High temperature, low cycle fatigue characterization of P91 weld and heat affected zone material

The high temperature low cycle fatigue behavior of P91 weld metal (WM) and weld joints (cross-weld) is presented. Strain-controlled tests have been carried out at 400 °C and 500 °C. The cyclic behavior of the weld material (WM) and cross-weld (CW) specimens are compared with previously published base material (BM) tests. The weld material is shown to give a significantly harder and stiffer stress–strain response than both the base material and the cross-weld material. The cross-weld tests exhibited a cyclic stress–strain response, which was similar to that of the base material. All specimen types exhibited cyclic softening but the degree of softening exhibited by the cross-weld specimens was lower than that of the base material and all-weld tests. Finite element models of the base metal, weld metal and cross-weld test specimens are developed and employed for identification of the cyclic viscoplasticity material parameters. Heat affected zone (HAZ) cracking was observed for the cross-weld tests

A microscale integrated approach to measure and model fibre misalignment in fibre-reinforced composite

peer-reviewedThe full text of this article will not be available in ULIR until the embargo expires on the 30/08/2021Computational micromechanics of fibre-reinforced polymers (FRPs) relies on the ability of the representative volume elements (RVEs) to take into account the different features that characterise the geometry of the material system under consideration. Fibre misalignment has been proven experimentally to have a significant effect on the mechanical properties at the macroscale, but is not currently taken into consideration in models at the individual fibre level, perhaps due to the difficulty in statistically characterising the fibre misalignment. In this work, an integrated approach is presented to measure and model fibre misalignments in FRPs. A computed tomography (CT) scan is used to identify the fibre geometry and statistically characterise the fibre misalignment angle distribution. Using a methodology recently developed by the authors, three-dimensional (3D) RVEs were generated by requiring their misalignment angle distribution to fit the empirical distribution. The methodology proposed provides a framework for the systematic numerical analysis of the influence of fibre misalignment on mechanical properties of FRPs

Influences of non-singular stresses on plane-stress near-tip fields for pressure-sensitive materials and applications to transformation toughened ceramics

In this paper, we investigate the effects of the non-singular stress ( T stress) on the mode I near-tip fields for elastic perfectly plastic pressure-sensitive materials under plane-stress and small-scale yielding conditions. The T stress is the normal stress parallel to the crack faces. The yield criterion for pressure-sensitive materials is described by a linear combination of the effective stress and the hydrostatic stress. Plastic dilatancy is introduced by the normality flow rule. The results of our finite element computations based on a two-parameter boundary layer formulation show that the total angular span of the plastic sectors of the near-tip fields increases with increasing T stress for materials with moderately large pressure sensitivity. The T stress also has significant effects on the sizes and shapes of the plastic zones. The height of the plastic zone increases substantially as the T stress increases, especially for materials with large pressure sensitivity. When the plastic strains are considered to be finite as for transformation toughened ceramics, the results of our finite element computations indicate that the phase transformation zones for strong transformation ceramics with large pressure sensitivity can be approximated by those for elastic-plastic materials with no limit on plastic strains. When the T stress and the stress intensity factor K are prescribed in the two-parameter boundary layer formulation to simulate the crack-tip constraint condition for a single-edge notch bend specimen of zirconia ceramics, our finite element computation shows a spear shape of the phase transformation zone which agrees well with the corresponding experimental observation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42782/1/10704_2004_Article_BF00018779.pd

Numerical investigation of 3-D constraint effects on brittle fracture in SE(B) and C(T) specimens

This investigation employs 3-D nonlinear finite element analyses to conduct an extensive parametric evaluation of crack front stress triaxiality for deep notch SE(B) and C(T) specimens and shallow notch SE(B) specimens, with and without side grooves. Crack front conditions are characterized in terms of J-Q trajectories and the constraint scaling model for cleavage fracture toughness proposed previously by Dodds and Anderson. The 3-D computational results imply that a significantly less strict size/deformation limit, relative to the limits indicated by previous plane-strain computations, is needed to maintain small-scale yielding conditions at fracture by a stress- controlled, cleavage mechanism in deep notch SE(B) and C(T) specimens. Additional new results made available from the 3-D analyses also include revised {eta}-plastic factors for use in experimental studies to convert measured work quantities to thickness average and maximum (local) J-values over the crack front

Effect of Thermomechanical Loading on Near Tip Constraint

Two parameter approaches to elastic-plastic fracture mechanics were introduced to remove some of the conservatism inherent in the one parameter approach based on the J integral, ([l]) and to account for constraint effects on fracture toughness. It was shown in [2], [3] and [4] among others that much of the dependence of fracture toughness on specimen geometry could be explained by two parameter fracture theories based on Q or T. This paper examines the effect of residual or thermal stress fields on constraint in a mechanically loaded body. Finite element analyses of representative thermal loadings which give rise to high and low constraint stress fields (high and low T or Q) have been conducted. The structure is then subjected to mechanical loading, both tension and bending dominated to assess the effect on the near tip constraint. It is observed that the two parameter structure of the fields is maintained and values of Q stress have been obtained from the analyses. The thermal loading initially has a strong effect on the Q value but at higher loads when the mechanical loading dominates, this effect is much weaker. The ability of a two parameter approach using the T stress to account for constraint is assessed

Computational modelling of crack initiation in a single crystal superalloy under fatigue –oxidation conditions

International audienceCrack initiation in a single crystal nickel base superalloy was studied under fatigue-oxidation conditions, using a crystallographic constitutive theory in conjunction with a mass diffusion model. Finite element (FE) analyses were carried out on a notched compact tension (CT) specimen with a void close to the notch surface. The number of cycles to crack initiation was predicted using a strain ratchetting based failure criterion. Microcracks were predicted to initiate from the void due to the more severe ratchetting at the void surface. The applied load level and ratio, as well as the void location, strongly affect the number of cycles to crack initiation from the void. A high temperature oxidising environment is predicted to reduce the number of cycles to crack initiation by enhancing the ratchetting in the vicinity of the void, a consequence of microstructural degradation of the material near the notch due to oxidation linked diffusion processes at the notch root

A generalised sliding wear model for inhomogeneous coatings

In this work, a recently proposed continuum mechanics based multiscale approach to study the sliding wear behaviour of homogeneous coatings is generalised to components coated with an initially inhomogeneous material. The approach is suitable for either single phase or multiphase materials with and without porosities, and relies on a representative volume element of the coated system and periodic unit techniques to incorporate explicitly second phase particles or porosities. A wear relation is derived by expressing the rate of material removed in terms of an average ratchetting strain measure integrated along the sliding direction, and the average depth at which the inelastic deformation within the coating localises. The model is used to investigate the sliding wear of a Mo coating containing 5% randomly distributed porosities