Several deburring drums that fractured were filled with abrasive, water, and small parts, such as roller bearing rollers, and rotated on their axis at 36 rpm. Cracks were discovered very early in the service lives of these high-chromium white iron cast structures. All of the fractures were through bolt holes in the mounting flange. The holes had a sharp edge and exhibited uneven wear on the inside diameter. In operation, the mounting bolts were frequently found to be loose and in at least one case broken off. A 25x scanning electron microscopy (SEM) fractograph from near this fracture-initiation area showed fatigue striations. No casting or metallurgical structural defects were found that could explain the failures. This evidence supports the conclusion that cracking was a result of the stress-concentration site at the bolt holes where a fatigue-initiated fracture occurred. Recommendations included that the radii be increased at the sharp corners and that lock-wiring be used to secure against bolt loosening.

New aircraft wing panels extruded from 7075-T6 aluminum exhibited an unusual pattern of circular black interrupted lines, which could not be removed by scouring or light sanding. The panels, subsequent to profiling and machining, were required to be penetrated inspected, shot peened, H2SO4 anodized, and coated with MIL-C-27725 integral fuel tank coating on the rib side. Scanning electron microscopy and microprobe analysis (both conventional energy-dispersive and Auger analyzers) showed that the anodic coating was applied over an improperly cleaned and contaminated surface. The expanding corrosion product had cracked and, in some places, had flaked away the anodized coating. The corrodent had penetrated the base aluminum in the form of subsurface intergranular attack to a depth of 0.035 mm (0.0014 in.). It was recommended that a vapor degreaser be used during cleaning prior to anodizing. A hot inhibited alkaline cleaner was also recommended during cleaning prior to anodizing. The panels should be dichromate sealed after anodizing. The use of deionized water was also recommended during the dichromate sealing operation. In addition, the use of an epoxy primer prior to shipment of the panels was endorsed. Most importantly, surveillance of the anodizing process itself was emphasized.

Practical examples of stress-corrosion cracking (SCC) and methods for its prevention were presented. Cracks in chloride-sensitive austenitic steels were very branched and transcrystalline. Etched cross sections of molybdenum-free samples showed chloride-induced cracks running out of the pitted areas. Alternatively polishing and etching micro-sections for viewing at high magnification made crack detail more visible. Optical and scanning electron micrographs showed cracking in austenitic cast steel and cast iron due to both internal tensile and critical residual stresses; the latter causes flake-like spalling. Measures to prevent SCC include stress reduction, use of austenitic steels or nickel alloys not susceptible to grain boundary attack, use of ferritic chromium steels, surface slag removal, control of temperature and chloride concentration, and cathodic protection.

An exhaust diffuser assembly failed prematurely in service. The failure occurred near the intake end of the assembly and involved fracture in the diffuser cone (Corten), diffuser in take flange (type 310 stainless steel), diffuser exit flange (type 405 stainless steel), expansion bellows (Inconel 600), and bellows intake flange (Corten). Individual segments of the failed subassemblies were examined using various methods. The analysis indicated that the weld joint in the diffuser intake flange (type 310 stainless steel to Corten steel) contained diffusion-zone solidification cracks. The joints had been produced using the mechanized gas-metal arc welding process. Cracking was attributed to improper control of welding parameters, and failure was attributed to weld defects.