Gauge Volume Research Articles

Neutron Surface Residual Stress Scanning Using Optimisation of a Si Bent Perfect Crystal Monochromator for Minimising Spurious Strains

For non destructive stress analysis of surface treated steel samples the application of laboratory X rays or high energy synchrotron radiation in reflection mode covers the region from some micrometers up to a depth of about 150 - 200 µm. To access depth regions deeper than 200 µm the incremental layer removal technique in combination with the repeated application of X‑ray stress analysis for the newly generated surfaces can be used. However, this procedure is destructive, laborious and furthermore, it has to be checked whether corrections have to be applied due to stress relaxation. By using neutron radiation penetration depths generally up to several millimetres can be achieved non destructively [1]. However neutron measurements are critical at the surface. When scanning a sample surface, aberration peak shifts caused by so called spurious strains arise due to the fact that the gauge volume defined by the primary and secondary optics is partially outside of the sample. These aberration peak shifts can be of the same order of magnitude as the peak shifts related to residual strains [2-6]. In this exemplary study it will be demonstrated that, by optimising the bending radius of a Si (400) monochromator, the spurious surface strains can be strongly reduced when compared to the values obtained with a traditional Ge (311) mosaic monochromator, even when the gauge volume is mainly out of the surface. The objective of the experiments is to find the optimal monochromator settings for the Si (400) monochromator at the STRESS-SPEC instrument at the research reactor FRM II, Munich, Germany. For the parametric studies a stress free steel sample of the fine grained construction steel, S690QL was used. The optimised conditions for the Si (400) monochromator that resulted from the systematic studies were applied to a shot peened plate of steel SAE 4140. The residual stress distribution is analysed by means of through surface strain scanning. The residual stress gradient obtained is in very good agreement with the well characterised residual stress depth profile obtained within a round robin test in the scope of the BRITE-EURAM-project ENSPED (European Network of Surface and Prestress Engineering and Design) [7]. The results indicated that surface residual stress profiles can be measured with neutrons up to 200 µm underneath the surface without time consuming and laborious surface effect corrections.

Cooperation of length scales and orientations in the deformation of bovine bone

Bone has a complex hierarchical structure. Combined wide angle X-ray diffraction and small angle X-ray scattering were used together with in situ tensile testing to investigate the deformation and failure mechanisms of bovine cortical bone at three material levels: (1) the atomic level, corresponding to the mineral crystal phase; (2) the nano level, corresponding to the collagen fibrils; (3) the macroscopic level. It was found that deformation was linear at all three levels up to a strain of 0.2% in the longitudinal tensile direction. At this critical strain a sudden 50% decrease in the fibrillar and mineral strains was observed. This suggests the presence of partial local damage that leads to inhomogeneous strain distributions within the probed gauge volume. This also gives rise to diffraction peak broadening in the mineral phase, as well as strain relaxation at the nanoscale. Above the critical strain the longitudinally oriented strains below the nanoscale remained constant at a reduced level until failure. This suggests that the lateral orientation of the nanostructures toughens the bone, while a higher material level dominated the subsequent deformation process, either by sliding between the lamellar layers or by the growth of microcracks. Analysis of the diffraction data also shows that the bone has compressive residual stress in the crystal phase. A better understanding of the basic mechanics of the hierarchical bone structure could be the basis to enhance research in biomimetics, developing new advanced materials, and to find solutions for orthopedic problems.

Precise Analysis of Near Surface Neutron Strain Imaging Measurements

The lateral resolution of Neutron Strain Imaging Instrumentation for high resolution applications is restricted. A realistic lower limit for reasonably fast measurements is 0.5mm. The reason is, that in order to achieve maximum neutron flux at the sample position, the beam must be focussed. This is often achieved by double focussing monochromators or focussing guides. As a result the beam-divergence sets a lower limit for gauge volume definition. However, the lateral resolution can be improved for near surface and interface analysis by entering the gauge volume only partially into the material. As a result, the sampled gauge volume is getting smaller and resolution is improved. As a drawback, errors of the detected peak position, usually called surface errors or pseudo-strain, are introduced in the measurements. Such errors are often corrected by repeating the measurement with the sample turned by 180°. This is inefficient, limited in precision and often not possible. This paper describes a more efficient and precise method for near surface stress analysis that requires only one reference measurement, which provides experimental parameters for an analytical model for data correction. The model takes into account curved surfaces and allows precise stress determination as close as 40 micrometres below a surface or interface.

New insights into alloy compositions: studying Renaissance bronze statuettes by combined neutron imaging and neutron diffraction techniques

Until recently the inside parts of hollow cast Renaissance bronze statuettes were concealed, yet they hold important information on the production techniques used in the manufacture of these fine works of art. For that specific reason the inside of the sculptures have been made visible using a neutron imaging technique (tomography) at the Paul-Scherrer Institut, Villigen, Switzerland. This method allowed us to study the internal structure of a bronze sculpture and provided an indication of different material compositions. Since sample taking is not advised on these precious works of art, different non-destructive methods had to be investigated to obtain more specific information on the compositions of the inner parts. This research focuses on analyzing pre-determined small volumes selected from neutron tomographies. With this approach it has become possible to study the material compositions and crystalline structures of these statuettes with a millimetre-sized gauge volume placed at any selected point within the object, using time-of-flight neutron diffraction in the setup of ENGIN-X at the ISIS facility at the Rutherford Appleton Laboratory, UK. Analysis of a Renaissance statuette from the Rijksmuseum, a Striding Nobleman, gives evidence of the different copper alloy compositions of superficial and internal parts but also shows small amounts of ferrite present which until recently was not reported for Renaissance bronzes. The alloy is magnetic due to the ferrite, and strong rare-earth magnets were used to establish whether or not other Renaissance bronzes and brasses contain ferritic iron. A combined application of neutron techniques leads to a better understanding of the production techniques and will, in general, help to advance the analytical studies of these marvelous objects.

Non-destructive evaluation of strain-stress and texture in materials science by neutrons and hard X-rays

Together with the microstructure residual stresses and crystallographic textures are important parameters in considering the application profile of engineering products. Both residual stress and crystallographic texture are related to identical grain arrangements. That means on one hand texture and strain influence each other and on the other hand both informations contribute to considering anisotropic materials properties, quality of welds, inhomogenous behaviour and so on. Non-destructive evaluations need experimental methods going deeply inside a material. Furthermore, it is necessary to handle complex and heavy samples. Neutrons and high-energy X-rays (about 100keV) have a high penetration power in the cm-range to fulfil these requirements. The instrumentation must be very flexible fitting the set-up for both investigations. The materials science diffractometer STRESS-SPEC at the Forschungsneutronenquelle Heinz Mayer-Leibnitz FRM II at Garching/Germany (neutron) and the HZG materials science beamline Harwi-II at Hasylab-Hamburg/Germany (synchrotron) are able for combined strain profile and texture gradient investigations non-destructively. The newly installed robot at STRESS-SPEC allows a very flexible sample manipulation for both types of experiments. First results are presented. Synchrotron radiations with higher brilliance are favoured for small gauge volume and fast measurements. That opens the field of in situ loading (tension-, compression-, cyclic loading) and in situ welding investigations.

Residual Stress Measurement in Aluminum Engine Block

Residual stress distribution at the cylinder block wall in 1500cc class aluminum engine was measured using RESA in JRR-3 of JAEA. In order to increase neutron flux at the gauge volume measured, newly developed vertically focused collimator was employed. Since the grain size in cast aluminum engine block was large, the oscillation method was applied. The residual stresses near the surface layers of cylinder block wall were compressive all in the tangential, the thickness and the cylinder axial directions, but tensile in the middle part of cylinder block wall. This means the residual stress distributions are likely to be hydrostatic. The residual stresses in the tangential direction were not balanced in the cylinder block wall. If the residual stress distribution in the hoop direction is approximated by 2 dimensional power equation, the residual stresses on the cylinder wall surface is estimated as about -140MPa and the mean residual stress in the cylinder wall is 15MPa.

Energy-dispersive diffraction stress analysis under laboratory and synchrotron conditions: a comparative study

For a feasibility study of energy-dispersive residual stress analysis under laboratory conditions, an X-ray diffractometer that has been operated so far in the angle dispersive diffraction mode was equipped with a commercial tungsten tube and an energy-dispersive solid-state germanium detector. Starting from systematic investigations to find the optimum configuration regarding geometrical resolution, measuring time and stability of the applied detector system, different materials were characterized with respect to the near-surface residual stress state. The results achieved with the modified laboratory equipment within reasonable measuring times are in good agreement with synchrotron measurements performed on the same samples. With the example of a shot-peened Al2O3ceramic with a highly non-uniform near-surface residual stress distribution it is furthermore shown that the different size and shape of the diffracting gauge volume used for the laboratory and synchrotron measurements might have a significant influence on the experimentally obtained Laplace-space residual stress depth profiles σ||(τ).

Neutron diffraction study of Levantine Middle Bronze Age cast axes

A neutron diffraction study on 6 Middle Bronze Age axes, cast from tin bronze or from arsenical copper, has been carried out using the ENGIN-X beamline at ISIS. The gauge volumes dimensions were 4x4x10mm3; data were collected along the lengths of the objects in their central parts, as well as on the blades, in order to establish the spatial phase contents. Average phase fractions were determined by Rietveld analysis. The main phases identified were solid solutions, corrosion phases and metallic Pb inclusions. We have observed distributions of lattice constants of the solid solutions Cu-Sn, and Cu-As inside each gauge volume in the central parts of the axes due to segregation, or liquation effects. However, the Cu-Sn variations were significantly less pronounced in comparison with typical inhomogeneity effects in as cast objects. The results indicate that the studied Middle Bronze Age axes were probably treated at high temperatures for homogenization necessary for generating sufficient hardness, especially on the blades.

A non-destructive method for the measurement of residual strains in semi-crystalline polymer components

A new non-destructive tool is presented, enabling the measurement of locked-in residual strains in semi-crystalline polymer-based components. The technique employs high-energy synchrotron X-rays to probe the variation of diffraction angle within a well-defined ‘gauge volume’ with a spatial resolution of the order of 1 mm. Lattice strain is calculated from the diffraction angles. An overview of the experimental methodology and underlying principles involved in the non-destructive evaluation of residual strain in polymer-based components is given. Preliminary results show that synchrotron X-rays can be used successfully to measure the internal elastic residual strain field in polymer components, being potentially applicable to other materials. The method was used successfully to measure residual strains in a commercial high density polyethylene gas pipeline sample.

Through-thickness texture profiling by energy dispersive synchrotron diffraction

Two new approaches that make use of the advantages of energy dispersive diffraction were applied to investigate through-thickness variations of the crystallographic texture: the `real-space' and the `Laplace-space' methods. The first consists of defining a small gauge volume by adding a pair of slits in the primary and the diffracted beams. Thus the depth resolution is achieved by a decoupledztranslation of the gauge through the sample. The second method is based on the Beer attenuation law, and the depth resolution is achieved by assigning pole figures of different order reflections (e.g.0002, 0004…) to different average information depths. Wrought rolled AZ31 magnesium alloy was selected as a `model' material for the experiments because of the low X-ray absorption of Mg. The through-thickness crystallographic texture variation was generated by sample bending. The results give a first insight into the possibilities of fast texture depth profile investigation applying nondestructive methods and represent a step forward towards a simultaneous evaluation of residual stress and texture gradients.

Dislocation-based plasticity model and micro-beam Laue diffraction analysis of polycrystalline Ni foil: A forward prediction

A physically-based, rate and length-scale dependent strain gradient crystal plasticity framework was employed to simulate the polycrystalline plastic deformation at the microscopic level in a large-grained, commercially pure Ni sample. The latter was characterised in terms of the grain morphology and orientation (in the bulk) by micro-beam Laue diffraction experiments carried out on beamline B16 at Diamond Light Source. The corresponding finite element model was developed using a grain-based mesh with the specific grain orientation assignment appropriate for the sample considered. Sample stretching to 2% plastic strain was simulated, and a post-processor was developed to extract the information about the local lattice misorientation (curvature), enabling forward-prediction of the Laue diffraction patterns. The ‘streaking’ phenomenon of the Laue spots (anisotropic broadening of two-dimensional (2D) diffraction peaks observed on the 2D detector) was correctly captured by the simulation, as constructed by direct superposition of reflections from different integration points within the diffraction gauge volume. Good agreement was found between the images collected from experiments and simulation patterns at various positions in the sample.

Texture crossover: Trace from multiple grains to a subgrain

Neutron and synchrotron X-ray diffractions were used to study the texture development in the face-centered-cubic nickel-based superalloy over gauge volumes ranging from hundreds of cubic millimeters down to sub-cubic micrometers. The bulk averaged results find a uniform texture development from collective slip. However, X-ray microbeam studies at microscale find that the plastic deformation within a single grain is mediated by limited slip, as evidenced by the local strain distribution. Polychromatic microdiffraction shows the formation of several distinct structural zones even in the same grain. A hierarchical heterogeneous geometrically-necessary dislocations arrangement and distinct slip bands are observed within each grain. A depth-dependent change of the stereographic projection within the slip band in the grain is demonstrated. Correlated oscillations of the lattice orientation at the submicron scale evolve into an overall texture at the macroscale. Possible reasons for this observed structural evolution are discussed.

Precise Determination of Residual Stresses in Large Specimens by Neutron Diffraction

Experimental errors in neutron residual strain imaging are often underestimated, mis-understood or simply ignored. The choice of beam defining optics can play a large role in the magnitude of these errors. In this paper we show that the use of a traditional slit beam defining system with large specimens can lead to several sources of errors. Large specimens usually require necessarily large distances between the slits and the instrument reference point (gauge volume). At large distances, the slit system can lead to significant underestimation of the residual strain magnitude as well as poor definition of the gauge volume. We show that for large specimens, the use of radial focussing collimators reduces these effects, leading to significant improvement in measurement reliability. Specifically, we show application of radial collimators at the SALSA instrument of the Institut Laue Langevin neutron source in Grenoble, France.

Kowari - OPAL's New Stress Diffractometer for the Engineering Community: Capabilities and First Results

Kowari is a neutron diffractometer at ANSTO’s research reactor OPAL. The instrument is dedicated to investigate residual strains and stresses in engineering samples or new materials. Besides its usage in engineering/strain-scanning it is more and more frequently being used for obtaining texture information related to materials science applications. In particular the possibility of using the gauge volume defining slit systems allows extracting local information from the diffraction peak, i.e. position, intensity, and width, without the need to cut the sample specimen. The instrument obtained its operating license in August 2008 to briefly describe the instruments parameters and capabilities, and give examples of two typical applications since it went from commissioning into user mode.

The Robot Concept at STRESS-SPEC for the Characterisation or Semi-Finished Products

The materials science diffractometer STRESS-SPEC located at Forschungsneutronen¬quelle Heinz Maier-Leibnitz (FRM-II) in Garching/Germany is a dedicated instrument for residual strain, texture and microstrain analysis. Comparably low gauge volumes of 1x1x1mm³ for gradient investigations in many types of materials can be investigated. A robot system consisting of a robot type Stäubli RX160, a laser tracker and a heavy vibration free basement was installed to overcome limitations in gradient investigations of residual stresses and crystallographic textures in semi-finished and finished products. The robot can carry up to 30kg with precise positioning. Al7020, an Al-alloy for airspace application, was used to perform a first global texture measurement using the robot.

Automation of a neutron diffractometer for analysis of residual stress inside complex engineering components

Residual stress measurement using neutron diffraction is becoming an increasingly important tool in engineering stress analysis. To this effect, a new generation of dedicated engineering strain instruments are being built at neutron sources, offering considerable improvements in both counting time and spatial resolution. Alongside these improvements, measurements in complex geometry prototype components are increasingly in demand. As a result, there is a strong drive towards integrated sample positioning systems that allow for simplified setup and operating of experiments on components with complex geometries. The present study details work carried out at the ENGIN-X instrument at the UK’s ISIS pulsed neutron source, on measurements in a prototype metal matrix composite (MMC) aircraft wheel, forged from a billet produced through a powder-metallurgy route. The measurement was designed to obtain the macrostress and misfit stresses developed in the matrix and in the reinforcement phase in the wheel during fabrication. The study also demonstrates the use of the SScanSS software for experimental design and implementation, which was developed to complement the advances in the instrumentation of new strain mapping diffractometers. SScanSS simplifies the precise spatial location of the measuring gauge volume inside such complex components.

Residual strain evolution in steel samples: tension versus torsion

Torsion provides a unique opportunity to probe mechanical behavior of materials subjected to pure state of shear stress. In this research, identical steel alloy (12L14) hollow cylinder samples are subjected to predetermined amounts of plastic axial and shear strain such that their octahedral shear strain (an invariant) are identical for comparison. Measurements were made at the residual stress measuring facility at the High Flux Isotope Reactor in Oak Ridge (NRSF2), using a small gauge area in the direction of strain gradients (0.5 mm×0.5 mm) through the hollow cylinder wall thickness. These orthogonal strains are obtained for BCC Fe for three hkl’s. Three normal strains in the hoop, radial, and axial directions are obtained as a function of centroid position of the gauge volume through the 2 mm wall thickness. Significant differences in measured residual strains are noted between the torsion and the tension samples. The largest differences are found for the Fe (200) planes while the smallest differences are observed for the Fe (211) planes. This research demonstrates the need for a systematic study of residual strain as a function of applied stress path moving beyond tensile testing for solving real world problems.

In-situ observation of creep damage evolution in Al–Al2O3 MMCs by synchrotron X-ray microtomography

Abstract Creep damage by pore nucleation and growth limits the lifetime of components subjected to loading at high temperature. In this work, the creep damage evolution in the aluminium matrix composite AA6061 + 22 % (Al2O3)P is observed in-situ by the fast-tomography technique using a miniature creep device. Reconstructions of the gauge volumes of the creep samples show particle delamination, pore formation and growth at 300 °C and additional particle fracture at 200 °C during the creep process. The results reveal that both the number of pores and the total pore volume show exponential growth with creep time, whereas the average volume of individual pores remains almost constant. The total pore volume increases linearly with the creep strain with a stress-dependent factor of proportionality.

The Effect of Grain Size on Strain Determination Using a Neutron Diffractometer

The determination of strain from neutron diffraction data is normally based upon the fit of a Gaussian function to a Bragg reflection. The error in the fit is assumed to be that based on ‘counting statistics’ and this error propagates through the analyses until the final stress evaluation. This relies on there being a big enough number of diffracting grains/crystallites within the gauge volume to ‘approximate’ to counting statistics. The number of grains however depends on the gauge volume size chosen and the average size of the grains (and hence diffracting grains) within the gauge volume and this should be taken into account. The aim of this work is to give an estimate of the uncertainty due to these ‘grain-size statistics’ due to grain size, gauge volume, FWHM of the Bragg reflection (for angular dispersive diffractometers), scattering angle (2), size of detector (and hence number of diffracting grains ‘seen’ on the detector), hkl multiplicity (m) and eventually texture.

Modelling and experimental characterisation of a residual stress field in a ferritic compact tension specimen

The aim of the work is to elucidate the influence of plasticity behaviour on the residual stress field in a ferritic reactor pressure vessel steel. To this end, we investigate two compressively pre-loaded compact tension (CT) specimens to generate a mechanical residual stress field. One specimen was subsequently pre-cracked by fatigue before both specimens were measured using high-energy synchrotron X-ray diffraction. A fine grain size microstructure (∼5–10 μm grain size) allowed a small X-ray beam slit size and therefore gauge volume. The results provide an excellent data set for validation of finite element (FE) modelling predictions against which they have been compared. The results of both mechanical testing and modelling suggest that the use of a combined hardening model is needed to accurately predict the residual stress field present in the specimen after pre-loading. Some discrepancy between the modelled crack tip stress values and those found by X-ray diffraction remain which can be partly explained by volume averaging effects in the presence of very high stress/strain gradients.