Comparison of fatigue data at room temperature obtained by the authors and Hirano

As stated in the introduction, Hirano reported S-N test results at RT for a G40-800/5260 CF/BMI composite material using a small number of specimens (Hirano, 2001). This section will compare his data with those obtained in the present study. He used OH specimens 38.1 mm wide, and with other dimensions such as the hole diameter, stacking sequence, and thickness the same as those in this study. The loading conditions of the fatigue tests were R=0.05 for tension fatigue, R=20 for compression fatigue, and R=-1 for tension-compression fatigue. The combination of R and the number of specimens tested, n, are {R=0.05, n=4(1)}, {R=20, n=3(1)}, and {R=-1, n=5}, where the figure in ( ) is the number of unbroken specimens. Since there are several differences between his and the authors’ investigations, the sets of data are compared in the following way: (1) Net section stress is used. (2) Stress range is used, because R values are different in tension fatigue and compression fatigue tests. (3) Only Hirano’s failure data are used. The results compared are as follows: (1) Both tension fatigue strength and compression fatigue strength in the present study are slightly higher than those given by Hirano; however, the difference is regarded as falling within the scatter band of data in the present study. (2) The tension-compression fatigue strength was equivalent in both investigations. Therefore, (3) it can be considered that the tension – compression fatigue strength was in agreement with the compression fatigue strength expressed by Srange as in the case of the present investigation. The results mentioned above are considered to indicate that the fatigue test results obtained by both investigations are effective and can be used as reference data. Moreover, the use of net section stress and stress range was effective in comparing the fatigue test results obtained for the different specimen width and R.

5. Conclusions

This study determined the static strength of NH (non-hole) and OH (open-hole) specimens and the fatigue strength of OH specimens made of a G40-800/5260 CF/BMI high – temperature polymer composite material with a quasi-isotropic layup at RT (room temperature) and high temperatures, mainly 150°C, where OH static and fatigue strengths were represented by net section stress. Major conclusions are as follows:

1. This study generated systematic test results of the static and fatigue strengths at RT and high temperatures and presented the original data in mathematical tables available for reference data.

2. NHT (non-hole tension) static strength, OHT (open-hole tension) static strength, and OHT fatigue strength (R=0.1) are classified as fiber-dominated properties; however, they have some weak matrix-dependence and are slightly influenced by temperature. Furthermore, due to its slight matrix dependence, OHT fatigue strength is weakly dependent on the number of load cycles.

3. NHC (non-hole compression) static strength, OHC (open-hole compression) static strength, OHC fatigue strength (R=10), and OHTC (open-hole tension-compression) fatigue strength (R=-1) are classified as matrix-dominated properties and are temperature-dependent. Because of the matrix dependence, OHC and OHTC fatigue strengths are fairly dependent on the number of load cycles.

4. The fiber fracture of 45° plies was also observed in the static tensile failure of an NH specimen, though the static tensile strength was considered to be a strongly fiber – dominated property. In the static compressive failure of NH and OH specimens, the rhombus shaped fracture and partial shear failure were observed on the side face. These failure modes were regarded as indicative of matrix-dominated fracturing.

5. An open hole greatly reduced static strength and this effect was larger for compressive strength than for tensile strength. The strength reduction due to the hole either for tension load or compression load was smaller at 150°C than at RT.

6. The static strength ratio (=compressive strength/ tensile strength) was fairly low for both NH and OH specimens, except for the comparatively high ratio for NH static strength at RT. The fatigue strength ratio (=OHC fatigue strength/ OHT fatigue strength) was generally lower than the static strength ratio, and it decreased to 50% at both RT and 150°C.

7. The Srange-N line of compression fatigue (R=10) and the Smax-N line of tension- compression fatigue (R=-1) gradually intersected each other at either RT or 150°C. Especially at RT the two S-N lines broadly overlapped. This fact indicates that the compression stress range mostly controlled fatigue lives in tension-compression fatigue tests at RT and 150°C.

8. The use of net section stress and stress range was effective in comparing the fatigue test results obtained for the different specimen width and R.

9. The static and fatigue strengths showed relatively small reduction from RT to 150°C except for NHC static strength. This is due to the use of a BMI resin for the material tested and demonstrates its practical utility at 150°C. However, structural designers should be careful about strength reduction when a structure has open holes and encounters compression loads in a high temperature environment.