A Procedure for Verifying the Structural Integrity of an Existing Pressurized Wind Tunnel

This paper describes the application of material test, stress-fatigue-fracture mechanics analyses, nondestructive examinations and repairs to verify the structural integrity and the remaining cyclic life in a large pressurized wind tunnel (65,000 ft³) (1840 m³). The tunnel with pressures up to 135 psig (0.93 MPa) was constructed in 1940 and has been in service to the present date. The only record of a non-destructive examination conducted on the vessel prior to this evaluation was a hydrostatic test-pressure at 1 1/2 times the maximum working pressure. The material tests were performed on a sample of material (A-70 steel) cut from the tunnel shell. These tests included fracture toughness (R-curve, Kc) as determined from a compact tension specimen, crack growth rate (da/dn vs ΔK), Charpy V-notch, dynamic tear (from which the nil-ductility temperature was determined), and tensile and chemical tests. The results and applications are presented and discussed. Stress analyses include computer programs based on finite element and numerical integration techniques. Fatigue analyses incorporating a fatigue reduction or stress amplification factor to account for a small flaw existing in a weld are presented. Fracture mechanics analyses of the tunnel shell were performed for (1) the general membrane regions, (2) regions of high bending stress, and (3) areas at tunnel penetrations. The critical flaw sizes at each location are determined. The use of the “leak before break” criterion is discussed. The non-destructive examinations (radiograph, ultrasonic, sonic, and magnetic particle) to verify the assumptions of fatigue-fracture mechanics analyses and ASME Code applications are documented. Penetrations in the tunnel shell that were fatigue limited are shown “before” and “after” repair. The remaining cyclic life as obtained by the fatigue-fracture mechanics analyses and the operating envelope which resulted from these studies for metal temperature vs pressure was determined to be approximately 10 years.