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ABSTRACT--The work presented here evaluates the validity of using the load at the point of linearity on the displacement load curve, as suggested by Elber, as an experimental measure of the crack-tip-opening load. Displacement-load behavior was investigated for a fatigue-cracked modified compact specimen of a nickel-base superalloy. Displacements were determined at the notch mouth using a standard clip-on gage, along the crack sufrace using a laser-interferometric displacement gage and, in the plastic zone ahead of the crack, using an opticalinterferometry technique. Acoustic-emission monitoring was employed as a means to detect potential crack extension during measurement-load cycles and to detect physicalcrack closure. The magnitude of the crack-tip-opening load, as determined from these measurements, is dependent on the distance from the crack tip at which the measurement is made. As an additional means of evaluating the crack-tip-opening load, crack-surface profiles are constructed from the displacementload measurements made behind the crack tip. A discussion is given concerning the significance of these results in evaluating the validity of using the toad at the point of linearity as a parameter to quantify crack closure.

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USAThe initial investigations by Elber [1,2] demonstrated the occurrence of crack tip closure and the utility of the concept of an effective stress intensity range in predicting the dependence of fatigue crack growth rate on the stress ratio R. This apparent success of the crack closure concept led to considerable effort to refine the experimental techniques for measuring the magnitude of crack tip closure. These techniques are generally divided into two categories: techniques which measure bulk specimen response and those which measure displacements on the specimen surface.Bulk measurements have been made using electrical potential methods [3], ultrasonics [4] and, for transparent materials, optical interferometry [5]. Specimen surface displacement measurements have been made with strain gages bonded across the crack line [6], displacement transducers attached across the crack line or ahead of the crack tip [7], optical interference techniques [8,9,10], and clip-on displacement gages at the notch mouth [ll].To quantify crack closure most researchers have attempted to determine the applied load at which the crack is fully open, i.e., no physical contact occurs between the cracked surfaces.The crack opening load Pop is defined in the idealized displacement-load curve, shown in Fig. i, as the load at the transition point from nonlinear to linear behavior.In his original analysis Elber [2] assumed that the crack surface displacement was a function only of load and crack length.For a partially closed crack the elastic displacement-load behavior is not linear but is a function of the effective crack length.Based on these assumptions, the determination of Pop in Fig. 1 defines the load at which the crack is fully open.The majority of opening load determinations, regardless of the experimental technique used, are based on the above assumptions concerning displacement-load behavior. However, despite the numerous sophisticated techniques utilized, a considerable amount of conflicting crack closure data exists in the literature. Conflicting results on the dependence of Pop on Kma x and R [2,3] question the validity of crack closure for incorporation into crack propagation models.It has also been observed that bulk measurements often yield a higher Pop than do measurements of displacements on the specimen surface [11,12].-Some efforts have been made to match bulk results with surface results by various schemes of extrapolating the bulk measurement data [4,13]. Such techniques assume a priori the accuracy of Pop as determined by specimen surface displacement measurements.The purpose of this report is to present preliminary results of a current investigation into the uniqueness of P~_ as determined from up speciment surface displacement measurements. Displacement measurements were made at room temperature on a modified compact specimen as shown in Fig. 2. The material used was Gatorized TM IN-I~0, a nickel-base superalloy, with o. = 1120 MNm -2 and E = 214xlO3~4~m -. Fatigue crack growth was conductedYin a closed loop servo-h...

Fatigue cracks grown in compact tension specimens of 2024-T851 aluminum at ΔK = 6.93 MN/m3/2 (R =0.1) were subjected to single-peak overloads of 17.31 MN/m3/2. After the overload, the specimens were held at zero load for periods of 3 min, 1 h, and 20 or more hours before recycling at ΔK = 6.93 MN/m3/2. Increasing the rest time had the effect of slightly reducing the crack retardation as measured on the specimen surface. The load at which the fatigue crack faces fully separate was measured on the specimen surface by laser interferometric techniques and in the specimen interior by through-transmission ultrasonic methods. Crack opening loads measured on the specimen surface were found to increase with application of an overload and then decrease to the original value when the specimen was allowed to rest at zero load, while opening loads measured through the sample by the ultrasonic method did not vary significantly with peak loads or rest times. Varying specimen thickness between 0.64 and 2.54 cm had little effect on surface measured retardation or opening loads. The relationship between applied load and crack surface displacement as measured by ultrasonics varied significantly with specimen thickness.