Hong Tae Kang

Fatigue life estimation of spot welded joints using an interpolation/extrapolation technique

A numerical interpolation/extrapolation technique is presented that can utilize the fatigue life and strength databases of spot welded joints that have been developed by previous researchers. These databases typically included multiple independent variables, such as specimen dimension and type, material property, R-ratio, and welding condition, such as are often encountered in a typical design of experiments approach to testing. In this study, Rules interpolation technique is modified and a new extrapolation scheme is adopted. The test data for multiaxial ultimate strength of spot welded joints under combinations of tensile and shear force are used to demonstrate the suitability of the interpolation technique in multivariable databases, and the technique is also applied to the calculation of total fatigue life of spot welded joints based on Swellams experimental data. The results show that the calculations and test values are reasonably matched. This approach is also compared the methods of Swellam and Sheppard.

Fatigue damage severity calculation for vibration tests

An analytical solution to calculate a fatigue damage spectrum for a linear single degree-of-freedom system is presented to assess fatigue damage severity for various vibration test specifications. This solution can be served as the fundamental to develop an accelerated vibration test specification based on the real measured customer usage data and the use of material. Fundamentals of vibration characteristics for sinusoidal and random vibrations are well described in this paper. The state-of-the-art frequency-based fatigue theories are also reviewed. Finally, this paper compares the calculation results for a product subject to the vibration loads defined in IEC-60068-2-6 and ISO 16750-3.

Fatigue Predictions of Various Joints of Magnesium Alloys

In this project, a front shock tower of a passenger vehicle is developed with various magnesium alloys and joining methods. To predict the fatigue life of the joints in the structure, fatigue tests of various joint specimens including friction stir linear welding, self-piecing rivet joint with and without adhesive, and friction stir spot welding were conducted. The magnesium alloys used for the specimens are AM60 (cast), AM30 (extrusion), and AZ31 (sheet). Various finite element modeling techniques were attempted for simulating the various joints. Fatigue life prediction method for the joints was performed using the stress-life curve approach. The finite element modeling technique and the fatigue prediction method will be verified with fatigue tests of the actual front shock tower structure subjected to variable amplitude loadings in near future.

Temperature Distribution during Friction Stir Spot Welding of Magnesium Alloy AM60B

This study investigated the effects of welding parameters on temperature distribution of magnesium alloy AM60B during Friction Stir Spot Welding (FSSW). The welding parameters included tool rotational velocity, plunge depth, and holding time. These parameters were selected from previous test results. The temperature distributions were measured with type-K thermocouples during FSSW with various welding parameter combinations. The measurement locations were placed along the centerline in longitudinal direction of the coupons. The temperature distributions during FSSW were also calculated with finite element analysis (FEA) for each combination of the welding parameters. The experimental results were compared with those obtained from FEA.

Fatigue prediction of lightweight thermoplastic fiber-metal laminates

Lightweight thermoplastic-based fiber-metal laminates were developed based on a self-reinforced polypropylene and a glass fiber-reinforced polypropylene composite material with two sheets of aluminum alloy 2024-T3. The laminates were manufactured using a fast one step cold press manufacturing procedure. The mechanical behavior of the laminates was then investigated under tensile and fatigue loading conditions. The tensile properties of the plain aluminum, the composite materials, and the thermoplastic fiber-metal laminates (TFML) were investigated at quasi-static rates of loading. The fatigue tests were also conducted under load control based on the ASTM E 466 standard procedure. Various loading cycles were employed for the fatigue tests in order to minimize the possibility of heat generation on the composite materials. Three different load levels were applied for the fatigue specimens with zero to max loading. Results have shown that the glass fiber-reinforced polypropylene hybrid systems exhibited higher fatigue strength than the self-reinforced polypropylene based fiber-metal laminates. Following this, a simple prediction method for fatigue life of the TFMLs has been introduced. It is clearly indicated that the stress ranges calculated using the proposed method for the monolithic AL2024-T3 and the TFMLs are very similar at the same cycles to failure.

Fatigue Behavior of AM60B Subjected to Variable Amplitude Loading

Magnesium alloys are considered as an alternative material to reduce vehicle weight due to their weight which are 33% lighter than aluminum alloys. There has been a significant expansion in the applications of magnesium alloys in automotives components in an effort to improve fuel efficiency through vehicle mass reduction. In this project, a simple front shock tower of passenger vehicle is constructed with various magnesium alloys. To predict the fatigue behavior of the structure, fatigue properties of the magnesium alloy (AM60B) were determined from strain controlled fatigue tests. Notched specimens were also tested with three different variable amplitude loading profiles obtained from the shock tower of the similar size of vehicle. The test results were compared with various fatigue prediction results. The effect of mean stress and fatigue prediction method were discussed.

Parametric modelling approach for development of an automotive bucket seat frame

This paper presents a design process of an automotive bucket seat frame using a parametric modelling approach. The design process involves, first, optimising a generic seat frame using OptiStruct optimisation software. Then, a parametric model of the seat frame was developed based on the optimisation analysis results using CATIA V5. The design parameters were changed in the parametric model and numerous static tests were also performed using CATIA V5. Design of Experiment (DOE) study was then conducted using MINITAB to optimise the design parameters. Finally, the optimised seat frame was evaluated based on the requirements of the Federal Motor Vehicle Safety Standards (FMVSS) using in LS-DYNA.

The effect of voids on the quasi-static tensile properties of carbon fiber/polymer-laminated composites

Static longitudinal/transverse tensile tests for unidirectional carbon fiber/polymer (T300/924) laminates and laminates with lay-ups [ 0 / ± 45 / 0 / 90 ] s at various void levels were conducted, and degradations in stiffness/strength were observed with the presence of voids. The void levels were controlled by compression pressure during the compression molding process. The characterization of voids was achieved by digital microscopy image analysis; the density distributions of equivalent diameters and aspect ratios were analyzed with respect to compression pressure. For the purpose of quantifying the effect of voids on the static mechanical properties of composites, a stiffness prediction method based on the Mori-Tanaka method and void geometric statistical data have been used with the implementation of a finite element model of the representative volume element for unidirectional composites; the prediction results show good correlation with experimental data. Finally, a modified continuum damage model for laminated composites with the presence of voids was proposed, the model is capable of capturing the effect of voids; and gradual damage analysis for carbon fiber/polymer composite laminates at different void levels was conducted to evaluate the effect of voids on their tensile properties.