ACOUSTIC EMISSIONEarly Uses of Acoustic Emission:-

The first acoustic emission used by an artisan may well have been in making pottery (the oldest variety of hardfired pottery dates back to 6,500 BC) . In order to assess the quality of their products, potters t raditionally relied on the audible cracking sounds of clay vessels cooling in the kiln. These acoustic emissions were accurate indications that the ceramics were defective and did indeed structurally fail

Acoustic Emission in Metalworking:-(Tin Cry)

It is reasonable to assume that the first observation of acoustic emission in metals was tin cry, the audible emission produced by mechanical twinning of pure tin during plastic deformation. This phenomenon could occur only after man learned to smelt pure tin, since tin is found in nature only in the oxide form. It has been established that smelting (of copper) began in Asia Minor as early as 3,700 BC. · The deliberate use of arsenic and then tin as alloying additions to copper heralde1 the beginning of the Bronze Age somewhere between the fourth and third millennium BC. The oldest piece of pure 'tin

found to date is a bangle excavated at Thermi in Lesbos. The tin has been dated between 2,650 and 2,550 BC. It is 41 mm (1.6 in.) in diameter and . consists of two strands of pure tin , on wrapped around the other and hammered flat at the end. During the manufacture af this bangle, the craftsman could have heard considerable tin cryEarly Documented Observations of Tin Cry:-The first documented observations of acoustic emission may have been made by the eighth century Arabian alchemist Jabir ibn Hayyan (also known as Geber). His book Summa Perfectionis Magisterii (The Sum of Perfection or The Sum of Perfect Magistery) was published in English translation in 1678; the Latin edition was published in Berne in 1545. In it he writes that Jupiter (tin) gives off a "harsh sound" or "crashing noise." He also describes Mars (iron) as "sounding much" during forging. This sounding of iron was most likely produced by the fonnation of martensite during cooling….Since the time of the alchemists, audible emissions have become known and recognized properties of cadmium and zinc as well as tin. Tin cry is commonly found in books on chemistry published in the last half of the nineteenth century. For example, Worthington Hooker in 1882 describes "the cry of tin" as owing to the friction on minute crystals of the metal against each other."

DETECTING AND RECORDING ACOUSTIC EMISSION:-

The transition from the incidental obsezvation of audible tin cry to the deliberate study of acoustic emission phenomena consisted of three separate and unrelated experiments in which instrumentation was used to detect, amplify and record acoustic emission events occurring in the test specimens. The first experiment instrumented specifically to detect acoustic emission was conducted in Germany and the results were published in 1936 by Friedrich Forster and Erich Scheil. They recorded the "Gerausche" (noises) caused by the formation of martensite in 29 percent nickel steel

In the United States, Warren P. Mason, H.J. McSkimin and W. Shockley performed and published the second instrumented acoustic emission experiment in 1948. At the suggestion of Shockley, experiments were directed toward obsezvation of moving dislocations in pure tin specime by means of the stress waves they generated. The

experiment's instrumentation was capable of measuring displacements of about 10-7

mm occurring in times of 10-6

seconds. The third instrumented experiment was performed in England by D.J. Millard in 1950 during research for his Ph.D. thesis at the University of Bristol. He conducted twinning experiments on single crystal wires of cadmium. Twinning was detected using a Rochelle salt transducer

Kaiser's Study of Acoustic Emission Sources:-

The early obsezvations of audible sounds and the three instrumented experiments were not directed at a study of the acoustic emission phenomenon itself, nor did the researchers carry on any further investigations in acoustic emission. The genesis of today's technology in acoustic e mission was the work of Joseph Kaiser at the Technische Hochschule Miinchen in Germany.

In 1950 Kaiser published his Doktor-Ingenieur dissertation where he reported the first comprehensive investigation into the phenomena of acoustic emission. Kaiser used tensile tests of conventional engineering materials to determine:

(1) what noises are generated from within the specimen;

(2) the acoustic processes involved;

(3) the frequency levels found; and

(4) the relation between the stress-strain curve and the frequencies noted for the various stresses to which the specimens we re subjected

His most significant discovery was the irreversibility phenomenon which now bears his name, the Kaiser effect. He also proposed a distinction between burst and continuous emission. Kaiser concluded that the occurrence of acoustic emission arises from frictional rubbing of grains against each other in the polycrystalline materials he tested and also from intergranular fracture.

Kaiser continued his research at the Institut fiir Metallurgie und Metallkunde der Technischen Hochschule Mlinchen until his death in March 1958. His work provided the momentum for continued activities at the Institut by several of his coworkers, including Heinz Borchers and Hans Maria Tensi, and also

furnished the impetusfor further research elsewhere in the world.

What is AEAcoustic emission is the technical term for the noise emitted by materials and structures when they are subjected to stress. Types of stresses can be (1) mechanical, (2) thermal or (3) chemical. This emission is caused by the rapid release of energy within a material due to events such as crack initiation and growth, crack opening and closure, dislocation movement, twinning, and phase transformation in monolithic materials and fiber breakage and fiber- matrix debonding in composites The subsequent extension occurring under an applied stress generates transient elastic waves which propagate through the solid to the surface where they can be detected by one or more sensors. The sensor is a transducer that converts the mechanical wave into an electrical signal (piezoelectric) . In this way information about the existence and location (triangulation by multi-transducers) of possible sources is obtained. Acoustic emission may be described as the "sound" emanating from regions of localized deformation within a material Until about 1973, acoustic emission technology was primarily employed in the non-destructive testing of such structures as pipelines, heat exchangers, storage tanks, pressure vessels, and coolant circuits of nuclear reactor plants. However, this technique was soon applied to the detection of defects in rotating equipment bearings

Acoustic Emission:-

Acoustic Emission (AE) refers to generation of transient elastic waves during rapid release of energy from localized sources within a material The source of these emissions in metals is closely associated with the dislocation movement accompanying plastic deformation and with the initiation and extension of cracks in a structure under stress. Other sources of AE are: melting, phase transformation, thermal stresses, cool down cracking and stress build up, twinning, fiber breakage and fiber- matrix debonding in composites.

AE TECHNIQUE:-

The AE technique (AET) is based on the detection and conversion of high frequency elastic waves emanating from the source to electrical signals. This is accomplished by directly coupling piezoelectric transducers on the surface of the structure under test and loading the structure. The output of the piezoelectric sensors (during stimulus) is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and further processed by suitable electronics. AET can non-destructively predict early failure of structures. Further, a whole structure can be monitored

from a few locations and while the structure is in operation. AET is widely used in industries for detection of faults or leakage in pressure vessels, tanks, and piping systems and also for on-line monitoring welding and corrosion.

Difference between AET and other non-destructive testing (NDT):-

The difference between AET and other non-destructive testing (NDT) techniques is that AET detects activities inside materials, while other techniques attempt to examine the internal structures of materials by sending and receiving some form of energy

Types of AET:-

Acoustic emissions are broadly classified into two major types namely:-

continuous type (associated with lattice dislocation)

burst type. (twinning, micro yielding, development of crack

The waveform of continuous type AE signal is similar to Gaussian random noise, but the amplitude varies with acoustic emission activity. In metals and alloys, this form of emission is considered to be associated with the motion of dislocations. Burst type emissions are short duration pulses and are associated with discrete release of high amplitude strain energy. In metals, the burst type emissions are generated by twinning, micro yielding, development of cracks.

What is Normal (Gaussian) distribution:-

In probability theory, the normal (or Gaussian) distribution is a very common continuous probability distribution. Normal distributions are important in statistics and are often used in the natural and social sciences to represent real-valued random variables whose distributions are not known he normal distribution is remarkably useful because of the central limit theorem. In its most general form, under mild conditions, it states that averages of random variables independently drawn from independent distributions are normally distributed. Physical quantities that are expected to be the sum of many independent processes (such as measurement errors) often have distributions that are nearly normal.[3] Moreover, many results and methods (such as propagation of uncertainty and least squares parameter fitting) can be derived analytically in explicit form when the relevant variables are normally distributed

The normal distribution is sometimes informally called the bell curve. However, many other distributions are bell-shaped (such as Cauchy's, Student's, and logistic). The terms Gaussian function and Gaussian bell curve are also ambiguous because they sometimes refer to multiples of the normal distribution that cannot be directly interpreted in terms of probabilities.