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Continuous monitoring of fatigue-crack growth by acoustic-emission techniques

Purpose of this investigation was to further explore the relationship between crack-growth characteristics and acoustic-emission variables such as instrumentation gain and sensor frequency. The threshold conditions for crack detection were also investigated

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Abstract

The application of acoustic emission to the detection of fatigue-crack propagation in 7075-T6 aluminum and 4140 steel is investigated. The relationship between crack-growth rate, cyclic stress-intensity factor, load-cycling rate and observed acoustic-emission behavior is presented. Crack-growth rates of less than 10−6 in./ cycle could be detected, and acoustic-emission counts per cycle were shown to be closely related to the energy released by crack extension per cycle. A quantitative relationship for the threshold conditions for detection of fatigue-crack growth is presented which agrees with experimental test results. The results also showed that fatigue-crack growth occurs in an accelerating and decelerating manner, even though the stress-intensity range remains uniform, and that the count rate posses through a peak that is believed to be associated with a plane strain-plane stress transition. The effects of instrumentation sensitivity and frequency bandpass are also investigated. The results obtained indicate that acoustic-emission techniques should be suitable for in-service monitoring of a variety of cyclically loaded structures, even in the presence of high background noises.

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Abbreviations

A :

crack area

a :

crack length

a′ :

crack-growth rate (=da/dn)

B :

specimen thickness

C :

proportionality constant

D :

proportionality constant in crack-growth relation

E :

modulus of elasticity

G :

amplifier gain

G :

strain-energy release rate

K :

stress-intensity factor

\(\hat K\) :

stress-intensity range (=K max-K min)

N :

acoustic-emission counts

N′ :

acoustic-emission count per cycle (=dN/dn)

n :

number of fatigue cycles

n :

cyclic-loading rate

n f :

cycles to failure

P :

load

\(\hat P\) :

load range (=P max-P min)

R :

load ratio (=K min/K max=P min/P max)

t :

time

t * :

time for amplified sensor signal to ring down below trigger level of counter

U :

energy released by an event

V :

voltage of amplified and filtered acoustic-emission signal

V o :

initial voltage of acoustic-emission signal due to a single event

V t :

minimum voltage required to trigger the counter

Y :

term in equation for stress-intensity factor

α:

a/w

β:

time constant of exponential decay of sensor signal

η:

number of counts from a single event

σ ys :

yield strength

ω:

frequency of acoustic-emission signals

References

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  1. Dunegan/Endevco, 94550, Livermore, CA

    D. O. Harris & H. L. Dunegan

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  1. D. O. Harris
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  2. H. L. Dunegan
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Harris, D.O., Dunegan, H.L. Continuous monitoring of fatigue-crack growth by acoustic-emission techniques. Experimental Mechanics 14, 71–81 (1974). https://doi.org/10.1007/BF02323130

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