Materials Week '97: Tuesday AM Session

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 Tuesday morning, September 16.

GEORGE R. IRWIN SYMPOSIUM ON CLEAVAGE FRACTURE: Session III: Micromechanical and Microstructural Modeling

Sponsored by: SMD Mechanical Metallurgy Committee, MSCTS Flow & Fracture and Computer Simulation Committees

Program Organizer: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238

Room: 211

Session Chairs: R.O. Ritchie, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720; W.W. Gerberich, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455

8:30 am INVITED

RECENT ADVANCES IN THE MICRO-MODELLING OF CLEAVAGE FRACTURE IN STEEL: J.F. Knott, School of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, UK

The paper covers two aspects of the modelling of cleavage fracture in steels. The first relates to events at the microstructural scale, where fracture initiators are smaller than 10 µm (usually, smaller than 5µm) in size. Here, the discussion focuses on the ways in which the local cleavage fracture stress depends on microstructure. Attention is drawn to behaviour in weld metals, where initiation sites can be clearly identified on fracture surfaces. The relation of local fracture stress to fracture toughness relies on the identification of a "critical distance" and the uniqueness of this "distance" when dealing with cleavage following warm prestressing, or cleavage ahead of a growing fibrous crack, is questioned. Comparisons are made with the so-called "local approach" which is statistically based. The second aspect covers events at the meso-scale: specifically, the ways in which fracture toughness results should be analysed, when the microstructure comprises a two-phase mixture of tough and brittle areas, of order 50-200 µm in size. The traditional Weibull approach is challenged, because it does not represent physical reality and because it produces ultra-pessimistic lower-bound values. The re-examination of the Weibull approach has implications with respect to micro-scale modelling.

9:00 am

CLEAVAGE-QUASI CLEAVAGE IN FERRITIC AND MARTENSITIC STEELS: G.R. Odette, K.V. Edsinger, G.E. Lucas, Department of Mechanical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-5070

Confocal microscopy-fracture reconstruction and SEM were used to characterize the sequence-of-events leading to cleavage in a low alloy pressure vessel steel and two 8-12Cr martensitic steels as a function of temperature. While differences between the steels were observed, they shared some common characteristics that differ from the conventional view of cleavage. Most notably cleavage does not occur as a single weakest link event; rather it is the consequence of a critical condition when a previously nucleated dispersion of microcracks suddenly coalesce to form a large, rapidly propagating macroscopic crack. It is argued that the critical event can be treated as a bridging instability. The stabilizing effect of the ductile ligaments separating the cleavage facets increases with increasing temperature. Indeed, even in the ductile tearing regime cleavage facets form a significant fraction of nuclei for larger microvoids.

9:20 am

STATISTICAL AND CONSTRAINT FACTORS IN CLEAVAGE INITIATION: G.R. Odette, K.V. Edsinger, G.E. Lucas, Department of Mechanical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-5070

The size dependence of effective cleavage initiation toughness K_ee(T) (defined by the load-displacement conditions at initiation) of steels are mediated by both statistical and constraint factors. Statistical effects are controlled by the total high stress volume even under plane strain, small scale yielding, e.g., K_Ic1/B^-1/4. Constraint loss and reductions in the stress fields occurs for shallow cracks, large scale yielding and deviations from plane strain. The interplay between these factors is examined by analyzing the observed K_ee(T) behavior for specimens with different W, B and a/W using FEM simulations of the crack tip fields and confocal microscopy, fracture reconstruction and SEM characterization of the sequence-of-fracture-events. Observed versus actual sequences and complications such as crack tip strain, the transition to ductile tearing and ultimate loss of specimen capacity are discussed.

9:40 am

THE FRACTOGRAPHY-MODELING LINK IN CLEAVAGE FRACTURE: Anthony W. Thompson, Materials Sciences Div., Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720

Cleavage fracture has historically been modelled, out of necessity, in rather idealized terms. In real materials, however, there are a number of difficulties in linking such models with metallographic and fractographic observations. Some of the most vivid examples occur for ₂ titanium aluminide alloys, in which, when the microstructure contains primary ₂ particles, the primary particles crack first. When "basketweave" or Widmanstätten structures of ₂ laths comprise the microstructure, it appears that individual laths crack first. And in colony structures, cracking occurs first across the ₂ lath colonies. Both detailed fractographic observations, and also a statistical model for brittle fracture by failure of weakest links, have been developed. The extent to which this can be interpreted in classical cleavage terms will be discussed. This work has been supported by the U.S. Air Force Office of Scientific Research.

10:10 am BREAK

10:20 am INVITED

MICROMECHANISMS OF CLEAVAGE FRACTURE: A.R. Rosenfield, Rosenfield and Rosenfield, 1650 Ridgway Pl., Columbus, OH 43212

This paper will review the history of crack-arrest technology, i.e. defining the conditions necessary to arrest a fast-moving cleavage crack in steel. Prof. Irwin's major contributions to the topic will be summarized and our research at Battelle stemming from these contributions will be discussed. The history of the concept of crack-arrest toughness, the development of a standard test method, and unresolved issues will be reviewed. Major emphasis will be placed on the observation and modelling of the microstructural contributions to toughness stemming from the unbroken ductile ligaments left in the wake of the advancing crack.

10:50 am

AN APPLICATION OF THE J-Q MODEL FOR ESTIMATING CLEAVAGE STRESS IN THE BRITTLE TO DUCTILE TRANSITION: John D. Landes, University of Tennessee, 310 Perkins Hall, Knoxville, TN 97996-2030; Carlos A.J. Miranda, IPEN, San Paulo, Brazil

A recent model has been proposed by the authors to predict cleavage failure in the transition for steels based on a weak link mechanism and a crack tip stress field modified for planar constraint by the J-Q theory. The model uses the distribution of toughness results at a single temperature to predict the same at a different temperature or for a different geometry. In this model a material cleavage stress is needed to predict when the weak link fracture is triggered. This cleavage stress is a key input for the application of the model but is not a property that is routinely measured and is hence not available for most steels. Using a characteristic of the model this cleavage stress can be estimated from the result of two distributions of toughness values tested at two different temperatures in the transition. In this paper the method to estimate a value of cleavage stress is presented and the result is used to predict the toughness distributions for structural components. Examples are given for several steels.

11:10 am

MODELING A CLEAVAGE-CHARACTERISTIC STRESS (S_co) OF FERRITIC STEELS: D.M. Li, School of Materials Science & Engineering, Harbin Institute of Technology, Box 433, Harbin 150001, China; M.Yao, Materials Science Department, Yanshan University, Qinhuangdau 066004, China

The cleavage fracture of ferritic steels has been studied under different specimen geometry (smooth, notched and cracked bars/sheets), loading mode (plain tension, three-/four-point notch/crack bending and center-cracked sheet tension) and strain rate (quasi-static to impact) conditions. The data of mechanical properties testing, fractographic observation and FEM simulation are incorporated to model and substantiate a "cleavage-characteristic stress," S_co, as representing a material's resistance to cleavage fracture. A concept of "effective plastic zone" has been emphasized in constructing the proposed probabilistic fracture model. Based on the parameter S_co, a unified cleavage fracture criterion is established accounting for the different conditions involved.

11:30 am

AN INVESTIGATION INTO THE MECHANISM OF CLEAVAGE FRACTURE IN A DUAL PHASE STEEL: Wei-Di Cao, Allvac, An Allegheny Teledyne Company, Monroe, NC 28110-0531

The cleavage fracture behaviors in notched and precracked specimens of a dual phase steel were studied as a function of grain size and test temperature. It was found that the relationship between critical fracture stress and bainite packet size can be described by _FF=_F^o + K_FF (d_pp)^-1/2, and _F is relatively temperature independent within the test temperature range. The cleavage fracture toughness decreased with increasing grain size, but rose again after passing a critical grain size, leading to a minimum in K_ICI-grain size dependence. A strong temperature dependence of K_ICIwas observed at higher test temperature near transition temperature of cleavage-dimple fracture. To explain the observed behaviors, a detailed fracture process observation was conducted by using acoustic emission monitor, cross section observation and fractography on interrupted test specimens. A comparison was made with major available theories and a modeling methodology is suggested to better explain the experimental phenomena.