Upon arrival at the erection site, an AISI type 316L stainless steel tank intended for storage of fast breeder test reactor coolant (liquid sodium) exhibited cracks on its shell at two of four shell/nozzle fillet-welded joint regions. The tank had been transported from the manufacturer to the erection site by road, a distance of about 800 km (500 mi). During transport, the nozzles were kept at an angle of 45 deg to the vertical because of low clearance heights in road tunnels. The two damaged joints were unsupported at their ends inside the vessel, unlike the two uncracked nozzles. Surface examination showed ratchet marks at the edges of the fracture surface, indicating that loading was of the rotating bending type. Electron fractography using the two-stage replica method revealed striation marks characteristic of fatigue fracture. The striations indicated that the cracks had advanced on many “mini-fronts,” also indicative of nonuniform loading such as rotating bending. It was recommended that a support be added at the inside end of the nozzles to rigidly connect with the shell. In addition to avoiding transport problems, this design modification would reduce fatigue loading that occurs in service due to vibration of the nozzles during filling and draining of the tank.

Four truck cross members intended for use in heavy-duty transport trucks were investigated. Two of the members had cracked on a prototype vehicle and two had been fatigue tested in the laboratory. The cross members were fabricated from SAE 950X plate and consisted of a formed channel section and an internal fillet-welded diaphragm. Sections from each of the cross members were subjected to a complete analysis, including chemical analysis, magnetic particle testing, mechanical testing, scanning electron microscope/fractography, and metallography. The primary mode of failure was found to be fatigue cracking that initiated at the toes of the fillet welds. Secondary fatigue cracking occurred at the torque rod mounting holes. Failure was attributed to cyclic stresses at the weld toes that exceeded the lowered fatigue strength at this location. A design change that eliminated the fillet welds alleviated the problem.

The impeller of a 4 ft. diam extraction fan driven by a 120 hp motor at 1,480 rpm. disrupted suddenly. The majority of the vanes had become detached where they were welded to the plates. At other locations, separation of the vanes was accompanied by tearing of the adjacent plate, failure being initiated at the weld fillets of the inner end of the vanes. An unusual feature was that the blades disclosed regions having a pronounced striated and stepped appearance. The etched microstructure was typical of a low carbon rolled plate having the usual banded appearance. A cross section through the fillet welds and zone showed lamellar tearing, which confirmed that failure had occurred in weld metal adjacent to the fusion face of the fillet to the vane. Results of the investigation indicated that the primary cause of failure of the impeller was the development of fatigue cracks from the unwelded roots of the fillet welds, by which the vanes were attached to the supporting plates. The impeller would have shown increased resistance to fatigue crack initiation if the T joint between the vanes and plates had been of the full penetration type.