Robust estimation of fracture parameters

Fracture mechanics is mainly concerned with the analysis of fracture-dominant failure. Various methods of analyses for cracks are used in the determination of fracture characterizing parameters, which in turn can be used to predict loads and crack sizes at which failure would occur. -- In engineering practice, robust methods for estimating fracture parameters are useful, given the lack of details of material data, geometric configuration and computational sophistication in an operational industrial plant. This thesis focuses on the approximate methods for the determination of (1) crack tip plastic zone size, (2) the J of circular-ended notches and (3) the J for three-dimensional pressure vessels and piping components. Furthermore, this thesis focuses on design perspectives based on robust methods. -- The assessment of the integrity of structures and components with defects has been shown to be possible using the robust methods developed in this thesis. The robust method for estimating the elastic-plastic energy release rate has been presented which is suitable for the purpose of design. In order to estimate the effect of out-of-plane loading which is parallel to the crack front, the so-called "2½-D" model is proposed in this thesis. This model has been developed to evaluate J for three-dimensional pressure vessels and piping components with defects such as pressure vessels and piping with a circumferential flaw or a longitudinal flaw

Geometry of power flows and convex-relaxed power flows in distribution networks with high penetration of renewables

AbstractRenewable energies are increasingly integrated in electric distribution networks and will cause severe overvoltage issues. Smart grid technologies make it possible to use coordinated control to mitigate the overvoltage issues and the optimal power flow (OPF) method is proven to be efficient in the applications such as curtailment management and reactive power control. Nonconvex nature of the OPF makes it difficult to solve and convex relaxation is a promising method to solve the OPF very efficiently. This paper investigates the geometry of the power flows and the convex-relaxed power flows when high penetration level of renewables is present in the distribution networks. The geometry study helps understand the fundamental nature of the OPF and its convex-relaxed problem, such as the second-order cone programming (SOCP) problem. A case study based on a three-node system is used to illustrate the geometry profile of the feasible sub-injection (injection of nodes excluding the root/substation node) region