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Automotive forgings and fatigue: a research paper.

The study excerpted below was concerned with fatigue performance evaluation of forged versus competing manufacturing process technologies using experimental, numerical and analytical tools. Most of the results have been presented at several conferences and published in conference proceedings and journals. The fully-detailed report, including the literature review, experimental data and analytical results, is available from the organization listed at the end of the excerpt.

Background and motivation for the study

Manufacturing processes face major competitions in automotive industry to produce lighter, cheaper and more efficient components of greater precision, while requiring less machining and less part processing. Material mechanical properties and manufacturing parameters play decisive roles, and the weaknesses and strengths of each manufacturing process need to be available to designers in these respects, to enable them to choose the optimum choice for the specific component and application.

In the automotive industry, designers have a wide range of materials and processes from which to select. Steel and aluminum forgings and castings, cast irons and powder forgings have found broad applications in automotive safety-critical systems. The competition is particularly acute in vehicle chassis, and it is not unusual to find a range of different materials and manufacturing technologies employed within modern chassis components.

Many safety-critical components in the vehicle experience time-varying loadings during a major portion of their service life. However, material selection for these components made by various manufacturing techniques is often based on monotonic rather than cyclic properties. The stress-strain behavior obtained from a monotonic tension or compression test can be quite different from that obtained under cyclic loading.

In addition, fatigue is the major cause of most mechanical failures in components. Fatigue behavior is, therefore, a key consideration in design and performance evaluation of automotive components, and to address the issue effectively and economically, engineers need to model and design for mechanical fatigue early in the product design stage. Overlooking fatigue behavior often results in inefficient design and/or over-designed parts from large safety factors.

This research was motivated by a practical need to assess and compare fatigue performance of components produced by competing manufacturing processes, to develop a general durability assessment methodology for automotive chassis (and similar) components, and to implement an optimization methodology that incorporates structural durability performance, material properties, manufacturing and cost considerations for such components.

Scope

Strain-controlled monotonic and fatigue tests were conducted on three specimens--a forged steel SAE Grade 11 V37 steering knuckle of the rear suspension of a 4-cylinder sedan weighing 2.4 kg; a cast aluminum ASTM A356-T6 steering knuckle of front suspension of a 6-cylinder minivan weighing 2.4 kg; and a cast iron ASTM A536 Grade 65-45-12 steering knuckle of the front suspension of a 4-cylinder sedan weighing 4.7 kg. ASTM standard test methods and recommended practices were conducted.

The data obtained made it possible to compare deformation response, fatigue performance, and failure mechanisms of the base materials and manufacturing processes, without introducing the effects and interaction of complex design parameters. Load control component tests for the forged steel and cast aluminum steering knuckles were also conducted.

The analytical work consisted of finite element analysis (FEA), durability assessment and optimization analysis. Linear and nonlinear finite element analyses of the steering knuckles were conducted to obtain critical locations and stress-and-strain distributions of each component. A general life prediction methodology for the subject components was developed, where material monotonic and cyclic data and results of the FEA were used in life prediction methods applicable to safety-critical automotive components. An analytical optimization study of the forged steel steering knuckle was also performed. Such optimization sought to minimize weight and manufacturing costs while maintaining or improving fatigue strength of the component by targeting geometry, material and manufacturing parameters.

Summary and conclusions

The effects of manufacturing process on fatigue design and optimization of automotive components using experimental, numerical and analytical tools were investigated. The findings of this study are summarized below.

Material fatigue behavior and comparisons

* Forged steel is considerably stronger and more ductile than cast aluminum and cast iron.

* The cyclic deformation curve of the forged steel is independent of the geometrical direction (i.e. isotropic behavior).

* Significantly better S-N fatigue resistance of the forged steel was observed, as compared with the two cast materials.

* Forged steel was found to be superior to cast aluminum and cast iron with respect to low cyclic fatigue. In automotive design, cyclic ductility can be a major concern when designing components subjected to occasional overloads, particularly for notched components, where significant local plastic deformation can occur.

* A Neuber stress versus life plot, which considers the combined effects of both stress and strain amplitudes, shows forged steel to have about two orders of magnitude longer life than cast iron and about four orders of magnitude longer life than cast aluminum.

Finite element analysis

* Even at a lower loading level, which can be considered as an indication of long-life service of the components, the material undergoes local plastic deformation. This is evidence that mere use of linear elastic FEA is not sufficient for reliable fatigue life predictions.

* FEA simulation for cyclic loading is important for fatigue analysis since cyclic deformation material response can be vastly different from monotonic deformation response.

Component fatigue behavior and comparisons

* Based on the component testing observations, crack growth life was found to be significant portion of the cast aluminum steering knuckle fatigue life (on the average, about 50% of the cast aluminum steering knuckle life is spent on macro-crack growth), while crack growth life was an insignificant portion of the forged steel steering knuckle fatigue life.

* Component testing results showed the forged steel steering knuckle to have about two orders of magnitude longer life than the cast aluminum steering knuckle, for the same stress amplitude level

* Failure locations in the component tests agreed with FEA predictions.

Fatigue life predictions

* The nominal stress approach cannot be used for complex component geometries, because nominal stress cannot be defined explicitly for complex geometries.

* The local stress or strain approaches in conjunction with the FEA results were found to provide better life predictions, as compared with the commonly used nominal S-approach.

* Simpler and less time consuming linear elastic FEA, when modified to correct for plastic deformation, proved an effective and capable approach for life prediction of components with complex geometries and/or loadings.

Optimization

* Manufacturing process considerations, material and cost parameters are major constituents of a general optimization procedure with durability constraints for automotive components.

* The proposed material alternatives (steel) provide higher fatigue strength for the component.

* Additional manufacturing operations, such as surface hardening and surface rolling to induce compressive residual stress, can be considered to improve fatigue strength of a forged steel steering knuckle at the spindle fillet area.

* Optimization of a steering knuckle can produce overall weight and cost reductions of at least 12% and 5%, respectively, with no performance degradation.

* The approach that was followed in the study is applicable to other forged automotive components.

Finally, it was concluded that components with fewer geometrical restrictions than the steering knuckle considered in this study have much higher potential for weight reduction and cost savings.

Circle 110--AISI, or connect directly at www.rsleads.com/508df-110

RELATED ARTICLE: Sweet sound of success.

Software supports automotive audio development

edited by Richard Mandel

Located in Hampshire, UK, EuroTec International plc provides audio design services for automakers, industrial customers, gaming companies, and makers of public address systems. With projects worldwide, the company is a key resource in the design and supply of loudspeakers, tweeters, subwoofers, grills and other audio devices.

Initially, the company was established because many automobiles arrived in their destination countries without any radio/CD/tape unit installed. For many customers, the sound system, used on a daily basis, becomes an important element of the car. The challenge is to engineer an audio system taking full advantage of a car's physical design. Many variables come into play, including the speaker placement, interior acoustics, volume of available air, and alignment of the speakers with respect to the listener.

Over the years, the company began using its expertise in sound and acoustics to influence the acoustical design earlier in the process. "Where we began as a second-tier supplier, we have now worked our way back up the customer chain so we are now a first-tier partner supplying the auto manufacturers themselves, influencing how the loudspeakers and grill fit into the vehicle," according to John Radford, EuroTec research and development manager.

One of the biggest challenges for the engineers was having access to the vehicle design at a very early stage in order to provide cost bids on a project. Since there is no single, central design software used throughout the industry, the company needed to read and comment quickly on designs executed in software systems such as CATIA, Delcam, EDS I-DEAS and Unigraphics.

The company elected to use SpinFire Pro from Actify. "Where SpinFire Pro has been so useful to us is the speed of access," says Radford. "It allows us to respond quickly to a complex request and to turn it around within the week, and frequently within the same day. In our own design efforts, we mainly use Catia V5, but also Pro E and I-DEAS. Even when you have the software, however, it doesn't make sense to have a CAD workstation used up as a viewer. It's not just the software cost, but also the hardware and the productivity of having that seat tied up. Through productivity savings alone, SpinFire has saved us tens of thousands of pounds. Many times, our questions are about costing or production issues and the person who needs the answer is an acoustics engineer or a cost analyst who did not need to use a complete CAD system. In fact, using the CAD system would slow down the process, and end up costing us several days."

SpinFire Pro has also allowed them to be more efficient in responding to potential new customers. Continues Radford, "Before using SpinFire Pro, in order to access the drawings we used to go to an outside company to translate the data for us--at a cost of 100 to 300. Or, we would have to write back to the customer asking for IGES data or another intermediary format.

"Later, we discovered that IGES files could be very risky to use. It was only after we began using SpinFire Pro that we realized how much data was being lost when incoming IGES files were being translated into other CAD packages. When we compared the IGES files to what we were seeing in SpinFire, we realized that the assembled nature of the data could be lost. The software is particularly good in bringing the data into position, and best when working with assemblies."

Radford then explains, "We can't make any mistakes in the costing process. We often work on fixed cost bids and both our reputations and our profits are on the line. If you're not absolutely sure of your costs and the data to make that part, then you haven't sharpened your pencil as much as is needed. With SpinFire, rather than getting a 2D drawing and not understanding it immediately, we have a 3D model with complete information.

"This is of paramount importance in the auto industry. We are normally working on quantities of 10,000 to 20,000 units or more per year, per job, and the contracts last from three years upwards, so a small cost savings are magnified through both scale and time.

"Our biggest gain has been through increased productivity," concludes Radford. "For example, recently an OEM came to us with parts they wanted us to produce. Normally it would have been a 14- to 16-month project, but because we were able to see the design and communicate our changes so quickly, we were able to turn it around in six months with SpinFire. It's been a great addition to our organization."

Circle 112--EuroTec International plc, or connect directly at www.rsleads.com/508df--112

Circle 113--Actify, or connect directly at www.rsleads.com/508df-113

Professor Ali Fatemi and Mehradad Zoroufi, research assistant Department of Mechanical, Industrial and Manufacturing Engineering University of Toledo
COPYRIGHT 2005 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2005 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Fatemi, Ali; Zoroufi, Mehradad
Publication:Designfax
Date:Aug 1, 2005
Words:2018
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