Cracks As Reference Standards

T.O. 33B-1-1 4-63 4.5.7.1 Cracks As Reference Standards. When calibrating an eddy current instrument for detection of cracks, some means must be provided to assure that the sensitivity of the test system is sufficient to detect the smallest required crack size. Ideally, the best standard would be a section of the same material containing a crack of this minimum size. Cracks of specified sizes are difficult to obtain. With few specimens to choose from, such situations are rare. Fatigue cracks of specified size can be grown under laboratory conditions, but this method is extremely expensive. The length of the crack along the surface and its width at the surface are easily measurable. The depth of the crack is generally unknown and must be approximated or assumed from other data. Because of difficulty in obtaining actual cracks for reference standards, a number of other standards may be used. These standards are discussed below. 4.5.7.2 Requirements For Reference Standards. The primary requirement for eddy current reference standards is that they provide uniformity of response which can be related to the condition or material property to be detected or measured. Two fundamental ideas are assumed by uniformity of response. First, this means that all tests can be done with the same sensitivity or that different levels of sensitivity can be compared on a quantitative basis. Second, standards fabricated to a specific design should be stable devices able to provide a repeatable response within certain specified limits. To be useful for flaw size and type evaluation, the reference standard must relate to the flaw to be detected. By means of correlation data, prior history or investigation, the response from the reference standard must relate to the response from the condition or material property of the part. To permit fabrication of standards at a number of locations, material, alloy, temper and dimensional tolerances which will provide the required response should be defined in the applicable Technical Order for the test being performed. Methods of fabrication which utilize simple tools should be specified when adequate uniformity and sensitivity can be obtained. Ideally, when an instrument has been adjusted for a specified response from the standard, a signal of approximately the same amplitude and phase (where applicable) should be obtained from the condition or material property with an eddy current instrument and probe of the same general type. 4.5.7.3 Standards For Specific Tests. Calibration standards must be designed for the specific material property or condition being tested. Specific standards are required for each type of test being performed. Calibration standards used to sort alloys must meet very specific conductivity requirements. Calibration standards for measuring coating thickness of conductive coatings would not be suitable for measuring coating thickness of paint or other nonconductive coatings or for detecting cracks around rivet holes. Drilled holes or EDM (electrical discharge machining) notches in an aluminum block should not be used to test for material thickness or alloy composition of titanium or stainless steel parts. 4.5.7.4 Artificial Defects For Standards. Due to the difficulty of obtaining the types and sizes of real flaws in parts for use as calibration standards, a variety of artificial flaws have been developed to simulate the real flaws. Fatigue cracks have been grown under laboratory conditions, but reproducible sizes in sufficient quantity for standards are impractical. Artificial flaws, such as drilled holes, EDM notches, saw cuts, two surfaces clamped together to simulate a crack, or chemically produced conditions to simulate pits or corrosion, can be produced in a variety of ways. Ideally an artificial flaw will produce an eddy current response identical to the response from a real flaw of the same size, orientation, and location. This ideal is seldom achieved with artificial flaws. Estimation of flaw size from the response to artificial flaws must be based upon correlating previous known flaw sizes with the response from the artificial flaws. To maintain the quality of this correlation, it is necessary to carefully specify the material properties and fabrication process of the artificial defect standard. 4.5.7.5 Simulated Conditions For Standards. When using eddy current techniques to measure conductivity, coating thickness, permeability, alloy sorting, and hardness, standards can usually be obtained which represent the materials and conditions being tested. These standards are used for direct comparison to the response seen on the part being tested. Great care must be exercised in handling these types of calibration standards. Scratches, dents, distortion, oxidation, or other conditions can alter the calibration standards making them useless for comparison and calibration purposes. In order to protect the quality of reference standards, a primary standard and secondary standards will be utilized. The primary standards are usually maintained under laboratory storage conditions and may be traceable to the National Bureau of Standards, ASTM (American Society for Testing Materials) or similar agency. The secondary standard, are compared to the primary standard for