The mechanics of ductile fracture is receiving increased focus as the importance of integrity ofstructures constructed from ductile materials is increasing. The non-linear, irreversible mechanicalresponse of ductile materials makes generalized models of ductile cracking very difficult to develop. Therefore, research and testing of ductile fracture have taken a path leading to deformation-basedparameters such as crack tip opening displacement (CTOD) and crack tip opening angle (CTOA). Constrained by conventional test techniques and instrumentation, physical values (e.g. crack mouthopening displacement, CMOD, and CTOA angles) are measured on the test specimen exterior and asingle through-thickness "average" interior value is inferred. Because of three-dimensional issuessuch as crack curvature, constraint variation, and material inhomogeneity, inference of averageparameter values may introduce errors. The microtopography methodology described here measuresand maps three-dimensional fracture surfaces. The analyses of these data provide direct extraction ofthe parameters of interest at any location within the specimen interior, and at any desired increment ofcrack opening or extension. A single test specimen can provide all necessary information for theanalysis of a particular material and geometry combination.

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.