J. C. F. Millett

Equation of state and mechanical response of NiTi during one-dimensional shock loading

The equation of state and the mechanical response (dynamic tensile strength and dynamic shear strength) of the shape memory alloy NiTi have been investigated using plate impact. The Hugoniot has been extended with additional data and a nonlinear behavior of the Hugoniot (shock velocity-particle velocity) has been noted. A bilinear representation has been proposed for the trend. These two behaviors were attributed to the shock-induced phase transformation from B2 to monoclinic. However, this phase transformation seems to have no influence on the dynamic tensile strength. A minimum impact stress value was found necessary to create the spallation in NiTi but the pull back stress remains near constant above this value. A negative strain-rate dependency was also noted on the spallation. The shear strength (τ) of NiTi appears to increase continuously with the impact stress. The evolution of τ behind the shock front seems to be linked to the phase transformation observed in determination of the Hugoniot. Indeed,...

Delayed Failure in a Shock Loaded Alumina

Manganin stress gauges have been used to measure the lateral stress in a shock‐loaded alumina. In combination with known longitudinal stresses, these have been used to determine the shear strength of this material, behind the shock front. The two‐step nature of the lateral stress traces shows a slow moving front behind the main shock, behind which shear strength undergoes a significant decrease. Results also show that this front decreases markedly in velocity as the HEL is crossed, suggesting that limited plasticity occurs during inelastic deformation. Finally, comparison of measured shear strengths with other aluminas shows a high degree of agreement.

Shear Stress in Nickel and Ni‐60Co under One‐Dimensional Shock Loading

The dynamic response of pure nickel (Ni), and its alloy, Ni‐60Co (by weight %), has been investigated during one‐dimensional shock loading. Few materials’ properties are different and the only significantly altered feature is the reduced stacking fault energy (SFE) for the Ni‐60Co. This paper considers the effect of this reduced SFE on the shear strength. Data (in terms of shock stress, particle velocity and shock velocity) are also presented. The influence on the shear stress, τ of cobalt additions in nickel are then investigated and presented. Results indicate that the lateral stress is increasing in both materials with the increasing impact stress. The shear stress was found to be higher in the nickel than in the Ni‐60Co. The progressive decrease of the lateral stress noted during loading indicates a complex mechanism of deformation behind the shock front.

Longitudinal and Lateral Stress Measurements in NiTi under One‐Dimensional Shock Loading

This paper investigates the influence of the impact stress on the magnitude of the shear stress under one‐dimensional shock loading. The shear stress is calculated from the measured longitudinal and the lateral stresses. New data in terms of shock stress, particle velocity and shock velocity has been gathered. Results indicate that the lateral stress has a positive dependence on the impact stress. A general decrease of the lateral stress was also observed immediately after the impact, while the longitudinal stress remains constant for the duration of the pulse length. This suggests that the shear strength increases behind the shock front. This decrease had been found to reach a constant value for the specimens impacted at lower stress. A complex mechanism of deformation behind the shock front during loading was thus reveals. This limit, related to the inflexion point noted on the Hugoniot (Us‐up), seems to be an effect of the martensitic phase transformation undergoes by the material.

High‐Rate Compaction of Aluminium Alloy Foams

The response of aluminium foams to impact can be categorised according to the impact velocity. Tests have been carried out at a range of impact velocities from quasi‐static to velocities approaching the speed of sound in the foam. Various experimental arrangements have been employed including pneumatic launcher tests and plate impact experimants at velocities greater than 1000 m s−1. The quasi‐static compression behaviour was approximately elastic, perfectly‐plastic, locking. For static and dynamic compression at low impact velocities the deformation pattern was through the cumulative multiplication of discrete, non‐contiguous crush bands. Selected impact tests are presented here for which the impact velocity is less than the velocity of sound, but above a certain critical impact velocity so that the plastic compression occurs in a shock‐like manner and the specimens deform by progressive cell crushing. Laboratory X‐ray microtomography has been employed to acquire tomographic datasets of aluminium foams b...

The Shock Response, Simulation and Microstructural Determination of an Inert Simulant

Assessing microstructural details in a polymer matrix composite is important in addressing safety issues in energetic materials. The generation of three‐dimensional microstructure, using a non‐invasive method of high resolution will advance knowledge in a range of fields. An inert composite analogous to plastic bonded explosives (PBXs) has been studied, and both X‐ray microtomography for microstructural investigation in 3‐D and a parallel series of shock experiments (with associated modelling) have been conducted. The experimental aims of this study lay in several areas. Firstly, to adequately define the bulk morphology, secondly, to determine the geometry of defects that might lead to sites for accidental ignition within the material and finally, to demonstrate a direct linkage into the finite element prediction of mechanical response. This work is the first step in providing a coordinated capability to understand accidental ignition within insensitive high explosives (IHEs).

One‐Dimensional Shock and Detonation Characterization of Ultrafine Hexanitrostilbene

A series of plate impact experiments was performed, using a single‐stage gas gun, on die‐pressed, high density (92 % theoretical maximum) samples of ultrafine hexanitrostilbene (HNS). This enabled investigation of the inert shock response and subsequent detonation of the material. Shock magnitudes up to ca. 6 GPa were investigated by varying the flyer and target plate materials, and impact velocities. In each case, the shock length was chosen to be longer than the pellet (ca. 3 mm). Shock wave profiles and transit times were diagnosed using embedded miniature (1 mm2) manganin stress gauges placed at the front and rear of the shock assemblies. The results have been interrogated to establish the non‐reactive Hugoniot of the HNS and deduce information on its run‐to‐detonation. Analysis of measured stresses and calculated pressures suggests that pressed HNS possesses little strength behind the shock front. These and other features are discussed and compared with existing data.

On the Unreacted Hugoniots of Three Plastic Bonded Explosives

There is a continuing interest in determining the detonation characteristics of loaded plastic‐bonded explosives (PBXs). The UK licensing agency for explosives, DOSG, wishes to better understand the response of insensitive high explosives. This has required more detailed investigation of the transit of reaction from the unreacted state to products. The starting condition, before application of a kinetic scheme to describe reaction, is thus the unreacted Hugoniot for the material. In this work three PBXs, manufactured by BAE Land Systems, are investigated and modelled. All contain RDX in differing quantities in an HTPB binder. One of them contains aluminium. Two of the materials have the same weight percentage of filler and binder but differ in the grain size distribution entrained. The experimental Hugoniots are presented, and a composite equation of state is derived using an engineering model and shown to describe the measurements well. Further applications of the technique are described and future uses ...

Spallation in NiTi under One-Dimensional Shock Loading

The dynamic response of the shape memory alloy NiTi has been of interest to a number of investigators because it displays a shape memory effect. The dynamic tensile (spall) strength of this material is measured under one‐dimensional shock loading. The loading stress pulse length and impact stress were varied to a peak stress of 15 GPa. The pull back stress (σpbs) was found to increase with the applied pulse length. This suggests that the dynamic tensile strength is dependent upon the generation of a deformation micro structure that evolves behind the shock front. In contrast, increasing stress levels result in a near‐constant pull back stress, although at the lowest applied stress, spallation did not occur.