The research presented herein is an effort to support computational modeling of ultra-high molecular weight polyethylene (UHMWPE) composites. An effort is made to characterize the composites and their constituents. UHMWPE, as a polymer, is time and temperature dependent. Using time-temperature superposition (tTSP), the constituent properties are studied as a function of strain rate. Properties that are believed to be significant are fiber tensile properties as a function of strain rate, as well as the through-thickness shear behavior of composite laminates. Obtaining fiber properties proved to be a challenge. The high strength and low surface energy of the fibers makes gripping specimens difficult. Several different methods of fixturing and gripping are investigated, eventually leading to a combination of friction and adhesion approaches where a fiber was wrapped on an adhesive coated cardboard mandrel and then gripped in the test fixture. Fiber strength is estimated using tTSP to equivalent strain rates approaching 10^6 sec^-1. Punch-shear testing of UHMWPE laminates is conducted at quasi-static strain rates and the dependence of the results on thickness and test geometry is investigated. / Master of Science

With increasing applications of composite materials, their health monitoring is of growing importance in engineering practice. Damage development in composite materials is more complex than for metallic materials, because in composite materials (a) multiple damage modes are simultaneously in play, and (b) individual 'damage events' that occur throughout a component's service life may neither noticeably affect its performance, nor suggest future failure. Therefore, informed health monitoring of composite components must include monitoring and analysis of their health state throughout their service life. A crucial aspect of the health monitoring process of composites is the development of tools to help with this goal of understanding the health state of composites throughout their life. This knowledge can lead to timely anticipation of future failure in composite components, and advance the state of current technology. One, timely maintenance can be planned in advance. Two, each component's service life can be determined based on its individual health information, rather than empirical statistics of previously failed components. This dissertation develops such tools and methods. Composite specimens of multiple ply-layups are subjected to tensile loading schemes until failure. Pencil Lead Breaks (PLBs) are used to simulate Acoustic Emission sources and generate acoustic waves that are acquired by installed piezoelectric sensors. A numerical method to estimate the arrival of wave modes from ultrasonic signals is presented. Methods are also presented that utilize PLB signals to indicate approaching failure of specimens under monotonic as well as cyclic loading. These processes have been developed prioritizing simplicity and ease-of-execution, to be adapted for practical deployment. / Ph. D.

The potential of many nondestructive inspection methods and evaluation procedures are often demonstrated under rather pristine conditions. Considerable difficulty may result from attempting to apply such methodologies to actual components. This is further complicated if the material is inhomogeneous and anisotropic. Nevertheless with the use of advanced materials in complex geometry critical components, it is necessary to develop methods for assuring their quality during or upon completion of manufacture. This presentation will examine the challenges associated with this task and discuss the feasibility of the acousto-ultrasonic material characterization method for meeting these challenges. Particular attention will be given to implementing this methodology in an automated, cost effective manner. Work that is in progress involving laser in/out ultrasonic generation and detection technology, along with robotic sensor manipulation, and advanced computerized data acquisition and analysis will be discussed. / Master of Science

The surface chemical properties of Sheet Molded Composite (SMC) materials, formulated to contain polyester binder, calcium carbonate filler, and glass fibers, have been investigated with emphasis on how the surface properties are related to adhesion. These surface properties, relative elemental concentratibns and their chemical states, were studied using x-ray photoelectron spectroscopy (XPS) before and after a series of surface treatments as well as after mechanical lap shear fracture tests for SMCs bonded with urethane adhesive. Surface chemical functionalities, including -C-0·R(X)/-NCOZR and -C=0 (ester carbonyl) groups, were found to promote good adhesion using urethane adhesives. These chemical functionalities_were particularly enhanced after treating the SMC surface with an isocyanate primer as well as after a solvent/abrasive surface pretreatment done with a Scotch Brite abrasive pad soaked in methylene chloride and after washing the SMC surfaces with hot aqueous chemicals at high pressures. / Master of Science

During vibrothermographic experimental testing of damaged composite plates, frequency dependent heat generation phenomena were observed. Local hot spots were formed around imperfection areas especially delaminations. Heat generation was also found to relate to the crack size. In order to explain the above observed phenomena, the dynamic behavior of undamaged and damaged composite plates was studied over a broad frequency range. The analysis was carried out using the finite element method based on the concepts of the three dimensional theory of anisotropic elasticity. Delaminations were modeled, and the local crack resonance’ was justified. Two NDE methods namely, Vibrothermography and SPATE were used to verify the numerical predictions. Experiments performed for both undamaged and damaged specimens, and good correlation between theory and testing was achieved. / Master of Science

Findings made through the indentation testing of composites are presented in this thesis. The concept was developed as an attempt to evaluate the interfacial shear strength at a mesolevel, possibly overcoming the deficiencies of present techniques. Vickers Microhardness Testing and Continuous Ball Indentation Testing (CBIT) of composite materials provided data for assessing the sensitivity of indentation techniques to interfacial characteristics and engineering properties. Both methods proved capable of discerning the level of fiber-matrix adhesion. The CBIT presented the greatest potential for making quantitative measures of interfacial shear strength. A unique micromechanics model of the contact situation predicted failure events and trends consistent with the observed data from the CBIT. The present elastic model predicted an interfacial shear strength slightly higher than those reported in the literature. However, the interface strength obtained through the CBIT provides more of an engineering assessment of the interfacial quality when compared to other techniques. Both experimental and analytical results suggest that indentation testing of composites is most sensitive to shearing characteristics of the system. Vickers and ball penetration results displayed some correlation to global laminate properties. Vickers hardness shows a close relationship to IITRI compression strength only when fiber compressive failure is observed in the laminate test. The CBIT provides the best opportunity for exploring fiber composite stress-strain information. / Master of Science

A study of the literature pertaining to test methods for advanced composite materials has been carried out. Several test methods were discussed and compared for each of three areas of interest. These areas were uniaxial tension, uniaxial compression and in-plane shear. Test methods were selected for tension, compression and shear and guidelines set for the entry of material property data into a comprehensive mechanical property database being undertaken by Virginia Tech's Center for Composite Materials and Structures (CCMS). According to the findings, recommendations for future work were made. / Master of Science

The resin transfer molding (RTM) process has been identified as a cost-effective fabrication technique for producing composite materials from geometrically complex reinforcements. Processing models can be used to determine the temperature and pressure cycles which will produce a finished part with the best properties in the shortest time. This work involved the development and verification of a processing model for RTM. The processing model is based on the assumption that infiltration can be described as flow through a porous medium. Flow through porous media, as governed by D’Arcy’s law, depends on the viscosity of the fluid and the microstructure of the interconnected pores. Infiltration by thermosetting resin systems is assumed to behave as a Newtonian fluid with a time and temperature dependent viscosity. The kinetics of the resin can be described by mathematical expressions determined from standard thermal analysis techniques. The reinforcement is assumed to be a homogenous, anisotropic material which exhibits strain stiffening, hysteresis and plastic deformation. D’Arcy’s law describes the porous material in terms of the material permeability. Kozeny-Carman’s relationship is used to relate the porosity to the permeability. Solution of D’Arcy’s law is accomplished in a quasi-steady state manner by an evolving mesh finite element technique. After infiltration is completed, the model continues to predict the temperature, degree of cure and viscosity of the resin. The equations governing the unsteady heat transfer are solved with an existing cure model by the finite difference method. Results of the processing model include estimates of infiltration, gel and cure times as well as the cured thickness and fiber volume fraction. Test laminates were fabricated, mechanically tested, and compared to prepregged laminate results. Construction of one of the test laminates was simulated with the processing model to verify the accuracy of the simulation. / Master of Science

The significance of the role of advanced composite materials in many engineering applications is increasing.Environmental effects on the material properties of composites is an important aspect of design considerations . To determine the long term influence of the individual contributions of time, temperature, and humidity on composite materials would require extensive numbers of specimens an:f tests. In order to isolate significant variables experiments have been designed for maximum utilization of specimens. Due to wide variation in the information obtained from experimentation, a statistical analysis of the data was conducted. Multiple regression techniques were employed and the significance of the individual variables was tested. This experimental program resulted in the development of response surfaces for the complex moduli of composite materials. / Master of Science

This investigation concerns the frequency dependent heat generation behavior and the heat generation mechanisms for the thermal patterns of delamination in fiber reinforced composites during a vibrothermographic test, which uses real time thermography as a nondestructive evaluation of a structure or a component excited with mechanical vibration. A local resonance model was proposed in the past to describe the frequency dependent heat generation behavior during a vibrothermographic test, and this model was used as a basis for writing software for calculating the natural frequencies of a plate with the size of delamination. Vibrothermographic tests were performed on three glass-epoxy panels that each contained four different sized simulated delaminations. Comparison between the observed vibrothermal peak frequencies and the natural frequencies predicted by the local resonance model, and investigations of the thermoelastic emission field in the delamination region using SPATE, were made to determine the validity of the local resonance model. A significant conclusion of the results is that the local resonance is indeed the mechanics model for the frequency dependent heat generation behavior. A careful measurement of the degree of heating of both sides of [0₅] glass-epoxy panel with delaminations on the 2-3 ply interface, and comparison between the predicted heat patterns generated from a finite difference heat transfer program and observed heat patterns, was made to identify the heat generation mechanism. The results show that the majority of heat generation during vibrothermographic testing results from higher stresses or strains due to local resonance. The heat generation was affected by the combination of the principal strains and shear strain for the lower modes of resonant vibration, and was dominated by the shear strain for the higher modes of resonant vibration. Impact damaged graphite-epoxy panels were also inspected constituting an application of vibrothermography on real damaged components. The degree of heating of the damage were measured through a frequency range, and the damage severity was inspected by ultrasonic C-scan and edge replication. From comparison of two plots of the degree of heating versus exciting frequency, either the area under the curve or the number of vibrothermal peak frequencies, the severity of the damage can be qualitatively identified. / Ph. D.