Crack closure at positive stress levels [i] is currently being investigated as a mechanism to explain variations in fatigue crack growth rate caused by changes in the testing conditions. Compliance [i-3], resistivity [4][5][6], optical [7,8], and acoustical [9,10] measurements, extensively used to monitor crack growth, yield information about crack closure. Correlation between the various results is quite difficult, however, since different monitoring techniques, sample geometries, materials, etc. have been involved.In the present investigation two of the above crack monitoring techniques are used on two different sample geometries, PTC [I0] and CT [ii], and two heats of A1 2219-T851, but otherwise equal testing conditions, to characterize closure more fully.Crack closure in PTC samples was monitored [9] by an acoustic surface wave technique (500kHz) and in CT samples using longitudinal bulk waves (2.25 MHz), utilizing both the transmitted and the reflected acoustic beam, see Fig. i. Two compliance gauges, the conventional mouth opening clip-on gauge and an Elber-type gauge [i-3], were mounted on the CT specimen.Testing was done in air (5% relative humidity) at R = Omin/Sma x values of 0.08, 0.30, and 0.50.A comparison of closure curves as observed using an Elber gauge and acoustic bulk wave signals is shown in Fig. i. In all cases, closure is initiated at some load (the closure load, P8) and increases rapidly as the load decreases.The previously used method of defining P0 for the acoustic signals [i0] has been to use the" intersection of tangents to the two extreme segments of the curve. At low R values this agrees well with the point where nonlinearity [1][2][3][4][5][6][7][8] is first observed in the Elber gauge signal. One might argue then, that there is good agreement of the Elber gauge and the acoustic methods of determining Po" However, as the R value was increased it became more difficult to aetermine P^ using the Elber gauge. Closure was observed U . .at R = 0.5 by the acoustlc technlques in both sample geometrles, but not by the Elber gauge.Both the sensitivity and the closure load varied with relative location of the Elber gauge. At low R, agreement was best with the acoustic technique if the Elber gauge was approximately 2.5 mm behind the crack tip. The compliance gauge at the CT specimen mouth does not have the location variability problem of course, but the sensitivity was lower than with the Elber gauge.The acoustic technique allowed a qualitative measurement of the area over which closure is occurring.By moving the acoustic transmitter with the sample fully loaded and then unloaded the length of crack over which closure occurs may be determined:The differenceInt Journ of Fracture ii (1975)

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This paper describes experiments on fatigue crack growth, Δa/ΔN, in part-through crack specimens of 7075-T651 aluminum alloy. The effects of dry and humid environment in combination with single-spike overloads on Δa/ΔN, the crack growth delay, ND, and the “effective” stress intensity range, ΔKeff, are discussed. Using an acoustic monitoring technique, it was observed that the crack closure not only reflects the influence of the environment, but also of the spike overload on the plastic deformation at the crack tip and therefore on Δa/ΔN. The relationship Δa/ΔN = A (ΔKeff)n describes both environmental and spike overload effects, uniquely. Delay is more pronounced in a dry environment and increases strongly with increasing overload ratio.

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