Neutron Strain Measurements Research Articles

Minimizing and Characterizing Uncertainties in Neutron Strain Measurements with Special Attention to Grain Size Effects

The accurate determination of strain during measurement using neutron diffraction depends on many factors. The statistical uncertainty of the diffraction data is not always the most important contributor to the total uncertainty in the measured strain. Other contributors, such as sample positioning, size and shape of the sampling (gauge) volume and the size and distribution of grains within the sampling volume, often play an important role as well. Grain size issues have been the least studied and their impact is often ignored even though the potential uncertainty contribution can be large. Certain methods such as oscillating the sample during measurement can help in reducing the magnitude of the grain size effect and hence also that of the related uncertainty contribution. A thorough characterization of uncertainties due to grain size effects however, in terms of absolute values that should be added to the statistical peak fitting uncertainties has not yet been implemented. This paper will present an improved method to characterize and estimate absolute uncertainty values due to grain size effects.

Absorption correction of peak positions for neutron strain measurements

In angle-dispersive neutron strain scanning the information about residual strain comes from the whole gauge volume that is defined by slits in the incoming and diffracted beams. Since the intensity of the neutron beam decreases with the amount of material it has travelled, neutrons diffracted from different locations within the gauge volume contribute with different intensities to the recorded diffraction peak. This can lead to peak shifts, and thus apparent strains. The magnitude of this peak shift depends mostly on the beam attenuation and the size of the gauge volume, but also on the sample geometry and position of the gauge volume within the sample. The peak shift plays a significant role when the size of the gauge volume becomes large because of peak broadening by the sample. An analytic expression for the peak shift was derived for a simple geometry to evaluate a numerical simulation. The numerical simulation was developed to quantify necessary corrections in detail. The attenuation-induced peak shift was demonstrated by measurements on a strain-free powder sample and the results were compared with the numerical predictions.

中性子回折法による溶接残留応力測定精度に関する考察 ―無ひずみ状態の格子定数の影響―

It is required to evaluate a strain-free reference, α0, to perform accurate stress measurement using neutron diffraction. In this study, accuracy of neutron stress measurement was quantitatively discussed from α0 evaluations on a dissimilar metal butt-weld between a type 304 austenitic stainless steel and an A533B low alloy ferritic steel. A strain-free standard specimen and a sliced specimen with 10 mm thickness taken from the dissimilar metal butt-weld were utilized. In the lattice constant evaluation using the standard specimen, average lattice constant derived from multiple hkl reflections was evaluated as the stress-free reference with cancelling out an intergranular strain. Comparing lattice constant distributions in each reflection with average lattice constant distribution in the standard specimen, αFe211 and γFe311 reflections were judged as a suitable reflection for neutron strain measurement to reduce intergranular strain effects. Residual stress distribution in the sliced specimen evaluated using α0 measured here exhibited higher accuracy than that measured using strain gauges. On the other hand, α0 distributions were evaluated using the sliced specimen under the plane-stress condition. Existence of slight longitudinal residual stresses near the weld center decreased accuracy of the α0 evaluations, which means that it is required to optimize the thickness of the sliced specimen for accurate α0 evaluation under plane strain condition. As a conclusion of this study, it was confirmed that procedures of accurate α0 evaluation, optimization of the measurement condition, and multiple evaluations on the results play an important role to improve accuracy of the residual stress measurement using neutron diffraction.

Minimization of spurious strains by using a Si bent-perfect-crystal monochromator: neutron surface strain scanning of a shot-peened sample

Neutron strain measurements are critical at the surface. When scanning close to a sample surface, aberration peak shifts arise due to geometrical and divergence effects. These aberration peak shifts can be of the same order as the peak shifts related to residual strains. In this study it will be demonstrated that by optimizing the horizontal bending radius of a Si (4 0 0) monochromator, the aberration peak shifts from surface effects can be strongly reduced. A stress-free sample of fine-grained construction steel, S690QL, was used to find the optimal instrumental conditions to minimize aberration peak shifts. The optimized Si (4 0 0) monochromator and instrument settings were then applied to measure the residual stress depth gradient of a shot-peened SAE 4140 steel sample to validate the effectiveness of the approach. The residual stress depth profile is in good agreement with results obtained by x-ray diffraction measurements from an international round robin test (BRITE-EURAM-project ENSPED). The results open very promising possibilities to bridge the gap between x-ray diffraction and conventional neutron diffraction for non-destructive residual stress analysis close to surfaces.

Neutron Strain measurement

The use of neutrons has extended the technique of diffraction strain measurement from an essentially two-dimensional, near-surface tool using X-rays to a true three-dimensional method. The depth scale has gone from micrometres to millimetres. This initially gave rise to subsurface measurements of type I residual macrostresses in weldments and type II residual microstresses in anisotropic and multiphase metals, and composites. Soon the possibilities of in situ applied stress measurements became apparent and today they represent a significant portion of the work being carried out. This perspective focuses on the current state of the art, the prospects and developments necessary for further progress. The transition from two- to three-dimensions raised the issues of stress free reference values, methodology for general tensor measurements, beam optics and experimental design. Workers invested great effort in resolving these issues. In addition, the development of pulsed source instruments has enabled in situ measurements of slip, fatigue, load partitioning, twinning, high T stress-strain response, thermal cycling and the shape memory effect. Most recently, the SNS instrument, VULCAN, has introduced torsion and continuous recording of data which can later be binned in the shortest statistically significant time intervals.

OS1210 中性子ひずみ測定用極低温引張試験装置の開発(X線,放射光,中性子による材料・材料強度評価,オーガナイズドセッション)

For an investigation of stress and strain effects for superconducting wires, neutron strain measurements under a loading are required. In order to make in-situ measurements of lattice strain for the superconducting wires, we developed a tensile frame with a GM refrigerator for the neutron diffraction measurements.

Neutron and X-ray diffraction studies and cohesive interface model of the fatigue crack deformation behavior

The crack-tip deformation behavior during a single overload, fatigue test of ferritic stainless steel, and Ni-based HAYNES 230 superalloy is studied at different structural levels using (1) neutron-diffraction, from which both the elastic-lattice strain and volume-averaged total dislocation densities are obtained, (2) polychromatic X-ray microdiffraction to probe the geometrically necessary dislocations and boundaries distribution, and (3) an irreversible and hysteretic cohesive interface model which has been implemented into a finite element framework to simulate the stress/strain evolution near the fatigue crack tip. Neutron strain measurements and finite element simulations are in qualitative agreement on the macroscopic length scale. Large plastic deformation induced by the overload and the resulting compressive residual strains are observed in front of the crack tip after the overload, and are the principal reason for the fatigue-crack-growth retardation. Strong strain gradients surrounding the crack propagation result in the formation of a high density of geometrically necessary dislocations near the fractured surface and cause local lattice rotations on the submicron level.

Measurement of residual stresses in rails by neutron diffraction

Rolling contact fatigue damage is accumulated in rails during the repeated passage of trains over the rails, and an incipient crack may initiate due to the rolling contact fatigue. Rail failures can occur due to crack growth in the rails. In order to prevent such rail failures, the estimation of the behavior of internal rail cracks is required on the basis of an exact engineering analysis model as well as a method to detect rail defects before the crack grows to critical size. It is expected that the residual stress distribution in a rail has a significant influence on the growth rate of the crack. The purposes of this study are to measure the residual stress distribution in rails utilizing the neutron strain measurement. The test samples were taken from rails used in a service line in Japan for about six years (222 million gross tons). The neutron measurement was conducted using the Residual Stress Analyzer (RESA) at the Japan Atomic Energy Agency (JAEA). This report presents the results of the neutron strain measurements on a longitudinal-cut slice of a rail.

A deconvolution method for the reconstruction of underlying profiles measured using large sampling volumes

A deconvolution method for diffraction measurements based on a statistical learning technique is presented. The radial-basis function network is used to model the underlying function. A full probabilistic description of the measurement is introduced, incorporating a Bayesian algorithm based on an evidence framework. This method allows predictions of both the convolution and the underlying function from noisy measurements. In addition, the method can provide an estimation of the prediction uncertainty,i.e.error-bars. In order to assess the capability of the method, the model was tested first on synthetic data of controllable quality and sparsity; it is shown that the method works very well, even for inaccurately measured (noisy) data. Subsequently, the deconvolution method was applied to real data sets typical of neutron and synchrotron residual stress (strain) data, recovering features not immediately evident in the large-gauge-volume measurements themselves. Finally, the extent to which short-period components are lost as a function of the measurement gauge dimensions is discussed. The results seem to indicate that for a triangular sensor-sensitivity function, measurements are best made with a gauge of a width approximately equal to the wavelength of the expected strain variation, but with a significant level of overlap (∼80%) between successive points; this is contrary to current practice for neutron strain measurements.

Recent developments for strain measurements at the LLB

DIANE (Diffractomètre Italo-français pour l'Analyse Neutronique des Efforts) is the Italian–French diffractometer for the strain/stress analysis at the Laboratoire Léon Brillouin. It was funded 50% by the Italian INFM (Istituto Nazionale Fisica della Materia) and 50% by the LLB. In the last years some improvements have been done, in particular we report here on the installation of a new sample line for very heavy samples and a new tensile test device for “in situ” mechanical loading of sample during neutron strain measurements.

Complementary X-ray and neutron strain measurements of a carburized surface

Lattice spacing profiles through a carburized layer in a cylindrical 5120 steel specimen were recorded using X-ray and neutron diffraction. The neutron measurements were performed non destructively, whereas the X-ray measurements employed sequential layer removal by electropolishing. The X-ray measurements provided a base determination of residual stress and microstructure, against which the neutron results were critically compared. Several methods were considered for determining the unstrained lattice parameter variation, necessary for interpretation of the neutron results.

Neutron Strain Measurement of Internal Strain in Metal and Ceramic Matrix Composites

Neutron Strain Measurement of Internal Strain in Metal and Ceramic Matrix Composites