The Significance of Tensile and Other Mechanical Test Properties of Metals

Abstract

Critical consideration of the conventional quantities obtained from the tensile test reveals their limitations for design purposes, especially where notch-fatigue effects predominate. Against such notch-fatigue the importance of high work-hardening capacity in a metal is emphasized, and its relation to tensile elongation behaviour outlined. It is suggested that even conventional tensile records may yield some rough measure of work-hardening capacity prior to cracking by means of quantities which have here been called “plasticity ratio” and “plasticity value”. Attention is drawn to certain refined methods of plotting tensile diagrams. A review of published results indicates a basic generalization for plastic deformation. If a graph of true stress P against one of the several forms of true strain ∈ be obtained, then the basic deformation diagram is in general the same for tension, compression, torsion, and indentation. It consists first of a curved portion extending up to the point of critical plasticity which corresponds with the onset of “necking” in tension. The curve afterwards flattens out and continues upwards almost as a straight line. The complete diagram may be considered very approximately as of the logarithmic form P = κε^m, and its gradient (or the value of m) will be high in materials with very high strain-hardening capacity, and low in those which have been cold-worked. Since work-hardening capacity has hitherto been largely determined by ball indentation tests, a correlation has been made between these and tensile tests. This has revealed some shortcomings of the Meyer n value for indentation. Heat treatment and crystal grain size influence the plastic properties of a metal and the effects of both have been studied. For specimens of various crystal sizes there is similarity in form between certain tensile and indentation diagrams. From these an appropriate ultimate true stress value may be obtained which is practically independent of crystal grain size and therefore enables comparisons of materials to be made. Similarly there is a fundamental limiting “pressure of fluidity” for metals which is independent of their initial content of cold work. Values for this pressure have been determined. The pressure of fluidity obtained by linear extrapolation of a Stead true-stress tensile diagram is lower than that determined by heavy cold-rolling experiments. The deformation and strain-hardening which take place up to the point of “critical plasticity” (i.e. “necking” in the tensile test) appear to differ in internal crystallographic mechanism from that developing beyond this point. It is found that several physical properties reach limiting values at the “necking” stress. Investigation shows that heat-treated steels with sorbitic microstructures may have a relatively low capacity for strain-hardening. Their employment under conditions of notch-fatigue is therefore not attractive. On the other hand their resistance to wear may be relatively good. Wear remains a complex problem, but examples are given where relatively good wear resistance has corresponded with relatively high “plasticity values”.