Browsing by Author "Czarnek, R."
Now showing 1 - 6 of 6
Results Per Page
Sort Options
- Experimental method of analyzing stress intensity factors and singularity order in rocket motor geometryChe-Way, Chang (Virginia Tech, 1990)A series of frozen stress experiments were conducted on surface flaws of varying aspect ratios in pressurized cylinder with star-shaped cutout in order to study the stress intensity factor distribution along the flaw border and to estimate the loss of the inverse square root singularity when the crack border intersects the inner star surface at right angles. By applying a refined optical method, the photoelastic data are converted into classical stress intensity factors resulting from the three dimensional stress state existing at the inner surface and compared with a numerical analysis to indicate the nearly uniform distribution of the stress intensity factor along the crack border. Based upon this result a two dimensional weight function approach is demonstrated to yield accurate values of the maximum stress intensity factor for the motor grain test geometry.
- An experimental study of frictional phenomena around the pin joints of plates using moire interferometryJoh, Duksung (Virginia Polytechnic Institute and State University, 1986)Although contact problems with friction have received considerable attention in recent years, analytical as well as experimental limitations have tended to obscure some of their essential features. All the experimental techniques employed in the past lacked either required sensitivity or adequate spatial resolution for local measurements of deformation near the contact surface. Further, most techniques also did not allow the use of prototype material which is crucial for investigation of contact stress problems with friction. In the present study, a relatively new experimental method, which has been developed at VPI & SU, is employed: high-sensitivity moiré interferometry. Using a clearance pin-joint model made of prototype structural material, Aluminum 7071-T6, studies on frictional phenomena between the pin and plate are conducted to provide a comprehensive treatment of the following subjects: slip-stick phenomena, variation of contact zone, distribution of frictional force at the contact region, effects of frictional shear stress on stress concentration, and identification of slip amplitude. Experimental techniques and algorithms of analysis necessary for the research are further developed. The results showed a strong influence of friction, including significant differences in the load-increasing and load-decreasing phases.
- Integration and processing of high-resolution moiré-interferometry dataLin, Shih-Yung (Virginia Tech, 1992-05-05)A new hybrid method combining moire interferometry, high resolution data-reduction technique, two-dimensional datasmoothing method, and Finite Element Method (FEM) has been successfully developed. This hybrid method has been applied to residual strain analyses of composite panels, strain concentrations around optical fibers embedded in composites, and cruciform composite shear test. This hybrid method allows moire data to be collected with higher precision and accuracy by digitizing overexposed moire patterns (U & V fields) with appropriate carrier fringes. The resolution of the data is ± 20 nm. The data extracted from the moire patterns are interfaced to an FEM package through an automatic mesh generator. This mesh generator produces a nonuniform FEM mesh by connecting the digitized data points into triangles. The mesh, which uses digitized displacement data as boundary conditions, is then fed to and processed by a commercial FEM package. Due to the natural scatter of the displacement data digitized from moire patterns, the accuracy of strain values is significantly affected. A modified finite-element model with linear spring elements is introduced so data-smoothing can be done easily in two dimensional space. The results of the data smoothing are controlled by limiting the stretch of those springs to be less than the resolution of the experimental method. With the full-field hybrid method, the strain contours from moire interferometry can be easily obtained with good accuracy. If the properties of the material are known, the stress patterns can also be obtained. In addition, this method can be used to analyze any two-dimensional displacement data, including the grid method and holography.
- New in-plane and interlaminar shear test methods for fiber reinforced compositesIfju, Peter G. (Virginia Tech, 1989-06-05)A new compact double-notched specimen for in-plane shear testing of isotropic, anisotropic and composite materials was developed. A preliminary test of the specimen was performed on a cross-ply laminate. High sensitivity moire interferometry was used to determine the shear and normal strains on the entire face of the specimen. The results of the test were compared to those of the Iosipescu specimen on the same material. The compact specimen produced a more uniform and more pure shear distribution than the Iosipescu specimen. Stiffness measurements of the composite material were made. A detailed investigation of the specimen was performed and it revealed important attributes and deficiencies of the specimen. Recommendations for future work are cited. The interlaminar shear response of a cross-ply composite was investigated. Shear strains on a ply-by-ply basis were measured using moire interferomety. Qualitative and quantitative information of the interlaminar shear characteristics of the material were obtained. The interlaminar shear modulus of individual plies and the effective modulus for the laminate were determined. Variations of nominally equal plies were observed.
- Real-Time Optical Differentiation For Moire InterferometryPatorski, K.; Post, Daniel; Czarnek, R.; Guo, Y. F. (Optical Society of America, 1987)Real-time methods for differentiation of in-plane displacement fields produced by the moire interferometry technique are introduced. Two approaches are developed: (1) optical shearing of displacement patterns and (2) superposition of two lateral shear interferograms of wavefronts from 2 diffraction orders of the specimen grating. Coherence problems are circumvented by polarization effects. In both cases additive-type moire fringes give the map of displacement derivatives of the object under load. The issue of carrier patterns and extraneous fractional fringe order is clarified. Experimental verification of the principle is presented.
- A study of near tip phenomena for cracks in a particulate compositeRezvani, Mohamad A. (Virginia Polytechnic Institute and State University, 1989)An experimental investigation using grids with a frequency of 125 lines/in. (5 lines/mm) was performed on inert propellant and pure binder at two different global head rates of 0.1 in./min (2.5 mm/min) and 1.0 in./min (25.4 mm/min). From the extracted data, displacements, strains, and dominant eigenvalue for displacement were calculated. An idealized model was used to explain the high strain zone ahead of inert propellant that caused severe blunting at the crack tip. Using the available algorithms and three dimensional photoelasticity, the dominant stress singularity order values were calculated in a four point single edged cracked bend specimen with both straight front and thumbnailed cracks. The free surface values are the same as for the inert propellant and in good agreement with analytical values. A boundary layer is observed in the singularity order which extends towards the mid-plane of the specimen. This region is about twenty percent of the distance from the free surface to mid-depth of the fractured body. The slow and fast head rates alter the global behavior of the specimen as well as the density of the displacement and strain contours. However, the near tip mechanisms are not altered.