Non-destructive testing invisibly holds together the gigantic structures that make up our civilised world. Invisible because the cohesive power of matter remains hidden from the human eye. Seemingly solid and unbreakable steel structures can collapse at the slightest mistake, causing incalculable material damage, not to mention catastrophic loss of human life.
Invisible, because non-destructive material testing detects these defects, negative deviations (deviations outside the test standard, outside the defect limit), material continuity defects without damaging the material. As the structures that make up the modern world have become more complex as technology has advanced, materials testing technologies have had to keep pace with the rapid pace of change. This is the breeding ground for innovation. But one thing has not changed over time: the potential for human error and failure remains.
It's human to make mistakes, but in some industries, a mistake or oversight can be fatal. Mistakes are a natural consequence of the passage of time, the weather and certain technological processes, whether it's a material defect, a seam defect or a non-conformity in mass-produced products. The defect in the piece being tested becomes a fatal defect if it is not detected in time. Non-destructive material testers therefore have a huge responsibility to identify these defects in time, so efficiency is no longer a mere quality issue, but a matter of life. And time is gained by making the non-destructive testing process as efficient as possible - innovation helps.
The purpose of this of this article is to show that innovation is more than just the presence of a state-of-the-art instrument in the materials testing laboratory's toolbox: true innovation permeates the whole process of non-destructive testing, and is embedded in the way materials testers approach and schedule materials testing.
Conditions for effective use of innovation
In the early 1800s, the railway construction fever catalysed the need for materials testing, in the early 1900s the Titanic disaster triggered the spread of ultrasonic materials testing, among others, and from the turn of the millennium to the present day, a myriad of new challenges and needs have emerged, to the extent that structural integrity has become a separate science, of which materials testing is one of the main areas.
The choice of test method can be influenced by many factors: material thickness, material quality, test situation, economics, size of the defect, its location and nature. This requires considerable expertise. The key is to determine which method is the most reliable to detect the suspected defect. This can often involve a combination of several material testing methods. In order for the test to be truly effective and for the innovation to be used effectively, three conditions must be met:
- Modern and innovative methods
Non-destructive material testing technology has developed exponentially in recent decades. Each of these materials testing methods is increasingly moving towards digitisation and automation, and the combination of materials testing methods has opened up new horizons. Immediate results, rapid analysis and documentation are achievements that are invaluable for industries burdened by downtime and tight deadlines.
- Experienced and highly trained material testers
Innovative instrumentation only truly serves innovation and efficiency if it is selected and operated by the skilled hands of materials scientists, depending on the specific testing task.
- Scheduling the test process and cost-effectiveness
Although this is also the competence of the materials inspector, it deserves a special mention, as the whole process needs to be seen through to maximise the efficiency of innovation. Different material testing situations require different organisation and scheduling, with the inspection of tens of thousands of defective products requiring a different schedule to the inspection of welded seams in a power plant during the annual mandatory shutdown. Furthermore, on-site risk assessment, adherence to health and safety guidelines and fast and transparent documentation are also the responsibility of the materials inspector - innovation is present throughout the process from needs assessment, through preparation of the work, to its follow-up, not just during the inspection.
The basics of non-destructive testing
Before going into the three conditions in more detail, it is important to highlight the most common defects and non-destructive testing situations, as this will determine which materials testing procedure is the most appropriate.
Another type of material tests are destructive material tests, such as tensile or bending tests. Non-destructive material testing is used for testing tasks where, by definition, the integrity of the material must remain intact. A welded bridge structure element, if the test finds no material integrity defects, is installed, and the welded seams of a power plant must also be detected intact to detect any defects.
Most common defects
The two large groups in terms of location are defects on the surface of the material and defects inside the material.
In terms of origin, the two major groups are:
- Technological defects that occur during production:
- casting and welding defects (shrinkage cracks, gas inclusions, slag inclusions, etc.)
- defects caused by ductile deformation (flattening, ruptures)
- heat treatment defects (hardening cracking)
- chipping defects (grinding cracks)
- In-service failures that occur over time:
- cracks caused by fatigue
- corrosion and stress corrosion cracking
- creep damage - this is the deformation of a structural material as a function of time and temperature under constant, sustained load
The most serious failure in non-destructive testing is cracking, which occurs most frequently in welded structures, so one of the most common material testing tasks is to assess the safety of welds.
The inspection situations can also be divided into two large groups in terms of the most common failures:
One group covers the life cycles of the manufacturing process and investigates possible negative deviations in the machining of materials for new products at the start of production, during production and at the final acceptance. This includes, for example, the inspection of thousands of mass-produced automotive parts to ensure that rejects are safely detected by the materials inspector. Immediate results, transparent documentation and flexible availability are the prerequisites for minimising production downtime.
The other large group are failures that occur during operation, where the passage of time can cause failures, such as the testing of welded seams in power plants.
For both groups, the material quality, size, geometrical conditions and environmental conditions of the test piece must also be taken into account - they must not be polluting and must be feasible at the test site.
Non-destructive testing methods and innovation
The choice of test procedure and specific instrumentation requires considerable expertise, safety, cost-effectiveness and a clear understanding of the most common applications, advantages and limitations of the particular method. Thanks to innovation, more and more barriers to non-destructive testing are being overcome, and new solutions are emerging that were only a figment of the imagination a few years ago.
Ultrasonic material testing
Physical principle: In ultrasonic materials testing, the ultrasound conducted into the surface to be tested propagates evenly, but if it reaches a defective part, some of it is reflected back. The defect location is called the acoustic boundary, whose geometry and angle of incidence play an important role in the detection of negative deviation. The material makes itself known in the form of an echo that can be easily detected by instruments.
Fields of application: It is used to measure wall thickness and to detect in-plane defects in welded joints, base materials, castings and formed steel products.
Benefits: Provides immediate results. The test does not require much preparation.
Disadvantages: Small volume defects are difficult to detect with this method. The evaluation of the signals obtained requires great expertise.
Innovation: There are three main types of ultrasonic material testing: transmission, pulse and time-of-flight scattering echo. When inspecting welds, the combination of the latter two methodsis increasingly being used to detect defects and pulse echo and 2D matrix technology to more easily detect so-called dead-space defects that would be hidden under conventional ultrasonic testing.
Magnetic materials testing
Physical principle: The test can determine the location of negative deviations by visualising the magnetic force lines. There are two main variants: the dry powder method and the wet method.
Fields of application: It is suitable for the detection of surface or near-surface planar defects and is one of the most commonly used methods for crack detection, along with penetration testing, and is the preferred method for magnetizable materials.
Benefits: It is particularly sensitive and fast at detecting crack-like defects.
Disadvantages: Magnetic crack detection can only be performed on ferromagnetic materials. Also, the test is directional, access to narrow test sites can be problematic.
Innovation: High temperature is a factor that negatively affects the effectiveness of magnetic materials testing, but innovative methods have appeared on the market that can now eliminate this.
Radiographic material testing
Physical principle: The intensity of the X-ray, gamma or neutron radiation used changes as it passes through the object, depending on the properties of the material.
Fields of application: detection of volumetric continuity defects (cavities, inclusions), accurate detection of 3D defects
Benefits: can be used regardless of material quality, provides a well-documented and clear image, no surface preparation required
Disadvantages: Traditional X-ray examination is a time-consuming procedure due to the image development processes and is not the most reliable method in case of planar defects, so it is worthwhile to use ultrasound material analysis as a complementary method. It requires considerable training to use for accurate evaluation of test results and radiation protection.
Innovation: With the advent of digital radiography, a digitised, innovative version of the analogue, traditional X-ray process, the range of applications has also expanded: from the large-scale inspection of automotive and other mechanical parts and the immediate detection of results, to the more cost- and time-effective inspection of castings and welds. Highly mobile, easy to document. More on this innovative technique below.
Liquid penetrant or penetration testing
Physical principle: The low surface tension (capillary) fluid penetrates the crack open to the surface and draws out the shape of the defect as it leaks out (the depth and width of the crack cannot be measured).
Fields of application: detection of surface defects in welds, base materials, formed products and castings
Benefits: It is a cheap and simple process, requiring no major equipment.
Disadvantages: It is difficult to use on porous surfaces, requires intensive surface cleaning and after-cleaning is essential.
Innovation: As with magnetic materials testing, one of the limitations of this technique is that it can only be performed within a certain temperature range, but the latest penetration fluids can now be used at high temperatures. In addition, the advent ofrobotics and automated penetration systems has made this technique much more efficient and safe.
Visual material testing
Physical principle: The non-destructive material tester carries out the material test with the naked eye, supplemented by devices to improve the perception of the eye (magnifying glass, video scope, endoscope).
Fields of application: It is only suitable for identifying surface defects and is usually used as a complement to other procedures.
Benefits: fast, simple, cheap
Disadvantages: subjective, difficult to document
Innovation: Although magnification devices are in a constant state of technological development, the protagonist of visual inspection, as in the case of other procedures, is the non-destructive material tester himself, who, in the midst of innovative instruments and procedures, must judge which methods will be the most appropriate for a given inspection situation, since as a general rule there is no 100% certain, universal material testing procedure.
The ideal non-destructive materials tester
The use of the most modern instrumentation will not produce results if the non-destructive material tester is not the right person for the job. In addition to knowledge of the various occupational safety guidelines, in the case of radiography, completion of a course in radiation protection and obtaining a Materials Testing Certificate 1, 2 or the highest, 3, you need other qualifications.
In this profession, it is not enough to have precise theoretical knowledge, you also have to perform well in mental stress situations. Often you have to work in a completely alien environment, far from your family. Resilience, adaptability and ingenuity are the drivers of real innovation, and must be the qualities of a dedicated materials tester.
And the most important thing is professional humility. This can be said of any profession, but in a profession where human lives are at stake, it is even more true. The NDT Video Library, one of the largest online platforms for non-destructive materials testing training, does a great job of walking you through the qualities a materials tester should have, including professional humility, and the video can be viewed here.
Scheduling and cost-effectiveness of non-destructive testing
The purpose of non-destructive material testing is to provide information on the conformity of the product under test, such as the verification of welding technology according to a given standard system, in the most cost-effective and quickest way possible. Optimisation of the inspection technology process is a key factor in reducing quality costs.
There are many industries and testing situations, accredited material testing, OK/NOK testing for large volume product screening, technical changeover support with expert advice, but there are some basic rules to follow that are essential for good scheduling and cost-effective testing processes, whatever the testing task:
- Accurate needs assessment: A lot of time can be saved if the material inspector is sure of the product to be tested, its material, the type of product (casting, welded structure, electromechanical part) and the test unit (material defect, weld defect, wall thickness defect), and it is also recommended to know well in advance about the documentation needs. Only then can the materials testing company provide an accurate quotation, avoid mutual misunderstandings and prepare for the testing work.
- Preliminary preparations: Proper on-site risk assessment, adherence to health and safety guidelines, fencing off the radiation zone for radiography and preparation of the material to be tested if the procedure requires it - it is important to include these elements in the schedule, and it is worth making a checklist of the process to make sure you don't waste time.
- Responsibilities: It is important that all members of the team have a clear understanding of their responsibilities, which is one of the reasons why it is worth working with a materials testing laboratory where a cohesive team works together.
- Documentation and follow-up: Accurate maintenance of a material test logbook, accurate preparation of the test report and/or test protocol in a format that is easy for the client to consume, with transparent test evaluation - all essential to optimise the process and reproducibility of the test, so proper follow-up saves time and effort for both partner and material tester, and the digitisation of test methods can be a major advantage.
These basic rules, while not eternal, can generally be said to form the basis of cost-effective materials testing. At the same time, it is important to note that the introduction of innovative procedures requires a rethinking of the established schedules and workflows, because innovation is never isolated "only" within the test procedure, but also affects the whole process of materials testing and the coordination of tasks. Thus, real innovation and efficiency can be said to exist when all the elements of the triad mentioned above - state-of-the-art equipment, a dedicated materials tester and an inspection schedule - support innovation and contribute to efficiency.
This makes it possible to solve challenges that seemed impossible in the past: scanning thousands of products in a short time, with immediate results, is now easy thanks to digital radiography.
Exponential progress in radiography - DDA at the forefront of innovation
We have already mentioned digital radiography as an effective and innovative materials testing technique. From the point of view of the future of non-destructive materials testing, the development of radiography is the most tangible demonstration of the exponential scale of this development. Digital imaging has transformed our entire world, it is the basis of our information society, digitisation is a key driver in all disciplines, and digital imaging tools are increasingly used in non-destructive testing, and consequently in optimising and improving workflow efficiency.
Radiography, or X-ray materials testing, is a prime example because it is one of the most complex and demanding materials testing procedures because of the radiation hazard. It is also a good illustration of the exponential development, especially if we consider the last 5 years.
Analogue vs digital imaging
Although it is almost a hundred years since the advent of X-ray testing, it was relatively recently that only the traditional, analogue method of testing materials existed. The analogue method, although still in use today because it is still the most suitable for certain materials testing tasks, is polluting, both in terms of imaging (longer exposure times are needed) and image processing (chemicals are used in a dark room). The process is also extremely time-consuming. In addition, the films have to be physically stored, making archiving and follow-up difficult.
The phosphor plates used in digital radiography are reusable, there is no need to develop each image on a separate film, because a laser scanner can be used to digitise them, allowing much faster evaluation, although even here it is still indirect, secondary digitisation. Digital radiography is generally understood as this method, although there is also a more modern version.
Ten years ago, a new imaging system, direct digital radiography (DDA), appeared on the market, where primary digitisation is already performed, and a digital image is formed immediately after the radiation. It was first used in the military industry, but its efficiency and wide range of applications have led to its entry into the non-destructive materials testing market. Over the last 5 years, the instrumentation has been fine-tuned by specialised manufacturers to take into account the specificities of materials testing. Direct and instant digital imaging allows for fast evaluation and is a more environmentally friendly solution, as its speed greatly reduces radiation time and replaces the chemicals used for development with fast digital imaging.
The evolution of radiography illustrates the 3 key factors that underpin innovation:
- Overriding the limits of inquiry: advances in science are erasing more and more trade-offs that were a disadvantage or limiting factor, fuelling and innovating the human mind. In radiography, the slowness of the examination and the inconvenience of retrieving and storing results, as well as the environmental pollution, have been transformed and have evolved over the last 5 years into a cost- and time-effective, sustainable tool: direct digital radiography.
- Digital imaging: traceability and archivability is one of the major benefits of digital imaging, which optimises the whole workflow. Analogue films, which are difficult to trace, have been replaced by digital films, and in the last 5 years, the use of phosphor plates has been eliminated by the direct digital radiography imaging system, which allows us to obtain results that can be evaluated even faster.
- Sustainability:A safer working environment for materials testers who can say goodbye to chemicals and minimise exposure time thanks to DDA. The whole process is faster, more efficient and more sustainable.
Innovation as a system of tools
The instrument used for digital imaging serves innovation and progress when it is used as both a tool and a system. When used solely as a tool, direct digital radiography is not a universal method that supersedes analogue or phosphor plate. In fact, depending on the task, there are times when the traditional method is more appropriate. It is more robust and easier to use in some extreme locations, analogue X-ray is the best choice, while in other cases the phosphor plate version is the best choice, for example when examining pipe seams, as the plate can be wrapped flexibly around the pipe, whereas DDA may not be the most suitable for this purpose due to the rigidity of the panel.
Direct digital radiography is time- and cost-efficient and a true innovation when used as a system of instruments, taking into account the inspection criteria and the site, completely optimised for the task. At Control Laboratory, direct digital radiography is most often used for the inspection of mass-produced automotive components. The main inspection factors are:
- this task requires mobility, so we use a lightweight 3 kg pulsed X-ray machine as a source to make the inspection site at the factory as easy as possible
- often thousands or tens of thousands of products need to be inspected in a short period of time, so this task requires accuracy and precision, while the imaging system of direct digital radiography can detect defective products in seconds.
In addition to using the latest instrumentation when choosing a materials testing tool, the whole process must be optimised for the task and the goal, which is the only way innovation can show its true power, the basis for a safer future.
The future of non-destructive materials testing
As we have already shown in the case of radiography, most innovations are driven by the three forces that fuel progress, and from this we can infer the innovations of the future:
- Resolving the compromises: What is a drawback and a barrier to testing in a given technology can be overridden by science - solutions thought unimaginable are being developed every day, and the limits of individual tests are increasingly being broken down in the face of progress.
- Digitalisation: Today's information society requires the most efficient storage, processing and tracking of information, and digital, automatable processes are increasingly used in materials testing.
- Sustainability: Safety is not just about inspecting the elements in the structure, but also about minimising the environmental impact of the work process and protecting the material inspectors - the future is sustainable.
Non-destructive testing is responsible for the quality of the structures that hold our world together, a major line of defence for a safe world, as long as it remains at the forefront of innovation, efficiency and safety. With new testing technologies and dedicated professionals, our future is in safe hands.