186 Testing techniques
120.0, r -1.80.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5The curves associated with these results are shown
above. The strength parameters are read from these
curves, and have the following values:
(i) yield strength ay 83 MPa (this is where the
axial stress-axial strain curve becomes visibly
non-linear);
(ii) peak strength a,,, % 97 MPa (this is the max-
imum axial stress the specimen sustained);
(iii) residual strength a, % 49 MPa (this is the final
stress value given, which may have reduced
further had the test been continued);
(iv) tangent Young’s modulus Et, at 50% peak
axial stress is computed from the axial stress
and axial strain values immediately above and
below 50% of the peak axial stress. The value is
given by
58.70 - 44.80 13.9
1.171 - 0.893 0.278
=-- - 50.00 GPa.
Axial Axial Diametral
stress strain strain
0.00 0.000 0.000
10.05 0.397 -0.020
20.01 0.694 -0.060
32.03 0.992 -0.100
44.79 1.289 -0.159
58.67 1.587 -0.239
73.05 1.884 -0.319
81.97 2.083 -0.438
93.45 2.578 -0.717
96.13 2.777 -0.916
96.80 2.975 -1.056
90.85 3.173 -1.155
69.32 3.471 -1.295
58.34 3.768 -1.414
51.34 4.066 -1.534
49.34 4.363 -1.653
48.84 4.661 -1.653In practice, we would compute this value from
several ranges of values to estimate any vari-
ation.
(v) tangent Poisson’s ratio urn at 50% peak axial stress is similarly
computed from the diametral and axial strain values immediately
above and below 50% of the peak axial stress. It is given by
0.159 - 0.100 0.059
1 .I71 - 0.893 0.278
- 0.21.
QI 1.8 Explain why the introduction of servo-controlled testing ma-
chines in the early 1970s revolutionized rock mechanics laboratory
testing, and why we are now able to test rocks under virtually any
loading conditions.
A17.8 The principle of a servo-controlled testing machine is that a
feedback signal representing some experimental value is continuously