monochromator setup using three different monochromators: Ge (511), bent silicon (400) and pyrolitic graphite (PG). This range of monochromators and the possibility to vary the take-off angles from 2θM = 35º to 110º allows wavelength adjustment such that measurements can be performed around a scattering angle of 2θS ~ 90º. This is important in order to optimise neutron flux and resolution, especially for stress analysis on components, since the gauge volume element in that case is cubic and large vertical divergences due to focusing monochromators do not affect the spatial resolution. The instrument is now available for routine operation and here we will present details of recent experiments and instrument performance.

Abstract: SALSA (Strain Analyser for Large Scale engineering Applications) is a novel instrument for strain imaging at the high flux neutron reactor of the Institut Max von Laue – Paul Langevin (ILL) in Grenoble, France. It is the first of its kind that uses a precise robotic sample stage for sample manipulation. In addition to standard xyz-translation it provides tilts up to ±30° about any horizontal axis. Its load capacity is more than 500 kg and samples up to 1.5 m in length can be scanned with high accuracy. Thanks to a double focusing monochromator and supermirror guide count times are very short. A broad wavelength range, variable beam optics, including radial focusing collimators for high lateral resolution, make it a very flexible instrument for a large variety of strain imaging applications in small and large specimens. The instrument has been commissioned in 2005 and is open for access by a peer review proposal system and as well by industry. The paper describes the important aspects of the instrument and results from the first experiments.

Abstract: Recent progress in engineering includes the development of new materials and innovations in their processing and treatments. Material technologies, like the study of metals, alloys, ceramics and composites, especially non-destructive analyses of residual stresses profiles and textures, have gained an increasing importance. The dedicated residual stress diffractometers E3 and E7 at BENSC, HMI, Berlin are already equipped with new two-dimensional position sensitive detectors. An upgrade of the monochromator system is planned for 2006 which includes perfectly bent silicon crystals in order to optimise both intensity and angular resolution yielding a large gain of the diffractometer efficiency for strain measurements. A range of equipment for sample positioning is available, such as a closed Eulerian cradle for samples with weights of up to 5 kg, a second cradle for heavy samples (up to 50 kg) with the ability to tilt the samples up to 90° and a translation table carrying samples of up to 300 kg and 1000 mm in diameter. Gauge volumes can be adjusted by a new computer controlled variable slitsystem in a range from 1x1x1 mm³ up to several mm³. In-situ residual stress analysis can be performed within industrial components during mechanical or thermal loading (up to 2000 K). Rapid data visualization as well as evaluation is performed by the specially designed software. The powder diffraction pattern is calculated by summation over the scattering angle dependent Debye- Scherrer lines on the two-dimensional 400*400 mm² planar area detector. A large amount of beam time is exclusively used for industrial research. Among the components that were investigated are crankshafts, impellers, pistons, cylinder heads, turbine blades and welds. Both instruments are similarly designed, where E3 is set up for higher flux and therefore penetration depths and E7 is designed for higher angular resolution.

Abstract: Although x-ray diffraction techniques have been applied to the measurement of residual stress in the industry for decades, some of the related details are still unclear to many production and mechanical testing engineers working in the field. This is because these details, specifically those associated with the transition between diffraction and mechanics, are not always emphasized in the literature. This paper will emphasize the appropriate calculation methods and the steps necessary to perform high quality residual stress measurements. Additionally, details are given regarding the difference between mechanical and x-ray elastic constants, as well as the true meaning of stress and strain from both diffraction and strain gage point of view. Cases where the material is subject to loading above the yield limit are also included.

Abstract: A new method for calculation of the diffraction elastic constants, based on the selfconsistent model, is proposed and tested. This method is especially useful in the interpretation of the results of X-ray measurements since the ellipsoidal inclusion near the sample surface is considered. In X-ray diffraction the information volume of the sample is defined by absorption, causing unequal contribution of different crystallites to the intensity of the measured peak. Consequently, the surface grains participate more effectively in diffraction than the grains which are deeper in the sample.

Abstract: Residual stress can be found in engineering components as a result of non-uniform plastic strain introduced through a variety of manufacturing processes such as rolling, casting, hot forging, cold working, shot-peening, laser shock peening, welding, etc. The numerical simulation of the resulting residual stress field requires the use of sophisticated coupled microstructural and thermomechanical models that rely on profound understanding of the constitutive laws and detailed knowledge of material parameters. In practice this level of understanding is not generally available, leading to the use of simplified models that are unable to reproduce or predict reliably the real residual stress distributions. This leads to the necessity of using increased safety factors and utilising overly conservative designs. A rational approach to the description of residual stress states is proposed that relies on the use of eigenstrain distributions as sources of residual stress. The problem of residual stress evaluation can then be replaced by the problem of determining the underlying eigenstrain distribution. An approach to this problem is proposed based on a simple variational formulation. Some examples of its application are shown, and the difficulties and challenges that may arise are discussed.

Abstract: In order to study the method of the neutron stress measurement using the cosα method, a numerical simulation study was performed and the result was compared with author's former experiment. The results of the simulation study agreed with those obtained in the previous experiment, which suggests the validity of the present method for neutron stress measurement.

Abstract: The stress distribution on the midsection of a pure bending beam where tensile strain localization band initiates on the tensile side of the beam and propagates within the beam is analyzed. Using the static equilibrium condition on the section of the midspan of the beam and the assumption of plane section as well as the linear softening constitutive relation beyond the tensile strength, the expressions for the length of tensile strain localization band and the distance from the tip of the band to the neutral axis are derived. After superimposing a linear unloading stress distribution over the initial stress distribution, the residual stress distribution on the midsection of the beam is investigated. In the process of strain localization band’s propagation, strain-softening behavior of the band occurs and neutral axis will shift. When the unloading moment is lower, the length of tensile strain localization band remains a constant since the stress on the base side of the beam is tensile stress. While, for larger unloading moment, with an increase of unloading moment, the length of tensile strain localization band decreases and the distance from the initial neutral axis to the unloading neutral axis increases. The neutral axis of midsection of the beam will shift in the unloading process. The present analysis is applicable to some metal materials and many quasi-brittle geomaterials (rocks and concrete, etc) in which tensile strength is lower than compressive strength. The present investigation is limited to the case of no real crack. Moreover, the present investigation is limited to the case that the length of strain localization band before unloading is less than half of depth of the beam. Otherwise, the residual tensile stress above the elastic neutral axis will be greater than the tensile strength, leading to the further development of tensile strain localization band in the unloading process.