Leonardo Gunawan

Measurement of Mechanical Properties of St 37 Material at High Strain Rates Using a Split Hopkinson Pressure Bar

The dynamic mechanical properties of a material are important keys to investigate the impact characteristic of a structure such as a crash box. For some materials, the stress-strain relationships at high strain rate loadings are different than that at the static condition. These mechanical properties depend on the strain rate of the loadings, and hence an appropriate testing technique is required to measure them. To measure the mechanical properties of a material at high strain rates, ranging from 500 s-1 to 10000 s-1, a Split Hopkinson Pressure Bar is commonly used. In the measurements, strain pulses are generated in the bars system, and pulses being reflected and transmitted by a test specimen in the bar system are measured. The stress-strain curves as the material properties of the test specimen are obtained by processing the measured reflected and transmitted pulses. This paper presents the measurements of the mechanical properties of St 37 mild steel at several strain rates using a Split Hopkinson Pressure Bar. The stress-strain curves obtained in the measurement were curve fitted using the Power Law. The results show that the strength of St 37 material increases as the strain rate increases.

Numerical and Experimental Impact Analysis of Square Crash Box Structure with Holes

Numerical and experimental study of the effects of center holes located at opposite sides on dynamic axial crushing of thin-walled square aluminum extrusions column are presented in this paper. The results showed that, by inserting the holes, the impact energy absorption characteristic in a progressive buckling can be improved as the starting location of the plastic deformation is always from holes and peak crush force can be decrease, so that the deceleration does not exceed the limit that can injure the passenger when frontal impact occurs. Here, the results of numerical simulations, conducted using an explicit finite element code, are compared with experimental results for various hole diameter. The results shows that the peak crushing force is decrease, while the mean crushing force is relatively constant.

Low Velocity Impact Analyses of Prismatic Columns Using Finite Element Method

This paper presents the study of prismatic columns of different cross sections subjected to low velocity impact, which are commonly used as energy absorber components in vehicles. The impacts of the columns were numerically analyzed using FEM. Four cross sections were considered, i.e. square, hexagonal, octagonal and circular. For each cross section, columns with several combinations of perimeters and thicknesses were analyzed. The results showed that, for columns with equal perimeter and thickness, those with circular cross sections have the highest mean crushing force and those with square cross sections have the lowest crushing forces. Furthermore, keeping all other parameters constant, columns with thicker wall have significantly higher crushing force while columns with longer perimeter have only slightly higher crushing force. This parametric information will be very useful for modern automotive industry in designing front longitudinal members within an acceptable safety level.

Numerical simulation of SHPB to measure the mechanical properties of aluminium foam material at high strain rate by using MAT 163 modified crushable foam

Aluminium foam is a kind of metal foam material which has large energy absorption capability. The mechanical properties of aluminium foam material in high strain rates could be measured by using SHPB. Numerical simulation is used as the initial step to measure mechanical properties of this material. MAT 163 modified crushable foam used as material model in the SHPB numerical simulation of aluminium foam. Numerical simulation showed a quite close results to experimental data.

The Influence of Sheet Metal Forming on The Axial Crushing Analysis of Top–Hat Columns

Reducing the rate of casualty in the crash events is always on the top priority of car manufacturers and customers. It is therefore necessary to make accurate predictions of car structural behavior during the crash events. To achieve this goal, the axial crushing behavior of the thin-walled top hat column needs to be understood thoroughly so that the crashworthiness performance of the column can be predicted accurately. The effect of sheet metal forming should be considered since many car crashworthiness components are fabricated by sheet metal forming. This paper presents a numerical study to investigate sheet metal forming effects such as: thickness distribution, residual stress and plastic strain change to the crushing force characteristics of the top-hat columns. First, the design of the top-hat column was generated by using deep drawing forming process simulation. Then, the forming parameters (geometry, residual stress, plastic strain, thickness distribution) were transferred to the non-linear finite element dynamic analysis model. The axial crushing simulations for the top-hat column with forming parameters were then performed and analyzed. The results showed that the sheet metal forming has a considerable effect on the crush behavior and performance of the thin-walled top-hat columns.

Measurements of flutter derivatives of a bridge deck sectional model

Aeroelastic phenomena should be considered during the design phase of long span bridges. One of the aeroelastic problems is flutter, the dynamic instability that may cause structural failure at a wind speed called the flutter speed. The prediction of flutter speed of a bridge needs a thorough modelling of bridge stiffness, inertias, and especially its unsteady aerodynamic forces. The potential flow theory is not applicable to calculate unsteady aerodynamics of oscillating bridges due to their non-streamlined complex geometry, and the non-avoidable flow separation. For these reasons, a semi empirical model proposed by Scanlan is used to describe unsteady aerodynamic forces on an oscillating bridge deck. In this model, relation between unsteady aerodynamic forces and motion of the bridge is modelled using parameters known as flutter derivatives. The values of flutter derivatives can be identified from the free vibration responses of an elastic bridge at several wind-speeds. This paper presents wind tunnel tests and flutter derivatives identification of a sectional aeroelastic bridge model. Modified Ibrahim Time Domain method was applied to identify the eigenvalues and eigenvectors of the model at each wind speed, from which the flutter derivatives can be calculated. The results show that the measurement procedure is able produce flutter derivatives, which are in good agreement with those obtained by other researchers.

Stress Analysis of the Impactor Assembly of the Dropped Weight Impact Testing Machine

This paper presents the stress analysis of an impactor assembly of a dropped weight impact testing machine. A finite element analysis was performed to calculate the stress occurred on the impactor during the impact which should be less than its yield strength. By varying the level of the reaction load from the specimen to the impactor, the maximum load that can be withstand by the impactor assembly without plastic deformation was predicted. Then, several simulations were carried out to find the best way to increase the load limit of the impactor. It was found out that increasing the thickness of box to 7 mm and both the diameter of arm and frame-column to 40 mm will increase the load limit by 236%.

The Effects of Spot Weld Pitch to the Axial Crushing Characteristics of Top-Hat Crash Box

Numerical study of the effect of spot weld pitch with respect to top-hat crash box crushing characteristics are presented in this paper. Belytschko-Lin-Tsay shell element was used for modeled columns wall with Piecewise Linear Plasticity material model. The impactor was modeled using hexahedral solid elements and assumed as a rigid body. Spot weld joints used to connect mild steel St37 plates of the columns were modeled using beam element and solid element. Impact characteristics related to the spot weld pitch and models were evaluated from simulation results in the form of crushing force vs axial deformation of the column. The results show that spot weld pitch does not significantly affect the crushing characteristics for top-hat crash box with beam element spot weld model, while solid element spot weld model show otherwise. The difference between beam element spot weld model and solid element spot weld model is larger at spot weld pitch 0.50H – H, and tend to close at higher spot weld pitch. Top-hat crash box model becomes stiffer with solid element applied as spot weld model.