Advanced NDT methods are developed versions of conventional NDT methods whereby advanced technology coupled with digital signal and image processing techniques are used to render the technique more flexible and most of the time easy to interpret.
Amongst the popular advanced NDT methods we find:
1. AUTOMATED ULTRASONIC TESTING SERVICES
AUT is a non-destructive testing method utilizing the advantages of conventional ultrasonic techniques in an automated process with computerized data acquisition and data storage. A distinct advantage of AUT is the creation of baseline inspections with a database and hard copy scan images for reporting and customer review for each scan image. Data collection is accomplished in the A-Scan, B-Scan, B-Prime (D-Scan) and C-Scan formats with an overlay capability of the data.
The advantages of the AUT process also include the versatility, speed and coverage attainable on the test specimens. Large areas of materials are scanned quickly and accurately. Multiple scanner configurations and transducer applications including; angle beam, straight beam, TOFD and other specialty transducers including Phased Array may be utilized. Sophisticated data analysis software is implemented for the resolution, interpretation and sizing of AUT indications. Other customer advantages include in-service examinations, thuseliminating downtime costs by bringing the equipment out of service and equipment cleaning costs for internal inspections and fitness-for-service evaluations. AUT is a non-hazardous procedure which may be performed in conjunction with other activities. Access to only one side of test material is required for the inspection.
Automated Ultrasonic testing has been proven and gained acceptance in all industries as an acceptable and preferred inspection technique saving customers time, costs, increasing inspection coverage and inspection integrity. The object of this nondestructive testing method is to utilize all the advantages of Ultrasonic techniques in an Automated process with data acquisition and storage.
Benefits of AUT include:
- Optimizes your inspection spend
- Reliable, repeatable and accurate results
- Minimizes costly internal entry
- Minimizes unnecessary repairs
- Reduces or eliminates downtime
- Potentially reduces outage/turnaround schedules
- "On-Line" inspection provides data for advanced planning
- Accepted by regulatory and industry standards and specifications
- Supports RBI, FFS and remaining life programs
Below please find a list of damage mechanisms and the technique(s) best suited to identify the type of damage in question:
- Erosion/Corrosion of large areas: Large Structure Inspection
- Erosion/Corrosion of small or inaccessible areas: Pocket UT
- Flaw Detection and Sizing: Phased Array or P-Scan
- Hydrogen Induced/Stress Oriented Hydrogen Induced Cracking (HIC&SoHIC): P-Scan or Phased Array
- Ammonia/Chloride Stress Corrosion Cracking: Phased Array or P-Scan
- Intergranular Stress Corrosion Cracking: P-Scan
- Creep Cracking: P-Scan
- Microbiologically Induced Corrosion: Phased Array or P-Scan
- Vessels/Tanks: Large Structure Inspection, Time of Flight Diffraction, Phased Array, Pocket UT
- Point of Contact Corrosion: Pocket UT with Touch Point fixture
- Bullets: Large Structure Inspection or Pocket UT
- Spheres: Large Structure Inspection or Pocket UT
- Piping: Guided Wave, Touch Point Corrosion, Phased Array, P-Scan
2. Long Range Guided Ultrasonic Testing ( LRGUT)
Guided Wave testing (GWT) is one of latest methods in the field of non-destructive evaluation. The method employs mechanical stress waves that propagate along an elongated structure while guided by its boundaries. This allows the waves to travel a long distance with little loss in energy. Nowadays, GWT is widely used to inspect and screen many engineering structures, particularly for the inspection of metallic pipelines around the world. In some cases, hundreds of meters can be inspected from a single location. There are also some applications for inspecting rail tracks, rods and metal plate structures.
Although Guided wave testing is also commonly known as Guided Wave Ultrasonic Testing (GWUT) or Long Range Ultrasonic Testing (LRUT), it is fundamentally very different to conventional ultrasonic testing. Guided wave testing uses very low ultrasonic frequencies compared to those used in conventional UT, typically between 10~100kHz. Higher frequencies can be used in some cases, but detection range is significantly reduced. In addition, the underlying physics of guided waves is more complex than bulk waves. Much of the theoretical background has been addressed in a separate article. In this article, the practical aspect of GWT will be discussed.
3. Alternating Current Field Measurement (ACFM)
ACFM is used for detecting and sizing surface breaking flaws. Technical Software Consultants (TSC) developed ACFM as an extension of the successful Alternating Current Potential Drop (ACPD) technique. It was initially conceived for use under water to detect flaws in offshore structures and to overcome the fact that ACPD was unsuitable for such applications because of the need for good electrical contact between probes and the structure's surface. Now, however, ACFM is also applied to structures both in and out of the water. (It has the advantage over some other techniques that the structure requires minimal cleaning and that it can be applied over paint and other coatings up to several millimetres in thickness.)
ACFM is an electromagnetic technique. A sensor probe is placed on the surface to be inspected and an alternating current is induced into the surface. When no defects are present the alternating current produces a uniform magnetic field above the surface. Any defect present will perturb the current forcing it to flow around and underneath the defect; this causes the magnetic field to become non-uniform and sensors in the ACFM probe measure these field variations.
Two components of this magnetic field are measured - one provides information about the depth or aspect ratio of the defect(s), and the
other shows the positions of the defects' ends. The two signals are used to confirm the presence of a defect and, together with a sizing algorithm, measure its length and depth.
The advantages of ACFM are that it:
- Works equally well on parent material or welds, ferritic or non-ferritic metals.
- Can be used on hot surfaces, underwater, or in irradiated environments.
- Provides both depth and length information. Accurate sizing of defects up to 25mm (1") in depth.
TWI owns the state-of-the-art Amigo ACFM system supplied by TSC.
Selected clients and applications
ACFM is particularly suited to the detection and sizing of fatigue cracks at the toes of welds, including all butt, fillet, node and nozzle welds. TWI has applied its AMIGO system in this role for inspection of fillet welds under the orthotropic decks of several highway bridges. These have a paint coating so that ACFM was particularly suitable. In some cases inspections are made to a routine schedule to determine whether crack growth is occurring. TWI has also used ACFM to inspect fillet welds in Mobile Offshore Drilling Units (MODUs) to detect any possible original fabrication hydrogen cracking present at weld toes. It has also been successfully deployed in the laboratory in a joint industry project to detect stress corrosion cracking in duplex stainless steel pipe welds.
4. DIGITAL RADIOGRAPHY
Digital Radiography is one of the newest forms of radiographic imaging. Since no film is required, digital radiographic images are captured using either special phosphor screens or flat panels containing micro-electronic sensors. Captured images can be digitally enhanced for increased detail and are easily archived.
There are a number of forms of digital radiographic imaging including:
- Computed Radiography (CR): digital imaging process that uses a special imaging plate which employs storage phosphors.
- Real-Time Radiography (RTR): a form of radiography that allows electronic images to be captured and viewed in real time.
- Direct Radiography (DR): a form of real-time radiography that uses a special flat panel detector.
- Computed Tomography (CT): uses a real-time inspection system employing a sample positioning system and special software.
MISTRAS Services employs a wide array of digital radiographic systems to solve specific industrial problems. Thickness profiles of piping systems, both insulated and uninsulated, are performed using computed radiography, while large production runs of smaller parts are inspected using direct radiography. Real time radiography is utilized for large "real time" inspections of insulated piping systems looking for areas of pipe degradation.