Flaw detection and processes.
Inspection personnel should know where flaws occur or can be expected to exist and what effect they can have in each of the aerospace NDT test methods. Misinterpretation and/or improper evaluation of flaws or improper performance of aerospace NDT can result in serviceable parts being rejected and defective parts being accepted.
All NDT personnel should be familiar with the detection of flaws such as: corrosion, inherent flaws, primary processing flaws, secondary processing or finishing flaws, and in-service flaws. The following paragraphs classify and discuss the types of flaws or anomalies that may be detected by aerospace NDT. Visit our website to get free information about SalemNDT testing systems
a. Corrosion detection. This is the electrochemical deterioration of a metal resulting from chemical reaction with the surrounding environment. Corrosion is very common and can be an extremely critical defect. Therefore, NDT personnel may devote a significant amount of their inspection time to corrosion detection.
b. Inherent Flaws. This group of flaws is present in metal as the result of its initial solidification from the molten state, before any of the operations to forge or roll it into useful sizes and shapes have begun. The following are brief descriptions of some inherent flaws.
Primary pipe is a shrinkage cavity that forms at the top of an ingot during metal solidification, which can extend deep into the ingot. Failure to cut away all of the ingot shrinkage cavity can result in unsound metal. called pipe, that shows up as irregular voids in finished products.
Blowholes are secondary pipe holes in metal that can occur when gas bubbles are trapped as the molten metal in an ingot mold solidifies. Many of these blowholes are clean on the interior and are welded shut into sound metal during the first rolling or forging of the ingot. However, some do not weld and can appear as seams or laminations in finished products.
Segregation is a non-uniform distribution of various chemical constituents that can occur in a metal when an ingot or casting solidifies. Segregation can occur anywhere in the metal and is normally irregular in shape. However, there is a tendency for some constituents in the metal to concentrate in the liquid that solidifies last.
Porosity is holes in a material’s surface or scattered throughout the material, caused by gases being liberated and trapped as the material solidifies.
Inclusions are impurities, such as slag, oxides, sulfides, etc., that occur in ingots and castings. Inclusions are commonly caused by incomplete refining of the metal ore or the incomplete mixing of deoxidizing materials added to the molten metal in the furnace.
Cooling cracks can occur in casting due to stresses resulting from cooling, and are often associated with changes in cross sections of the part. Cooling cracks can also occur when alloy and tool steel bars are rolled and subsequently cooled. Also, stresses can occur from uneven cooling which can be severe enough to crack the bars. Such cracks are generally longitudinal, but not necessarily straight. They can be quite long, and usually vary in depth along their length.
Shrinkage cracks can occur in castings due to stresses caused by the metal contracting as it cools and solidifies.
c. Primary Processing Flaws. Flaws which occur while working the metal down by hot or cold deformation into useful shapes such as bars, rods, wires, and forged shapes are primary processing flaws. Casting and welding are also considered primary processes although they involve molten metal, since they result in a semi-finished product. The following are brief descriptions of some primary processing flaws:
Seams are surface flaws, generally long, straight, and parallel to the longitudinal axis of the material, which can originate from ingot blowholes and cracks, or be introduced by drawing or rolling processes.
Laminations are formed in rolled plate, sheet, or strip when blowholes or internal fissures are not welded tight during the rolling process and are enlarged and flattened into areas of horizontal discontinuities.
Flakes are internal ruptures that can occur in metal as a result of cooling too rapidly. Flaking generally occurs deep in a heavy section of metal. Certain alloys are more susceptible to flaking than others.
Forging laps are the result of metal being folded over and forced into the surface, but not welded to form a single piece. They can be caused by faulty dies, oversized dies, oversized blanks, or improper handling of the metal in the die. They can occur on any area of the forging.
Forging bursts are internal or external ruptures that occur when forging operations are started before the material to be forged reaches the proper temperature throughout. Hotter sections of the forging blank tend to flow around the colder sections causing internal bursts or cracks on the surface. Too rapid or too severe a reduction in a section can also cause forging bursts or cracks.
A hot tear is a pulling apart of the metal that can occur in castings when the metal contracts as it solidifies.
Cupping is a series of internal metal ruptures created when the interior metal does not flow as rapidly as the surface metal during drawing or extruding processes. Segregation in the center of a bar usually contributes to the occurrence.
A cold shut is a failure of metal to fuse. It can occur in castings when part of the metal being poured into the mold cools and does not fuse with the rest of the metal into a solid piece.
Incomplete weld penetration is a failure of the weld metal to penetrate completely through a joint before solidifying.
Incomplete weld fusion occurs in welds where the temperature has not been high enough to melt the parent metal adjacent to the weld.
Weld undercutting is a decrease in the thickness of the parent material at the toe of the weld caused by welding at too high a temperature.
Cracks in the weld metal can be caused by the contraction of a thin section of the metal cooling faster than a heavier section or by incorrect heat or type of filler rod. They are one of the more common types of flaws found in welds.
Weld crater cracks are star shaped cracks that can occur at the end of a weld run.
Cracks in the weld heat-affected zone can occur because of stress induced in the material adjacent to the weld by its expansion and contraction from thermal changes.
A slag inclusion is a nonmetallic solid material that becomes trapped in the weld metal or between the weld metal and the base metal.
Scale is an oxide formed on metal by the chemical action of the surface metal with oxygen from the air.
d. Secondary Processing or Finishing Flaws. This category includes those flaws associated with the various finishing operations, after the part has been rough-formed by rolling, forging, casting or welding. Flaws may be introduced by heat treating, grinding, and similar processes. The following are brief descriptions of some secondary processing or finishing flaws.
Machining tears can occur when working a part with a dull cutting tool or by cutting to a depth that is too great for the mate rial being worked. The metal does not break away clean, and the tool leaves a rough, tom surface which contains numerous short discontinuities that can be classified as cracks.
Heat treating cracks are caused by stresses setup by unequal heating or cooling of portions of a part during heat treating opera tions. Generally, they occur where a part has a sudden change of section that could cause an uneven cooling rate, or at fillets and notches that act as stress concentration points.
Grinding cracks are thermal type cracks similar to heat treating cracks and can occur when hardened surfaces are ground. The overheating created by the grinding can be caused by the wheel becoming glazed so that it rubs instead of cutting the surface; by using too little coolant; by making too heavy a cut; or by feeding the material too rapidly. Generally, the cracks are at right angles to the direction of grinding and in severe cases a complete network of cracks can appear. Grinding cracks are usually shallow and very sharp at their roots, which makes them potential sources of fatigue failure.
Etching cracks can occur when hardened surfaces containing internal residual stresses are etched in acid.
Plating cracks can occur when hardened surfaces are electroplated. Generally. they are found in areas where high residual stresses remain from some previous operation involving the part.
e. In-Service Flaws. These flaws are formed after all fabrication has been completed and the aircraft. engine, or related component has gone into service. These flaws are attributable to aging effects caused by either time, flight cycles. service operating conditions, or combinations of these effects. The following are brief descriptions of some in-service flaws.
Stress corrosion cracks can develop on the surface of parts that are under tension stress in service and are also exposed to a cor rosive environment, such as the inside of wing skins, sump areas, and areas between two metal parts of faying surfaces.
Overstress cracks can occur when a part is stressed beyond the level for which it was designed. Such overstressing can occur as the result of a hard landing, turbulence, accident, or related damage due to some unusual or emergency condition not anticipated by the designer, or because of the failure of some related structural member.
Fatigue cracks can occur in parts that have been subjected to repeated or changing loads while in service, such as riv eted lap joints in aircraft fuselages. The crack usually starts at a highly-stressed area and propagates through the section until failure occurs. A fatigue crack will start more readily where the design or surface condition provides a point of stress concentration. Common stress concentration points are: fillets; sharp radii; or poor surface finish, seams, or grinding cracks.
Unbonds, or disbonds, are flaws where adhesive attaches to only one surface in an adhesive-bonded assembly. They can be the result of crushed, broken, or corroded cores in adhesive-bonded structures. Areas of unbonds have no strength and place additional stress on the surrounding areas making failure more likely.
Delamination is the term used to define the separation of composite material layers within a monolithic structure. Ultrasonic testing is the primary method used for the detection of delamination in composite structures.