What is Non-Destructive Testing
Non-Destructive Tests (NDT) are conducted to determine the defects appearing at the surface or inside the components of machines without destroying them.
Main purpose of applying NDT is to ensure material reliability and prevent premature material failure. It is carried out on finished or semi-finished materials to check for discontinuities or defects. It is essential for flight safety and for keeping inspection cost low.
NDT ensures material reliability and prevents premature material failure. A product is designed to perform the assigned functions well and give trouble free service to the user for a stipulated period. The trouble-free service given by the product is termed as reliability, which comes through by improving the quality level of the product.
Factors Affecting Non-Destructive Test
Various factors affecting NDT are:
(a) Trained personnel
(b) Approved procedures.
(c) Correct Equipment.
(d) Clearly defined acceptance criteria.
(e) Suitable facilities and environmental conditions.
Advantages of NDT
The main advantages of applying NDT for material inspection during manufacturing process and for in service inspections are as follows:
(a) Ensures product quality, reliability and safety.
(b) Aids in optimum product design.
(c) Controls manufacturing processes within specified tolerances.
(d) Ensures uniform quality levels.
(e) Ensures customer satisfaction.
(f) Predicts impending failures, thus preventing costly shutdowns and aids in plant life extension.
Types of NDT Techniques
There are different types of NDT techniques such as:
(a) Visual Inspection
(b) Magnetic Particle Inspection
(c) Eddy Current Testing
(e) Liquid Penetrant Testing
Selection of various method of NDT to detect cracks/flaws depends upon following factors:
(b) Manufacturing process
(c) Product phase
(d) Discontinuity characteristics
(e) Portability of the test equipment
(f) Test cost
(g) Execution time
The cracks and flaws that can be seen by naked eyes are visually inspected by means of magnifying glasses. Normal visual inspection cannot be relied upon to disclose very fine surface cracks nor can it reveal internal faults, which may develop in highly stress prone components. Therefore, special components must be subjected to a crack detection test. The various methods used for non-destructive tests are follows:
(a) Flight method
(b) Workshop method
(c) Specialist method
There are two types of flight method for crack detection, which are as follows:
(a) Hot Fluid Chalk Test
(b) Cold Fluid Method
Hot Fluid Chalk Test
This process consists of heating a mixture of kerosene (3 parts) and lubricating oil (one part) in a bath to 90º C. The smaller component which is removable is to be immersed in the fluid to soak until it attains the bath temperature. The component is then removed, cleaned and dried as quickly as possible and while the component is still hot, it is relied with French chalk. Surplus chalk is removed by tapping and when the component is cooled off, the contraction of the component on cooling will force the oil to cut off any crack and strain the French chalk yellowish colour.
Cold Fluid Method
This method should be used for the components, which cannot be readily dismantled. The method is same as previous one, except that the fluid is applied on the parts to be tested, cleaned and then the parts are painted with a mixture of methylated spirit and French chalk. When the spirit evaporates, the component can be examined for cracks by strains on the thin coating of French chalk.
There are three types of workshop method for crack detection, which are as follows:
(a) Dye Penetrant Crack Detection Method
(b) Magnetic Crack Detection Test
(c) Fluorescent Dye Penetrant Crack Detection Method
Dye Penetrant Crack Detection Method
This method is used to detect the flaws in ferrous and non-ferrous metals, a/c components, ceramics and plastics. Ardrox kit consists of three aerosol cans of fluid; one each of penetrant remover is contained in a wooden box. The penetrant is deep red liquid and is slow to evaporate. It has the ability to flow into any cracks or breaks in a surface. Having cleaned and dried the area, spray the film of thin dye penetrant on to the area with the spray gun provided. Leave this film for a period of 10 minutes to act. Re-spray again and leave for 20 minutes, in cold condition leave for one hour. After painting, remove all visible red traces. Dry the area with low air pressures. Now apply a thin film of developer on the treated area. The developer should be held about 8” to 10” away from the area. After 5 minutes examine, as the developer drives, a smooth white coating will form over the treated area. If flaws are present, they will appear as red traces in the white coating. Clean off all traces of developer with trichloroethylene and give the protective finish.
Magnetic Crack Detection Test
It is restricted to only ferrous metals and their alloys. The piece to be tested is magnetized, magnetic powder is sprinkled on the surface under testing. The excess powder is then dusted or blown-off. The remaining magnetic powder particles adhere to the cracks and to the outer line even after the blowing off operation.
Fluorescent Dye Penetrant Crack Detection Method
This method of testing for flaws and surface cracks is like the Penetrant Crack Detection Method but the component is placed in the wire basket and immersed in a tank of fluorescent solution for one to ten minutes, the time depending upon the material of the component. When testing metallic components, the temperature of the solution is maintained at 40ºC.After removal from the solution the component is allowed to dry for 3 to 4 minutes and then washed in running water. After washing for a further period of at least 4 minutes, it should elapse before the component is examined under a ultra-violet lamp in a dark room. The fluorescent solution remaining in the flaws or cracks will reveal these clearly as bright lines or patches of light.
There are four types of specialist method normally used for crack detection, which are as follows:
(a) Ultrasonic Testing
(c) Magnetic particle testing
(d) Eddy current- testing
(e) Microscopic Method
In this inspection, high frequency sound waves are sent into the object under test. The sound waves travel through the material. During their path of travel, they suffer loss of energy and are reflected at the interface. A receiver probe picks up the reflected wave and an analysis of this signal is done to locate flaws in the object under inspection. Sound waves follow the laws of optics in their propagation. Further the velocity of propagation of sound in various metals has been very accurately determined. The time taken by a sound pulse to travel through a material is a direct measure of the length of path traveled by it. In ultrasonic inspection, both through transmission and pulse echo techniques are used.
Most of the Ultrasonic Testing inspections are done in the mega cycle range at frequencies between 0.5 and 25 MHz, which is well above the audible range. The probes used for the inspection contain a piezoelectric crystal. The transmitter probe generates ultrasonic pulse and transmits them into the material while receiver probe receives the reflected pulse.
Ultrasonic Testing can detect defects such as cracks, laminations, shrinkages, cavities, flakes and other discontinuities reliably. This technique is widely used for quality control and material inspection in all major industries. It is used extensively for in service inspection in the nuclear and processing industry.
Radiography is nothing but formation of an image on fluorescent screens or photographic materials by short wave length radiations like X-rays, gamma rays to detect cracks, corrosion, and porosity in variety of materials.
It can be used to inspect most types of the solid materials both ferrous and nonferrous as well as materials and composites and is extensively used for castings, welding, forging and for internal flaw detection.
It is suitable for semiconductor devices for detection of cracks, broken wires, unsoldered connectors, foreign materials and misplaced components. However linear flaws are difficult to detect by this technique. Also compared with other NDT techniques radiography is expensive.
Living tissues absorb radiations liberated by ionization. It can result in damage or destruction of cells. Response of radiation exposure to tissues and organs of the body is called as radio sensitivity, which is proportional to reproductive capacity of the cells.
Radiation injury falls in two general categories:
(a) Prompt. Expressed in short time after absorbing radiations.
(b) Delayed. Potential delayed effects of radiations include genetic defects in off springs, increased risk of cancer.
In this method, a source of penetrating radiation is used. When these radiations pass through an object, they are differentially absorbed. Variations in the beam intensity are detected by a detector, normally a sheet of photographic film. On processing the film, a permanent image is formed. The developed film is called radiograph. Possible imperfections are indicated on the film as density changes. The commonly used sources are X-rays or gamma rays.
Radiography is widely used for the inspection of welding, castings assemblies and adhesive bonded structure, plastics, wood, composite materials and electronic components. This technique is best suited for detection of volumetric defects such as for AN-32 aircraft. Few critical components undergoing radiography are:
(a) Bilge beam
(b) Supporting strut
(c) Shackle, pipe, yoke of Engine mount.
Magnetic Particle Testing (MPT)
MPT is a sensitive test method used for the detection of surface and sub-surface defects in ferromagnetic materials and components. It is based on the principle that when a ferromagnetic material is placed in a magnetic field, it gets magnetized. The lines of forces through the material are continuous from one place to another. When any surface or sub surface defect is present, the magnetic field and associated lines of forces get deflected and forms a leakage field. This leakage field is present at and above the surface of the magnetized component. Its presence can be visibly detected by the utilization of finely divided magnetic particles.
The ‘magnetic indication’ so formed can be clearly seen and reveals the location, size and shape of the discontinuity. Magnetization of a component is accomplished using alternating current (AC), direct current (DC), Half Wave DC (HWDC). Magnetic particle application method includes:
(a) Dry method. In this method, dry magnetic particle powder is used.
(b) Wet method. In this method particles are suspended in water or oil base.
Detected discontinuity must be evaluated according to applicable acceptance criteria. Any discontinuity that exceeds the acceptance criteria must be rejected. Parameters of the crack considered for acceptance or rejection. If colored particles are used, inspection should be made under white light. If fluorescent particles are used, inspection should be made under black light (UV light).
After evaluation, material must be demagnetized otherwise it may affect the instrumentation of the ac system or subsequent manufacturing process. It is done through an alternate decreasing magnetic field or by placing the part in the field of an AC coil and withdrawing it slowly about 1.2 to 2 mm away.
MPT is very effective for the detection of surface and sub surface cracks, grinding cracks, heat treatment cracks, stringer type nonmetallic inclusions, porosities, laps and folds. The method can be applied to finished articles, billets, hot rolled bars, castings and forgings. Few critical components, which are being tested by MPT during overhaul of AN-32 aircraft, are as follows:
(a) Bolts of wing centre section
(b) Universal joints of folding struts
(c) Bolts, Brackets of NLG
(d) Bolts, Nuts, Washer of stabilizer
(e) Axle, Sprocket, Crank of Ramp
(f) Rod, Eye lug, Fork of Engine control linkage
Eddy Current- Testing (ECT)
This technique is based on induction of currents in electrically conducting material being inspected and observing the interaction between these currents and the materials. When magnetic flux through a conductor changes, induced currents are set up in closed paths on the surface of the conductor. These currents are in a direction perpendicular to the magnetic flux and are called eddy currents. When an alternating current is passed through the coil, a magnetic field is setup around it. The direction of magnetic field varies with each cycle of the alternating current. This is known as excitation field.
When a conductor is brought near this field, eddy currents are introduced in it. The direction of these eddy current changes with the direction of magnetic field during each cycle of AC. These eddy current produce their own magnetic field, which oppose the excitation field. Thus, the resultant field is changed which changes the coil impedance. This change in impedance is suitably displayed using a meter or a CRT screen.
The test coil also referred to as the probe is the main link between the test instrument and the test object and serves two main functions. The first is to establish a varying electromagnetic field which induces eddy currents within the test object and the second is to feed the response due to the electromagnetic field which induces eddy currents within the test object and the second is to feed the response due to the electromagnetic field to a signal analysis system. Various types of probes are available suitable for the inspection of flat surfaces and tubular products.
ECT is widely used to identify or differentiate between a wide variety of physical, structural and metallurgical conditions in conductive materials. It is used to measure or identify such properties as electrical conductivity, magnetic permeability, grain size, heat treatment conditions, hardness and physical dimensions. It is used to detect seam, laps cracks, voids and inclusions and other surface discontinuities in non-ferromagnetic materials and also to measure the thickness of a nonconductive coating on a conductive metal.
This technique is difficult to use on ferromagnetic materials. False indications are possible because of mixed variables, edge effects and lift off effects. Extensive technical knowledge is required for the development of inspection techniques; design of specific probes and to interpret the inspection data. Few critical areas, which are being tested by ECT during overhauling of AN-32 ac, are as follows:
(i) Area of LG actuating cylinder attachment
(ii) Wing center section spar
(iii) Aileron in board section
(iv) Slat fittings eye lugs
It is important to find the cause for potential failure of the component. Replica method is used to asses’ crack shape and size. Fig shows schematic representation of various types of cracks in a steel structure.
Each crack has its own characteristics and it is often possible to make a crack determination of crack type. It is important to know that the crack is original defect or caused due to surface conditions. Once the crack type is identified proper corrective action such as eliminating a corrosive environment or reducing the stress level can be taken.