Nozzle Quality Assurance
Nozzle quality is assured by good process controls, nondestructive testing, destructive sample testing of components, and leak testing of the nozzle assembly. In addition, movable nozzles are vectored while pressurized.
Nondestructive testing such as X-ray, alcohol wipe, ultrasonic, dye penetrant, and hardness testing generally is performed on all components. Reinforced-plastic composites usually are subjected to X-ray (tangential) and alcohol wipe; no cracks are allowed, and delaminations and voids are limited to specific sizes depending on part size and location of the defect. Graphites normally are subjected to 100-percent X-ray inspection and 100-percent dye-penetrant inspection; no cracks are allowed, foreign inclusions are limited to few in number with none penetrating the surface and none in a region that erosion might reach, and voids are limited to six times the average natural void size. Elastomeric materials are checked for hardness and subjected to X-ray; no voids, delaminations, or foreign inclusions are allowed. Metal components are normally subjected to dye penetrant, magnetic particle, X-ray, or ultrasonic test, with no cracks allowed. All critical welds are subjected to 100-percent X-ray inspection.
Destructive sample testing usually is accomplished by one of three methods: (1) random selection and destructive testing of one actual component of every so many produced (1 of 20 is typical); (2) destructive testing of an excess part of the component or "tag end"; or (3)
destructive testing of an extra test slab of component material that has been processed with a batch of components (e.g., hydroclaved with them or heat treated with them). Method (1) has the advantage that an actual part is tested; it has the disadvantages of expense and the statistical probability that a part not tested is bad. Method (2) has the advantage that a test is conducted for each part, but it is questionable that the tag end is truly representative of the part; end effects in processing may change the properties. Method (3) likewise is questionable as to whether the test slab is truly representative. Method (1) is practical only in production quantities; therefore, methods (2) and (3) normally are used in concept-demonstration programs. If factors of safety are large, destructive testing often is not included in test and development programs.
The destructive tests usually performed on plastics include density, volatile content, resin content, acetone-soluble content, tensile strength and modulus, compressive strength and modulus, interlaminar shear, hardness, flexure strength, and modulus. Tests for density, compressive strength, and tensile strength are performed on graphites. Metals are checked for density, hardness, tensile and compressive strength, modulus, percent elongation, and percent reduction of area. Metal quality assurance is covered in more detail in reference 81.
Leak testing of nozzle assemblies usually is conducted as part of a motor leak check at pressures typically in the range of 30 to 50 psia. The throat is plugged for this test. Nozzles sometimes are tested at full pressure prior to installation on the motor. Movable nozzles are vectored while at full pressure to ensure sealing by the dynamic O-ring and to measure dry torque to check for abnormal values (ref. 2).
Reference 134 reports tests of the effects of selected defects on nozzle performance.
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