We now discuss the ratio of static compressive strength to static tensile strength in NH and OH specimens, and the ratio of compression fatigue strength (R=10) to tension fatigue strength (R=0.1) in OH specimens. That is, this section evaluates the weakness of the material tested for compression loading on the basis of the ratio of compression strength to tension strength.
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The fatigue strength ratio for arbitrary N is defined by
where Scomp min(N) is compression fatigue strength (R=10), and Stens max(N) tension fatigue strength (R=0.1). The static strength ratio is defined for N=1, rCT(1), where Scomp min(1) is the mean compressive strength and Stens max(1) the mean tensile strength. Fatigue strength is calculated by Eq. (2) and the parameters given in Table 4.
Table 7 indicates the calculated strength ratio, rCT (N). The static strength ratio of NH specimens is 81% at RT and 68% at 150°C. As described below, this ratio at RT is considered reasonable; however, it is fairly small at 150°C. This tendency is reflected in the compressive strength reduction presented in Fig. 3. On the other hand, the static strength ratio of OH specimens is 65% at RT and 56% at 150°C. That is, an open hole reduces this ratio further. Moreover, the following published examples obtained at RT were found for the static strength ratio. For NH specimens of CF/epoxy composites, the static NHC/NHT strength ratio was 77% (JAXA, 2007) and 84% (Nagao et al., 2007) for two cases of T800S/3900-2B, and 83% for T800H/3900-2 (JAXA, 2007). For OH specimens, the static OHC/OHT strength ratio was 57% and 66% for T800S/3900-2B and T800H/3900-2 CF/epoxy composites, respectively (JAXA, 2007), and 67% (Shimokawa et al., 1999-a) and 68% (Hirano, 2001) for G40-800/5260 CF/BMI composites. Therefore, the numerical values of the static strength ratio in Table 7 are considered reasonable as compared with other test results.
As shown in Table 7, the fatigue OHC/OHT strength ratio is in the range of 50% to 60% at RT and 150°C. It is lower than that of static strength and has a tendency to decrease with the increase in the number of load cycles. Moreover, there is a tendency for the ratio at 150°C to be slightly lower than that at RT. Since compression strength is expected to be lower than
tension strength because of being a matrix-dominant property, these quantitative values are very important from an engineering perspective. This property should be fully taken into account in the design of composite structures.
Comparison |
Temp. |
Static strength ratio |
Fatigue strength ratio |
|||
N=104 |
N=105 |
N=106 |
N=107 |
|||
NHC/NHT |
RT |
81 % |
– |
– |
– |
– |
150°C |
68 % |
– |
– |
– |
– |
|
OHC/OHT |
RT |
65 % |
60 % |
57 % |
53 % |
50 % |
150°C |
56 % |
54 % |
53 % |
51 % |
50 % |
Table 7. The ratio of compression strength to tension strength, rCT (N), for NH and OH specimens: static strength ratio=mean compressive strength/mean tensile strength, and fatigue strength ratio=compression fatigue strength (R=10)/tension fatigue strength (R=0.1) |