Hole-Drilling Strain Gauge Method

The initial stress state at any point P(R,a) can be expressed in polar coordinates by the set ofEqs. (7.15a-c)  

On introduction of a hole the stress state changes by relaxation since ar and rrfl must be zero over the hole surface. Then the expression for the stress state in the vicinity of the whole yields the following set of Eqs. (7.16a-c)  

Substitution of Eqs. (7.15 a to c) and (7.16 a to c) into Eq. (7.17 a to c) yields the full expression for the relaxed stress. Assuming linear-elastic behaviour of the material these equations can then be substituted into the biaxial Hooke s law and solved for the relaxed normal strains er and ee at point P(l ,a). The measuring procedure involves the following steps (Vishay Precision Group, 2010)  

1) A special three- (or six-) element strain gauge rosette is installed on the test part at the point where residual stresses are to be determined. 

2) The gauge grids are wired and connected to a multi-channel static strain gauge. 

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Arguably the most widely used modern technique to measure residual stress is the hole-drilling strain-gauge method of stress relaxation. Figure 7.43 shows a thin sample with stress states at a point PfR,a) indicated before (top) and after (bottom) the introduction of a hole. 

Stress Analysis by the Hole-Drilling Strain Gauge Method... 

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