Shear banding is a deformation phenomenon that involves intense localized shear deformation in a material. The intense shear strains signify localization of energy dissipation as deformation progresses. The eventual outcome of localized deformation is ductile rupture and material separation. For this reason, the combination of shear strain localization and eventual rupture is called shear failure. This phenomenon occurs and plays an important role in many applications. For example, shear bands are observed in ballistic impact, explosive fragmentation, high speed machining, metal forming, grinding, interfacial friction, powder compaction, granular flow, and seismic events. The formation of shear bands is a self-feeding process. Plastic dissipation generates heat which causes the material to soften as temperature increases. The thermal softening causes the material to deform at higher strain rates, resulting in further dissipation, temperature increase and thermal softening. This catastrophic cycle does not occur uniformly in the material. The result is the formation of narrow and distinct bands of high rates of shear deformation and high temperature. An important outcome of this process is the decrease of the material's stress-carrying capability, leading to precipitous drop in stress in later stages of the process. If deformation is allowed to continue, the eventual result is the ductile rupture of the material and total separation through the center of the shear band.
Constitutive and Failure Behavior of Concrete and Mortar under Impact Loading
The need to understand the dynamic behavior of concrete and mortar at high strain rates is of critical importance in a range of applications including airport runways and structures subject to blast or penetration. Under such dynamic conditions, the strain-rate dependence of material response and high levels of hydrostatic pressure cause the material behavior to be significantly different from what is observed under quasistatic conditions. This part of the research focuses on two aspects of the behavior of concrete and mortar: (1) the dynamic stress-carrying capacity at strain rates above 104 s-1 and pressure above 1 GPa and (2) the dynamic failure behavior under conditions of uniaxial strain generated by planar impact. The materials used are a G-mix concrete and a mortar having the same composition and processing condition as the concrete. The experimental analyses use plate impact and split Hopkinson bar. Normal impact experiments involving impact velocities up to 482 ms-1 are conducted.
Current research in the group focuses on the effects of microstructure in heterogeneous materials, nanoscale deformation, continuum representations of atomistic systems, and fracture. Research projects emphasize both high-performance computational modeling using finite elements and molecular dynamics as well as experimental characterization using laser interferometry and novel digital diagnostics. Using novel micromechanical models, the computations strive to outline microstructural designs that may improve the performance of composites in applications in which mechanisms such as shear banding and fracture play important roles. Another research thrust is the development of equivalent continuum (EC) representations for atomistic deformation events. This direction leads to the multiscale characterization of material behavior "from the ground up", beginning with atomistic models. The experimental part of Dr. Zhou's research uses an intermediate-to-high strain rate material test facility which includes a split Hopkinson pressure bar apparatus, a tension bar apparatus, and a combined torsion-tension/torsion-compression bar apparatus.
This facility allows material testing to be conducted for strain rates up to 106 s-1, under a range of multiaxial states of stress through normal and pressure-shear (inclined) plate impact experiments. The gas gun has an 80 mm diameter, 8 m long gun barrel connected to a 28.5 liter test chamber. A double diaphragm separates the test chamber and a large soft-recovery catcher tank. Impact velocities between 100 and 1200 m/s can be achieved and recorded. The large gun barrel diameter allows observations to be made under pure plane strain conditions for up to 15 ms. Controlled experiments are performed with impact planarities better than 50 milli-radians. The facility has state-of-the-art interferometric and stress-gauge diagnostic capabilities along with instrumentation for high-speed digital data acquisition. Time-resolved in-situ pressure measurements can be obtained using piezo-electric (PVDF) and piezo-resistive (Manganin) stress gauges. A VISAR (Velocity Interferometer System for Any Reflector) is available for free surface velocity measurements. This system is capable of making four simultaneous measurements with a sensitivity of 2 m/s and a depth of field of 12 mm. Velocities in the range of 30-50,000 m/s can be measured. Three four-channel Tektronix digital oscilloscopes (Model TDS784A) with a sampling rate of 4 Giga points per second are available for high-speed data acquisition with sub nanosecond resolutions.
The Georgia Tech Hopkinson Bar Facility has three Split Hopkinson Bar configurations: Compression Bar, Tension Bar, combined Tension-Torsion or Compression-Torsion Bar.
The compression bar has been used in studies on the dynamic behaviors of a variety of materials including structural steels, laminate composites, PZT ceramics, concrete and ceramics. It is capable of achieving strain rates in the range of 102 - 103 sec-1. Two bar diameters are used, 0.75 inch diameter and 0.5 inch diameters bars with various lengths to control the loading duration. Typical loading durations range between 15 usec - 250 usec. The bars are manufactured from Vascomax C-350 maranging steel heat-treated to a hardness of 59 on the Rockwell C scale. The uni-axial compressive yield strength of the bars is approximately 2.7 GPa.
Strain gauges with a nominal resistance of 1000 ohm are used to record the passing of the stress waves in the Hopkinson bar. The signals form the strain gauges are recorded on high speed digital oscilloscopes. The laboratory uses a Nicolet Pro 42 and a Nicolet Integra 40 high speed digital oscilloscope.The compression bar is also equipped with a recovery system that allows researches to study damage evolution under impact conditions and even allow researchers to apply a 1 cycle load on materials.
The local computer environment in the dynamic properties research group consists of Unix and NT workstations. One unix cluster is based on a central SUN Ultra-30 server. The second is served by an SGI Origin2000. In addition, a Windows NT server provides file serving capabilities for our Windows NT workstations. The workstations serve as the work environment for code development, for pre- and post-processing (graphics generation and video animation) and for access to supercomputers. The research group also has access to the following high performance computing facilities:
NPACI (National Partnership for Advanced Computational Infrastructure) and NCSA (National Computational Science Alliance) Resource through the San Diego Supercomputer Center (SDSC), allocations are for Cray T90 vectorized system and T3E parallel system from both the NCSA National Resource Allocation Committee (NRAC) and the NPACI allocation committee;
Pentium-based high performance computing facility:
Acquired through an Intel grant worth $3.7M to several Georgia Tech units including the mechanics of materials group. This facility consists of 160 quad Pentium II computers. A variety of application software and graphics packages are available, including ABAQUS, IDEAS, PATRAN, ALGOR, and TECPLOT.
The split Hopkinson bars provide material testing capabilities in compression, tension, torsion, torsion/compression and torsion/tension at strain rates of the order of 102 to 104 s-1. This facility is equiped with two Polytec OFV-3001 laser vibrometer systems and two OFV-512 optical fiber laser heads for measurement of surface velocities between 0 to 10 m/s. A set of Tektronix high speed digital oscilloscopes are available for data acquisition at sampling rates up to 50 nanoseconds per point.
The gas gun facility (a joint ME and MSE facility) allows material testing to be conducted at strain rates up to 106 s-1, under a range of multiaxial states of stress through normal and pressure-shear (inclined) plate impact. A state-of-the-art VISAR (Velocity Interferometer System for Any Reflector) for free surface velocity measurement is available. This system is capable for making four simultaneous measurements with a sensitivity of 2 m/s and a depth of field of 12 mm. Velocities in the range of 30-50,000 m/s can be measured. This facility is also equipped for internal stress measurement using PVDF stress gauges. High speed digital oscilloscopes are capable of sampling at rates of up to 4 million data points per second.