Effect of an open hole on static strength and its temperature dependence

This section discusses the ratio of static OH strength to static NH strength and its temperature dependence, where this ratio is calculated from the mean strength based on the test results in Table2 or Fig. 3. Since the cross section of OH specimens was equal to that of NH specimens and the static strength was represented by the net section stress in this study, the experimental results are directly comparable.

Table 5 shows the ratios of the mean OH strength SUO to the mean NH strength SUN, where SUO is represented by the net section stress, calculated from the data at RT and 150°C.

S

r = UO (3)

‘ notch

SUN

This table indicates that an open hole greatly reduces static strength for both tensile and compressive loading. The effect of stress concentration produced by the open hole is clearly apparent. Moreover, comparison of the reduction in tensile strength and compressive strength shows that compressive strength decreases more than tensile strength does at either RT or 150°C. Furthermore, the strength reduction ratio due to the open hole at RT is larger than that at 150°C. This fact is explained by the relaxation of stress concentration around the open hole due to resin softening at 150°C, as described above.

Comparison

Temp.

rnotch (%)

Tensile strength ratio, OHT/NHT

RT

66

150°C

71

Compressive strength ratio, OHC/NHC

RT

53

150°C

59

Table 5. Effect of an open-hole on static strength represented by the ratio of the mean static strength of OH specimens to that of NH specimens

The following published data obtained at only RT were found. OHT/NHT strength ratios are 57% for T650-35/Radel 8320 CF/thermoplastic resin (Pilato & Michno, 1994), 58% for T800S/3900-2B and 56% for T800h/3900-2 CF/epoxy (JAXA, 2007). OHC/NHC strength ratios are 52% for T800H/F655-2 Cf/bMI (Marais et al., 2001) and 51%-53% for T800S/3900- 2B and T800H/3900-2 CF/epoxy (Nagao et al., 2007). Therefore, the numerical values presented in Table 3 are considered reasonable. The authors discussed the effect of an open hole on the static strength of CFRPs in detail based on theoretical formulae and test results (Shimokawa et al., 2008).