Time-of-flight diffraction (TOFD) method of ultrasonic testing is a sensitive and accurate method for the nondestructive testing of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddingtonin 1975 which paved the way for TOFD. Later works on this technique are given in a number of sources which include Harumi et al. (1989), Avioli et al. (1991), and Bray and Stanley (1997).
Bray and Stanley (1997) summarized TOFD as tip-diffraction techniques which utilized the principle that the tips of a crack when struck by a wave will diffract the signals back to the other location on the surface. The depth of these tips can be determined from the diffracted energy.
TOFD was invented in the UK in the 1970s initially as a research tool.The use of TOFD enabled crack sizes to be measured more accurately, so that expensive components could be kept in operation as long as possible with minimal risk of failure.
Measuring the amplitude of reflected signal is a relatively unreliable method of sizing defects because the amplitude strongly depends on the orientation of the crack. Instead of amplitude, TOFD uses the time of flight of an ultrasonic pulse to determine the position and size of a reflector.
In a TOFD system, a pair of ultrasonic probes sits on opposite sides of a weld. One of the probes, the transmitter, emits an ultrasonic pulse that is picked up by the probe on the other side, the receiver. In undamaged pipes, the signals picked up by the receiver probe are from two waves: one that travels along the surface and one that reflects off the far wall. When a crack is present, there is a diffraction of the ultrasonic wave from the tip(s) of the crack. Using the measured time of flight of the pulse, the depth of a crack tips can be calculated automatically by simple trigonometry.
Phased Array + TOFD combination is commonly used to inspect pipeline welds.
One of the limitations of TOFD is the "dead zone' created by the lateral wave signal just below the inspection surface (or OD surface in case of the pipe).The dead zone is approximately 5 mm and there is no flaw detection in this zone. Calibration blocks with side drilled holes as shown in Reference and ISO 10863 used to validate the "dead zone" and sizing accuracy.
Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage. The terms nondestructive examination (NDE), nondestructive inspection (NDI), and nondestructive evaluation (NDE) are also commonly used to describe this technology. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research. The six most frequently used NDT methods are eddy-current, magnetic-particle, liquid penetrant, radiographic, ultrasonic, and visual testing. NDT is commonly used in forensic engineering, mechanical engineering, petroleum engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and art. Innovations in the field of nondestructive testing have had a profound impact on medical imaging, including on echocardiography, medical ultrasonography, and digital radiography.
Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.
Laser-ultrasonics uses lasers to generate and detect ultrasonic waves. It is a non-contact technique used to measure materials thickness, detect flaws and carry out materials characterization. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.
Acoustic emission (AE) is the phenomenon of radiation of acoustic (elastic) waves in solids that occurs when a material undergoes irreversible changes in its internal structure, for example as a result of crack formation or plastic deformation due to aging, temperature gradients, or external mechanical forces.
Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.
Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen atoms. Note that molecular gaseous hydrogen does not have the same effect as atoms or ions released into solid solution in the metal.
Phased array ultrasonics (PA) is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing. Common applications are to noninvasively examine the heart or to find flaws in manufactured materials such as welds. Single-element probes, known technically as monolithic probes, emit a beam in a fixed direction. To test or interrogate a large volume of material, a conventional probe must be physically scanned to sweep the beam through the area of interest. In contrast, the beam from a phased array probe can be focused and swept electronically without moving the probe. The beam is controllable because a phased array probe is made up of multiple small elements, each of which can be pulsed individually at a computer-calculated timing. The term phased refers to the timing, and the term array refers to the multiple elements. Phased array ultrasonic testing is based on principles of wave physics, which also have applications in fields such as optics and electromagnetic antennae.
Rail inspection is the practice of examining rail tracks for flaws that could lead to catastrophic failures. According to the United States Federal Railroad Administration Office of Safety Analysis, track defects are the second leading cause of accidents on railways in the United States. The leading cause of railway accidents is attributed to human error. The contribution of poor management decisions to rail accidents caused by infrequent or inadequate rail inspection is significant but not reported by the FRA, only the NTSB. Every year, North American railroads spend millions of dollars to inspect the rails for internal and external flaws. Nondestructive testing (NDT) methods are used as preventive measures against track failures and possible derailment.
A scanning acoustic microscope (SAM) is a device which uses focused sound to investigate, measure, or image an object. It is commonly used in failure analysis and non-destructive evaluation. It also has applications in biological and medical research. The semiconductor industry has found the SAM useful in detecting voids, cracks, and delaminations within microelectronic packages.
Electromagnetic acoustic transducer (EMAT) is a transducer for non-contact acoustic wave generation and reception in conducting materials. Its effect is based on electromagnetic mechanisms, which do not need direct coupling with the surface of the material. Due to this couplant-free feature, EMATs are particularly useful in harsh, i.e., hot, cold, clean, or dry environments. EMATs are suitable to generate all kinds of waves in metallic and/or magnetostrictive materials. Depending on the design and orientation of coils and magnets, shear horizontal (SH) bulk wave mode, surface wave, plate waves such as SH and Lamb waves, and all sorts of other bulk and guided-wave modes can be excited. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries, in which they are typically used for nondestructive testing (NDT) of metallic structures.
Weld quality assurance is the use of technological methods and actions to test or assure the quality of welds, and secondarily to confirm the presence, location and coverage of welds. In manufacturing, welds are used to join two or more metal surfaces. Because these connections may encounter loads and fatigue during product lifetime, there is a chance they may fail if not created to proper specification.
Alternating current field measurement (ACFM) is an electromagnetic technique for non-destructive testing detection and sizing of surface breaking discontinuities. It was derived from the methods used in eddy-current testing and works on all metals, ferrous or non-ferrous. Since it doesn't require direct electrical contact with the surface it can work through thin coatings such as paint. This practice is intended for use on welds in any metallic material.
Thermographic inspection refers to the nondestructive testing (NDT) of parts, materials or systems through the imaging of the temperature fields, gradients and/or patterns ("thermograms") at the object's surface. It is distinguished from medical thermography by the subjects being examined: thermographic inspection generally examines inanimate objects, while medical thermography generally examines living organisms. Generally, thermographic inspection is performed using an infrared sensor.
Hot plate welding, also called heated tool welding, is a thermal welding technique for joining thermoplastics. A heated tool is placed against or near the two surfaces to be joined in order to melt them. Then, the heat source is removed, and the surfaces are brought together under pressure. Hot plate welding has relatively long cycle times, ranging from 10 seconds to minutes, compared to vibration or ultrasonic welding. However, its simplicity and ability to produce strong joints in almost all thermoplastics make it widely used in mass production and for large structures, like large-diameter plastic pipes. Different inspection techniques are implemented in order to identify various discontinuities or cracks.
Microwave imaging is a science which has been evolved from older detecting/locating techniques in order to evaluate hidden or embedded objects in a structure using electromagnetic (EM) waves in microwave regime. Engineering and application oriented microwave imaging for non-destructive testing is called microwave testing, see below.
Carbon fiber testing is a set of various different tests that researchers use to characterize the properties of carbon fiber. The results for the testing are used to aid the manufacturer and developers decisions selecting and designing material composites, manufacturing processes and for ensured safety and integrity. Safety-critical carbon fiber components, such as structural parts in machines, vehicles, aircraft or architectural elements are subject to testing.
Active thermography is an advanced nondestructive testing procedure, which uses a thermography measurement of a tested material thermal response after its external excitation. This principle can be used also for non-contact infrared non-destructive testing (IRNDT) of materials.
Robotic non-destructive testing (NDT) is a method of inspection used to assess the structural integrity of petroleum, natural gas, and water installations. Crawler-based robotic tools are commonly used for in-line inspection (ILI) applications in pipelines that cannot be inspected using traditional intelligent pigging tools.
Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.
A variety of non-destructive examination (NDE) techniques are available for inspecting plastic welds. Many of these techniques are similar to the ones used for inspecting metal welds. Traditional techniques include visual testing, radiography, and various ultrasonic techniques. Advanced ultrasonic techniques such as time of flight diffraction (TOFD) and phased-array ultrasonics (PAUT) are being increasingly studied and used for inspecting plastic pipeline welds. Research in the use of optical coherence tomography (OCT) and microwave reflectrometry has also been conducted.