A summarizing description of a statistical modeling method partly based on earlier publications is given to predict the total loading and breaking process of fiber bundles generated by tensile tests. This method uses some types of idealized fiber bundles, the so-called fiber bundle cells (basic types: E, EH, ES, and ET) to model the structure of real fibrous structures. Using one of these bundle cells or a composite bundle made of some bundle cells connected in parallel, the expected value and standard deviation of the whole damage process of this bundle can be calculated up to the breakage of the last intact fiber during a mechanical test. As a new application the fracture process of unidirectional composite beam is modeled during the 3P bending test considering the beam is built up of elementary embeddedfiber layers considered as E-type fiber bundle cells in the first step. Formulae for calculating the expected value and standard deviation processes of the bent specimen are elaborated assuming that the fiber breakages determined the failure of layers.
Unidirectional carbon fiber-epoxy composite specimens were produced and three-point bending tests were carried out at spans of 10 and 80 mm. During the tests images were taken using a CCD camera system and the type of damage was studied. The fracture process of unidirectional composite beams were modeled during the 3P bending test considering the beam built up of elementary embedded-fiber layers considered as E-type fiber bundle cells using the formulae developed in the first part of this paper. The fracture processes obtained by measurements were compared to the modeled expected value processes completed with deviation field or confidence interval.
Foreign object damage (FOD) behavior of AS800 silicon nitride was determined using four different projectile materials at ambient temperature. The target test specimens rigidly supported were impacted at their centers by spherical projectiles with a diameter of 1.59 mm. Four different types of projectiles were used including hardened steel balls, annealed steel balls, silicon nitride balls, and brass balls. Post-impact strength of each target specimen impacted was determined as a function of impact velocity to better understand the severity of local impact damage. The critical impact velocity where target specimens fail upon impact was highest with brass balls, lowest with ceramic ball, and intermediate with annealed and hardened steel balls. Degree of strength degradation upon impact followed the same order as in the critical impact velocity with respect to projectile materials. For steel balls, hardened projectiles yielded more significant impact damage than annealed counterparts. The most important material parameter affecting FOD was identified as hardness of projectiles and was correlated in terms of critical impact velocity, impact deformation, and impact load.
Foreign object damage (FOD) phenomena of two gas-turbine grade silicon nitrides (AS800 and SN282) were assessed at ambient temperature applying impact velocities from 20 to 300 m/s using 1.59-mm diameter hardened steel ball projectiles. Targets in a flexural configuration with two different sizes (thicknesses) of 1 and 2 mm were ballistic-impacted under a fully supported condition. The severity of impact damage, as well as the degree of post-impact strength degradation, increased with increasing impact velocity, increased with decreasing target size, and was greater in SN282 than in AS800 silicon nitride. The critical impact velocity where targets fractured catastrophically decreased with decreasing target size and was lower in SN282 than in AS800. Overall, FOD by steel projectiles was significantly less than that by silicon-nitride ceramic counterparts, due to much decreased Hertzian contact stresses. A correlation of backside cracking velocity versus target size was made based on a simplified elastic foundation analysis.
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