Numerical analysis of the effect of weld-induced residual stress and plastic damage on the ballistic performance of welded steel plate

The current paper presents numerical analyses that elucidate the effects of post-weld residual stress and associated plastic damage on the ballistic performance of 316L austenitic steel plate. Impact simulations of an 18 mm thick plate with a centreline three-pass slot weld by hemispherical-nosed and flat-nosed projectiles are performed, with initial velocities in the range of 300-800 m/s. The numerical framework consists of three interdependent stages: (i) a weld model was developed in Abaqus/Standard and validated using two independent experimental data sets; (ii) a Johnson-Cook material model is calibrated and validated along with the shear failure fracture criterion available in Abaqus/Explicit for impact models; and (iii) the weld modelling results were transferred to an impact model built in Abaqus/Explicit, which employs the validated material and fracture models to predict the ballistic performance of welded plate. It is shown that the associated plastic strain damage accumulated during the welding process - and its distribution - has an adverse effect on the ballistic performance. It has also been determined that a fracture criterion that accounts for pre-existing damage in the weldment must be used for accurate impact analyses of welded structures. Crown Copyright (C) 2012 Published by Elsevier B.V

Life assessment methodologies for high temperature branch pieces.

Branch pieces in high temperature steam circuits are a common feature of power generating plants both conventional and nuclear. A simple inverse code methodology based on BS1113 [1-3] exists for estimating base rupture life in cylinder to cylinder configurations (branches) under constant pressure and temperature. This does not cover the complex issue of estimating the mixed creep-fatigue effects of cycling which can have a significant influence on damage especially under the current practice of multi-shifting as utilities follow fluctuating energy markets. The current work is primarily aimed at extending the inverse code methodology for base rupture to include cycled loading due to startups and shutdowns. This is achieved under the guidelines of the R5 assessment code by the use of an analytical expression for metastable thermal stresses [4,5] and mapped thermal stress response. System loads are not considered in this work other than by the simple method suggested using the inverse code method. Only 90° non protruding branches are considered in the current work. Examples of 90° branches are examined showing the significance of cyclic loading on a variety of branch configurations. It was observed that base rupture dominates most configurations up to a "cliff edge" in thermal ramp rate followed by rapid accumulation of creep-fatigue damage at higher rates. The results are a useful aid when assessing the optimal operating conditions for individual power stations. © 2009, Australian Institute for Non-Destructive Testing (AINDT

Use of a simplified analytical expression for metastable thermal stress analysis and its application to creep-fatigue damage of a 2.25Cr 1Mo thick walled component.

Thick walled pressure vessels are of considerable importance in a wide range of industries. The evaluation of stresses is necessary not only from a design point of view but also for fitness for service analysis of ageing infrastructure. The accumulation of creep–fatigue damage over time is the principal damage mechanism which will eventually lead to crack initiation in critical high temperature fossil plants. Many power stations are being subjected to two-shift operation due to changes in demand and competition from cheaper energy sources, and in the future from added carbon taxes. To assess high temperature components for creep–fatigue damage for example, under faster ramp rates and additional cycles, as a first pass it would be useful to explore the feasible operational envelope using simplified calculations. These are, however, generally not available and more complex finite element analysis is necessary. This paper uses a simplified closed form solution for metastable thermal stresses in thick walled pressure vessels. This form of solution can if necessary be used with either stress concentration factors or superposition of polynomials for more complex components derived from FEA analysis, such that the closed form solution can be used to estimate any ramp rate on the unit. In this case the ramp rates are considered to provide sufficient time to become metastable. Many existing units rely on heavy section 2.25Cr 1Mo steel (P22) pipe-work and tubing, and hence for two shifting can be subjected to high levels of cyclic strain. Based on the simplified expression developed, an operational envelope is explored for thick walled cylinders constructed using P22 steel. Creep–fatigue damage is calculated based on the R5 methodology. The analysis shows that for thick walled components with minimal stress concentrations, creep will dominate the life of the component. However, complex interaction between base rupture, onset of significant cycling, creep, and fatigue dictates the upper bound on feasible ramp rates, as a result it is possible to construct screening curves based on the effective elastic stress intensity range. © 2010, Elsevier Ltd

Residual stresses in a welded zircaloy cold neutron source containment vessel

Zirconium alloys are widely used in the nuclear industry because of their relative high strength, neutron transparency, resistance to high neutron-irradiation environment and corrosion resistance. One application for Zirconium alloy Zr-2.5Nb is the vacuum confinement vessel utilised in the cold neutron source of the OPAL research reactor at ANSTO. Having a total length of more the 3 meters, it is made of two sections joined using electron beam welding. The weld and the nearby regions are critical for the performance and integrity of the component and therefore understanding of the residual stresses development within the weld is important in connection to (i) evolution of fine dual phase α/b microstructure and crystallographic texture (ii) and stress-related radiation induced phenomena, such as grain growth, creep and sub-critical crack growth by delayed hydride cracking. The stresses were measured in and around an electron beam weld produced during the development of this component of the OPAL Cold Neutron Source. The effects of a large grain size in the weld were reduced by taking advantage of rotational symmetry and rotating the sample to increase the swept volume. Due to the heat-treatment after welding, the stresses were very low, less than 10% of the yield strength of the material, in both the hoop and axial directions. As a result of phase transformation effects during the welding process the final stresses are compressive in the weld, which reduces the likelihood of fracture or of hydride formation in this region. The highest stresses are in the parent material adjacent to the weld where the toughness is expected to be higher than in the weld material

Using advaced weld modelling to ensure the structural integrity of plant in the nuclear industry

Not available. Physical copy available in ANSTO Library at 621.48/17Using advaced weld modelling to ensure the structural integrity of plant in the nuclear industr

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 problem in an extensive programme of measurement and simulation extending from 2002 to 2008. Thus, the NeT bead on plate forms an ideal benchmark problem for the development of weld residual stress simulation techniques. It is often difficult to separate the influence of different analysis variables in a large collaborative study such as NeT Task Group 1. This paper examines sensitivity to mesh design, element type, and heat source characteristics in a closely controlled study using several different mesh designs, element types (both tetrahedral and hexahedral), and heat sources, but the same material constitutive model and finite element analysis code. It complements a companion paper that varies material constitutive models. A dedicated heat source modelling tool with a semi-automatic interface to the ABAQUS finite element code has been used to vary the heat source characteristics, thus facilitating rapid and controlled sensitivity studies without the need for bespoke heat source coding within ABAQUS

Special testing equipment and validation of measurement methodologies for high temperature low cycle fatigue testing of miniature metallic specimens

A technique for high temperature low cycle fatigue testing of metallic materials has been developed, to determine fatigue behaviour through the testing of miniature specimens. The miniature specimen geometry was specifically designed, such that it could be manufactured from a small volume of material removed by chain-drilling extraction. An extensometry method to measure and control strain at the specimen shoulders during testing was adopted. This was undertaken to minimise the deleterious contact effects that can occur via extensometry attached at the gauge length of specimens, hence leading to premature failure and inaccurate fatigue data. By the application of this technique, the high temperature low cycle fatigue behaviour of 2.25Cr-1Mo steel was successfully characterised at 540 °C, under a fully reversed strain-controlled regime. The fatigue properties of the steel obtained from testing miniature specimens were shown to correlate well with existing literature for the material under comparable conditions, as determined by the testing of conventional standard-sized specimens. © 2016 Society for Experimental Mechanic

Crack initiation and crack growth assessment of a high pressure steam chest.

Extensive cracking had occurred in a number of high pressure steam chests. An assessment was undertaken based on the R5 British Energy methodology to assess the components for both creep–fatigue damage initiation and crack growth analysis to determine fitness for purpose. The analysis determined that the remaining base rupture endurance life of the component was greater then 1 million hours, however, due to the start-up and shutdown ramp rates, creep–fatigue damage greater then unity has occurred leading to crack initiation in a number of locations. These cracks were confirmed during internal inspection of the steam chest. A subsequent crack growth analysis determined that the component could safely be returned to service for the expected future life of the station. © 2011, Elsevier Ltd

Assessment of creep damage models in the prediction of high-temperature creep behaviour of Alloy 617

This study aims to predict the creep response of Alloy 617 and assess the accuracy of three established creep damage models in predicting the creep time-to-failure (tf) and strain-to-failure (εf) in the temperature range of 800–1000 °C. Idaho National Laboratory (INL) creep data were used to calibrate (1) Ductility Exhaustion (DE), (2) Stress-Modified Ductility Exhaustion (SMDE), and (3) Strain-Energy Density (SED) creep damage models using multiple linear regression (LR) and reverse damage (RD) approaches. In order to predict the creep response of Alloy 617 these creep damage models are then coupled with (secondary) creep strain model calibrated using Idaho National Laboratory (INL), Korean Atomic Energy Research Institute (KAERI), and Argonne National Laboratory (ANL) experimental data. Direct comparisons made between the investigated creep damage models revealed that the SED model with parameters calibrated through RD approach captures the creep response of Alloy 617 the most accurately and it thus produces the most accurate prediction of time-to-failure (tf) and strain-to-failure (εf) across different temperature/stress (creep) conditions. However, it is also shown that none of the employed creep damage models are able to fully capture the material creep response at 1000 °C. This is attributed the strong oxidation of Alloy 617 at 1000 °C (tested in air) leading to the formation of a thick oxidation layer, which might affect the failure mechanism of the alloy at this temperature

Comprehensive numerical analysis of a three-pass bead-in-slot weld and its critical validation using neutron and synchrotron diffraction residual stress measurements

The current paper presents a finite element simulation of the residual stress field associated with a three pass slot weld in an AISI 316LN austenitic stainless steel plate. The simulation is split into uncoupled thermal and mechanical analyses which enable a computationally less expensive solution. A dedicated welding heat source modelling tool is employed to calibrate the ellipsoidal Gaussian volumetric heat source by making use of extensive thermocouple measurements and metallographic analyses made during and after welding. The mechanical analysis employs the Lemaitre-Chaboche mixed hardening model. This captures the cyclic mechanical response which a material undergoes during the thermo-mechanical cycles imposed by the welding process. A close examination of the material behaviour at various locations in the sample during the welding process, clearly demonstrates the importance of defining the correct hardening and high temperature softening behaviour. The simulation is validated by two independent diffraction techniques. The well-established neutron diffraction technique and a very novel spiral slit X-ray synchrotron technique were used to measure the residual stress-strain field associated with the three-pass weld. The comparison between the model and the experiment reveals close agreement with no adjustable parameters and clearly validates the used modelling procedure. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved