Acoustic emission from superconducting magnets

We have shown experimentally that the acoustic emission (AE) is an effective means of monitoring the operating conditions of superconducting magnets and can be used to prevent catastrophic quenching. the signal-to-noise ratio of AE transducers at liquid helium temperature has been found to be about 18 dB higher than that at room temperature; AE originating from a superconducting magnet and those from the liquid helium shower can be clearly discriminated by observing their wave forms and frequency spectra. These results are also discussed from a theoretical view point based on an elastic medium model for the superconducting magnet.     

Acoustic emission in brittle solids

A signal/source correlation study of the stress waves emitted during unstable microscopic Hertzian fracture in glass is described. A theoretical analysis of the variation in excess strain energy with applied load is made and the results compared with experimental data covering a wide range of crack sizes.     

Acoustic emission sources modelled by dislocations

Acoustic emission sources representing dislocations and cracks under tensile forces, shear forces or a combination of these forces are modelled by a single circular dislocation loop and by a continuous distribution of such loops. The medium is assumed to be linear, elastic isotropic, homogeneous and unbounded. The quasistatic and the point source approximations are assumed. The wave displacement induced by the expansion of a loop and of a continuous distribution of loops is calculated for a tensile and a shear source on the basis of the dynamic dislocation theory by using the Green's function formalism. The wave is given in terms of L-, SV- and SH-waves as a function of the expansion law of the source and is calculated in a simple case. The wave form, radiation pattern and the diameter of the source are calculated; the influence of the expansion time of the source and of the distance between the source and the observation point is discussed. The results are used to check the assumptions of linearity and elasticity. They are compared with experiments and applied to characterise the source from measurements.     

Acoustic emission in carbon fibre composites

Acoustic emission analyses have been performed on carbon fibre-epoxy composites in order to correlate acoustic activity with well defined fracture processes. Different types of laminates were studied to discriminate the basic processes of fracture within the laminates. Test specimens were subjected to three-point flexural tests so as to separate fracture initiation and propagation. Acoustic emission signals were recorded by digital techniques for off-line analysis.

Results show that during fracture initiation, high amplitude signals are correlated with fracture processes controlled by fibre rupture, and low amplitude signals with matrix fracture. For laminates characterized by major fracture propagation, acoustic emission is generated by the processes induced by the friction of the fracture surfaces created. This acoustic activity, associated with abnormally long duration and high amplitude signals, exceeds the emission generated by the fracture processes themselves.     

Acoustic emission sensors for OEM applications

The acoustic emission (AE) technique is concerned with the detection of structure-borne ultrasonic activity that is naturally generated during the operation of all structures, machinery and processes. Most examples of its application involve the purchase and use of commercially available instrumentation by end-users. To satisfy these demands AE instrumentation has evolved into various forms for use by researchers, engineers and craftpersons. The increasing credibility of AE as a robust industrial measurement technique is creating opportunities for AE sensing to be built into machinery by original equipment manufacturers. In this paper, the evolution of AE technology towards an OEM technique is described and illustrated with examples of some of the information AE can provide in the industrial environment     

Acoustic emission from subsonic turbulent jets

A method is proposed for determining the noise intensity of subsonic jets. The results are given from a calculation of the far-field intensity level of a jet for various Mach numbers at the nozzle orifice. The results of the calculation are compared with the experimental data.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 4, pp. 711–716, April, 1971.     

Acoustic emission from low-cycle high-stress-intensity fatigue

This paper presents the findings of an Advanced Research Projects Agency (ARPA) study, the overall objective of which was to develop a nondestructive testing technique to determine flaw criticality based on acoustic emission. The research included an evaluation of sensors and instrumentation systems, using several materials and material conditions loaded in low-cycle, high-stress-intensity fatigue.The materials used for the study were D6aC tempered at 600 and 1100°F, annealed and solution-treatedand-aged 6A1-4V titanium and 7075-T6 aluminum. The test specimen was the precracked, single-edgenotch tension specimen; macrocracking was detected by crack-opening-displacement (COD) gage and micro-cracking by acoustic emission. The acoustic-emission system utilized 400 and 1000 kHz band-pass filtering at 100db gain. The output signals of the totalizer and the COD gage were recorded on a single strip chart using a dual-pen recorder. The specimens were subjected to low-cycle, high-stress-intensity fatigue at 6 c/min. In some tests, cycling was begun in air and finished in water.Acoustic emission was demonstrated to be highly effective as a non-destructive test method for following crack growth in low-cycle high-stress-intensity fatigue; acoustic emission confirmed the existence of periods of dormancy punctuated by periods of active fatigue crack growth. Using a dual-pen, strip-chart recorder displaying both crack-opening-displacement and stress-wave count on the same chart, it was a simple matter not only to observe if there was crack growth in each individual cycle but also where in the cycle it occurred. Moreover, the process of stress-corrosion cracking during low-cycle, high-stress-intensity fatigue was readily detected by a marked increase in the stress-wave count rate.The utility of acoustic emission as a precursor of imminant failure was demonstrated for low-cycle, high-stress-intensity fatigue as well as for the case of environmentally assisted fatigue. Plots of cumulative stress-wave count vs cycle number consistently showed a marked increase in count rate several (10–20 or more) cycles before fracture.     

Acoustic emission mechanisms during high-cycle fatigue

Acoustic emission was obtained during fatigue crack propagation in a D6 tool steel and a 1015 mild steel using two different load ratios, R. It was found that the slope of the acoustic emission count rate vs stress intensity factor was higher when R = 0.2 than when R = 0.4. These results are described in terms of several models of acoustic emission mechanisms available in the literature as well as by a proposed new model. It is concluded that the observed R dependence can be explained by the combination of the contribution of two different models. These two models are the new plastic yielding as well as the crack tip fracture processes.     

Acoustic emission testing the F-111

Significant portions of the F-111 aircraft were fabricated of D6AC steel, which is now known to have a fairly small critical crack size. To prove structural flight-worthiness the Air Force built a chamber at McClellan AFB, where all F-111 aircraft are periodically chilled to 40°C and stressed to 7.3g and3.0g. Recently the chamber was modernized, and Physical Acoustics Corporation was selected to supply an acoustic emission system to locate any sources of structural failure. The new F-111 Cold Proof Test Station Acoustic Emission Monitoring System has several innovative features, including a colour CRT which displays the exact location of AE events in real time on simultaneous overhead and side views of the F-111. The events are coloured green, yellow and red according to their severity, as calculated from their amplitude and energy. A monochromatic CRT is used concurrently with the colour CRT to display severity information on AE events which only arrive at one sensor. Alarms also audibly alert the operator to crucial events, using two tones to distinguish the degree of severity.