Last modified 6/3/97
|Valeo Systemes D'Essuyage||Chatellerault, France||Engineering Intern, Quality Control Office||5/91-8/91|
|David Taylor Research Center||Annapolis, MD||Engineering Aide
Division of Metals & Welding, Fatigue & Fracture Branch
|5/90-8/90, 5/89-8/89, 12/88, 5/88-8/88, 5/87-8/87|
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Dissertation Proposal to Graduate School (November 1994)
Continuous fiber-reinforced composite materials are an important class of engineering materials. They offer outstanding mechanical properties, excellent strength-to-weight ratios, and unique flexibility in design capabilities. The strength of composite materials depends not only on the strengths of the constituent materials but also on the processing history of the material. The load carrying capabilities of fiber-reinforced composites are very sensitive to the distribution and linearity of the fibers. In a unidirectional laminate composed of many plys or layers of composite prepreg, the fibers should be perfectly aligned with respect to one another, aligned from layer to layer, and, ideally, they should be linear. However, the fibers in real composite materials exhibit small angular imperfections which can be categorized as either fiber misalignment or fiber waviness.
An improved explanation of how this fiber waviness develops is necessary not only for the development of processing techniques which reduce fiber waviness, but also for the more accurate prediction of material strength. Past researchers have linked the existence of fiber waviness to a number of variables in the fabrication of composite parts. There is still a need to investigate the thermo-mechanical mechanisms at work at the microscale which really drive the initiation and growth of fiber waviness. A number of mechanisms have been postulated to be involved in driving the development of fiber waviness during autoclave processing. A better understanding of the mechanisms which lead to the development of fiber waviness and how these mechanisms are linked to the processing will enhance one's ability to predict the amount of waviness in a product without destructive testing and enable one to predict waviness levels before rigorous (and often expensive) experimental testing is required.
The objective of this thesis is to investigate the many phenomenae that occur leading up to the development of fiber waviness. It is proposed to conduct a series of model experiments which address the different waviness inducing mechanisms in isolation to study their various effects in the development of fiber waviness for an array of different composite material systems.
Past researchers have experienced difficulty quantifying fiber waviness throughout composite parts; therefore, there is a need to develop an improved method for characterizing fiber waviness for qualification and/or comparison purposes. The other objective then, of this experimental program is to establish a method for characterizing the process-induced fiber waviness found in unidirectional composite laminates. A major obstacle to investigating fiber waviness is determining how to quantify fiber waviness. Since sinusoidal models are frequently adopted or assumed, wavelength and amplitude are the traditional geometric parameters used to define waviness. However, even these relatively simple parameters are not easily measured when examining the entangled fibers in a real composite laminate. In a composite, each fiber may have its own amplitude and wavelength. Therefore, a statistical analysis describing the waviness distribution throughout the part yields the most comprehensive characterization available using sectioning and polishing techniques with optical microscopy. If a non-destructive test for local fiber waviness can be found that is applicable to the entire part this dependence on statistical sampling can be overcome.
An experimental methodology for characterizing the process-induced fiber waviness in thermoplastic (T300/P1700) composite laminates using computer assisted optical microscopy has been developed [see (Joyce et. al 1994)]. Several experiments are left to be performed that will help to characterize the fiber waviness in composite laminates using optical microscopy. More importantly a non-destructive evaluation (NDE) technique for detecting and measuring fiber waviness is desired. The secondary thrust of the proposed work is to seek after an appropriate NDE technique for characterizing fiber waviness in composite parts. I am looking specifically at ultrasonic inspection and x-ray computed tomography for their potential capability to detect/measure fiber waviness in composite parts. If a suitable NDE technique could be implemented on-line it would be a significant contribution to the quality assurance of composite parts in which the presence of fiber waviness plays a key role in failure.
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