Ratio of compression strength to tension strength in static and fatigue strengths

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.

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.