Publications

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Delaminations are of great concern to any fiber reinforced polymer composite (FRPC) structure. In order to develop the most efficient structure, designers may incorporate hybrid composites to either mitigate the weaknesses in one material or take advantage #of the strengths of another. When these hybrid structures are used at service temperatures outside of the cure temperature, residual stresses can develop at the dissimilar interfaces. These residual stresses impact the initial stress state at the crack tip of any flaw in the structure and govern whether microcracks, or other defects, grow into large scale delaminations. Recent experiments have shown that for certain hybrid layups which are used to determine the strain energy release rate, G, there may be significant temperature dependence on the apparent toughness. While Nairn and Yokozeki believe that this effect may solely be attributed to the release of stored strain energy in the specimen as the crack grows, others point to a change in the inherent mode mixity of the test, like in the classic interface crack between two elastic layers solution given by Suo and Hutchinson. When a crack is formed at the interface of two dissimilar materials, while the external loading, in the case of a double cantilever beam (DCB), is pure mode I, the stress field at the crack tip produces a mixed-mode failure. Perhaps a change in apparent toughness with temperature can be the result of an increase in mode mixity. This study serves to investigate whether the residual stress formed at the bimaterial interface produces a noticeable shift in the strain energy release rate-mode mixing curve.

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Woven fiber, laminated composites allow the design engineer to create high strength parts, but the effectiveness of the final processed part is greatly diminished through weak or nonexistent bonds between the composite and the substrate to which it is bonded. Additionally, these layered laminates are commonly made by curing the resin infused carbon fiber fabrics in predefined layers and then bonding them to another composite or a metallic structure using either a pre-cure or a co-cure method. The focus of this study is the identification of the defect caused by a disbond or a delamination located at the interface between a composite laminate stack and the substrate to which it is bonded. We present a nondestructive approach using various ultrasonic methods to identify the existence of the bond between composite and composite-to-metal interface. This paper explores contact and immersion ultrasound methods using pulse-echo for evaluating the composite material and adhesive bondline and the signal attenuation undergone by the wave as it propagates through the composite. Finally, a summary of the detection and analysis techniques developed to identify disbonds, including Fast Fourier Transform analysis of the immersion data, is presented. Lastly, each of the methods evaluated in this study is able to detect the transition from bonded to unbonded sections at the bondline from either side of the bonded part, with the immersion technique providing a significantly higher resolution of the edge of the bondline.

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Ultrasonic analysis is being explored as a way to capture events during melting of highly dispersive wax. Typical events include temperature changes in the material, phase transition of the material, surface flows and reformations, and void filling as the material melts. Melt tests are performed with wax to evaluate the usefulness of different signal processing algorithms in capturing event data. Several algorithm paths are being pursued. The first looks at changes in the velocity of the signal through the material. This is only appropriate when the changes from one ultrasonic signal to the next can be represented by a linear relationship, which is not always the case. The second tracks changes in the frequency content of the signal. The third algorithm tracks changes in the temporal moments of a signal over a full test. This method does not require that the changes in the signal be represented by a linear relationship, but attaching changes in the temporal moments to physical events can be difficult. This paper describes the algorithm paths applied to experimental data from ultrasonic signals as wax melts and explores different ways to display the results.

Ultrasonic analysis is being explored as a way to capture events during melting of highly dispersive wax. Typical events include temperature changes in the material, phase transition of the material, surface flows and reformations, and void filling as the material melts. Melt tests are performed with wax to evaluate the usefulness of different signal processing algorithms in capturing event data. Several algorithm paths are being pursued. The first looks at changes in the velocity of the signal through the material. This is only appropriate when the changes from one ultrasonic signal to the next can be represented by a linear relationship, which is not always the case. The second tracks changes in the frequency content of the signal. The third algorithm tracks changes in the temporal moments of a signal over a full test. This method does not require that the changes in the signal be represented by a linear relationship, but attaching changes in the temporal moments to physical events can be difficult. This paper describes the algorithm paths applied to experimental data from ultrasonic signals as wax melts and explores different ways to display the results.

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