Handbook of nondestructive testing of concrete:Acoustic Emission Methods.pdf
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1、16-1 0-8493-1485-2/04/$0.00+$1.50 2004 by CRC Press LLC 16 Acoustic Emission Methods 16.1Introduction 16-1 16.2Historical Background 16-2 16.3Theoretical Considerations. 16-3 16.4Evaluation of Acoustic Emission Signals. 16-4 16.5Instrumentation and Test Procedures 16-6 16.6Parameters Affecting Acous
2、tic Emissions from Concrete . 16-8 The Kaiser Effect Effect of Loading Devices Signal Attenuation Specimen Geometry Type of Aggregate Concrete Strength 16.7Laboratory Studies of Acoustic Emission 16-9 Fracture Mechanics Studies Type of Cracks Fracture Process Zone (Crack Source) Location Strength vs
3、. Acoustic Emission Relationships Drying Shrinkage Fiber Reinforced Cements and Concretes High Alumina Cement Thermal Cracking Bond in Reinforced Concrete Corrosion of Reinforcing Steel in Concrete 16.8Field Studies of Acoustic Emission 16-14 16.9Conclusions . 16-14 Acoustic emission refers to the s
4、ounds, both audible and subaudible, that are generated when a material undergoes irreversible changes, such as those due to cracking. Acoustic emissions (AE) from concrete have been studied for the past 30 years, and can provide useful information on concrete properties. This review deals with the p
5、arameters affecting acoustic emissions from concrete, including discussions of the Kaiser effect, specimen geometry, and concrete properties. There follows an extensive discussion of the use of AE to monitor cracking in concrete, whether due to externally applied loads, drying shrinkage, or thermal
6、stresses. AE studies on reinforced concrete are also described. While AE is very useful laboratory technique for the study of concrete properties, its use in the fi eld remains problematic. 16.1Introduction It is common experience that the failure of a concrete specimen under load is accompanied by
7、a considerable amount of audible noise. In certain circumstances, some audible noise is generated even before ultimate failure occurs. With very simple equipment a microphone placed against the specimen, an amplifi er, and an oscillograph subaudible sounds can be detected at stress levels of perhaps
8、 50% of the ultimate strength; with the sophisticated equipment available today, sound can be detected at much lower loads, in some cases below 10% of the ultimate strength. These sounds, both audible and subaudible, are referred to as acoustic emission. Sidney Mindess University of British Columbia
9、 16-2Handbook on Nondestructive Testing of Concrete: Second Edition In general, acoustic emissions are defi ned as “the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material.”1 These waves propagate through the materi
10、al, and their arrival at the surfaces can be detected by piezoelectric trans- ducers. Acoustic emissions, which occur in most materials, are caused by irreversible changes, such as dislocation movement, twinning, phase transformations, crack initiation, and propagation, debonding between continuous
11、and dispersed phases in composite materials, and so on. In concrete, since the fi rst three of these mechanisms do not occur, acoustic emission is due primarily to: 1. Cracking processes 2. Slip between concrete and steel reinforcement 3. Fracture or debonding of fi bers in fi ber-reinforced concret
12、e 16.2Historical Background The initial published studies of acoustic emission phenomena, in the early 1940s, dealt with the problem of predicting rockbursts in mines; this technique is still very widely used in the fi eld of rock mechanics, in both fi eld and laboratory studies. The fi rst signifi
13、cant investigation of acoustic emission from metals (steel, zinc, aluminum, copper, and lead) was carried out by Kaiser.2 Among many other things, he observed what has since become known as the Kaiser effect: “the absence of detectable acoustic emission at a fi xed sensitivity level, until previousl
14、y applied stress levels are exceeded.”1 While this effect is not present in all materials, it is a very important observation, and it will be referred to again later in this review. The fi rst study of acoustic emission from concrete specimens under stress appears to have been carried out by Rsch,3
15、who noted that during cycles of loading and unloading below about 70 to 85% of the ultimate failure load, acoustic emissions were produced only when the previous maximum load was reached (the Kaiser effect). At about the same time, but independently, LHermite4,5 also measured acoustic emission from
16、concrete, fi nding that a sharp increase in acoustic emission coincided with the point at which Poissons ratio also began to increase (i.e., at the onset of signifi cant matrix cracking in the concrete). In 1965, however, Robinson6 used more sensitive equipment to show that acoustic emission occurre
17、d at much lower load levels than had been reported earlier, and hence, could be used to monitor earlier microcracking (such as that involved in the growth of bond cracks in the interfacial region between cement and aggregate). In 1970, Wells7 built a still more sensitive apparatus, with which he cou
18、ld monitor acoustic emissions in the frequency range from about 2 to 20 kHz. However, he was unable to obtain truly reproducible records for the various specimen types that he tested, probably due to the diffi culties in eliminating external noise from the testing machine. Also in 1970, Green8 repor
19、ted a much more extensive series of tests, recording acoustic emission frequencies up to 100 kHz. Green was the fi rst to show clearly that acoustic emissions from concrete are related to failure processes within the material; using source location techniques, he was also able to determine the locat
20、ions of defects. It was this work that indicated that acoustic emissions could be used as an early warning of failure. Green also noted the Kaiser effect, which suggested to him that acoustic emission techniques could be used to indicate the previous maximum stress to which the concrete had been sub
21、jected. As we will see below, however, a true Kaiser effect appears not to exist for concrete. Nevertheless, even after this pioneering work, progress in applying acoustic emission techniques remains slow. An extensive review by Diederichs et al.9 covers the literature on acoustic emissions from con
22、crete up to 1983. However, as late as 1976, Malhotra10 noted that there was little published data in this area, and that “acoustic emission methods are in their infancy.” Even in January, 1988, a thorough computer-aided search of the literature found only some 90 papers dealing with acoustic emissio
23、ns from concrete over about the previous 10 years; while this is almost certainly not a complete list, it does indicate that there is much work to be carried out before acoustic emission monitoring becomes a common technique for testing concrete. Indeed, there are still no standard test methods whic
24、h have even been suggested for this purpose. Acoustic Emission Methods16-3 16.3Theoretical Considerations When an acoustic emission event occurs at a source with the material, due to inelastic deformation or to cracking, the stress waves travel directly from the source to the receiver as body waves.
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