The creep and creep recovery behavior of a random fiber composite (SMC-R50) at elevated temperature and constant humidity were investigated experimentally and theoretically. The short time creep response for four constant stress levels at each of four selected temperature levels was experimentally determined. It was found that repeatable results can be obtained by applying a mechanical conditioning prior to each creep and creep recovery test. Creep data were modelled using the Findley equation which contains three parameters, ε_O (the instantaneous creep response), m (the amplitude of transient creep), and n (the time exponent). It was found that the time exponent is a function of time but approaches to an asymptotic value when the duration of creep is long. Thus, at a constant temperature level, one long-time creep test and four short-time creep tests were conducted. The long-time creep results were used to determine the proper time exponent n. The short time creep data for constant load were used to determine the Findley parameters ε_O and m. It was found that the Findley equation represented the creep results very accurately. Based on the short-time creep results, the Findley equation was used to predict the long time creep response and the creep response due to multiple step loadings. Five long time creep experiments were conducted. Four of them were 10,000 minutes long and were conducted at the same stress level (6,510 psi) but different temperature levels. The fifth creep experiment was conducted at 5,425 psi and 185°F over a three week period. Three multiple step creep experiments were conducted. These tests were of different load steps and durations. In all cases, it was found that the Findley equation predicted both long time creep response and multiple step creep response very accurately. Since repeatable results were obtained from conditioned specimens, the test results were compared to experimental data obtained from unconditioned specimens. It was found that experimental results of the conditioned specimens fell within the scatter band of the data for the unconditioned specimens. A free energy based failure criterion (proposed by Reiner and Weisenberg) was coupled with the Findley equation to predict the creep rupture time of SMC-R50. It was found that the critical free energy at the time of failure is temperature dependent. For a constant temperature, the critical free energy required for rupture is essentially a constant. It is also concluded that, for limited data, the Reiner-Weisenberg failure criterion provj.dl!S overall good prediction of the time to failure for SMC-R50. / Ph. D.

The final tensile fracture of any composite structure is primarily due to the failure of its constituents, namely fibers and matrix in the present case. To date, no experimental data exists, to the author’s knowledge, to define the behavior of constitutive fibers of a composite structure throughout its life span. The prime candidate for a fiber-based investigation is unidirectional zero-degree composite coupons. But unidirectional coupons do not demonstrate any significant loss of stiffness during fatigue cycling compared to other lay-ups. Even if stiffness degradation was significant, due to the nature of damage in this material system it would be impossible, practically, to monitor that change using conventional techniques (e.g. an extensometer) because the damage and failure process destroys the integrity of the contact between those devices and the material, under cyclic conditions. This investigation presents the findings of a fiber-based investigation of unidirectional composite material systems. In particular, a unidirectional graphite/epoxy system was studied, and the influence of applied load level on fiber fractures, and their influence on damage growth documented. A damage monitoring technique (patent pending) was developed to accurately record the state of damage in this material system without the usage of extensometers or strain gages. Following this method, two new damage norms were introduced, namely, “percent phase damage” and “percent gain damage”. Fiber fracture, strength degradation, and the life of unidirectional specimens were investigated and recorded as a function of various load levels. Fiber fracture, in general, showed no definitive growth pattern during fatigue cycling. It appears that the majority of the broken fibers that occur over nearly 90% of the life are due to the initial applied load cycle. This is one of the key findings of this investigation. “Proof testing” which is a common practice in industry for “verifying” the integrity of a structure, could very well be causing significant subsequent reductions in life. With these findings as a base, it is now possible to postulate the first well-founded mechanistic model of fiber-dominated fatigue degradation under tensile loading. / Ph. D.

This investigation concerns the frequency related behavior of delaminations in fiber reinforced composites during vibrothermography, the use of active thermography with a mechanical excitation for the nondestructive evaluation of a structure or part. Two models, one where the size and geometry of the flaw control a local resonance and the other where the part or panel is undergoing structural resonance with the flaws dissipating the mechanical energy, are proposed for this frequency related behavior and tested on simulated and service produced delaminations in coupons, panels, and a machine part of complex geometry. The behavior predicted by the local resonance model is compared with experimental observations. The vibration state of the panels or coupons is determined during the vibrothermal tests, and compared with the frequencies which cause vibrothermographic heating of the flaws as a test of the structural resonance model. The usefulness of vibrothermography is demonstrated in glass and graphite reinforced epoxy components. Impact damage sites are located in graphite epoxy panels using vibrothermography. The size of the damage is indicated not only by the size of the hot region but also by the temperature rise in the center of the flawed region. A glass epoxy machine part, which was damaged during service, was subjected to interrogation by ultrasonic C-scans, X-ray radiography, and then compared with the vibrothermographic NDE. / Ph. D.

ln the past twenty years a great deal of research has been conducted on the use of externally bonded CFRP sheets for strengthening RC beams. It has been found that various anchorage designs at the ends of the bonded sheets could lead to different load-deflection characteristics and failure modes in the beams. This study presents results from an experimental study on new anchorage designs for strengthening RC Beams in flexure with externa1ly bonded CFRP sheets. Nine specimens were tested in four-point bending. The test variables include anchorage design, bond length, and mid-span bond. Anchorage is achieved by a mechanism that clamps the sheet-ends into a groove that is cut into the tension side of the RC beam. Two clamping approaches are adopted. One employs a bolted steel bar and rubber bearing pad; the other employs a plug and epoxy filling.

No description available.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 134-139). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 134-139). Also available on the Internet.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2009. / Includes bibliographical references (p. 261-275).

Filament wound composite structures are becoming more and more attractive to designers in the aircraft and aerospace industries due to increasing strength- and stiffness-to-weight ratios and falling fabrication costs. However, the interaction of some of the manufacturing process variables such as mandrel stiffness and thickness, winding tension and pattern, and cure cycle characteristics can lead to common defects such as delamination, matrix cracking and fiber buckling. A model of the filament winding process was developed to better understand the behavior of wound structures during fabrication. Specifically, the residual stress state at the end of winding, heat-up and cool-down was determined. This information is important because adverse stress states are the mechanism through which the process variables cause fabrication defects. The process model utilized an incremental finite-element analysis to simulate the addition of material during winding. Also, the model was refined and extended to include changes that occur in the material behavior during the cure. A fabrication analysis was performed for an 18 in. (457 mm) graphite/epoxy filament wound bottle. Two different mandrel models were examined, a rigid steel and a soft sand/rubber mandrel. At the end of winding, the composite layers in the model retained all of their initial winding tension for the steel mandrel but did exhibit significant loss of tension for the sand/rubber mandrel. The composite layers experienced a large increase in tension during heating for the steel mandrel but showed no significant recovery of tension for the sand/rubber system. / M.S.

Dynamic mechanical analysis (DMA) has been used extensively to characterize polymeric materials. This thesis investigates the use of this technique to characterize composite materials. Four material systems, all having the same fiber and the same matrix material, were systematically altered with different interphase regions. Initial run data indicates that the variation in fiber sizings create different interphase regions that are detectable by DMA. However, some post-cured specimens revealed that those differences are reduced with further heat treatment. Fiber sizing variation also affects the material response to thermal and mechanical cycling. Different fiber surface treatments have little effect on the dynamic mechanical response of the materials. It is shown that DMA is a test method that yields repeatable results and is capable of detecting small changes in composite constituents. / Master of Science