Suchergebnisse

Detonation Shock Dynamics (DSD), involved in an underwater explosion scenario is numerically simulated by an in-house computer code, 'DSSDYN'. The simulation is based on Chapman-Jouguet (CJ) theory, formulated in Arbitrary Lagrangian Eulerian frame work. Specifically, the propagation of detonation front is simulated with'Burn Fraction Model'. The classical burn fraction model is improved for achieving better computational efficiency. The simulation capability of DSS-DYN is demonstrated through a case study on explosion of PETN charge under the deep-water medium.Through this study, the salient features of DSD with better insight have been brought out. Besides, the physical parameters, such as work potential of PETN, are predicted efficiently.The apportionment of energy distributions indicates that about 70 % of chemical energy of explosive is transmitted to the surrounding water that is the major contribution of damage potential of the explosive. The predictions of peak velocity and peak pressure values by DSS-DYN and LS-DYNA show satisfactory comparison.DSS-DYN consumes lesser computational time (~1h), compared to LS-DYNA (~3h).

Ceramic materials used in mechanical applications show variations in their properties due to the difference in the presence of cracks and various defects. The micro-crack length, orientation, geometry and wing crack formation and propagation within the ceramic material define the strength of the ceramic material. In this study, a micro-mechanics-based model that accounts for micro-cracks is developed. Unlike other micromechanics-based models, the current model defines failure based on entropy. Entropy generated with various micro-crack lengths, orientations and wing crack extensions is calculated using the energy approach.The Unified Mechanics Theory (UMT) is used to define the damage in the ceramic material, which can include all possible failure mechanisms. A representative volume element (RVE) with a pre-existing flaw is simulated to generate stress-strain curves. The effect of different initial crack lengths and orientations on alumina peak strength is also investigated.

Ballistic impact of a steel projectile on armour steel plate is examined by numerical simulations using 3-D nonlinear dynamic explicit finite element code ANSYS LS-DYNA. Simulations are attempted using a simple strain rate dependent plasticity model that can capture large strain, strain rate hardening and fracture encountered at high velocity ballistic impacts. Initial simulations are carried out for a cylindrical bullet with a semi-spherical nose shape impacting a military vehicle door at two different velocities as a test problem. This is then extended to simulate a real problem of armour piercing shot impact on a thick armour steel plate at ordnance velocity regime. The former is compared with results reported in published literature while the latter is assessed with the experimental findings. The deformation pattern generated in the deformed armour plate, residual projectile velocity and displacement of the projectile are taken as the necessary parameters for evaluating the results of simulation. The study presented in this paper demonstrates the effectiveness of the adopted simple plasticity model to simulate a highly nonlinear phenomenon to reasonably predict the physically measurable impact parameters.Defence Science Journal, Vol. 64, No. 1, January 2014, DOI:10.14429/dsj.64.4521

Airframe construction in conical form is the most desired shape of flight hardware due to their low drag profileand are located at the fore-end region of flight vehicles encountering high drag loads. Owing to their tailoringcapability, materials with orthotropic mechanical properties are preferred choice. Delamination defects formed inthem while manufacturing or when subjected to loads would unfavorably influence the mechanical performanceof the orthotropic airframe. In the current work, FE simulation of delamination which is embedded in orthotropiccone shaped shells under external pressure load is performed as per the method cited in published literature. A layer wise element based on shell theory has been used and the effect of delamination size and its through the thickness position on the mechanical performance of the cone shaped shell is investigated. Circumferential and rectangular shapes of defects have been simulated. The investigation is performed for metal and composite materials with 3 types of stacking sequences generally used in practical designs. Verification of the procedure is carried out by equating with the procedure cited in published studies on shells of thin orthotropic cylinders. The eigen value of the first mode is taken as the critical buckling factor under external pressure. The buckling factor of the delaminated cone is normalized with the buckling factor of the ideal cone. The normalized buckling factor is showed graphically with the normalised defect size. Global, as well as local buckling and also symmetric as well as asymmetric buckling shapes, are observed in the results of the simulation. Shift from global mode to local mode of buckling is also observed in certain cases. Drastic reduction in buckling capability with the local mode is observed when the defect location is close to the surface and more prominent for an outer surface case.

Unified Mechanics Theory's (UMT) entropy-based damage parameter, also known as the "Thermodynamic State Index" has been proven to be consistent and useful in predicting the fatigue life of different metal alloys. In recent times, studies have also demonstrated its applicability towards creep damage in nickel-based superalloys under a limited set of conditions. However, the usefulness of the "Thermodynamic State Index" in estimating damage at different temperatures, and creep loads for different metal alloys has not been evaluated yet. In this paper, creep in INCONEL 600 alloy is modeled using Norton's creep law modified with entropy-based damage (Thermodynamic State Index). The model is calibrated to predict both damage and creep strains for any given input of stress, temperature, and time. The available database on INCONEL 600 is used in parts to both calibrate and validate the prescribed model. The damage evolution for different cases is compared and imminent conclusions are drawn.

In order to simulate and predict material's real-time responses for a component under complex mechanical and thermal loads, continuum damage mechanics (CDM) is employed. However, majority of the models found in the literature are phenomenological and primarily based on curve fitting, which offer limited understanding of the underlying physics of the problem. A few physics-based models have been developed that provide greater insights. Unified mechanics theory (UMT) is one such approach that captures entropy generation due to various dissipative mechanism which aims to explain the physics of the problem. During hold time in strain-controlled creep-fatigue interaction loading, stress relaxation is observed. This study attempts to capture stress relaxation response due to creep-fatigue interaction of nickel-based superalloys using UMT, which is regarded as a more scientific method than simply fitting curves. The evolution of creep strain energy with hold time is used to understand how material ages over time due to stress relaxation during creep-fatigue interaction loading.