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Ndt students guide NON-DESTRUCTIVE TESTINGIntroduction to Non-Destructive Testing Many mechanical parts are subject to stresses and strains that may eventually lead to part failure In order to prevent the costly delays associated with equipment down- time

NON-DESTRUCTIVE TESTINGIntroduction to Non-Destructive Testing Many mechanical parts are subject to stresses and strains that may eventually lead to part failure In order to prevent the costly delays associated with equipment down- time many parts are tested for weaknesses and defects when machinery is dismantled for maintenance The procedures used are called non- destructive testing because aws may be detected without damaging the part Four types of non-destructive testing are routinely used magnetic particle inspection ultrasonic testing radiographic inspection and liquid penetrant testing OBJECTIVES ? Explain the purpose of non-destructive testing ? List four types of non-destructive tests ? Explain the importance of properly preparing parts to be tested Magnetic Particle Inspection Magnetic particle inspection techniques are frequently used to detect racks and aws in ferrous metal parts parts of iron or steel which can be Magnetized When the part is magnetized electrically aws in the metal part cause disturbances in the magnetic ?eld These disturbances can be detected by the pattern that is created when iron particles are dusted over the part Ultrasonic Testing Ultrasonic testing uses high-frequency sound waves to create a picture of the internal characteristics of parts The ultrasonic beam is re ected by defects within the part Indications are displayed on a screen similar to a sonar scope or on a digital readout The locations and sizes of aws can be determined by comparing the relative sizes and positions of pulses on the screen of an ultrasonic aw detector Radiographic Testing The technique of radiographic testing is similar to that of a medical xray Radiation passes through the material to be tested and registers an image on a piece of ?lm or a uorescent screen A trained examiner can determine the internal state of the material by inspecting the radio-graphic image just as a medical radiologist can diagnose a Cbroken bone from an x-ray Liquid Penetrant Testing Liquid penetrant testing is a commonly used method of detecting surface aws in all kinds of materials A dye is mixed with a thin liquid that readily penetrates surface cracks in the material The penetrant is applied to the part and allowed time to be drawn into the surface cracks After the excess dye is removed from the surface of the part the part is sprayed with a developer that draws the dye out of the cracks and makes it visible on the surface These visible indications show the sizes and locations of the cracks Preparing Parts for Non- Destructive Tests Non-destructive tests reveal the presence of aws or foreign matter in parts Accurate interpretations of test results frequently depend on the cleanliness of the part being tested All non-destructive tests involve comparative measurements so it is especially important that test conditions be as uniform as possible Rust dirt or corrosion can lead to false indications They might disguise a aw or indicate a defect where none actually exists To prevent these potential problems parts must be prepared by thorough cleaning prior to testing Solvents detergents or abrasive