The re-characterisation of complex defects: Part II: cleavage

The re-characterisation of complex defects with re-entrant sectors has been addressed for cracks extending by fatigue, ductile tearing and cleavage. In Part I crack extension by fatigue and ductile tearing was discussed. In Part II cleavage data are presented for a family of complex defects with re-entrant sectors. Experimental tests on complex and re-characterised profiles are analysed using deterministic and probabilistic approaches. The work addresses the conservatism of re-characterisation procedures when applied to cleavage failure on the lower shelf and in the ductile–brittle transition.     

Behaviour of fibre metal laminates subjected to localised blast loading—Part II: Quantitative analysis

This two part paper examines the behaviour of fibre-metal laminates (FMLs) subjected to localised explosive blast loading. Part I presents the experimental observations of diamond and cross-shaped back face deformation, as well as pitting, global displacement and ring-buckling of the front face are also discerned. Part II presents quantitative analysis of the experimental data. When expressed in terms of non-dimensional parameters front and back face displacements fall within one plate thickness of a linear trend line. The threshold impulse for the onset of tearing is found to increase linearly with panel thickness.     

Effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens

The effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens were studied by experiments and FEM calculations. The results show that the local stress concentration around the voids promotes the cleavage initiation and decreases the notch toughness and cleavage fracture stress. The fibrous cracks ahead of notch tips caused by the ductile tearing in the WPS obviously raise the normal stress in front of their tips and decrease fracture load and notch toughness. When the beneficial effects of WPS on improving apparent fracture toughness for specimens or structures are used, the loads in WPS need to be limited so that no obvious void damage and ductile tearing are produced in front of defects.     

Probabilistic treatment of fracture using two failure models

A probabilistic methodology for brittle fracture based on two local failure models is presented. Probabilistic fracture parameters are obtained using a weakest link and a chain-of-bundles formulation. Both models define limiting distributions for the fracture stress described by a two-parameter Weibull distribution. Numerical procedures employing measured toughness data and finite element solutions are also described to calibrate the Weibull parameters. An application of the methodology then follows to predict geometry and stable crack growth effects on the distribution of macroscopic fracture toughness (J_c) for a high-strength steel. Measured fracture toughness values for a high-constraint geometry that exhibit no prior ductile tearing are effectively ‘transferred' to a different geometry having much lower constraint and in which tearing precedes cleavage. The inherent difficulty in predicting the scatter of experimental fracture toughness, as well as constraint and ductile tearing effects, within the scope of conventional procedures appears greatly reduced in the framework presented in this work.     

Dynamic ductile crack growth and transition to cleavage – a cell model approach

The fracture behavior of ferritic steel in the transition regime is controlled by the competition between ductile tearing and cleavage. Many test specimens that failed by catastrophic cleavage showed significant amounts of ductile tearing prior to cleavage fracture. The transition from ductile tearing to cleavage has been attributed to the increase in constraint and sampling volume associated with ductile crack growth. This work examines the role of dynamic ductile crack growth on the fracture mode transition by way of a cell model of the material. The cell model incorporates the effects of stress triaxiality and strain rate on material failure characteristics of hole growth and coalescence. Loading rate and microstructure effects on the stress fields that evolve with rapid (ductile) crack growth are systematically studied. The stress fields are employed to compute the Weibull stress which provides probability estimates for the susceptibility to cleavage fracture. A center-cracked panel subjected to remote tension is the model problem under study. The computational model uses an elastic-viscoplastic constitutive relation which incorporates enhanced strain rate hardening at high strain rates. Adiabatic heating due to plastic dissipation and the resulting thermal softening are also accounted for. Under dynamically high loading rate, our model shows the crack speed achieves its peak value soon after crack initiation and quickly falls off to slower speeds with further crack growth. Remarkably, the Weibull stress follows a similar pattern which suggests that the transition to the cleavage fracture is most likely to occur, if at all, at the peak speed of ductile crack growth. Key words: Dynamic fracture, ductile tearing, crack growth, transition regime, cleavage fracture, cell model, finite element.     

Behaviour of fibre–metal laminates subjected to localised blast loading: Part I—Experimental observations

This two-part article examines the behaviour of aluminium alloy–glass fibre-reinforced polypropylene-based fibre–metal laminates (FMLs) subjected to localised explosive blast loading. Part I presents observations from the experiments on samples of varying thickness and material distribution, and investigates the influence of stacking configuration. This extensive study examines panels which have between two and five layers of aluminium, and up to eight plies of composite between each pair of aluminium layers. Diamond and cross-shaped back face damage is observed and varies according to panel thickness. Pitting, global displacement and ring buckling of the front face are also discerned. Some observations are related to wave propagation effects. Part II reports a quantitative analysis of the experimental data. Expressed in terms of non-dimensional parameters, front and back face displacements fall within one plate thickness of a linear trend line. The threshold impulse for the onset of tearing is found to increase linearly with panel thickness.     

Use of R5 in plant defect assessment

The R5 defect assessment procedures are widely used to assess components in Nuclear Electric's Advanced Gas-Cooled Reactor plant. In this paper, the use of R5 is illustrated by calculating the creep- fatigue crack growth in a specific component. The resulting crack size is compared with the limiting crack size calculated using R6. The implications of considering ductile tearing in the latter calculation, the possible interaction of tearing with creep crack growth, and the use of leak before break arguments are discussed.     

Analysis of ductile to cleavage transition in part-through cracks using a cell model incorporating statistics

This paper describes an approach to study ductile/cleavage transition in ferritic steels using the methodology of a cell model for ductile tearing incorporating weakest link statistics. The model takes into account the constraint effects and puts no restriction on the extent of plastic deformation or amount of ductile tearing preceding cleavage failure. The parameters associated with the statistical model are calibrated using experimental cleavage fracture toughness data, and the effect of threshold stress on predicted cleavage fracture probability is investigated. The issue of two approaches to compute Weibull stress, the 'history approach' and the 'current approach', is also addressed. The numerical approach is finally applied to surface-cracked thick plates subject to different histories of bending and tension, and a new parameter, ψ, is introduced to predict the location of cleavage initiation.     

The deformation and tearing of thin square plates subjected to impulsive loads—An experimental study

Experimental results for clamped mild steel square plates subjected to impulsive loads are reported. The strain rate-sensitive plates exhibit mode I (large ductile deformation), mode II (tensile-tearing and deformation) and mode III (transverse shear) failure modes. Three phases of mode II are identified as mode II¹ (partial tearing), mode IIa (complete tearing with increasing mid-point deformation), and mode IIb (complete tearing with decreasing mid-point deformation). For the cases where complete tearing occurs (modes II and III) the velocity of the square disc torn from the base plate is measured. An attempt to measure the speed of rotation of the disc is discussed. Threshold for the onset of failure modes II and III are given. Comparisons between the response of square and circular plates are discussed.     

A practical method for fatigue limit prediction in ductile cast irons

A simple, reasonably accurate method was proposed for fatigue limit prediction in ductile cast irons (DCIs) containing small defects, employing an easy‐to‐use prediction equation. A technique was also presented for the estimation of the statistical effects of complex structural discontinuities, as characterized by graphite and casting defects. The validity of these approaches was confirmed by the experimental results obtained via rotating‐bending fatigue tests on six DCIs, with varying distributions of graphite nodule size and different matrix structures.     

The effect of specimen dimensions on mixed mode ductile fracture

The results from a series of experiments are presented to determine the effect of specimen dimensions on the ductile tearing resistance of A508 Class 3 forged steel at ambient temperature. Single edge notch tension specimens were subjected to Mode I, Mode II and combination of Modes I and II. Mode I tests on various specimen sizes reveal characteristic features found in earlier work, such as decreasing slope of the tearing resistance with increasing constraint (or specimen size). In contrast, for Mode II the tearing resistance is shown to be independent of specimen size, although dependent on initial crack length. The tests show that there is a competition between void growth and shear localisation as mechanisms for ductile crack extension. The dominance of one mechanism over the other is shown to be related to the local Mode I and Mode II components of the J-integral.     

STRUCTURAL COLLAPSE, THE FAILURE ASSESSMENT DIAGRAM AND "OFF-THE-SHELF" OPERATION

Abstract— The criteria for determining whether ferritic material exhibits fully ductile behaviour are generally based on the fracture toughness vs temperature relationship determined from standard laboratory test pieces (e.g. Charpy V Impact tests or fracture toughness specimens). This relationship defines a ductile-brittle transition region. When fully ductile, microvoid coalescence behaviour is experienced, and fracture toughness is described as being on the “upper shelf”. At “off the shelf” temperatures brittle, cleavage fracture is experienced. On the lower shelf the material is entirely brittle, exhibiting 100% crystallinity on the fracture face. As the temperature increases, initiation of tearing by microvoid coalescence occurs and some stable tearing can occur prior to the cleavage event. Material toughness increases with temperature until the upper shelf condition is achieved. The characteristics of fracture toughness tests in terms of the toughness level exhibited and the extent of ductile tearing experienced have been used as a guide to whether the structural application (e.g. a pressure vessel) will behave in a brittle or a ductile manner. This paper reports on a feasibility study where various worked examples have been performed to examine the concept of using a “cut off” on the failure assessment diagram, determined from the conditions required to cause plastic collapse of a pressure vessel, as a criterion for defining effective “ductile” operation. Fracture assessment procedures (R6 revisions 2 and 3 and PD6493 levels 2 and 3) have been utilised to determine the influence on pressure vessel performance of the behaviour of fracture toughness test specimens. The procedure of plotting a structural collapse “cut off” on a failure assessment diagram enables the assessment of whether a particular flaw geometry would result in gross deformation of the structure at failure. The use of this procedure provides an unambiguous demarcation between “fracture dominated” and “collapse controlled” conditions. This procedure facilitates judgements on the level of toughness necessary to ensure ductile operation and whether a “tearing plus toughness” requirement is necessary. It is recommended that consideration be given to including structural collapse into fracture assessment procedures carried out using R6 revision 3 or PD 6493:1991 procedures in order to determine the conditions when enhanced toughness no longer influences structural performance (i.e. when effective “upper shelf” conditions are attained).     

Ductile to brittle transition of an A508 steel characterized by Charpy impact test: Part I: experimental results

This study is devoted to the ductile-brittle transition behavior of a French A508 Cl3 (16MND5) steel. Due to its importance for the safety assessment of PWR vessels, a full characterization of this steel with Charpy V-notch test in this range of temperature was undertaken. The aim of this study is to provide a wide experimental database and microstructural observations to supply, calibrate and validate models used in a local approach methodology. Mechanical and fracture properties of the steel have been investigated over a wide range of temperatures and strain-rates. Effects of impact velocity on ductile-brittle transition curve, on ductile tearing and on notch temperature rise are presented and discussed. A detailed study of ductile crack initiation and growth in Charpy specimens is also carried out. From fractographic investigations of the microvoids nucleation around carbide second phase particles, a plastic strain threshold for nucleation is determined for this material. A508 Cl3 steels undergo a transition in fracture toughness properties with temperature, due to a change in fracture mode from microvoids coalescence to cleavage fracture. A systematic investigation on the nature and the position of cleavage triggering sites and on any change in the ductile to brittle transition (DBT) range has been carried out. This leads to the conclusion that manganese sulfide inclusions do not play an increasing role with increasing test temperature as recently mentioned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [Tanguy et al., Engng. Fract. Mech., in press], the numerical simulation of the Charpy test in the ductile-brittle transition range using fully coupled local approach to fracture is presented.     

Thermomechanical transformation fatigue of TiNiCu SMA actuators under a corrosive environment – Part I: Experimental results

In this two-part paper, the thermomechanical fatigue of TiNiCu shape memory alloy (SMA) wire actuators undergoing thermally induced martensitic phase transformation in a corrosive environment is investigated. The main objective of this work is to evaluate the cyclic response and fatigue behavior of TiNiCu SMA wire under corrosive conditions and to compare it to results obtained for fatigue testing in a corrosion-free environment. Part I focuses on the various experimental aspects of this work, including the presentation of fatigue results as a function of various testing parameters. The variable test parameters are five applied stress levels from about 50 MPa to about 250 MPa, and two different actuation strains, one corresponding to full actuation or complete transformation and the other to partial transformation. The results from fatigue testing in a corrosive environment show a consistent reduction of the fatigue life compared to corrosion-free fatigue results, in both complete and partial transformation conditions. It is also observed that corrosion-assisted fatigue leads to more scattered fatigue data and this spread is mostly attributed to enhanced and accelerated damage mechanisms due to corrosion. From these conclusions, a microstructure evaluation is performed to understand the damage that contributes to lower fatigue limits under corrosion and is presented in Part II of this work. Fracture surfaces, development of fatigue cracks and effect of corrosion are presented and discussed. The conclusion from the microstructure analysis has led to the formulation of a damage accumulation model accounting for a cyclic corrosion mechanism. This modeling approach allows for determining the fatigue life reduction of SMA wire actuators in a corrosive environment. All results of the microstructure analysis and fatigue life modeling are presented in Part II.     

Assessment of effect of ductile tearing on cleavage failure probability in ductile to brittle transition region

The fracture behaviour of ferritic and ferritic martensitic steels in ductile to brittle transition (DBT) region has been extensively studied in recent years and a probabilistic approach of master curve method is generally used to describe the fracture toughness of BCC steels in DBT region as a function of temperature. The assessment of cleavage failure probability however is still untouched in the upper region of ductile to brittle transition, although various extensions of master curve approach and various local approaches has been explored. Additionally the geometry and loading in tension and bending also adds up to the difficulties when cleavage failure is assisted with prior ductile tearing. In this work the cleavage fracture is investigated in upper region of DBT and a modified master curve approach is presented which can satisfactorily describe the fracture toughness as a function of temperature as well as amount of ductile tearing preceded by cleavage.     

Prediction of residual fatigue life under interspersed mixed-mode (I and II) overloads by Artificial Neural Network

Mixed-mode (I and II) overloads are often encountered in an engineering structure due to either alteration of the loading direction or the presence of randomly oriented defects. Prediction of fatigue life in these cases is more complex than that of mode-I overloads. The objective of this study is to explore the use of an artificial neural network (ANN) model for the prediction of fatigue crack growth rate under interspersed mixed-mode (I and II) overload. The crack growth rates as predicted by the ANN method on two aluminium alloys, 7020 T7 and 2024 T3 have been compared with the experimental data and an Exponential Model. It is observed that the predicted results are in good agreement and facilitate determination of residual fatigue life.     

Pull-out of a ductile fibre from a brittle matrix

The pull-out of a ductile fibre from a brittle matrix was analysed in Part I [1] using a shear-lag model. However, the analysis is formidable due to the consideration of Poisson's effect along the sliding length. This consideration is essential when the debonded fibre-matrix interface is subjected to Coulomb friction during fibre pull-out. To simplify the analysis, Poisson's effect is treated in an average sense in the present study, whereas it was treated pointwise in Part I. The present simplified solutions are in excellent agreement with the previous more rigorous and more complex solutions. The simplified model thus provides adequate solutions for the pull-out of a ductile fibre from a brittle matrix, and can be readily used for further applications.