Residual Stress Factors

Sign of the Residual Stress. It has frequently been stated that a compressive state of residual stress enhances the fatigue behavior of a finished component. Crack initiation and propagation in ground components of aerospace or automotive engines are indeed slowed down tremendously when negative (compressive) residual stress values are present. In a few instances zero residual stress is the desired goal, as in the case of read/write heads of hard disks, because all residual stresses lead to magnetically inactive layers.

Magnitude and Direction of the Residual Stress. The loading conditions to which the finished part will be submitted are an important consideration. The total applied stress (mechanical, thermal, etc.) plus the residual stress determine the mechanical behavior of a finished part, such as its fatigue resistance. Quantitative analysis of residual stress with both destructive techniques (e.g., hole drilling, deflection technique, indentation, etching) and nondestructive techniques (e.g., x-ray or neutron diffraction, Eddy current, Barkhausen noise analysis) are being used more often to help characterize the magnitude and direction (Ref 21). Applications have been reported in gear grinding assessment (Ref 22), bearings manufacturing (Ref 23), camshaft grinding monitoring (Ref 24), rotor shaft examination (Ref 25), welding (Ref 26), and even production of rail components for the French fast speed TGV train (Ref 27).

Residual Stress Field. When considering the type of residual stress field present in the part, it is important to know about the specific stress tensor component concerned. Many studies focus only on the principle stress component, ¿7",,. when the shear component, (7b, should also be considered. Indeed, many experimental techniques assume a perfectly biaxial stress when the various characteristics of the finishing method being considered can lead to a highly triaxial residual stress field. X-ray or neutron diffraction techniques can be valuable nondestructive tools in quantifying such biaxial and/or triaxial stress fields (Ref 28).

Stress Below the Surface. X-ray techniques emphasize surface measurement of residual stress, because x-ray penetration depths are typically less than 20 /Jm in steels. However, the critical areas often appear much deeper in the part (150 to 200 /'m being typical), emphasizing the need for careful plotting of residual stress versus depth, possibly with the help of electropolishing techniques.

The four residual stress factors are summarized in Table 1. Examples of residual stress introduced by grinding, milling, turning, planing, shaping, electrical discharge machining (Ref 29), abrasive tumbling, rolling, and shot peening can be found in Ref 2 and 19, as well as in the very good survey paper by Brinksmeier et al. (Ref 30).

Table 1 Parameters that characterize a state of residual stress in a finished component

Table 1 Parameters that characterize a state of residual stress in a finished component

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