No description available.

No description available.

No description available.

An experimental evaluation of the mixed-mode fracture behavior of bonded composite joints is presented. Commonly used experimental techniques for characterizing the mode I, mixed-mode I/II, mode II, and mode III fracture behavior have been employed for the purpose of developing a fracture envelope to be utilized in the automotive design process. These techniques make use of such test geometries as the double cantilever beam (DCB), asymmetric double cantilever beam (ADCB), single-leg bend (SLB), end-loaded split (ELS), and split cantilever beam (SCB) specimens. Symmetric versions of the DCB, SLB, and ELS specimens produced mode mixities of 0°, 41°, and 90° respectively, while the testing of ADCB specimens allowed for mode mixities of 18°, 31°. Pronounced stick-slip behavior was observed for all specimen test geometries under both quasi-static and dynamic loading conditions. Due to the nature of the adhesive studied, a limited number of data points were obtained under mode I loading conditions. A significant increase in the number of measurable crack initiation events was observed for mixed-mode I/II loading conditions, where stick slip behavior was less pronounced. Additionally, a comparison of the measured fracture energies obtained under mixed-mode I/II loading conditions reveals that the addition of a small mode II component results in a decrease in the mode I fracture energy by roughly 50%, as the crack was driven to the interface between the adhesive layer and composite adherends. Furthermore, the propensity of debonds to propagate into the woven composite laminate adherends under mode II loading conditions limited the number of crack initiation points that could be obtained to one or two usable data points per specimen. A limited number of experimental tests using the SCB specimen for mode III fracture characterization, combined with a numerical analysis via finite element analysis, revealed a significant mode II contribution toward the specimen edges. Similarly, FE analyses on full bond width and half bond width SCB specimens was conducted, and results indicate that by inducing a bond width reduction of 50%, the mode II contribution is greatly decreased across the entire width of the specified crack front. To provide a means for comparison to results obtained using the standard DCB specimen, an alternative driven wedge test specimen geometry was analyzed, as this geometry provided a significant increase in the number of measurable data points under mode I loading conditions. A three-dimensional finite element analysis was conducted to establish ratios of simple beam theory results to those obtained via FEA, GSBT/GFEA, were of particular interest, as these ratios were used to establish correction factors corresponding to specific crack lengths to be used in correcting results obtained from an experimental study utilizing a driven wedge technique. Corrected results show good agreement with results obtained from traditional mode I double cantilever beam tests. Finally, bulk adhesive experiments were conducted on compact tension specimens to establish a correlation between adhesively bonded composite joint and bulk adhesive fracture behavior under mode I loading conditions. Measured fracture energy values were shown to gradually drop across a range of applied loading rates, similar to the rate-dependent behavior observed with both the DCB and driven wedge specimens. Application of the time-temperature superposition principle was explored to determine whether or not such techniques were suitable for predicting the fracture behavior of the adhesive studied herein. Good correlation was established between the fracture energy values measured and the value of tan d obtained from dynamic mechanical analysis tests conducted at corresponding reduced test rates. / Master of Science

This research explores what type of verbal/text survey content translates effectively between survey modes within a Computer Assisted Telephone Interview (CATI) and Computer Assisted Web Interview (CAWI) mixed-mode survey project within unified mixed-mode survey design. This phenomenon is researched through the implementation of online probes within an existing CAWI survey project for sections close to an original CATI design. This research provides insights into how respondents understand and conceptualize verbal content within a unified mixed-mode CAWI survey in today’s online survey environment. This research is timely as the penetration of mobile has increased drastically, which may have implications on CAWI/CATI mixed-mode research considering the conflicting best practices for mobile CAWI survey design and CATI survey design. The results show that respondents do not engage with introductory pages enough to reliably recall their information, even when significantly shortened and simplified. Additionally, respondents are sensitive to high amounts of text and topics within survey question spaces. This means that some aspects of CATI survey design do not translate to CAWI and may even be cause for data quality concerns. These findings highlight some key issues faced by researchers and practitioners, provide some guidance on how design choices may impact results, and presents rich opportunities for future research.

This study concerns the identification of the particular competences required by education, training and development practitioners (ETD practitioners) in the South African National Defence Force to develop suitable and appropriate career and training strategies. An applied research approach and a primarily quantitative approach were used. Questionnaires were completed by the commanding officers or the training managers, as well as the ETD practitioners at the education, training and development providers in the South African National Defence Force to determine the actual utilisation of ETD practitioners. Descriptive statistics were used to determine the roles, core competences, levels of competences and clusters of competences required by ETD practitioners in the South African National Defence Force. In addition, the actual utilisation of ETD practitioners was compared with a proposed competence profile that was based on the literature study in order to determine the competence gap that has to be addressed by means of career and training strategies. / Educational Studies / M.Ed.(Didactics)

The growth in the number of wireless devices and applications underscores the need for characterizing and mitigating interference induced problems such as distortion and blocking. A typical interference scenario involves the detection of a small amplitude signal of interest (SOI) in the presence of a large amplitude interfering signal; it is desirable to attenuate the interfering signal while preserving the integrity of SOI and an appropriate dynamic range. If the frequency of the interfering signal varies or is unknown, an adaptive notch function must be applied in order to maintain adequate attenuation. This work explores the performance space of a phase cancellation technique used in implementing the desired notch function for communication systems in the 1-3 GHz frequency range. A system level model constructed with MATLAB and related simulation results assist in building the theoretical foundation for setting performance bounds on the implemented solution and deriving hardware specifications for the RF notch subsystem devices. Simulations and measurements are presented for a Low Noise Amplifer (LNA), voltage variable attenuators, bandpass filters and phase shifters. Ultimately, full system tests provide a measure of merit for this work as well as invaluable lessons learned. The emphasis of this project is the on-wafer LNA measurements, dependence of IC system performance on mismatches and overall system performance tests. Where possible, predictions are plotted alongside measured data. The reasonable match between the two validates system and component models and more than compensates for the painstaking modeling efforts. Most importantly, using the signal to interferer ratio (SIR) as a figure of merit, experimental results demonstrate up to 58 dB of SIR improvement. This number represents a remarkable advancement in interference rejection at RF or microwave frequencies.

A new nonlinear fracture analysis framework is developed for the mode-I and II fracture response of thick-section fiber reinforced polymeric (FRP) composites. This framework employs 3D micromechanical constitutive models for the nonlinear material behavior along with cohesive elements for crack growth. Fracture tests on various cracked geometries are used to verify the prediction of the failure loads and the crack growth behavior. A commercially available pultruded E-glass/polyester and vinylester thick-section FRP composite material was used to demonstrate the proposed fracture approach along with the nonlinear constitutive modeling. A new Infra-red thermography technique is derived to measure the surface strain field near the crack tip in the linear response range. Mode I and II fracture toughness tests for pultruded composites are also examined using the eccentrically loaded, single-edge-notch tension, ESE(T), single-edge-notch tension, SEN(T), and a butterfly specimen with an Arcan-type fixture. Material nonlinearity and crack growth effects were observed during the tests and investigated using the proposed analysis framework. The effect of material orthotropy on the stress intensity factor solutions was addressed using the virtual crack closure technique. The analytic and experimental results support the use of the ESE(T) specimen for the measuring the mode-I fracture toughness and the butterfly shaped specimen for measuring the mode-II toughness. The calibrated cohesive models were able to predict the measured crack growth in both modes I and II for various crack geometries. A mixed mode failure criterion is proposed and verified with test results. Examples are presented for using this criterion and crack growth analyses. The experimental and analytical results of this study can form a foundation for using fracture-based methods for the design of structures using these materials.

科学研究費補助金 研究種目:基盤研究(B)(2) 課題番号:07455052 研究代表者:田中 啓介 研究期間:1995-1996年度

This study concerns the identification of the particular competences required by education, training and development practitioners (ETD practitioners) in the South African National Defence Force to develop suitable and appropriate career and training strategies. An applied research approach and a primarily quantitative approach were used. Questionnaires were completed by the commanding officers or the training managers, as well as the ETD practitioners at the education, training and development providers in the South African National Defence Force to determine the actual utilisation of ETD practitioners. Descriptive statistics were used to determine the roles, core competences, levels of competences and clusters of competences required by ETD practitioners in the South African National Defence Force. In addition, the actual utilisation of ETD practitioners was compared with a proposed competence profile that was based on the literature study in order to determine the competence gap that has to be addressed by means of career and training strategies. / Educational Studies / M.Ed.(Didactics)