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1854 Hartford, Connecticut: a boiler at the Fales and Gray Car works explodes, killing 21
people and seriously injuring 50. Within a decade, the State of Connecticut passes a law
requiring annual inspection (in this case visual) of boilers.
1880 - 1920 The "Oil and Whiting" method of crack detection is used in the railroad industry
to find cracks in heavy steel parts. (A part is soaked in thinned oil, then painted with a white
coating that dries to a powder. Oil seeping out from cracks turns the white powder brown,
allowing the cracks to be detected.) This was the precursor to modern liquid penetrant tests.
1895 Wilhelm Conrad Röntgen discovers what are now known as X-rays. In his first paper he
discusses the possibility of flaw detection.
1920 Dr. H. H. Lester begins development of industrial radiography for metals.
1924 — Lester uses radiography to examine castings to be installed in a Boston Edison
Company steam pressure power plant.
1926 The first electromagnetic eddy current instrument is available to measure material
thicknesses.
1929 Magnetic particle methods and equipment pioneered (A.V. DeForest and F.B. Doane)
1930s Robert F. Mehl demonstrates radiographic imaging using gamma radiation from
Radium, which can examine thicker components than the low-energy X-ray machines
available at the time.
1935 - 1940 Liquid penetrant tests developed (Betz, Doane, and DeForest)
1935 - 1940s Eddy current instruments developed (H.C. Knerr, C. Farrow, Theo Zuschlag,
and Fr. F. Foerster).
1940 - 1944 Ultrasonic test method developed in USA by Dr. Floyd Firestone.
1950 The Schmidt Hammer (also known as "Swiss Hammer") is invented. The instrument
uses the world’s first patented non-destructive testing method for concrete.
1950 J. Kaiser introduces acoustic emission as an NDT method.
Acoustic emission testing (AE or AT)
The application of acoustic emission to non-destructive testing of materials in the
ultrasonic regime, typically takes place between 100 kHz and 1 MHz. Unlike conventional
ultrasonic testing, AE tools are designed for monitoring acoustic emissions produced within
the material during failure or stress, rather than actively transmitting waves, then collecting
them after they have travelled through the material. Part failure can be documented during
unattended monitoring. The monitoring of the level of AE activity during multiple load cycles
forms the basis for many AE safety inspection methods, that allow the parts undergoing
inspection to remain in service.
The technique is used, for example, to study the formation of cracks during the welding
process, as opposed to locating them after the weld has been formed with the more familiar
ultrasonic testing technique. In a material under active stress, such as some components of an
airplane during flight, transducers mounted in an area can detect the formation of a crack at
the moment it begins propagating. A group of transducers can be used to record signals, and
then locate the precise area of their origin by measuring the time for the sound to reach
different transducers. The technique is also valuable for detecting cracks forming in pressure
vessels and pipelines transporting liquids under high pressures. Also, this technique is used
for estimation of corrosion in reinforced concrete structures.
In addition to non-destructive testing, acoustic emission monitoring has applications in
process monitoring. Applications where acoustic emission monitoring has successfully been
used include detecting anomalies in fluidized beds, and end points in batch granulation.
Electromagnetic Testing (ET), as a form of non-destructive testing, is the process of
inducing electric currents or magnetic fields or both inside a test object and observing the
electromagnetic response. If the test is set up properly, a defect inside the test object creates a
measurable response.

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