Understanding the negative thermal expansion in planar graphite–metal composites

The addition of graphitic fibers and flakes as fillers is commonly used to control the thermal expansion of metals. Sintered metal matrix composites with a planar distribution of graphite flakes show a low or negative thermal expansion coefficient perpendicular to the orientation plane of the graphite (z-CTE). Since the metal matrix has a positive isotropic expansion and graphite has a high z-CTE, this effect cannot be explained by a simple model of stapled metal–graphite layers. Instead, a mechanical interaction between graphite and matrix must be considered. With neutron scattering measurements, we show that there is little or no strain of the graphite flakes caused by the matrix, which can be explained by the high modulus of graphite. Instead, we suggest that a macroscopic crumpling of the flakes is responsible for the low z-CTE of the composite. The crumpled flakes are thicker at low temperature and get stretched and flattened by the expanding matrix at high temperature, explaining the reduction in the thermal expansion across the orientation plane

Aspects of neutron residual stress analysis

This thesis is concerned with the physical principles, methodology and applications of neutron diffraction in the measurement of residual stress. Work on three main areas is presented. 1) Carbon steels 2) Data and Peak Broadening analysis and 3) Single lap glue shear joints. The Carbon steels section shows the drastic effect of the content of carbon on the measured stress. This is an aspect which has been somewhat neglected in the past. The carbon is in the form of cementite, which is a hard compound and causes the carbon steel to act like a composite material, the ferrite acting as a soft matrix and the cementite as a reinforcement. The consequence of this is that the two components develop high microstresses with plastic deformation. This is clearly illustrated in the work of [Bon 97] where values of approx. 460 MPa in the residual stress in the ferrite are balanced by negative residual stresses of 2300 MPa in cementite yielding an overall macro residual stress of zero. In this work it has been shown that even knowledge of the cementite and ferrite residual stresses and fractions may not be sufficient to accurately calculate the macro stress since the ferrite unloading curve is non linear. The use of a single valued constant modulus to convert from strain to stress is hence not valid. Peak shape analysis enables dislocation density and cell size estimates to be made. The thesis examines several methods of data weighting and deconvolution in order to asses the best means of extracting this information from standard residual stress data. Care should be taken for the peaks with very low backgrounds when finding the Gaussian and Lorentzian components. A weighting that avoids the strong bias of zero and I counts in the detector channels should be used e.g. W = I / ( 10 + Y). Lorentzian and Gaussian components can be successfully extracted from asymmetrical peaks (of peaks that broaden symmetrically), using deconvolution method 1, although the data should be of good quality. Reproducibility has been shown in the Gaussian, Lorentzian and FWHM for different instruments at different institutes. This is extremely important for the use of these values for peak broadening analysis and for estimation of the plastic deformation within a sample. The neutron diffraction technique has been used to investigate the longitudinal stresses in the adherend produced as a result of cure and due to the application of a tensile load in a single lap shear joint. The results throw doubt on widely used finite element predictions

E3: Residual Stress Neutron Diffractometer at BER II

The E3 residual stress neutron diffractometer operated at Helmholtz-Zentrum Berlin (HZB) is designed for studies in material science and engineering applications. Recent upgrade activities have made the instrument faster and more adaptable to different types of measurement. Thus, E3 has become more attractive to a broad user community, including industry, and increased substantially its scientific output

Neutron diffraction measurements of weld residual stresses in three-pass slot weld (Alloy 600/82) and assessment of the measurement uncertainty.

This paper describes in detail two neutron diffraction residual stress measurements, performed on the ENGIN-X neutron scattering instrument at the ISIS facility in the UK and on the SALSA instrument at the Institut Laue-Langevin in Grenoble, France. The measurements were conducted as part of the NeT Task Group 6 (TG6) international measurement round robin on an Alloy 600/82 multi-pass weldment - a slot in an Alloy 600 plate filled with three Alloy 82 weld beads, simulating a repair weld. This alloy/weld combination is considered challenging to measure, due to the large grain size and texture in the weld, and large gradients in the stress-free lattice parameter between the parent and weld metal. The basic principles of the neutron diffraction technique are introduced and issues affecting the reliability of residual stress characterization are highlighted. Two different analysis strategies are used for estimation of residual stresses from the raw data. Chemical composition studies are used to measure the mixing of parent and weld metal and highlight the steep lattice parameter gradients that arise as a consequence. The inferred residual stresses are then compared with three sets of measurements performed on the same plate by other NeT partners on E3 at the HZB in Berlin, STRESS-SPEC at the FRM II in Munich and KOWARI in Sydney. A robust Bayesian estimation average is calculated from the combined five-instrument data set, allowing reliable best estimates of the residual stress distribution in the vicinity of the weldment. The systematic uncertainties associated with the residual stress measurements are determined separately in the weld and parent materials, and compared with those in the NeT TG4 benchmark. This is a three-pass slot-welded plate fabricated from American Iron and Steel Institute AISI 316L(N) austenitic stainless steel, and is normally considered less challenging to measure using diffraction techniques than all nickel welds. The uncertainties in the stress measurements by neutron diffraction for these two weldments seem to be comparable. [Abstract copyright: © Vasileios Akrivos et al. 2020.

PVP2011-57695 STUDY OF CREEP RELAXATION BEHAVIOUR OF 316H AUSTENITIC STEELS UNDER MECHANICALLY INDUCED RESIDUAL STRESS

ABSTRACT Compact tension 316H austenitic steel specimens, extracted from an as-received ex-service pressure vessel header, have been pre-compresse

The DREAM Dataset: Supporting a data-driven study of autism spectrum disorder and robot enhanced therapy

We present a dataset of behavioral data recorded from 61 children diagnosed with Autism Spectrum Disorder (ASD). The data was collected during a large-scale evaluation of Robot Enhanced Therapy (RET). The dataset covers over 3000 therapy sessions and more than 300 hours of therapy. Half of the children interacted with the social robot NAO supervised by a therapist. The other half, constituting a control group, interacted directly with a therapist. Both groups followed the Applied Behavior Analysis (ABA) protocol. Each session was recorded with three RGB cameras and two RGBD (Kinect) cameras, providing detailed information of children’s behavior during therapy. This public release of the dataset comprises body motion, head position and orientation, and eye gaze variables, all specified as 3D data in a joint frame of reference. In addition, metadata including participant age, gender, and autism diagnosis (ADOS) variables are included. We release this data with the hope of supporting further data-driven studies towards improved therapy methods as well as a better understanding of ASD in general.CC BY 4.0DREAM - Development of robot-enhanced therapy for children with autism spectrum disorders

The NeT Task Group 4 residual stress measurement and analysis round robin on a three-pass slot-welded plate specimen

Accurate prediction and measurement of residual stresses in welds is an important part of assuring their short and long-term structural performance in high value, safety critical engineering components and structures. However, both measurements and predictions of weld residual stresses often exhibit high levels of variability that are not widely appreciated. The mission of the European Network on Neutron Techniques Standardization for Structural Integrity (NeT) is to develop experimental and numerical techniques and standards for the reliable characterisation of residual stresses in structural welds.The NeT Task Group 4 project examined residual stresses in a three-pass slot-welded plate specimen fabricated from AISI 316L(N) austenitic stainless steel plate. Several nominally identical specimens were fabricated under closely controlled conditions, with detailed records kept of the manufacturing history, weld process parameters, transient temperatures during welding, and the resulting geometric distortions. Comprehensive stress-strain material property characterisation was then undertaken, extending to the isothermal cyclic tests necessary to calibrate the mixed isotropic-kinematic material hardening models required for accurate weld residual stress prediction. Parallel residual stress measurement and simulation round robins were performed by a large number of participants from around the world.Residual stresses were measured using neutron and high energy synchrotron diffraction, surface X-ray diffraction, surface and deep hole drilling, the contour method, and ultrasonics. Neutron diffraction measurements were made at eight different instruments. The diffraction measurements database alone is large enough to generate reliable mean profiles, to identify clear outliers, and to establish that there is no statistically significant difference in the residual stress field in the specimens used for the non-destructive measurements. NeT Task Group 4 gives a unique insight into the real-world variability of diffraction-based residual stress measurements, and forms a reliable foundation against which to benchmark other measurement methods.NeT Task Group 4 is also a unique test bed for the development and validation of weld residual stress simulation techniques in austenitic stainless steel. Its combination of extensive materials characterisation, accurately characterized welding temperature transients, and reliable residual stress and distortion measurements is currently unrivalled. About thirty finite element simulations were submitted to the network over the course of the project, giving insights into the required accuracy of welding thermal solutions, the mechanical solution accuracy achievable using optimized material constitutive models, and the level of acceptable error in finite element residual stress simulation results for use in structural integrity assessments of high integrity engineering components.</p