The Forging Industry Educational and Research Foundation has taken full advantage of the positive findings of recent research to promote forged steel for safety-critical automotive components over other materials and processes. Summary information on the project was distributed earlier this summer to more than 4,000 forging customers and prospects.
The study was conducted by the University of Toledo for the Bar Applications Group of the American Iron and Steel Institute and FIERF. In the study, the fatigue-performance characteristics of steering knuckles made of steel, aluminum, and cast iron were examined using experimental, numerical, and analytical tools.
Professor Ali Fatemi and research assistant Mehrdad Zoroufi, Dept. of Mechanical, Industrial, and Manufacturing Engineering at the University of Toledo, conducted the study and prepared the report for FIERF and AISI.
Rationale for the study
The sponsors provided the funding for the study to document how the properties of forged steel stacked up against competitive materials and processes in safety critical applications. This was important because designers in the automotive industry have a wide range of materials and processes to select from, including steel and aluminum forgings and castings, cast irons, and powder forgings. The competition is particularly acute in the chassis, and it is not unusual to find a range of different materials and manufacturing technologies employed within modern chassis components.
The automotive industry is continually striving to produce lighter, less expensive, and more efficient components that exhibit precise dimensions, need less machining, and require less part processing. Material mechanical properties and manufacturing parameters have guided the design engineer in making the optimum choice for his specific component and application.
In automotive design, component durability evaluation based exclusively on experimental assessments is time-consuming and expensive, so analytical approaches that include a limited number of component verification tests have gained more attention. This approach reduces design cycle time due to reduced testing, allows inexpensive evaluation of changes in geometry, material, loading, and manufacturing process through performance simulation, and provides evaluation techniques for product optimization and failure analysis.
Accordingly, the FIERF-AISI 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.
Study objectives and scope
The overall objectives of the research program were:
The study consisted of several main topics:
For specimen testing, strain-controlled monotonic and fatigue tests of forged steel, cast aluminum and cast iron steering knuckles based on 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.
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 of, stress and strain distributions for 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. The strengths and deficiencies of methods were evaluated. An analytical optimization study of the forged steel steering knuckle was 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.
FIERF/AISI study results … and conclusions
Material Fatigue Behavior:
Finite Element Analyses:
Component Fatigue Behavior
Fatigue Life Predictions: