Fracture mechanics, broadly speaking, represents knowledge of the influence of loading and geometry on fracture. Figure 1 shows the size range of significant events involved in the crack-extension process. Until the ion and electron cloud configuration shown schematically at the far left can be analyzed from wave mechanics, and the results integrated through ten orders of magnitude, it will be necessary to work with a series of different models, each appropriate to its own size range. Boundary conditions for each model are found from the next largest and next smallest scales. The smaller the size of the model from which we can integrate through larger sizes to obtain the fracture load for a given geometry, the broader the range of conditions which can be predicted or correlated from given experimental data. From whatever level we do start, it will be necessary to introduce test data to substitute for lack of knowledge (or cost of analysis) in the smaller regions.

Under monotonic loading, structures should ideally be ductile enough to provide continued resistance during crack growth. Such fully plastic behavior is of interest in design against collisions, tank car accidents, earthquakes, and ship groundings. For fully plastic crack growth in low strength alloys, existing asymptotic solutions for elastic-plastic growing cracks are not applicable because they reach the fracture strain only in regions small compared to the inhomogeneities of the actual fracture process.

The crack tip opening angle (CTOA) is seeing increased use to characterize fracture in so-called"low constraint" geometries, such as thin sheet aerospace structures and thin-walled pipes. Withthis increase in application comes a need to more fully understand and measure actual CTOAbehavior. CTOA is a measure of the material response during ductile fracture, a "crack tipresponse function". In some range of crack extension following growth initiation, a constantvalue of CTOA is often assumed. However, many questions concerning the use of CTOA as amaterial response-characterizing parameter remain. For example, when is CTOA truly constant?What three-dimensional effects may be involved (even in thin sheet material)? What are theeffects of crack tunneling on general CTOA behavior? How do laboratory specimenmeasurements of CTOA compare to actual structural behavior?Measurements of CTOA on the outer surface of test specimens reveal little about threedimensionaleffects in the specimen interior, and the actual measurements themselves arefrequently difficult. The Idaho National Engineering and Environmental Laboratory (INEEL) usetheir microtopography system to collect data from the actual fracture surfaces following a test. Analyses of these data provide full three-dimensional CTOA distributions, at any amount of crackextension. The analysis is accomplished using only a single specimen and is performed entirelyafter the completion of a test. The resultant CTOA distributions allow development of full andeffective understanding of CTOA behaviors. This paper presents underlying principles, varioussources of measurement error and their corrections, and experimental and analytical verificationof CTOA analysis with the microtopography method.

Model weldment fracture specimens have been fabricated, tested, and analyzed using finite elements. The specimens consist of an interleaf of commercially pure titanium diffusion-bonded to a harder alloy titanium. A deep edge crack is introduced symmetrically into the interleaf, and the specimens are loaded in pure bending. Variation of the thickness (2h) of the soft interleaf layer provides insight into effects of weld geometry in strongly undermatched weldments tested in plane strain bending. Ductile crack growth (beyond blunting) initiated at loads giving J ? 95 kJ/m2 in all specimens. In the thickest interleaf geometries, stable tearing was obtained, but in the thinnest interleaf (2h ? 3 mm), crack initiation resulted in a massive pop-in of 5.4 mm across an initial ligament of 12 mm. Finite element studies show that the thinnest interleaf geometry had slightly higher peak stress triaxiality at the beginning of cracking, and that the highest triaxiality extended over a larger region than in the thicker interleaf specimens loaded to the same initiation J-values. More importantly, the blockage of plastic straining above and below the crack tip in the 3 mm interleaf specimen forced higher values of plastic strain to spread forward into the ±45‡ sector of highest stress triaxiality directly ahead of the crack tip. The higher strains, in conjunction with the slightly higher stress triaxiality, led to the unstable pop-in initiation.

Identifer : Dislocations. Conditions for determining the motion of dislocation arrays from plasticity solutions were established. The predicted mode of deform tion was observed in Al and in AgCl specimens under torsion. Observations of AgCl under plastic reversed stress showed the slip usually did not reverse on a given plane, that further forward slip could occur on one pla e, that co sid r ble disorganization of the crystal developed before fracture, th t cracking can occur on planes of high shear stress without appreciable motion normal to the surface, and that structural inhomogeneities reached a scale of the order of 0.1 mm. Crack initiation and propagation rates were reasonably well correlated with a crack growth theory based on continuum plasticity. (Author).

The most dominant singular stress distributions caused by grain-boundary sliding accommodated by power-law creep have been determined at hard grain-boundary particles and at triple grain junctions, where cavitation and eventual creep cracking are frequently observed. Finite element computations have been used to connect these local stresses to the far-field stresses. These finite element analyses incorporate a specially formulated power-type singularity element, a new method of simulating periodicity and symmetry boundary conditions, a scheme for automatic variable step selection for stable time integration, and a method to bypass the influence of incompressibility. The deviatoric stresses obtained from the finite element results show very good agreement with the dominant singularity solutions, and provide accurate estimates of the generalized stress intensity factors.

A comparison of the slip line flow field at the tip of a sharp crack and of a blunted crack shows that a sharp crack can be extended by alternating shear until it becomes blunted. Crack tip blunting is associated with high strain concentration and fracture by hole growth and coalescence. Crack extension by alternating shear has been obtained in fully plastic, plane strain sharply notched, singly and doubly grooved tensile specimens. Electron fractography shows crack extension by alternating shear at the tip of cleavage and fatigue cracks. It is shown that fatigue crack propagation is the result of repeated cyclic crack extension by alternating shear. (Author).