Jonas Faleskog

Effects of local constraint along three-dimensional crack fronts—a numerical and experimental investigation

Abstract A two-parameter characterization of the crack front state in a variety of geometries (mode I) is explored by means of three-dimensional finite element analysis, including finite strain effects. In particular the approximate J-Q theory is scrutinized and it is found that Q appears to be a good measure of the deviation in stress triaxiality ahead of a crack tip as compared to the highly constrained plane strain SSY-solution, also in cases where the crack front is relatively curved. The implications for cleavage fracture in the upper transition region are elucidated by appraising the results from an extensive experimental program, where both tension and bending type of plane specimens as well as surface cracked plate specimens had been tested. It appears that the J-Q concept together with some cleavage failure criteria, e.g. the RKR-model, can be applied locally along three-dimensional crack fronts in a structure in order to assess cleavage fracture. To the extent that one dominating cleavage fracture spot could be located at a three-dimensional crack border, this was in general found at the position which had undergone the most critical J-Q sequence, in the light of the RKR-criteria. Microstructural features of both ductile and cleavage fracture are elucidated by a fractographical survey performed.

A probabilistic model for cleavage fracture with a length scale - parameter estimation and predictions of stationary crack experiments

This study presents a large experimental investigation in the transition temperature region on a modified A508 steel. Tests were carried out on single-edge-notch-bend specimens with three different ...

A probabilistic model for cleavage fracture with a length scale : influence of material parameters and constraint

A probabilistic model for the cumulative probability of failure by cleavage fracture with a material related length scale is developed in this study. The model aims at describing the random nature of fracture in ferritic steels in the brittle-to-ductile transition temperature region. The model derives from use of an exponential function to describe the distribution of microstructural entities eligible to take part in the fracture initiation process, where also a dependence on effective plastic strain is incorporated. A nonlocal stress measure, calculated as the average stress in a spherical volume, drives the contribution to failure probability of an infinitesimal material volume. The radius of the spherical volume enters as the material length in this model. This length has a significant influence on failure probability predictions in geometries exposed to strong stress gradients as found ahead of cracks. The material length is associated with a fracture toughness threshold value. In a fracture application three model parameters need to be estimated based on testing; a parameter directly related to the mean fracture toughness, a parameter that primarily is related to crack-tip constraint effects and the material length parameter. The model is explored in a parametric study showing model features in concord with typical features found in toughness distributions from fracture mechanics testing in the transition region.

Near-tip field characterization and J-integral evaluation for nonproportional loads

Abstract The J -integral is calculated numerically using the domain integral method and a contour integral according to the definition of the J . It was conjectured that the methods could give conflicting results for nonproportional loads. It is found, however, that both methods give satisfactory results conforming even for highly nonproportional loads. In plane strain, the near-tip field could successfully be characterized by a two-parameter description according to the J-Q theory. Remotely applied nonproportional loads corresponded to different paths in the J-Q space. This was demonstrated in the range of small-scale yielding up to fully yielded situations.

An experimental and numerical investigation of ductile crack growth characteristics in surface cracked specimens under combined loading

Fracture mechanics tests have been performed in the upper shelf region of a steel on large surface cracked plate (SCT) specimens and on small compact tension (CT) specimens. Some of the SCT specimens were subjected to combined tension and bending in such a way that the loading was strongly non-proportional. Crack growth characteristics were compared between the specimens in order to assess possible influences of geometry and nonproportional loading. The differences observed could unambiguously be reduced to and correlated with differences in constraint. Constraint parameters were evaluated by detailed three-dimensional finite element computations and quantities for growing cracks were interpreted on the basis of deformation theory considerations, in analogy with the commonly used JR-philosophy. It was noted that the initiation of ductile crack growth along a three-dimensional crack front appears to be independent of the degree of local constraint. However, the increase in toughness for a growing crack was markedly affected by the degree of local constraint. Some estimates of the constraints effects regarding stability considerations were also made.

Probabilistic cell modelling of cleavage fracture

Various aspects on probabilistic modelling of cleavage fracture are discussed. The investigation involves consideration of a unit cell with an explicitly modelled void. The results from this model are compared with results for the case when the void content is accounted for in the sense of a Gurson-Tvergaard law. It is found that explicit modelling of the void can give substantially different results for the fracture probability. The effect depends on the exponent in the assumed Weibull distribution, the threshold stress, the constraint conditions and the hardening of the material.

Evaluation of the influence of residual stresses on ductile fracture

In this work, the significance of residual stresses on ductile fracture is investigated by a set of experiments that are analyzed by finite element simulations. The treatment of residual stresses as expressed in fracture assessment procedures such as R6 is believed to be very conservative for ductile materials, when fracture occurs at high primary loads. Earlier numerical studies have reinforced this belief. This is supported in the current study. Tests on notched 3PB specimens with and without residual stresses were conducted on two ferritic steels. The residual stresses were introduced by applying a compressive preload on notched specimens. The tests were designed to achieve crack initiation at load levels around the plastic limit load. The crack growth in the tests was measured by a compliance method and by color marking of the crack surface. The crack tip driving force J was evaluated numerically for specimens with and without residual stresses. The experimental results show that the residual stresses clearly contribute to J at low primary loads. However, this contribution diminishes as the primary loads increase. The experimental results were also compared with results evaluated using the R6 procedure. These comparisons revealed overly high conservatism in R6 for cases with residual stresses compared to the ones for cases without residual stresses where less conservatism was evident.

An experimental investigation of the transferability of surface crack growth characteristics

The goal of the study was to compare J R -curves from testing on relatively small conventional specimens of a pressure vessel steel with results from large slightly curved surface cracked (SCT) plates subjected to different loading histories. Six SCT experiments were carried out at 20°C and three at 60°C. Unstable crack growth occurred very shortly after crack growth initiation in four of the tests at 20°C, while some amount of stable crack growth was observed in the remaining two and in all of the small specimens. The analysis showed that initiation occurred at roughly the same value of J for all the SCT specimens. For all three SCT experiments carried out at 60°C ductile crack growth was observed. The J R -curves from the SCT tests coincided qualitatively with each other as well as with the results from CT testing. It seems that the J R -philosophy can be used if the temperature is well above the transition temperature. The initiation point is well predicted while some deviations can occur at larger crack growth increments if the loading is mainly of tensile character. This also holds for the initiation event at the lower temperature while for the further growth the situation is less clear.

Evaluation of the Influence of Residual Stresses on Ductile Fracture

In this work the significance of residual stresses for ductile fracture was investigated. The treatment of residual stresses as expressed in fracture assessment procedures such as the R6 method is ...

Micromechanics of Rupture in Combined Tension and Shear

A micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice [1] is developed. The model employed consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The micromechanics model is applied to analyze the rupture mechanisms associated with mixed mode ductile fracture. The stress state is characterized by the stress triaxiality T and the Lode parameter μ, which adequately describe the stress state ahead of a crack tip under mixed mode loading of an isotropic elasto-plastic material. The main focus is the influence of μ on void growth and coalescence behavior. It is shown that the Lode parameter exerts a strong influence upon this behavior.