Issue 38

T. Sawada et alii, Frattura ed Integrità Strutturale, 38 (2016) 92-98; DOI: 10.3221/IGF-ESIS.38.12 94 F D 1 2 4     (1) T D 12 3 16     (2) Axial-tension stress σ 1 was loaded in proportion to torsion stress τ 12 in accordance with a combined stress ratio α , which is defined in Tab. 2, on the gauge section surface. Static tests were controlled by Eq. 3 so as to achieve the stress loading speed (   ,   ) of 10 MPa/sec. Fatigue tests were conducted at the stress ratio R defined as the ratio of minimum stress to maximum stress of 0.1 and applied sinusoidal waves combined between axial-tension stress and torsion stress with cyclic frequency f of 2 Hz. Axial-tension stress was loaded to the specimen so as to be in the same phase with torsion stress in accordance with defined the combined stress ratio α in Tab. 2. τ 12 σ 1 α = τ 12 / σ 1 1 0 1/0 (pure torsion) 1 1 1/1 1 2 1/2 0 1 0/1 (pure tension) Table 2 : Definitions of combined stress ratio α . 2 2 10 [MPa / sec]       (3) T EST RESULTS AND DISCUSSION Multiaxial static strength evaluation xternal load acting on a structure is commonly considered to be not only a uni-axial stress but also a multiaxial stress state. Therefore, we discuss a multiaxial strength criterion in terms of the axial tension strength and the torsional shear strength in this chapter. From the micro observation and frequency analysis for fibre orientation in C-SGP and I-SGP, we deduced that they will have an orthotropic property. Then, we assumed the SGP multiaxial failure was followed by Tsai-Hill failure criterion given by Eq. 4 [8]. L S 2 2 1 12 2 2 1       (4) where σ L and τ S are tension and torsional strength in material reference axes, and σ 1 , and τ 12 are applied axial stress and torsional stress on the specimen surfaces. Suffixes 1 and 12, which are written in subscript to the right of strength parameters σ and τ , indicate the axial-stress direction and the torsional-stress direction, respectively. σ L and τ S are the material strengths determined by the static test on the basis of the experimental results in α = 0 / 1 and α = 1 / 0 in Tab. 2. Fig. 2 shows the static strength criterion for SGP in the multiaxial stress state, which is plotted on σ L - τ S plane. The white and gray plots on each chart indicate the strength data of C-SGP and I-SGP, respectively. The solid lines on the σ - τ plane represent the failure criterion given by Eq. 4 for each V f . Broken lines represent α given in Tab. 2. As C-SGP is molded by pressuring and heating after raw materials described at Tab. 1 are filled into the die, a microscopic void tends to be easily retained as a manufacturing defect in C-SGP. Furthermore, the authors [9] previously suggested that the strength properties of SGP depend on both short-fibres orientation and manufacturing defects in matrices. From these perspectives, we deduced that these manufacturing defects affected experiment variations in SGP. Considering such manufacturing defects, the static fracture strength of C-SGP approximately accords with the Tsai-Hill failure theory criteria in Eq. 4. E