David G. Carr

Optimisation of Mixed Hardening Material Constitutive Models for Weld Residual Stress Simulation Using the NeT Task Group 1 Single Bead on Plate Benchmark Problem

A single weld bead deposited on a flat plate is a deceptively simple problem that is in practice a challenge for both measurement and prediction of weld residual stresses. Task Group 1 of the NeT collaborative network has examined this problem in an exten

Defect dynamics in polycrystalline zirconium alloy probed in situ by primary extinction of neutron diffraction

After α + β-zirconium has fully transformed into β-phase upon heating, the intensities of all β-Zr Bragg reflections decrease simultaneously as a function of time. It is shown that this effect represents a transition from the kinematic to the dynamic theory of diffraction due to the ever increasing crystal perfection driven by thermal recovery of the system. The best fitting coherent crystallite size of 30 μm and other microstructural features are verified by in situ laser scanning confocal microscopy. This effect of primary extinction in neutron diffraction has been employed to further investigate the crystal perfection kinetics. Upon further heating, crystal recovery is identified as a process of dislocation annihilation, suffering from lattice friction. Upon cooling, precipitating α-Zr induces strain into the perfect β-crystallites, re-establishing the kinematic diffraction intensities. An Avrami analysis leads to the estimations of nucleation time, consumption of nucleation sites and lower-dimensional growth. Such technique bears great value for further investigation on all metal systems annealed close to the melting temperature.

Weld Toe SCF and Stress Distribution Parametric Equations for Tension (Membrane) Loading

The results of linear elastic finite element analyses of stress concentration factor (SCF) and through-thickness stress distribution for 80 weld toe T-butt plate geometries are presented in parametric form for tension (membrane) loading. The closed-form solutions, which describe the stress state of the two-dimensional plane stress models studied in terms of weldment angle, weld toe radius, weld attachment width and plate thickness, are accurate and wide ranging. The SCF is presented in full parametric form and also as a simple reduced expression, quantifying the degree of error from raw data in each case. An expression for the stress distribution through the potential Mode I crack plane of the uncracked geometries is also presented in full parametric form. It is anticipated that these HBC equations will be particularly useful in the calculation of weight functions for stress intensity factors (SIFs) of fatigue cracks emanating from weld toes in T-butt welded joints, especially in the presence of known residual stresses, or those resulting from peening.

Sensitivity of predicted weld residual stresses in the NeT Task Group 1 single bead on plate benchmark problem to finite element mesh design and heat source characteristics

A single weld bead deposited on a flat plate is a deceptively simple problem that is, in practice, a significant challenge for both measurement and prediction of weld residual stresses. Task Group 1 of the NeT collaborative network has examined this probl

An Analysis of Deformation and Fracture Behaviour of Zircaloy-4 Alloy Using Small Punch Test

The small punch (SP) test is a novel technique that uses a relatively small volume of material, to enable a measure of load versus deflection to determine the mechanical behaviour of the alloy. This study aims to investigate the mechanical properties of Zircaloy-4 (Zr-4) alloy which will be used for construction of many of the core components in ANSTO’s replacement research reactor at Lucas Heights in Australia. The Zr-4 alloy was hydrided under different conditions to simulate the radiation-induced reduction in fracture toughness over the service life of the reactor. The SP test was applied to determine the deformation and fracture behaviour of the hydrided materials. It was found that as the hydride formation increased, the equivalent fracture strain (eqf) of the alloy decreased to lower values.

Surveillance of the Fracture Behavior of Zircaloy-4 Welds Using the Small Punch Test

Zircaloy-4 (Zr-4) is a zirconium alloy that has been used extensively within the nuclear industry in both power generation and research reactors. The potential for welded reactor core components to undergo embrittlement during neutron irradiation is an important materials property issue to be considered throughout the service life of the reactor. Of particular interest is the performance of welded joints which depend on the properties of both the weld heat-affected zone (HAZ) and the parent metal. For this work Zr-4 HAZ was simulated using a Gleeble weld simulator to rapidly bring the material into the β-Zr zone at peak temperatures of 1200 and 1400°C. The small punch (SP) test was then used to investigate the variation in fracture behavior of these narrow zones of the weld and compare them with the parent metal. A finite element analysis (FEA) model of the SP test was developed which can be used to predict the deformation and fracture behavior of Zr-4 welds and is compared with the experimental data obtained.

In Situ Neutron Diffraction Studies of the Pseudoelastic-Like Behaviour of Hydrostatically Extruded Mg-Al-Zn Alloy

In-situ neutron diffraction has been used to study the pseudoelastic-like behaviour of hydrostatically extruded AZ31 magnesium alloy during stress-strain cycles in compression and tension along the extrusion direction. It has been confirmed that the activation of reversal twinning processes during unloading is responsible for the macroscopically observed hysteresis effect. Moreover, neutron diffraction data reveals the existence of high tensile stresses in grains which have just experienced significant twinning activity prior to the start of the unload cycle. It is thus proposed that this tensile stresses provides the necessary driving force for the activation of untwinning in already twinned grains.

Residual Stress Measurements in Australia: Present and Future

The Australian Nuclear Science and Technology Organisation, ANSTO, (http:\\www.ansto.gov.au) has initiated a “Neutrons for Engineering” project to provide an integrated residual stress service to Australian industry and academia. The service is based around measurements of residual stress using neutrons on a newly-refurbished instrument on the HIFAR research reactor. In addition to the neutron measurements there is a range of expertise available on the ANSTO site to solve residual stress problems using other techniques including hole-drilling, strain-gauging, and x-ray diffraction, as well as capabilities for finite element modeling and mechanical testing. In this paper we describe briefly the existing and future facilities at ANSTO for neutron strain scanning and present some benchmark results for the HIFAR strain scanner.

Combined in situ neutron diffraction and acoustic emission of twin nucleation & twin growth in extruded ZM20 Mg alloy

In the present work in situ neutron diffraction and acoustic emission were used concurrently to study deformation twinning in two ZM20 Mg alloys with significantly different grain sizes at room temperature. The combination of these techniques allows differentionation between the twin nucleation and the twin growth mechanisms. It is shown, that yielding and immediate post-yielding plasticity in compression is governed primarily by twin nucleation, whereas the plasticity at higher strains is governed by twin growth. The current results further suggest that yielding by twinning happens in a slightly different manner in the fine-grained as compared to the coarse-grained alloy.

Residual Stresses in a Zircaloy-4 Weld

The macroscopic stress distribution across a Zircaloy-4 (Zr-4) GTAW weld was measured by time-of-flight neutron diffraction at the SMARTS diffractometer at Los Alamos National Laboratory. Time-of-flight diffraction enabled the measurement of strain for all the available reflections permitted by the rolling texture of the plate and its modification in the weld (melted) and heat-affected zones. Reference lattice spacings were measured on coupons cut from an adjacent section of the weld. A maximum longitudinal stress of [Formula: See Text] was observed along the weld centre line, compared with the plate 0.2% proof stress of 390 MPa. A maximum transverse stress of [Formula: See Text] was observed on the weld centre-line falling to zero at the edge of the plate.