C Lakshmana Rao

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.

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.

The main objective of this study is to introduce a 'dissipationless band' to model inner hysteresis loops of response of shape memory alloys (SMAs). Dissipation that occurs when the material undergoes phase transformation is critical to the modeling of hysteretic behavior. Emphasis is placed on modeling such dissipation in the proposed methodology. Using a dissipationless virtual response of the material, a logical framework for the onset transformation under reversal of cycles is presented. Characteristics of the material transformation with reference to a dissipationless band model the true inner hysteresis loops. It is identified that this dissipationless band occurs due to the difference between the starting states of forward and reverse transformations. The construction of the generalized driving force for the transformation along with the rate of dissipation function is formulated. A numerical example is presented to highlight the qualitative prediction capabilities of the model. The example involves simulating hysteresis loops for different kinds of partial and complete loading cycles in the pseudoelastic state of the material. The predictions show that the proposed one-dimensional model is capable of representing the actual hysteresis behavior of the stabilized shape memory alloys, by effectively incorporating the dissipation effects due to the loading history. © IOP Publishing Ltd.

Asphalt concrete used in flexible highway pavements has 5-8% air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of voids concentration on the mechanical behaviour of the material. In the present study the theory of mixtures is used to model asphalt concrete. Asphalt concrete is considered to be a mixture of aggregate matrix, asphalt and air in a purely mechanical system in which the thermal effects and chemical reactions are ignored. Constitutive relation for each component of the mixture is assumed to be dependent only on the kinematical quantities associated with each component. The resulting hyperbolic conservation equations are solved by an upwind finite volume scheme coupled with an operator splitting technique for a quasi-static type of loading. The numerical scheme is used to simulate the variation of air voids content across the thickness of a typical road pavement.

It is necessary to size the cracklike defects accurately in order to extend the life of thin-walled (< 10 mm) components (such as pressure vessels) particularly for aerospace applications. This paper discusses the successful application of ray techniques to simulate the ultrasonic time-of-flight diffraction experiments for platelike structures. For the simulation, the diffraction coefficients are computed using the geometric diffraction theory. The A and B scans are simulated in near real time and the different experimental parameters can be interactively controlled due to the computational efficiency of the ray technique. The simulated results are applied to (1) defect signal identification for vertical defects, (2) inspection of inclined defects, and (3) study the effect of pulse width or probe frequency on experimental results. The simulated results are compared with laboratory scale experimental results. Copyright © 2005 by ASME.

Few investigations have been carried out with bamboo fibers despite its high strength, biodegradability, and low cost. The overall objective of this work was to investigate fiber extraction from bamboo and the use of these bamboo fibers as reinforcement in polymeric composites. A combination of chemical and mechanical methods was used for the extraction of bamboo fibers. Conventional methods of compression molding technique (CMT) and roller mill technique (RMT) were explored for the mechanical separation. Fiber population from both the techniques were characterized. Mechanical properties of the fibers also were evaluated. Bamboo fibers obtained from CMT and RMT were used to make unidirectional composites of polyester. High values of tensile strength were observed in all the composites. The predominant mode of failure for the composite was shown to be the cracking of the fiber-matrix interface. Quantitative results from this study will be useful for further and more accurate design of bamboo reinforced composite materials.

The flow of granular materials between rotating cylinders is studied using a continuum model proposed by Rajagopal and Massoudi (A method for measuring material moduli for granular materials: flow in an orthogonal rheometer, DOE/PETC/TR90/3, 1990). For a steady, fully developed condition, the governing equations are reduced to a system of coupled non-linear ordinary differential equations. The resulting boundary value problem is non-dimensionalized and is then solved numerically. The effect of material parameters, i.e., dimensionless numbers on the volume fraction and the velocity fields are studied. Published by Elsevier Science Ltd.

Wheel excitations measured on a heavy commercial vehicle by driving it through extreme road operating conditions, are considered as inputs to perform dynamic response analysis in a simulated laboratory and computational environment. From initial modal analysis results using finite elements, critical vehicle frame rail locations are identified for dynamic laboratory strain measurements on a six poster road load simulator that employs dynamic wheel excitations as input. Dynamic stresses calculated from measured strain values are then compared with computationally obtained stress results on each of these locations. This study also points out all geometric locations and vibration modes that may affect the design behavior of the frame members under extreme road operating conditions. The results obtained from this work can be considered for further fatigue life prediction and design optimization of chassis frame rail assembly. © 2009 IOP Publishing Ltd.

The ultrasonic Time-of-Flight Diffraction (TOFD) technique is a well-known technique for defect sizing. This technique has been applied to thick sections (>15 mm). The application of the TOFD technique to thin sections like pressure vessels and piping requires simulation of this technique. Simulation gives ideas about the expected results from experiments where real experiments are not possible or are difficult to conduct. Further, simulation helps us to derive the optimum choice of experimental parameters. This paper discusses the application of the finite element technique to simulate the ultrasonic time-of-flight diffraction (TOFD) technique. The diffracted and reflected signals in TOFD techniques for vertical and inclined defects were simulated using plane strain elements. The simulated results are compared with the experimental observations. FEM simulation of wave propagation in complex joints such as a T-joint with embedded flaws is also discussed.

Ultrasonic time of flight diffraction (TOFD) for sizing defects is based on the time of flight of the diffracted echo that is generated when a longitudinal wave is incident on a crack tip. This technique has the limitation during near-surface inspection due to signal superposition. Here, this limitation is overcome by using the shear wave-diffracted signal (instead of longitudinal wave) and hence called S-TOFD. Experiments were conducted on samples with defect tip closer to the surface of a flat plate sample to illustrate the utility of the S-TOFD technique. An increase in the flaw sizing accuracy, by using the shear wave-diffracted echoes from the tip and through the application of a signal processing technique (ESIT), was demonstrated. © 2006 Elsevier Ltd. All rights reserved.