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40 3.4 Sample B1 100,000 hrs 50 yrs 3.3 100 yrs Log Stress (psi) 3.2 3.1 1124 psi y = -0.0459x + 3.3237 1242 psi R2 = 0.9527 1161 psi 3 REFERENCE TEMPERATURE - 23C 2.9 0 1 2 3 4 5 6 7 8 Log Time (Hrs) Figure 39. Long-term yield strength of Sample B1. temperature steps. The test was continued until the specimen is less than about 1,000 psi, the material will not fail in a duc- yielded. The results from the three loads were analyzed accord- tile manner. The long-term tensile strength is largely governed ing to the standard and master creep rupture curves were by the short-term strength. Therefore, to ensure a 1,000 psi prepared. The rupture point for each load was defined by yield strength after 50 years, the short-term strength should the intersection of two tangent lines drawn before and after be over 3,500 psi. yielding. It would be interesting to compare these results with others The results for Sample B1 (100% VR1) are shown in Fig- obtained through long-term hydrostatic testing, either at room ure 39. Tabulated results for the six candidate formulations temperature or elevated temperatures to verify that the two are shown in Table 16. methods produce similar results. These predicted values assume that the material will stay basically unchanged over the 50 or 100 years of service life- Long-Term Creep Strain and Modulus by SIM time. This, of course, is not true; all materials undergo aging effects during service. However, these are the best models cur- The long-term modulus values on the six final candidate rently available and give a good approximation of the time- resins were also determined in accordance to ASTM D6992. dependent tensile strength. These results basically suggest Initially, the elastic limit of the samples was determined by that the yield strength of the material will be about 30% of the two short-term (15 min) creep experiments. This is impor- initial strength after 50 years. So, as long as the applied stress tant for defining when nonreversible creep actually begins. Table 16. Long-term tensile stress of six formulations. Long Term Tensile Stress (psi) Sample/ 100,000 h 50 years 100 years Formulation B1 100% VR1 1,242 1,161 1,124 A2 85% VR1 + 15% MCR 1,227 1,145 1,108 L5 50% VR1 + 20% MD + 30% MCR 1,184 1,103 1,067 A5 40% VR1 + 30% MD + 30% NAT 1,192 1,105 1,066 B3 20% VR1 + 40% MD + 24% MCR + 16 NAT 1,064 973 934 B5 40% MD + 36% MCR + 24% NAT 1,164 1,082 1,045