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.

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.

Coiling of steel sheet is one of the easiest methods adopted by steel mills for the compact transportation of bulk sheet material to truck manufacturers for making 'C' shaped side frame rails. The coiling-uncoiling operation induces residual stresses in the steel sheet due to plastic strain hardening. This initial imperfection in the form of residual stresses remains in the plain web and flange areas of the rail sections. In corner areas of frames, increase in residual stresses are observed due to severe plastic deformation from forming operation than the coiling-uncoiling operation. More accurate consideration of the residual stresses becomes necessary when the behaviour of truck side frame rail web and flange sections, derived from different coil diameters are assessed. In this work, micro-hardness measurement is employed to predict the extent of plastic deformation at different coil diameter locations. The residual stresses induced by the coiling-uncoiling operation is assessed using the X-ray diffraction technique for steel coil diameters ranging from 600 mm to 1800 mm. Fatigue tests on samples extracted from different coil diameters indicate that fatigue life increases with the increase in coil diameter of the sample. The fatigue life of the annealed flat test sample was found to be higher than the fatigue life of samples derived from different coil diameter locations of steel coils received from steel mills.

In this article, forming residual stresses in cold-formed small-radius corner sections are analytically predicted with the consideration of the shift in the neutral axis and the nonlinear strain-hardened material model. The predicted forming stress results in the transverse direction show a trend of increased compressive residual stress in the outer surface and reduced tensile residual stress in the inner surface, as the corner radius-to-thickness ratio increases in small-radius bends. In the longitudinal direction, there is no significant change in the residual stress values observed in the inner and outer surfaces with respect to an increase in corner radius-to-thickness ratios. But a considerable decrease in compressive residual stress and an increase in tensile stress values are observed in the midsection areas, with an increase in the corner radius-to-thickness ratio. It is observed that the analytical peak compressive residual stress values are always higher than the experimental results. Also, the through-thickness residual stress from the numerical model is in close agreement with the analytical results. The magnitude of the maximum compressive stress in the inner half thickness is observed to be more than the magnitude of the maximum tensile stress in the outer half thickness of the corner section. The shift in the neutral axis towards the inner corner surface is much severe for lower corner radius-to-thickness ratio sections. The new approach provides a more accurate definition of initial conditions for further nonlinear structural behavior analysis of cold-formed structures.

Cold formed carbon steel C sections are often employed as load carrying structural members in heavy commercial trucks. The cold forming operations employed during the making of these members generate certain amount of residual stresses throughout the sections. As the residual stresses play a significant role in determining the structural behavior of truck frame rail members, a careful assessment of residual stresses resulting from cold forming operation is needed. In the present investigation, residual stresses in frame rail corner sections were numerically predicted with the help of non-linear Finite Element (FE) analysis in ABAQUS and compared with the experimentally measured residual stress values using X-ray diffraction technique. It has been observed that the numerically predicted residual stresses are in agreement with the experimentally measured residual stresses in forming direction. In the direction perpendicular to forming, while the trends of numerical and experimental residual stresses were observed to follow the same pattern, some deviation in stress values were observed in the inner half of the corner sections. The deviation in residual stress values are mainly due to the effect of steel sheets undergoing coiling-uncoiling processes prior to forming operation, which is not considered as part of numerical analysis; whereas the coiling-uncoiling residual stresses are inherited in experimentally measured residual stresses. The measured residual stresses along with corresponding equivalent plastic strains and virgin material properties in terms of stress-strain relationships can be considered as initial conditions in numerical models for further fatigue life analysis of truck frame rail structures. © 2013 SAE International.