314 J. Ma et al./ Materials Letters 61(2007)312-315 Table 2 Table 3 Tension test data of the composites Bending test data of the composites Specimen Tensile Strength(MPa) Failure strain(%)Initial m Loading Flexural Strain at flexural Strain at fracture direction strength modulus ngth (%) direction 326(35) 06920.07) 153(16) Weft direction 145(16) 0.70500.04 522(48) (11) Standard deviations are given in parentheses. 313 1(4.6)0.780.052)0.790.054 deviations are fracture of weft yams occurred at random positions. When the loads s are subjected to tension along the warp direction, the tensile strength, it was just the reverse. The relatively lower strain at the flexural loads are primarily carried by warp yams, thus the straightening strength of the warp-direction specimen further confirms large stress tendency of warp yarns will cause high values of stresses in yam concentrations in the yarn crossover areas crossover areas. Such stress concentrations can trigger localized damage in warp yarn even when the overall applied tensile stress is 3. 4. Shear tests much lower than the ultimate tensile strength. This can well explain why the fracture of warp yarns mostly occurred in yam crossover areas. In Considering that the thermostructural components will undergo ontrast, when the specimens are subjected to tension weft-wise, the complex loads during service, it is valuable to carry out shear tests, be- tensile loads are primarily carried by weft yams. Obviously, almost no cause shear tests can provide information on the strength and stress concentrations exist in yarn crossover areas, therefore the weft deformation of materials under shear stresses yarns fractured at random positions. Typical in-plane shear stress-strain curves are shown in Fig. 4. It can be seen that the specimens cut along the warp and weft directions 3.3. Bending tests had comparable shear strength (listed in Table 4)and exhibited similar stress-strain trends. All the curves displayed nonlinearity from o The typical flexural strength-displacement curves are shown in Fig. 3, the very beginning due to the processing-induced matrix cracks With me of the test data are summarized in Table 3. It is seen that all the increasing the applied loads the degree of nonlinearity was increased curves exhibited similar trends except for the stress history after peak due to the damage accumulation, including the multiplication of stresses For the specimen cut along the warp direction, the curve showed matrix cracks, the propagation of cracks along the fiber directions and nonlinearity to a displacement value of 1.01 mm to reach the peak stress the failure of some fibers by tension in the transverse-loading (522 MPa). After the peak stress, the stress dropped a little and then directions. At the last stage, the applied loads were maintained due to remained in a large range of displacement until the primary load drop the friction of fiber and fiber bundle pullout until the final failure of occurs at a displacement of about 1.37 mm. The crucial phenomenon is the specimens believed to be attributed to a lockup mechanism involving yam waviness To further investigate the shear properties of the composites, short- and pinching feature of the adjacent warp yams. In the case of the beam shear tests were conducted to determine the interlaminar shear specimen cut along the weft direction, the curve displayed similar strength. The test results are presented in Table 4. The specimen that nonlinearity to a displacement value of 1.25 mm to reach the peak stress was cut along the warp direction had much higher interlaminar shear (313 MPa). However, after the peak stress, the stress descended rapidly in strength than the specimen cut along the weft direction, which a steep manner. It is also noted that the strain at the flexural strength of the attributed to the difference in density between warp and weft yarns. It is men cut along the warp direction was much lower than that of the evident that the composites displayed higher interlaminar shear men cut along the weft direction, while for the strain at fracture properties relative to their 2D counterparts reported in literature [8], warp direction 雪20 weft direction 0.00.20.40.60.81012141.61.8 002040.60.81.012141.61.8 Displacement(mm) Strain (%) and, weft direct isnrengthi-displacement curves tor specimens cut along the warp wilt d inctiane shear stressestrain curves for specimens cut aiong the warp anfracture of weft yarns occurred at random positions. When the specimens are subjected to tension along the warp direction, the tensile loads are primarily carried by warp yarns, thus the straightening tendency of warp yarns will cause high values of stresses in yarn crossover areas. Such stress concentrations can trigger localized damage in warp yarn even when the overall applied tensile stress is much lower than the ultimate tensile strength. This can well explain why the fracture of warp yarns mostly occurred in yarn crossover areas. In contrast, when the specimens are subjected to tension weft-wise, the tensile loads are primarily carried by weft yarns. Obviously, almost no stress concentrations exist in yarn crossover areas, therefore the weft yarns fractured at random positions. 3.3. Bending tests The typical flexural strength–displacement curves are shown in Fig. 3, and some of the test data are summarized in Table 3. It is seen that all the curves exhibited similar trends except for the stress history after peak stresses. For the specimen cut along the warp direction, the curve showed nonlinearity to a displacement value of 1.01 mm to reach the peak stress (522 MPa). After the peak stress, the stress dropped a little and then remained in a large range of displacement until the primary load drop occurs at a displacement of about 1.37 mm. The crucial phenomenon is believed to be attributed to a lockup mechanism involving yarn waviness and pinching feature of the adjacent warp yarns. In the case of the specimen cut along the weft direction, the curve displayed similar nonlinearity to a displacement value of 1.25 mm to reach the peak stress (313 MPa). However, after the peak stress, the stress descended rapidly in a steep manner. It is also noted that the strain at the flexural strength of the specimen cut along the warp direction was much lower than that of the specimen cut along the weft direction, while for the strain at fracture strength, it was just the reverse. The relatively lower strain at the flexural strength of the warp-direction specimen further confirms large stress concentrations in the yarn crossover areas. 3.4. Shear tests Considering that the thermostructural components will undergo complex loads during service, it is valuable to carry out shear tests, be￾cause shear tests can provide information on the strength and deformation of materials under shear stresses. Typical in-plane shear stress–strain curves are shown in Fig. 4. It can be seen that the specimens cut along the warp and weft directions had comparable shear strength (listed in Table 4) and exhibited similar stress–strain trends. All the curves displayed nonlinearity from the very beginning due to the processing-induced matrix cracks. With increasing the applied loads the degree of nonlinearity was increased due to the damage accumulation, including the multiplication of matrix cracks, the propagation of cracks along the fiber directions and the failure of some fibers by tension in the transverse-loading directions. At the last stage, the applied loads were maintained due to the friction of fiber and fiber bundle pullout until the final failure of the specimens. To further investigate the shear properties of the composites, short￾beam shear tests were conducted to determine the interlaminar shear strength. The test results are presented in Table 4. The specimen that was cut along the warp direction had much higher interlaminar shear strength than the specimen cut along the weft direction, which is attributed to the difference in density between warp and weft yarns. It is evident that the composites displayed higher interlaminar shear properties relative to their 2D counterparts reported in literature [8], Table 2 Tension test data of the composites Specimen Tensile Strength (MPa) Failure strain (%) Initial modulus (GPa) Warp direction 326 (35) 0.692 (0.07) 153 (16) Weft direction 145 (16) 0.705 (0.04) 62 (2.8) Standard deviations are given in parentheses. Fig. 3. Flexural strength–displacement curves for specimens cut along the warp and weft directions. Table 3 Bending test data of the composites Loading direction Flexural strength (MPa) Flexural modulus (GPa) Strain at flexural strength (%) Strain at fracture strength (%) Warp 522 (48) 109 (11) 0.62 (0.071) 0.86 (0.093) Weft 313 (22) 51 (4.6) 0.78 (0.052) 0.79 (0.054) Standard deviations are given in parentheses. Fig. 4. In-plane shear stress–strain curves for specimens cut along the warp and weft directions. 314 J. Ma et al. / Materials Letters 61 (2007) 312–315