performance between the 20 Mrad-37 % material (Fig. 12.4) and the 5 Mrad-0 % material (Fig. 12.6). Figures 12.5 and 12.6 show the absolute peak load degradation with load cycling, and also the normalized drop from the first cycle at each condition. Both materials appear to effectively shake down up to 67 % strain, though longer tests are really required to clarify the extent to which a stable state really is reached (see Fig. 12.4). However, there are clear differences in the responses. There is apparently less plastic degradation at intermediate cyclic strains in the 20 Mrad-37 % material, which could be an effect of irradiation or precompression set. Note that, at 75 % strain, the 20 Mrad-37 % strain material undergoes a sudden, discontinuous degradation at cycle 15, while at the same conditions the 5 Mrad 0 % are either stable or may start to degrade weakly around cycle 20. At 87 % strain the 5 Mrad-0 % material appears to undergo a similar sudden degradation, while the response for the 20 Mrad-37 % material is likely progressive damage (decreasing slope) with some small-step discontinuous effects past 20 cycles. The interpretation from this data is that the 20 Mrad-37 % strain precompressed material undergoes apparently severe damage at a less severe cyclic compression than the 5 Mrad-0 % material. Deleterious changes to the onset conditions for damage and failure of foams due to environment and storage strain history are the target observable of this work, and maybe be framed here sufficiently well to motivate their further study. It is interesting to observe that the normalized degradation plots show that, above 50 %, the nonlinear degradation appears to behave very similarly with little sensitivity to increasing compressions. To be sure, there are differences between 50–70 % and 75–87 %, but these appear be relatively constant within those groupings, and less different outside of them than one might expect. This apparent normalized insensitivity may be due to the process being essentially driven by viscoplasticity, in which plastic strain accumulation will increase with increasing applied load. Thus, in absolute terms the nonlinear degradation is indeed quite different between groups, but when normalized this difference largely disappears. Future work on the viscoplastic deformation of both solid RTV and these foams will likely reveal a flow law that matches these observations. Peak Cyclic Load (N) # Cycles # Cycles Normalized Peak Load 0 1 6 11 16 21 26 31 1 6 11 16 21 26 31 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 50% 62% 67% 75% 87% 50% 62% 67% 75% 87% 0.4 0.5 0.6 0.7 0.8 0.9 1 Fig. 12.6 5 Mrad, 0 % strain irradiation/precompression. Here the response at 75 % is stable and may start to degrade slightly at cycle 20, but severe degradation does not set in until 20 cycles at 87 % strain. The lowest strain test that experienced sudden degradation (87 %) is highlighted # Cycles 50% 62% 67% 75% 87% 50% 62% 67% 75% 87% Normalized Peak Load 0 1 6 11 16 21 26 31 # Cycles 1 6 11 16 21 26 31 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Peak Cyclic Load (N) 0.4 0.5 0.6 0.7 0.8 0.9 1 Fig. 12.5 20 Mrad, 37 % strain irradiation/precompression. Note the discontinuous degradation of the peak load at 75 % cyclic strain at cycle 15. The lowest strain test that experienced sudden degradation (75 %) is highlighted 12 Damage of Rubber Foams During Large Cyclic Compression 105
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