PSI - Issue 13

Mor Mega et al. / Procedia Structural Integrity 13 (2018) 123–130

126

4 M. Mega et al. / Structural Integrity Procedia 00 (2018) 000–000 Table 2: Volume fraction, as well as mechanical and thermal properties for the woven fabric with fibers oriented in the + 45 ◦ / − 45 ◦ -directions. V f E 11 = E 33 E 22 G 13 G 21 = G 23 ν 13 ν 21 = ν 23 α 11 = α 33 α 22 (GPa) (GPa) (GPa) (GPa) ( × 10 − 6 / ◦ C ) ( × 10 − 6 / ◦ C ) 0.36 7.5 5.4 16.3 1.7 0.79 0.073 7.3 85.8 software. In addition, the specimen location in the loading frame is examined so that the load line and the delamina tion center approximately intersect. After the specimen is positioned in the frame, as shown in Fig. 2b, the relative humidity RH and temperature ϑ , in the Instron work area are continuously recorded. As the test begins, the frame is loaded in compression in a uniaxial Instron testing machine (model number 8872; High Wycombe, England), with a load cell whose capacity is 25,000 N and a resolution of ± 0 . 25% of the reading, greater than 250 N. The cross-head displacement of the Instron is increased quasi-statically at a displacement rate of 0 . 008 mm / sec. The load and cross-head displacement are monitored with a computer and synchronised with the specimen images obtained during the test. The fracture toughness for the described interface was obtained for each specimen tested using the critical load P c and critical delamination length a c found from the tests. To this end, the first term of the asymptotic expansion of the displacement field for the considered interface was derived based on the Stroh [31] and Lekhnitskii [32] formalisms as they are described in Ting [33]. The three-dimensional DE method and the mechanical and thermal M -integrals were used to obtain the stress intensity factors resulting from mechanical loads, as well as from residual thermal curing stresses along the delamination front. Verification of the software written for each method was carried out with FE analyses of three mechanical Benchmark problems and one thermal problem. The mechanical Benchmark problems produced excellent results; whereas, the results obtained for the thermal problem were satisfactory. It should be pointed out that the DE method is easy to apply, but is less accurate than the energy based M -integral. Hence, the former was used for verification of the latter. In Table 3, the dimensions of the specimens are presented, namely the specimen thickness B and diameter 2 R which were obtained using a digital micrometer with a resolution of 0.001 mm and a digital caliper with a resolution of 0.01 mm, respectively. The loading angle ω was measured with the Vision Assistant [30] software from an image of the specimen obtained by the LaVision 5 MP Imager Pro SX camera with a resolution of 2456 × 2058 pixels. The critical delamination lengths at failure 2 a c were measured after each test as the length of the smooth part on the fracture surface. Five locations were measured along the specimen thickness on each side of the fractured specimen. From the ten measurements obtained, the inner six, three from each of the lower 3. Test results

composite strip

loading frame

Al

Al

Brazilian disk specimen

( a )

(b)

Fig. 2: (a) Illustration of a Brazilian disk specimen with a delamination between two fiber reinforced composite plies; (b) Brazilian disk specimen placed in the loading frame .