Finite Element Simulation of Ice Axe Pick Impact on a Semi-Rigid Surface

Abstract

The aim of this paper is to investigate, using finite element analysis, a possible cause of fatigue failure due to the pick of an ice axe impacting a semi-rigid surface This simulates the pick striking a thin layer of ice covering rock. The technical ice axes used by climbers for ice wall climbing are subjected to a number of different loading situations in use. These can result in high stresses being generated. The geometry of the ice axe pick consists of a series of teeth along the bottom surface, which bite into the ice to give the mountaineer purchase when climbing. The tooth profile is such that, at the root, they would act as points of high stress concentrations in the presence of tensile stresses. However the stresses at the root of the tooth are generally compressive for most load cases. However, there have been recorded instances where an ice axe pick has failed in use due to fatigue with the crack being initiated at the root of a tooth. The requirement for a fatigue failure is a cyclic load which has at some point a tensile stress present. As part of an initial study a transient dynamic finite element analysis was carried out on the geometry of an ice axe pick striking a semi-rigid surface. The pick was assumed to strike at velocity of 9m/s at an angle to the tip of 20°. It was assumed that the pick was manufactured from steel and both elastic and plastic properties modelled. The impact surface was given the physical properties of granite and was considered to be flat. A two-dimensional analysis was carried out. Only the pick of the axe was modelled, the adze and handle ignored. It was assumed that the pick was rigidly held at the junction with the ice axe handle. To model the impact the surface was given an initial velocity and a mass of 0.7kg, which was the total mass of the ice axe. The analysis revealed that a compressive failure occurred at the tip of the ice axe pick causing the characteristic rounding of the pick tip (blunting). The stress at the root of the teeth nearest to the tip of the pick yielded in compression, which resulted in a tensile residual stress in the unloaded pick. Each successive impact would therefore result in a tensile-compressive stress cycle which is conducive to fatigue failure.