Additive layers to suppress delamination in composite laminates

  • Miguel Ubago Torres

Student thesis: Doctoral Thesis

Abstract

Delamination originating from the free edges is a common failure mechanism of composite laminates, as the free edges in composite laminates are high-stress concentration areas. This thesis has studied a solution that could suppress delamination starting from the free edges in composite laminates in different loading scenarios. The solution consists of thin additive layers of prepreg carbon/epoxy composite material, which are added to the composite laminate’s free edges to enhance the through-thickness strength of the laminate. Additionally, the risk of delamination is reduced by constraining the out-of-plane and shear movements of different plies at the free edge. The solution was explored in four different loading scenarios, which are easily encountered in the life cycle of a composite laminate, as are, composite laminates under tension, composite laminates under edge impact, composite laminates with open holes under tension, and pin-loaded composite laminates. Both numerical and experimental methods have been complementarily used in each loading case to verify the solution’s effectiveness. The use of these additive bindings for suppressing free-edge delamination was successfully proven in the case of composite laminates under tension.For example, when used to bind the edges of a [(202/ − 202)2]s laminate, the tensile failure strain and load were increased by about 50%, and the failure mode changed from free-edge delamination to in-plane shear. However, it did not change the failure load and final failure mode of a substrate less susceptible to free-edge delamination as a [(452/ − 02/ − 452/902]s laminate. The use of additive bindings in composite laminates under edge impact successfully enhanced the through-the-thickness strength of the laminates and stiffened the initial response of the laminate to the impact. As a result, higher average peak load values were observed in the specimens with bindings. Additionally, the delamination length in the perpendicular direction to the impactor displacement was reduced by about 30% for low and medium-impact energies and by about 20% for high-impact energies. However, the damage in the parallel direction to the impactor displacement was not significantly affected, and similar delamination length occurred in specimens with binding. Therefore, further investigations are suggested to test the capabilities of additional binding configurations to enhance the damage tolerance of composite laminates under edge impact.Additive narrow binding stripes were added to the edges of holes of composite laminates with open holes. Tensile and fatigue testing was conducted, but the results obtained were inconclusive. Further work is suggested for composite laminates with open holes. The remarkable point was that the specimens with binding had a more explosive failure and presented a smaller coefficient of variation of the strain and load values. Last, in the case of pin-loaded laminates, narrow additive binding stripes were added at the edge of the hole of the laminate. Both quasi-static and fatigue tension-tension testing was carried out. The use of additive bindings successfully enhanced the bearing damage resistance, increasing the offset bearing strength by about 38%. In addition, additive bindings reduced the hole diameter size; hence, different diameters were used for the loading pins of the specimens without and with binding to achieve a tight fit between the pin and the hole. Smaller pin diameters in specimens with binding increased the stress concentration at the contact point, and a more defined ”pear-like” shape was obtained at the deformed hole. Thus, the design of specimens with larger holes is encouraged for testing the solution in this type of specimen to avoid the effects of using different pin sizes.
Date of Award10 May 2023
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorLiu Yang (Supervisor) & James Thomason (Supervisor)

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