EMBEDDED HIGH FREQUENCY SIGNAL EFFECTS ON FAILURE MECHANISMS AND MODELS

Abstract

Embedded high frequency signal effects can have a deleterious effect on the fatigue resistance of structures. For example, ship structures can be subject to many operational loads (wind, pressure, temperature, etc.), one of which is the structural effects from the surrounding sea environment. Typically, the wave environment applies an ordinary wave component, which drives the primary bending stress of the vessel, along with a more stochastically driven element that manifest itself as wave impacts. To account for these effects, designers have relied on simplified assumptions, such as safety factors and/or margins of safety. Existing academic research centered on capturing a simplified sinusoidal response associated with the primary loading event and the embedded high frequency response, but has not addressed logarithmic decay, signal frequency, or frequency of occurrence. All these factors have associated uncertainty and cause impact on fatigue life and failure mechanisms exhibited by structures. This research effort focuses on a more fundamental understanding of the effects of embedded high frequency loading on fatigue crack propagation in Aluminum 5xxx material. Carried out by accounting for the signal’s characteristics, and through an experimental evaluation assess its impact on the local failure mechanism and life cycle models. In particular, the use of Digital Image Correlation to quantify the effects of the embedded high frequency on the plastic zone that develops ahead of the fatigue crack, and the subsequent changes in crack growth. This enabled the following four primary contributions: (1) development of a unique test configuration protocol and process to investigate HF pulse effects on fatigue crack growth in small scale specimens, (2) measured a 35% decrease in COD due to crack closure from the residual stresses associated with a larger plastic zone caused by HF loading, (3) development of a unique model that couples crack kinking and retardation behavior, and (4) elucidation on the effects of sequencing of HF pulses on crack kinking and retardation. The findings of this research can be used in future investigative efforts to develop analytical models that address secondary material effects, such as welds, provide underpinnings for high fidelity numerical modelling, and to reduce the dependency of designers on the use of safety factors and enable them to account more rigorously for failure mechanisms in digital twins.