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Materials Engineering
 


Materials Engineering

Stuctural Integrity research projects - Dr. Jon James

Engin X

Stress measurement by neutron diffraction is a non-destructive technique that uniquely provides insights into stress fields deep within engineering components and structures.

Over the last two decades the diffractometers available for engineering neutron diffraction have evolved from adapted traditional ‘all-purpose' instruments, designed to balance the competing requirements of the various user communities, to dedicated engineering strain scanning instrument optimized for engineering measurements.

The ISIS facility, at the Rutherford Appleton laboratory near Oxford in the UK , was home to ENGIN, one of the first neutron stress diffractometers dedicated to engineering use.

Following the success of ENGIN and the rapidly rising demand for engineering neutron diffraction facilities, an EPSRC Grant (GR/M51963), was awarded to a consortium of universities led by the Open University, and including University of Manchester, University of Salford and Imperial College, London, as well as ISIS, to design and build ENGIN-X the successor to ENGIN.

ENGIN-X was designed with the sole aim of making engineering strain measurements: essentially the accurate measurement of polycrystalline material lattice parameters, at a precisely determined location in an object. The resulting optimization led to considerable improvements, both in increased intensity and reduced background noise and resulted in an overall improvement over ENGIN in performance of approximately 25X

 

Comparison in performance between ENGIN-X and its predecessor ENGIN: (a) Ceria (200) diffraction peak as measured by both instruments. (b) Contributions to the instrument resolution: moderator (circles) and geometrical (triangles). (c) Gain in intensity: scattering spectra for a vanadium specimen. The fall of the ENGIN-X spectrum at low wavelength is due to the curvature of the neutron guide.

Strain Scanning Simulation Software, (SScanSS)

The technique of Neutron strain scanning (NSS), for determining the stress field deep inside engineering components or test samples, has evolved rapidly since its inception in the early 1980s. NSS is now an established tool for both academia and industry that commands substantial world-wide investment. Over 20 million US$ are currently being invested by the UK/US alone to produce 3rd generation, state of the art neutron stress diffractometers, ENGIN-X at ISIS in the UK, and SALSA at the ILL in Grenoble, France, and in the US at Los Alamos and Oak Ridge. Complimentary developments are also underway in Europe, Australia and Japan . The emphasis at these new facilities is on the efficient and routine execution of measurements within a range of demanding scenarios, including high spatial resolution mapping within complex specimens.

The ENGIN-X instrument at ISIS , was one of the first instruments designed within this ethos in mind, however it was recognized from the start that hardware improvements alone would not be sufficient to fully realize the potential of these instruments. In particular routine problems of experimental method needed to be overcome such as the difficulty of sample positioning and alignment and of estimating the time needed for an experiment. In addition the avoidance of collisions between the sample and elements of the instrument hardware was of prime importance. The possibility of using modern software techniques in the solution of these problems was realized in the development of the SScanSS software suite. SScanSS utilizes virtual reality methods to provide comprehensive planning and execution tools for strain scanning experiments. The software provides comprehensive facilities for positioning measurement points with the sample and automating instrument movements to realize these measurements. In addition the neutron path length through the sample may be calculated and combined with the instrument gauge volume and material attenuation coefficient to provide estimates of the likely measurement time. In this way the temporal and spatial viability of the experiment can be verified in a advance and maximum use made of the beam-time through planning and automation.

 

 

The ENGIN-X virtual laboratory: Three-dimensional models of the laboratory and sample are used for planning of experiments. (a) Simulated measurement of the axial direction of the strain tensor in a large pipe sample. (b) Once the position of the pipe has been measured the positioner movements for the actual measurement are determined automatically. (c) Measurement positions are placed within the three-dimensional model of the sample which is produced either from simple primitive objects or from detailed descriptions of the actual surface of the specimen obtained using the ENGIN-X coordinate-measuring machine and laser scanner. (d) Count time estimates are produced by combining knowledge of the material attenuation properties with calculated path lengths, (calculated intensity (symbols), measured intensity (continuous).

 

Application of robotics methods to Neutron and Synchrotron diffraction instrumentation

The SScanSS software tools for planning and executing neutron strain scanning experiments were initially written specifically for the ENGIN-X engineering diffractometer. However, recognition that a majority of the specimen positioning systems in use at strain scanning facilities are effectively serial robot manipulators, suggested that the methods of serial robot kinematic modelling might provide a means to generalize these tools for other facilities.

 

The robotics formulation enables the accurate modeling of arbitrary positioning systems:

A pipe sample with neutron access hole mounted for measurement on (a) the ENGIN-X (x,y,z, w ) table, (b) the KOWARI instrument at ANSTO (Australian Nuclear Science and Technology Organisation ), (c) the NRSF2 instrument at ORNL (Oak Ridge National Laboratory USA).

 

The numerical solution of the inverse kinematic problem allows specimens to be automatically positioned and orientated so that pre-determined strain components are measured. Using this approach the measurement positions and required strain components are established, prior to an experiment, on a virtual reality model of the sample to be measured. The software will then calculate the positioner movements that are required to execute these measurements, either in “simulation mode” for planning purposes, or for automated instrument control at the time of the experiment.

A positioning system with sufficient degrees of freedom, such as the ENGIN-X (x,y,z, w ) table with the addition of 3-axis goniometer provides considerable flexibility and the option to i) measure the three orthogonal strain components typically required for stress determination to be measured consecutively at each measurement point ( see video clip ), or, ii) optimize a secondary characteristic of the measurement position such as the measurement count time.

Alignment of a complex sample on the ENGIN-X goniometer: (a) Measurement points and strain components are defined on the virtual sample model, (b) in the alignment step measurement vectors are aligned coincidentally with the instrument Q-vectors so that the required components are measured at each measurement point, (c) The ENGIN-X (x,y,z, w ), ?table with the addition of a triple axis goniometer provides a very flexible positioning system enabling the three orthogonal strain components typically required for stress determination to be measured consecutively at each measurement point.

 

Archaeometry:

Residual stresses are those stresses which exist in an object when no external load is applied, often introduced during manufacture their examination can reveal important information about manufacturing processes, including details of cold working, or just simply whether an object was cast or forged.

The ENGIN-X neutron diffraction instrument at the Rutherford Appleton Laboratory in Oxfordshire represents the state of the art in the non-destructive measurement of residual stresses. Not only does it utilise the world's most powerful pulsed neutron source, but more specifically it incorporates a unique combination of hardware and software that enables it to provide services of particular appropriateness to the archaeometry community. Archaeological artefacts presented for measurement are often of complex shape, of delicate construction and of sometimes of extreme value. The ability to position such samples in complex orientations, while ensuring their safety through careful simulation and collision prevention is of great value.

Expertise in these techniques has resulted in an invitation for The Open University to collaborate in the European ANCIENT CHARM project. The purpose of this project is to utilise a range of European facilities and expertise in the development of established and novel neutron imaging techniques as non-invasive techniques for 3D tomographic imaging for use in cultural heritage research.

Archaeometry: (a) Accurate, virtual models of complex artefacts may be obtained at ISIS using a combination of coordinate measurement machine and laser scanner. (b) objects may be positioned accurately and simulations undertaken to ensure the accuracy of the measurement and the safety of the object, (c) neutron methods may be combined with other imaging techniques to establish sample properties such as, composition, method of manufacture or authenticity.

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