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Experiments were conducted on smooth specimens to study the closure behavior of short cracks at high cyclic strains under completely reversed cycling. Testing procedures and methodology, and closure measurement techniques, are described in detail. The strain levels chosen for the study cover from predominantly elastic to grossly plastic strains. Crack closure measurements were made at different crack lengths. The study reveals that, at high strains, cracks close only as the lowest stress level in the cycle is approached. The crack opening was observed to occur in the compressive part of the loading cycle. The applied stress needed to open a short crack, under high strain was found to be less than for cracks under small scale yielding. For increased plastic deformations, the value of σ_op/σ_max is observed to decrease and approaches the value of R. Comparison of the experimental results with existing analysis has been made and indicates the limitations of the small scale yielding approach where gross plastic deformation behavior occurs. / M.S.

An experimental program is developed to investigate the three dimensional nature of the stress field surrounding the border of semi-elliptical surface flaws, particularly the singularity exponent. Stress freezing photoelasticity is employed to generate experimental data from nearly incompressible, elastic material. The technique of optical fringe multiplication is utilized to collect data from thin, closely spaced photoelastic slices. A new quasi-linear algorithm for data analysis is developed and verified. The algorithm is implemented using the interactive and graphics capabilities of a microcomputer and digitizing tablet, saving time and reducing errors in photoelastic data analysis. By utilizing CRT graphics, the measurement zone producing the most consistent results is delineated. Results obtained from a series of tests on both surface flaws and straight-front cracks bracket analytical values of the singularity exponent at the flaw border-free surface intersection. Suggestions to decrease variance in the results and possibly cause the results for all tests to coalesce to the analytical value(s) at the free surface are presented. An algebraic formula is developed to account for the singularity exponent variation (3-D effect) by adjusting the magnitude of the classical LEFM mode I stress intensity factor, K_I. A discussion of the need to recognize and account for these effects in high-tech materials is also included. / M.S.

Ph. D.

Polymeric methylenebis(phenylisocyanate) (pMDI) has increasingly been used in the wood particulate composite industry. Wood composites, especially oriented strand board (OSB) are made with many variations of wood species. Little research has been done to investigate how pMDI adhesion has been affected by species. The present is divided into two parts. First, mode I fracture mechanics and surface free energy analysis was performed to investigate differences in adhesion between southern yellow pine and yellow-poplar bonded with pMDI. Secondly, an improvement in the synthesis of 13C, 15N labeled pMDI is discussed. Mode I fracture results show that pMDI adhesion was affected differently by southern yellow pine compared yellow poplar. The shear energy release rate was significantly higher in pine/pMDI composites than in yellow poplar/pMDI composites. The total surface energy of southern yellow pine was shown to be significantly greater than yellow poplar. The free energy of adhesion (DG) of the pine/pMDI and the poplar/pMDI was investigated. The DG indicated that the pine/pMDI system would take more energy to separate compared to the poplar/pMDI system. Lastly, a double-labeled 13C, 15N pMDI adhesive was successfully synthesized to produce Solid-State NMR composites. / Master of Science