CHAPTER-10 DYNAMIC SIMULATION USING LS-DYNA

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1 DYNAMIC SIMULATION USING LS-DYNA CHAPTER Introduction In the past few decades, the Finite Element Method (FEM) has been developed into a key indispensable technology in the modeling and simulation of various engineering systems. In the development of an advanced engineering system, engineers have to go through a very rigorous process of modeling, simulation, visualization, analysis, designing, prototyping, testing and finally fabrication. As such, techniques related to modeling and simulation in a rapid and effective way play an increasingly important role in building advanced engineering systems, and therefore the application of the FEM has multiplied rapidly. Commercial softwares, like PAM-Crash, implement algorithms that include modeling of contact and are capable of simulating impact conditions Hypermesh Software Altair Hypermesh is a high-performance finite element pre-and post-processor tool for major finite element solvers, allowing engineers to analyze design conditions in a highly interactive and visual environment. Some of the benefits of Hypermesh are: Reduces time and engineering analysis cost through high-performance finite element modeling and post-processing Reduces learning time and improve productivity with an intuitive user-interface and best-in-class functionality Reduces redundancy and model development costs through the direct use of CAD geometry and existing finite element models. To simplify the modeling process for complex geometry through high-speed, high-quality auto-meshing. In this chapter, finite element analysis is carried out using LS-Dyna finite element code to predict the load-displacement response of sandwich panels and its component materials, silk-cotton wood and honeycomb core subjected to dynamic loading. The FE models of these were constructed using Hypermesh pre-processor. 139

2 Processing is done using LS-Dyna and post-prossing is carried out using Hyperworks software. The dynamic simulation results of these materials are then compared with their respective dynamic experimental results for degree of convergence General Methodology for Dynamic Simulation LS-Dyna explicit methods are those in which the information at time step n+1 can be obtained in terms of previous time steps and there is no dependence on the current time step. Explicit methods are very fast as there is no matrix inversion and the mass matrix is lumped or diagonal. Linear and highly non linear problems can be effectively solved. This is the reason that, explicit codes are being used for crash analysis. The dynamic FE simulation involves the following steps a) Part definition b) FE meshing and material properties c) Contact analysis d) Boundary conditions and loading e) Results 10.4 Dynamic simulation of silk-cotton wood The compressive stress-strain behavior of wood and crushable foam is identical [1]. Therefore, crushable foam model has been idealized as wood material model in the present study. The quasi-static test properties of silk-cotton wood along the grains are input to the material model. 3-D model geometry was constructed for the nominal dimensions of 45 x 45 x 45 mm with Hypermesh pre-processor FE Meshing and Material properties Following input data is used for developing FE model for silk-cotton wood. LS-Dyna material type : 63 (MAT_CRUSHABLE_FOAM) This material is used for modeling crushable foam with optional damping. Unloading is fully elastic. The behavior is treated as perfectly elastic-plastic. Density : 327 kg/m 3 : 7.35 GPa 140

3 Ratio : 0.23 Yield Stress : 350 MPa Element Size : 2.5 mm Number of integration points through thickness: 02 Specimen Length : 16 mm (each) Element Type : 8-Node Hexahedron solid No. of Elements : 9800 No. of Nodes : 24,080 FE discretization of the wood model was done using two point integration 8-nodal hexahedron elements. Figure (10.1) shows the sketch of an 8-nodal hexahedron element which is used to mesh wood model, each node of the element has three degree of freedoms. Figure 10.1 Eight nodal hexahedron element which is used to mesh the wood model FE Model Data for Impactor Plate and Base Plates The velocity input is effected through a defined rigid impactor plate. The specimen is supported on a rigid base plate. The same material model is used to construct both the impactor and the base plates. Figure (10.2) shows the meshed model of silk-cotton wood with the impactor and base plates. The material model details are as below. 141

4 LS_Dyna material type : 20 (*MAT Rigid) Parts made of this material are considered to be a rigid Ratio : 0.3 Element Type : No: 2, Elastic form shell No. of Elements : 70,000 No. of Nodes : 60,000 Figure 10.2 Meshed model of silk-cotton wood with impactor and base plates Contact Treatment Contact occurs between the components or within the component itself when the components try to come towards each other during the plastic deformation. When the components touch each other, a force is transmitted across the common interface between them due to friction. This gives rise to a contact pressure and shear stress. A high end computational process is required due to the sever discontinuity with respect to boundary conditions. If there were no contacts defined between the components, then the components would simply penetrate into each other and this is unphysical. LS-Dyna software by default has number of defined contact algorithms to detect and establish the contact between the components automatically. The following line command enables the automatic contact between the components. The present analysis uses the, contact type: Contact_Automatic_General. This establishes the physical 142

5 contact automatically between the impactor, wood, within the wood itself and with base plate Boundary conditions The base plate is constrained in all the directions Enabled the automatic contact generation The velocity input of 3.8 m/s is given through the command in the load collector Figure 10.3 shows the simulated model of silk-cotton wood specimen. Figure 10.3 Simulated model of silk-cotton wood specimen 10.5 Dynamic Simulation of Aluminum Honeycomb The model geometry of honeycomb was constructed for (100 x 100 x 50) mm nominal size with Hyper mesh pre-processor. Figure (10.4) shows the 3-D geometric model of aluminum honeycomb. LS-Dyna defined honeycomb material model 24 is selected for the present study to accommodate the elastic-plastic behavior of the honeycomb material. This model represents the piecewise linear plasticity to consider the strain hardening modulus. The quasi-static test properties of aluminum honeycomb are input to the material model. 143

6 Figure 10.4 Geometric model of aluminum honeycomb specimen FE Meshing and Material properties The FE material model input data are as below. LS-Dyna material type : 24 (*MAT_PIECEWISE LINEAR_PLASTICITY) This model is for Elasto-plastic material with an arbitrary stress-strain curve and arbitrary strain rate dependency. Density : 2700 kg/m 3 Yield Stress Element Type Element Size : 69 GPa Ratio : 0.3 : 165 MPa : Co_rotational, Belystchko Lin Tsay Shell : 2 mm Number of integration points through thickness: 02 Thickness of the Shell Specimen Length : 0.068mm. : 50 mm FE discretization of the honeycomb model was done using two point integration 4- nodal shell elements. Figure (10.5) shows the sketch of a 4-nodal shell element which is used to mesh honeycomb model. Each node of the element has 6 degree of freedom, three translational and three rotational. 144

7 Figure nodal shell element used for meshing of honeycomb model FE Model of Honeycomb Figures (10.6) shows the meshed model of aluminum honeycomb along with the impactor and base plates Figure 10.6 Meshed model of aluminum honeycomb with impactor and base plates Boundary Conditions 1. The specimen was supported on a rigid surface (base plate). 2. Base plate was fixed in all directions 3. The impactor plate is constrained to move only in Z-direction 4. Enabled the automatic contact generation 5. The velocity input of 4.7 m/s is given through the command in the load collector 145

8 Figure 10.7 Simulated aluminum honeycomb specimen 10.6 Dynamic Simulation of Sandwich Panel Part Definition The model geometry was constructed with Hyper mesh pre-processor. The FE model of the sandwich panel consists of a base plate, two silk-cotton wood skins, honeycomb core and an impactor plate. A 3-D geometric model was constructed for the similar dimensions to that of experimental specimens of sandwich panels. Figure (10.8) shows the geometric model of the sandwich panel. Figure 10.8 Geometric model of the sandwich panel. 146

9 FE Meshing and Material Properties The model is then meshed with Hypermesh pre-processor. The procedure was repeated for FE meshing of the components of the sandwich panel FE Model for Sandwich Panel Figure (10.9) Shows meshed model of sandwich panel with base and impactor plates and Figure (10.10) shows simulated model of the crushed specimen. Figure 10.9 FE Model of sandwich panel with base plate and impactor plates Boundary Conditions and Loading 1. The specimen was supported on a rigid surface (base plate). 2. Base plate was constrained in all directions 3. The impactor plate is constrained to move only in Z-direction. 4. For each run, one face of the top wood skin was constrained for all degrees of freedom. 5. Enabled the automatic contact generation 6. The impactor plate was given with a velocity of 6.7 m/s. 147

10 Figure Simulated model of sandwich panel specimen 10.7 Results and Discussion Figures [10.11 to 10.13] shows the comparative load-displacement responses obtained through the dynamic simulation of silk-cotton wood, aluminum honeycomb and Type-I sandwich panels respectively. The simulated behavior of these materials closely resembles when compared with the behavior of these materials tested under dynamic experiments. Table 10.1 shows the comparison of results obtained by FE dynamic simulation and dynamic experiment. The results of dynamic simulation shows a decreasing trend when compared with the experimental results. For the case of silk-cotton wood, aluminum honeycomb, and Type-I sandwich panel the decrease in the energy absorption capacity with respective to the experiment is about 6.5%, 4% and 4.4% respectively. 148

11 Figure Comparative load-displacement responses of wood using simulation and experiment. Figure Comparative load-displacement responses of aluminum honeycomb using simulation and experiment. 149

12 Figure Comparative load-displacement responses of Type-I Panel using simulation and experiment. Table 10.1 Comparison of simulation and the dynamic experimental results Cellular Material/Structure Energy Absorbed (J) Dynamic Experiment Dynamic Simulation Silk-cotton wood (along the grain) Honeycomb (out-of-plane) Sandwich panel (Type-I)