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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conventional lay-up methods, the automated fabrication involves compressing, heating and melting the material which significantly maximizes most of the previous RSs types. This is mainly because of the permanent deformations that occur by the applied pressure as well as the change in the cooling rates between the layers. As a consequence, the RFP and the related induced RSs have a significant e ff ect on the composite structure quality and performance. For example, if the RSs aligned with external stresses’ direction, they will contribute to each other and the performance of the composite structure could deteriorate. These may also cause fiber waviness which yields a deviation from the unidirectional composites and leads to a degradation of the compression strength. The transverse cracking is another defect caused by the RSs. The cracks are initiated inside the matrix when the RSs exceed the yield strength of the matrix which brings out local failures in the composite parts. Moreover, based on the condition of matrix-fiber bonding, the cracks may be propagated parallel to the fiber direction. These microcracks provide damage initiation which significantly a ff ects the material performance at cyclic loading and fatigue. They may also reduce the composite strength and sti ff ness and cause delamination inside the laminate. Additionally, an interlaminar debonding and premature delamination could be also resulted between the layers when considering cross-ply laminate with di ff erent directional layers. The inhomogeneous and non-symmetric interlaminar RSs also bend the laminate and cause distortion, warpage, and dimension instability El-Gizawy and Kuan (2003). Generally, neglecting the RSs when designing the composite structures may result in a deviation of shapes from the desired geometry. The scales and e ff ects of RSs are reported in detail in Figure 1.

Induced RS in composite structures

Fiber-matrix scale

Structure scale

Lamina scale

Laminate scale

Levels

Low stiffness and strength Low fracture toughness Low fatigue strains Delamination Laminate warpage Micro-cracks

Cracks Deformations Spring-in effect

Fiber waviness Fiber-matrix debonding Micro-cracks

Interlaminar cracks Transverse cracking in plies

Effects

Fig. 1: Schematic of the composites induced RSs types and e ff ects.

3. Experimental

This study is a part of an ongoing project that focuses on investigating and understanding the relationship between the manufacturing factors and the mechanical properties of the final thermoset composite products. The material utilized in this study is the carbon fiber reinforced epoxy (i.e. prepreg tows of type T700G). The robotic fabrication process Ammar et al. (2021b) is utilized in this study to perform the production of the specimens. A Yaskawa industrial manipulator attached to a fully controlled fiber placement device is used in this work. Fig. 2 shows the components of the workcell. The prepregs are cooled down when moving through the workcell, subsequently, they are subjected to an instantaneous preheating before the placement. Then the compression force is applied to make the consolidations and to construct the final panels. A steel compaction roller is used in this study to ensure superior surface quality as proposed in the citation Ammar and Shirinzadeh (2022a). The compaction module is established and passive compliance motion is developed to maintain the magnitude and direction of the compression