M.K. Singha

Thermal postbuckling analysis of laminated composite plates

Abstract The thermal postbuckling behavior of graphite/epoxy multi-layered rectangular plates of various boundary conditions is studied using the finite element method. Temperature dependent thermal and elastic properties of the material are used in the analysis. The nonlinear finite element equations are solved as a sequence of linear eigenvalue problems to trace the thermal postbuckling paths of 15-layered symmetric angle-ply plates. The presence of secondary instability with an unsymmetric deformation mode has been identified for symmetric laminates under uniform temperature rise. In the case of linearly varying temperature rise through the thickness of the plate, the nonlinear equilibrium equations are solved by the modified Newton–Raphson technique to get the temperature-displacement curves.

Optimum Design of Laminated Composite Plates for Maximum Thermal Buckling Loads

Buckling temperatures of graphite/epoxy laminated composite plates are maximized for a given total thickness considering fiber-directions and relative thicknesses of layers as design variables. Thermal buckling analysis is carried out using the finite element method with 4 node shear deformable plate element, while genetic algorithm (GA) is employed to optimize as many as ten variables for the five layered plates. In addition to traditional three-operator approach, i.e., reproduction, crossover and mutation, other variants of GAs such as the elitist model, two-point crossover models are also discussed. The study includes composite plates of three different aspect ratios, two support conditions and three different numbers of layers. The presented results reveal that the buckling loads can be increased significantly with appropriately orienting the fiber directions and varying the thickness of different layers. The authors also recommend the judicious selection of specific thicknesses and fiber orientations needed for the practical implementation highlighting the importance of their theoretical values so that they may also be made available in near future for fabrication.

Stability and Strength of Composite Skew Plates Under Thermomechanical Loads

The buckling and postbuckling analysis of shear deformable composite skew plates subjected to combined uniaxial compression and uniform temperature rise has been carried out using the finite element method. The governing nonlinear finite element equation is posed as a sequence of linear eigenvalue problems, and each one of them is solved for different amplitudes of deflection to trace the thermal postbuckling path. A maximum strength criterion is used to identify the laminates that have failed in strength, and appropriate modifications are made to the stiffness matrix. The first-ply failure of laminates has also been predicted by Tsai-Hill and Tsai-Wu criteria. Postbuckling paths are traced for different boundary conditions of the plate. Specific numerical studies have been reported showing the effects of skew angle, initial uniaxial compression, and thickness-to-span ratio on the thermal postbuckling behavior of the eight-layered [45/-45/0/90 deg] s symmetric plate. The presence of secondary instability has been identified while tracing the postbuckling path.

Dynamic Tensile and Compressive Behaviors of Mild Steel at Wide Range of Strain Rates

AbstractThe purpose of the present paper is to investigate the mechanical behavior of mild steel at quasi-static (0.001 s−1) and different rates of dynamic tensile (5–750 s−1) and compressive (125–2,350 s−1) strain rates. Quasi-static experiments are conducted on a universal testing machine to study the stress-strain behavior of mild steel. A hydropneumatic machine and a modified Hopkinson bar are used to investigate the dynamic tensile behavior of mild steel specimens at medium and high strain rates, respectively, whereas the specimens are tested on a split Hopkinson pressure bar to acquire understanding of the strain rate sensitivity of mild steel under dynamic compression. The effects of a pulse shaper and gauge length of the specimen in the dynamic compression tests are investigated. High-speed photography has been used to monitor the deformation of the specimen at high strain rate experiments. The applicability of the existing Cowper-Symonds and Johnson-Cook material models to represent the mechanica...

Nonlinear Dynamic Thermal Buckling of Functionally Graded Spherical Caps

isotropic/orthotropic/functionally graded material spherical shells suddenly exposed to thermal environment is rather meager in the literature and such studies are important to the structural designers. In the present work, the nonlinear dynamic thermal buckling of functionally graded spherical caps is investigated using a threenoded shear flexible axisymmetric curved shell element based on field-consistencyprinciple[6].Geometricnonlinearityisassumedin thepresentstudy,usingvonKarman’sstrain-displacement relations. In addition, the formulation includes in-plane and rotary inertia effects. The material properties are graded in the thickness direction according to the power-law distribution in terms of volume fractions of the constituents of the material. The nonlinear governing equations derived are solved employing Newmark’s numerical integration method in conjunction with the modified Newton– Raphson iteration scheme. The critical dynamic buckling temperature difference is taken as the temperature difference between the shell surfaces corresponding to a sudden jump in the maximum average displacement in the time history [2,12]. Numerical results arepresented that considerdifferent values ofgeometrical parameter and power-law index.

Quasi-Static and Dynamic Tensile Behavior of CP800 Steel

An experimental investigation on the tensile behavior of Complex Phase 800 (CP800) steel at different strain rates (0.001 to 750 s−1) is reported here. The material is tested on a ZWICK universal testing machine to obtain the stress-strain relationship under quasi-static condition, and on a Hydro-Pneumatic machine and modified Hopkinson bar to study the mechanical behavior at medium and high strain rates, respectively. The failure surfaces of the tested specimens at different strain rates are studied from their fractographs. Finally, the applicability of the existing Cowper-Symonds and Johnson-Cook material models to represent the mechanical behavior of CP800 is examined.

Thermomechanical Postbuckling Response and First-Ply Failure Analysis of Doubly Curved Panels

The thermomechanical postbuckling response of graphite/epoxy multilayered doubly curved (spherical and elliptic paraboloid) shell panels having rectangular planform is obtained within the framework of the finite element method. The nonlinear equilibrium paths are predicted using the displacement control method and the temperature-dependent material properties are used in the analysis. The structural model is based on a first-order shear deformation theory incorporating geometric nonlinearities. The first-ply failure of laminates is predicted with the Tsai-Wu failure criterion. Specific numerical results are reported that show the effects of radius-of-curvature-to-span ratio and thickness-to-span-ratio on the stability and strength characteristics of doubly curved shell panels subjected to combined thermal and mechanical loads. Moreover, numerical results are presented showing the effect of temperature dependence of material properties on limit loads and snap-through response of shallow curved panels.

Strain Rate Sensitivity of an Aluminium Alloy under Compressive Loads

An experimental investigation on the dynamic compressive behaviour of the aluminium alloy, AA6063-T6 in the strain rate range from 0.001s-1 to 850s-1 is reported here. Cylindrical specimens of AA6063-T6 are tested under universal testing machine at quasi-static (0.001s-1) condition, whereas, experiments at high strain rates (110s-1,400s-1,550s-1,700s-1 and 850s-1) are conducted on the traditional split Hopkinson pressure bar setup. The strain hardening in the material is found to increase with increasing strain rate. It is observed that the existing Johnson-Cook material model with appropriate material parameters predicts the dynamic compressive flow stress of AA6063-T3 aluminium alloy precisely.

Mechanical Behavior of a Structural Steel at Different Rates of Loading

The purpose of this chapter is to investigate the mechanical properties of a structural steel under quasi-static and dynamic loads. Specimens of as-received low-carbon mild steel are tested on universal testing machine to study their stress-strain behavior under quasi-static tension (0.001s−1) and compression (−0.001 s−1). Then, the specimens are tested under split Hopkinson pressure bar (SHPB) and modified Hopkinson bar (MHB) to study their material properties under dynamic compressive (−550, −800s−1) and tensile (250, 500s−1) loading, respectively. The material parameters of the existing Johnson-Cook model are determined. Finally, the applicability of the existing Johnson-Cook material model to represent the mechanical behavior of mild steel in plastic zone is examined.

Mechanical Characterization of Multi Phase Steel at Different Rates of Loading

The purpose of the present paper is to investigate the mechanical properties of multi phase 800 high yield strength (MP800HY) steel under compressive loading at different strain rates (-4700s-1 to-0.001s-1). Specimens of MP800HY steel are tested on universal testing machine to study their stress-strain behavior under quasi-static (-0.001s-1) condition. Then, the specimens are tested under split Hopkinson pressure bar (SHPB) to study the strain rate sensitivity of the material under different rates of compressive loading (-4700s-1, -4300 1/s, -3800 1/s, -2900s-1 and-1600s-1). The effect of pulse shaper in SHPB experiments has been studied. Thereafter, the applicability of the existing Johnson-Cook material model to represent the flow stress of MP800HY is examined.