Nadir Yilmaz

The Effects of Specimen Geometry on the Plastic Deformation of AA 2219-T8 Aluminum Alloy Under Dynamic Impact Loading

The effects of specimen geometry on shear strain localization in AA 2219-T8 aluminum alloy under dynamic impact loading were investigated. The alloy was machined into cylindrical, cuboidal and conical (frustum) test specimens. Both deformed and transformed adiabatic shear bands developed in the alloy during the impact loading. The critical strain rate for formation of the deformed band was determined to be 2500xa0s−1 irrespective of the specimen geometry. The critical strain rate required for formation of transformed band is higher than 3000xa0s−1 depending on the specimen geometry. The critical strain rate for formation of transformed bands is lowest (3000xa0s−1) in the Ø5xa0mmxa0×xa05xa0mm cylindrical specimens and highest (>xa06000xa0s−1) in the conical specimens. The cylindrical specimens showed the greatest tendency to form transformed bands, whereas the conical specimen showed the least tendency. The shape of the shear bands on the impacted plane was also observed to be dependent on the specimen geometry. Whereas the shear bands on the compression plane of the conical specimens formed elongated cycles, two elliptical shaped shear bands facing each other were observed on the cylindrical specimens. Two parallel shear bands were observed on the compression planes of the cuboidal specimens. The dynamic stress–strain curves vary slightly with the specimen geometry. The cuboidal specimens exhibit higher tendency for strain hardening and higher maximum flow stress than the other specimens. The microstructure evolution leading to the formation of transformed bands is also discussed in this paper.

Mechanical Properties of Ultrafine Grain 2519 Aluminum Alloy

The effect of percentage thickness reduction and annealing time on the mechanical properties of cryo-rolled AA 2519 aluminum (Al) alloy were examined. Tensile tests was performed on samples in the longitudinal, transverse and at 45° to the rolling direction. The mechanical properties such as the Yield Strength (YS) and the Ultimate Tensile Strength (UTS) were observed to improve when compared to as-received sample of the 2519 alloy. This is in agreement with the Hall-Petch relationship. The highest variations in these properties were observed in the longitudinal direction, followed by the 45° and the lowest values were obtained in the transverse direction. However, the difference between the mechanical properties in the various directions decreased with an increase in annealing time showing homogeneous distribution of the fine particles.

A Prediction of Aerodynamics of Arbitrary Shape Non-fragmenting Space Debris During Decay Without Ablation

Along with many technological and economic benefits brought by the space exploration to the humankind, there were damaging impacts of these achievements to the space environment. As a result of the activities in space, an enormous amount of uncontrolled space objects have been left in Earth’s orbit which poses a serious endangerment to the sustainability of outer space. The objective of this research was to develop an engineering method to predict an aerodynamics of arbitrary shape non-fragmenting space debris without ablation. Using a complex variable method (“linearization of single-bonded area”), a universal formula for velocity of arbitrary shape fragments was derived. This technique allows describing the space fragments (debris) of various shapes, sizes, and masses.

Dynamic Behavior of AA2519-T8 Aluminum Alloy Under High Strain Rate Loading in Compression

In this study, the effects of strain rate on the dynamic behavior, microstructure evolution and hence, failure of the AA2519-T8 aluminum alloy were investigated under compression at strain rates ranging from 1000 to 3500xa0s−1. Cylindrical specimens of dimensions 3.3xa0mmu2009×u20093.3xa0mm (L/Du2009=u20091) were tested using the split-Hopkinson pressure bar integrated with a digital image correlation system. The microstructure of the alloy was assessed using optical and scanning electron microscopes. Results showed that the dynamic yield strength of the alloy is strain rate dependent, with the maximum yield strength attained by the material being 500xa0MPa. The peak flow stress of 562xa0MPa was attained by the material at 3500xa0s−1. The alloy also showed a significant rate of strain hardening that is typical of other Al–Cu alloys; the rate of strain hardening, however, decreased with increase in strain rate. It was determined that the strain rate sensitivity coefficient of the alloy within the range of high strain rates used in this study is approximately 0.05xa0at 0.12 plastic strain; a more significant value than what was reported in literature under quasi-static loading. Micrographs obtained showed potential sites for the evolution of adiabatic shear band at 3500xa0s−1, with a characteristic circular-shaped surface profile comprising partially dissolved second phase particles in the continuous phase across the incident plane of the deformed specimen. The regions surrounding the site showed little or no change in the size of particles. However, the constituent coarse particles were observed as agglomerations of fractured pieces, thus having a shape factor different from those contained in the as-received alloy. Since the investigated alloy is a choice material for military application where it can be exposed to massive deformation at high strain rates, this study provides information on its microstructural and mechanical responses to such extreme loading condition.