C Lakshmana Rao

A simple two-dimensional Unit cell (UC) with three different mesoscopic phases may be geometrically modeled as a combination of two concentric circles or squares embedded in a square. In this paper, a two-dimensional three-phase UC with a similar design is developed and utilized for modeling the full deformation and failure response of cement based quasi-brittle cementitious composites under compression. It was identified from the numerical investigation that the three-phase design greatly improved the prediction capability of the UC in suitably capturing the material nonlinearity observed experimentally.

The current trend in automotive design is to optimize components for weight. To achieve this, automotive designers need to have complete understanding of various stresses prevalent in different areas of the component. The chassis frame assembly of a heavy truck used for long distance goods hauling application is chosen for this investigation and dynamic stress-strain response of the component due to braking and cornering maneuvers are experimentally measured and reported. A quasi-static approach that approximates the dynamic maneuvers into number of small processes having static equilibriums is followed to carry out the numerical simulation, approximating the dynamic behavior of frame rail assembly during cornering and braking. With the help of commercial finite element package ANSYS, the quasi-static numerical simulations are carried out and compared with experimental results. This study helps in understanding prevailing stresses in truck frame rails especially during cornering and braking maneuvers and brings out all geometric locations that may be potential failure initiation locations. This study makes a case for further investigation on the effects of residual and assembly stresses on frame rails.

The void growth model parameter for Modified 9Cr-1Mo steel has been identified based U-notched tensile tests, scanning electron microscopy study on fracture surface and parallel 3D FEM simulations. Damage Index (DI) symptomatic of Rice-Tracy VGM parameter at a large number of sampling points in micro void coalescence zone is calculated for increase in applied loads. The calculated DI has been calibrated with experiments on specimens with through wall central notches oriented at different angle to loading axis.

In this study, we investigate the monotonic deformational response of Polyvinylidene fluoride (PVDF). Monotonic uniaxial and biaxial experiments are conducted and deformations are monitored using non-contact speckle monitoring method. The mechanical response of PVDF is observed to exhibit finite strains which are also anisotropic in nature. A hyper-elastic finite deformation transversely isotropic model is used to model the biaxial response of PVDF. Experimental data was shown to fit well with the proposed model. The model parameters obtained from biaxial tests were used to capture uniaxial response in two orthogonal directions and the ability of the model to predict any arbitrary mechanical response is assessed.

Modal parameters namely frequency, mode shape and generalized mass are required to determine the forced response of structures using mode summation method. The structure may be thin shell element of aircraft, rocket, submarine, pipeline and pressure vessel. The initial estimates are generally carried out using Euler-Bernoulli (EB) beam theory or classical beam theory since it provides reasonable engineering solution to the slender beams only as it introduces significant error for the calculation of modal parameters for short beams and also for higher modes. The predictions are improved by using Timoshenko Beam (TB) model which is based on First Order Shear Deformation Theory (FSDT). The accurate prediction of model parameters for thin isotropic shells shall be carried out by using shell equations based on Fliigge formulations and Finite Element Analysis (FEA). The objective of the present paper is to compare the modal parameters of the first three beam type modes (n= 1) of thin isotropic shells using both beam and shell models considering the slenderness ratio for various boundary conditions. The results shall be useful for the selection of suitable model for the estimation of modal parameters of beam type modes for shell structures. The errors in modal parameters predicted by EB beam theory for the general assumption of slender beam (L/D = 10) is also reported.

In this paper, compression behavior and energy absorption of aluminum alloy AA7005 tubes in T6 with and without holes are investigated numerically. True stress-plastic strain curves from the tensile test are used in the static simulation of AA7005 tubes. The deformed modes from the numerical simulation are compared between tube with and without holes. Energy absorption values and peak loads with and without holes are compared to develop the understanding on the size of hole to be used in tube for energy absorption control. © (2013) Trans Tech Publications, Switzerland.

Buckling control of space structures using piezoelectric actuators is an emerging area of research. In particular, cylindrical shells present several challenges as they exhibit multiple buckling modes. This work focuses on placement of ring actuators on cylindrical shells exhibiting axisymmetric buckling. A new method based on the characteristic wavelength of the cylindrical shell is proposed for actuator placement. It is shown by means of numerical studies that passive control of these shells shows a distinct advantage over the conventional actuator placement. It is possible to obtain a stiffer load-axial shortening response as well as a peak load enhancement of the shell using the present approach. The results obtained give important insights into the actuator placement problem for cylindrical shells undergoing axisymmetric buckling.

Distress due to water induced damage and bleeding accelerates the failure of an asphalt concrete pavement due to damage mechanisms such as fatigue cracking, rutting, stripping etc. One of the important variables used while modeling water induced damage for asphalt concrete pavement is permeability. This study uses the framework developed to model the movement of air voids in asphalt concrete in a previous study by the authors. The assumption of constant permeability in the theory of consolidation by Terzaghi is relaxed and a linear relation is proposed between permeability and air voids. Voids filled with asphalt (VFA) are proposed as a parameter to model bleeding of asphalt concrete, and its variation for different loading and mixture condition is studied.

Presence of delamination causes reduction in strength and stiffness of laminated composite structure and detrimental to the safety and stability of structures made of composite materials. Determination of the residual strength and stiffness with respect to a finite size of delamination would be helpful in assessment of structures fitness to usage whenever the defects are identified by non-destructive procedures. Experiments of in-plane compression testing of multidirectional laminate (150mm × 100mm × 4.4mm) with an artificial delamination (030 and 040mm) at the center are carried out. For the purpose of reference, tests of an ideal laminate with identical ply sequence and without delamination are carried out. Each experiment is performed till failure measuring strains and displacements. Simulation of the experiment are carried out by finite element analysis using cohesive zone material model assuming bonded contact with bilinear material behavior. Simulation is carried out to establish cohesive zone material parameters for further usage in analyzing real structures for the structural integrity assessment when identified with defects and testing is not probable. Significant reduction in strength is observed due to the defect. It is observed that failure load and strain at failure are decreasing from ideal to larger defect size. Strain data comparison with FEA is carried out and % error is below 10% in the case of ideal laminate whereas it is above 20% for the cases of defective laminates.

Elastomeric seals are devices that are widely used to prevent fluid leakage through the interface of mating parts. In this paper, we study the leak characteristics of a representative elastomeric seal system using an experimental setup that has been developed to measure the leak rate of gas through an elastomer-metal interface as a function of sealing load intensity and fluid pressure. These experiments are carried out on nitrile butadiene rubber (NBR), hydrogenated nitrile butadiene rubber (HNBR) and fluoro-elastomer (FKM) specimens using nitrogen gas at pressures ranging from 40 kPa to 800 kPa. The experiments revealed that the leak rate increases rapidly with gas pressure and decreases with sealing load intensity. When leak rate was plotted against normalised gas pressure, it was observed that all the data points fall reasonably on one single curve irrespective of sealing load intensity and the material. These results will be useful for further analysis in developing a mathematical model for characterising fluid leak through elastomer-metal alloy interfaces.