Steel Processing Properties and Their Effect on Impact Deformation of Lightweight Structures

 

Project Description



Background/Purpose

It is clear that in order to achieve the PNGV goal of 3X fuel efficiency, significant weight reduction must be achieved. Significant efforts are underway to develop an alternative to replace steel as the main structural material. However, introduction of new materials in automotive application leads to a long development cycle because structural performance, durability, aging, life-time performance, and other extended lead-time factors need to be well understood prior to entering the four- to five-year development cycle. Time constraints on vehicle development, the need for optimized components, the high cost of gearing up, and very high demands on performance, appearance, and durability indicate that new materials and processes will have a very difficult time breaking the barriers of experience and knowledge base held by traditional materials.

An alternative approach is to explore application of improved steels coupled with an innovative approach to design, as demonstrated by UltraLight Steel Auto Body (ULSAB) activities of AISI. By using material, such as steel, that has a proven track record relative to cost and performance, the numerous unknowns associated with new materials, changes in production and infrastructure can be avoided. ULSAB has clearly demonstrated the potential for developing a steel autobody design that provides an economical, technological and environmentally viable alternative route for reaching fuel efficiency goals.

At the same time, the auto industry has identified key focus areas, including product lead times, development costs, and quality where rapid improvements in the production cycle must be made. Traditional methods for new product development involve building and testing prototype hardware. These tests involve a number of design criteria, such as structural strength verification, ride quality, and noise suppression, and are tremendously expensive and time consuming. Issues of production, cost, and quality will be addressed by reducing the dependence on traditional empirical approaches and by increasing the use of accurate analytical vehicle modeling techniques. This process is based on a rational approach to minimizing the number of physical structures required to be built and tested to prove a design. The technology developed within this research project is aimed at addressing these issues and includes predictive and modeling capabilities to aid in accelerated development.

The objective of the research is to perform a comprehensive computational analysis of the effects of advanced material processing, forming and joining techniques on performance of ULSAB vehicles. The research addresses numerous material related effects, impact conditions as well as analyze the performance of the ULSAB vehicles in crashes against designs representing the current US vehicle fleet. Current vehicle designs will be analyzed to determine the applicability of ULSAB technology to different vehicle classes and the resulting effect on structural performance.

Technical Issues

The real challenge of reducing vehicular weight is to maintain and improve performance compared to current designs and to meet and exceed safety standards without sacrificing affordability. ULSAB is an aggressive attempt to achieve such a weight reduction. It utilizes new steel technologies such as high and ultra high strength steels, hydroforming, tailor-welded steel blanks, steel sandwich materials, laser welding, and applies them at performance-critical regions.

ULSAB uses high strength steel and ultra high strength steel for more than 90 percent of the body structure to improve structural performance and save mass. One challenge posed by these steels is that they for differently from the mild steel to which many component fabricators are accustomed. High strength steel stampings have greater springback and require different draw angles, so each different grade must be treated by the design engineer and manufacturing engineer as a unique material.

Nearly half of ULSAB's mass consists of parts that require tailored blanks, which enable the design engineer to locate various steels within the part precisely where their attributes are most needed, thereby removing mass that does not contribute to performance. Tubular and sheet hydroforming and their cold working effect produce high dimensional stability and increase effective yield strength in formed components. Laser welding was used in ULSAB to improve dynamic strength of joints, for areas where access was available on only one side and for good aesthetic appearance of joint areas. Laser welding also has the benefit of a small heat-affected zone, which reduces dimensional distortion and material property changes.

These advanced materials and processes enabled the design engineers to consolidate functions in fewer parts, reducing ULSAB's part count to 96 major parts and 158 total parts, as compared with more than 200 total parts for an existing typical body structure in the same class. Reduced part count leads to reduced tooling and assembly costs. The function consolidation also leads to mass savings and improved structural performance.

Variety of computational models and simulations have been used in ULSAB design process. The type of computational model depends on the modeled physical process and on the accuracy that is required. For example, general stiffness, strength and cost can be analyzed using modestly large, tractable finite element models, the dynamic impact aspects require large, detailed, computational models that can have in excess of fifty to one hundred thousand finite elements. The crash is a violent, dynamic process where large amounts of energy are dissipated in less then 150 milliseconds. Characteristic like engine mounts, suspension, thickness variations, parts interaction, contact and entrapment of parts can have substantial influence on the overall load paths, energy dissipation, vehicle safety and integrity.

The ULSAB vehicle design is a sophisticated, technically advanced engineering product that requires adequately advanced materials processing support system. The reduced number of parts and joints also means that there is less redundancy in the structure. Parts need to be manufactured in closer tolerances than it is the case in current vehicles. The materials processing conditions and their corresponding effect on structural properties need to be addressed with more attention than in mild-steel based vehicles.

As a result, detailed and numerous computational simulations will need to be performed that can take into account the effects of advanced material processing on the ULSAB performance so that the cost and number of prototypes needing to be physically tested are minimized.

The expense of physical testing and the availability of supercomputers have motivated the development of sophisticated computer programs to model such complex physical phenomena. In recent years, massively parallel computer designs have emerged and presented a new technology aimed at meeting the ever increasing need for scalable processing speed. Massively parallel computers can combine hundreds or even thousands of processors which are able to operate concurrently on a problem. These computers to provide a viable route for performing detailed and realistic vehicle performance analyses and consequently enabling the assessment of material processing and manufacturing conditions on performance of ULSAB automobiles.

Scope

The scope of the project is to accelerate the development and introduction of lightweight steel autobodies through the use of advanced computational simulations for the assessment of advanced materials processing techniques on design and performance. The project will :

  • Analyze existing ULSAB designs
    • develop structural finite element computer models that integrate advanced material processing
    • computationally analyze various dynamic loading situations including with rigid and deformable barriers and current US fleet vehicles
    • evaluate the effect of advanced material processing, forming and joining techniques on ULSAB structural performance
  • Analyze vehicle models representing the current US car fleet
    • analyze applicability of ULSAB for different vehicle classes
    • develop general design modification for ULSAB
    • determine weight reduction potential using ULSAB
    • evaluate the effect of advanced ULSAB material processing for different vehicle classes
  • Document the developments and findings.

This project will take advantage of ongoing automotive related projects at ORNL aimed at developing vehicle models that are representative of the US car and truck fleet. These models will be used as starting platforms for this project and only the activities specific to this project will have to be performed.

Project Approach


Phase I.

The initial activities will concentrate on establishing a partnership that will define loading and materials processing conditions to be analyzed in order to evaluate material processing effects on performance of lightweight steel designs. The issues of target weight reduction, cost, and performance will be analyzed in the context of advanced material processing required for USLAB design.

Phase II.

The second phase of the project will be dedicated to performing detailed computational analyses of the problems defined in the first phase. Two main directions will be followed:

2.1. Analysis of material processing and manufacturing effects on existing ULSAB designs

The computer models of existing ULSAB designs that incorporate the structural effect of advanced material processing and joining techniques of will be developed and modified in order to be analyzed in various dynamic loading situations. The impact situations to be considered will include:

  • frontal, offset, oblique, side, and rear impacts
  • rigid and deformable barriers, standard US fleet cars and ULSAB vehicles

This analysis will provide a comprehensive performance evaluation of ULSAB designs, evaluate the effects of materials processing and manufacturing, compare it with the standard mild steel based vehicles, and identify the limitations and advantages that are resulting from ULSAB materials processing and manufacturing techniques.

2.2. Analysis of vehicles models representing the current US car fleet

The models representing the current US car fleet will be used to evaluate the potential of ULSAB designs for the wide range of vehicle classes. Each of the models will be analyzed to investigate the applicability of ULSAB for the particular vehicle classes. General design modifications for achievement of targeted weight reduction will be determined. The potential, limitation, and effects of material processing on the performance of ULSAB vehicles will be analyzed.

3. Documentation of developments and findings

The vehicle models, processing models and impact situations analyzed through the course of the project will be organized into a parametric set of models that will allow for simple modification of processing conditions and their influence on the structural performance of the ULSAB vehicles.

Project Schedule and Milestones


Year 1

Task 1. Develop a partnership for ULSAB vehicle study

Task 2. Determine ULSAB vehicle designs to be modeled and analyzed.

Task 3. Determine the US car fleet models to be used in impact with ULSAB vehicles.

Task 4. Determine the impact scenarios to be analyzed.

Task 5. Determine material processing and manufacturing conditions which are going to be incorporated into ULSAB structural models and analyzed.

Task 6. Document the developments and findings of the project to date.

Year 2

Task 7. Develop models that simulate the effect of material processing and manufacturing (defined in Task 5) on structural properties of finished material.

Task 8. Incorporate material processing related structural models into computational models of ULSAB vehicles.

Task 9. Combine different ULSAB, barrier, and fleet car models to develop impact situations (defined in Task 4) to be analyzed.

Task 10. Perform computational analysis of impact scenarios developed in Task 9 using the vehicle models that incorporate materials processing conditions and were developed in Task 8.

Task 11. Evaluate the effect of advanced material processing, forming and joining techniques on ULSAB structural performance.

Task 12. Document the developments and findings of the project to date.

Year 3

Task 13. Determine the weight reduction potential of ULSAB design for different vehicle classes that are represented by US car vehicle fleet model.

Task 14. Determine the applicability of the ULSAB design for different vehicle classes.

Task 15. Develop general ULSAB design modifications for different vehicle classes.

Task 16. Evaluate the effect of the advanced material processing and manufacturing associated with ULSAB design on structural performance.

Task 17. Develop a library of computational models used in the project.

Task 18. Document the developments and findings of the entire project.