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24th Forging Industry Technical Conference, Cleveland, Ohio, 2002

FATIGUE PERFORMANCE EVALUATION OF FORGED VERSUS COMPETING PROCESS TECHNOLOGIES: A COMPARATIVE STUDY ALI FATEMI AND MEHRDAD ZOROUFI PROFESSOR AND RESEARCH ASSISTANT, RESPECTIVELY DEPARTMENT OF MECHANICAL, INDUSTRIAL, AND MANUFACTURING ENGINEERING THE UNIVERSITY OF TOLEDO, TOLEDO, OHIO 43606 ABSTRACT To increase the usage and competitiveness of forged ferrous components, comparative mechanical and metallurgical properties for ferrous forged components and similar components produced by other manufacturing technologies must be evaluated. Among the mechanical properties, fatigue properties and performance are key considerations in design and performance evaluation of many components. The overall objective of this study is therefore to compare fatigue performance of forged components with those of components in which forging is not a process step. Steering knuckle was chosen as an example representative component, because it is a common automotive component. In addition to a literature survey, the overall study includes both analytical as well as experimental evaluations. This paper describes the analytical and experimental methods to be employed and presents some of the findings from the literature survey conducted. INTRODUCTION AND PROJECT DESCRIPTION Fatigue is a major consideration in the design and performance evaluation of materials, components, and structures since about 90 percent of all mechanical failures are attributed to fatigue fractures (1). A comprehensive study of the cost of fracture in the United States indicated a $119 billion (in 1982 dollars) cost occurred in 1978, or about 4% of the gross national product (2). The greatest portion of these failures were found to occur in motor vehicle and parts. The investigation emphasized that this cost could be significantly reduced by using proper and current technology in design, and this includes fatigue design. In automotive component design and analysis, efforts have been made to determine the fatigue behavior of components made of forgings as well as competing processes (3-6), to implement methodologies for stress analysis and fatigue life predictions (7-13), and to optimize such components (14-16). Only a small number of studies have been devoted to manufacturing process comparisons with a focus on durability aspects. Such studies are necessary to enhance the competitiveness of the forged components and their application in the automotive industry. Accordingly, the overall objective of this research program is to compare fatigue performance of forged components with that of components in which forging is not a process step.

Steering knuckle was chosen as an example representative component, because it is a common automotive component. The main competitions to forged steel steering knuckle are cast iron and cast aluminum steering knuckles. The overall study includes a literature survey, analytical evaluations, as well as experimental work. The literature survey includes both a survey on comparison of forging with competing manufacturing processes and potentials for improving competitiveness, as well as a survey of durability and optimization studies on steering knuckle. The analytical work includes stress analysis, durability analysis, and optimization analysis. The experimental work includes both specimen testing to characterize material monotonic deformation and fatigue performance, as well as component testing to compare with life predictions. This paper describes the aforementioned analytical and experimental methods to be employed and presents some of the findings from the literature survey conducted. Literature Survey A literature review is conducted to determine the processes and example parts that represent competition to forgings and compare mechanical properties of forgings and competitive products such as castings. Mechanical properties evaluated mainly include strength, ductility and fatigue. Similar alloys and heat treatment conditions are evaluated and benefits of forgings (in particular regarding fatigue properties) are compared to other products. A specific literature survey is also conducted for vehicle steering knuckle, which is used as an example part in this study. This survey includes material selection and manufacturing, stress analysis, fatigue analysis and life prediction, and optimization analysis. As mentioned earlier, some of the results from the literature survey are presented, after the analytical and experimental methods to be employed in the overall project are described. Stress Analysis Finite element analysis of the steering knuckle will be conducted to obtain stress distributions for the component. Forces and moments that the steering knuckle experiences include static reactions imparted via the tire patch, dynamic force due to steer motion, dynamic lateral loading, off-center loading when the tire is loaded laterally on the tire sidewall, and oscillatory response that the knuckle exhibits due to impact loading. As recommended by Conle and Chu (9), an elastic unit load analysis combined with a superposition procedure for each load point in the service history will be utilized to produce each elements stress history. This approach uses strength of materials method and an elastic FEA model to obtain stress-strain relation, and subsequently load-strain relation, which is the norm for most vehicular durability analyses. This approach contrasts a plastic FEA analysis which would compare load-strain curve directly. Nonlinear analysis will also be used in this study, if appropriate, where deformations may be inelastic such as at stress concentrations.

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Durability Analysis Based on the finite element analysis, fatigue life predictions will be performed. Basically, four fatigue life models exist (1). These consist of the nominal stress-life (S-N) and the local strain-life (ε-N) models which are based on crack nucleation, the fracture mechanics model which is based on fatigue crack growth (da/dN-∆K), and the two-stage model which consists of combining the strain-life and crack growth models to incorporate both crack nucleation and growth. In the stress-life analysis the material properties are obtained from load-controlled fatigue tests, while in the strain-life analysis, the material properties are obtained from strain-controlled fatigue tests. The strain-life approach is also called local strain approach since nominal stresses and strains are translated to local stresses and strains by using experimental methods such as strain gauges, computational methods such as FEA, or analytical models such as Neuber’s rule. In the fatigue crack growth analysis, the governing parameter is the stress intensity factor which is obtained from the component geometry and loading. The material properties used are fatigue crack growth rate of the material which is obtained from fatigue crack growth tests, and material fracture toughness. Figure 1 provides a fatigue design flow chart, often used for automotive components (1). In this flow chart, a number of inputs are gathered in order to select the configuration of the component, the material(s) of construction and the manufacturing process(s). These include component size and geometry (such as stress concentrations and notches), material properties obtained from specimen testing, the environment (such as corrosion, temperature, and fretting), and manufacturing/processing/treatment, which affect material properties and introduce effects such as surface roughness and residual stresses. The type of loading the component is subjected to can be classified as constant versus variable amplitude, uniaxial versus multiaxial (such as combined bending and torsion), and with or without mean stresses. A cycle counting method (such as rainflow cycle counting) and a cumulative damage model (such as the Miner linear rule) are needed for variable amplitude loading. An equivalent multiaxial stress/strain parameter (such as von Mises or Fatemi-Socie parameters) is needed for multiaxial loading. A mean stress correction parameter (such as SWT parameter) is required for mean stress correction, if present. A fatigue design criterion is also selected depending on the functionality, design requirements, cost, and safety critical nature of the component. These criteria include infinite-life, safe-life, fail-safe, and damage tolerant designs. Based on the selected criterion and the above considerations, the fatigue life of the component is then estimated and verified against component bench testing. Optimization Based on the results of stress and durability analyses and the testing conducted, an analytical optimization study of the forged steel steering knuckle will be performed. Such optimization seeks to minimize stress, maximize fatigue life, and minimize manufacturing weight. The same model used for the finite element analysis will be used to perform the optimization analysis. Recommendations will be made on the optimized geometry. The optimization problem is made up of three basic considerations. It

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includes an objective function to be minimized or maximized, which for this case would be stress in a particular region of the component and fatigue life of the component. Another consideration is a set of design variables that affect the value of the objective function, which are the variables used to define the geometry and material of the knuckle in this study. Finally, A set of constraints are specified that allows the design variables to have certain values but exclude others. For this study it is intended to limit the component’s weight, while other constraints might be fatigue life or crack nucleation size. The optimization aspects include optimization analysis, manufacturing of the optimized part if possible, and testing/verification of the optimized part. Such analysis and testing allow production of lighter steering knuckles, resulting in more efficient engines. The optimized parts for testing are manufactured by the component manufacturer. Specimen Testing Strain-controlled monotonic and fatigue tests of specimens obtained from steering knuckles will be conducted. From these experiments, both static as well as baseline cyclic deformation and fatigue properties of materials are obtained. Such data provide a direct comparison between deformation, fatigue performance, and failure mechanisms of the base materials, without introducing the effects and interaction of complex design parameters such as surface finish, component size, residual stress, and stress concentration. They also provide the required baseline data for life prediction analysis to predict component fatigue life and performance under actual service loading conditions. ASTM standard test methods and recommended practices will be followed for all tests. The forged steel data are available from the AISI fatigue database, while these properties for the other two materials (cast iron and cast aluminum) will be generated in this study. Component Testing Component testing is more challenging than specimen testing because of the large load requirements and complexities of setups and fixturing, to be able to cycle the most important loading of the steering knuckle over long periods. A limited number of load or stroke-control fatigue tests of steering knuckles made of forged steel and alternative materials/processes (i.e. cast iron and cast aluminum) will be conducted. The stress history amplitudes will be increased (magnification factors to be determined) compared with those recorded on the corrugated proving ground in order to reduce testing time. Strain gages will be positioned on those locations found critical during FEA stress analysis. Data from such tests provide a direct comparison between fatigue performances of the components made of each base material and manufacturing process. This comparison inherently includes effects such as surface finish, component size, residual stresses, stress concentrations, etc. In addition, such data provide validation for the life predictions performed based on the baseline material fatigue data generated by specimen testing.

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SUMMARY OF THE LITERATURE SURVEY Manufacturing Processes Competing with Forging Various mechanical and metallurgical properties, environmental considerations, and above all cost competitiveness aspects are the main driving forces shaping the future direction of the forging industry. In the ground vehicle industry, the power train components, suspension components, and steering system components are the main application of steel forgings. Such conventional forged components include crankshaft, camshaft, connecting rod, piston crown, steering lever, suspension arm, steering knuckle and spindle, wheel hub, drive flange and axle beam. Some of these components are also manufactured by die-casting, and more recently by powder forging and composite technologies. Powder metallurgy processes using sintering offers netshaped products and have been used to improve productivity. Composites offer lightweight and directional properties, but are comparatively expensive. A number of selected examples from literature are provided here to indicate the drivers for conversion from forging process to competing technologies and to help improving forging competitiveness. Forged products are compared to cast competitor products in Table 1. The cost of forging a part compared to that of making it by various casting techniques, machining, or by other manufacturing methods is a key consideration and often depends on the production volume. A cost comparison for a typical automotive component (connecting rod) is provided in Figure 2 (17). As can be seen, all other factors being the same, and depending on the number of pieces required, manufacturing a certain part by, say, expandable mold casting may well be more economical than doing so by forging and on the other hand, for large quantities forging is more economical. Powder metallurgy offers high precision and low weight tolerances leading to less machining operations in comparison to classically forged or cast components. On the other hand, fatigue resistance and toughness are not generally as good as steel forgings (18). Powder metallurgy may not be satisfactory for parts with small precision geometrical details. Also, large components can be forged, whereas only relatively small parts can be manufactured by P/M to achieve high density. Jang et al. (19) conducted a study on powder materials and production processes by producing the clutch disk spline hub of automobile, to replace with the existing forged component. They also investigated mechanical properties and microstructure along with the performance of a dynamic test of three types of powder materials. They concluded that one of the produced powder metallurgy samples which is a diffusion alloy powder and is treated with carburizing-tempering, performed better in torsion durability tests and wear resistance than that of existing forged steel component. They also concluded that if powder metal is sintered and treated with adequate condition, toughness could be improved to the same level as forged metal. In general, the main advantages of P/M parts are elimination of waste material as well as machining operations and low unit cost when mass-produced, while their main disadvantages are high cost of dies,

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typically lower physical properties, higher cost of materials, limitations on the design, and the limited range of materials which can be used. Thixoforming has come to play an increasing role as an alternative to forging, especially to obtain high-strength aluminum components for lightweight automotive designs. In this process a semi-solid metal is injected into a closed die. Thixo-formed aluminum components are often intended to replace steel forgings to form near-net shape components, or nodular cast iron components to reduce the solidification shrinkage. Hirt et al. (20) used a pilot thixoforming system to redesign, thixoform and test an aluminum steering knuckle, as a thin walled structural component subject to high loads, and to compare the product with the original steel-forged knuckle. They combined steering knuckle thick-walled and thin-walled areas with stiffening ribs and undercuts to gain necessary yield strength, fracture toughness and stiffness. As a result, the weight of the new part was 50% below that of conventional forged steel design, despite identical functional capabilities. The positive trend for the application of cast components is mainly due to lower cost incentives. However, weaker mechanical properties of cast components due to a wide variety of flaws and their low ductility have always been a matter of concern. Houshito et al. (21) performed a feasibility study on the application of high strength ductile iron to automotive chassis parts, namely steering knuckles. They state that while the shape of forged part is limited by the manufacturing process, the shape of casting part can be optimized by balancing the stress distribution. They intended to reduce weight and cost of the steering knuckle by replacing the forged part by a cast part with optimized shape and comparable strength. In this regard, stress and rigidity analysis and fatigue and impact testing were conducted on the knuckle. They concluded that the Young's modulus of castings is lower than that of forging by 20%, and fatigue strength of the cast knuckle was lower than the original forged knuckle by 23%. The influence of surface roughness and defects on fatigue life for various manufacturing processes is also a key consideration in durability performance. Processes such as powder metallurgy have sometimes been more appealing, if they require less machining after production (22, 23). Surface effects also include differences in microstructure, chemical composition, and residual stresses. Figure 3 shows a comparison of fatigue strength in various manufacturing processes for a front suspension arm. Even though for the hot forged steel surface defects of blank surfaces reduce fatigue strength by 30% from that of the machined surface, the fatigue strength is still considerably better than that of the nodular cast iron arm. Strain hardening of the surface layer has a strong influence of fatigue behavior which depends on the depth of the deformed layer. Surface decarburization can occur after hot forging, which can cause different defects on the surface layer, reduce strain hardening of the surface layer, and consequently reduce the fatigue strength. Excessive strain hardening resulting from large deformations can also produce cracking and flaking of the surface and significantly reduce the fatigue strength.

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Benefits of Forgings and Some Potentials for Improving Competitiveness Application of forged parts made of microalloyed steels to automobile parts is becoming increasingly common due to their superior properties compared to conventional quenched and tempered carbon steels. Microalloyed steels do not require heat treatment and, therefore, no additional machining for correcting distortions after forging is necessary. The fatigue properties and toughness of microalloyed steel forgings have been demonstrated to be fit for purpose. However, compared with heat treated low alloy steels their fracture toughness is somewhat lower, even though still significantly superior to castings. To consider fatigue durability, Kuratomi et al. (24) developed lightweight connecting rods based on fatigue resistance analysis of microalloyed steels. Rotating bending fatigue on smooth and notched specimens as well as component buckling and load-life fatigue tests were conducted. Figure 4 shows the fatigue test results obtained with actual connecting rods made of SV40CL1 microalloyed steel (0.4% carbon steel) and S40C quenched and tempered (equivalent to SAE 1040) steel. It was concluded that connecting rods made of the forged microalloyed steel exhibit 25% higher fatigue limit than the similar forged Q& T steel and are 10% lighter in weight. Following the same goal, Farsetti and Blarasin (3) investigated the possibility of replacing forged quenched and tempered steels by forged microalloyed steels of appropriate composition and microstructure. They concluded that replacing Q&T steels with microalloyed steels is possible considering the following: • For mechanical components that can be made from 800 MPa class steels, microalloyed steels could be used. With low-C high-Mn steels, satisfactory strength values can be attained, keeping good toughness properties. • The higher-strength class microalloyed steel lends itself to an increase in strength and toughness by optimizing the microstructural parameters. • Small ferrite grain and presence of bainitic phase will increase strength of microalloyed steels significantly. Anisotropy, if present, is an importance consideration in durability performance. It could have beneficial or detrimental effects, depending on the loading direction. During initial breakdown of cast ingots or subsequent working nonuniformities in alloy chemistry, second-phase particles, inclusions, and crystalline grains are aligned in the directions of the greatest metal flow, known as the grain-flow pattern. Grain flow produces directional characteristics in properties such as strength, ductility, and resistance to impact and fatigue. The variation in yield strength and tensile strength is not usually as significant as that in impact and fatigue resistance. The forging process can use this directionality to provide a unique and important advantage by orienting grain flow within the component so that it lies in the direction requiring maximum strength. The maximum load-bearing capacity of a forging is realized when the component is loaded along the grain-flow direction. Properly developed grain flow in forgings closely follows the outline of the component. In contrast, bar stock and plate have grain flow in only one direction, and changes in contour require that flow lines be cut, exposing grain ends and rendering the material more liable to fatigue failure and more sensitive to stress corrosion.

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Fatigue crack growth resistance is an important consideration, when evaluating fatigue performance. Fatigue crack growth behavior of quenched and tempered steels, which are often used to produce forged products, are compared to cast steel in Figure 5. As can be seen, the quenched and tempered 4140 steel exhibits superior fatigue crack propagation behavior compared to the cast SAE 0030 steel. To improve fatigue performance of forged components, reducing surface defects and/or their effects play an important role. One way to achieve this goal is to induce surface compressive residual stresses. Applying localized inelastic deformation through processes such as shot peening or surface rolling, are the common available methods of inducing residual stresses. Fifty percent greater fatigue strength has been reported in rolled threads, compared with cut or ground threads made of high strength steel (1). Figure 6 illustrates the beneficial effect of shot peening on fatigue resistance of gears. Thermal processes can also induce residual stresses. For example, in surface hardening of steel by processes such as induction hardening, carburizing, or nitriding, in addition to the hard surface produced, a beneficial compressive residual stress is also created on the surface. This compressive residual stress can very effectively prevent the formation and growth of cracks. In order to keep pace with the new competitors, steel forgings must satisfy the automotive manufacturers requirements in terms of weight, cost, durability, recycleability and overall performance. Traditionally, forged components have been produced from heat treated carbon and low alloy steels. Although heat treated steels are still widely used, air cooled forging steels are becoming increasingly popular. Through a reduction in energy consumption, fewer process steps and lower inventories, these forgings can offer significant cost savings. Cristinacce et al. (25) provide some recent examples of the range of components produced from air cooled forging steels. An air cooled 0.53%C steel was used in the production of hubs and spindles, where an approximately 400% increase in hardness was achieved compared to the heat treated forgings. In another case, a swivel hub was redesigned as a forging in place of a steelcasting that had been proposed originally. The steel casting exhibited unacceptable distortion of the steering arm in heat treatment, surplus material leading to high machining cost, and excessive weight affecting the unsprung mass of the suspension design. A redesigned swivel hub was forged and control air-cooled. The results of mechanical tests on both the forging and the casting are given in Table 2. It can be seen that the forging had superior strength, ductility, and hardness, compared to the heat treated casting. The use of the forging also resulted in lower weight by 21%, better dimensional control, less machining, and avoidance of heat treatment costs. Steering Knuckle Studies Steering knuckle is selected as an example part for this study. Fatigue analysis and life prediction features and optimization of vehicle steering knuckle and similar automotive components were reviewed. The results of selected studies on determination of local stresses and strains, notch analysis, force and moment measurements, multiaxial stress/strain paths, fatigue failure diagnosis and analysis guidelines, and fatigue life assessment procedure are briefly discussed here.

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Lee et al. (12) describe the durability design process of a cast steering knuckle. Their objectives included:1) assessing quantitatively fatigue lives of steering knuckle, 2) identifying critical and non-damaging areas for design enhancement, and 3) weight reduction. They assumed that effects of dynamic frequency could be negligible and that a uniaxial notch stress/strain estimation procedure could be used for plasticity correction. The elastic stress components due to numerous static loads were combined to derive the time history for each stress component. The dynamic loads (i.e. variable field forces) and the static loads (i.e. clamp bolt forces and forces due to change in pressures and temperatures) were taken into account. The finite element models of the preliminary and optimized knuckles are shown in Figure 7. They used the critical plane approach to account for multiaxial stresses. The search for the plane where the fatigue crack initiates, called the critical plane, can be very time-consuming, but necessary, because the principal stress/strain axes vary with time for non-proportional loading problems. They noted that the SWT parameter was especially satisfactory for analyzing automotive suspension and engine components made of cast materials. knuckle strain gage measurements were made for loads in elastic and non-elastic load ranges. As a result of analyzing fatigue behavior of a vehicle axle steering arm based on local stresses and strains, to determine the effect of surface roughness and residual stress on fatigue life and to verify local strain approach, Savaidis (6) concluded that there was a significant detrimental effect caused by surface roughness and residual stress state on fatigue behavior. This detrimental effect for the surface roughness is shown in Figure 8. In addition, local strain approach for fatigue resistant design was shown to have good agreement with experimental results. Multiaxial fatigue is an important factor that should be considered in the fatigue analysis of components. Kocabicak and Firat (26) proposed a bi-axial load-notch strain approximation for proportional loading to estimate the fatigue life of a passenger car wheel during the cornering fatigue test under plane stress conditions. In their work the elasto-plastic strain components were calculated analytically using the total deformation theory of plasticity. In addition, the input for the load–notch strain analysis was the measured or calculated plastic strain state at the notch together with the materials stabilized cyclic stress–strain curve evaluated from unnotched axial specimens. The damage accumulation was based on the Palmgren–Miner rule. Conle and Chu (9) developed a 3-D stress-strain model to simulate reversed multiaxial stress/strain paths to assess damage in complex vehicular structures. They indicate that a Neuber plasticity correction method must be used to correlate plastic behavior. In addition elastic unit load analysis should be used combined with a superposition procedure of each load points service history. Devlukia and Bargmann (11) conducted fatigue assessment of a suspension arm using deterministic and probabilistic approaches. Their approach had the following considerations and results: • The strength reduction effect due to surface roughness is accounted for by representing the surface as a collection of notches and making use of Neuber's rule. The strength reduction effects due to the surface roughness are similar under constant and variable amplitude loading.

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Residual stress demonstrates a more pronounced effect under constant amplitude loading as compared to variable amplitude loading. • Cumulative damage under variable amplitude loading sequence of long duration on simple specimens is non-conservative by a factor of about 2 as compared to measured data. • The prediction of component lives based on specimen data is conservative by a factor of about 2. Conle and Mousseau (10) used vehicle simulation and finite element results to generate fatigue life contours for chassis components using automotive proving ground load history results combined with computational techniques. They concluded that the combination of vehicle dynamics modeling, finite-element analysis, and fatigue analysis is a viable technique for the fatigue design of automotive components. Witter et al. (27) converted a steering knuckle into a 6-DOF transducer to be able to estimate the operating wheel translation force and moment inputs to a Mercury Sable steering knuckle. To reach this goal, a 6-DOF load cell was used in the array calibration procedure to provide an estimate of the 6 forces and moments’ inputs at a point on the plate bolted to the steering knuckle. They concluded that the calibration matrix did not vary significantly with suspension height, but did vary significantly with large steering angles. Moreover, the strain gage responses were sensitive to moments. It was suggested to place vehicle on a 4-poster and apply forces and moments through the 4poster exciter to prevent the effect of un-suspended vehicle array calibration problems. The results of their analysis was compared with two cornering tests on the same design, showing an 11% error with respect to the physical test results conducted on the prototypes. Thermal shape vectors were used by Krishna and Fetcho (15) in finite element shape optimization of a steering knuckle for weight reduction. They found that thermal displacements could be used as shape vectors to derive shape optimization. The load and displacement methods of generating the shape vectors are cumbersome to apply to complex castings such as steering knuckles. With 100 iterations and 118 hrs of CPU, the steering knuckle was redesigned using thermal shape vectors and its weight was reduced by 7.6%. Botkin (14) used a shape design modeling with fully automatic three-dimensional mesh generation to model and optimize vehicle suspension components, namely suspension arm and steering knuckle. In the model the control arm geometry was assembled from two parts: BOSS and ARM and similarly the steering knuckle consisted of BOSS, SLAB and HUB (see Figure 9). A preliminary set of design primitives were developed which could be assembled into complete solid models. The resulting models were associated with design variables, which could be easily changed. SUMMARY This paper described the details of the analytical and experimental methods to be employed to compare fatigue performance of forged components with those of components in which forging is not a process step. Some of the findings from the

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literature survey conducted were also reported. Forging process as a major manufacturing process in the automotive industry, and its advantages in comparison with other manufacturing techniques and potential improvements were investigated. This comparison emphasized material selection and manufacturing, and mechanical properties, in particular with regards to fatigue performance. Durability and optimization of steering knuckle as a typical vehicle component were also reviewed. Selected examples from the literature were provided to determine some of the driving forces for conversion from forging to competing technologies, such as casting. Some important factors that could influence the properties of forged products, such as anisotropy and surface defects, were discussed. Machined, as well as as-forged steering knuckles of a small 4-cylinder vehicle have been obtained to be used as the forged steel component to be evaluated in this study. The machined knuckles will be used for component testing, while the as-forged steering knuckles will be used to prepare fatigue-testing specimens. Finite element method is currently being implemented to analyze the available forged steel component, and work is currently underway to prepare testing specimens from these components. Vehicles equipped with cast aluminum and cast iron steering knuckles, even though in a larger vehicle, have also been identified to be used for comparisons with the forged steel knuckles as the competing process technologies. REFERENCES 1. Stephens, R. I., Fatemi, A., Stephens, R. R., Fuchs, H. O., “Metal Fatigue in

2. 3. 4.

5.

6. 7.

Engineering,” 2nd ed., John Wiley & Sons, Inc., 2001. Reed, R. P., Smith J. H., and Christ, B. W., “The Economic Effects of Fracture in the United States,” U.S. Department of Commerce, National Bureau of Standards, Special Publication 647, March 1983. Farsetti, P., Blarasin, A., “Fatigue Behavior of Microalloyed Steels for Hot-Forged Mechanical Components,” International Journal of Fatigue, Vol. 10, No. 3, 1988, pp. 153-161. Gunnarson, S., Ravenshorst, H., Bergstorm, C. M., “Experience with Forged Automotive Components in Precipitation Hardened Pearlitic-Ferritic Steels,” Fundamentals of Microalloying Forging Steels, Proceedings, Metallurgical Society of AIME, 1987, pp. 325-338. Lee, S. B., “Structural Fatigue Tests of Automobile Components under Constant Amplitude Loadings,” Fatigue Life Analysis and Prediction, Proceedings, International Conference and Exposition on Fatigue, Goel, V. S., Ed., American Society of Metals, 1986, pp. 177-186. Savaidis, G., “Analysis of Fatigue Behavior of a Vehicle Axle Steering Arm Based on Local Stresses and Strains,” Material wissenschaft und Werkstoff technik, Vol. 32, No. 4, 2001, pp. 362, 368. Beranger, A. S., Berard, J. Y., Vittori, J. F., “A Fatigue Life Assessment Methodology for Automotive Components,” Fatigue Design of Components, ESIS Publication 22,

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Proceedings of the Second International Symposium on Fatigue Design, FD’95, 5-8 September, 1995, Helsinki, Finland, Marquis, G., Solin, J., Eds., 1997, pp. 17-25. 8. Blarasin, A., Farsetti, P., “A Procedure for the Rational Choice of Microalloyed Steels for Automotive Hot-Forged Components Subjected to Fatigue Loads,” International Journal of Fatigue, Vol. 11, No. 1, 1989, pp. 13-18. 9. Conle, F. A., Chu, C. C., “Fatigue Analysis and the Local Stress-Strain Approach in Complex Vehicular Structures,” International Journal of Fatigue, Vol. 19, No. 1, 1997, pp. S317-S323. 10. Conle, F. A., Mousseau, C. W., “Using Vehicle Dynamics Simulation and FiniteElement Results to Generate Fatigue Life Contours for Chassis Components,” International Journal of Fatigue, Vol. 13, No. 3, 1991, pp. 195-205. 11. Devlukia, J., Bargmann, H., “Fatigue Assessment of an Automotive Suspension Component Using Deterministic and Probabilistic Approaches,” Fatigue Design of Components, ESIS Publication 22, Proceedings of the Second International Symposium on Fatigue Design, FD’95, 5-8 September, 1995, Helsinki, Finland, Marquis, G., Solin, J., Eds., 1997, pp. 436-445. 12. Lee, Y. L., Raymond, M. N., Villaire, M. A., “Durability Design Process of a Vehicle Suspension Component,” Journal of Testing and Evaluation, Vol. 23, 1995, pp. 354363. 13. Taylor, D., Esmen, E., Pan, J., “Determination of Assembly Stresses in Aluminum Knuckles,” SAE transactions, Technical Paper no. 1999-01-0345, 1999, Section 1, pp. 302-306. 14. Botkin, M. E., “Shape Design Modeling Using Fully Automatic Three-Dimensional Mesh Generation,” Finite Elements in Analysis and Design, Vol. 10, No. 2, pp. 165181. 15. Krishna, M. M. R., Fetcho, M. R., “Thermal Shape Vectors in Finite Element Shape Optimization of a Steering Knuckle,” Recent Advances in Solid and Structures, ASME PVP, Chung, H. H., ed., Vol. 381, 1998, pp. 165-170. 16. Lin, J., “Shape Optimization of Nonlinear Structures under fatigue Loading,” Ph.D. Dissertation, Department of Mechanical, Industrial and Nuclear Engineering of the College of Engineering, University of Cincinnati, 2001. 17. Kalpakjian, S, Shmid, S. R., “Manufacturing Engineering and Technology,” 4th ed., Prentice-Hall, Inc., Upper Saddle River, New Jersey, 2001. 18. Esper, F. J. and Sonsino, C. M., “Fatigue design for PM Components,” European Powder Metallurgy Association, England, 1994. 19. Jang, G. B., Hur, M. D., Kang, S. S., “A Study on the Development of a Substitution Process by Powder Metallurgy in Automobile Parts,” Journal of Materials Processing Technology, Vol. 100, 2000, pp. 110-115. 20. Hirt, G., Cremer, R., Witulski, T., Tinius, H. C., “Lightweight Near Net Shape Components Produced by Thixoforming,” Materials and Design, Vol. 18, No. 4, 1997, pp. 315-321. 21. Houshito, S., Watanabe, Y., Goka, M., Ishihara, Y., “Feasibility Study on the Application of High Strength Ductile Iron to Automotive Chassis Parts, International Journal of Materials and Product Technology, Vol. 4, No. 3, 1989, pp. 285-299.

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22. Blarasin, A., Giunti, T., “Manufacturing Processes Influence Fatigue Life,” Automotive Engineering, Vol. 105, July 1997, pp. 93-95. 23. Rice, R. C., ed., “SAE Fatigue Design Handbook,” prepared under the auspices of the SAE Fatigue Design and Evaluation Committee, Society of Automotive Engineers, Inc., 1997. 24. Kuratomi, H., Uchino, M., Kurebayashi, Y., Namiki, K., Sugiura, S., “Development of Lightweight Connecting Rod Based on Fatigue Resistance Analysis of Microalloyed Steel,” SAE transactions, Technical Paper no. 900454, 1990, 487-491. 25. Cristinacce, M., James, D. E., Milbourn, D. J., “The Future Competitiveness of Automotive Forging the Steelmakers' Contribution,” Forging and Related Technology-ICFT’98, IMechE Conference Transactions, 1998, pp. 37-50. 26. Kocabicak, U., Firat, M., “Numerical Analysis of Wheel Cornering Fatigue Tests,” Engineering Failure Analysis, Vol. 8, 2001, pp. 339-354. 27. Witter, M. C., Dumbacher, S. M., Brown, D. L., “Converting a Steering Knuckle into a 6-DOF Force Transducer,” Proceedings, The 17th International Modal Analysis Conference - IMAC, 1999, pp. 1622-1632.

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Table 1: Summary of some characteristic comparisons between forging and casting processes. Process

Forging

Casting

Strength

High

Medium

Ductility

High

Low

Toughness

High

Medium

Fatigue crack growth resistance

Good

Poor

Yes

None

Good

Requires close control

Possible

Many

Production volume

High

High

Production rate

High

Initial tooling cost

High

Low (sand casting) to high (die casting) Medium

Production cost

Low

Low

Limited

High (in die-casting)

Dimensional versatility

High

Limited

Dimensional accuracy

Medium

Medium

Good to poor

Poor

High (ferrous and non-ferrous)

Limited

Property

Directional strength capability Heat treatment response Internal defects

Shape complexity

Surface finish Material versatility

14

Table 2. Properties of cast and forged steels swivel hub (25).

Casting (wide arm) Forging (wide arm)

C

Si

Mn

S

V

0.31

0.45

1.34

0.014

-

0.2% PS Lower YS 2 (N/mm ) 440

0.39

0.26

1.28

0.075

0.099

669

15

UTS (N/mm2)

El (%)

655 969

17

R/A (%)

3mm U (J)

Hv

15-25

63

190-205

46.6

9

290-300

Figure 1. Fatigue design flow chart (1).

Figure 2. Relative unit costs of a small connecting rod made by various forging and casting processes (17).

16

Figure 3. Changes in fatigue performance of vehicle front suspension arm due to surface defects from forged and cast manufacturing processes (22).

Figure 4. Fatigue strength of microalloyed (SV40CL1) and Q&T (S40C) forged connecting rods (24).

17

Figure 5. Constant amplitude fatigue crack growth behavior of Q&T vs. cast steels (23).

Figure 6. The effect of shot-peening on fatigue behavior of carburized gears (1).

18

Figure 7. Finite element model of the preliminary production (left) and the proposed knuckle (right) (12).

Figure 8. Estimation of the influence of roughness on fatigue life (6).

19

Figure 9. Design model (left) and meshes (right) of the steering knuckle (14).

20