Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Thursday morning, September 18.
Program Organizer: Prof. Wole Sobojeyo, The Ohio State University, Dept. of Materials Science and Engineering, Columbus, OH 43210
Session Chairs: Prof. T.S. Srivatsan, University of Akron, Dept. of Mechanical Engineering, Auburn Science Center, Akron, OH 44325-3903; Prof. Don Koss, Pennsylvania State University, Dept. of Materials Science & Engineering, 209 Steidle Bldg., University Park, PA 16802-7501
HIGH-CYCLE FATIGUE AND FRACTURE BEHAVIOR IN SQUEEZE CAST 356 Al/SiCW MMCS AND 390 Al/SiCW MMCS: D.A. Koss*, Sang-Beom Kim, Department of Material Science and Engineering, Penn State University, University Park, PA 16802; *On leave from Daewoo Heavy Industries Ltd.
Specimens of 356 Al/SiCw and 390 Al/SiCW metal matrix composites, produced by squeeze casting, were tested under fully reversed high cycle fatigue conditions. A staircase method was employed to determine the mean fatigue strength of the material at ten million cycles. Presence of 17% SiC whiskers in 356 Al alloy increases mean fatigue strength at 107 cycles by 30%. However, in 390 Al alloy, the increase of mean fatigue strength by the presence of 17% SiCw is relatively low compared with that of 356 Al. Fracture surface analysis of both composites shows that fatigue cracks are initiated primarily at regions which are characterized by a low volume fraction of SiCw. The influence of such local "discontinuity" on the fatigue life is modeled. In addition, it has been found that there are large differences between the tensile yield strength and the compressive yield strength in both composites. This result is indicative of the presence of thermally induced residual stresses. The difference of the fatigue strength increase in 356 Al/SiCw and in 390 Al/SiCw is interpreted in terms of a combination of local "discontinuity" and thermal residual stresses. This research is supported by General Motors Corp.
8:25 am INVITED
THE EFFECT OF SOLIDIFICATION RATE ON THE GROWTH OF SMALL FATIGUE CRACKS IN A CAST 319 ALUMINUM ALLOY: M.J. Caton, J.W. Jones, Dept. of Materials Science and Engr., Univ. of Michigan, Ann Arbor, MI; J.E. Allison, Ford Research Laboratories, MD 3182 SRL, Dearborn, MI 48121
Increased use of cast aluminum alloys in automotive applications has necessitated development of a better understanding of fatigue crack growth behavior in these materials. Of particular interest in materials containing casting porosity is the fatigue behavior of small cracks. This paper will describe the influence of solidification rate and microstructure on fatigue crack growth of small cracks in 319 Al, a common Al-Si-Cu alloy used in automotive castings. A replication technique capable of detecting cracks as small as 10 mm in length was used to monitor the growth of small, naturally-initiated fatigue cracks and establish da/dN vs. DK curves. A comparison between these results and those from large cracks will be made. The paths which these fatigue cracks take through the dendritic microstructure will be described.
THE EFFECT OF SOLIDIFICATION RATE ON THE MICROSTRUCTURE AND FATIGUE PROPERTIES IN CAST 319 AL: J.M. Boileau, J.E. Allison, Ford Research Laboratories, MD 3182 SRL, Dearborn, MI 48121
As the automotive industry increases its use of cast aluminum components, the need for more detailed information relating the impact of casting practice on mechanical behavior also increases. Therefore, a study characterizing the influence of solidification rate on the microstructure and fatigue properties in a cast 319 Al alloy was conducted. A wedge-shaped casting was developed with a dendrite arm spacing range of 10 - 100 mm and a porosity range of 0.01 - 1%. To decouple the combined influence of dendrite arm spacing and porosity, hot isostatic pressing was performed on several wedges to eliminate the porosity; all castings were then subsequently heat-treated to a T7 condition. Multiple fatigue tests were conducted at selected stress levels so that valid statistical comparisons could be made on the effect of dendrite arms spacing and of porosity. Extensive metallographic and fractographic characterization was performed to understand the influence of porosity on fatigue life. This data, along with the microstructural features initiating failure in these cast Al alloys, will be discussed.
RELATION OF PORE SIZE AND LOCATION TO FATIGUE FAILURE IN Al ALLOY A356 CAST SPECIMENS: M.E. Seniw, M.E. Fine, M. Meshii, Dept. of Materials Science and Engr., Northwestern Univ., Evanston, IL 60628; E.Y. Chen, GE Corporate R&D, Schenectady, NY 12301
Because of weight and cost savings motor vehicle manufacturers are making more extensive use of cast aluminum alloys to replace forged and cast iron or steel components. The presence of pores in certain locations may reduce the fatigue life. This is of special concern for safety critical components. This study is an examination of the influence of porosity morphology and location on the fatigue life of die cast aluminum A356 alloy specimens. The castings are first x-rayed to locate pores. Fatigue specimens were then machined from the cast test bars so that pores of various sizes are located various distances from the nearest free surface in the gage length surface. After fatigue failure the pore resulting in crack formation is carefully measured in size and distance from the surface. A statistical relation between pore size and distance from the free surface and fatigue life under constant amplitude loading was established. Distance from the surface was found to be the most important variable. This work was supported by the NIST under Contract No. 70NNANB9H0916.
CYCLIC STRESS RESPONSE, CYCLIC STRAIN RESISTANCE AND FRACTURE BEHAVIOR OF AN Al-Zn-Mg-Cu ALLOY, INFLUENCE OF TEMPERATURE: T.S. Srivatsan, S. Anand, Dept. of Mechanical Engineering, The University of Akron, Akron, OH 44325
The design of structural components for the newer generation of civilian and military aircraft demands satisfactory performance from the material under conditions of cyclic stress amplitude and strain amplitude control, and an extended service life. In this connection, a study has been made to understand the influence of test temperature on cyclic stress response characteristics, cyclic strain resistance, fatigue life and fracture behavior of a high strength aluminum alloy 70SS. Test specimens of the alloy were cyclically deformed over a range of strain amplitudes, giving less than 104 cycles to failure, at both ambient and elevated temperatures. In this presentation, the cyclic stress response characteristics, cyclic stress-strain characteristics, cyclic strain resistance and resultant low-cycle fatigue properties and fatigue fracture characteristics of the alloy will be highlighted in light of competing and mutually interactive influences of cyclic plastic strain amplitude, concomitant response stress, intrinsic microstructural effects and matrix deformation characteristics.
9:50 am INVITED
HIGH CYCLE FATIGUE BEHAVIOR OF PARTICLE REINFORCED ALUMINIDE COMPOSITES: J.E. Allison, Ford Research Laboratories, Ford Motor Company, Dearborn, MI 48124; J.W. Jones, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48105
The high cycle fatigue response of aluminum alloys can be significantly altered by the addition of high modulus particle reinforcements. This talk will review the current understanding of the influence high modulus particles have on cyclic deformation, fatigue crack initiation and fatigue lifetime of unnotched samples of aluminum alloys. Fatigue behavior in these materials can be described in terms of combinations of classical composite strengthening (direct strengthening) and changes in matrix microstructure and deformation characteristics (indirect effects) which arise from the presence of the reinforcement. Accelerated aging and refinement of slip length are two particularly important indirect phenomena affecting high cycle fatigue behavior of DRA. The improvements in stress-controlled fatigue performance that generally result from reinforcement can be understood using this framework, as can strain controlled fatigue results and elevated temperature fatigue response.
HIGH CYCLE FATIGUE STRENGTH OF AUSTEMPERED DUCTILE CAST IRON: S.K. Putatunda, P. Prasad Rao, Rachan Rao and Nathan Clark, Departments of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202
Austempered Ductile Cast Iron (ADI) has attracted considered interest in recent years because of its excellent properties such as high strength with good ductility, good wear resistance and fracture toughness. It is therefore, considered as an economic substitute for steel in several structural applications especially for the automotive industry. In the investigation, the influence of austempering time and temperature on the high cycle fatigue strength of an alloyed ADI (1.5% Ni and 0.3 % Mo) was investigated. The effect of the carbon content and the volume fraction of austenite on the high cycle fatigue strength of ADI was also studied. The crack growth mechanism in fatigue was also examined.
THE EFFECTS OF SURFACE OXIDE FILMS ON THE FATIGUE BEHAVIOR OF ALUMINUM-LITHIUM ALLOYS: A. Inchekel, Adv. Engr. Consulting Services, Wichita, KS 67277; J.E. Talia, Dept. of Mechanical Engr., Wichita St. Univ., Wichita, KS 67260
The fatigue behavior of oxide film coated Al-Li alloy samples was investigated. Adherent anodic oxide films of varying thickness were applied electrolytically onto the surface of the specimens. Tensile tests shown that the flow stresses increased as the oxide film thickness increases. However, fatigue life tests presented a reduction in life of the samples at a critical thickness of oxide and an increase in the life at a larger thickness of oxide coatings. These results are more pronounced during high cycle fatigue testing. A mechanism, based on dislocation structures changes in the surface and subsurface region introduced into the material by the presence of oxide films, is proposed.
10:55 am INVITED
THE EFFECTS OF SCRATCHES AND SHOT PEENING ON THE HIGH CYCLE FATIGUE CRACK GROWTH OF ALUMINUM ALLOY 2024-T3: J.E. Talia, Dept. of Mechanical Engineering, Wichita State Univ., Wichita, KS 67260; M. Talia, Dept. of Mechanical Engineering, Univ. of Kansas, Lawrence, KS 66045
An experimental study was performed to determine the crack growth rates of scratched-then-shot peened Aluminum alloy 2024-T3 specimens under high cycle fatigue conditions. Tests were conducted at a constant stress amplitude with different stress ratios. As expected, the crack growth rate of a scratch specimen increased with increasing the scratch depth. However, glass shot peening significantly reduced such crack growth rates. The rates of scratched-then-shot peened specimens were similar to those of as-received specimens at low and intermediate stress intensity factors' ranges. The increase of the positive stress ratio augments the crack growth rates of specimens. Nevertheless, the change of the negative stress ratio resulted in a different effect. In addition, it was found that the scratch exhibited small crack growth behavior. This behavior is related to a crack's path change occurring before the crack propagates across the specimens. An analytical model based on the stress intensity factor was developed for scratched samples.
FATIGUE BEHAVIOUR OF ADI-SOME SPECIFIC FEATURES: J. Svéjcar, Technical University of Brno-Pisek Institute of Materials Engineering, 616 69 Brno, Czech Republic
Ausformed ductile iron (ADI), with the structure of upper, transition, and lower bainite, exhibits anomalous behaviour in comparison with other materials (e.g., the form of Heigh diagram, decrease of the fitigue limit with increasing yield point). Microfractographical analyses and additional experiments, the results of which are presented in this paper, were carried out with the aim to explain this anomalous behaviour.
FATIGUE CRACK GROWTH BEHAVIOR OF SELECTED ALUMINUM ALLOYS-VARIABLE AMPLITUDE LOADING: N. Ranganathan, E.N.S.M.A., Teleport 2, BP 109 Futuroscope Cedex, France
The different kinds of loading studied are: single overload, block programs and transport aircraft wing loading spectra. Firstly, the micromechanisms and the associated effects are studied based on single overload tests (with an overload ratio of 2) covering a large ÆK range from near threshold to medium ÆK ranges (Paris law regime). It is shown from tests conducted in vacuum that the slip behavior (planar or multiple slip) can strongly affect the delay after an overload, the best behavior is obtained for the underaged alloy. In air all the studied microstructures show a similar delay after an overload and the micromechanisms are similar. In the Aluminum Lithium alloy, 8090 T651, which exhibits a mixed (planar and multiple slip) behavior in air, the delay is higher in air than in vacuum for tests at R=0.1. This peculiar behavior is shown to be associated to the activation of a large slip band along a (111) plane while the conditions at the crack-tip prevailing after the overload are not sufficient to sustain this slip activity. In the case of flight simulation tests, the longest life is observed for the 2024 alloy while the shortest life is obtained for the 7075 T351 alloy. In the 2024 alloy (for which the fracture surface analysis was carried out in detail) the global fracture surface appearance is again governed by the Kmax value and the most predominant R ratio as for the Block programmed tests. In the Al-Li surface covered by a new kind of striations not observed under constant amplitude loadings. Under such conditions the crack growth is accelerated four folds as confirmed from equivalent block load tests. In vacuum, no such change of micromechanism is observed for variable amplitude tests in the Al-Li alloy and the crack growth life is three times as high as that for the 2024 alloy.
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