On notch fracture mechanics

Different stress distributions for an elastic behavior are presented as analytical expressions for an ideal crack, a sharp notch and a blunt notch. The elastic plastic distribution at a blunt notch tip is analyzed. The concept of the notch stress intensity factor is deduced from the definition of the effective stress and the effective distance. The impacts of the notch radius and constraint on the critical notch stress intensity factor are presented. The paper ends with the presentation of the crack driving force Jρ for a notch in the elastic case and the impact of the notch radius on the notch fracture toughness Jρ,c. The notch fracture toughness Jρ,c is a measure of the fracture resistance which increases linearly with the notch radius due to the plastic work in the notch plastic zone. If this notch plastic zone does not invade totally the ligament, the notch fracture toughness Jρ,c is constant. This occurs when the notch radius is less than a critical one and there is no need to use the cracked specimen to measure a lower bound of the fracture resistance.

Blunt Notch Strength of AA2024 3-3/2-0.4 Fibre Metal Laminate Under Biaxial Tensile Loading

Fibre metal laminates are utilized in lightweight structures, such as aircraft fuselages, as fibre metal laminates provide outstanding fatigue and damage tolerance capabilities, together with a reduced weight compared to monolithic metallic structures. One critical feature of fuselage structures is their strength reduction that owes to riveting, i.e., a state-of-the-art joining technique in aircrafts. In the present work, the blunt notch strength of fibre-laminate panels with rivet holes is investigated under service-relevant biaxial loading conditions. To this purpose, cruciform specimens with a five-hole pattern were produced. These specimens were tested under various biaxiality ratios and fibre orientations. All tests were supported by three-dimensional digital image correlation to obtain the deformation field in the gauge area. Moreover, the displacement fields obtained during deformation were used in an elasto-plastic finite element model as boundary conditions to determine the maximum strains in the vicinity of the blunt notch holes and thus extend the application of the experimental results. The asymmetric strain fields obtained by digital image correlation reveal the interaction of the fibres with the blunt notch holes. Finally, it is shown that the biaxial loading conditions do not significantly influence the blunt notch strength.

Study of Fatigue Initiation of Austenitic Stainless Steel in a High Temperature Water Environment and in Air Using Blunt Notch Compact Tension Specimens

Fatigue life assessment procedures for components exposed to high temperature pressurised water environments are typically based on NUREG/CR-6909 or broadly similar codes (e.g. proposed ASME code case N-792). The effects of a high temperature water environment on the fatigue life are accounted for by simply adjusting the fatigue life in ambient temperature air by an environmental factor (FEN). This adjustment assumes that the environment affects both initiation (nucleation) and propagation equally, which is potentially over-conservative. Blunt notch compact tension (CT) specimens (along with direct current potential drop (DCPD) crack detection) have been proposed as a means of determining the “true” fatigue initiation life, enabling the relative impact of the environment on initiation and growth to be characterised and the level of conservatism in the FEN approach assessed. The current work uses a combination of finite element analysis and fatigue testing in both air and water to assess the feasibility of blunt notch CT testing to detect initiation and to quantify the environmental impact. This work indicates significant difficulties with the blunt notch CT test methodology both in terms of quantifying the applicable strain and in terms of detection of the very early stages of initiation which preclude the quantitative application of the technique to study true initiation. Qualitatively, the results suggest that there is still a significant impact of the high temperature water environment on the earliest detectable stages of crack growth in austenitic stainless steels; however the earliest defects detectible by DCPD techniques still involve a significant contribution from short crack growth. Nevertheless, the technique provides a valuable insight into initiation and residual life of components subjected to through wall strain gradients.