Non-Destructive Testing

Overview

 

Conventional NDT Services 

Advanced NDT Services

 

Ultrasonic Testing (UT)

Traditional Ultrasonic inspection uses high frequency sound energy to conduct examinations and perform measurements. Considerable information may be gathered during ultrasonic testing such as the presence of discontinuities, material or coating thickness. The detection and location of discontinuities is enabled by the interpretation of ultrasonic wave reflections generated by a transducer. These waves are introduced into a material and travel in a straight line and at a constant speed until they encounter a surface. The surface interface causes some of the wave energy to be reflected and the rest of it to be transmitted. The amount of reflected vs. transmitted energy is detected and provides information on the size of the reflector, therefore the discontinuity encountered. Three basic ultrasonic techniques are commonly used:

 

1. Pulse-echo and through transmission

 

  • In pulse-echo testing a transducer sends out a pulse of energy and the same or a second transducer listens for reflected energy, also known as an echo. Pulse echo is especially effective when only one side of a material is accessible.

 

  • Through transmission is performed using two transducers on opposing sides of the specimen.     One acts as a transmitter and the other as a receiver. Through transmission is useful detecting discontinuities that are not good reflectors when signal strength is weak.

 

2. Normal/Angle Beam

 

Normal beam testing uses a sound beam that is introduced at 90 degrees to the surface, while angle beam utilizes a beam that is introduced into the specimen at some angle other than 90 degrees. The choice between the two is made based on:

 

  • The orientation of the feature of interest so that the sound may produce the largest reflection from the feature.             

  • Obstructions on the surface of the specimen that must be avoided.

 

3. Contact and Immersion

 

To get useful levels of sound energy into the material, the air between the transducer and the specimen must be removed. This is referred to as coupling. Two types of coupling are utlizied:

 

  • In contact testing, a couplant such as water, oil or a gel is applied between the transducer and the specimen.           

  • In immersion testing, the specimen and the transducer are placed in a water bath. This allows better movement of the transducer while maintaining consistent coupling.

 

Some of the most common Ultrasonic applications are:

 

  • Flaw detection (cracks, inclusions, porosity, delaminations etc.)

  • Erosion/Corrosion thickness gauging

  • Assessment of bond integrity

  • Estimation of grain size in metals

  • Estimation of void content in composites and plastics

 

Info from ultrasonic inspection can be presented in a number of formats:

 

  • A-Scan displays the amount of received ultrasonic energy as a function of time.

  • B-Scan displays a profile view (cross-sectional) of a specimen.

  • C-Scan displays a plan type view of the specimen and discontinuities.

  • Hybrid/Stitched displays a C-Scan plan view with A and/or B Scan views along with C-Scan views that have been woven together to illustrate a clearer picture of the damaged areas of a specimen. The stitched views are used for larger specimens and surface areas.

 

Some of the major advantages of ultrasonic testing are:

 

  • Detects surface and subsurface defects.

  • Depth of penetration vs. other test methods is superior.

  • Only single sided access is required with a pulse-echo technique.

  • High accuracy regarding estimating discontinuity size and shape.

  • Minimal specimen preparation is required.

  • Instantaneous results produced by using electronic equipment.

  • Detailed images can be produced with automated systems.

 

Major limitations of ultrasonic testing are:

 

  • Surface must be accessible.

  • Skill training is more extensive that with some other methods.     

  • Normally requires couplant to promote sound transfer.

  • Surface roughness, complex geometries, small parts or exceptionally thin materials are difficult to inspect.

  • Coarse grained materials i.e. cast iron are difficult to inspect due to low sound transmission and high signal noise.

  • Linear defects oriented parallel to the sound beam go undetected     

  • Reference standards are required for equipment calibration

 

 

 

 

Magnetic Particle Testing (MT)

Used for finding surface/near surface defects in ferromagnetic material, Magnetic Particle testing is a versatile inspection method used for field and shop applications. Magnetic particle testing works by magnetizing a ferromagnetic specimen using a magnet or special magnetizing equipment. If the specimen has discontinuity, the magnetic field flowing through the specimen is interrupted and leakage field occurs. Finely milled iron particles coated with a dye pigment are applied to the specimen. These are attracted to leakage fields and cluster to form an indication directly over the discontinuity. The indication is visually detected under proper lighting conditions.

 

The basic procedure that is followed to perform magnetic particle testing consist of the following:

 

1.  Pre-cleaning of component

2.  Introduction of Magnetic field

3.  Application of magnetic media

4.  Interpretation of magnetic particle indications

 

It is essential for the particles to have an unimpeded path for migration to both strong and weak leakage fields. Therefore, the component in question should be clean and dry before beginning the inspection process. The presence of oil, grease or scale may compromise the inspection. The introduction of the magnetic field can be introduced a number of ways including use of a permanent magnet, flowing of electrical current through the specimen or flowing an electrical current through a coil of wire around the part or through a central conductor running near the part. Two types of magnetic fields can be established within the specimen. These are a longitudinal magnetic field that runs parallel to the long axis of the part or a circular magnetic field that runs circumferentially around the perimeter. Longitudinal magnetic fields are produced using a magnetic coil or a permanent magnet called a magnetic particle yoke. Circular magnetic fields are produced by passing current through the part or by placing the part in a strong circular magnetic field.

 

Magnetic particle inspection can use either wet or dry magnetic media. The dry method is more

portable, while the wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility.

 

Indications that are formed after applying the magnetic field must be interpreted by a skilled inspector. This requires the individual to distinguish between relevant and irrelevant indications.

 

The following are the advantages of magnetic particle inspection:

 

  • Can detect both surface and near subsurface indications

  • Can inspect parts with irregular shapes easily

  • Pre-cleaning is not as critical as for some other inspection methods

  • Fast method of inspection and indications are visible directly on the specimen surface

  • Considered low cost compared to many other NDT techniques

  • Very portable inspection especially when used with battery powered equipmen

 

Liquid Penetrant Testing (PT)

Penetrant testing based on the properties of capillary action, or the phenomenon of a liquid rising or climbing when confined to a small opening due to surface wetting properties of the liquid. Penetrant testing is used for finding surface breaking discontinuities on relatively smooth, nonporous surfaces.

 

The types of defects that can be found with penetrant inspection are:

 

  • Rolled Products: penetrant identifies anomalies (cracks, seams or laminations)

  • Castings: cold shuts, hot tears, porosity, blow holes or shrinkage

  • Forgings: illuminating cracks, laps or external bursts

  • Welds: to identify cracks, porosity, undercut, overlap, lack of fusion or lack of penetration

 

There are two main types of penetrant; fluorescent or visible. Within each method there are several methods including water washable, postemulsifiable-lipophilic, solvent removal and postemulsifiable-hyperdrophilic. The type and penetrant method are chosen based on sensitivity levels 1-4 and are based on job site conditions and other variables.

 

There are six main steps involved with penetrant testing:

 

1. Pre-Clean: Parts must be free of dirt, grease, rust, scale, oil or grease.

2. Application of Penetrant Material: The penetrant material may be applied by brushing, spraying, dipping/immersing or flow on the material.

3. Dwell Time/Penetrant Removal: The solution must be allowed to "dwell" on the surface to allow the penetrant to fill any defects that are present. Dwell times vary according to penetrant type, temperature and material types and finishes. Removal technique depends upon the type of penetrant used i.e. Solvent Removable, Water Washable or Post-Emulsifiable.

4. Developer Application

5. Inspection/Evaluation: In almost all cases the inspector evaluates the penetrant indications a specified accept/reject criteria and attempts to determine the origin of the indication.

6. Post Clean: The final step is to remove all penetrant processing materials from the component.

 

The main advantages of Penetrant testing are:

 

  • Relatively easy to use

  • Used on a wide range of material types

  • Large areas or large volumes of parts/material can be inspected rapidly and at low cost

  • Parts with complex geometries can be inspected easily

  • Indications are produced directly on the surface of the part providing a visual image of the anomaly

  • Aerosol spray cans can make equipment very portable

 

Radiographic Testing (RT)

Industrial radiography is used for a variety of applications but is commonly performed using two different sources of radiation, X-Ray and Gamma ray sources. The choice of radiation sources and their strength depends on a variety of factors including size of the component and the material thickness. Within the broad group of X-Ray and Gamma ray sources are a variety of camera choices with varying radiation strengths. Tesko X-Ray capabilities run the gamut from 4 MEV units utilized to radiograph extremely large and thick castings and forgings, to portable X-Ray cameras used for field weld applications and thin wall material inspection. Gamma sources vary from very low level fluoroscopic units to perform real time corrosion under insulation surveys, to Iridium (Ir192) and Selenium (Se 75) sources used for a variety of weld inspections, to Cobalt (Co 60) inspections for thick component testing.

 

There are many advantages to radiography including:

 

  • Inspection of a wide variety of material types with varying density

  • Ability to inspect assembled components

  • Minimum surface preparation required

  • Sensitivity to changes in thickness corrosion, voids, cracks and material density changes

  • The ability to detect both surface and subsurface defects

  • The ability to provide a permanent record of the inspection.

 

The disadvantages of radiography are:

 

  • Safety precautions are required for the safe use of radiation,

  • Access to both sides of the specimen are required

  • Orientation of the sample is critical

  • Determining flaw depth is impossible without additional angled exposures

 

Tesko supply a complete line of radiographic services for both shop and field applications. Our staff of qualified, certified, professional radiographers operate within strict safety parameters and produce high quality radiographs that allow us to utilize our interpretation skills honed through many years of experience to determine if an anomaly is actually a defect or can be accepted per code requirements. 

 

Visual Testing (VT)

A visual inspection or visual examination of objects, parts or components is the oldest and reliable non-destructive testing method. The test method is applied to almost every product as a quality assurance tool. The most detrimental unacceptable discontinuities in the objects or items are the surface opening discontinuities. Visual scanning, inspection or testing can successfully detect these unacceptable surface discontinuities without applying expensive test methods. Tesko perform visual examination of welded joints or fabricated components, castings, forgings, rolled products and several other wrought products in accordance with AS 3978, ASME V Article 9, ASE IX QW 194, EN 970, ISO 10042, ASTM other similar standards. 

 

Ultrasonic Thickness Measurement (UTM)

One of the most widespread NDT methods in mechanical equipment of industrial installations for the characterization of erosion and deterioration is the thickness measurements with the ultrasonic method. Tesko offers high level thickness measurement services on pipes, pressure vessels, boilers, tanks etc, by its experienced and certified inspectors. Thickness measurement is achieved by placing the UT probe on the object surface. Local or general reductions of thickness can be located and measured with high precision. The instruments that are used for the measurement are portable and light with possibilities of saving the measurements data in a data logger. The use of different probes offers the capability to perform thickness measurements in inaccessible surfaces, on very thin plates, on high temperature environments, above paint as well as on surfaces with intense local corrosion without any surface smoothing required.

 

Advantages of the method:

 

  • Quick measurements with direct results

  • High precision measurements

  • The inspected object can be in-service.

  • No particular surface preparation required.

  • Measurements can be made without removal of the paint.

  • Measurements can be made in inaccessible regions using suitable probes.

  • In many cases there is the possibility of measuring corroded surfaces without the need of cleaning the surface (using special probes).

  • Corrosion rate calculations can be made with thickness reduction observations with repeated measurements in the same points.

  • Measurements on high temperature surfaces possible.

  • Display of corrosion profile on B-Scan form. 

 

Ferrite Measurement

Austenitic, Duplex, Super-duplex stainless steels require adequate proportion of ferrite in the product to obtain acceptable corrosion resistance and strength and especially resistance to Stress Corrosion Cracking  (SCC). The laboratory based destructive method such as microstructural analysis on the test specimen or sample obtained from the batch of the product or components provides statistical quality assurance. 

 

In order to guarantee the acceptable ferrite content in the final product, ferrite measurements are performed on the actual product using feritscope. Ferrite measurement using feritoscope is a non-destructive test method which assures the quality of the product without the need of expensive laboratory based destructive test methods. The test is performed in accordance with the manufacturer's instructions, contractors specification and laboratory based validated methods.

Experienced metallurgist, engineers and eechnologist at Tesko provide accurate and reliable ferrite measurement on-site and in-situ products. 

 

 

Positive Material Identification (PMI)

Inclusion of material test reports (MTRs) has been the accepted method over the years; however, it has been proven that mill certifications and heat markings alone can be unreliable. Since MTRs are generated at the mills that provide the raw stock, by the time the material reaches a manufacturers’ facility, it may have been through many handling processes increasing the chances of error in the heat markings, therefore, the MTR may not be always trusted. This is becoming more common in the stainless steel and nickel alloy fabrication industries. So, Tesko have utilized the latest technology using X-Ray Fluorescence (XRF) hand-held alloy analyzers to perform the positive material identification (PMI).

 

XRF alloy analyzers are portable hand-held devices that can perform a non-destructive test on the material at any time in merely seconds. XRF works by exposing the material to a flux of x-rays. The atoms then absorb the energy and become temporarily excited and they fluoresce, or emit x-rays. The x-rays emitted by the sample’s atoms possess clearly defined energies that are unique to the elements present in the sample. By measuring the intensity and energy, the XRF instrument can provide qualitative and quantitative analysis. In other words, it can identify the elements, measure the concentration of each and display them on the unit. The data can be downloaded from the unit and saved for reference or creating reports. 

 

 

Time of Flight Diffraction (ToFD)

Time of Flight Diffraction (ToFD) is an advanced method of Ultrasonic inspection. It is a very sensitive and accurate method for non-destructive testing of discontinuities in welded joints. ToFD ultrasonic test method can perform crack sizing very precisely and allow the owner to run the equipments or plants until the critical crack depth and length  is not reached with  a minimal risk of failure. Conventional ultrasonic test method employs measuring of the amplitude of reflected signal which is relatively less reliable method of sizing defects because the amplitude strongly depends on the orientation of the crack. ToFD ultrasonic test method uses the time of flight of an ultrasonic pulse to determine the position of a discontinuity of interest. 

 

ToFD ultrasonic test system consists of a pair of probes placed on opposite sides of a weld. One of the probes emits an ultrasonic pulse that is collected by the probe on the other side. In undamaged pipe, 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. These ultrasonic waves are diffracted from the tip of the discontinuities, if present. The size of the discontinuity is calculated by measuring time of flight of the ultrasonic pulse. Compared to conventional ultrasonic testing, ToFD ultrasonic testing instruments and probes are complex and expensive and requires highly trained, skilled and experienced technicians. 

 

Phased Array Ultrasonic Testing (PAUT)

Phased Array ultrasonic testing (PAUT) is an advanced method of ultrasonic testing. Phased array ultrasonic method can reveal  defects embedded in the material that cannot be easily resolved with conventional ultrasonic testing. 

 

Compared to conventional ultrasonic testing, Phased Array ultrasonic testing instruments and probes are complex and expensive and requires highly trained, skilled and experienced technicians. The Phased Array Ultrasonic Testing is highly recommended for critical discontinuities detection, sizing and monitoring in aerospace industry, power general industry, nuclear industry and petrochemical industry.

 

Eddy Current Testing

Multi-frequency eddy current systems refer to equipment that can drive inspection coils at more

than two frequencies. This type of instrumentation is used extensively for tubing inspection in Power Generation and the Oil and Gas industries. Major advantages of this inspection are the ability to increase inspection information collected from one probe pull, comparison of same discontinuity signal at different frequencies, mixing of frequencies that helps to reduce or eliminate sources of noise and improves detection, interpretation and sizing capabilities.

 

A critical component of any eddy current examination is the ability to calibrate the unit based on reference standards manufactured from the same or very similar material as the test specimen. In the case of tubing inspection an ASME tubing pit standard is required.

 

The advantages of Eddy Current inspection are: sensitive to small cracks and defects, detects surface and near surface defects, immediate results are available, equipment is portable, minimum part preparation is required, probes do not need to contact the part and the ability to inspection complex shapes and sizes of conductive materials. The limitations of Eddy Current include; only conductive materials can be inspected, skill and training required is more extensive than other techniques, surface finish and roughness may affect the test. 

 

Videoscope Inspection Testing

Videoscopes Inspection offers the best image quality available in flexible scopes. The scopes are flexible so that they can be inserted into many applications, from gas turbines to process and high purity piping. They include distal tip articulation and interchangeable optical tip adapters to maximize image quality in the specifics of your application.

 

Available systems include :

 

  • High Resolution Display Camera

  • High Intensity Light Source

  • Camera Control Unit (CCU)

  • Digital Storage or Videotape

 

Videoscope applications:

 

  • Inspection/approval for pharmaceutical, chemical, food, power plant industries

  • Inspection/approval drinking water pipes and waste water technology

  • Inspection for heat exchangers

  • Examination of walls for deposits, erosion, corrosion, crack formations

  • Examination of electropolished piping, orbital weld seams, longitudinal weld seams

 

NDT by X-Ray Crawlers

The X-Ray Crawler is similar to conventional radiography however an x-ray source tube on a crawler device is run inside the pipe to each weld. The technique is quick and can inspect on average 150 welds per day. The biggest advantage of x-ray crawlers is their speed. 

 

NDT by Scar Projector

 A Portable gamma radiographic exposure device is specifically designed as a Small Controlled Area Radiography (SCAR) system. The SCAR projector is used primarily for the radiography of standard or difficult joint geometries without disruption and the need for large barrier areas. The SCAR system is comprised of a projector, a pneumatic control unit and a series of clamps to fit a variety of pipe sizes.

The system may be used with a maximum of 15 curies of Iridium-192 or 81 curies of Selenium-75. Lower activity sources with smaller focals are available. 

 

Features:

 

  • Allows for 24 hours radiography

  • Highly directional beam

  • Reduces radiation dose

  • Sealed source does not leave the device during exposures

  • No flash dose during exposure or retraction of sealed source     

  • More production time for all trades

  • Locks to prevent unauthorized operation

  • Inspection through insulation for corrosion  

 

Magnetic Flux Leakage (MFL)

The basic principle of this Non-Destructive Test is that a strong magnet induces a magnetic field in the material. At areas where there is corrosion or missing metal, a leakage field will arise. The output from the detector can be electronically digitalized for automated inspection systems. All indications is stored as well as their location. Bad areas or spots in plates can easily be identified and repaired. In the range 6 to 20 mm, all ferro-magnetic tank bottoms can be inspected, and (limited) coated surfaces also can be tested.

 

Magnetic Flux Leakage (MFL) is a method of Non-Destructive Testing, which is used to detect corrosion and pitting in steel structures, most commonly pipelines and storage tanks. MFL is a detection technique, which detects volumetrically changes. The disadvantage of Magnetic Flux Leakage is that no absolute values but relative volumetrically changes are reported. However it is a very suitable tool for detecting bad spots in the plates. After the fast MFL inspection is done, only the “suspicious” areas of the tank bottom surface will be quantified by the slow but more accurate Ultrasonic Testing.