Vasant Joshi

Constitutive Model Constants for Al7075-T651 and Al7075-T6

Aluminum 7075-T651 and 7075-T6 are characterized at quasi-static and high strain rates to determine Johnson-Cook (J-C) strength and fracture model constants. Constitutive model constants are required as input to computer codes to simulate projectile (fragment) impact or similar impact events on structural components made of these materials. Although the two tempers show similar elongation at breakage, the ultimate tensile strength of T651 temper is generally lower than the T6 temper. Johnson-Cook strength model constants (A, B, n, C, and m) for the two alloys are determined from high strain rate tension stress-strain data at room and high temperature to 250°C. The Johnson-Cook fracture model constants are determined from quasi-static and medium strain rate as well as high temperature tests on notched and smooth tension specimens. Although the J-C strength model constants are similar, the fracture model constants show wide variations. Details of the experimental method used and the results for the two alloys are presented.

RECENT DEVELOPMENTS IN SHEAR IGNITION OF EXPLOSIVES USING HYBRID DROP WEIGHT‐HOPKINSON BAR APPARATUS

The sensitivity and mechanical behavior of energetic material is highly dependent on its constituents. Cast and cast‐cured explosives have mechanical properties significantly different from metals and the assumption of isotropic behavior may not be valid beyond a finite strain. While Split Hopkinson Pressure Bar (SHPB) can be successfully used to obtain mechanical properties of these soft and compliant explosives, ignition conditions are seldom achieved in SHPB tests. If ignition occurs in very small sample at extremely high strain rates, it would be very difficult to calculate the energy and energy rate that led to successful ignition. In contrast to the SHPB test, the standard drop‐weight test to measure the sensitivity of explosives, is intended to obtain ignition at impact, but is only a go‐no go test. Due to lack of quantifiable parameters, the result from this test is not suitable for modeling, which is important in development of new explosive formulation. In order to overcome this barrier and allo...

Modeling of Bullet Penetration in Explosively Welded Composite Armor Plate

Normal impact of high‐speed armor piercing bullet on titanium‐steel composite has been investigated using smooth particle hydrodynamics (SPH) code. The objective is to understand the effects of impact during the ballistic testing of explosively welded armor plates. These plates have significant microstructural differences within the weld region, heat‐affected zone and the base metal. The variances result in substantial ductility, hardness and strength differences, important criteria in determining the failure mode, specifically whether it occurs at the joint or within the virgin base metal. Several configurations of composite plates with different material combinations were modeled. The results were used to modify the heat treatment process of explosively welded plates, making them more likely to survive impact.

High strain rate behavior of polyurea compositions

High-strain-rate response of three polyurea compositions with varying molecular weights has been investigated using a Split Hopkinson Pressure Bar arrangement equipped with aluminum bars. Three polyurea compositions were synthesized from polyamines (Versalink, Air Products) with a multi-functional isocyanate (Isonate 143L, Dow Chemical). Amines with molecular weights of 1000, 650, and a blend of 250/1000 have been used in the current investigation. These materials have been tested to strain rates of over 6000/s. High strain rate results from these tests have shown varying trends as a function of increasing strain. While higher molecular weight composition show lower yield, they do not show dominant hardening behavior at lower strain. On the other hand, the blend of 250/1000 show higher load bearing capability but lower strain hardening effects than the 600 and 1000 molecular weight amine based materials. Results indicate that the initial increase in the modulus of the blend of 250/1000 may lead to the los...

CONSTITUTIVE MODEL CONSTANTS FOR LOW CARBON STEELS FROM TENSION AND TORSION DATA

Low carbon C1010 steel is characterized under tension and torsion to determine Johnson‐Cook (J‐C) strength model constants. Constitutive model constants are required as input to computer codes to simulate projectile (fragment) impact on structural components made of this material. J‐C model constants (A, B, n, C, and m) for the alloy are determined from tension and torsion stress‐strain data. Reference tension tests are performed at a strain rate of ∼1/s at room temperature. Tests at high strain rates are performed at temperatures to 750 °C. Torsion tests at quasi‐static and high strain rates are performed at both room and high temperatures. Equivalent plastic tensile stress‐strain data are obtained from torsion data using von Mises flow rule and compared directly to measured tensile data. J‐C strength model constants are determined from these data. Similar low carbon steels (1006, 1008, and 1020) have their J‐C constants compared.

Resolving Mechanical Response of Plastic Bonded Explosives at High Strain‐Rate Using Split Hopkinson Pressure Bar

The mechanical properties of two explosives (PBXN‐110 and PBXW‐128) were determined using a split‐Hopkinson pressure bar at strain rates between 103 /s and 104 /s. The stress‐strain data for 1, 2 and 3‐wave analysis were compared to determine when stress equalization was achieved in the test samples. PBXN‐110 behaved similar to conventional Hopkinson bar samples, i.e., stress equalization was maintained for most of the loading cycle. Stress equalization was not achieved until late in the loading cycle for PBXW‐128. This behavior eventually terminates during the compression process yielding a uniform response.

A NOVEL METHOD OF RESOLVING IGNITION THRESHOLD IN STEVEN TEST USING HYBRID DROP WEIGHT‐HOPKINSON BAR

Sensitivity of energetic material is traditionally evaluated in a drop weight test by a go‐no‐go ignition condition on an unconfined sample. In contrast to this, the Steven test uses a semi‐confined energetic material to evaluate the post ignition violence. Frictional conditions can alter the results of both these tests. As a consequence, both these tests cannot be modeled accurately. Recently developed Hybrid Hopkinson Bar apparatus is well suited for modeling the impact initiation of energetic materials. In an effort to extend the capability of this apparatus, simple modification of bar geometry enables the Hybrid Hopkinson Bar apparatus to quantify ignition threshold as a subscale Steven test, and also evaluate the contribution of frictional changes leading to ignition. Comparison of a standard Hybrid Hopkinson Bar test to that of a Steven type test is being made using new diagnostic techniques to simultaneously quantify mechanical properties and ignition conditions.

Measurement of Ignition and Reaction Parameters in Non‐Ideal Energetic Materials

Two small‐scale tests were performed to measure ignition and reaction parameters in non‐ideal energetic materials. Hydrocode modeling underway will determine the effectiveness of this approach. The time to reaction and the ignition conditions are derived from the newly developed hybrid Hopkinson bar experiments, whereas the growth criteria are based on the recently developed small‐scale shock reactivity test (SSRT). The hybrid Hopkinson bar test simultaneously measures the mechanical behavior and ignition conditions of explosives. The reactivity test measures the potential of a material to be an explosive regardless of its sensitivity, thus avoiding the problem of scale, inherent in most small‐scale explosive tests.

Dynamic characterization and modeling of potting materials for electronics assemblies

Prediction of survivability of encapsulated electronic components subject to impact relies on accurate modeling, which in turn needs both static and dynamic characterization of individual electronic components and encapsulation material to generate reliable material parameters for a robust material model. Current focus is on potting materials to mitigate high rate loading on impact. In this effort, difficulty arises in capturing one of the critical features characteristic of the loading environment in a high velocity impact: multiple loading events coupled with multi-axial stress states. Hence, potting materials need to be characterized well to understand its damping capacity at different frequencies and strain rates. An encapsulation scheme to protect electronic boards consists of multiple layers of filled as well as unfilled polymeric materials like Sylgard 184 and Trigger bond Epoxy # 20-3001. A combination of experiments conducted for characterization of materials used Split Hopkinson Pressure Bar (SH...

Strain rate behavior of magnetorheological materials

Strain rate response of two Hydroxyl-terminated Polybutadiene/ Iron (HTPB/Fe) compositions under electromagnetic fields has been investigated using a Split Hopkinson Pressure bar arrangement equipped with aluminum bars. Two HTPB/Fe compositions were developed, the first without plasticizer and the second containing plasticizer. Samples were tested with and without the application of a 0.01 Tesla magnetic field. Strain gauge data taken from the Split Hopkinson Pressure Bar has been used to determine the extent of change in mechanical properties by inducing a mild electromagnetic field onto each sample. Raw data from strain gages was processed using commercial software (Signo) and Excel spreadsheet. It is of particular interest to determine whether the mechanical properties of binder systems can be manipulated by adding ferrous or Magnetostrictive particulates. Data collected from the Split Hopkinson Pressure bar indicate changes in the Mechanical Stress-Strain curves and suggest that the impedance of a binder system can be altered by means of a magnetic field.