Microtopography for ductile fracture process characterization Part 2: application for CTOA analysis

Abstract

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. With this increase in application comes a need to more fully understand and measure actual CTOA behavior. CTOA is a measure of the material response during ductile fracture, a "crack tip response function." In some range of crack extension following growth initiation, a constant value of CTOA is often assumed. However, many questions concerning the use of CTOA as a material 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 the effects of crack tunneling on general CTOA behavior? How do laboratory specimen measurements of CTOA compare to actual structural behavior?Measurements of CTOA on the outer surface of test specimens reveal little about threedimensional effects in the specimen interior, and the actual measurements themselves are frequently difficult. The Idaho National Engineering and Environmental Laboratory (INEEL) use their 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 crack extension. The analysis is accomplished using only a single specimen and is performed entirely after the completion of a test. The resultant CTOA distributions allow development of full and effective understanding of CTOA behaviors. This paper presents underlying principles, various sources of measurement error and their corrections, and experimental and analytical verification of CTOA analysis with the microtopography method.

Fracture and damage mechanics modelling of thin-walled structures – An overview

“…By this method (see [45][46]) the fracture surfaces are topographically analyzed post-mortem. It is based on the assumption that the CTOA is preserved in the plastic deformations of the fracture surface.…”

Strip Yield Model Numerical Application to Different Geometries and Loading Conditions

“…Another method of measuring crack opening is the microtopography method like the one described by [13,14]. This method measures and maps three dimensional fracture surfaces.…”

Correlation of crack-tip opening angle for stable crack propagation with charpy and drop-weight tear test properties in high-toughness API X70 pipeline steels

“…[10] The application of the CTOA to pipeline industries was extensively started in the late 1980s by Southwest Research Institute, the Centro Sviluppo Materiali, and the Societa Nazionale Metanodatti. [9,13,14] In this study, the relationship between the CVN and DWTT properties of high-toughness (CVN USE Ն 300 J) API X70 pipeline steels was investigated, and the CTOA sc results measured by the TSCT method were correlated with those of the CVN test and DWTT. In this model, a CTOA max was calculated as a crack driving force in a structure or a fluid of pipelines and was compared with a measured critical CTOA to obtain the reliable and predictable results on characteristics of dynamic ductile fracture propagation and arrest.…”