Why NDT is essential in any inspection?
It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture. During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods.
NDT classification
1) MPI - Magnetic particle inspection
2) PT - Penetrate testing.
3) RT - Radiographic testing.
4) UT - Ultrasonic Testing.
5) PMI - Positive material identification.
6) HT - Hardness Testing.
1) MPI - Magnetic particle inspection
2) PT - Penetrate testing.
3) RT - Radiographic testing.
4) UT - Ultrasonic Testing.
5) PMI - Positive material identification.
6) HT - Hardness Testing.
Magnetic particle inspection (MPI)
MT: Magnetic particle inspection (MPI) can be used for the detection of surface and near-surface flaws in ferromagnetic materials. Using a permanent magnet, electromagnet, flexible cables or hand-held prods a magnetic field is applied to the item under test. If a flaw is present the magnetic flux is distorted and 'leaks'. Fine magnetic particles, (normally in spray form in carrier fluid) can be applied to the surface of the specimen, are attracted to the area of flux leakage creating a visible flaw indication. It is recommended that the inspection surface is magnetised in at least two perpendicular directions at 90° to each other, due to lack of disturbance to the magnetic field if the crack runs parallel to the magnetic field
MT: Magnetic particle inspection (MPI) can be used for the detection of surface and near-surface flaws in ferromagnetic materials. Using a permanent magnet, electromagnet, flexible cables or hand-held prods a magnetic field is applied to the item under test. If a flaw is present the magnetic flux is distorted and 'leaks'. Fine magnetic particles, (normally in spray form in carrier fluid) can be applied to the surface of the specimen, are attracted to the area of flux leakage creating a visible flaw indication. It is recommended that the inspection surface is magnetised in at least two perpendicular directions at 90° to each other, due to lack of disturbance to the magnetic field if the crack runs parallel to the magnetic field
Penetrate testing (PT)
PT: This method involves applying a visible or fluorescent dye to the surface. After application by immersion or spray the dye enters any discontinuities via capillary action. The component is wiped dry and any subsequent seepage from fissures is detected by drawing the liquid out into a white absorbent coating applied after drying off (See Diagrams Below). This method is suitable for any non ferrous components or material that is non absorbent. Typical applications are forged, cast or welded products.
PT: This method involves applying a visible or fluorescent dye to the surface. After application by immersion or spray the dye enters any discontinuities via capillary action. The component is wiped dry and any subsequent seepage from fissures is detected by drawing the liquid out into a white absorbent coating applied after drying off (See Diagrams Below). This method is suitable for any non ferrous components or material that is non absorbent. Typical applications are forged, cast or welded products.
The basic principle of liquid penetrant testing is that when a very low viscosity (highly fluid) liquid (the penetrant) is applied to the surface of a part, it will penetrate into fissures and voids open to the surface. Once the excess penetrant is removed, the penetrant trapped in those voids will flow back out, creating an indication. Penetrant testing can be performed on magnetic and non-magnetic materials, but does not work well on porous materials. Penetrants may be "visible", meaning they can be seen in ambient light, or fluorescent, requiring the use of a "black" light. The visible dye penetrant process is shown in Figure 7. When performing a PT inspection, it is imperative that the surface being tested is clean and free of any foreign materials or liquids that might block the penetrant from entering voids or fissures open to the surface of the part. After applying the penetrant, it is permitted to sit on the surface for a specified period of time (the "penetrant dwell time"), then the part is carefully cleaned to remove excess penetrant from the surface. When removing the penetrant, the operator must be careful not to remove any penetrant that has flowed into voids. A light coating of developer is then be applied to the surface and given time ("developer dwell time") to allow the penetrant from any voids or fissures to seep up into the developer, creating a visible indication. Following the prescribed developer dwell time, the part is inspected visually, with the aid of a black light for fluorescent penetrants. Most developers are fine-grained, white talcum-like powders that provide a color contrast to the penetrant being used.
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Radiographic testing (RT)
A source of ionising radiation positioned at on one side of item to be inspected, and a photographic film placed in close proximity to the other side. The radiation is partly absorbed during transmission and differences in material thickness or absorption qualities are recorded on the film giving a full-size image showing internal detail. The higher the Material density more radiation absorption will occur.
Processed films are called radiographs, Electrically generated x-radiation (X-rays) are commonly used, and for thicker or denser materials, gamma radiation is generally used. Gamma radiation is given off by decaying radioactive materials, with the two most commonly used sources of gamma radiation being Iridium-192 (Ir-192) and Cobalt-60 (Co-60). IR-192 is generally used for steel up to 2-1/2 - 3 inches, depending on the Curie strength of the source, and Co-60 is usually used for thicker materials due to its greater penetrating ability.
Radiography's main benefits are that it provides a non-destructive method of detecting hidden flaws in materials and fabrications and provides a permanent record. Radiography is particularly good at detecting volumetric flaws such as voids, gas pores and solid inclusions, It is also good at determining the nature and dimensions (length and width) of flaws - however it cannot be used to measure through-thickness of defects.
A source of ionising radiation positioned at on one side of item to be inspected, and a photographic film placed in close proximity to the other side. The radiation is partly absorbed during transmission and differences in material thickness or absorption qualities are recorded on the film giving a full-size image showing internal detail. The higher the Material density more radiation absorption will occur.
Processed films are called radiographs, Electrically generated x-radiation (X-rays) are commonly used, and for thicker or denser materials, gamma radiation is generally used. Gamma radiation is given off by decaying radioactive materials, with the two most commonly used sources of gamma radiation being Iridium-192 (Ir-192) and Cobalt-60 (Co-60). IR-192 is generally used for steel up to 2-1/2 - 3 inches, depending on the Curie strength of the source, and Co-60 is usually used for thicker materials due to its greater penetrating ability.
Radiography's main benefits are that it provides a non-destructive method of detecting hidden flaws in materials and fabrications and provides a permanent record. Radiography is particularly good at detecting volumetric flaws such as voids, gas pores and solid inclusions, It is also good at determining the nature and dimensions (length and width) of flaws - however it cannot be used to measure through-thickness of defects.
- Types of scans- pulse echo (A-Scan), Side view (B-Scan), Top view (C-Scan) on the area of inspection.
A Scan A scan is an amplitude modulation scan. It gives the information in the form of one dimensional. it is used to detect the presence of flaws in the materials. A-scan ultrasound biometry, commonly referred to as an A-scan.
A data presentation method by which intelligence signals from a signal object located are displayed. As generally applied to pulse echo ultrasonics, the horizontal and vertical sweeps are proportional to time or distance and amplitude or magnitude respectively. Thus the location and magnitude of acoustical interface are indicated as to depth below the transducer.
A data presentation method by which intelligence signals from a signal object located are displayed. As generally applied to pulse echo ultrasonics, the horizontal and vertical sweeps are proportional to time or distance and amplitude or magnitude respectively. Thus the location and magnitude of acoustical interface are indicated as to depth below the transducer.
B scan A Single Value B-scan is commonly used with conventional flaw detectors and corrosion thickness gages to plot the depth of reflectors with respect to their linear position. The thickness is plotted as a function of time or position while the transducer is scanned along the part to provide its depth profile. Correlating ultrasonic data with actual transducer position allows a proportional view to be plotted and allows the ability to correlate and track data to specific areas of the part being inspected. This position tracking is typically done through the use of electromechanical devices known as encoders. These encoders are used in fixtures which are either manually scanned or in automated systems that move the transducer by a programmable motor-controlled scanner. In either case the encoder records the location of each data acquisition with respect to a desired user-defined scan pattern and index resolution.
C Scan The C-scan presentation provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer. C-scan presentations are produced with an automated data acquisition system, such as a computer controlled immersion scanning system. Typically, a data collection gate is established on the A-scan and the amplitude or the time-of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.
D Scan The D-scan view is perpendicular to both the C-scan and the B-scan. It refers to the image produced when the data collected from an ultrasonic inspection is plotted on an end view of the component. The true definition according to BS EN 1330-4:2000 is the 'Image of the results of an ultrasonic examination showing a cross-section of the test object perpendicular to the scanning surface and perpendicular to the projection of the beam axis on the scanning surface'. D-scans are often used in the presentation and analysis of TOFD data.
However, like the C-scan, the D-scan usually shows ultrasonic data collected through the whole or part of the inspection volume.The D-scan allows quick discrimination of indications along a weld by presenting their position in depth from the scanning surface.
However, like the C-scan, the D-scan usually shows ultrasonic data collected through the whole or part of the inspection volume.The D-scan allows quick discrimination of indications along a weld by presenting their position in depth from the scanning surface.
Time Of Flight Diffraction (TOFD)
The TOFD technique usually use a two probe (one transmitter; one receiver) arrangement. When sound is introduced into the material via the transmitter the defect will oscillate. Each defect edge works as a source point of ultrasound signals. These very weak signals are called diffracted waves and their appearance does not relate to the orientation of flat or spherical defects. These diffracted signals are received via the receiver probe. The diffracted signals are evaluated with the systems to clear gray scale B-scan or D-scan images (transversal- or longitudinal projection of the object being tested). The amplitude of the signal is not displayed , but the position of the signals on the time scale are. Thus it is possible to determine the defect location exactly - length, and defect height. Therefore the Probability of Detection (POD) increases greatly (up to 90 % !) for flat or spherical defects when compared to traditional techniques.
Applications for TOFD
The main TOFD applications are:
o In-service defect monitoring.
o Defect detection, documentation and evaluation during the production.
The dead zone under the outside surface has always been a limitation of TOFD. Defects close to the surface could not be detected (surface breaking cracks are detectable). TOFD can be applied down to 6 mm wall thickness. On the upper end it is usual to inspect up to 350 mm wall thickness.
Advantages of TOFD
• TOFD defect detection does not depend on the defect orientation, in contrast to the pulse echo technique .
• In contrast to the radiography method, planar defects and cracks, which are not perpendicular to the measured surface can be detected .
• Defect height can be exactly determined.
• Higher POD improves risk reduction and calculation.
• The evacuation of areas because of radiation is not necessary. That means less interruption in the production process less during pre-service or in-service inspections and fewer logistical problems for the manufacturer.
• The inspection results are immediately available, as is a permanent record and a permanent print as longitudinal or transversal projection of the weld is available.
• When Engineering Critical Assessment (ECA) is applied, only the relevant defect has to be cut, thereby preventing needless repairs which could harm the integrity of the weld.
• Because of the high test speed the costs are less than those for radiography for wall thickness above 25 mm.
• The inspection can be performed above 200° C.
• Since the Microplus-System is easy to transport, it is possible to perform test on all feasibly accessible areas.
• TOFD saves costs, if applied during construction, since it is possible to distinguish pre-service and in-service defects. That means the unit can stay longer in production, and is safe.
The TOFD technique usually use a two probe (one transmitter; one receiver) arrangement. When sound is introduced into the material via the transmitter the defect will oscillate. Each defect edge works as a source point of ultrasound signals. These very weak signals are called diffracted waves and their appearance does not relate to the orientation of flat or spherical defects. These diffracted signals are received via the receiver probe. The diffracted signals are evaluated with the systems to clear gray scale B-scan or D-scan images (transversal- or longitudinal projection of the object being tested). The amplitude of the signal is not displayed , but the position of the signals on the time scale are. Thus it is possible to determine the defect location exactly - length, and defect height. Therefore the Probability of Detection (POD) increases greatly (up to 90 % !) for flat or spherical defects when compared to traditional techniques.
Applications for TOFD
The main TOFD applications are:
o In-service defect monitoring.
o Defect detection, documentation and evaluation during the production.
The dead zone under the outside surface has always been a limitation of TOFD. Defects close to the surface could not be detected (surface breaking cracks are detectable). TOFD can be applied down to 6 mm wall thickness. On the upper end it is usual to inspect up to 350 mm wall thickness.
Advantages of TOFD
• TOFD defect detection does not depend on the defect orientation, in contrast to the pulse echo technique .
• In contrast to the radiography method, planar defects and cracks, which are not perpendicular to the measured surface can be detected .
• Defect height can be exactly determined.
• Higher POD improves risk reduction and calculation.
• The evacuation of areas because of radiation is not necessary. That means less interruption in the production process less during pre-service or in-service inspections and fewer logistical problems for the manufacturer.
• The inspection results are immediately available, as is a permanent record and a permanent print as longitudinal or transversal projection of the weld is available.
• When Engineering Critical Assessment (ECA) is applied, only the relevant defect has to be cut, thereby preventing needless repairs which could harm the integrity of the weld.
• Because of the high test speed the costs are less than those for radiography for wall thickness above 25 mm.
• The inspection can be performed above 200° C.
• Since the Microplus-System is easy to transport, it is possible to perform test on all feasibly accessible areas.
• TOFD saves costs, if applied during construction, since it is possible to distinguish pre-service and in-service defects. That means the unit can stay longer in production, and is safe.
Positive Material Identification (PMI)
Upon receiving material for fabrication, traceability of proper material becomes important. Compatibility issues, several other reasons can exist for material specification including design, corrosion resistance, and compliance to codes and standards such as ASME Boiler and Pressure Vessel Code.
Material traceability record are documented list of materials used in fabrication . In fact , materials as they are received and as they move through the production process are in doubt . Why?. Are those raw materials from the mill to service centers, from processing plants (e.g., pipe, tube and fittings) to subcontractors are properly checked and marked?. Each time raw material changes hands , the opportunity for error increases, resulting in questionable material quality. If a material certificate is missing or/and you need to be certain about the type of material used, PMI is the best NDT method . Positive Material Identification is particularly used for high quality metals like stainless steel and high alloy metals. PMI is done prior fabrication and HOLD points listed in the ITP especially to determine actual alloy composition as per ASME Sect II part D such as (SS304 or SS304L / SS316 / SS316L) as claimed in the mill cert are properly distingushed.
Upon receiving material for fabrication, traceability of proper material becomes important. Compatibility issues, several other reasons can exist for material specification including design, corrosion resistance, and compliance to codes and standards such as ASME Boiler and Pressure Vessel Code.
Material traceability record are documented list of materials used in fabrication . In fact , materials as they are received and as they move through the production process are in doubt . Why?. Are those raw materials from the mill to service centers, from processing plants (e.g., pipe, tube and fittings) to subcontractors are properly checked and marked?. Each time raw material changes hands , the opportunity for error increases, resulting in questionable material quality. If a material certificate is missing or/and you need to be certain about the type of material used, PMI is the best NDT method . Positive Material Identification is particularly used for high quality metals like stainless steel and high alloy metals. PMI is done prior fabrication and HOLD points listed in the ITP especially to determine actual alloy composition as per ASME Sect II part D such as (SS304 or SS304L / SS316 / SS316L) as claimed in the mill cert are properly distingushed.
Hardness Testing (HT)
HT: The hardness of steel is generally determined by testing its resistance to deformation. A number of methods are employed including Brinell, Vickers and Rockwell. The steel to be tested is indented by a hardened steel ball or diamond under a given load and the size of the impression is then measured. For steel there is an empirical relationship between hardness and tensile strength and the hardness number is often used as a guide to the tensile strength, e.g. 229 Brinell = 772N/mm2 (50 tons/sq.in).
BRINELL HARDNESS TEST
VICKERS HARDNESS TEST
ROCKWELL HARDNESS TEST
HT: The hardness of steel is generally determined by testing its resistance to deformation. A number of methods are employed including Brinell, Vickers and Rockwell. The steel to be tested is indented by a hardened steel ball or diamond under a given load and the size of the impression is then measured. For steel there is an empirical relationship between hardness and tensile strength and the hardness number is often used as a guide to the tensile strength, e.g. 229 Brinell = 772N/mm2 (50 tons/sq.in).
BRINELL HARDNESS TEST
VICKERS HARDNESS TEST
ROCKWELL HARDNESS TEST
The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.
F= Load in kgf
d = Arithmetic mean of the two diagonals, d1 and d2 in mm HV = Vickers hardness Vickers hardness (e.g. HV/30) value should normally be expressed as a number only (without the units kgf/mm2). Rigorous application of SI is a problem. Most Vickers hardness testing machines use forces of 1, 2, 5, 10, 30, 50 and 100 kgf and tables for calculating HV. SI would involve reporting force in newtons (compare 700 HV/30 to HV/294 N = 6.87 GPa) which is practically meaningless and messy to engineers and technicians. To convert a Vickers hardness number the force applied needs converting from kgf to newtons and the area needs converting form mm2 to m2 to give results in pascals using the formula above. |
The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation.
The diameter of the impression is the average of two readings at right angles and the use of a Brinell hardness number table can simplify the determination of the Brinell hardness. A well structured Brinell hardness number reveals the test conditions, and looks like this, "75 HB 10/500/30" which means that a Brinell Hardness of 75 was obtained using a 10mm diameter hardened steel with a 500 kilogram load applied for a period of 30 seconds. On tests of extremely hard metals a tungsten carbide ball is substituted for the steel ball. Compared to the other hardness test methods, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which will more accurately account for multiple grain structures and any irregularities in the uniformity of the material. This method is the best for achieving the bulk or macro-hardness of a material, particularly those materials with heterogeneous structures. |
The Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor load F0 (Fig. 1A) usually 10 kgf. When equilibrium has been reached, an indicating device, which follows the movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a datum position. While the preliminary minor load is still applied an additional major load is applied with resulting increase in penetration (Fig. 1B). When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained. Removal of the additional major load allows a partial recovery, so reducing the depth of penetration (Fig. 1C). The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell hardness number.
HR = E - e F0 = preliminary minor load in kgf F1 = additional major load in kgf F = total load in kgf e = permanent increase in depth of penetration due to major load F1 measured in units of 0.002 mm E = a constant depending on form of indenter: 100 units for diamond indenter, 130 units for steel ball indenter HR = Rockwell hardness number D = diameter of steel ball |
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FURTHER READINGS
http://www.corrosionsource.com/FreeContent/Corrosion+Knowledge
https://www.asnt.org/MajorSiteSections/NDT-Resource-Center/Introduction%20to%20Nondestructive%20Testing#VT
http://www.gordonengland.co.uk
http://www.corrosionsource.com/FreeContent/Corrosion+Knowledge
https://www.asnt.org/MajorSiteSections/NDT-Resource-Center/Introduction%20to%20Nondestructive%20Testing#VT
http://www.gordonengland.co.uk