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In this book, leading scientists share their vision on the Kolsky-Hopkinson bar technique, which is a well-established experimental technique widely used to characterize materials and structures under dynamic, impact and explosion loads. Indeed, the Kolsky-Hopkinson bar machine is not a simple experimental device. It is rather a philosophical approach to solve the problem of measuring impact events. The split Hopkinson pressure bar conventional device is mainly limited to test homogeneous ductile non-soft materials under uni-axial compression. Extending the use of this device to more versatile applications faces several challenges such as controlling the stress state within the specimen and mastering the measurement of forces and velocities at the specimen-bar interfaces and then the material properties. Thus, the topics discussed in this book mainly focused on the loading and processing parts.



Chapter 1. The Origins of the Hopkinson Bar Technique

Although Bertram Hopkinson had a research interest in the dynamic strengths of materials, the bar technique he published in 1914 was developed to roughly determine the shapes of stress pulses produced by bullet impact and explosions. His pressure bar proved useful to the British explosives and armour industry during the Great War allowing them to improve their munitions. It also allowed the phenomenon of spall of metal plates produced by ballistic impact or by explosions to be understood. Another greater war had to occur before Hopkinson’s pressure bar was applied by Taylor, Davies and Volterra in the UK to the measurement of the dynamic compressive properties of explosives and polymers. Unlike after the First World War, why this research was thought important to the British armed forces during the Second World War was not explained in the declassified reports published once the war was over. Soon after the end of the Second World War, Kolsky made a number of improvements to the double Hopkinson bar system including using detonators to substantially increase the stresses that could be applied to specimens. This made possible for the first time the accurate determination of the dynamic compressive stress-strain curves of metals. Detonators are, however, impractical for all but defence labs to obtain and use. So an alternative method of loading was needed, and this was developed by Krafft and co-workers at the Naval Research Laboratory in Washington DC in the early 1950s. Their design used a gun to launch a projectile rod against one end of the double bar system and this American method eventually became standard in all laboratories that use the split Hopkinson pressure bar technique. However, it was not until the end of the 1970s that split Hopkinson bars began to be built in laboratories in an ever increasing number of countries around the world.
Stephen M. Walley

Chapter 2. Tensile Testing Using the Kolsky-Hopkinson Bar Machine

The uniaxial stress-strain curve provides an important piece of information within several fields of engineering. Design of civil engineering and transport-related structures requires knowledge of stiffness properties, yield strength, and possibly also hardening behaviour and fracture characteristics for the material at hand. People working with machining and forming processes have similar demands. Accurate information about the material properties has become even more important during the last decades because the finite element method now is the primary design tool.
Arild H. Clausen

Chapter 3. Shear Testing Using the Kolsky-Hopkinson Bar Machine

The use of uniaxial tension and compression tests to characterise the mechanical behaviour of materials is widespread, also at dynamic strain rates. Indeed, multiple techniques which impose an axial load on a specimen have been developed.
Patricia Verleysen

Chapter 4. Dynamic Brazilian Test Using the Kolsky-Hopkinson Bar Machine

Brittle materials, such as rock, concrete and ceramic, break without significant deformation when subjected to static or dynamic load. The tensile strength, which is much smaller than the compressive strength, is a key mechanical parameter for the brittle materials. The tensile fracture is an important failure mode. Due to its brittleness, a direct uniaxial tension test using a conventional dumbbell sample is difficult to be conducted for a brittle material, and this may become even more difficult in dynamic conditions.
Pengwan Chen, Baoqiao Guo, Jingjing Chen

Chapter 5. Comparative Study of the Dynamic Fracture Toughness Determination of Brittle Materials Using the Kolsky-Hopkinson Bar Machine

Due to its high strength and hardness, high temperature resistance and low cost, alumina ceramics are widely used in a large range of applications such as armor systems, aerospace industry. In these cases, the ceramic materials inevitably subjected to dynamic loading, its dynamic mechanical properties consequently become the criterion. Fracture toughness is the key parameter in fracture mechanics, which defines a material’s resistance to crack propagation for plain strain loading. Measuring this parameter requires knowledge of the specimen geometry and a preset crack within the material. Metals are ductile materials and have traditionally used pre-notch methods to grow a natural crack starter. However, it’s difficult to machine a three-point bending (TPB) specimen for brittle materials because of low tensile strength.
Pengwan Chen, Baoqiao Guo, Jingjing Chen

Chapter 6. Wave Dispersion in Kolsky-Hopkinson Bar Machine

This chapter is interested in the wave dispersion effects in Kolsky-Hopkinson bar machine. First, Section 6.2 introduces how to assess wave dispersion parameters in three-dimensional elastic and viscoelastic bars using analytical analysis. Subsequently, Sect. 6.3 introduces experimental techniques used to measure the wave dispersion.
Ramzi Othman

Chapter 7. Wave Separation Techniques

Wave separation techniques are methods that use mathematical tools to process at least two measurements in order to separate the effects of two or more different types of waves. They are first introduced to the Kolsky/Hopkinson bar machine by Lundberg and Henchoz (Exp Mech 17:213–217, 1977 [1]) and Yanagihara (Bull Jpn Soc Mech Eng 21:1085–1087, 1978 [2]). The use of wave separation techniques overcomes the limitation of the test duration caused by the waves’ overlap in strain gauge cross-sections. The wave separation techniques are either based on the time expressions of waves or their frequency expressions. First, we will discuss the wave separation techniques that are based on the wave solutions that are written in the time domain. Later, we will discuss the wave separation techniques which are written in the frequency domain.
Ramzi Othman

Chapter 8. Inertial and Frictional Effects in Dynamic Compression Testing

An inevitable consequence of testing materials at high strain rates is rapid acceleration of at least part of a specimen. As well as inducing waves that propagate in the specimen, at least during the initial stages of loading, this rapid acceleration gives rise to inertial stresses that affect the force measurements at the interfaces between the specimen and the input and output Hopkinson bars. In this chapter, we will be mainly concerned with the effects of macro-inertia on the observed specimen response. Inertial effects due to any internal structure of, for example, a foam will not be considered. Another cause of departure from simple compression (upsetting) is friction between the faces of the specimen and the ends of the bars. Friction produces three effects: (i) it changes the state of stress from uniaxial to triaxial; (ii) the specimen bulges (barrelling); (iii) deformation localizes producing (in cross-section) an X-shaped shear band. Various lubricants and lubrication techniques have been developed over the years, but none have been found to reduce friction to zero for metals. However for polymers, hydrocarbon greases have been found to eliminate friction up to true strains of around 0.4. The ring and aspect ratio tests are discussed as two methods of quantifying friction in compression. Lubricants suitable for high and low temperatures are also discussed.
Clive R. Siviour, Stephen M. Walley

Chapter 9. Very High Strain Rate Range

The classical Split Hopkinson Pressure Bar (SHPB) system is considered to be able to perform tests at strain rates ranging from 102 to 104 s−1 (Zhao and Gary, Mater Sci Eng A 207:46–50, [1]). However, some modifications can be carried out to extend the reachable strain rate within the specimen. The mean strain rate defined within the specimen
Thomas Heuzé, Xiaoli Guo, Ramzi Othman

Chapter 10. Testing of Adhesively Bonded Joints by Split Hopkinson Bar Technique

Application of adhesively bonded joints has recently been expanding in many fields such as automotive and airspace industries, in which the products may be subjected to impact loading.
Chiaki Sato
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