Some Remarks on Various Concepts of Micro-Cracks

Abstract

In connection with the structural changes taking place during the fatigue test of metals, the various concepts of micro-cracks do not seem to have been made clear, and various innovations of technical terms have been proposed by different investigators to designate these concepts.
Since no systematic studies have been made of the relationships between these terms much confusion has been occasioned in the use of these terms.
In this paper these relationships are argued and a new system of nomenclature is proposed in order to make this matter clear. The concepts and appellations argued here are the surface notch (or top), the fissure, the surface disturbance, the persistent slip band, the intrusion (or extrusion) and the sub-microscopic crack.
The main conclusions from the present research are as follows:
(1) The surface top and notch suggested by Prof. Wood on the metallographic taper section or the approximate taper section of polycrystalline copper specimens are equivalent to the extrusion and the intrusion of small size. Because they cannot be removed by annealing but are connected with the fissure, their occurrence implies the initiation of fatigue damage.
(2) In the views of the authors the fissure, the persistent slip band and the deep intrusion are the phenomena that would also appear whatever different things were tried by the same method of observation, and their origination means the occurrence of the decisive fatigue damage.
Consequently, the authors suggest that they must be micro-cracks.
(3) The block movements observed between the blocks divided by the micro-cracks can be suppressed more or less by silver plating.
(4) The assignment of concepts for origination of sub-microscopic cracks is to be made when the phenomena of hardening are saturated. This assignment of concepts is not applicable to the fatigue of copper polycrystal.