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.

The simulation program FWCURE [1] models the curing process and layer tension loss of axisymmetric filament wound composite cases during fabrication. For a specified temperature cure cycle, the model predicts the temperature distribution, resin degree of cure, viscosity, layer compaction, and fiber motion throughout the composite case during cure. The scope of the simulation program developed by Tzeng [1] has been extended, and the modifications to the FWCURE program are the goals of this investigation. Major modifications to FWCORE include a more general 2-D layer tension loss model, additions to an element curvature calculation routine, a new cure reaction kinetics model and viscosity model for a Fiberite-974 epoxy resin system, and modifications and additions to Input/Output (I/O) throughout the program. Modifications and additions to FWCORE are implemented in the analysis of an 18 inch diameter test bottle. Results of the simulation are compared with test data obtained during winding and cure of a graphite-epoxy 18 inch test bottle. Excellent agreement was obtained between the results of the model and data. Another major accomplishment involved coupling FWCURE with a thermo-mechanical stress simulation program called WACSAFE. When combined, the coupled program forms an improved comprehensive structural model which characterizes the thermal, chemical, physical, and mechanical processes occurring during winding and cure of filament wound composite cases. The complete simulation program should provide the process engineer with a resource to help select an optimum fabrication cycle, assess the processing characteristics of new matrix reein eystems, and act as a simulator to yield real time, closed loop process control. FWCURE should also provide information on the processing parameters that have the greatest effect on the final filament wound composite structure. / Master of Science

Histotripsy is a non-invasive, focused ultrasound ablation modality that uses the precise control of acoustic cavitation to disintegrate tissue. Acoustic cavitation is the transient expansion of pre-existing nuclei to dimensions orders of magnitude beyond their original size due to large increases in the tensile (peak-negative) pressure of the medium. The peak-negative pressure required to generate a cavitation bubble is defined as the cavitation threshold. Once the cavitation threshold has been reached by an applied acoustic pressure, the remaining internal pressure of the bubble results in violent expansion. Histotripsy utilizes focused ultrasound to tightly control the region in which the acoustic pressure exceeds the cavitation threshold forming cavitation bubbles within a precise focal volume. The rapid expansion and collapse of the cavitation bubbles cause large deformations to the surrounding medium at high strain rates capable of mechanically disrupting cellular structures. During histotripsy therapy, numerous cavitation events are generated from a single pulse within the tightly bound focus of the transducer, defined as the bubble cloud. The disintegration of the targeted tissue volume occurs from the rapid expansion and collapse of bubble clouds over the course of many pulses. By maneuvering the transducer array through the use of robotics or by electronically steering the focus of the array, volumetric ablation of tissue can be performed non-invasively. Histotripsy was recently granted de novo clearance from the FDA for the ablation of liver cancer establishing its clinical relevancy in the field of medicine. This work is inspired by the amazing impact of this therapy and fulfilled with the desire to expand the knowledge of the underlying mechanics of histotripsy to allow for the treatment of malignancies that have not been previously investigated. This dissertation proposal outlines the development of high-rate, non-invasive ablation of soft and fibrous tissue using single-cycle histotripsy. My Ph.D. thesis is motivated to develop histotripsy for the ablation of larger volumes than previously considered feasible and tissues that are more resistant to histotripsy-induced damage by utilizing high pulse repetition frequencies (PRF). In this work, (1) I propose histotripsy for the ablation of uterine fibroids, which are large, fibrous tumors that would require unsuitably long treatment times using traditional histotripsy methods. To deliver higher doses and treat larger volumes within a clinically relevant treatment time, (2) I investigate the effects of PRF on reiterative bubble cloud dynamics in single-cycle histotripsy, and (3) demonstrate how bubble clouds form under high PRF conditions. In the last chapters of this work, (4-5) I developed a high PRF pulsing strategy that can efficiently ablate large volumes of soft tissue and uterine fibroids. The findings and implications found in this document aim to increase the robustness of histotripsy as a non-invasive ablation therapy for many new applications by developing faster ablations and furthering the understanding of the underlying mechanics of histotripsy at clinically relevant pulsing rates. / Doctor of Philosophy / Histotripsy is a new medical therapy that uses the precise control of acoustic cavitation to destroy tissue. Acoustic cavitation is the rapid expansion and collapse of stable bubbles in a fluid based medium. The pressure required to cause cavitation bubbles is called the cavitation threshold. Once the cavitation threshold has been reached, the bubbles result in a violent expansion and collapse. Histotripsy forms cavitation bubbles within a precise area by utilizing focused ultrasound to tightly control the region in which the acoustic pressure exceeds the cavitation threshold. The rapid expansion and collapse of the cavitation bubbles cause the surrounding medium to push away from the growing bubble causing high stress levels capable of breaking down cellular and other tissue structures. During histotripsy therapy, many cavitation events are generated from a single ultrasound pulse within a tightly bounded focal volume, which we define as the bubble cloud. The destruction of the targeted tissue volume occurs from the rapid expansion and collapse of bubble clouds over the course of many pulses. By moving the bubble cloud, entire tumors can be destroyed without any incisions or implants in the body, completely non-invasive. Histotripsy was recently granted clearance from the FDA for the ablation of liver cancer and is in development for many more applications where the complete removal of malignant tissue is desired. This work is inspired by the amazing impact of this therapy and fulfilled with the desire to expand the knowledge of the underlying mechanics of histotripsy to allow for the treatment of malignancies that have not been previously investigated. This dissertation proposal outlines the development of high-rate, single-cycle histotripsy for the ablation of soft and fibrous tissue. My Ph.D. thesis is motivated to develop histotripsy for the ablation of larger volumes than previously considered feasible and tissues that are more resistant to histotripsy-induced damage by increasing the rate at which the therapy is delivered, or the pulse repetition frequencies (PRF). In this work, (1) I propose histotripsy for the ablation of uterine fibroids, which are large, fibrous tumors that would require really long treatment times using traditional histotripsy methods. To deliver the higher doses necessary to treat fibrous tissues or larger volumes, (2) I investigate the effects of PRF on reiterative bubble cloud dynamics in single-cycle histotripsy in vitro, which has not been previously explored, and (3) demonstrate the difference in how bubble clouds form under high rate pulsing regimes. The results of these experiments were used to (4) develop a high PRF pulsing strategy that can efficiently ablate large volumes of soft tissue. In the final chapter, (5) I apply these high-rate ablation methods to uterine fibroids. The findings and implications found in this document aim to increase the robustness of histotripsy as a non-invasive ablation therapy for many new applications by developing faster ablations and furthering the understanding of the underlying mechanics of histotripsy at clinically relevant pulsing rates.

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.