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Session Chairpersons: Professor Ali S. Argon, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; Dr. K. Sadananda, Naval Research Laboratory, Code 6323, 4555 Overlook Drive SW, Washington D.C. 20375
OPENING STATEMENTS: Dr. K. Sadananda, Head, Deformation and Fracture Section, Naval Research Laboratory, Code 6323, Washington D.C.
INTRODUCTION OF PROFESSOR EMERITUS FRANK A. McCLINTOCK: Professor Ali S. Argon, Department of Mechanical Engineering, Massachusetts Institute of Technology, Room 1-306, Cambridge, MA 02139
8:50 am KEYNOTE
DUCTILE FRACTURE: HIGHLIGHTS AND PROBLEMS: Professor Emeritus Frank A. McClintock, Massachusetts Institute of Technology, Department of Mechanical Engineering, Room 1-304, Cambridge, MA 02139
The developing understanding of fracture by hole growth is reviewed, with mention of unsolved problems: Tipper in the late 1940's showed not only the idealized limiting micro-mechanisms of fracture by cleavage and by hole growth, but also the current problems of linkages of holes by irregular fine cracks and cleavage with concurrent plastic deformation. Both micromechanisms can lead to either ductile or brittle structures; the Irwin-Williams stress intensity concept K; the Paris fatigue crack correlation; elastic-plastic fracture instability in sheet, the Hutchinson, and Rice and Rosengren annular J-fields for initial and early growth of cracks in power-law materials; The K-T and J-Q extensions; fracture mechanics for predicting the behavior of large structures from tests on small, perhaps fully plastic specimens; traditional and fitness-for-service design and maintenance; the finite element method; its strength, current limitations, and needs; history effects, including the wolf's ear fracture, very low cycle fatigue, and torsion; micro-models of crack formation and growth; McClintock and Gurson; extreme value idealizations in statistics; cooperative and multi-stage problems; crack roughening and three-dimensional effects; recent work on rigid-plastic; non-hardening mechanics of initial and continuing growth of cracks, both with and without symmetry.
9:20 am INVITED
ELASTIC-PLASTIC FRACTURE MECHANICS OF STRENGTH-MISMATCHED INTERFACE CRACKS: David M. Parks, S. Ganti, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
Recent progress in rationalizing and predicting the diverse ductile and brittle cracking behaviors seen in a given structural alloy under monotomic loading (i.e., shallow versus deep cracks; tension versus bending; ductile growth versus cleavage, etc.) has rested critically upon improved characterizations of elastic-plastic crack-front stress and deformation fields. For macroscopically homogeneous elastic-plastic materials, these improved descriptions of the crack-front fields typically include a parameter such as J or CTOD which scales the intensity of crack-tip deformation, as well as a parameter such as the T-stress or the Q parameter which accounts for the triaxiality of the crack-tip fields. When a crack lies along or near a planar material interface across which there is a significant gradient in plastic deformation resistance, the local stress and deformation fields exhibits features which differ both in magnitude and in kind from the homogeneous fields noted above. Such conditions are encountered in strength-mismatched weldments, in which the flow strength of the weld metal, Ywm, differs that of the baseplate, Ybp. Major features of such strength-mismatched interface fields are that (i) for a given macroscopic characterization of crack-tip deformation (in terms of J or CTOD), deformation preferentially focuses in the softer material, and (ii) stress triaxiality in the softer material exceeds that attainable in even the most severe of the homogeneous fields noted above. We review basic features of strength-mismatched interface crack fields as determined from finite element solutions under well-constrained and fully plastic conditions, for a range of strength mismatches and strain hardening behaviors. The fields can be well-described with slip-line fields appropriate to the level of strength-mismatch. Implications of these fields for the toughness of interface cracks are noted.
9:45 am INVITED
IN-PLANE CONSTRAINT EFFECTS IN ELASTIC-PLASTIC FRACTURE MECHANICS: John W. Hancock, J. Li, A.D. Karstensen, A. Nekkal, Department of Mechanical Engineering, University of Glasgow, Scotland G12 8QQ, UK
McClintock (1971) has shown that under plane strain conditions, the fully plastic flow fields are not unique, but depend on geometry and loading. In mode I, the loss of uniqueness is shown to originate from the nature of the elastic field and in particular the sign of the non-singular T stress. The nature of these fields is elucidated by constructing a family of plane straw slip line fields (Du and Hancock 1991). As such they belong to a family of fields which can be described by J and a second parameter which determines the level of crack tip constraint (Q/T). This family of fields are deviatorically similar but differ largely hydrostatically (O'Dowd and Shih 1991, Beteg6n and Hancock 1991). Boundary layer formulations have been used to infer the force on mixed mode slip line fields in contained yielding (Hancock, Karstensen and Nekkal 1996). These differ from those discussed by Shih (1974) in that plasticity does not surround the crack tip, due to the occurrence of an-elastic wedge on the crack flanks. Both mode I and mixed mode fields for weakly stain hardening materials can be interpreted as belonging to a single family such that constraint loss by mixed mode loading. This gives a family of fields which differ largely hydrostatically on the plane of maximum hoop stress. Finally slip line fields for cracks on the interface between a rigid subsume and a perfectly plastic material, subject to mixed mode loading have been constructed, and these are also discussed in terms of constraint effects (Li and Hancock 1996). Experimental data on the effect of in plane constraint on fracture toughness is discussed within the framework of two parameter fracture mechanics. Finally the methods by which constraint enhanced toughness can be used in safety cases based on failure assessment diagrams is discussed (MacLennan and Hancock 1995).
10:10 am BREAK
10:20 am INVITED
TOUGHENING OF METAL MATRIX COMPOSITES THROUGH CONTROL OF INTERFACE TOUGHNESS BETWEEN FIBERS AND MATRIX: Ali S. Argon, M. Seleznev, and C.F. Shih, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, Division of Engineering, Brown University, Providence, RI 02912
In Al alloys reinforced with Al203 fibers control of the size of interface precipitates of Al2Cu, through coarsening, permits control of the effective separation toughness of the interface under the deformation induced local mixed modes of interface fracture during the evolution of global composite fracture, which assures optimization of both transverse strength and axial toughness. Experiments and micromechanical modeling will be presented demonstrating the effectiveness of this strategy.
10:45 am INVITED
THE ROLE OF FRICTIONAL CRACK SLIDING ON THE COMPRESSIVE FAILURE OF ICE AND ON ITS BRITTLE-TO-DUCTILE TRANSITION: Erland M. Schulson, Department of Mechanical Engineering, Thayer School of Engineering, 8000 Cummings Hall, Darthmouth College, Hanover, NH 03755
In 1962 McClintock and Walsh pointed out that frictional sliding across the surface of closed cracks inclined to the direction of loading is an important element in brittle compressive failure. The friction reduces the effective shear stress on the crack plane and so lowers the stress concentrated at the crack tip. Frictional sliding thus increases the far-field stress for out-of-plane wing crack initiation and growth, thereby raising the failure stress (e.g., Brace and Bombolakis 1963, Nemat-Nasser and Horii 1982, and Ashby and Hallam 1986). This paper will show that the brittle compressive failure of ice can be understood in terms of the frictional crack sliding/wing crack mechanism, and that its brittle-to-ductile transition can be explained by incorporating crack-tip creep.
11:10 am INVITED
FRACTURE IN THE DUCTILE AND DUCTILE/BRITTLE REGIMES: CELL MODEL STUDIES: Robert H. Dodds and C. Fong Shi, Department of Civil Engineering, 2129 Newmark Laboratory, MC-250, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue, Urbana, IL 61801; Division of Engineering, Brown University, Providence, RI 02912
Mode I crack initiation and growth under plane strain conditions in tough metals have been successfully simulated using an elastic-plastic continuum model which accounts for void growth and coalescence ahead of the crack tip. A row of void-containing cell elements is placed on the symmetry plane ahead of the initial crack. These cell elements incorporate the softening characteristics of the hole growth and its strong dependence on stress triaxiality. Under increasing strain, the voids grow and coalesce to form new crack surfaces thereby advancing the crack. The material parameters are the Young's modulus, yield stress and strain hardening exponent of the metal and two additional parameters, D and fo, characterizing the dimension of the cell element and the initial volume fraction of the void centered within the cell. Once the above material parameters have been calibrated for the material under investigation, the model can be applied to compute relationships among load, load-line displacement and crack advance with no restrictions on the extent of plastic deformation and crack advance [Xia and Shih, (1995a and 1995b)]. The model has been applied to several specimen geometries which are known to give rise to significantly different crack tip constraints and crack growth resistance behaviors. Computed results are compared with sets of experimental data for two tough steels. Details of the load, displacement and crack growth histories are accurately reproduced [Xia, Shih and Hutchinson, 1995]. Suitably defined measures of crack tip loading intensity, such as those based on the J-integral can also be computed; however such crack growth resistance curves no longer play the central role in this approach. The above model has been employed in the transition regime where a crack initiates an grows by ductile tearing but final failure can occur catastrophic cleavage fracture. Crack growth causes significant alterations in the stress field, the process zone size, the competition between ductile and brittle processes and the sampling volume. The effects are accounted for by incorporating weakest link statistics into the cell element model. The cleavage fracture model also takes into account the increase of sampling volume with crack growth and the competition between void nucleation from carbide inclusions and unstable inclusion cracking which precipitates catastrophic cleavage fracture. Load-displacement behavior, ductile tearing resistance and transition to cleavage fracture are discussed for several different test geometries and a range of microstructural parameters. The model predicts trends in the ductile/brittle transition region that agree with experimental data [Xia and Shih, (1996)].
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