Alan Cocks

Ice-lens formation and geometrical supercooling in soils and other colloidal materials.

We present a physically intuitive model of ice-lens formation and growth during the freezing of soils and other dense, particulate suspensions. Motivated by experimental evidence, we consider the growth of an ice-filled crack in a freezing soil. At low temperatures, ice in the crack exerts large pressures on the crack walls that will eventually cause the crack to split open. We show that the crack will then propagate across the soil to form a new lens. The process is controlled by two factors: the cohesion of the soil and the geometrical supercooling of the water in the soil, a new concept introduced to measure the energy available to form a new ice lens. When the supercooling exceeds a critical amount (proportional to the cohesive strength of the soil) a new ice lens forms. This condition for ice-lens formation and growth does not appeal to any ad hoc, empirical assumptions, and explains how periodic ice lenses can form with or without the presence of a frozen fringe. The proposed mechanism is in good agreement with experiments, in particular explaining ice-lens pattern formation and surges in heave rate associated with the growth of new lenses. Importantly for systems with no frozen fringe, ice-lens formation and frost heave can be predicted given only the unfrozen properties of the soil. We use our theory to estimate ice-lens growth temperatures obtaining quantitative agreement with the limited experimental data that are currently available. Finally we suggest experiments that might be performed in order to verify this theory in more detail. The theory is generalizable to complex natural-soil scenarios and should therefore be useful in the prediction of macroscopic frost-heave rates.

The Response of Rigid Plates to Deep Water Blast: Analytical Models and Finite Element Predictions

One-dimensional analytical models and finite element calculations are employed to predict the response of a rigid plate, supported by a linear spring, to loading by a planar pressure shock wave traveling in water or in a similar inviscid liquid. Two problems are considered: (i) a spring-supported rigid plate in contact with fluid on one side and (ii) a spring-supported rigid plate in contact with fluid on both sides; for both problems, plates are loaded by an exponentially decaying shock wave from one side. Cavitation phenomena in the liquid, as well as the effect of the initial static fluid pressure, are explicitly included in the analytical models and their predictions are found to be in excellent agreement with those from FE calculations. The validated analytical models are used to determine the sensitivity of the plate’s response to mass, spring stiffness and initial static pressure. [DOI: 10.1115/1.4006458]

A review of the changes of internal state related to high temperature creep of polycrystalline metals and alloys

Abstract When polycrystalline metals and their alloys are used at high temperature, creep deformation leads to changes in their internal state. The change in internal state manifests itself in many ways, but the two ways that concern us in this review are (i) the creation of internal stress arising from the strain incompatibility between grains and/or the formation of cell/sub-grain structures and (ii) a change in the material resistance. This review aims to provide a clear separation of these two concepts by exploring the origin of each term and how it is associated with the creep deformation mechanism. Experimental techniques used to measure the internal stress and internal resistance over different length-scales are critically reviewed. It is demonstrated that the interpretation of the measured values requires knowledge of the dominant creep deformation mechanism. Finally, the concluding comments provide a summary of the key messages delivered in this review and highlight the challenges that remain to be addressed.

Energy-based non-local plasticity models for deformation patterning, localization and fracture

This paper analyses the effect of the form of the plastic energy potential on the (heterogeneous) distribution of the deformation field in a simple setting where the key physical aspects of the phenomenon could easily be extracted. This phenomenon is addressed through two different (rate-dependent and rate-independent) non-local plasticity models, by numerically solving two distinct one-dimensional problems, where the plastic energy potential has different non-convex contributions leading to patterning of the deformation field in a shear problem, and localization, resulting ultimately in fracture, in a tensile problem. Analytical and numerical solutions provided by the two models are analysed, and they are compared with experimental observations for certain cases.

Initiation and growth kinetics of solidification cracking during welding of steel.

Solidification cracking is a key phenomenon associated with defect formation during welding. To elucidate the failure mechanisms, solidification cracking during arc welding of steel are investigated in situ with high-speed, high-energy synchrotron X-ray radiography. Damage initiates at relatively low true strain of about 3.1% in the form of micro-cavities at the weld subsurface where peak volumetric strain and triaxiality are localised. The initial micro-cavities, with sizes from 10 × 10−6 m to 27 × 10−6 m, are mostly formed in isolation as revealed by synchrotron X-ray micro-tomography. The growth of micro-cavities is driven by increasing strain induced to the solidifying steel. Cavities grow through coalescence of micro-cavities to form micro-cracks first and then through the propagation of micro-cracks. Cracks propagate from the core of the weld towards the free surface along the solidifying grain boundaries at a speed of 2–3 × 10−3 m s−1.

Computational modelling of hydrogen embrittlement in welded structures

Abstract This paper deals with the modelling of the combined hydrogen embrittlement phenomena: hydrogen-enhanced local plasticity (HELP) and hydrogen-induced decohesion (HID) in dissimilar welds through a cohesive zone modelling approach (CZM). Fractured samples of dissimilar weld interfaces in AISI8630/IN625 show that cracks propagate in a region called the “featureless” region located in the Nickel side of the weld. This region is characterized by the presence of a distribution of fine carbides. We model the effect of hydrogen on the material toughness as the result of a synergistic effect of HELP and HID mechanisms where (i) hydrogen enhanced dislocation mobility promotes the development of dislocation structures at the carbides which increases the stress on the particles; while the presence of hydrogen also results in (ii) a reduction in the (a) cohesive strength of the carbide/matrix interface and (b) in the local flow stress of the matrix. The decohesion mechanism at the carbide/matrix interface is modelled through a two-dimensional user-defined cohesive element implemented in a FORTRAN subroutine (UEL) in the commercial finite element code ABAQUS and the effect of the hydrogen on the plasticity properties of the matrix is coded in a UMAT routine. Preliminary analysis on a unit cell representing the matrix/carbide system under plane strain shows that HELP and HID are competitive mechanisms. When the combined mechanism HELP+HID occurs microcracks form at the matrix/carbide interface due to decohesion process followed by localization of plastic flow responsible for the link-up of the microcracks.

A micromechanical image-based model for the featureless zone of a Fe–Ni dissimilar weld

This paper deals with the constitutive modelling of the ‘featureless’ region located on the Nickel side of a AISI8630/IN625 dissimilar weld interface. Fractography of failed weld interfaces show that cracks propagate in this carbides ()-rich region in the presence of hydrogen. In this paper, TEM images of the carbide-rich region are converted into a finite element mesh through an image-based mesh generation scheme. Simulations of the response of these structures show that in areas where the hydrogen content is high the matrix surrounding the carbides softens and plastic flow is localized. Moreover, the presence of hydrogen lowers the cohesive strength, giving rise to microcrack formation at the carbide-matrix interface. The amount of deformation then increases in a localized region adjacent to the region where (a) hydrogen content is high and (b) the carbide/matrix interface has debonded. As deformation proceeds the microcracks grow and link to form macrocracks, which generates the failure surface.

Physics Based Formulation of a Cohesive Zone Model for Ductile Fracture

This paper addresses a physics based derivation of mode-I and mode-II traction separation relations in the context of cohesive zone modeling of ductile fracture of metallic materials. The formulation is based on the growth of an array of pores idealized as cylinders which are considered as therepresentative volume elements. An upper bound solution is applied for the deformation of the representative volume element and different traction-separation relations are obtained through different assumptions.

A new numerical scheme for computer simulation of multiple cracking in ceramic films during constrained sintering

Heterogeneities in a green film made from a powder compact is considered to be one of the major reasons for the generation of sintering cracks. Stresses are generated in a film due to the constraint of the substrate. Instabilities in the sintering process can occur at sites of these heterogeneities resulting in the generation of multiple cracks, which can propagate through the thickness of the film. The classical finite element method is fundamentally ill-suited to studying this multiple-cracking problem. This paper presents a simple and robust numerical method for the computer modelling of sintering and multiple cracking. The method is based on the so-called material point method, which was initially developed for large deformation problems in plasticity. A parallel computing algorithm is implemented and a simple scheme for modelling the initiation and propagation of multiple cracks is proposed. The numerical scheme is then validated by simulating a simple geometric problem for which an analytical solution can be obtained. Finally, the robust performance of the numerical method is demonstrated by modelling the sintering response of a film which contains different types of heterogeneities.

An integrated framework for multi-scale multi-physics numerical modelling of interface evolution in welding

The project Modelling of Interface evolution in advanced Welding (MIntWeld) is a 4-year international research project funded by the European Commission under their FP7 programme. Its main target is to develop a numerical toolbox which can be used to predict the evolution of interfaces during welding. There are various interfaces involving multiple phenomena and different spatial scales, which can be simulated using corresponding numerical modelling methods respectively. The modelling methods include quantum dynamics, molecular dynamics, phase field, phase field crystal, computational fluid dynamics, phase transformation and heat transfer, thermodynamics, continuum mechanics and life and defects prediction. Although each modelling method is based on different physical theories and involves different scales, they are not isolated. Therefore, this project aims to design a common framework which couples each model with the upstream and/or downstream model at the relevant neighbouring length scales. The data exchange framework which underpins the coupling of the models is described, and typical examples addressing the solution to the challenges faced, such as those of data interpolation between one discretisation of the computational domain and another, are discussed. Initial successes from the model-linking efforts of the authors are also presented.