Both rods in a Harrington rod cervical stent failed after a short time in service. Metallurgical analysis revealed a significant number of notches as well as enlarged grain size in one of the two rods, rough shallow-cracked surfaces along the bend profiles, possible signs of corrosion, and fractures (on both rods) near indentations imparted by retaining clamps. The observations suggest that surface roughness and bending defects initiated cracking that led to the fatigue failure of the compromised rod, followed some time later by the overload fracture of the second rod.

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.

In January 1965, a Reaction Control System (RCS) pressure vessel (titanium alloy Ti-6Al-4V) on an Apollo spacecraft cracked in six adjacent locations. It used N2O4 for vehicle attitude control through roll, pitch, and yaw engines, and was protected from the N2O4 by a Teflon positive expulsion bladder. Investigation (visual inspection, pressure testing of 10 similar vessels, and chemical testing of the N2O4) supported the conclusion that the failure was due to stress corrosion from the N2O4, and specifically from a specification change in the military specification MIL-P-26539. Recommendations included revising the specification to require a minimum NO content of 0.6%.

During an acceptance test of the Apollo spacecraft 101 service module prior to delivery, an SPS fuel pressure vessel (SN054) (titanium Ti-6Al-4V, approximately 1.2 m (4 ft) in diam and 3 m (10 ft) long) containing methanol developed cracks adjacent to the welds. The test was stopped. This acceptance test had been run 38 times on similar pressure vessels without problems. The methanol was a safe-fluid replacement for the storable hypergolic fuels (blend of 50% hydrazine and 50% unsymmetrical dimethyl hydrazine). Investigation (visual inspection and 65X images) showed similarities to stress-corrosion resulting from contamination during misprocessing of the vessels. However, another vessel underwent a more severe testing procedure and failed catastrophically. Further investigation supported the conclusion that the failure cause was SCC of titanium in methanol. Attack is promoted by crazing of the protective oxide film. It was learned that minor changes in the testing procedures could inhibit or accelerate the reaction. Recommendations included replacing the methanol with a suitable alternate fluid. Isopropyl alcohol was chosen after considerable testing. This incident further resulted in the imposition of a control specification (MF0004-018) for all fluids that contact titanium for existing and future space designs.

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.

A Lynx helicopter from the Royal Netherlands Navy lost a rotor blade during preparation for take-off. The blade loss was due to failure of a rotor hub arm by fatigue. The arm was integral to the titanium alloy rotor hub. An extensive material based failure analysis covered the hub manufacture, service damage, and estimates of service stresses. There was no evidence for failure due to poor material properties. However, fractographic and fracture mechanics analyses of the service failure, a full scale test failure, and specimen test failures indicated that the service fatigue stress history could have been more severe than anticipated. This possibility was subsequently supported by a separate investigation of the assumed and actual fatigue loads and stresses.

TiN coated back surgery wires were made of Ti-6Al-4V. The reported failure was the presence of pits located in the uncoated area of the wires. The uncoated area of the wire is where the wire is fixtured in the coating chamber during coating. Examination and analysis of the pits using SEM/EDX detection unit revealed significant peaks of B, O, Zr and Fe. Moreover, the shape of the pits was similar to an arc crater. The formation of pits in the wire was caused during coating due to microarcing. A contaminated fixture used during the coating most likely caused the microarcing.

This paper summarizes several cases of metallurgical failure analysis of surgical implants conducted at the Laboratory of Failure Analysis of IPT, in Brazil. Investigation revealed that most of the samples were not in accordance with ISO standards and presented evidence of corrosion assisted fracture. Additionally, some components were found to contain fabrication/processing defects that contributed to premature failure. The implant of nonbiocompatible materials results in immeasurable damage to patients as well as losses for the public investment. It is proposed that local sanitary regulation agencies create mechanisms to avoid commercialization of surgical implants that are not in accordance with standards and adopt the practice of retrieval analysis of failed implants. This would protect the public health by identifying and preventing the main causes of failure in surgical implants.

A fluorescent liquid-penetrant inspection of an experimental stator vane of a first-stage axial compressor revealed the presence of a longitudinal crack over 50 mm (2 in.) long at the edge of a resistance seam weld. The vane was made of titanium alloy Ti-6Al-4V (AMS 4911). The crack was opened by fracturing the vane. The crack surface displayed fatigue beach marks emanating from the seam-weld interface. Both the leading-edge and trailing-edge seam welds exhibited weld-metal expulsions up to 3.6 mm (0.14 in.) in length. Metallographic examination confirmed that metal expulsion from the resistance welds was generally present. The stator vane failed by a fatigue crack that initiated at internal surface discontinuities caused by metal expulsion from the resistance seam weld used in fabricating the vane. Expulsion of metal from seam welds should be eliminated by a slight reduction in welding current to reduce the temperature, by an increase in the electrode force, or both.

A Ti-6Al-4V alloy pressure vessel failed during a proof-pressure test, fracturing along the center girth weld. The girth joints were welded with the automatic gas tungsten arc process utilizing an auxiliary trailing shield attached to the welding torch to provide inert-gas shielding for the exterior surface of the weld. A segmented backup ring with a gas channel was used inside the vessel to shield the weld root. The pressure vessel failed due to contamination of the fusion zone by oxygen, which resulted when the gas shielding the root face of the weld was diluted by air that leaked into the gas channel. Thermal stresses cracked the embrittled weld, exposing the crack surfaces to oxidation before cooling. One of these cracks caused a stress concentration so severe that failure of the vessel wall during the proof test was inevitable. A sealing system at the split-line region of the segmented backup ring was provided, and a fine-mesh stainless steel screen diffuser was incorporated in the channel section of the backup ring to prevent air from leaking in. A titanium alloy color chart was furnished to permit correlation of weld-zone discoloration with the degree of atmospheric contamination.

Nondestructive metallographic examination of materials frequently must be performed on-site when the component in question cannot be moved or destructively examined. Often, it is imperative that specific microstructural information (i.e., material type, heat treatment condition, homogeneity, etc.) be obtained either before initial use of a component, or before the use of a component can be safely resumed. In this paper, the use of standard metallurgical laboratory equipment, and the procedures required to conduct nondestructive on-site metallographic analyses of engineering materials, is presented. As an example, the materials and metallographic techniques employed in an actual on-site investigation of a gas tungsten-arc weldment joining two large diameter Ti-6Al-4V alloy cylinders are discussed in depth to illustrate what can be accomplished.