Fracture toughness testing was conducted on 81 SE(B)-specimens extracted from the weld metal of an aged pressurizer weld, of which 42 were deep-cracked and 39 shallow-cracked specimens. The crack tips were positioned in distinct zones in the weld metal, which was achieved by polishing and etching the material to reveal prior-austenite grain boundaries prior to specimen manufacturing. Deep-cracked specimens with crack tips located in the as-welded zone and where dendrites exhibit a low inclination to the pre-crack plane, frequently showed pop-in events during testing. The length of these pop-ins correlated directly with the length of the weld zone in front of the crack tip. Toughness was evaluated both at the pop-in and at final failure, and values were assigned to the corresponding weld zones. The ductile-to-brittle transition temperature was determined separately for each zone, confirming that the as-welded zone with low dendrite inclination is the most critical in the aged weld.

The fracture surfaces of 49 SE(B) toughness tests performed on five different geometries, were carefully investigated by SEM imaging and cross-section analysis. The specimens were extracted from a large multi-pass weld in T-S orientation. The failure characteristics were associated with three distinctly different zones of the weld. Transgranular fracture occurred primarily in the reheated zone and in the as-welded zone with a dendritic microstructure inclined relative to the crack plane. With a dendritic microstructure aligned with the crack plane intergranular fracture occurred. The toughness of the as-welded zone with a low inclination angle, was significantly lower than that obtained in the other two weld zones. Due to the relatively large size of the zones compared to the fracture process zones of the tests, it is appropriate to characterize the failure behavior as large-scale heterogeneity. Weakest-link modeling may be applied locally in each weld zone, giving rise to three different sets of model parameters. A new calibration technique is introduced and used to fit a local weakest-link model to the toughness distribution curves of the individual zones.

A non-local extension of the shear-modified Gurson model for porous plasticity is proposed, which is capable of preventing spurious strain localization and suitable for predicting crack initiation and growth under general loading conditions. In the extended model, the progression of shear failure is separated from flat dimple rupture by the assumption that these failure mechanisms are characterized by a deviatoric and a dilatational material length parameter, respectively. The two length parameters are introduced through a delocalization process based on the non-local integral approach evaluated on the current configuration. A comprehensive numerical investigation is conducted to disclose the influence of these length parameters. For this purpose, five different specimen geometries are considered covering a wide range of stress triaxiality including pure shear. It is observed that separation of the length parameters is essential for predicting crack branching driven by shear localization, e.g., the flat dimple-to-shear failure transition of a crack that approaches and penetrates a free surface.

In this work, we use the size-dependent Monchiet-Bonnet porous plasticity model to study the influence of void size distribution on ductile fracture. The size effect implies that the void growth depends on the material intrinsic length scale in addition to the plastic deformation and stress state of the material, and smaller voids grow more slowly than larger voids. Finite element-based representative volume elements (RVEs) are built where each element is given an initial porosity and initial void size according to the specified void size distribution. The RVEs are loaded plastically to fracture under different stress states to study the influence of the void size distribution on ductility. The results show that heterogeneity can trigger a macroscopic failure mode caused by localized plastic flow. The onset of localized plastic flow is sensitive to the material heterogeneity while the stress-strain response up to the point of localization is not.

This work presents a systematic investigation of the combined effect of hydrogen embrittlement and loss of constraint. The fracture mechanics experiments are performed on an advanced martensitic high strength steel using a single-edge-notch bend specimen, with different crack over height ratio, subjected to electrochemical in-situ hydrogen charging at various loading rates. It is found that the environmentally driven ductile-to-brittle transition region in fracture toughness is obtained for both the high and low constraint specimen configurations. This region is characterized by a change from transgranular dimple rupture to an intergranular mode of fracture. The transition region for the low constraint specimen is shifted towards longer hydrogen exposure times, which is an effect of the reduced hydrostatic stress ahead of the crack front compared to the high constraint specimen. The low constraint specimen exhibits significant plastic straining, which is reflected in a significant decrease in the fracture toughness due to hydrogen assisted transgranular dimple rupture.

We investigate the compressive failure mechanisms in flax fiber composites, a promising eco-friendly alternative to synthetic composite materials, both numerically and experimentally, and explain their low compressive-compared-to-tensile strength, the compressive-to-tensile strength ratio being 0.28 -0.6. We present a novel thermodynamically consistent continuum damage micromechanics model capturing events on the fiber-matrix scale. It describes the microstructure of a unidirectional composite and includes the instantaneous constitutive behavior of matrix and fibers. We show that flax fibers behave as elastic-plastic-damaged solids in compression. Furthermore, we show that fiber damage plays an utmost role in the compressive failure of flax fiber composites - it is a major determinant of the material's compressive stress-strain response. Using X-ray Computed Tomography (XCT) and Scanning Electron Microscopy (SEM), we identify the fiber damage as intra-technical fiber splitting and elementary fiber crushing. Due to micro -structural similarities among natural fibers, the same micro-mechanisms are likely to appear in other bio-based fibers and their composites.

The effect of heterogeneous microstructures on the macroscopic probability of failure is studied by use of weakest-link modeling. Heterogeneity is here associated with a local variation of toughness, where a size scale characteristic of this variation defines a length parameter. The ratio between this length parameter and the size of the active fracture process zone, defined as the heterogeneity ratio, is key to evaluating the impact of a heterogeneous microstructure. Two extremes are identified; small-scale heterogeneity (SSH) and large-scale heterogeneity (LSH). For these cases, it is possible to formulate analytical expressions based on the weakest-link concept, and references are made to existing models in the literature. Typically, heterogeneity along the crack front, where gradients of the mechanical fields are small, falls under the category of SSH. On the other hand, the effect of heterogeneity in a plane perpendicular to the crack front depends strongly on the heterogeneity ratio. Cases that can neither be identified with SSH nor LSH must be addressed with care. How this can be done is discussed, and examples are given for four different microstructure configurations of interest. The investigation is carried out by numerical analysis of a modified boundary layer model. The cumulative probability of failure by cleavage fracture is evaluated in a post-processing step, where two different statistical models are examined; the Beremin model and the Kroon–Faleskog model. Both models render the same conclusion about the alteration of the overall failure probability distributions caused by heterogeneity.

Tension-torsion tests were conducted on two 6000-series aluminium alloys with different area fraction of con-stituent particles. The two alloys, denoted alloy A and B, have previously been characterized and found to have similar matrix material, albeit the three times higher area fraction of constituent particles in alloy B than in alloy A. Single notch tube specimens of the two alloys were subjected to fifteen proportional load paths by varying the ratio of axial force and twisting moment, probing stress states from torsion to plane-strain tension. The overall failure strain in the notch was estimated analytically based on the experimental data, whereas finite element simulations were used to determine the stress and strain fields within the notch region and to estimate the local failure strain. The experiments showed that the increased particle content led to a reduction in the local failure strain of alloy B compared with alloy A that varied from 16% to 60%, depending on the stress state, with an average reduction of 39%. While the overall trend was an increasing failure strain with decreasing stress triaxiality, significant influence of the Lode parameter was observed, and thus the increase was not monotonic. Applying a porous plasticity model, localization analyses were conducted to examine the underlying mechanisms for the complex variation of the failure strain with stress state.

A thermally aged low alloy steel weld metal is investigated in terms of its fracture toughness and microstructural evolution and compared to a reference. The main purpose of the study is to investigate the effects of embrittlement due to thermal ageing on the brittle fracture toughness, and its effects on the influence of loss of crack tip constraint. The comparison of the investigated materials has been made at temperatures that give the same median fracture toughness of the high constraint specimens, ensuring comparability of the low constraint specimens. Ageing appears to enable brittle fracture initiation from grain boundaries besides initiation from second phase particles, making the fracture toughness distribution bimodal. Consequently, this appears to reduce the facture toughness of the low constraint specimens of the aged material as compared to the reference material. The microstructure is investigated at the nano scale using atom probe tomography where nanometer sized Ni-Mn-rich clusters, precipitated during ageing, are found primarily situated on dislocation lines.