In the Beginning...
A brief history of the technology of nondestructive testing
Nondestructive testing (NDT) features a broad, diverse, and interesting history. Each of the many examination methods had a unique beginning. There were many pioneers who discovered individual methods, and many more who developed and improved those methods out of necessity and the quest to find more — and smaller — discontinuities.
Fig. 1 — The Sultana, a steamship whose boiler exploded in 1865.
So exactly when did NDT begin? Many believe that the first, and probably the most important, NDT was performed as described in the biblical account found in the book of Genesis. In describing the process of creation, Chapter 1 repeats the expression "and God saw that it was good" six times. That first nondestructive test is known today as visual testing (VT). Yet it took until 1988 for VT to be recognized as an "official" NDT method when it was finally incorporated into SNT-TC-1A, the document for the qualification and certification of NDT personnel published by the American Society for Nondestructive Testing (ASNT).
Fig. 2 — An early rigid borescope.
While no precise dates are recorded as to when NDT began as a recognized technology, human beings have performed nondestructive tests from their beginning. Centuries before the expression NDT was first used, people "looked" at objects to determine size, shape, and even the presence of visual surface imperfections, and "listened" to metal being shaped by a blacksmith or the tone of a bell after it was cast.
Fig. 3 — A low-energy X-ray unit.
The late Dr. Robert McMaster, considered by some to be the grandfather of NDT, described the human body as "the most unique NDT instrument." Our sense of sight not only enables us to perform VT, but it is essential for conducting most other NDT methods. Our sense of hearing enables us to make decisions based solely on the nature of sound, for example, tap testing of aerospace components. We can determine the condition of different foods through our sense of smell before we eat it. Our sense of touch can indicate the surface condition of various objects as well as their relative temperatures (hot or cold), and we can quickly tell the flavor of food products through our sense of taste (although some would not consider that a nondestructive test).
Fig. 4 — A submarine section being X rayed during World War II.
Fig. 5 — The fish-pole technique for radiography using a radium source.
Before World War II, NDT efforts were fragmented. Visual testing was generally unorganized, rarely documented, and in many cases, simply a "go/no-go" effort. It is said that flour and oil were used in Roman times to find cracks in marble slabs. The "oil and whiting" test, the forerunner to penetrant testing (PT), was used on a very limited basis in the late 1800s. The first reference to magnetic particle testing (MT) appeared in 1868 when S. H. Saxby observed how magnetized gun barrels affected a compass. And even though X rays were discovered by Wilhelm Conrad Röntgen back in 1895, their practical uses for NDT were not realized until the development of higher-energy equipment in the 1920s and 1930s. The first practical uses of radium for gamma radiography weren't demonstrated until the 1930s by Dr. Robert F. Mehl, which was more than 30 years after the discovery of radium by Marie and Pierre Curie. In 1929, Sergei Y. Sokolov, a Russian, created high-frequency vibrations in materials using a quartz crystal. In the United States, the propagation of high-frequency sound waves in materials is attributed to the work of Dr. Floyd Firestone in the 1940s. Donald Sproule conducted parallel work in the United Kingdom. Michael Farraday observed and recorded electromagnetic induction, the basic principle of eddy current testing (ET), in 1831, but it wasn't until the mid-1900s when ET instrumentation was developed that its full potential began to be realized. In 1800, Sir William Herschel recorded the earliest observations of thermal infrared testing (TIR) and, in the 1950s, J. Kaiser introduced acoustic emission (AE) as an NDT method. However, the principles of acoustical energy propagation in materials had been known for decades.
These and the many other NDT methods have interesting beginnings. The individuals responsible for discovering these methods and those involved with their development should be remembered. Most of these methods required years of dedication and hard work. Many of those early NDT pioneers did not benefit directly from their efforts and, in many cases, never fully knew the results of their contributions. The NDT community owes them much.The Need for Inspections
Fig. 6 — An early MT wet horizontal unit.
History does not provide a specific starting date for NDT. However, accidents and failures, which have always afflicted mankind, grew rapidly during the Industrial Revolution that began in Great Britain in the 18th century and spread to America in the latter part of that century.
technological advancements during the Industrial Revolution involved
the use of steam power. Also, new fuels such as coal and petroleum were
incorporated into these new steam engines. The significant increase in
the production and use of boilers led to a greater number of failures
and accidents. This was a period of little or no NDT, and it was also a
time when thermodynamic principles were not fully understood. With the
increase in the number of boilers put into service, there was a
corresponding increase in boiler explosions. For example, in five years
alone, from 1898 to 1902, there were 1600 explosions nationwide.
Following are some of the more noteworthy catastrophic boiler failures
that resulted in significant injuries and deaths:
The Turning Point
Fig. 7 — The ‘Supersonic’ UT unit, which was one of the first.
Many consider the World War II period as the most significant turning point in the history and development of NDT. Visual testing remained the primary NDT method, although at that time, it was not recognized in the same context as the other methods. The major VT "instrument" was the eyeball. Of course, other aids were used such as mirrors, various measuring devices, and basic rigid borescopes — Fig. 2. Radiography had been primarily used in the medical field and used on a limited basis for NDT due to the material thickness restrictions with the low-energy equipment available at the time — Fig. 3. With the development of higher-energy X-ray tubes and equipment through the 1930s, many of the structures and components used during the war could be X rayed, such as the submarine section illustrated in Fig. 4. Gamma radiography was still being done with radium sources using the "fish pole" technique as depicted in Fig. 5.
Fig. 8 — An early production line eddy current instrument.
The "oil and whiting" technique had now been refined and fluorescent penetrants were used widely for the inspection of war-related products. The contrast (visible daylight) penetrant was developed during the 1940s and was used primarily for field applications where fluorescent penetrants were difficult or inconvenient to use due to the need for a darkened area. Magnetic particle tests could now be performed with a higher degree of reliability using the wet horizontal equipment (Fig. 6) developed during the 1930s, and the refined colored iron oxide particles and powders considerably improved sensitivity.
The use of ultrasonics for materials testing during the war saw limited use because the equipment was still in the development stage. The first pulse echo equipment wasn't devised until around 1942. Prior to then, the "through transmission" technique had been used but on a limited basis due to the limitations inherent to that technique. An example of an early pulse echo instrument is shown in Fig. 7.
Practical eddy current equipment and techniques were not realized until after the war in the late 1940s — Fig. 8. During WW II, thermal infrared technology began to expand and resulted in a number of unique and useful military applications and developments.
The Years Following WW II
Fig. 9 — A fiber-optic borescope.
The benefits of nondestructive testing became apparent during WW II. It began to be recognized as an independent technology, partly through the founding of The American Industrial Radium and X-ray Society in 1941, known today as ASNT. While the original focus of the organization was radiography, the other frequently used methods of the day were included.
The uses and applications of visual aids such as mirrors, telescopes, and rigid borescopes (at times referred to as endoscopes), as well as other measuring devices, expanded into other industrial fields. The limitation of straight-line access with the rigid borescopes was overcome in the 1950s with the introduction of glass fiber bundles (Fig. 9) and fiber-optic image transmission, which led to the development of the flexible fiberscope. Further developments included the addition of cameras and closed-circuit TV systems that permitted areas being examined to be displayed on a monitor. Ultimately, smaller, solid-state imaging sensors known as charge-coupled devices (CCDs) would be developed. The new generation of CCDs provides for extremely high-resolution imaging and smaller sensors, and allows the digital recording and storage of the images.
Fig. 10 — The first fluorescent penetrant system.
Water-washable penetrants and "wet" developers originated during WW II and were refined well into the 1950s. Post-emulsified penetrants, which provided for controlled excess surface penetrant removal, were introduced in 1953. Penetrant systems were created for faster processing of a quantity of parts — Fig. 10. Visible daylight dye penetrants and spirit developers were also being refined to provide higher "see ability" for portable and field applications. In the late 1950s and into the 1960s, the various penetrants and techniques were being classified and sensitivity levels established.
The detection of certain weld discontinuities such as surface cracks was one of the major limitations of RT. This was overcome by the use of MT, which was typically used in conjunction with RT. The benefits of MT were realized during the war and, in the following years, significant developments and refinements contributed to its increased use and expanded applications. The improvements in particles and the introduction of portable equipment such as AC yokes and DC prod sets further increased the versatility of MT, especially in the field — Fig. 11.
Fig. 11 — Magnetic particle testing being performed with an early DC prod unit.
Radiographic testing saw significant innovation after the war. The development of resonant transformers in the 1940s permitted the use of increased X-ray energies ranging from 250 kV to 4 MeV — Fig. 12. Radiographic testing could now be used for the examination of thicker materials — up to 8 in. of steel. Further developments followed with the betatrons, linear accelerators, and Van de Graff generators. Sources or "artificial isotopes" such as Iridium 192 and Cobalt 60 were made possible with the advent of nuclear reactors. Along with this equipment evolution, significant improvement in radiographic film and accessories led to unique technique refinement, making RT the leading volumetric examination method, a distinction it still holds today.
Fig. 12 — Early industrial X-ray unit with high-energy tube.
Fig. 13 — The first “portable” ultrasonic instrument.
The use of UT during WW II was primarily limited to compressional wave (straight beam) techniques — Fig. 13. In 1947, D. O. Sproule developed a transducer design that would generate shear waves in materials, which opened the door to many additional applications for weld inspection and other material forms. Through the 1950s, there was significant development of UT techniques, including the use of "tip diffraction," that provided for more precise location and sizing of the reflector. This period also saw the improvement and growth of immersion testing (Fig. 14) as well as the manufacture of smaller, portable equipment and transducers with crystals other than quartz. In the 1960s, the first battery-operated instruments were introduced (Fig. 15) and immediately gained wide acceptance because of their more efficient use in the field. Innovations continued with the introduction of the time-of-flight-diffraction technique (TOFD) in 1977, which could display the edges of a discontinuity, thereby providing greater dimensional accuracy. Shortly thereafter, microchips were incorporated into UT instrumentation permitting the user to store calibration data, examination results, and other signal information. Digital technology and the use of LCD panels have further enhanced this technology.
The use of UT for thickness measurement has been employed since the advent of the pulse-echo techniques. Early equipment also used the principles of resonance testing for thickness measurements. While somewhat accurate readings could be obtained from the conventional A-scan display, UT digital thickness gauges made the process much more rapid and precise.
Fig. 14 — Early UT immersion test system.
The growth of eddy current testing was slow until the late 1940s when Dr. Friedreich Foerster founded his institute for the development of practical ET equipment. By the late 1960s, the Institute had developed equipment for a wide range of applications and expanded its market to the United States. In 1974, Intercontrolle of France developed equipment that permitted the test coil to be driven at multiple frequencies — Fig. 16. In addition to suppressing the display of undesired test variables, it could also optimize performance variables and assist in identifying the characteristics of a particular test response. Since the mid-1980s, microprocessor-based ET instruments have enhanced the potential of this method by making it more user friendly. Newer developments include remote field ET, flux leakage, and modulation analysis inspection.
Fig. 15 — One of the first battery-operated UT instruments.
Thermal infrared testing (TIR) finally began to be used for nonmilitary applications in the 1960s. At that time, the systems were cumbersome, slow, and exhibited poor resolution. The first portable system for TIR nondestructive testing, which used a cooled scanner detector, was produced in the 1970s — Fig. 17. This led to wide acceptance by the end of the decade and new developments include focal plane array and pulsed TIR.
Acoustic emission testing continues its growth, especially after the development of selective filtering instrumentation, which permitted the differentiation of background noise from actual events resulting from discontinuity growth. The success of this technology is due in part to the ongoing investigation and understanding of the micromechanical processes that produce emissions from various materials.
Fig. 16 — A multifrequency ET system.
As new materials have been developed and products manufactured that demanded higher quality levels, along with the imposition of stricter standards, the challenges to NDT have been great. The NDT equipment available today should send the message that "we've come a long way." Those involved with NDT today should appreciate the significant improvements to this technology made possible through the innovative equipment at our disposal. Compare today's equipment with that of the early days and you can't help but realize how much better off we are today.
Those early practitioners had to deal with significant challenges, including a general absence of standards, archaic equipment with limited reliability, little or no formalized training programs, and an overall lack of acceptance of NDT as a technology. To many in those early days, NDT was considered a necessary evil. While NDT has grown to become a recognized and sophisticated technology, some concerns remain. Nondestructive testing is still generally unknown or misunderstood by many. The many different personnel qualification and certification programs can be confusing and complicated. It will be interesting to see if this next period in the history of NDT will be considered the dark ages or a period of enlightenment.
Fig. 17 — Early TIR camera using a cooled scanner/detector.
CHARLES J. HELLIER, P.E., (email@example.com) is a Consultant in Old Lyme, Conn. Prior to his retirement, he was founder and president of Hellier, a Rockwood company. He is an ASNT Level III in VT, PT, MT, UT, and RT, and is a Past National President of ASNT and a Past President of the Nondestructive Testing Management Association.