Proper stress analysis during component design is imperative for accurate life and performance prediction. The total stress on a part is comprised of the applied design stress and any residual stress that may exist due to forming or machining operations. Stress-corrosion cracking may be defined as the spontaneous failure of a metal resulting from the combined effects of a corrosive environment and the effective component of tensile stress acting on the structure. However, because of the orientation dependence in aluminum, it is the residual stress occurring in the most susceptible direction that must be considered of primary importance in material selection for design configuration. A Navy UH-1N helicopter main rotor blade grip manufactured from a 2014-T6 aluminum alloy forging failed because of a design flaw that left a high residual tensile stress along the short transverse plane; this in turn provided the necessary condition for stress corrosion to initiate. A complete failure investigation to ascertain the exact cause of the failure was conducted utilizing stereomicroscopic examination, scanning electron microscopy, metallographic inspection and interpretation, energy-dispersive chemical analysis, physical and mechanical evaluation. Stereomicroscopic examination of the opened crack fracture surface revealed one large fan-shaped region that had propagated radially through the thickness of the material from two distinct origin areas on the internal diam of the grip. Higher magnification inspection near the origin area revealed a flat, wood-like appearance. Scanning electron microscopy divulged the presence of substantial mud cracking and intergranular separation on the fracture surface. Metallographic examination revealed intergranular cracking and substantial leaf separation along the elongated grains parallel to the fracture surface. Chemical composition and hardness requirements were found to be as specified. The blade grip failed due to a stress corrosion crack which initiated on the inner diam and propagated in the short transverse direction through the thickness of the component. The high residual tensile stress in the part resulting from the forging and exposed after machining of the inner diam, combined with the presence of moisture, provided the necessary conditions to facilitate crack initiation and propagation.

Parts of 21Cr-6Ni-9Mn stainless steel that had been forged at about 815 deg C (1500 deg F) were gas tungsten arc welded. During postweld inspection, cracks were found in the HAZs of the welds. Welding had been done using a copper fixture that contacted the steel in the area of the HAZ on each side of the weld but did not extend under the tungsten arc. In SEM examination, the cracks appeared to be intergranular and extended to a depth of approximately 1.3 mm (0.05 in.). The crack appearance suggested that the surface temperature of the HAZ could have melted a film of copper on the fixture surface and that this could have penetrated the stainless steel in the presence of tensile thermal-contraction stresses. The cracks in the weldments were a form of liquid-metal embrittlement caused by contact with superficially melted copper from the fixture and subsequent grain-boundary attack of the stainless steel in an area under residual tensile stress. The copper for the fixtures was replaced by aluminum. No further cracking was encountered.

A failure analysis case study on railroad rails is presented. The work, performed under the sponsorship of the Department of Transportation, addresses the problem of shell and detail fracture formation in standard rails. Fractographic and metallographic results coupled with hardness and residual stress measurements are presented. These results suggest that the shell fractures form on the plane of maximum residual tensile stresses. The formation of the shells is aided by the presence of defects in the material in these planes of maximum residual stress. The detail fracture forms as a perturbation from the shell crack under cyclic loading and is constrained to develop as an embedded flaw in the early stages of growth because the crack is impeded at the gage side and surface of the rail head by compressive longitudinal stresses.

Schedule 80 low-carbon steel pipes used to transfer kraft liquor in a Kamyr continuous pulp digester failed within 18 months after installation. Visual and metallographic examinations established that the cracking initiated on the internal surfaces of the equalizer pipes in the welds and heat-affected zones (HAZs). Fracture/crack morphology was brittle and primarily intergranular and deposits at crack tips were primarily iron oxides with significant amounts of sodium compounds. On these bases, the cracking was characterized as intergranular stress-corrosion cracking (IGSCC). Corrosion-related deterioration was not found, indicating that the material was generally suitable for the intended service. High residual tensile stresses in the welds and HAZS, resulting from field welding under highly constrained conditions using inadequate weld procedures, were the most probable cause of the failures. Minimizing residual stresses through use of welding procedures that include appropriate preweld and interpass temperatures and postweld stress relief heat treatment at 650 deg C (1200 deg F) was recommended to prevent further failures.

This report covers case histories of failures in fixed-wing light aeroplane and helicopter components. A crankshaft of AISI 4340 Ni-Cr-Mo alloy steel, heat treated and nitrided all over, failed in bending fatigue. The nitrided layer was ground too rapidly causing excessive heat generation which induced grinding cracks and grinding burn. Tensional stresses resulting from grinding developed in a thin surface layer. On another crankshaft, chromium plating introduced undesirable residual tensile stresses. Such plating is an unsatisfactory finish for crankshafts of aircraft engines. Aircraft engine manufacturers and aeronautical standards require magnetic particle inspection to detect grinding cracks after reconditioning. Renitriding after any grinding is needed also, regardless of the amount of undersize as it introduces beneficial residual compressive stresses.

Initial investigation showed that a landing gear failure was the result of a hard landing with no evidence of contributory factors. The objective of reexamination was to determine whether there was any evidence of metallurgical failure. The landing gear was primarily an AISI type 6150 Cr-V steel flat spring attached at the top end to the fuselage and at the bottom end to the axle. Failure occurred at the clamping point near the top end of this spring. The failure showed evidence of severe brinelling at one corner in the clamping area. The fracture surfaces were clean, fresh, and indicative of a shock type of failure pattern. Closer examination, however, showed a fatigue crack at one corner. At this point, there was definite evidence of progression and oxidation. It was concluded that the corner in question was subjected to repeated brinelling resulting from normal landing loads, probably accentuated by looseness in the clamping device. The resulting residual tensile stress lowered the effective fatigue strength at that point against drag and side loads.

Admiralty brass (Alloy C44300) cooling tubes which were part of a heat exchanger in a turbogenerator that provided electricity to a manufacturing plant failed. A mixture of non-recirculating city and “spring pit” water flowed through bundles of tubes to cool the oil in which they are immersed. However, a problem developed when several of the brass tubes cracked transversely, allowing cooling water to mix with the oil. The presence of a tensile stress, intergranular cracks, and a corrosion product suggested the tube failures resulted from stress-corrosion cracking. The main corrosion product was cupric hydroxychloride. In addition to switching to a more corrosion-resistant alloy, extreme care should be taken in the manufacturing of the replacement tube bundles to avoid imparting any residual tensile stresses in the tubing. Analyses of city and spring-pit water were recommended also, to determine which contained the least-harmful corrosive chemicals.

A controllable pitch propeller (CPP) on a dynamic positioning ship failed after eight months of operation. The CPP design consists of a hollow propeller shaft and a concentrically located pipe that operates inside. The pitch of the propeller blades is controlled hydraulically through the longitudinal displacement of the inner (concentric) pipe. Fractography, microstructural, microhardness, and chemical analyses revealed that the concentric pipe failed due to fatigue. Fatigue cracks initiated along longitudinal welds where wire spacers attach to the external surface of the pipe. The effect of crack-like defects, stress concentration at the weld toe, residual tensile stress, and lack of penetration contributed to a shorter fatigue crack initiation phase and premature failure.

To samples of helical compression springs were returned to the manufacturer after failing in service well short of the component design life. Spring design specifications required conformance to SAE J157, “Oil Tempered Chromium Silicon Alloy Steel Wire and Springs.” Each spring was installed in a separate heavy truck engine in an application in which spring failure can cause total engine destruction. The springs were composed of chromium-silicon steel, with a hardness ranging from 50 to 54 HRC. Chemical composition and hardness were substantially within specification. Failure initiated from the spring inside coil surface. Examination of the fracture surface using scanning electron microscopy showed no evidence of fatigue. Final fracture occurred in torsion. X-ray diffraction analysis revealed high inner-diameter residual stresses, indicating inadequate stress relief from spring winding. It was concluded that failure initiation was caused by residual stress-driven stress-corrosion cracking, and it was recommended that the vendor provide more effective stress relief.

The power-type counterbalance spring, formed from hardened-and-tempered carbon steel strip and subsequently subjected to phosphating treatment, fractured at the two locations during fatigue testing. A rust colored dark band at the inside edge of the fracture surface was disclosed during investigation. Etch pits were revealed by the cleaned surface which were never observed on properly phosphated coating. It was interpreted that the spring had been subjected to an abnormal acid attack in pickling or phosphating which had resulted in considerable absorption of hydrogen by the metal and hence embrittlement. The part was concluded to have cracked during phosphating or excessive acid pickling before phosphating.

An irrigation pipe made of medium-density PE failed during service. This pipe was subjected to severe cyclic-bending strain of the order of 6% while under tensile stress of approximately 6.9 MPa (1000 psi) and a hoop stress of approximately 6.2 MPa (900 psi), far more stringent conditions than those encountered in most applications of PE pipes. Visual inspection and reflected-light optical micrographs were used to plot bandwidth as a function of crack length. The conclusion was that, contrary to the dominant belief that pipe failure initiates from surface defects, a critical size flaw within the pipe wall can also initiate failure as it did in this case. Recommendations included that similarity criteria should be established between the fracture behavior of a component in service and that observed in the laboratory.

A steam jacketed autoclave of orthodox design was fabricated from mild steel for a working pressure of 320 psi. The only unusual feature in its construction was a protective layer of weld metal, which was deposited on the internal surface of the upper half of the 1 in. thick shell. The first indication of latent trouble was provided by the bolts which attached the stirring paddles to the shaft and the stationary scraper blades to the shell, either failing in service or breaking off when an attempt was made to remove them. It was the practice to renew them all annually. Microscopic examination of a failed bolt showed the path of the fracture and the secondary cracking associated with it were intergranular, suggesting that failure resulted from stress corrosion. A steel of the rimming type had been used to make the bar from which the bolt was forged. Cracks which originate at the root of threads generally result from fatigue but, in this instance, their intergranular mode of progression indicated that they were due to stress-corrosion. Examination of shell material showed that the cracks in the vessel were wholly intergranular. It was apparent from this evidence that this cracking was also due to stress-corrosion.

The bottom flange of a vertical pipe coupled to an isolating valve in a steam supply line to a turbine failed. Steam pressure was 1,500 psi and the temperature 416 deg C (780 deg F). Multiple cracking occurred in the bore of the flange. A quarter-segment was cut out and examined. The cracks were located in the part of the flange that formed a continuation of the pipe bore. The majority of them originated at the end of the flange bore and extended axially along the pipe and radially across the flange face. Magnetic crack detection revealed a further number of cracks in the weld deposit. While the fracture in the weld metal was of the ductile type exhibiting a fine fibrous appearance, that in the flange material was of the cleavage type. Microscopic examination revealed that the cracks were blunt-ended fissures of the type characteristic of corrosion-fatigue. It was concluded that cracking was due to corrosion-fatigue, which arose from the combined effect of a fluctuating tensile stress in the presence of a mildly corrosive environment.

Examination of several fighter aircraft main landing gear legs revealed unusual cracking in the hard chromium plating that covered the sliding section of the inner strut. The cracking was associated with cracks in the 35 NCD 16 steel beneath the plating. A detailed investigation revealed that the cracking was caused by the combination of incorrect grinding procedure, the presence of hydrogen, and fatigue. The grinding damage generated tensile stresses in the steel, which caused intergranular cracking during the plating cycle. The intergranular cracks were initiation sites for fatigue crack growth during service. It was recommended that the damaged undercarriage struts be withdrawn from service pending further analysis and development of a repair technique.

Coaxial cable connectors made of brass were failing at a high rate after less than one year of service in an outdoor industrial environonment. The observed failures, which consisted of cracks in the body and end cap, were analyzed and found to be brittle fractures due to stress-corrosion cracking. Two common stress-corrosion cracking tests for copper materials were conducted on new connectors from the same manufacturing lot, confirming the initial determination of the fracture mode. Additional testing as was done in the investigation is often helpful when analyzing corrosion failures.

A segment of a stainless steel clad bottom cone of an acid sulfite pulping batch digester failed from severe corrosion loss. The digester was fabricated of 19 mm ( 3 4 in.) low-carbon steel with 3.8 mm (0.15 in.) type 317L stainless steel cladding. The manufacturing method for the cladding was unknown. Visual and metallographic analyses indicated that the failure was from transgranular stress-corrosion cracking (TGSCC), which caused extensive cracking and spalling of the cladding and was localized in a segment of the bottom cone. The remainder of the digester cladding was unaffected. The TGSCC was attributed to high, locked-in residual stresses from the cladding process. It was recommended that the bottom cone replacement segment be stress relieved prior to installation.

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.

Mechanical springs are used in mechanical components to exert force, provide flexibility, and absorb or store energy. This article provides an overview of the operating conditions of mechanical springs. Common failure mechanisms and processes involved in the examination of spring failures are also discussed. In addition, the article discusses common causes of failures and presents examples of specific spring failures, describes fatigue failures that resulted from these types of material defects, and demonstrates how improper fabrication can result in premature fatigue failure. It also covers failures of shape memory alloy springs and failures caused by corrosion and operating conditions.