Measurement of the fracture toughness of steel is important for the assurance of the safety of ships and offshore structures, especially when these structures are made of thick sections and/or applied in cold environments. One key factor that will affect the determination of the fracture toughness is a pop-in, which is a short event in which unstable fracture is initiated and then self-arrests. If the pop-in is large enough, it will be used to calculate the fracture toughness. Pop-ins are believed to be the products of local brittle zones, which occur randomly at crack tips and have finite sizes. Fracture toughness testing codes have ways of determining whether a pop-in is critical (thus, identifying the maximum force and displacement to be used in the determination of the toughness of the material) or not important (thus, allowing for the test to proceed). In an ongoing project on the use of small-scale fracture specimens to predict standard fracture toughness test results, we would like to know how pop-in acceptance criteria should be scaled for specimen size. It is expected that the physical size of the brittle zones that cause pop-ins is invariant of specimen size, meaning that the contribution of the pop-in will be proportionally more important for smaller specimens. An analytical method for relating the pop-ins on one specimen size to another specimen size is developed. This method is partially verified by observations on the size of a local brittle zone observed on a fracture surface and the effect of that pop-in on the force-displacement curve during a CTOD test. The analytical method showed that an equivalent pop-in for a small-scale specimen is indeed larger, but that the effect was subtle.

In a prior project, TNO has presented a low-cost way of finding fracture toughness of base materials for cleavage fracture. That method features a small-scale CTOD specimen, combined with simplified sensors, less fatigue pre-cracking, faster testing, and no need for a temperature chamber. This method has been extended to welds by considering the effect of pop-ins. This paper summarizes the prior method and the justifications for it before extending it to welded structures by introducing adjustments for pop-ins for small-scale specimens.

The Weibull stress (Beremin, 1983) relates the local first principal stress and plastically strained volume to the probability of fracture. It requires two constants (m, σu) as input, which are generally regarded as material parameters. As the Beremin approach is used in the background to structural analysis rules, the Beremin parameters are now being used in other situations, including engineering analysis. However, the currently accepted way to find the Beremin constants requires twenty tests, which is considered to be unacceptably expensive for industrial application. Less expensive ways of finding the Beremin parameters have been published in the literature, but they have never been compared to the de facto standard. In this paper, the Beremin parameters were found by the de facto standard method and two other ways, and a comparison is made. It was found that the Beremin parameters can be estimated with reasonable accuracy with a method that uses on just two sets (six specimens) of fracture specimens by careful application of the method of Andrieu (2012). Also, a proposal is made for another method which is based only on the Master Curve temperature T0.

The simulation of collisions of ships using FEA has a long history of rules and guidelines to which such simulations should conform. Nevertheless, there are still subjects that lack thorough understanding in literature and standards. With the rise in popularity of stress state dependent failure criteria and the ability to simulate with smaller element sizes, new questions arise. This paper is part of an ongoing sensitivity study of the effect parameters such as element size, choice of failure criterion, geometric placement of the striking vessel and friction have on the dissipated energy in a collision simulation. It is shown that the use of different mesh sizes and different failure criteria results in different failure mechanisms and deformation patterns and have a great effect on dissipated energy. The stress states in which most of the plastic work is performed (and therefore energy is dissipated) are shown to be in the triaxiality range of 0.27–0.35. The energy contributions of the regions above that, towards equi-biaxial tension, and below that, towards pure shear, are very significant on first contact and shape the initial deformation mechanism. Furthermore it is shown that the placement of the incoming V-bow with respect to element boundaries significantly influences the energy absorption. The parameters influencing energy dissipation are numerous and just a very small selection is discussed in this paper. No perfect deterministic approach is presented in this paper, but a base for a probabilistic approach is given.

The local approach to modelling ductile tearing is a useful technique to give insight into fracture mechanics. However, applications of the local approach have been stymied by the high cost of finding the parameters that characterize it because of the number of specimens and expensive post-processing that the testing requires. In this paper, a novel iterative method to extract a failure locus from one Crack Tip Opening Displacement (CTOD) specimen is presented. Material points fail under various different stress states in a CTOD specimen, so many different points on the failure locus can be found through thoughtful post-processing in FEA. A phenomological ductile failure locus is fitted through the stress triaxiality, Lode angle, and plastic strains that cause failure at material points in the CTOD test. Simulating a CTOD test with a different aspect ratio has shown that the failure locus found by this method can be predictive, giving both accurate force versus Crack Mouth Opening Displacement (CMOD) curves and realistic fracture surfaces featuring separate tunnelling and shear lips.