Nondestructive Testing Inspection Plan
In each new motor program, there are normally two methods for controlling motor case reliability. One method is to accumulate and analyze data on failures that occur during hydrostatic proof tests and motor static and service firings, then modify the design according to the results of the failure analysis. The other approach, used in combination with the method above, is to employ a detailed and comprehensive program of material and fabrication-process control throughout material procurement and fabrication. This program permits detection of potential causes of failure and the timely repair and correction of these areas. Proper inspection processes are key factors in the success of this approach.
Some pertinent thoughts on the development of a successful inspection plan, or nondestructive testing (NDT) plan, as it is sometimes called, are outlined in six steps in reference 123. Briefly, these six steps are as follows:
(1) Determine the types of defects that require detection
(2) Evaluate existing inspection (NDT) techniques for sufficient sensitivity and accuracy or develop new acceptable or adequate technique
(3) Verify that the inspection techniques obtain a valid indication or description of the actual defects
(4) Establish accept-reject standards for each type of defect and each inspection technique
(5) Establish an inspection plan
(6) Eliminate any redundant inspection as knowledge and experience are gained during case development and production
Additional guidelines in the development of a motor case NDT plan are given in reference 32, pp. 98-104.
2.5.2.2 Inspection Processes
Reference 124 presents the basic principles of inspection (nondestructive testing), expected results, and typical applications.
Current practice shows that radiographic, ultrasonic, magnetic-particle, and dye-penetrant inspections are commonly used in various degrees, depending on program requirements and the effect of a case failure on cost and schedule. The advantages and disadvantages of these inspection techniques (refs. 95 and 125) are summarized in the following paragraphs.
Radiographic.—Radiographic inspection has been used extensively in the past and is the most definitive inspection technique. Visual presentation of the flaw size and shape is obtained on a permanent record. Limitations exist in determining the actual depth of the defect and in detecting tight, cracklike defects adversely oriented (plane of minimum density-change perpendicular to the X-ray).
Ultrasonic.—Ultrasonic inspection is the most sensitive method of detecting thin, cracklike subsurface defects. The shear-wave method is sensitive to defects perpendicular and nearly perpendicular to the surface, and the longitudinal method is sensitive to defects parallel and nearly parallel to the surface. Limitations of the inspection process exist in areas of rapid dimensional change, where irregularities in the defect surface or its attitude with respect to the sound waves may decrease or mask the indication, and in the inadvertent failure of the operator to observe the defect indications. Also, limitations sometimes exist in the interpretation of indications for use with accept-reject criteria in the inspection of welds, particularly of welds with weld crowns.
Magnetic Particle.—The magnetic-particle inspection process is useful for inspection of the surface and near-surface. The wet inspection process is the most sensitive; however, the dry process is sometimes used when the inspection of heated parts is required (e.g., weld with preheating maintained). The inspection process is not useful on nonmagnetic materials including areas of retained austenite (generally associated with areas of multiple weld repair) in the nickel maraging steels. Also, magnetic leakage can occur at areas of sharp contour change.
Dye Penetrant.—Dye-penetrant inspection is a very rapid process for surface inspection and is particularly useful with nonmagnetic materials and for spot-check inspections. Limitations exist in the interpretation of small indications.
All of the above inspection methods are sensitive to surface finish and treatment, and to the skill and alertness of the operators (ref. 32, pp. 95-104).
Results of a cooperative nondestructive testing (inspection) program to evaluate NDT sensitivity are given in reference 95. Test plates 0.7 in. thick were prepared with fatigue cracks. After cracking, the faces of the plates were ground to remove any trace of the crack starter notches. The plates were then inspected using the ultrasonic and radiographic inspection techniques of two case fabricators, two Navy laboratories, and the NASA/Lewis Research Center. The study showed that—
(1) Radiographic inspection consistently failed to detect flaws 0.063 in. deep by 0.155 in. long and smaller;
(2) The largest flaw, 0.105 in. deep by 0.309 in. long, was not consistently detected by radiography; and
(3) Using ultrasonic inspection procedures with normal production sensitivity, flaws 0.063 in. deep by 0.155 in. long and smaller were not consistently detected.
2.5.2.3 Hydrostatic Proof Test
Hydrostatic proof tests of one or more cycles are accomplished to demonstrate operational structural integrity of the rocket motor case.
The significance of the proof test is adequately discussed in reference 126. The proof-test concept is based on the premise that the motor case that is proof tested at a pressure higher than the motor operating pressure cannot have any detrimentally oriented flaws greater than the critical flaw size at the proof-stress level. This concept is discussed in detail in references 28, 30, 61, 126, and 127.
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