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Session Chairpersons: Dr. Wilbur C. Simmons, Research Office, P.O. 12211, Research Triangle Park, NC 27709; Dr. A.B. Geltmacher, Naval Research Laboratory, Code 6380, 4555 Overlook Drive SW, Washington, D.C. 20375
8:30 am INVITED
DUCTILE FRACTURE IN STEELS AND ALUMINIUM ALLOYS: John F. Knott, School of Metallugy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Ductile fracture mechanisms in these alloys, of such importance to modern civilization, are set in the context of the main applications for which they are used. Thin-sheet applications encompass two main aspects: prevention of localized shear fracture in sheet-forming, promotion of reproducible shear fractures in the "ripping" of "ring-pulls" on beverage cans or in the stamping-out of milk-bottle tops from oil. Both aspects relate to the generation of through-thickness shear fractures. In thick-section applications, it is necessary to address local fracture mechanisms in the combination of steep strain-gradient and high hydrostatic tension ahead of a notch or pre-crack. The prime micro-mechanisms are microvoid coalescence and "fast (in-plane) shear" linkage resulting from a (plane-strain) localization of flow. Behaviour in steels and high-strength aluminum alloys is contrasted and recent findings on "mixed-mode" shear fracture criteria are discussed. Attention is drawn to a number of applications for which "plane-stress" and "plain-strain" methodologies must be combined.
8:55 am INVITED
MODELLING LARGE DEFORMATION AND FAILURE IN MANUFACTURING PROCESSES USING INTERNAL STATE VARIABLES: Doug Bammann, Mechanics and Simulation of Manufacturing Processes Department, Mail Stop 9405, Sandia National Laboratories, Livermore, CA 94551
A state variable model for describing the fine-deformation behavior of metals is described. A multiplicative decomposition of the deformation gradient into elastic, volumetric plastic (damage), and deviatoric plastic parts is considered. With respect to the natural or stress-free configuration defined by this decomposition, the thermodynamics of the state variable theory is investigated. This model incorporates strain rate and temperature sensitivity, as well as damage, through a yield surface approach in which the state variables follow a hardening minus-recovery format. The microstructural underpinnings of the model are presented along with a detailed description of the effects which each model parameter has on the predicted response. Issues associated with the implementation of this model into finite element codes are also discussed. A range of problems involving damage at various loading rates are presented including hydroforming, prediction of forming limit diagrams from limiting dome height studies and cracking during welding processes. In each case, experiments are well described by predictions based on this model.
9:20 am INVITED
THE EFFECT OF HYDROSTATIC PRESSURE ON THE ROOM TEMPERATURE FORMABILITY OF METAL SHEETS: Henry Piehler, Consortium for the Advancement of Deformation Processing Research, Department of Materials Science and Engineering, Carnegie Mellon University, Wean Hall 2323, Pittsburgh, PA 15213
The effect of pressure on the room temperature formability of 6111 T4 aluminum and ß 21 S titanium sheets was evaluated in stretching, drawing, as well as plane strain. The sheet formability tests were performed using the patented Carnegie Mellon hot triaxial deformation system in conjunction with a specially developed 2 in diameter hemispherical punch and forming fixture. The strain state was varied by using different sample widths and lubrication conditions. Tests were conducted at ambient pressure and constant pressures of up to 70 MPa (10 ksi). Increasing all pressures used in forming increased the formability of the al sheets along all forming paths. The largest pressure-formability increase in the Al sheets occurred in plane strain. Biaxial stretching tests on the ß 21 S sheets revealed that the formability actually decreased under a pressure of 10 ksi compared to the formability at ambient pressure. This pressure-induced formability decrease was accompanied by a change in fracture path and its mechanism is currently being investigated.
9:45 am INVITED
MICROSTRUCTURAL EFFECTS ON CAVITATION AND FRACTURE IN SUPERPLASTIC METALS: Amit K. Ghosh, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109
Cavitation leads to flow localization and fracture during superplastic forming process. In this paper, several approaches on the micromechanical aspects of cavitation are critically examined. With plasticity effects present, constrained diffusional and plasticity-induced growth of cavities have been proposed in the dislocation creep regime. In the superplastic regime, however, cavitation behavior is considerably complicated by the predominance of grain boundary sliding and grain rotation effects. Consequently the amount of grain boundary sliding strain has a major effect on the degree of cavitation through cavity initiation, growth and coalescence. Dynamics of cavitation and its effect on strain localization leading to fracture has been examined. Also detailed quantitative aspects of cavity initiation on cavity growth have been carefully documented to separate the phenomenon of continuous nucleation of new voids from the cavity growth process. It appears that dynamic grain growth which influences grain boundary sliding in a major way, also influences cavitation. Thus flow localization and fracture are critically affected by microstructural evolution effects such as dynamic recrystallization and dynamic grain growth. Illustrative examples will be given from various metallic system.
10:10 am INVITED
MECHANISMS OF DUCTILE FRACTURE IN CRACK-TIP FRACTURE PROCESS ZONES: Peter F. Thomason*, Department of Metallurgy and Materials Science, University of Cambridge, Cambridge CB2 3QZ; *on leave from University of Salford
The strength and toughness of engineering alloys, under conditions where general yielding and subcritical crack growth precede catastrophic fracture, are critically depend on nucleation, growth and coalescence of microvoids in fracture process zones that are generally subject to high mean-normal stresses. It is shown that, at these high mean-normal stress levels, microvoid nucleation and coalescence can become the controlling process of ductile fracture in crack-tip plastic zones, with negligible dilatational void-growth prior to microvoid coalescence on the fracture surface. This effect is direct result of high mean-normal stresses promoting the microvoid coalescence process when the microvoids are still relatively small and widely spaced; thus virtually all dilational void-growth is confined to the final void-coalescence fracture surface, and the result of plastic limit-load failure (or internal microscopic necking) between adjacent microvoid nuclei.
10:35 am BREAK
10:45 am INVITED
IMPROVED PROPERTIES OF HSLA AND DUAL-PHASE STEELS: Gareth Thomas, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 97420-1760 and National Center for Electron Microscopy, Lawrence Berkeley Laboratory, Berkeley, CA 94720
The utilization of controlled rolling and quenching from the finish roll allows the design of composite microstructures, whereby the advantages of the second phase are optimized while the less desirable features of this phase are simultaneously mitigated by the presence of the other constituent phase. The size, morphology, distribution, shape and volume fracture of the second phase critically control the mechanical properties, especially fracture and fatigue of the dual phase systems. As a consequence, these structures offer a degree of metallurgical flexibility that is absent in single phase structures or in many precipitation strengthened systems, for attaining optimum sets of mechanical properties. Examples are presented here of martensite/austenite (~2 - 5%) mixtures designed for optimum combinations of high strength, toughness, and wear properties in medium carbon steels, e.g., for mining and agricultural applications; low carbon martensite/ferrite (~80%) structures for high strength, cold formability, improved low temperature ductility, and attractive improved corrosion resistance in concrete. In all cases these steels can be produced on line in a hot mill by controlled rolling and direct quenching.
EXPERIMENTAL AND THEORETICAL ANALYSIS OF THE CAVITATION AND FAILURE BEHAVIOR OF A SUPERPLASTICALLY DEFORMED NEAR-GAMMA TITANIUM ALUMINIDE ALLOY: Carl M. Lombard, Perikles D. Nicolaou, Amit K. Ghosh, S. Lee Semiatin, US Air Force Wright Laboratory, WL/MLLM, Room 101, Building 665, Wright Patterson Air Force Base, OH 45433; UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432; Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109
The uniaxial hot tension behavior of a near-gamma titanium aluminide alloy sheet (Ti-45.5Al-2Cr-2Nb) was determined in the as-rolled condition (initial grain size ~3 to 5 µm) and a rolled-and-heat treated (1177°C/4 hours) condition (initial grain size ~10 to 12 µm). Microstructure evolution, cavitation rates, and failure modes were established via constant strain rate tests at 10-4 to 10-2 sec-1 and test temperatures between 900 and 1200°C. For both initial microstructural conditions, the failure mode was established as predominantly cavitation/fracture controlled. Cavity growth rates was greatest at lower temperatures and in the heat treated specimens; the higher cavitation rates in the heat treated specimens led to elongations only approximately one-half those of the as-rolled material. Experimental results were then compared with a 'force equilibrium approach' theoretical analysis of the isothermal hot tension test under cavitating conditions. Model results delineated the competition between failure controlled by localized necking versus fracture, the latter being defined by a critical volume fraction of cavities. The validity of the modeling approach was confirmed through the analysis of the experimental results and data from the literature.
THE EFFECT OF HYDROSTATIC AND SHEAR STRESSES ON THE HOT WORKING BEHAVIOR OF BULK METALS: Henry Piehler, Consortium for the Advancement of Deformation Processing Research, Department of Materials Science and Engineering, Carnegie Mellon University, Wean Hall 2323, Pittsburgh, PA 15213
The effect of pressure on the hot workability of a Ti-49.5Al-2.5Nb-1.1Mn (a/o) titanium aluminide was evaluated using both isothermal compression and isothermal hydrostatic upsetting tests. Tests were conducted at 1050°C to strains of the order of one with and without pressure of 260 MPa to evaluate the effect of pressure on flow stress response, fracture retardation, and hot worked microstructure. The samples deformed without pressure exhibited noticeable cracks on the bulged free surface as well as internal deformation-induced voids; the superimposition of pressure suppressed both these types of deformation damage. Pressure was observed to have a neglible effect on the flow stress response, which could be adequately described by the von Mises criterion in all cases. Similarly, pressure had a negligible effect on dynamic recrystallization and microstructure under the conditions investigated. Preliminary results on pore closure in Osprey processed IN 718 subsequently deformed by isothermal hydrostatic upsetting are also presented.
THE EFFECT OF MATERIALS PROCESSING ON FRACTURE OF METALS: Clyde L. Briant, Division of Engineering, Brown University, Providence, RI 02912
Thermomechanical processing of metals is often used to improve the fracture properties of materials. When materials are processed at room temperature, one can often achieve good control over resulting properties because one can avoid dynamic recovery and recrystallization. However, when processing is carried out at elevated temperature, dynamic changes can occur and make control difficult. In this paper we report on work in which refractory metals were processed at elevated temperatures try to achieve optimum fracture strength as measured in four point bending. The results will show that there is an optimum processing range to achieve fracture resistance. If the material is processed at too low a temperature internal cracks that occurred during processing will cause failures. If the processing temperature is too high, recrystallization occurs and the material will fail by low energy brittle fracture.
THE INFLUENCE OF INCLUSIONS ON THE FRACTURE TOUGHNESS OF AerMet 100: J.W. Morris, Jr., J. Chan and K. Sato, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 97420-1760 and, Center for Advanced Materials, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
AerMet 100 is an ultra-high strength alloy recently that was recently commercialized by Carpenter Technology. The alloy is a secondary hardening, Fe-Ni-Co-Cr-Mo alloy that is age-hardenable to yield strength above 250 ksi, and fractures in a ductile mode at ambient temperature even after hardening to peak strength. The alloy is currently under investigation for potential use in aircraft landing gear. As expected from its ductile fracture mode, the fracture toughness of AirMet 100 is strongly dependent on the density of inclusions with sizes greater than about 2 µm. As the inclusion spacing increases from ~0.2 to ~0.8 mm, the fracture toughness rises by roughly 70%, from ~110 ksiin. Metallurgical analysis shows that the large inclusions in these alloys are primarily rare earth (La, Ce, Nd) oxysulfides. The rare earth elements are present in the low concentration in the alloy, and, presumably, represent intentional additions to getter metalloid impurities such as sulfur. Research has shown that substantial improvements in toughness can be obtained by filtration before solidification. The toughness of AerMet 100 can be further improved by modifying, the alloy heat treatment the alloy heat treatment to alter the distribution of hardening precipitates. A brief pre-age at 510°C prior to the standard aging at 485°C significantly increases the fracture toughness, particularly in the case of the low-toughness sample.
STEREO-SECTION FRACTOGRAPHIC STUDY OF FRACTURE BEHAVIOR OF A Ti6Al4V ALLOY: Xian J. Zhang and R.L. Tregoning, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742; Fracture and Fatigue Branch, Naval Surface Warfare Center, 3A Leggett Circle, Annapolis, MD 21402
The stereo-section fractography (SSF) technique has been employed to study the relationship of surface microfractographic features and internal microstructures. In this way, a direct correspondence is established between various fracture modes and microstructural details beneath the fracture surface of fracture toughness and fatigue specimens of an () Ti6Al4V alloy. The measurements show a one to one relation between surface and internal features. It was found that ductile holes formed along grain boundaries and also, cleavage facets cut through grains. It was evident that the microscopic texture formed in the solidification process affects the fracture mechanisms and hence, the fracture toughness of the alloy. Thus, the SSF technique is demonstrated as a very useful tool for the study of fracture behavior of materials.
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