Energy balance growth

Abstract

The problem of predicting residual strength of sheet metal construction is of considerable importance in damage-tolerant or fail-safe design of aircraft. The phenomenon of stable crack growth prior to fracture instability is an important factor in the determination of residual strength of cracked sheet metal, which generally fractures under plane stress conditions.

The phenomenon of stable crack growth is not yet clearly understood; it is intimately related to plastic energy dissipation at the tip of a crack as it extends in a stable manner under increasing applied stress. This thesis partly provides an insight into the phenomenon of stable crack growth. An approximate analysis is presented to obtain the plastic energy dissipation rate during stable crack growth in a centre-cracked infinite plate subjected to monotonically increasing, remotely applied, equibiaxial and non-equibiaxial loading. The effect of loading path on the plastic energy dissipation rate is highlighted. Using the results of this analysis, an energy balance equation is formulated for stable crack growth with a critical rate of energy flow into a process zone as the criterion for continued crack extension. The energy balance equation has been used to obtain stable crack growth curves for an infinite plate subjected to equibiaxial loads.

Following this, an energy balance formulation of fatigue crack growth under constant amplitude loading is presented, including the effects of crack closure. This formulation results in a relation for fatigue crack growth rate in a form similar to Paris’ empirical relations. Fatigue crack growth has been considered both under plane stress and plane strain conditions, and the growth rate relations obtained have been suitably combined to evolve a growth rate relation for finite-thickness plates. Thus, the growth rate relation obtained accounts for crack closure, fracture mode transition, thickness effects, mixed pop-in fracture, and fatigue crack growth situations. The structure of the energy balance formulation is shown to have the capability to deal effectively with stress corrosion and corrosion-fatigue crack growth.

Thus, a unified treatment of the problem of ductile fracture and fatigue crack growth has been achieved in the thesis.

Finally, an experimental study of the effect of overaging on fatigue crack growth rates in four commercial Al-Zn-Mg and Al-Cu-Mg alloys is presented. Master crack growth rate curves have been obtained using a parameter involving plastic zone size and yield strength for all the alloys investigated.