Date of Award

Winter 12-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Roberto Lopez-Anido

Second Committee Member

Keith A Berube

Third Committee Member

Senthil S. Vel

Abstract

Composite materials have been adopted into primary aircraft structures by virtue of their great strength-to-weight and stiffness-to-weight ratios, fatigue insensitivity, and corrosion resistance. These characteristics are leveraged by aircraft designers to deliver improved fuel effciency and reduced scheduled maintenance burdens for their customers. These benefits have been impressively realized in the Boeing 787 and Airbus A350 XWB, with airframes utilizing about 50% composites by weight. Tempering these successes, however, are the inherent vulnerabilities of carbon-fiber reinforced composites. When compared to conventional metallic structure, composite laminates are more sensitive to stress concentrations at mechanical fastenings and damage due to low-velocity impact. In the case of low-velocity impact, the delamination failure mode presents a unique potential for critical strength and stiffness reduction with little visible indication that damage has occurred. This Barely Visible Impact Damage (BVID) is a critical design case and is typically addressed in the aircraft design process by reducing material allowables to account for undetected damage, as well as defining specific maintenance and inspection plans to be carried out by operators during service. The next generation of composite materials are being developed which effectively eliminate the delamination failure mode using three-dimensional pre-formed fiber architectures. So-called 3D woven composites offer enhanced through-the-thickness performance, and exhibit improved damage containment when exposed to out-of-plane impact loading. This study follows on to work by Warren [1-4] and London [5], who characterized the response of similar 3D woven composites to other critical design cases such as open hole compression, single and double shear bearing, and fatigue of bolted connections. This study adds critical information to the knowledgebase on 3D woven composites by comparing the impact damage resistance and tolerance performance of a 3D woven composite with that of an industry-typical multi-directional laminate of comparable thickness, stiffness, and strength. Both materials were first characterized using standard ASTM tension, compression, V-notched rail shear, fiber-volume fraction by acid digestion, and mode-1 fracture toughness tests. Damage resistance and tolerance were evaluated using standard drop-weight impact and quasi-static compression-after-impact tests under a range of BVID-type conditions from 5 to 130 Joules. The AITM 1-0010 test standard was used with minor adaptations. Post-impact non-destructive inspection (NDI) methods included damage area measurement using ultrasonic C-scan, damage characterization using micro-computed tomography (μCT,) and manual indentation measurements for comparison with industry data.

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