Author

Robin James

Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Victor Giurgiutiu

Abstract

Advanced composite materials have begun to be used extensively in the manufacturing of aerospace structures. Composite aerospace structures can develop complex types of damages during the manufacturing stages and operational lifetime. This creates an indispensable demand for appropriate nondestructive evaluation (NDE) and structural health monitoring (SHM) methods that are tailored to specific types of damage. This dissertation addresses innovative methods for NDE and SHM of manufacturing flaws and operational damage in composite structures.

For the NDE of manufacturing flaws in carbon fiber reinforced polymer (CFRP) composites, an eddy current testing (ECT) NDE method has been developed by conducting multiphysics modeling and simulation of ECT detection of different types of manufacturing flaws. In addition, extensive experiments have also been conducted on in-house manufactured composite specimens with embedded manufacturing flaws, and specimens obtained from Boeing which had realistic manufacturing flaws.

To validate NDE and SHM methods, controlled impact testing experiments have been conducted on quasi-isotropic CFRP composites of increasing thicknesses (2-mm, 4-mm and 6-mm) to create approximately 1" impact damage diameter. The impact testing experiments were conducted with increasing energy and it was observed that it was easier to experimentally obtain desired impact damage size in thin composites compared to thicker composites. Each impact damage was characterized using ultrasonic NDE, X-ray micro computed tomography (CT) and contact profilometry methods. A pure mode guided wave excitation method using a variable angle beam transducer (ABT) as the excitation, and a phased array transducer (PAT) as the receiver, has been developed. This method has been used for exciting pure SH0 mode guided wave in quasi-isotropic composites for the detection of impact damage. Experiments have demonstrated that the presence of impact damage in thin composites leads to amplitude drop in the received signal. On the other hand, in thicker composites, in addition to amplitude drop we can also observe mode conversion.

An in-situ acoustic emission recording method for impact damage ascertainment has been implemented. This method utilizes a quasi-isotropic composite coupon which has been instrumented with four piezoelectric wafer active sensors (PWAS) to record real-time acoustic emission signals during an impact event. The impact event is created by conducting drop weight impact tests. Through this method, it is possible to ascertain if an impact event has indeed caused damage to the composite or not.

The dissertation finishes with concluding remarks which include summary, conclusions and suggestions for future work.

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