Wednesday, AM Room: Orange County 1
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: TBA
AN INTERNAL VARIABLE APPROACH TO SUPERPLASTIC DEFORMATION OF Pb-Sn EUTECTIC ALLOY: Tae Kwon Ha, Young Won Chang, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea
A series of load relaxation and tensile tests were conducted on superplastic Pb-62wt%Sn eutectic alloy at room temperature. The internal variable theory of structural superplasticity proposed recently was employed to analyze the results. The flow curves obtained from load relaxation tests were shown to consist of the contributions from boundary sliding, such as grain boundary sliding (GBS) and phase boundary sliding (PBS), and the accommodating plastic deformation. A new attempt to deduce the optimum strain rate, one of the most important parameters of superplastic (SPF) forming process, has been also made, which makes it possible to quantitatively introduce the nature of boundary sliding into the SPF process.
SUPERPLASTIC BEHAVIOR OF STAINLESS STEEL NITRONIC 19D: C. K. Syn, D. R. Lesuer, Lawrence Livermore National Lab., P. O. Box 808, Livermore, CA 94550; O. D. Sherby, Dept. of MSE, Stanford University, Stanford, CA 94305
Superplastic forming of stainless steels can be an economically viable manufacturing process for complicated structures for applications in aggressive environment such as automotive exhaust parts. A commercial stainless steel known as Nitronic 19D with a duplex microstructure and a nominal chemistry of 21Cr-4Mn-2Ni-0.3C-0.2N-Fe was repeatedly hot-rolled to develop a micro-structure containing fine elongated grains. Tensile tests were performed at various strain rates and temperatures to examine the plastic deformation behavior at elevated temperature. A tensile ductility in excess of 250% at 950°C was obtained at constant true strain rate of 3x10-4sec-1. Cavitation was observed at ferrite-austenite grain boundaries. During tensile testing, the elongated grains were broken up into randomly oriented equiaxed grain structure with high angle grain boundaries. A detailed description of microstructural evolution and plastic deformation behavior will be presented. Work performed under the auspices of the U.S. Dept. of Energy by the Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.
EXPERIMENTAL AND NUMERICAL INVESTIGATION OF FRACTURE IN DOUBLE EDGE NOTCHED STEEL PLATES: R. Knockaert, I. Doghri, Universiteé Catholique de Louvain, CESAME, Bâtiment Euler, 4-6 Avenue G. Lemaître, B1348 Louvain-La-Neuve, Belgium
Double edge notched (DENT) steel plates were pulled until complete rupture and several experimental observations were made (including profilometry and electronic microscope measurements). The essential work to fracture (EWF) model was found to be well verified; indeed the specific work to rupture, the maximum load and the maximum displacement are linear functions of the ligament length. Numerical simulations of the experiments taking into account geometric and material non-linearities (large deformation elasto-plasticity) were performed using the finite element method (FEM). Excellent agreement was found between experimental observations and numerical computations of load-displacement curves, thickness reduction and stored plastic work. The FEM also allowed to check the shape of the plastic zone against the one assumed in the EWF model. In order to determine the size of the plastic zone surrounding the crack tip, several models have been tested, ranging from simple criteria to a fully coupled approach based on continuum damage mechanics and using the Lemaitre-Chaboche ductile damage model.
9:30 am BREAK
FATIGUE PROPERTIES OF GRAPHITE-EPOXY COMPOSITE AFTER PLASTIC MEDIA BLASTING PAINT REMOVAL: M. Shao, A. Inchekel, J. E. Talia, Wichita State University, Department of Mechanical Engineering, Wichita, KS 67208
An experimental study was conducted in order to investigate the effects of plastic media blasting (PMB) on the fatigue properties of woven graphite-epoxy composite material. Plastic media blasting was used to remove paint from the samples. A procedure consisting of fatigue cycling, blasting, and repainting (FCBP) was devised. Specimens subjected to one to four cycles of FCBP were tested for their fatigue life and residual strength. The results showed that plastic media blasting (PMB) reduced the fatigue life of the composite. Furthermore, FCBP cycles reduced significantly the fatigue life of samples and this effect is more detrimental as the number of FCBP increases.
STRESS MAPPING AT FATIGUE FRACTURE INTERFACES: David J. Dingley, H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL UK; David P. Field, TSL, Inc., 226 W. 2230 North Provo, UT 84604
Through the use of the new technique of Orientation Imaging Microscopy, OIM, it is possible to construct micrographs depicting the changing cryrstallography over the surface of single crystal or polycrystalline bulk materials. This technique has been used in the investigation of fatigued nickel base alloy single crystals. The specimens were cut and polished normal to the fracture surface and examined in an SEM fitted with the OIM system. Orientation image micrographs were obtained over three sections of the fatigue crack and over an area sufficiently far from thc fracture surface as to represent undamaged material. Two types of micrograph were produced. The first showed the variation of crystal damage as measured by the contrast observed in diffraction patterns obtained at each point. The second contoured regions across which there were small lattice rotations of 0.5 degrees or more. These contours depict the locations of dislocation clusters. It was observed that the fatigue damage extended only microns from the fracture surface.
EFFECT OF SCRATCH AND SHOT PEENING ON FATIGUE CRACK GROWTH OF AL 2024-T3: M. Shao, J. E. Talia, A. Inchekel, Wichita State University, Department of Mechanical Engineering, Wichita, KS 67208
The effects of scratch and shot peening on fatigue crack growth in Al 2024-T3 alloy specimens have been investigated. The results indicated that the scratch accelerated the crack growth. However, shot peening reduced the fatigue crack growth in scratched specimens to those of as-received specimens at low K. As K increased, crack growth rates gradually merged with those of scratched specimens. The specimens with the across-width scratch showed small crack growth behavior.
INFLUENCE OF Cr-Ti CONTENT AND MICROSTRUCTURE ON IMPACT BEHAVIOR OF VANADIUM ALLOYS: A. N, Gubbi, A. F. Rowcliffe, D. J. Alexander, M. L. Grossbeck, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
A vanadium alloy containing 4 wt.% Cr and 4 wt.% Ti has been identified as a promising candidate structural alloy for liquid metal cooled blankets in advanced fusion systems. Charpy impact testing was carried out on a large heat (~500-kg melt) of V-4Cr- 4Ti and small heats (~15-kg melt each) of compositional variants with Cr and Ti contents ranging from 3 to 6 wt.% which were used for optimization studies. One-third- size Charpy impact specimens were employed for impact testing. In a fully recrystallized condition with a grain size of ~16 m the large heat of V-4Cr4Ti exhibited a high level of resistance to cleavage failure with a DBTT at ~-190°C. The small (15 kg) heat of V-4Cr4Ti heat treated to produce the same microstructural condition exhibited similar Charpy impact properties. The small heats containing higher concentrations of Cr and Ti, in a fully recrystallized condition exhibited a DBTT at around -100deg.C, whereas the V-3Cr3Ti alloy failed by pure ductile shear at liquid nitrogen temperatures. Research sponsored by the Office of Fusion Energy, U.S. Dept. of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
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