Data Reduction and Interpretation (Blind Hole): Residual Stress Determination

Data Reduction and Interpretation

Blind Hole
As recommended in ASTM E837, it is always preferable to drill the hole in small increments of depth, recording the observed strains and measured hole depth at each increment. This is done to obtain data for judging whether the residual stress is essentially uniform with depth, thus validating the use of the standard full-depth coefficients

and

for calculating the stress magnitudes. If incremental measurements are not taken, there is no means for making references about stress uniformity, and the calculated residual stress may be considerably in error. In such cases, when the stress varies with depth, it should be realized that the calculated stress is always lower than the actual maximum.

There is currently no absolute criterion for verifying stress uniformity from the surface of the test piece to the bottom of a full-depth hole. However, the incremental data, consisting of relieved strain versus hole depth, can be used in two different ways to aid in detecting a nonuniform stress distribution. The first of these is to calculate, for each depth increment, the sums and differences of the measured strain data,

and

respectively (Ref. 1 ). Express each set of data as fractions of their values when the hole depth equals 0.4 times the mean diameter of the strain gage circle. Plot these percent strains versus normalized hole depth. These graphs should yield data points very close to the curves shown below.

Percent strain versus normalized hole depth for uniform stress with depth (Ref. 1 ).

Data points which are removed from the curves shown above indicate either substantial stress nonuniformity or strain measurement errors. In either case, the measured data are not acceptable for residual stress calculations using the full-depth coefficients

and

.

When a principal residual stress direction is closer to the axial direction of gage no. 2 in the strain gage rosette arrangement than to either gage nos. 1 or 3, the strain sum

, will be numerically larger than

. In such a case, the percent strain data check should be done using

instead of

.

NOTE : This graphical test is not a sensitive indicator of stress field uniformity. Specimens with significantly nonuniform stress fields can yield percentage-relieved-strain curves substantially similar to those shown above. The main purpose of the test is to identify grossly nonuniform stress fields. Further, the graphical comparison test using or becomes ineffective when the residual stress field approaches equal biaxial tension or compression ( ) as expected in surface blasting and heat treating procedures. Comparison to the plot is ineffective when (pure shear); however, this condition is relatively uncommon in the practical industrial setting.

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