Often hailed as the father of ultrasonic testing, Sergei Y. Sokolov, famed Soviet scientist at the V.I. Ulyanov (Lenin) Electrotechnical Institute, Leningrad was born in 1897 in the same Russian city. He proposed in 1928, and a few years later demonstrated a through-transmission technique for flaw detection in metals. He advanced his idea in the late 1920s, at a time when the required technology did not exist. He proposed that such technique could be used to detect irreguarities in solids such as metals.

As with much of the early exploitation of ultrasonics, extensive work on flaw detection in metals took place in Germany in the early 1930s. In 1933, O Muhlhauser patented a system for transmitting ultrasonic energy into the work piece to be tested, detecting it as it emerged by means of a second transducer. In 1935 Sokolov published details of the experimental design of quartz generators, methods of coupling the generators to the test piece in order to achieve maximum energy transfer, and also various methods of detecting the ultrasonic energy after transmission through the test piece. He proposed the first 'ultrasonic camera' in which an actual image of a flaw could be obtained. He envisaged that by using sound waves with a frequency of 3,000 megahertz (MHz), a resolution equal to that of the optical microscope could be obtained. His method used a large piezoelectric receiving transducer upon which the charges generated by the arriving acoustic signals remained isolated. The face of transducer is then scanned by a mechanical contact or beam, building up a 'picture' of the flaw. The resolution of the experimental devices which he fabricated was however not good enough to be used at a practical level.

In Germany, by 1936, Raimar Pohlman had developed an ultrasonic imaging method based on transmission using acoustical lenses and conversion of the resulting acoustical image into a visually observable image in the same volume of interest. In Germany, his image converting apparatus becomes known as the "Pohlman cell". Used extensively near the end of world war 2 in Germany when ultrasonic material testing became essential in the selection of ammunitions for the flak defense of Berlin. Pohlman had been experimenting along similar lines with the propagation of ultrasound through metals containing defects, but had employed an entirely different approach to the method of detection of the emerging beam. His detector utilised a suspension of colloidal aluminium flakes in xylene sealed in a transparent container. The ultrasonic beam emerging from the test piece acted on the aluminium flakes causing them to align themselves normally to the direction of the beam. In the case of any defects being present in material under test, shadow areas would be cast where flakes maintained their original orientation.

In 1939, investigators at the British Iron and Steel Institute decided to investigate the possibility of detecting hairline cracks in steel using ultrasonic wave carried out using a Pohlman type apparatus, but it was found to be insufficiently sensitive. In 1942 Donald Sproule successfully adapted the principle of echo-sounding to the detection of defects in steel. His apparatus used separate transmitting and receiving transducers. Almost simultaneously but independently an American scientist Floyd Firestone devised a 'reflectoscope', an apparatus working on similar principles, but utilising a single transducer for transmission and reception].

In the reflection technique, a pulsed sound wave is transmitted from one side of the sample, reflected off the far side, and returned to a receiver located at the starting point. Upon impinging on a flaw or crack in the material, the signal is reflected and its traveling time altered. The actual delay becomes a measure of the flaw's location; a map of the material can be generated to illustrate the location and geometry of the flaws. In the through-transmission method, the transmitter and receiver are located on opposite sides of the material; interruptions in the passage of sound waves are used to locate and measure flaws. Usually a water medium is employed in which transmitter, sample, and receiver are immersed. Similar technology had also been investigated in Germany and Japan, but developments had been curtailed because of the second world war.

The key-persons, Floyd Firestone, Donald Sproule and Adolf Trost had no knowledge of each other as they worked strictly in secret. Not even their patent-applications were published. Sproule and Trost used transmission-technique with seperate transmitter- and receiver-probes. Trost invented the so-called "Trost-Tonge". The 2 probes were contacted on opposite sides of a plate, held in same axis by a mechanical device - the tonge - and coupled to both surfaces by continuously flowing water. Sproule placed the 2 probes on the same side of the workpiece. So he invented double-crystal probes. He used this combination also with variing distance from each other. Firestone was the first to realize the reflection-technique. He modified a radar instrument and developed a transmitter with short pulses and an amplifier with short dead-zone. Sproule eventually gave up the transmission method and filed a patent in 1952 entitled "the improvements in/ or relating to apparatus for flaw detection and velocity measurement by ultrasonic echo methods".

Firestone had made the following presentations and/or registered the following patents between 1940 and 1951:

  • 1940 Flaw detecting device and measuring instrument
  • 1943 Resonance inspection method; Surface and shear wave method and apparatus
  • 1945 Supersonic reflectoscope, an instrument for inspecting the interior of solid parts by means of sound waves
  • 1946 Refinements in supersonic reflectscope, Polarized sound
  • 1948 Tricks with supersonic reflectscope
  • 1951 Method and means for generating and utilizing vibrational wave in plate

Sproule and Firestone found industrial partners for their instruments: Kelvin-Hughes and Sperry Inc. Kelvin-Hughes produced their first commercial machine in 1952. Several years later, in Germany in 1949 two persons received information about the Firestone-Sperry-Reflectoscope by publications in technical papers: Josef Krautkrämer in Cologne and Karl Deutsch in Wuppertal. Both started developments - without knowledge of each other. Josef Krautkrämer and his brother Herbert were physicists, working in the field of oscilloscopes. They could develop ultrasonic instruments alone. Karl Deutsch, a mechanical engineer needed a partner for the electronics and found him with Hans-Werner Branscheid who had got some technical experience in radar-technique during the war. Within only one year both young and tiny companies could present their Ultrasonic testing-flaw-detectors, starting a competition still existing today.

Early pipe testing with Krautkrämer apparatus (1950s)

Later on more ultrasonic testing units came on the international markets: Siemens and Lehfeldt in Germany, Kretztechnik in Austria, Ultrasonique in France and Kelvin-Hughes in Britain. They all stopped their production before the 1970s. Kelvin-Hughes also stopped at the same time, Sperry was later renamed Automation Industries Inc.

Krautkrämer became world-wide market-leader in the early 60-ies and has kept this position until today. Besides Karl Deutsch new names came up: Nukem in Germany, Panametrics and Stavely (after Sonic and Harisonic) in USA, Sonatest and Sonomatic in Briatain, Gilardoni in Italy.

Four German Flaw detectors in the 1950s.
upper left: Krautkramer, right: Siemens.    lower left: Lehfeldt, right: Karl Deutsch.

Research into ultrasonics and metal-flaw detection in Japan was considerably curtailed when World war II broke out in 1941, at which time the Americans and the Germans were both diligently researching into ultrasonics and the development of the Radar. The study into Radar techniques in Japan was also in the disadvantage. As the war ended in 1945, research into high-power electronics was prohibited in Japan for some time (up till 1948, when developments of non-military electronics resumed).

Japanese enterprises took on the research from the U.S. and England and soon developed its own flaw detectors in non-destructive testing. In about 1949, four Japanese companies started to manufacture their own flaw dectors. These were: the Mitsutbishi Electric Corporation, the Japan Radio Company ( later became the Aloka Company), the Shimadsu Manufacturing Company and the Toyko Ultrasonic Industrial Company. Only Mitsubishi continued to expand in the field of non-destructive testing and the other companies moved on to other areas and in particular diagnostic medical ultrasound applications. The Japanese Society for Non-Destructive Inspection officially recognised the year 1952 as the first year non-destructive testing was implemented in Japan.

Information courtesy of NDT.net and other sources.

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