Multiaxial fatigue testing generates curved cracks that are extremely difficult to characterize using standard
compliance based and potential drop-based methods. Therefore, an automated online system was
developed to monitor the crack tips positions in plate cruciform specimens. The system periodically
evaluates the deformation fields at small areas around the crack tips by performing digital image correlation
(DIC) on images obtained by a moving camera triggered at desired phases of the loading cycle. The
displacement field obtained by DIC is fitted by a simple model that specifies the crack propagation direction
and enables it to iteratively find new crack tip positions. Moreover, the model is capable of computing the
crack opening displacements near the crack tip and thus characterizes the local loading. This approach
enables fully automated multiaxial testing with controlled crack length and crack tip loading. The method
was successfully tested on an AA5754 aluminium alloy sheet..Multiaxial fatigue testing generates curved cracks that are extremely difficult to characterize using standard
compliance based and potential drop-based methods. Therefore, an automated online system was
developed to monitor the crack tips positions in plate cruciform specimens. The system periodically
evaluates the deformation fields at small areas around the crack tips by performing digital
EXTENDED ABSTRACT

The field of fracture mechanics started with Griffith’s energy concept for brittle fracture in 1920. In 1963, Paris et al. used a fracture mechanics’ parameter to introduce an equation for the fatigue crack growth rate in ductile materials and this equation is now commonly known as the ‘Paris law’. However, the Paris law and the semi-empirical models that followed ever since do not fully account for the main intrinsic and extrinsic properties involved with fatigue crack growth in metallic alloys. In contrast, here we introduce a dimensionally correct fatigue crack growth rate equation that is based on the original crack driving force as introduced by Griffith and the presence of plasticity in a metal to withstand crack propagation.
EXTENDED ABSTRACT

The present work is dedicated to a comparative analysis of strain accumulation and damage initiation in a forged Ti-6Al-4V alloy subjected to either fatigue or dwell-fatigue condition. To this end, high-resolution digital image correlation analyses were carried out on fatigue specimens interrupted at different number of cycles to clarify the grain-scale strain activity and correlate it with local micro-mechanical features and crack initiation sites.
EXTENDED ABSTRACT

The major challenge in the mechanics of elastic-plastic fatigue crack growth (FCG) is to find a physically based driving force to correlate the crack growth rates under stress-controlled and strain-controlled conditions. Specifically, a parameter capable of providing a single-valued correlation of crack growth rate, regardless of applied fatigue stress/strain values, is needed. Approaches of the past used either cyclic strain (strain intensity factor) or nonlinear fracture mechanics based (cyclic J-integral, ∆J) parameter, to correlate fatigue crack growth. The latter, however, requires experimental load-deflection curve after every crack length increment and geometry correction factors, which are complex. In the present work, it is shown that a new and physically based approach, based on the cumulative change in the cyclic strain energy of the net-section, is used to successfully correlate fatigue crack growth in a variety of loading elastic-plastic loading situations. The change in the cyclic strain energy is determined analytically from tensile elastic-plastic behavior of material and from the relative sizes of cracked and uncracked sections in the crack plane. Remarkably, excellent correlations of fatigue crack growth data in a variety of specimen geometries and stress/strain levels have been found for both stress- and strain-controlled fatigue conditions.
EXTENDED ABSTRACT