Detonation loading of tubes in the modified shear wave speed regime
We performed experiments and numerical simulations on a thin-wall aluminum tube with internal gaseous detonation loading in the modified shear wave speed regime. Strain gauges were used to determine the time-dependent hoop strain on the surface of the tube. The experimental results were compared with analytical models and numerical simulations with explicit finite element computation treating the detonation as a traveling load. Hoop strain amplification factors (defined as the ratio of the dynamic strain to the equivalent static strain) close to two were obtained both in the present experiments and simulations. No resonance phenomena in the hoop strain were found in either experiments or simulations. Computations reveal the existence of a resonance in the shear strain.
Detonation induced strain in tubes
When a detonation wave propagates through a piping system, it acts as a traveling pressure load to the pipe wall. The detonation wave must be followed by an expansion wave in order to bring the combustion products to zero velocity at the ignition end. When it reaches a closed end-wall, a reflected shock is formed which propagates back into the tube with a decaying pressure. The present study aims to develop predictive models for the stresses and strains produced in such a situation. To this end, two series of experiments are discussed. The first series used strain gauges and a laser vibrometer to measure the elastic response of the tube to the incident detonation in thin aluminum tubes. The second series used strain gauges and high speed video to measure the plastic response of steel tubes to incident detonations and reflected shocks. In these experiments a novel mode of plastic deformation was discovered in which the residual plastic deformation in the tube wall had a periodic sinusoidal pattern. A semi-empirical model of the pressure history was developed for use as a boundary condition in models of the mechanical response of the tube. This model was tested against experiment, and it was found that the pressure and arrival time could not be simultaneously predicted from the simple model. This and the general form of the pressure traces in the experiment seem to suggest an interaction between the reflected shock and the boundary layer behind the detonation resulting in a possible bifurcation in the reflected shock wave. With these considerations in mind, the model was applied to single degree of freedom and finite element models of the tube wall. The ripples observed in the experiment were present in the 1-D single degree of freedom models, indicating that they are a result of the interaction of the reflected shock wave with the elastic oscillations set in motion by the detonation wave. Strain-rate hardening was found to be an important consideration under detonation loading conditions. With proper consideration of rate hardening, a single material model may be used to arrive at reasonable predictions the plastic strains resulting from detonations and reflections at initial pressures of 2 and 3 bar initial pressures.
Strain Concentrations in Pressurized Dented Pipes
Knowledge of the strain distribution in a dent is crucial in assessing the severity of damage to pipes. Accordingly this paper presents the results of a series of tests to measure strains and displacements in previously dented aluminium pipes subject to increasing internal pressure. Indentation of unpressurized and pressurized pipes was caused by a rigid sphere and resulted in short smooth dents of depths up to 16 per cent of pipe diameter. Strain changes on the external pipe surface were monitored by strain gauges and a photoelastic coating. The results identify two stationary crescent-shaped regions of high external strain at the axial extremities of the initial dent, where strain concentration factors of 10 were recorded in the early (elastic) stages of pressurization, increasing to a maximum of 28 in one particular dent. The results have immediate application to the assessment of a gouge within a dent, which has long been recognized as the most severe form of mechanical damage to pipelines and can result in failures at very low internal pressures. A parallel series of burst tests by the authors revealed that gouges located near the axial extremities of the initial dent may cause premature failure. The strain concentration results presented here help to explain this behaviour, and indicate the size, shape and location of the regions of high external strain.