Spot weld failure has a great influence on the crashworthiness of a vehicle since an automotive body is mostly assembled by spot welding. Spot weld failure was not a serious problem when using low strength steel, but as the strength of steels increases, spot weld failure became a hot issue for crash performance due to the low spot weld strength compared to material strength. Nowadays, the car design is based on CAE, and the crashworthiness is evaluated from crash simulation. Spot weld failure is a critical factor causing the discrepancy between the actual crash performance and simulation result. Of course, car designers want to get accurate simulation results and design to avoid spot weld failure based on simulation. There are a lot of studies on spot welding failure, but It is necessary to further enhance the accuracy. In this paper, we study how to predict accurate spot weld failure by macroscopic analysis of spot weld failure. Normal, shear, bending, and torsional load components act on spot welds, and many spot weld failure model consider that they act independently, destroying the spot weld. In this paper, normal and bending load components are considered together because loading direction and plane of normal and bending components are same. Spot weld failure model that normal and shear load components act independently and there is the interaction of normal and bending components, is newly proposed. Here, torsional component is ignored because of low influence on an automotive body. Spot weld failure tests are performed for various automotive steels, and coefficients of spot weld failure models are derived. Since an automotive body has mostly heterogeneous stack-ups of the strength and the thickness, the spot welding failure tests for heterogeneous stack-ups are also performed and it is verified that the new model describes dissimilar stack-ups well. Compared to conventional models, the new model has an advantage in the simplicity and the accuracy. Finally, the predicting method of coefficients of spot weld failure models is developed to consider spot weld failure in the crash simulation without experiments.
Increasing demands for the simulation of complex, multi-physics problems in crashworthiness and manufacturing process analyses have necessitated new developments in the structural conjugate heat transfer solver in LS-DYNA®. Some of the most recent extensions and new implementations are presented and discussed in this contribution. The first block addresses the relatively new field of battery abuse simulations. Focus is put on a novel thermal composite thick shell element that is defined using *PART_COMPOSITE_TSHELL. On the one hand, the implementation allows for a relatively easy input definition. On the other hand, the formulation adds new temperature degrees of freedom for each layer of the composite structure and, thus, accurately resolves the internal lay-up of the structure, i.e. the battery cell. The reconstructed lay-up is also accounted for in the thermal contact routines. Consequently, the heat transfer through a stack of solid elements can be reproduced exactly by a single composite thick shell element with the corresponding lay-up definition. The second block presents the work on different thermal boundary conditions. A recent enhancement enables the “standard” boundary conditions (convection, radiation, and flux) to be transferred to newly exposed surfaces after element erosion. In general, this is sufficient for modeling laser cutting with a flux boundary condition, but the input of such a model can become very complex. Therefore, a new thermal boundary condition *BOUNDARY_FLUX_TRAJECTORY is introduced in the second part of this block, which is tailored for moving heat sources acting on the surface of a structure. In contrast to the standard flux boundary condition, the new implementation also accounts for the tilting of the heat source. The boundary condition is applicable in coupled thermal-structural and thermal-only simulations. The second block is completed by the presentation of a new temperature boundary condition *BOUNDARY_TEMPERATURE_RSW that is devised as a simplified modeling strategy for resistive spot welds (RSW). With the keyword, the temperature distribution in a weld nugget is defined directly.
To fulfill recent regulations for automobile fuel economy great demand on saving weight of automobiles is growing. Since making a lighter car with conventional material loses occupant safety, at least stiffer materials with the same weight are needed. For example, use of CFRP (Carbon Fiber Reinforced Plastics), Aluminum, Magnesium or Titanium is attracting our attention in these days. But technique to handle these materials is still under developmental stage. High-tensile steels made by hot forming is one of the most promising candidate since it can realize better balance between cost and weight saving. In hot forming technique, heated blank material is pressed by tools and then quenched by various methods to cause martensitic transition of the blank to obtain high tensile steel. It is not only stiff but also has good shape freezing property, causing smaller springback of the stamped materials. As an another advantage of hot forming, steels as raw materials can be easily obtained all over the world, compared with other materials listed above. On the other hand its disadvantages is relatively large investment in plant and equipment such as chiller or cooling tower and costs for prototyping production of tools with pipes to run cooling water. In order to cause the martensitic transition of the blank materials, one needs to quench it sufficiently fast. We, JSOL, think that a CAE tool to calculate and predict the stiffness of high-tensile materials contributes to ensure their strength in mass production stage. Important points in accurate prediction are following: (i) to calculate phase transition of the materials correctly (ii) to predict cooling performance of the tools to ensure (i). By making these uncertainties clear it is expected that CAE is capable of reducing trials-and-errors on prototyping, causing reduction of tool designing costs. LSTC, Dynamore and JSOL have been working on formulating a manufacturing CAE solution to the hot forming techniques. For example, development of phase transition material models (*MAT_244, *MAT_254) will overcome the uncertainty (i) described above. We also have been investigating simulation technique for thermal-structural-fluid coupling calculation to demonstrate the behavior of cooling water flowing in pipes of the tools, corresponding to item (ii) above. In this paper we report the results of recent solution developments on the latter point.
Thermal radiation problems are gaining interest in the automotive industry. Examples include paint drying and curing processes, determining material characteristics and deformation due to heat treatment, temperature distributions in muffler systems and heat shields in engine compartments. LS-DYNA has capabilities to couple the thermal solver with mechanical and multi physics solver. Solving for thermal convection, conduction and contact in three dimensions are already available in all parallel models LSTC are offering, namely shared memory parallel (SMP), massive parallel processor (MPP) and the combination of both models (HYBRID). Lately the thermal radiation feature has been extended to be used with massive parallel processor (MPP) version and a new solver to solve for radiosity. New developments are tested with verification examples and small test cases to determine the code functionality and expected results. Furthermore they have to show their applicability with validations of numerical models with experimental data. They also need to be evaluated regarding their scalability of wall clock time to reduce costs of compute resources. This contribution addresses two of these subjects, the scalability and the validation. The validation example used here is a part of a B-pillar which is heated up in an oven. Temperatures were measured at several locations of the sheet metal. Test data was provided by Honda R&D Americas, Inc. The test was modeled as a thermal radiation problem in an enclosure. Thermal radiation was modeled using the keyword *BOUNDARY_RADIATION_ENCLOSURE and was performed in LS-DYNA MPP. An LS-DYNA MPP scalability study was performed. Due to missing data for the thermal parameters, the heat capacity, thermal conductivity and emissivity were determined with LS-OPT.
Heat-exchangers are devices used to transfer heat between two or more fluids and can be found in both heating and cooling processes. At high temperatures during operation, thermal induced stresses occur and can lead to the failure of the device. The Technische Hochschule Mittelhessen has, in cooperation with the company WK, started a research project for the development of a HT-HE, which is designed for operating temperatures up to 1100°C and the contact with aggressive chemical media. In order to develop an efficient HT-HE regarding the heat recovery, semi analytical calculations have been carried out to optimize the geometry of the heat exchanger. This study focusses on the validation of these semi-analytical calculations by using Multiphysics simulation. Due to the time costly simulation of transient fluid-structure-interactions (FSI) while taking high temperatures into account, special emphasis has been placed on the reduction of simulation time without losing accuracy. Initially, a number of simplified models were set up to control the bug-free operation of the relatively new ICFD-Solver. It has been shown that the necessary workarounds, due to some implementation errors, only had a minor effect on the heat transfer. Modifications have been added to the input data in order to significantly reduce simulation time without affecting the quality of results. The results of the simulations done with LS-DYNA show a qualitatively good correlation with the semi analytical calculations and improve the understanding of the thermo- and fluid dynamic processes inside the HT-HE during operation.
In the Finite Element Modeling community there is a trend to use models with increasing modeling details which raises the numbers of elements and solution variables. The increase in solution variables has a big impact on the run time of the analysis. Reducing wall clock time is an important item in using numerical analysis in production. The wall clock time can be reduced by using improved CPU technology and hardware with a higher throughput and lower latency for memory, storage and interconnect. On the software side, the use of parallel models to utilize more cores in an analysis reduces the wall clock time. Key measure for reducing wall clock time is scalability, which is in general expressed as the reduction of the run time due to an increase of cores used for the analysis. LSTC is currently offering LS-DYNA in three different parallel models, namely shared memory parallel (SMP), massive parallel processor (MPP) and the combination of both models (HYBRID). The focus on these developments is scalability for all three parallel models. Scalability is influenced by several factors. Beside the already mentioned hardware environment, main contributors are the decomposition (MPP and HYBRID) of the model, the model size and application type. Scalability can not only be evaluated on a global implementation level. It needs to be evaluated on the application at hand and the features utilized in this analysis. This contribution discusses the scalability of thermal solvers offered by LS-DYNA MPP using a surrogate engine model from Rolls-Royce. Three thermal solver types are used with three different MPP rank count (4, 8 and 16). The scalability is measured using the wall clock time summary of the LS-DYNA runs found in the d3hsp files.
Several Arctic waters are no longer ice-covered throughout the year. As a result, the Northern Sea Routes are getting into the focus of the maritime industry [1]. In addition less ice coverage in other sea areas such as the Baltic Sea leads to increased shipping traffic in the winter season. This repeatedly leads to damages to ships when sailing in ice-covered waters, but also when colliding with ice floes and icebergs but also with ships, such as icebreakers, in convoys [2, 3]. It is of great importance for the structural simulation of these events to model the material properties of the ship structure under consideration of the environmental conditions. These material properties such as yield strength and tensile strength as well as fracture strain, however, are strongly influenced by the material temperature [4]. Therefore the question arises how cold a ship structure can actually become in winter and in arctic waters and how this affects the structural response in the event of a collision. In the rules and guidelines of the classification societies -60 °C can be found as the lowest temperature for material tests on steels used in shipbuilding [5]. This value corresponds well with different temperature measurements where extreme values below -50 °C were measured in the area of the Northern Sea Route [6, 7]. In contrast, liquid seawater cannot become colder than -2 °C [8]. If the interaction with ice is considered, the structural temperature in the waterline area is of particular interest. It is influenced by both water and air temperature. Therefore, the structure temperature is estimated by thermal simulations in order to determine suitable temperature depended material curves and to predict the influence on the structural response in the collision scenario.
Prediction of Spot Weld Failure for Automotive Steels