It is shown that acoustic second harmonic generation is a useful tool for studying surface microcrack development during fatigue of a high-strength aluminum alloy. A fundamental (5 MHz) surface acoustic wave (SAW) was transmitted across the gauge section of flexural fatigue specimens of Al 7075-T6. The second harmonic amplitude was determined after several increments of fatigue, as a function of external load and the amplitude of the fundamental. It was found that the second harmonic signal is at a maximum close to zero external load and increases with progressing fatigue. Harmonic generation, attributable to microcracking at the surface, has been observed as early as 10–20% of the expended fatigue life. A simple analysis to obtain a coefficient of harmonic-generation efficiency versus applied surface stress is described. This analysis considers the effect of changes in attenuation of the fundamental and harmonic waves associated with degree of surface microcrack opening as a function of surface stress. It is shown that acoustic harmonic generation may be a useful tool for reliable monitoring of the state of fatigue of a structural material.

No abstract

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)

Some deductions have been made from fractographic evidence about mechanisms of low-cycle mechanical fatigue in plain woven graphite/SiC composites at room and high temperature in vacuum. At both room temperature and 830°C, fatigue appears to be confined to the crack wake, where attrition reduces the efficacy of bridging fibers. It is inferred that the crack tip advances at some critical value of the crack tip stress intensity factor, as in monotonic growth, rather than by any intrinsic fatigue mechanism in the matrix. However, the manifestations of attrition are very different at room and high temperatures. At high temperature, wear is greatly accelerated by the action of S i c debris within the crack. This distinction is rationalized in terms of the temperature dependence expected in the opening displacement of a bridged crack. This argument leads in turn to plausible explanations of trends in load-life curves and the morphology of cracks as the temperature rises.

A machine capable of fatigue testing high strength alloys at an ultrasonic frequency (20 kHz), and a range of temperatures likely to be encountered by such materials in aerospace and power generation applications and in basic research, is described. The machine is assembled entirely from commercially available components used in ultrasonic joining processes. Basically, it consists of a power supply module and heavy duty transducer capable of delivering up to 1.2 kW of acoustic energy at peak-to-peak amplitudes ranging from 10 to 20 /-L, which is further amplified by tuned acoustic horns. The machine sets up a 20 kHz standing wave in the resonant test specimen. Peak-to-peak displacement amplitudes in the specimen of up to 300 /-L (typical stress levels up to 1400 MN/m2) can be achieved. Provisions have also been made to accurately monitor the frequency of testing and the displacement amplitude during the test. As an example of the capabilities of the machine, results of high-frequency fatigue tests performed over a range of temperatures are presented. Lastly, it is indicated that ultrasonic fatigue testing, by virtue of its extremely high rate of testing, can be conveniently used for the study of very slow fatigue crack propagation in engineering alloys.