Effects of defects on laminate quality and mechanical performance in thermoplastic Automated Fiber Placement-based process chains

Abstract

Automated Fiber Placement of thermoplastic unidirectional tape materials offers several advantages over conventional organosheets, such as enhanced part performance through tailored fiber architecture, and economic and ecological benefits due to scrap reduction. Because material is cut perpendicular to the feeding direction in state-of-the-art machine technology, triangular gaps and overlaps occur when geometrically complex layups are fabricated. Their effect on part properties is unknown for thermoplastic materials. This study investigates the influence of various defect configurations on laminate quality and mechanical performance for different consolidation processes. Analysis of microsections prepared from post-consolidation specimens shows out-of-plane undulations in defect areas. The undulation extent is quantified by angle and deflection. Tensile and compressive testing is performed. Gaps reduce ultimate tensile and compressive strength significantly for variothermal press and autoclave consolidation. Digital-image-correlation-based strain measurement during tensile testing shows strain concentration in the defect area for these specimens. Specimens consolidated in an isothermal stamp forming process show no comparable stress concentration, as well as no reduced ultimate strength. Specimens containing overlaps generally show a better performance in terms of ultimate strength compared to those containing gaps. Even though no full factorial design of experiments was used, the results obtained from this study can be used as a baseline for sector-boundary design strategies. The definition of defect-specific knock-down factors would be a next step towards the solid engineering of thermoplastic Automated Fiber Placement parts.

Ply-by-ply inline thermography inspection for thermoplastic automated tape layup

“…The experimental investigation of effects of gaps and overlaps is conducted by Woigk et al [14], with a distinction between lamina and laminate level by Croft et al [15]. Zenker et al [16] conducted statistical analysis with multiple null-hypotheses investigating the effects of consolidation process, defect type, defect size and stacking sequence on the mechanical performance of the thermoplastic laminate. Although the consensus is that defects are detrimental to the mechanical performance, some experimental results point otherwise.…”

“…Threshold of large and small defects according to aerospace standards are 7.35 mm for large defects for both gaps and overlaps, 2.5 mm for small gap and 1 mm for small overlap [16]. The foreign object inclusions for defect introduction are chosen to conform to those standards.…”

Effects of defects in automated fiber placement laminates and its correlation to automated optical inspection results

“…Specimen containing overlaps generally show a better performance in terms of ultimate strength compared to those containing gaps. 18…”

“…Specimen containing overlaps generally show a better performance in terms of ultimate strength compared to those containing gaps. 18 In addition to the experimental studies described above, other studies have been presented in the last years that have focused on the modeling and simulation of AFP induced defects. [19][20][21] The general conclusion that can be drawn from literature is that the mechanical knockdown of an AFP laminate results mainly from the induced fiber waviness.…”

Optimization of the laser-assisted tape winding process using an inverse kinematic-optical-thermal model

“…The process temperature, speed, compaction pressure [1][2][3][4][5], fiber path planning [6][7][8][9], and processinduced defects [10][11][12] are the important factors for developing a proper bonding quality at the interface between the tape and substrate to manufacture products with high mechanical performance. In addition, optimal ranges of the tape winding and placement parameters such as tape tension and roller force are necessary to reduce the void content [13,14] and residual stress [15,16] and to increase the tensile strength [13,15], interlaminar shear strength [17,18], peel strength [19,20], and production rate [16,18,19] The nip point temperature serves as an indicator for the process temperature and might vary during the LATW process, possibly resulting in a substantial variation in the consolidation quality.…”