Technical Paper by J.H. Greenwood - IGS - International ...

DISCUSSION AND CLOSURE D Designing to Residual Strength Instead of Stress-Rupture
T R = 1 − T S t ∕ 10
(7)
The result is a group of curves which are plotted against log time, t in Figure 9. The
residual strengths at higher applied loads fall more rapidly than those at lower loads.
Each test ends with rupture when T S =T R ,or:
T R = 1 ∕ (1 + t ∕ 10)
(8)
This is the stress-rupture graph and is plotted as a thick line in Figure 9.
Noting the lack of data, it is now possible to quote the following results. Fifteen specimens
of various polyester yarns had been placed under a constant load at 50% of their
respective tensile strengths for a period of 55700 hours (6.4 years; log t = 4.75) when the
tests were interrupted in August 1997 and the tensile strengths measured (the question
of the duration of short-term overloading was not considered). The creep strain values
ranged between 5.6 and 7.2%. The mean log t value for the same yarns under a sustained
load of 60% of the tensile strength at 20_C was measured as 3.84 (t = 6950 hours).
Greenwood and Yeo (1996) presented the stress-rupture graph of Fortrac geogrid,
which is made from similar yarns, and showed the gradient of the applied load plotted
against log t to be -3.83% of the tensile strength. The sustained load corresponding to
55,700 hours is thus 60 - 3.83×(4.75 - 3.84) = 56.5%; thus, the yarns were exposed at
50÷56.5×100 = 88% of the load that would have led to rupture at the end of the test.
The residual strengths of the yarns are given in Table 1. The test results in Table 1 indicate
that the strength is effectively unchanged under these conditions.
1
0.9
0.8
Percent Tensile tensile strength strength ratio
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
residual strength for applied load 0.1
residual strength for applied load 0.2
residual strength for applied load 0.3
residual strength for applied load 0.4
residual strength for applied load 0.5
residual strength for applied load 0.6
residual strength for applied load 0.7
residual strength for applied load 0.8
residual strength for applied load 0.9
stress-rupture
0.1 1 10 100
Time (hours)
Figure 9. Residual strength and stress-rupture plotted on a logarithmic scale for a
reduction in strength that depends linearly on time and sustained load.
676 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NO. 6