Lasing Toward a Mainstream Nondestructive Testing Technique
Nondestructive Testing (NDT) based on laser technology has witnessed incremental acceptance in the NDT industry since the early 1990’s with shearography and holography interferometry as the most widely recognized techniques based on this technology. Over the past decade, integration of lasers with ultrasonic testing has seen commercial acceptance in a number of different applications. Laser profilometry is another technique that has experienced growing adoption as a tool for surface mapping and when coupled with advanced software capabilities, this technique can be used for external surface corrosion assessment. These laser-based NDT techniques are the ones that have gained visibility in the industry with a niche market developing for laser visual inspection. Integration of lasers with cameras to offer a faster, more accurate technique for visual inspection capable of providing an image of the surface under test has created an important niche in the visual inspection market. However, considering the conservative nature of end users of NDT and the slow rate of change of technology in the NDT industry, realistically laser NDT, depending upon the technique, still has 5-10 years to go before it is widely accepted.
Laser Shearography and Holography
Although shearography and holography are spoken of in the same breath by many experts, there are subtle differences between the two techniques with holography in use for NDT applications earlier than shearography. Shearography gives the first derivative of the out of plane displacement, whereas holography measures absolute out of plane displacement of an object in response to an applied load.
“Holography serves as a displacement transducer, since it gives a direct measurement on displacement, whereas shearography serves as a strain gauge as it gives a direct measurement of the displacement gradient,” explained Robert Sammaritano, NDT shearography specialist at Dantec Dynamics Inc.
Holography—Not in demand due to technical limitations
Holography as an NDT tool was introduced by Dr. Karl Stetson through his invention of holographic interferometry. Although holography had limited applications in the 1960’s and 1970’s, the technique grew in importance with film-based tire testing in the 1970’s. However, since the development of electronic speckle interferometry (ESPI), film-based systems have been replaced by electronic systems with the recent use of mega-pixel charge-coupled device (CCD) cameras significantly improving systems speed and reliability. Having experienced incremental acceptance in its early years with Pratt & Whitney introducing an ultrasonic holography system for disbond detection in 1982, holography was tipped for greater things. However, this technique has a key limitation that restricts its application base; holography is sensitive to external vibrations introduced in field or production applications. Hence, holography needs to be carried out on vibration free or vibration isolation tables. As a result, holography does not find many applications in the production or field applications and the technology is mainly being used in lab environments. That being said, Norcom Systems Inc, a sister company of Laser Technology Inc, has been able to develop a production application for holography in the electronics industry. The company’s system for testing hermiticity in critical electronics components packaging has gained significant popularity.
“This optical leak testing system uses a type of holography called superhetrodyne holography, which coupled with advanced software is extremely sensitive to leak grade in electronics packaging,” said John Newman, President at both Laser Technology Inc and Norcom Systems Inc. “Our systems are made on vibration isolation tables, but for factory environments. So, we overcome the key limitation of holography to provide a true production environment system that works exceptionally well to view tiny deformations.”
Although holography was introduced in the 1960’s and early 1970’s, its technological limitations have not allowed the technique to gain large scale market adoption.
Shearography—A mature technology, yet an emerging market
Research in shearography NDT was pioneered by Dr. Michael Hung in the late 1970’s and early 1980’s. He developed a film based shearography system for tire testing, but did not gain large scale acceptance for this system. In early 1980’s, Dr. Hung invented electronic shearography and licensed this technology to Laser Technology Inc, that in turn introduced the first commercial product for electronic laser shearography in 1986. The company’s first customer was Northrup Grumman who used this technology on the B2 stealth bomber program. Since then, the technology has evolved significantly and has gained incremental acceptance in the industry.
Technologically, shearography has witnessed a few landmark developments throughout the years. The electronic shearography invention in the early 1980’s kick started the shearography evolution. This was carried forward in the early 1990’s with the integration of Michelson interferometery in the systems. The late 1990’s saw the development of real-time phase mapping or phase stepping, which allowed a constant picture of different details transpiring through the whole course of a test to be visible, which is considered by many industry experts as the last major technological breakthrough of shearography. Recently, over the past few years, key developments have revolved around refining the functionality of the product through software advancement and feature-rich offerings capable of addressing key end user needs and, as with holography, integration of CCD cameras significantly improved the speed and reliability of the system.
The major applications for shearography are in the aerospace industry and the tire industry with a few applications developing in the wind energy and automotive industry. Since the first system for shearography was sold to Northrup Grumman, the aerospace industry and more specifically, the military and defense programs adopted shearography as a mainstream inspection tool for testing disbonds, voids and delaminations. Since, the technology was expensive in its early days, the commercial aircraft industry was apprehensive to adopt shearography. The 1990’s witnessed acceptance from the spacecraft industry with NASA using shearography to support development in its space shuttle programs. After the Columbia disaster, NASA worked with a number of technologies and companies to develop a system for testing the foam on external tanks of space shuttles and avoid another incident. Laser Technology Inc was among those companies invited to present a laser shearography system for foam inspection. Eventually, due to the benefits of the technology in detecting damages to the foam and predicting the foam that could possibly fall off, shearography became a standard tool for this application. Laser shearography is the most suitable technology for detecting disbonds, voids and delaminations in the composite materials and due to the use of composites in the aerospace industry, laser shearography has found immense acceptance.
In the tire industry, shearography has made great strides and is a standard tool for inspecting the carcass of the tires. This has significantly reduced the number of tire failures in aircrafts, trucks and also the number of tire bursts on the road. Laser Technology Inc. developed a system for tire testing, which was licensed to a European car manufacturer. Currently, SDS Systemtechnik GmbH and Steinbichler Vision Systems, Inc (Steinbichler) offers a shearography system for tire testing and have been successful in the market.
“Although this is not a large volume market, we have been able to develop a shearography system for tire testing that has gained large-scale adoption,” said Hans Weigert, Head of Sales and Marketing at Steinbichler. “We estimate the total market for tire testing in the $10 to $15 million range with us having a major share of this market.”
In wind energy, due to the use of composites, there have been applications for shearography with it being extensively used during production. “Wrinkles or waves are a real problem in the wind industry and have led to numerous blade failures. Shearography is able to detect these wrinkles and is a great production screening tool thanks to the large area inspection capability,” said Matt Crompton, National Sales Manager for NDE at Dantec Dynamics Inc. However, until recently a system for the inspection of wind blades and turbines in field were considered highly challenging. Dantec Dynamics has been able to bridge this gap by developing a system that can inspect the blades for wrinkles, up-tower, removing the need to lower blades to the ground which is a huge saving on time and cost.
“Dantec is the only company right now which has an up tower blade inspection shearography system that can be deployed using rope access or gantry systems,” explained Sammaritano. “The system can provide 100% inspection of a blade. The unique design allows for inspections to be carried out in most weather conditions, at which access to the blade is permitted, and removes the challenge of blade movement which is a substantial problem if your system is not moving with the blade.”
Personnel Certification and Operating Standards for Shearography
In the mid 1990’s, shearography was included in ASNT’s SNT-TC-1A certification scheme for level I and level II qualification. However, the major landmark was achieved in 2005 when ASNT included shearography and holography in its SNT-TC-1A certification scheme for level III qualification. In Europe, there is also EN 473, which now superseded by ISO 9712. Apart from these, there are certification schemes against Standards, such as the National Aerospace Standard NAS 410 for United States which is fully harmonized with the European EN 4179 standard. These have been adopted by most major aerospace and defence manufacturers and include a strict set of requirements.
For aerospace composites inspection using shearography there is ASTM, E2581-07.
”Apart from this, a new set of standards have passed voting and will be published by the fall,” added Newman, who also chairs the laser NDT committee at ASTM. “This will act as a guide for people to perform shearography.”
Laser Profilometry—Evolving applications provide growth potential
The use of laser profilometry dates back to the 1980’s with Qi2 developing a system for the US Navy in 1985 to inspect internal surface of boiler tubes. This system known as laser optic tube inspection system or LOTIS is the first traceable system applying laser profilometry in an NDT application. Following this, Qi2 has been able to develop LOTIS for application in other industries, such as oil and gas, automotive, petrochemical and industrial parts manufacturing for a variety of applications. The inherent advantages that laser profilometry offers are high speed inspection, accuracy and repeatability. These advantages offer significant productivity gains for end users resulting in lower operational expenditure for testing. As a result, the industrial applications for laser profilometry as a standalone tool or in conjunction with other techniques have evolved significantly.
Applus RTD, a leading provider of third party NDT inspection services, developed a system in collaboration with the Pipeline Research Council International (PRCI) called the laser pipeline inspection tool (LPIT). This tool, offered as an active service since the early 2000’s, is capable of dimensioning corrosion in pipelines and has been well received in the industry.
Dr. Casper Wassink, Chief Scientist at Applus RTD explained the application of this service:
“Corrosion is detected using an in-line inspection tool such as a pipeline inspection gauge (PIG) and then we do field validation using laser profilometry of the detected corrosion,” said Dr. Wassink. “Our service can include any number of other NDT techniques to measure and identify the kind of corrosion and the extent of it.”
Using LPIT, an informed decision can be made regarding the extent of corrosion and how it may propagate in the future. Applus RTD has also been able to build additional capabilities into this tool that can measure the remaining burst pressure in the pipeline according to standardized calculations. The traditional method involves dividing the pipeline surface into a grid and then using ultrasonic NDT measure the wall thickness at every grad of the pipeline surface. This is an extremely time consuming method and laser profilometry saves precious inspection time by speeding up the wall thickness measurement aspect. Considering the amount of money involved in excavating a pipe and the performing inspection on it, every hour saved can contribute significant cost savings for the end user.
There are a number of other companies in the market that offer a laser profilometry based corrosion assessment tool in the industry and a recent addition, with an extremely impressive tool, is Creaform Inc. A company known for their low-cost 3D laser scanners used in the metrology industry for dimensional measurements, Creaform, integrated their 3D laser scanners with powerful software to develop a technique for external corrosion assessment on pipelines called Pipecheck.
Although, laser profilometry for corrosion assessment is an extremely powerful application, there are challenges and limitations.
“The main technical challenge involves knowing the original geometry of the pipe. An accurate assessment needs to make on the shape of the pipe and distortion on the surface,” Explained Dr. Wassink.
To overcome this challenge, other NDT techniques like ultrasonic are used to estimate the physical characteristics of the pipeline.
There is also an inherent limitation of laser profilometry in that it only measures the surface and not the volume.
“The limitation is that it measures only the surface and not the wall thickness which is what the clients are interested in,” said Dr. Wassink.
It is expected that with evolving technology this limitation may also be addressed.
Laser Visual Inspection
Although laser visual inspection is currently a niche market, it has immense potential and is expected to become a mainstream NDT technique in the future. One company leading development in this field is Servo Robot Inc. In 1996, the company was involved in a U.S. government research project along with Caterpillar to improve the fatigue life and develop a portable laser visual weld inspection and measurement system. Over the years, Servo Robot has evolved its product line and recently introduced a truly innovative tool in this market called WiKi-Scan.
“WiKi-Scan is the only portable weld inspection product in the market that measures a weld like a human inspector by comparing the dimensions to tolerance limits based on weld quality standards. Other systems in the market compare actual weld to a perfect weld and allow a certain variation. However, this does not constitute visual weld inspection as defined by codes such as AWS D1.1 or ASME Section 9,” Jeff Noruk, President at Servo Robot described the product.
The WiKi-Scan has received extremely positive feedback and Servo Robot hopes to introduce more such products in the future to continue its leadership in the laser visual inspection equipment market.
Most research and development work in laser NDT started with holography in the 1960’s and 1970’s. Other tools such as shearography and laser profilometry started witnessing commercial feasibility from the 1980’s. In the last decade, emerging techniques, such as laser visual inspection and laser ultrasonics have further added to the laser NDT portfolio. The inherent advantages associated with lasers, such as accuracy, speed of inspection and repeatability of measurement has led to a number of NDT equipment manufacturers pursuing integration of laser to develop NDT tools. Of these technologies, holography and shearography are considered mature technologies with established training schemes and standards for using these techniques published. These techniques are key steps toward greater market acceptance and wide-scale adoption. Laser profilometry is still witnessing technological evolution in its various applications and is still not a mature technology. On the other hand, laser visual inspection and laser ultrasonics are emerging technologies with limited applications.
Laser NDT or laser-based NDT inspection systems offer qualities that not many traditional NDT techniques can offer and hence this discipline is expected to witness incremental growth in its various technology segments.