The hardness-flow stress correlation in metallic materials

Hardness is a measure of the resistance of a material to indentation and a wide variety of indentation tests have been devised to measure the hardness of materials. In the case of hardness tests which utilize spherical balls as the indentor, it is also possible to derive flow stress-strain relationships from hardness tests carried out either over a range of loads (static test) or over a range of impact velocities (dynamic test). This paper first describes the experimental procedure for obtaining stress-strain curves from hardness tests. In addition, the paper also analyzes in detail, the indentation test conditions under which the conversion of the hardness-average strain data to flow stress-strain data is simple and straightforward in the sense that the constraint factor which is the correlating parameter for the above conversion is not only independent of strain but also easily computable on the basis of known mechanical property data of the test material

A comprehensive analysis of the static indentation process

The nature of the indentation process in a low alloy steel heat treated to four different hardness levels with a tungsten carbide ball as the indenter has been investigated. In particular, the effects of load and steel hardness on the constraint factor, the size of the plastic zone, the hardness and strain profile within the plastic zone, the pile-up height around the indenter and the extent of the elastic relaxation of the indentation surface have been studied. These results have been rationalized either in terms of the existing static indentation models or in terms of empirical equations suggested by others

The strain-rate sensitivitity of flow stress and strain-hardening rate in metallic materials

The objectives of the present investigation are to characterize the high strain rate (≅ 104s−1) plastic flow behaviour of a number of metals and alloys and also to compare the dynamic flow behaviour with that at static strain rates (≅ 10−2s−1). Such a comparison has provided information on the strain-rate sensitivity of both the flow stress and the strain-hardening rate for a number of pure metals, solid solutions and dispersion-strengthened alloys. On the basis of the present work, qualitative conclusions have been inferred on the effect of strain rate on the resistance offered by various types of thermally activatable obstacles to dislocation motion

The localization of plastic flow under dynamic indentation conditions: II. Analysis of results

In this paper, the data from the companion paper has been analyzed within the framework of localization of plastic flow. First, it has been demonstrated that the decrease in Hd beyond the critical strain (εc) is the result of the onset of localization of plastic flow in the material being indented and that such a localization is triggered by the decrease in flow stress due to the temperature rise in the plastic zone due to the adiabatic nature of deformation. However, it is further shown that the extent of decrease of Hd with increasing strain (I^av) beyond I^avc can be explained only if it is additionally postulated that the plastic zone itself shrinks in size with increasing strain. It is then demonstrated through an indirect analysis of the experimental data, that such a shrinkage of plastic zone does occur. Finally, a composite expression for hardness under dynamic indentation conditions has been derived

On the constraint factor associated with the indentation of work-hardening materials with a spherical ball

It is generally accepted that the constraint factor (CF) associated with the indentation of metallic materials by a much harder indentor is in the range of 2.8 to 3.0. Invariably, the CF is assumed to have a constant value in the above range irrespective of the material indented while correlating the hardness of the material indented with its uniaxial strength properties. The objective of the present investigation is to assess the above assumption by evaluating the CF associated with the indentation process as a function of strain in the case of metallic materials exhibiting a wide range with respect to elastic modulus, strength, and strain-hardening rate. The results indicate that the CF is not really a constant but is dependent on the various material properties. The experimental CF-strain relationship observed in all of the test materials has been rationalized on the basis of elastic-plastic and fully plastic indentation models. While the theoretical models can explain the trend of the data, they are not capable of making adequately accurate quantitative predictions

Evaluation of microhardness correction procedures

The major objective of the present investigation is to develop empirical procedures for correcting microhardness values for the size effect. Such correction procedures can be expected to be useful in the determination of equivalent bulk hardness values in the highly strained regions just beneath worn or eroded surfaces. In this paper two empirical correction procedures were defined and their usefulness was evaluated on three single-phase materials, namely iron, Cu-2Si alloy and 7075 aluminium. It is found that the hardness subtraction method (one of the two correction procedures) predicts most accurately the equivalent bulk hardness values. Finally, the methodology for converting the corrected microhardness-depth profile beneath a Brinell indentation to an equivalent strain-depth profile is illustrated and the validity of such a procedure is demonstrated

A dynamic indentation technique for the characterization of the high strain rate plastic flow behaviour of ductile metals and alloys

The objective of the paper is to describe a dynamic indentation (DI) technique suitable for the determination of the high strain rate flow behaviour of ductile metals and alloys and illustrate its use by characterizing the high strain rate flow behaviour of iron and OFHC copper. The DI technique is first described in detail and the dynamic hardness-strain data of iron and copper obtained using the technique is presented. It is also demonstrated that it is a suitable technique for characterizing the high strain rate flow behaviour as long as certain validity conditions are met. It is shown that these validity conditions are fully met in the case of copper and at low strain levels in iron. The reliability of the DI technique is finally demonstrated by comparing the present data with the literature data on similar materials and finally a critique of the DI technique is provided

The volume of the crater formed by the impact of a ball against flat target materials- the effect of ball hardness and density

The objective of this investigation is to study the effect of projectile (a ball) hardness and density in relation to target material hardness, on the volume of the crater formed by the impact of the target material with the ball over a range of velocities (20-200 m/s). For this purpose, copper (HV48) and steel (HV551) targets, representing the extreme with respect to hardness, have been impacted by a variety of balls having a wide range of hardness and density. The results indicate that the crater volume is determined by the kinetic energy of the ball irrespective of its density and hardness. However, if the hardness of the ball is less than 1.5 times the target material hardness, the ball also undergoes plastic deformation during impact and, a result, the crater volume now becomes a function of the ball hardness also