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Session Chairpersons: Professor Viggo Tvergaard, Department of Solid Mechanics, Technical University of Denmark, DK-2800 Lyngby, Denmark; Professor Peter K. Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996
8:30 am INVITED
DAMAGE ACCUMULATION AND DUCTILE FRACTURE--GLOBAL AND LOCAL PERSPECTIVES: F. Zok*, D. Lahaie**, and John D. Embury**; *Department of Materials Science, University of California, Santa Barbara, Santa Barbara, CA 93106; **Department of Materials Science and Engineering, McMaster University, 1280 main Street, Hamilton, Ontario, L8S 4L7, Canada
This presentation will consider the quantification of damage accumulation in metals and composites. Consideration will be given to the influence of hydrostatic pressure and strain path on nucleation and damage accumulation process. Models of fracture based on the influence of damage accumulation on both the stability of plastic flow and the attainment of critical damage levels will be discussed. In the final portion of the presentation, the use of damage accumulation in relation to the design of hydrostatic extrusion processes will be discussed.
8:55 am INVITED
PREDICTION OF DUCTILE BEHAVIOUR IN WELDED STRUCTURES (DUCTILE TO BRITTLE TRANSITION BEHAVIOUR, SHEAR LIP, NOTCH ACQUITY, AND SPECIMEN SIZE): James R. Matthews, Dockyard Laboratory, Department of National Defence, Defence Research Establishment, Atlantic, Dartmouth, Nova Scotia B2Y 3Z7, Canada
Design of ships against fracture requires, at the simplest level, that the minimum service temperature of the ship coincide with the upper shelf of the structural ductile to brittle transition curve for the steel and weldments. Material specifications in shipping standards often have no requirement for notch toughness and when they do specify toughness the specimen is of insufficient size and notch acquity to reflect the structural transition behaviour and to guarantee that the structure will be ductile at the lowest service temperatures. Consequently minimum standards should be adopted and specimen size and notch acquity requirements should be sufficient to predict ductile behaviour. In this paper, it will be shown that the transition behaviour for Charpy six specimens can be 50 to 100°C below that for larger DT specimens. It will he shown that replacing the standard notch with an EDM notch in Charpy speci mens moves the transition curve towards the DT curve but as little as 20 degrees for some materials. This leaves the DT specimen as the simplest existing specimen whose transition behaviour would approximate that of a structure. Data will be presented for 350WT and A517 steels. Refinement of the DT specimen to include an EDM notch as opposed to a machined and pressed notch may marginally reduce cost of testing. Relying on a shear lip width transition curve as opposed to an energy based transition curve can reduce the cost considerably. Data will be presented to show that shear lip width is related to the fracture energy of specimens over a wide range of temperatures, compositions and weldment locations. Finally data will also be presented to show that the relationship between shear lip and energy is independent of specimen thickness between l6 mm and 25 mm thickness.
9:20 am INVITED
A COMPUTER SIMULATION OF THE EFFECTS OF LOADING PATH ON THE MECHANICAL PROPERTIES OF POROUS COPPER: Anthony Kee, Peter Matic, Jennifer Morris and Andrew Geltmacher, Mechanics of Materials Branch, Code 6382, Naval Research Laboratory, 4555 Overlook Drive SW, Washington D.C. 20375
The present research examines the local stress and strain rates generated during loading in porous copper specimens produced by the GASAR gas-eutectic process, developed by Shapovalov and Timshenko. Two-dimensional explicit finite element simulations are performed on a representative microstructure taken from a 21.5 percent pore volume fraction bulk sample which contains high aspect ratio pores. The fraction bulk sample which contains high aspect ratio pores. The models investigate the role of multiple pore interactions and their effect on the mechanical properties of the porous materials. Previous research has shown the dependence of bulk strength and ductility on factors such as pore density, spacing, constraint and axial offset for uniaxial tensile loading. The present study extends the effort by applying biaxial and shear boundary conditions to the specimen, which simulates the effects of strain path on the deformation and fracture behavior of the porous materials. From these finite element models, the shape of the yield surface has been determined for the porous materials.
HIGH SPEED TEMPERATURE AND OPTICAL MEASUREMENTS OF DUCTILE FAILURE EVENTS IN METALS: Ares J. Rosakis, G. Ravichandran, Raman P. Singh, Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125
An experimental investigation has been conducted to study dynamic crack initiation in precracked highly ductile steel specimens. The specimens are fabricated out of 4340 mild steel and 304 stainless steel and subjected to dynamic three point bend loading by impacting them in a drop weight tower at various loading rates. This dynamic impact of the pre-cracked steel specimens results in deformation followed by fracture initiation at different loading rates. During the dynamic deformation and failure process the time history of the transient temperature and deformation fields around the crack tip are recorded experimentally. The transient temperatures are recorded using high speed infrared detectors, while the deformation fields are obtained using the optical method of Coherent Gradient Sensing (CGS) in conjunction with high speed photography. The experimental information is used to extract fracture parameters of interest, such as the time history of the dynamic J-integral (Jd(t)) and its critical value at initiation (JdInitiation) This is the first time that a noncontact temperature measurement is used to evaluate Jd(t). The applicability and accuracy of this technique is validated through comparison with the simultaneously performed CGS optical measurements. Finally, the data provided by the two techniques is used to identify various ductile failure mechanisms and quantify the fracture initiation toughness in terms of JdInitiation at different loading rates.
ANALYSIS OF FAILURE MODES IN IMPULSIVELY LOADED PRE-NOTCHED STEEL PLATES: Ramesh C. Batra, N.V. Nechitailo, Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0219
We analyze transient plane-strain thermomechanical deformations of a pre-notched 4340 steel plate impacted by a 4340 steel projectile in the direction of two parallel notches, and study the influence of the impact speed and notch tip radius on the localization of the deformation. The plate configuration is identical to that in Kalthoff's experiments (1987, 1988). There is no failure or fracture criterion included in our work. However, the computed evolution of stress and plastic strain fields strongly suggest that with an increase of impact speed and decrease in notch tip radius, there is a failure mode transition from a tensile crack opening at approximately 70° to the notch ligament to an adiabatic shear band propagating at (-5°) - (-15°) to the notch ligament. This is in qualitative agreement with Kalthoff's findings.
10:25 am BREAK
EFFECT OF STRAIN RATE ON THE FRACTURE TOUGHNESS REFERENCE TEMPERATURE To FOR FERRITIC STEELS: Kim Wallin, Materials and Structural Integrity Department, VTT Manufacturing Technology, P.O. Box 1704, FIN-02044 VTT, Finland
The new master curve concept for describing materials fracture toughness in the transition region, with the help of a reference temperature To, is a candidate for structural integrity assessment codificaton. Normally, To is determined for (quasi) static strain rates, while often dynamic values are required. The master curve concept can of course be applied also to dynamic tests, but this would require a double amount of testing. Therefore, if the effect of strain rate on To can be quantified with sufficient accuracy, the applicability of the master curve concept for structural integrity assessment codification would be strongly enhanced. For this purpose, fracture toughness data found in the literature was analyzed with the master curve concept, and, using the Zeller-Hollomon strain rate parameter, a simple semiemperical expression for the strain rate dependence of To was developed. The error of the expression is only of the order ±20% covering yield strength levels from 200 to 1400 MPa.
DYNAMIC PLASTICITY OF DUCTILE MATERIALS AT THE MESOSCOPICAL SCALE LEVEL: Yuri I. Mescheryakov, Institute of the Mechanical Engineering Problems, Russian Academy of Sciences, Saint-Petersburg, V. O. Bolshoi 61, 199178, Russia
Dynamic tests of ductile materials (copper, aluminum, ductile steels) were carried out under uniaxial strain conditions to determine the criterion of transition between translational (shear banding) and rotational modes of deformation. All the basic processes defining the mode of plastic deformation were found to occur at the mesoscopical scale level (0.1 - 10 µm) and depend on the particle velocity distribution at that level. The later has been recorded with a velocity interferometer modified for measuring both average mesoparticle velocity and particle velocity dispersion simultaneously. The kind of kinematical mechanism (translational or rotational) is determined by difference between longitudinal and transverse components of the mesoparticle velocity dispersion. This conclusion results from theoretical analysis of rotational motion of medium by using the mesomechanics approach.
FLOW AND FAILURE OF HIGH DENSITY MATERIALS IN BALLISTIC IMPACTS: Lee S. Magness1, Jr., Deepak Kapoor2 and Moon Chung2, 1U. S. Army Research Laboratory, Aberdeen, MD 21005; 2U. S. Army Armament Research, Development and Engineering, Picatinny Arsenal, Dover, NJ 07806
High-density alloys and composites are employed as the penetrator core materials in modern armor-piercing projectiles. During ballistic impact with an armored target, the length of the penetrator core is eroded or back-extruded as a cavity is opened in the armor. The core material is deformed to very large strains at strain-rates exceeding 104 per second. The hydrostatic component of the stresses on the penetrator can exceed 5 GPa (1 Msi), suppressing void nucleation and growth as a fracture mechanism. However, plastic localization and failures (shear bands) are promoted by the high rate, adiabatic deformation. The ballistic performances of a number of high-density alloys and composites are compared. Deformation and failure behaviors are categorized via optical metallographic examinations of penetrators recovered from ballistic impacts.
ANALYTICAL MODELING OF THE SHEAR MODE AND OPENING MODE OF DUCTILE FRACTURE: G. Rousselier and G. Barbier, Electricité de France, Research Division, Les Renardières, F-77250 Moret-sur-Loing, France
Strain localization in the vicinity of a surface is a precursor to ductile fracture of materials. It can be analyzed by considering the stability of a linear perturbation. For a broad class of materials: plastic, rate-dependent, with void-growth damage and loading conditions: three-dimensional, thermomechanical and dynamic, a closed-form solution is obtained for the stability condition. From this condition, a mode of ductile fracture can be predicted: shear or opening. In a plastically incompressible material, the shear mode of fracture only is obtained; it results from a competition between strain hardening and thermal softening. In a void-growth damaging material, the opening mode is promoted by stress triaxiality. The analytical results are in agreement with experimental observations and numerical results from the literature.
LOADING RATE INFLUENCE OF FRACTURE TOUGHNESS IN INSTRUMENTED PRECRACKED CHARPY-TYPE TESTING: Thomas Varga and Friedrich Loibnegger, Technische Versuchs- und Forschungsanstalt, Technische Universität Wien, A-1040 Wien, Karlspaltz 13, Austria
The usual loading rate of impact testing is near to 0.5 m/s. As it will be demonstrated, force-time and force-deflection diagrams show at least at the beginning strong oscillations. Investigations on realistic loading rates showed, however, in most practical cases effective loading rates below 0.1 m/s. The loading rate of precracked Charpy-type testing is by using the Schnadt pendulum 0.1 m/s. Because of this factor in nearly every situation a loading rate of 0.1 m/s seems to be sufficient. If the loading rate is reduced from about 5 m/s to 0.1 m/s the force-deflection-diagram can be evaluated without parasitic oscillations. The measurement of fracture toughness, Kid, Jd and the calculation of CTOD becomes therefore easier. If the steels tested are sufficiently brittle, an extrapolation to the fracture toughness of larger sections becomes possible. Otherwise side grooving may help in obtaining realistic fracture toughness values. A comparison between fracture toughnesses measured at high and at low loading rates will be demonstrated. HSLA-steels, thermomechanically treated, will be used for the comparison.
EFFECTS OF INCLUSIONS ON DUCTILE FRACTURE OF AN Fe-42PCT Ni ALLOY IN TENSION AND SHEAR: N. Yuki*, R. Foley** and G. Krauss***, *Nippon Mining and Metals Company Ltd, 3 Kurami, Samukawa, Koza, Kanagawa 253-01, Japan; **Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL 60616; ***Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401
Effects of inclusions on ductile fracture have been examined with laboratory produced Fe-42 pct Ni alloy. The inclusion volume fraction was varied between approximately 0.01 and 3.00 pct. Axisymmetric tensile bars and cylindrical, double shear test samples were tested in the recrystallized condition. In the tensile test, increasing inclusion volume fraction significantly reduced post-uniform elongation and strain to fracture . In the shear test, fracture displacement obtained from load-displacement data and strain to fracture calculated from shear test samples decreased with increasing inclusion volume fraction. The fracture behavior in tension and in shear is related to inclusion distribution. Applicability of different models is considered.
DUCTILE FRACTURE TOUGHNESS EVALUATION AT HIGH STRAIN RATES USING STRETCH ZONE: M. Nari Bassim* James R. Matthews**; *Department of Mechanical and Industrial Engineering University of Manitoba, Winnipeg, Manitoba, Canada R3T SV6; **Department of National Defence, Defence Research Establishment Atlantic, Dartmouth, Nova Scotia, Canada B2Y 3Z7.
Fracture studies of high strength low alloy steels, at very high loading (strain rates) of up to equal to 107 were conducted using specially designed Split Hopkinson Bar equipped with a swing arm mechanism capable of fracturing CT specimens with a thickness of 12.7 mm. While it was possible to reproduce the load-displacement curves for the specimens, evaluation of the fracture toughness was mostly obtained from post-test examination of the stretch zone ahead of the crack using scanning electron microscopy. Several factors affecting the stretch zone were identified including the effect of fatigue precracking as well as the method of measurement of the stretch zone (nine point vs. three point approach). Significant differences in the stretch zone were observed which are attributed to the extent of the constraint factor ahead of the crack due to fatigue precracking which demonstrates the occurrence of some nonlinearity in the initial part of the J-Stretch Zone Width relationship.
NEUTRON IRRADIATION EFFECTS ON THE DUCTILE-BRITTLE TRANSITION OF FERRITIC/MARTENSITIC STEELS: Ronald L. Klueh and D. J. Alexander, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
Below ~450°C, neutron irradiation hardens the Cr-Mo ferritic/martensitic steels considered for future fusion power plants. Hardening reduces ductility, but the major effect is an increase in the ductile-brittle transition temperature (DBTT) and a decrease in the upper-shelf energy (USE), as measured by a charpy impact test. After irradiation, DBTT values can increase to well above room temperature, thus increasing the chances of brittle rather than ductile fracture. Such a shift in DBTT could eliminate certain steels for nuclear applications. Steels are being developed for fusion applications that have a low DBTT prior to irradiation and then show only a small shift after irradiation. Low-chromium (3% Cr) and high-chromium (9% Cr) Cr-W steels are being investigated. A martensitic 9Cr-2WVTa (nominally Fe-9Cr-2W-0.25V-0.07Ta-0.1C) steel had a much lower DBTT than the conventional 9Cr-1MoVNb (Fe-9Cr-1Mo-0.25V-0.06Nb-0.1C) and 12Cr-1MoVW (Fe-12Cr-1Mo-0.25V-0.5W-0.5Ni-0.2C) steels prior to neutron irradiation and showed a much smaller increase after irradiation. The tantalum in 9Cr-2WVTa was concluded to affect the fracture stress. For the 3Cr steels, the type of bainitic microstructure formed during heat treatment affected the impact behavior. Granular bainite had inferior properties compared to an acicular bainite. Alloying was used to promote the acicular bainite. Improved toughness makes the steels candidates for both nuclear and non-nuclear applications.
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