Date of Award

5-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Civil Engineering

First Advisor

David Fleming

Second Advisor

Ronnal Reichard

Third Advisor

Razvan Rusovici

Fourth Advisor

Shengyuan Yang

Abstract

Delamination between composite layers is one of primary weaknesses in composite material structure. Various researchers have developed techniques to control delamination in laminated structures. One of these techniques is “interleaving,” adding high toughness material to key interfaces in a laminate. This research studies using polyester veil as a low-cost interleaf alternative to other materials and focuses on a non-woven, polyester spunbond material. Two different interleaf thicknesses, 0.18 mm and 0.74 mm are primarily used. In addition, fine 4 g/m2 polyester was also compared. Carbon/epoxy composites are manufactured using 2x2 Twill 24”-12k carbon fibers embedded in an epoxy resin, with polyester interleaves at key interfaces. Specimens are fabricated using wet hand layup and cured at room temperature in a vacuum bag. Mode I fracture toughness is measured using the Double Cantilever Beam (DCB) test and Mode II fracture toughness is examined using the End-Notched Flexure (ENF) test. Further evaluation is made using static indentation and full penetration impact testing. Toughness is compared, and the resulting fracture surfaces are investigated. Significant improvement is seen in Mode I testing. Up to a factor of four increase in propagation energy per unit area resulted from the inclusion of the interleaf material. Smaller improvements were observed in Mode II, with the best cases showing an increase in propagation energy to maximum load by about a factor of two compared with control cases. The polyester interleaf significantly influences the fracture morphology observed in static indentation and full penetration tests. Tests of Mode I and Mode II delamination specimens were modeled with the finite element to simulate the behavior of composite interleaved structures. The effects of the relatively thick interleaf layers on the basic stiffness on composite laminates is studied using finite element models generated using ANSYS, while delamination behavior is investigated using cohesive zone models (CZM) in LS-DYNA. The use of CZM allows accurate simulation of crack initiation, though propagation response is not well captured.

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