Accelerated aging tests on detonator assemblies, to verify the compatibility of gold bridgewire and Pd-In-Sn solder with the intended explosives, revealed an unusual form of corrosion. The tests, conducted at 74 deg C (165 deg F) and 54 deg C (130 deg F), indicated a preferential attack of the gold. To investigate the problem, a matrix of test units was produced and analyzed. Scanning electron microscopy, EDX analysis, and x-ray diffraction techniques were used to determine the extent of the corrosion and identify the corrosion products. The results indicated that the preferential attack of the gold was due to HCN formed by decomposition of the explosive powder at high temperatures. Other associated reactions were also observed including the subsequent attack of the solder by the gold corrosion product and degradation of the plastic header.

Following the crash of a Mirage III-0 aircraft (apparently caused by engine failure), a small crack was detected in a bolt hole in the wing main spar (AU4SG aluminum alloy). Because this area was considered to be critical to aircraft safety and similar cracking was found in other spars in service, the Royal Australian Air Force requested that the crack growth rate during service be determined. The loading history of the aircraft was made available in the form of flight by-flight records of the counts from the vertical accelerometer sensors fitted to the airframe and a series of “overstress” events recorded during the life of the aircraft. The bolt hole was examined by eddy current testing, visual examination, high-powered light microscope, and scanning electron microscope. Simulation tests were also conducted. The use of simulation specimens permitted actual crack growth rate data to be determined for the configuration of interest.

A failed laser mirror and another complete mirror of the same construction were analyzed. The laser mirror consisted of three layers of material brazed together to form channels through which the cooling water flows. Samples were analyzed with light optical and scanning electron microscopy. The corrosion product contained molybdenum and copper with a trace of gold. The base material was analyzed as molybdenum with negligible alloying additions. The primary mode of corrosion attack on the base material appeared to be intergranular, although uniform corrosion was evident also. It was concluded that corrosion attack sufficiently weakened the base material and the brazed joints, allowing catastrophic failure of the mirror due to the pressure of the cooling water. It was recommended that the mirrors be cleaned of all corrosion products present as a result of past service conditions and proof tested. It was recommended that the water system consisting of deionized water and formaldehyde be replaced with water having a low oxygen content and a cathodic inhibitor (oxygen scavenger).

Waspaloy (AMS 5586) fabricated inner ring of a spray-manifold assembly failed transversely through the manifold tubing at the edge of the tube and support sleeve brazed joint. The assembly was brazed with AWS BAu-4 filler metal (AMS 4787). Fatigue beach marks propagating from extremities of a granular gold-tinted surface region adjacent to the tube-to-sleeve brazed joint and extending circumferentially were revealed by microscopic examination. Embrittlement of the tube caused by molten braze metal penetration along grain boundaries was evidenced by micrographs of a granular portion of the fracture. It was revealed by the initial fracture profile that fatigue cracks begun as an intergranular separation and subsequently became transgranular. It was concluded that failure of the tube was caused by excessive alloying between the braze metal and the Waspaloy. Reduced temperatures during torch debrazing or rebrazing were recommended to minimize molten braze metal penetration.

Diode leads that were to be made from OFHC copper were instead made from copper with high oxygen content. The leads had a nickel underplating, a gold final plating, and were brazed to the diode package in a hydrogen atmosphere. After brazing, the leads became embrittled. SEM examination of the fractured leads revealed voids and some oxidized areas surrounded by ductile fracture areas. High pressure steam pockets observed as voids in the microstructure caused hydrogen embrittlement of the leads. The obvious corrective action was to ensure that the lead material was OFHC copper.

Electroless nickel plating separation from copper alloy CDA175 retaining clips used on printed circuit boards was caused by a copper oxide layer that reduced adhesion of the nickel plating on the clips. Stresses that developed during module insertion caused flaking to occur at the oxidized copper surface. Electroless nickel plating separation from OFHC copper leads was caused by improper handling rather than a plating anomaly per se. Tin plating separation from copper underplating on a hybrid package lid occurred because of a four-week delay between the copper plating and tin plating steps. It was recommended that tin plating should follow the copper underplating within 24 h and a cleaning step of bright dipping after copper plating be performed.

Several surface-mount chip resistor assemblies failed during monthly thermal shock testing and in the field. The resistor exhibited a failure mode characterized by a rise in resistance out of tolerance for the system. Representative samples from each step in the manufacturing process were selected for analysis, along with additional samples representing the various resistor failures. Visual examination revealed two different types of termination failures: total delamination and partial delamination. Electron probe microanalysis confirmed that the fracture occurred at the end of the termination. Transverse sections from each of the groups were examined metallographically. Consistent interfacial separation was noted. Fourier transform infrared and EDS analyses were also performed. It was concluded that low wraparound termination strength of the resistors had caused unacceptable increases in the resistance values, resulting in circuit nonperformance at inappropriate times. The low termination strength was attributed to deficient chip design for the intended materials and manufacturing process and exacerbated by the presence of polymeric contamination at the termination interface.

A helicopter main rotor bolt failed in the black-coated region between the threads and the taper section of the shank during assembly. The torque applied was approximately 100 N·m (900 in.·lbf) when the bolt sheared. No other bolts were reported to have failed. The failed bolt material conformed to AISI E4340 steel, as specified. The microstructure was tempered martensite, with hardness ranging from 41 to 45 HRC. Failure was in the shear ductile mode. The crack initiated in the area of slag inclusions. Inspection of other bolts from the same shipment was recommended.

Six transformer brackets failed in service, sending a group of three pole-mounted transformers to the ground below. The brackets were made from acrylonitrile-butadiene-styrene (ABS) resin and had been in service for more than 30 years. Remnants of the fractured brackets were analyzed using optical and scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The exterior surfaces of all six brackets were alike and shared similar features, including witness marks, discoloration, mechanical deformation, and secondary cracking, along with crack networks. Both FTIR and TGA analyses indicated that the surface material was in a highly degraded state, likely due to weathering and thermal and ultraviolet exposure. This, in turn, led to the formation of cracks that propagated under the cyclic forces of vibration and wind. As the cracks grew larger, the weight of the transformer eventually overloaded the brackets, resulting in failure.

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.

The scanning electron microscopy (SEM) is one of the most versatile instruments for investigating the microstructure of metallic materials. This article highlights the development of SEM technology and describes the operation of basic systems in an SEM, including the electron optical column, signal detection and display equipment, and vacuum system. It discusses the preparation of samples for observation using an SEM and describes the application of SEM in fractography. If the surface remains unaffected and undamaged by events subsequent to the actual failure, it is often a simple matter to determine the failure mode by the use of an SEM. In cases where the surface is altered after the initial failure, the case may not be so straightforward. The article presents typical examples that illustrate these points. Image dependence on the microscope type and operating parameters is also discussed.

The scanning electron microscope (SEM) is one of the most versatile instruments for investigating the microscopic features of most solid materials. The SEM provides the user with an unparalleled ability to observe and quantify the surface of a sample. This article discusses the development of SEM technology and operating principles of basic systems of SEM. The basic systems covered include the electron optical column, signal detection and display equipment, and the vacuum system. The processes involved in the preparation of samples for observation using an SEM are described, and the application of SEM in fractography is discussed. The article covers the failure mechanisms of ductile failure, brittle failure, mixed-mode failure, and fatigue failure. Lastly, image dependence on microscope type and operating parameters is also discussed.