The location of crack closure with respect to crack wake and specimen thickness under different loading conditions was determined. The rate of increase of K_. in the crack wake was found to be significantly higher for plasticity induced closure 1n comparison to roughness induced closure. enRoughness induced closure was uniform throughout the thickness of the specimen en en ™ while plasticity induced closure levels were 50 percent higher in the near uu surface region than in the mid-thickness. The influence of state of stress on low-high load interaction effects was also examined. Load interaction effects differed depending upon the state of stress and were explained in terms of AK ,_. eff

Suggestions for testing of notched or cracked three-point bend specimens are presented that (1) correct displacement measurement errors resulting from misalignment between the load applicator and specimen; (2) account for coincidental strains not associated with the work of crack extension; (3) simplify record analysis and processing; and (4) extend displacement gage range without sacrifice of sensitivity or accuracy. These testing details are particularly applicable to procedures in which the crack extension force J1 is determined from the work done on the specimen.