Instabilities in Novel Alloys

Numerous applications demand high strength steels with high flow stability and uniform ductility in torsion and shear. Blast and fragment protection in military and civilian applications, e.g., transportation, requires maintaining the integrity of structures while simultaneously minimizing their weight.

In the investigation of impulsive loads, it is important to monitor and characterize dynamic shear bands (DSB) resulting from damage-induced or thermo-mechanical instabilities. These appear at high strain rates after an initial uniform plastic deformation. The formation of DSB represents a critical failure mode for many structural metals under dynamic loading conditions, as a result of shear localization, and should be avoided or delayed to improve the integrity of structures subjected to extreme loadings. One example, the “plugging” mode observed in ballistic penetration, of relevance to fragment protection, is shown to operate by plastic instability in a local stress state near pure shear.

To characterize the shear behavior and instabilities of materials, we conduct high strain rate torsion experiments with a Kolsky bar apparatus which allows strain rates up to 2000s-1. Novel steel alloys designed by the Olson group at Northwestern University are being investigated. The specimen surface is monitored during the experiment by means of a high speed camera and a long distance microscope to assess localization and shear banding onset and evolution. Post mortem analysis is conducted to identify failure mechanisms leading to fracture, such as void growth or intergranular fracture. These fracture signatures are then correlated with the material microstructure and chemistry. One of the aims of this study is to provide insights to material designers to achieve materials with superior flow stability, uniform ductility and delayed instabilities.

Figure 1: Dynamic shear band on a martensitic high strength steel tested in torsion with a Kolsky bar apparatus. Void growth in shear triggers the instability.

A critical piece of information to assess the deformation behavior of sandwich structure is the dynamic collapse, through compressive instabilities, of the core materials in uniaxial compression and shear at various strain rates. We have investigated the compressive behavior of various metallic cellular core materials, such as open cell aluminum alloy foams, stainless steel textile cores, stainless steel tetrahedral trusses, and pyramidal trusses, at three different strain rate regimes. A sub-miniature loading frame for quasi-static loading, a Kolsky bar for strain rates up to 100-700 s-1, and a light gas gun for high strain rate up to 104 s-1 were employed. For all the tests, real time imaging of the specimen allows the determination of failure and deformation modes through digital image correlation. A transition in failure mode was identified when deformation rates in excess of 1000 s-1 were imposed on the specimens.

Figure 2: Quasi static and dynamic collapse of a cellular core material.







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