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Effect of loading variables on the acoustic emissions of fatigue-crack growth

Objective of research described in this paper was to conduct a preliminary assessment of the effect of loading variables on acoustic emissions during fatigue-crack growth. Work was also aimed at presenting methods of utilizing the results in a practical manner to assure safe-life of engineering structures

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Abstract

A series of experiments has been conducted to determine the effect of loading variables such as cyclic frequency, load ratio, and material on acoustic emission from fatigue-crack propagation. It is shown that the applied-stress intensity range (ΔK) is the controlling parameter for all materials studied while the other parameters have lesser effects. Two potential methods for engineering application of acoustic emission during fatigue loading are described.

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Abbreviations

a :

crack length or surface-crack depth (in.) (mm)

a 1 ora 1 (conventional):

worst-case initial flaw size after a conventional proof test of a structure

a 1 (acoustic emission):

the flaw size detected in an acoustic-emission proof test

a 2 :

failure flaw size in service for the proof-tested structure

C 0,C 1,C 2,C 3 :

experimental constants

da/dn :

fatigue-crack growth rate (μin./cycle) (μm/cycle)

dΣN/dn :

acoustic-emission rate in fatigue (counts/cycle)

H :

test-specimen half height

K :

stress intensity:K ∝ σ (πa)1/2 (ksi-in.1/2) (Pa-m1/2)

K AE :

stress intensity for which one count/cycle is detected in acoustic-emission proof test (ksi-in.1/2) (Pa-m1/2)

K c ,K Ic :

stress intensity required for unstable fracture of a particular material thickness (ksi-in.1/2) (Pa-m1/2)

K Ii :

maximum permissible stress intensity at the beginning of service for a structure with assured safe operating life. TheK Ii value incorporates potential crack growth from corrosion and/or fatigue. (ksi-in.1/2) (Pa-m1/2)

K max :

maximum stress intensity in a fatigue cycle (ksi-in.1/2) (Pa-m1/2)

n :

number of cycles in fatigue

R :

ratio of minimum fatigue load/maximum fatigue load

t :

test-specimen thickness (in.) (mm)

W :

test-specimen test length (in.) (mm)

X 0,X 1,X 2,X 3 :

experimental constants

ΔK :

applied range of stress intensity in fatigue: ΔK ∝ (σ max -σ min) (πa)1/2·(ksi-in/1/2) (Pa-m1/2)

ΣN :

total number of acoustic-emission counts

σ:

gross applied stress (ksi) (Pa)

σ max :

maximum applied stress in fatigue (ksi) (Pa)

σ min :

minimum applied stress in fatigue (ski) (Pa)

σ PROOF :

maximum stress level in a conventional proof test (ksi) (Pa)

σAE PROFF :

maximum stress in an acoustic-emission proof test (ksi) (Pa)

σ OPER :

maximum operating stress for a structure (ksi) (Pa)

References

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Authors and Affiliations

  1. Lockheed Palo Alto Research Laboratory, Lockheed Missiles & Space Company, Inc., 94304, Palo Alto, CA

    T. M. Morton, S. Smith & R. M. Harrington

Authors
  1. T. M. Morton
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  2. S. Smith
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  3. R. M. Harrington
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Additional information

This research was funded by the Lockheed Independent Research Program.

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Morton, T.M., Smith, S. & Harrington, R.M. Effect of loading variables on the acoustic emissions of fatigue-crack growth. Experimental Mechanics 14, 208–213 (1974). https://doi.org/10.1007/BF02323071

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