PSI - Issue 42

Mohamed M.A. Ammar et al. / Procedia Structural Integrity 42 (2022) 1328–1335 Mohamed Ammar / Structural Integrity Procedia 00 (2019) 000–000

1332

5

Fig. 3: CFRP multi-axis flat panels.

in total. All the specimens are produced under the same conditions with identical curing procedures. The average thickness of the laminates is 1.2 mm.

4.1. Structural analysis

Cross-sectional images have been taken using scanning electron microscopy (SEM) analysis. The samples micro imaging is conducted using the high-resolution FEI Quanta electron microscope. The laminate internal gaps and the cross-sectional porosity are investigated using the Everhart Thornley detector (ETD). The specimens are cut into small square samples using water jet cutting process and prepared for analysis. The small samples are then exposed to nitrogen to remove any dust or fine particles. Subsequently, the samples are coated with carbon or sometimes iridium to avoid the buildup of electrons on the surface in order to avoid charging the samples during the analysis. Di ff erent micrographs are taken for the multi-axis laminates with di ff erent configurations and fiber directions. Fig.4 shows SEM micrographs that were taken for the cross-sectional area of the laminate with successive layers in the same direction and in di ff erent directions. It is evident that internal cracks increased and more voids and higher porosity are detected when maintaining the fiber directions in the successive layers before changing it. On the contrary, fewer cracks are detected when changing the direction of fibers between every two successive layers. This occurs due to the large temperature gradients and the compression forces that align with the fiber directions in each lamina. Having the same lamina’s direction in the successive layers would build up the stresses in the same direction. Subsequently, more residual stresses are developed which leads to the formation of internal cracks and gaps between the lamina, which agrees with Alimardani et al. (2009); Salmi et al. (2018); Vrancken et al. (2014). Therefore, the lay-up of the successive layers in opposite directions would assist in decreasing or canceling out the residual stresses in the other direction. As mentioned in Section 2, this type of stress occurs in the lamina scale along with fiber-matrix level. Thus, the inter-laminar cracks and transverse cracking in the plies are the main consequences of these types. This section presents the measured residual stresses inside the internal layers of the angle-ply thermoset laminates. Each laminate contained 8 layers bonded together. The stresses are measured in the first 4 layers of the samples. Fig s . 5 and 6 compare the estimated residual stresses between the laminates that possess consecutive layers with the same fiber directions and the laminates that have consecutive layers with opposite directions for the fibers. Based on the scales reported in Section 2, these stresses are formed in the fibers and matrix which represented by the fiber-matrix and lamina scales. The maximum stresses are estimated in samples that have every two layers from the same directions with values of 90 MPa in the [-45 o , -45 o , 45 o , 45 o ] laminate and 110 MPa in [0 o , 0 o , 90 o , 90 o ] the laminate. Introducing 4.2. Residual stresses