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

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Daniel et. al Daniel and Liber (1977) have investigated the impact of the sequence of fiber orientation as well as ply stacking on the induced stresses inside graphite / polyimide laminates. They implemented the embedded-gage techniques to estimate the curing and post-curing thermal strains. Further, they conducted a comparison with the unidirectional samples to show the influence of the laminate orientation and stacking sequence on the generated stresses. Another study on the ply orientation e ff ect on the residual stresses is performed by Bajaj et al. (2020). They presented a relationship between residual stresses and the processing parameters of two-layered carbon fiber reinforced polymer composites. They implemented the X-ray di ff raction technique to measure the residual stresses of the laminate. They found an increase in the residual stresses followed by a reduction in tensile stresses when increasing the orientation angle of the second layer. De Garcia et. al De Gracia et al. (2018) conducted a study on the combination of ply’s angle and residual stresses on fracture behavior of symmetric and non-symmetric laminates. They found that the orientation can a ff ect the stresses and subsequently the crack propagation modes are impacted. In order to provide more insights on the e ff ect of the ply orientation on the developed residual stresses of the multi axis laminates, the present study is conducted on the thermoset composites (i.e. carbon fiber reinforced epoxy). The specimens are manufactured using robotic manufacturing technique. The manufacturing conditions are maintained the same for all samples. The structural analysis is performed to show the e ff ect of the angle-ply laminate’s config uration in the micro-scale of the material. Additionally, the residual stresses are measured based on the hole-drilling methodology. The samples are processed and analyzed to understand the behavior of having di ff erent orientations on their mechanical characteristics. RSs play a major role in determining the performance and quality of the composite materials. The generated thermal and mechanical RSs due to the plastic deformation after processing of the material, could have a significant e ff ect on manufacturing and design operations of composite structures. The RSs are induced inside the composite laminate after material processing and cooling-down. The sources of RSs within a composite component are classified as extrinsic which are related to the tooling and processing, and intrinsic which are related to the lay-up sequence, material, and the structure geometries. However the RSs induced from the RFP process are related to the tooling and processing stage, it also a ff ects the other stresses generated due to material properties, lay-up sequence and structural geometry. In order to understand the e ff ect of the RFP on these stresses, this section briefly describes all the sources and types of the induced RSs inside the composite laminate. The RSs can be categorized into four di ff erent scales; micro-scale, meso-scale, macro-scale, and global-scale. The micro-scale is induced at the fiber and matrix level due to their di ff erent mechanical and thermal properties. The mismatch between the elastic and plastic deformations, besides thermal expansions for both fiber and matrix, are the main causes of this level of the RSs. These di ff erences in the fiber and matrix properties induce di ff erent stresses in each component. The meso-scale occurs at the lamina level due to the projection of defects such as micro-cracks, interaction with another lamina, and stresses from the micro-scale. While macro-level is created in the composite laminate, and is a result of including layers with di ff erent directions and accordingly di ff erent directional properties. Other reasons for this scale of RSs are the change in the cooling rate through the laminate thickness, and the interaction between laminate and manufacturing tool, in addition to the projection of RSs from lower levels. Finally, the global scale considers the entire structure which includes various laminated parts, e.g., cones, pressure vessels, and V-shaped structures. The main reasons for these types of RSs are the non-uniform mechanical processing and cooling rates at di ff erent areas of the structure, the interaction between the part and manufacturing tools, and induced stresses from lower scales. For instance, the mid-layers of a thick laminate mostly have a lower cooling-down rate than the surface layers. In some circumstances with a certain temperature, the plies at the center may remain soft while the other plies close to the surface are solidified. Thus, any further cooling imposes a constraint by the solid outer layers on the shrinkage of layers at the center and then generates RSs. Another important source of the global RSs is the di ff erence in thermal conductivity and thermal expansion of the mold and the composite structure. Whereas the interaction between them generates RSs at the cooling-down as the mold is isotropic, unlike the composites. Generally, the majority of RSs are developed during the curing cycle, however, the automated manufacturing process has a considerable contribution to the development of the RSs inside the composite structures. Unlike the 2. Residual Stresses (RSs)