Computations and Measurements of the Stress Intensity Factors of Cracked/Notched Plates by the Finite Element and Digital Image Correlation Methods

Abstract

Combined with the theory of linear elastic fracture mechanics, finite element analysis provides a numerically advantageous technique for studying crack initiation and propagation, and fracture in structures and components. Single edge-cracked specimens subjected to mode I loading are typical and popular objectives for fatigue crack growth rate testing. The stress intensity factor (SIF) plays an important role in assessing crack initiation and propagation, and must be determined accurately in order to accurately represent the stress singularity at the crack tip. Frequently, in SIF computations of cracked/notched plate specimens, pinned boundary conditions are assumed, which do not authentically reflect real-world scenarios in engineering fields. However, in many practical applications, clamped boundary conditions are more appropriate. Therefore, the main objective of this work was to compute SIF values for single edge-cracked/notched plate specimens with pinned and clamped boundary conditions. Thus, SIF values for clamped specimens which are rare in the literature would be provided as complementary to the SIF values of pinned specimens which are generally available in the literature. Whilst the use of the SIF values of pinned specimens leads to the conservative (over)design of plate-like structures, which is acceptable for Earth-bound structures, they lead to heavy structures, which are not desirable for aerospace and space applications. In these latter cases, light-weight design is essential; this is achievable by using the SIF values of clamped specimens. The SIFs of single edge-cracked plate specimens were evaluated from the displacement and stress fields predicted by finite element analysis (FEA) using displacement and stress extrapolation methods, node displacement method and J-integral method. The computed SIFs are compared and validated with the SIFs published by other researchers. Subsequently, a parametric analysis was conducted to investigate how the SIF values vary with different parameters of crack length, crack position and plate height for the single edge-cracked specimens subjected to pinned/clamped boundary conditions. Whilst there is an abundance of published SIF values for cracked plates, there is a general lack of published SIFs for slanted cracks and notches, even though there are many components with sharp V-notches. Therefore, the work presented includes computations of SIF values for slanted cracks in plane stress plates, and single slant-edge-notched plates subjected to tension loading. The mixed mode I/II SIFs, for different notch length, notch slant angle and notch initial opening angle, were derived from the strain field computed by FEA using the strain energy approach. Moreover, the Three-Dimensional Digital Image Correlation (3D-DIC) experimental method was used to determine the SIF values for the single edge-cracked/notches specimens subjected to pinned/clamped boundary conditions experimentally. This involved the derivation of the experimental SIF values from the measured displacement fields using the Strain Energy Approach (SEA) and the Finite Element Over-Deterministic (FEOD) method. The experimental SIFs are shown to be in close agreement with the numerical SIFs and, therefore verify and establish the feasibility and accuracy of the experimental 3D-DIC method.

Date of Award	4 Jul 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhenmin Zou (Supervisor) & Sunday Oyadiji (Supervisor)