Plunger shafts machined from 4150 steel bar stock were involved in a series of fatigue failures. The fractures consistently occurred at two locations on the shafts: the shaft fillet and either side of a machined notch. The material specification for the shafts required 41xx series steel with a carbon content of 0.38 to 0.53%, a hardness of 35 to 40 HRC for the shaft, and a hardness of 50 to 55 HRC for the notch (which was case hardened). Analysis (visual inspection, chemical analysis, hardness testing, and magnetic particle inspection) supported the conclusions that all the fractures were fatigue-induced failures due to sharp radii in the fillets. The stress-concentrating effects of the fillets caused fatigue cracks to initiate and grow under cyclic loading until the crack depth was critical, causing the shaft to fail and rendering the assembly inoperative. Recommendations included increasing the radii of the notch and shaft fillets. If fatigue cracking had continued to be a problem with this component, shot peening of the subject radii would be appropriate. This process produces residual compressive stresses in the surface of the part, thereby retarding initiation of fatigue cracks.

After several years' use, an eyebolt suffered brittle fracture in the first turn of the thread. The fracture started at the notch at the root of the thread. Neither localized material defect nor an old crack were present. The investigation showed that instead of the specified steel quality St 37-2 N, a steel with about 0.5% C had been used. The microstructure with the coarse ferrite network indicated that the forged eye bolt had been normalized either at too high a temperature or not at all. In any case the anneal at 900 deg C produced a considerably more finely grained structure. In addition, the nature of the fracture and the results of the notched bar impact tests showed that in spite of the high C-content, the eye bolt had become brittle as a result of aging.

Several thousands of new 16 mm diam alloy steel sling chains used for handling billets failed by chain-link fractures. No failures were found to have occurred before delivery of the new chains. It was observed that the links had broken at the weld. It was found that all failures had occurred in links having hardness values in the range of 375 to 444 HRB. It was revealed by the supplier that the previous hardness level of 302 to 375 HRB was increased to minimize wear which made the links were made notch sensitive and resulted in fractures that initiated at the butt-weld flash on the inside surfaces of the links. A further reduction in ductility was believed to have been caused by lower temperatures during winter months. Thus, the failure was concluded to have been caused in a brittle manner caused by the notch sensitivity of the high hardness material at lower temperatures. The chains were retempered to a hardness of 302 to 375 HRB as a corrective measure and subsequently ordered chains had this hardness as a requirement.

Three bearing bosses from the cover of scrap shears were sent in for examination. They had torn off the base plate to which they had been welded by fillet welds all around. Two of these were examined. They showed entirely the same symptoms. The bosses had broken away on three sides along the welds. The cleaved fractures in the burned notches propagated partially above and partially below several incipient cracks which may have been fatigue fractures. Metallographic sections showed that the fractures had occurred either at the burned notches near the transition from the weld to the sheet, or else they ran in the sheet material next to the weld. The quality of the welds could not be judged because the opposite fracture pieces to which they adhered had not been sent in. It was concluded that the breakaway of these bosses was at least favored by overheating and hardening.

A reversible four-way carbon steel flap valve in a thermal power station failed after 7 years of service. The flap had been fabricated by welding two carbon steel plates to both sides of a carbon steel forging. The valve was used for reversing the flow direction of seawater in the cooling system of a condenser. Visual examination of the flap showed crystalline fracture, indicating a brittle failure. Metallographic examination, chemical analyses, and tensile and impact testing indicated that the failure was caused by the notch sensitivity of the forging material, which resulted in low toughness. It was recommended that fully killed carbon steel with a fine-grain microstructure be used. Redesign of the flap to remove the step in the forging that acted as a notch was also recommended.

An anchorage plate which fractured was one of a pair used as intermediate members through which the boom suspension ropes were attached to the jury-mast of an excavator. Failure of the plate released the ropes on one side of the boom, resulting in extensive damage to the latter and also bending of the other anchorage plate. The anchorage plates were 23 x 9 in. and had been flame-cut from mild steel plate. Collars were fillet-welded on each side at both ends to provide extra bearing area for the pins. Holes had then been flame-cut slightly under size and bored to final dimensions. The plates were given a slight set after flame-cutting to provide a more direct line of pull for the ropes. The fracture surface was bounded by narrow lips, indicative of shear failure. Failure of the anchorage plate was attributed to cracks present at the junctions of the fillet welds, and deficient notch-ductility of the material from which the plates were made.

The US Army Research Laboratory performed a failure investigation on a broken main landing gear mount from an AH-64 Apache attack helicopter. A component had failed in flight, and initially prevented the helicopter from safely landing. In order to avoid a catastrophe, the pilot had to perform a low hover maneuver to the maintenance facility, where ground crews assembled concrete blocks at the appropriate height to allow the aircraft to safely touch down. The failed part was fabricated from maraging 300 grade steel (2,068 MPa [300 ksi] ultimate tensile strength), and was subjected to visual inspection/light optical microscopy, metallography, electron microscopy, energy dispersive spectroscopy, chemical analysis, and mechanical testing. It was observed that the vacuum cadmium coating adjacent to the fracture plane had worn off and corroded in service, thus allowing pitting corrosion to occur. The failure was hydrogen-assisted and was attributed to stress corrosion cracking (SCC) and/or corrosion fatigue (CF). Contributing to the failure was the fact that the material grain size was approximately double the required size, most likely caused from higher than nominal temperatures during thermal treatment. These large grains offered less resistance to fatigue and SCC. In addition, evidence of titanium-carbo-nitrides was detected at the grain boundaries of this material that was prohibited according to the governing specification. This phase is formed at higher thermal treatment temperatures (consistent with the large grains) and tends to embrittle the alloy. It is possible that this phase may have contributed to the intergranular attack. Recommendations were offered with respect to the use of a dry film lubricant over the cadmium coated region, and the possibility of choosing an alternative material with a lower notch sensitivity. In addition, the temperature at which this alloy is treated must be monitored to prevent coarse grain growth. As a result of this investigation and in an effort to eliminate future failures, ARL assisted in developing a cadmium brush plating procedure, and qualified two Army maintenance facilities for field repair of these components.

Two type 316L stainless steel orthopedic screws broke approximately 6 weeks after surgical implant. The screws had been used to fasten a seven-hole narrow dynamic compression plate to a patient's spine. The broken screws and screws of the same vintage and source were examined using macrofractography, SEM fractography, and hardness testing. Fractography established that fracture was by fatigue and that the fatigue cracking originated at corrosion pits. Hardness while below specification, still indicated that the screws were in the cold-worked condition and notch sensitive during fatigue loading. Use of a steel with a higher molybdenum content (317L) in the annealed condition was recommended.

A bolt breaks along a change in cross section well below its rated capacity. An anchoring screw spins freely in place, having snapped at its first supporting thread. A motor unexpectedly disengages its load, its driveshaft having fractured near a keyway. Such failures – involving axles, leaf springs, engine rods, wing struts, bearings, gears, and more – can occur, seemingly without cause, due to vibrational fracture. Vibrational fractures begin as cracks that form under cyclic loading at nominal stresses which may be considerably lower than the yield point of the material. The fracture is proceeded by local gliding and the development of cracks along lattice planes favorably orientated with respect to the principal stress. This non-reversible process is often misleadingly called “fatigue” and presents significant challenges to engineering teams that ill-advisedly take to searching for material faults. Several examples of notch-induced vibrational fractures are presented along with guidelines for investigating their cause.

A stainless steel pipe transferring hot white liquor solution of sodium hydroxide and sodium sulfite, developed leaks adjacent to the welds within four years of service. The stainless steel pipe was AISI type 304 and welded with E308 weld electrodes. The service temperature was 190 deg C (375 deg F) and the solution contained approximately 700 ppm chlorides. Liquid penetrant inspection of the pipeline showed the leaks were numerous and confined adjacent to the welds. A metallographic specimen from the circumferential weld showed the cracks initiated at the inside surface. In addition to the base metal, SCC also had initiated at a notch at the weld root due to improper welding procedures. Failure was attributed to chloride-induced SCC with secondary contributory factors, including improper welding procedures. It was recommended that the pipeline be replaced with a material more resistant to SCC. The candidate materials are commercial grade unalloyed titanium or Inconel 600, which have superior resistance to SCC compared to austenitic stainless steels.

A combination of adverse factors was present in the disruption of a turbo-alternator gearbox. The major cause was the imposition of a gross overload far in excess of that for which the gearbox was designed. The contributory factors were a rim material (EN9 steel) that was inherently notch-sensitive and liable to rupture in a brittle manner. Discontinuities were present in the rims formed by the drain holes drilled in their abutting faces, and possibly enhanced by the stress-raising effect of microcracks in the smeared metal at their surfaces It is probable that the load reached a value in excess of the yield point within the delay time of the material so when the fracture was initiated, it was preceded by several microcracks giving rise to the propagation of a brittle fracture.

A steel socket pipe conduit NW 150 cracked open during pressure testing next to the weld seam almost along the entire circumference. The crack occurred in part in the penetration notch and in part immediately adjacent to it. While the uncracked pipe showed the light etch shading of a low-carbon steel in which the zone heated during welding was delineated only slightly next to the seam, the other pipe was etched much darker, i.e., higher in carbon, and the heated zone appeared to stand out darkly against the basic material. The overlapping weld was defect-free and dense. The uncracked pipe consisted of soft steel that obviously was made for this purpose, while the cracked pipe consisted of a strongly-hardenable steel which contained not only more carbon and manganese than customary but also a considerable amount of chromium. Therefore, the damage was caused by a mix-up of materials that allowed an unsuitable steel to be used for the weldment.

A crane hook was stamped S.W.L. 3 tons and, while its main dimensions were in approximate accordance with those specified in B.S. 482 for a hook of this capacity, its shape in some respects was not exactly in conformity with that recommended. At the time of fracture, the load being lifted was slightly under 10 cwts. Fracture occurred away from the normal wearing surface where the hook makes contact with the lifting slings. There was no evidence that fracture was preceded by any appreciable deformation locally or in the region of the failure. A sulphur print, taken on a cross section of the hook adjacent to the plane of fracture, showed the hook was made from a killed steel free from major segregation. Microscopic examination showed the material to be a mild steel in the normalized condition, the carbon content being of the order of 0.25%. Bend tests showed the material at the intrados of the hook would deform in a ductile manner both under slow and impact-loading conditions if in the form of an unnotched test piece, but if notched, it failed in a brittle manner under impact, though not under slow loading.

A forged, cadmium-plated electroslag remelt (ESR) 4340 steel mixer pivot support of the rotor support assembly located on an Army attack helicopter was found to be broken in two pieces during an inspection. Visual inspection of the failed part revealed significant wear on surfaces that contacted the bushing and areas at the machined radius where the cadmium coating had been damaged, which allowed corrosion pitting to occur. Optical microscopy showed that the crack origin was located at the machined radius within a region that was severely pitted. Electron microscopy revealed that most of the fracture surface failed in an intergranular fashion. Energy dispersive spectroscopy determined that deposits of sand, corrosion and salts were found within the pits. The failure started by hydrogen charging as a result of corrosion, and was aggravated by the stress concentration effects of pitting at the radius and the high notch sensitivity of the material. The failure mechanism was hydrogen-assisted and was most likely a combination of stress-corrosion cracking and corrosion fatigue. Recommendations were to improve the inspection criteria of the component in service and the material used in fabrication.

Numerous flaws were detected in a steam turbine rotor during a scheduled inspection and maintenance outage. A fracture-mechanics-based analysis of the flaws showed that the rotor could not be safely returned to service. Material, samples from the bore were analyzed to evaluate the actual mechanical properties and to determine the metallurgical cause of the observed indications. Samples were examined in a scanning electron microscope and subjected to chemical analysis and several mechanical property tests, including tensile, Charpy V-notch impact, and fracture toughness. The material was found to be a typical Cr-Mo-V steel, and it met the property requirements. No evidence of temper embrittlement was found. The analyses showed that the observed flaws were present in the original forging and attributed them to lack of ingot consolidation. A series of actions, including overboring of the rotor to remove indications close to the surface and revision of starting procedures, were implemented to extend the remaining life of the rotor and ensure its fitness for continued service.

A large crack developed at a girder-truss joint area of the Fremont bridge in Portland, OR, on 28 Oct 1971. It occurred during a positioning procedure involving a junction piece welded to a girder, starting as a brittle fracture and terminating in plastic hinges in the girder web welds. The arch rib top plate, as it met the main girder, formed a composite beam of A588/A36 composition. Investigation showed the original design of the failed component called for an angle of high geometric stress concentration (90 deg with no radius) in a region of substantial transverse weld joints. While the material met chemical and mechanical property requirements, tests showed it had low fracture toughness and critical-sized flaws oriented normal to the principal stress in the failed junction piece. Fabrication procedures resulted in high residual stresses and a metallurgical notch at the radius in the junction piece. Stresses induced during jacking (the procedure used to raise bridge components into position) applied the stresses in the critical radius that triggered the cracking.

Two cases of failure of centrifuge baskets were investigated. The first involved a centrifuge running at approximately 1000 rpm. The basket was constructed from a perforated sheet of stainless steel rolled into a cylinder and joined by a single vee longitudinal weld. Detailed examination showed the weld had not completely penetrated the full depth of the section. The fracture faces showed a gradually progressing fatigue crack developing from a notch, formed by the lack of penetration, at the root of the weld. Microscopic examination of the parent plate showed it was a typical titanium stabilized austenitic steel. It is probable that had the basket been subjected to a periodic inspection by a competent person, this failure would not have occurred. The second case concerned a continuous duty centrifuge operating at 2200 rpm. Fracture had occurred at the circumferential weld attaching the stainless steel skirt to the basket rim and also in the region of the vertical weld which was made when the skirt was formed into a cone. Stress-corrosion cracking of the skirt material, which contained residual stresses due to cold-rolling, had been caused by the presence of sodium chloride.

A cooler of an ammonia synthesis plant was destroyed after three years of service due to the rupture of a distribution manifold. Synthesis gas under high pressure and at about 300 deg C, consisting of 10% NH3 and unconverted gas of 25% N2 and 75% H2 content, was water-cooled externally to room temperature in this unit. The fracture had the typical flat-gray fibrous structure of a material destroyed by hydrogen. Specimens for the metallographic investigation showed that the structure appeared to have been loosened by intergranular separations. DVM notched impact specimens from the affected area yielded low specific impact energy values. These are the significant characteristics of hydrogen attack. The attack penetrated to a depth of 13 to 16 mm. It was recommended that the manifolds be made of hydrogen-resistant steel instead of the unalloyed steel used.

Failure analysis was performed on threaded Ti-6Al-4V fasteners that had fractured in the threads during installation. Scanning electron microscopy (SEM) and optical metallography revealed that the fractures initiated in circumferential shear bands present at the thread roots. The fractures propagated by microvoid coalescence typical of that observed in notched tensile specimen fractures of the same material. For comparison, Ti-6Al-4V fasteners from various commercial sources were tested to failure in uniaxial tension and examined in the SEM. In all cases, the fracture appearances were similar to that exhibited by the fasteners that failed during installation. In addition, results of optical microscopy indicated that the geometry and extent of the shear bands appeared to depend on the fabrication process employed by the individual manufacturers. Causes of shear band formation are discussed along with potential methods to eliminate these microstructural in homogeneities.