Steel Forgings are Superior, according to safety-critical application study

Among other advantages, FIERF and AISI demonstrated that forged steel is considerably stronger and more ductile than cast aluminum and cast iron, and that optimization of a forged steering knuckle can achieve weight and cost reductions.

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:

  • To assess fatigue life and compare fatigue performance of competing material and manufacturing process technologies.
  • To develop a general durability assessment methodology for safety critical automotive components.
  • To develop a method for efficient and reliable optimization of such components That satisfies performance criteria and considers geometry, material, manufacturing parameters and costs.

    The study consisted of several main topics:

  • A background study on forging and its competing manufacturing processes, and vehicle engine and chassis components that are produced by these competing processes.
  • A literature review that focuses on the comparison of competing material and manufacturing process technologies and durability assessment and optimization of performance critical automotive components.
  • Experimental work including specimen and component testing.
  • Analytical work including durability assessment and optimization analysis.

    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:

  • Forged steel is considerably stronger and more ductile than cast aluminum and cast iron.
  • Forged steel exhibits higher cyclic strength and higher resistance to plastic deformation.
  • The SN fatigue resistance of forged steel is significantly better than the two cast materials.
  • With respect to low cyclic fatigue, forged steel is superior to cast aluminum and cast iron.


    Finite Element Analyses:

  • Linear elastic FEA is not sufficient for reliable fatigue life predictions.
  • FEA simulation for cyclic loading is important for fatigue analysis.


    Component Fatigue Behavior

  • For the same stress amplitude level the forged steel steering knuckle exhibits approximately two orders of magnitude longer life than the cast aluminum part.
  • Failure locations in the component tests agreed with FEA predictions.
  • Crack growth life was found to be a significant portion of the cast aluminum steering knuckle fatigue life but insignificant for the forged steel steering knuckle.


    Fatigue Life Predictions:

  • The nominal stress approach cannot be used for complex component geometries.
  • Local stress or strain approaches in conjunction with FEA results were found to provide better life predictions,
  • Linear elastic FEA, when modified to correct for plastic deformation, is an effective and capable approach for life prediction of components with complex geometries and/or loadings.


    Optimization:

  • Material and manufacturing process considerations are major constituents of a general optimization procedure with durability constraints for automotive components.
  • Material alternatives identified in the study provide higher fatigue strength.
  • Added manufacturing operations such as surface hardening and surface rolling to induce compressive residual stress can improve fatigue strength of forged steel components in critical stress areas.
  • Optimization of the steering knuckle can achieve weight and cost reductions of at least 12% and 5%, respectively.
  • The approach followed in the study is applicable to other forged components.
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