During installation, a clamp-strap assembly, specified to be type 410 stainless steel-austenitized at 955 to 1010 deg C (1750 to 1850 deg F), oil quenched, and tempered at 565 deg C (1050 deg F) for 2 h to achieve a hardness of 30 to 35 HRC, and used for securing the caging mechanism on a star-tracking telescope, fractured transversely across two rivet holes closest to one edge of the pin retainer in a completely brittle manner. Comparison with a non-failed strap using microscopic examination, spectrographic analysis, and slow-bend tests showed that both fit the 410 stainless steel specs, but hardness and grain size were different. Reheat treatment of full-width specimens showed that coarse grain size (ASTM 2 to 3) was responsible for the brittle fracture, and excessively high temperature during austenitizing caused the large grain size in the failed strap. The fact that the hardness of the strap that failed was lower than the specified hardness of 30 to 35 HRC had no effect on the failure because that of the non-failed strap was even lower. Recommendation was that the strap should be heat treated as specified to maintain the required ductility and grain size.

An investigation was conducted to determine why 16 out of 139 pipe bends cracked during hot induction bending. The pipe conformed to API 5L X65 PSL2 line pipe standards and measured 1016 mm (40 in.) in diam with a wall thickness of 18.5 mm. A metallurgical cross section was removed along a crack on the extrados to document the crack morphology using optical microscopy. In addition to cracking, golden-yellow streaks were visible at the extrados, and the composition was examined using scanning electron microscopy with energy dispersive spectroscopy. Based on the results, investigators concluded the pipe was contaminated with copper at the mill were it was produced.

Mechanical testing is an evaluative tool used by the failure analyst to collect data regarding the macro- and micromechanical properties of the materials being examined. This article provides information on a few important considerations regarding mechanical testing that the failure analyst must keep in mind. These considerations include the test location and orientation, the use of raw material certifications, the certifications potentially not representing the hardware, and the determination of valid test results. The article introduces the concepts of various mechanical testing techniques and discusses the advantages and limitations of each technique when used in failure analysis. The focus is on various types of static load testing, hardness testing, and impact testing. The testing types covered include uniaxial tension testing, uniaxial compression testing, bend testing, hardness testing, macroindentation hardness, microindentation hardness, and the impact toughness test.

A sand-cast gray iron flanged nut was used to adjust the upper roll on a 3.05 m (10 ft) pyramid-type plate-bending machine. The flange broke away from the body of the nut during service. Analysis (visual inspection and 150x micrographs of sections etched with nital) supported the conclusions that brittle fracture of the flange from the body was the result of overload caused by misalignment between the flange and the roll holder. The microstructure contained graphite flakes of excessive size and inclusions in critical areas; however, these metallurgical imperfections did not appear to have had significant effects on the fracture. Recommendations included carefully and properly aligning the flange surface with the roll holder to achieve uniform distribution of the load. Also, a more ductile metal, such as steel or ductile iron, would be more suitable for this application and would require less exact alignment.

Cold-drawn type 303 stainless steel wire sections, 6.4 mm (0.25 in.) in diameter, failed during a forming operation. All of the wires failed at a gradual 90 deg bend. Investigation (visual inspection and 5.3x/71x/1187x SEM views) supported the conclusion that the wires cracked due to ductile overload. The forming stresses were sufficient to initiate surface ruptures, suggestive of having exceeded the forming limit. Recommendations included examining the forming process, including lubrication and workpiece fixturing.

After 14 months of service, cracks were discovered in castings and bolts used to fasten together braces, posts, and other structural members of a cooling tower, where they were subjected to externally applied stresses. The castings were made of copper alloys C86200 and C86300 (manganese bronze). The bolts and nuts were made of copper alloy C46400 (naval brass, uninhibited). The water that was circulated through the tower had high concentrations of oxygen, carbon dioxide, and chloramines. Analysis (visual inspection, bend tests, fractographs, 50x unetched micrographs, 100x micrographs etched with H4OH, and 500x micrographs) supported the conclusions that the castings and bolts failed by SCC caused by the combined effects of dezincification damage and applied stresses. Recommendations included replacing the castings with copper alloy C87200 (cast silicon bronze) castings. Replacement bolts and nuts should be made from copper alloy C65100 or C65500 (wrought silicon bronze).

Some 99.90 pure tin tubes (0.15 mm thick) used for packaging a chemical compound cracked on bending and underwent brittle fracture prior to filling, while others remained ductile and showed no sign of failure. Examination showed that specimens prepared by mechanical methods such as electrolytic and hand polishing and the vibration method resulted in poor edge and crack edge definition due to material thickness. Etching experiments involved a grain surface attack and hence produced a rather strong surface relief from which the grain boundary cracks could again not clearly be differentiated. The sections were therefore examined unetched in polarized light. The microstructure of the cracked tubes was shown to have much smaller grains than the ductile and showed cracks from the surface down along the grain boundaries. Material hardness also differed between the unusable tubes and the ductile, and chemical analysis showed a higher level of aluminum in the brittle specimens. Failure obviously occurred due to the high material aluminum content that increased hardness which then caused embrittlement at the surface which led to cracks or fracture on bending. Since no explanation of how the aluminum entered the tin was available, no recommendations could be made.

Several of the aluminum alloy 6061-T6 drawn seamless tubes (ASTM B 234, 2.5 cm (1.0 in.) OD with wall thickness of 1.7 mm (0.065 in.)) connecting an array of headers to a system of water-cooling pipes failed. The tubes were supplied in the O temper. They were bent to the desired curvature, preheated, then solution treated, water quenched, and then aged for 8 to 10 h. Analysis (visual inspection, slow-bend testing, 65x macrographic analysis, macroetching, spectrographic analysis, hardness tests, microhardness tests, tension tests, and microscopic examination) supported the conclusions that bending of the connector tubes in the annealed condition induced critical strain near the neutral axis of the tube, which resulted in excessive growth of individual grains during the subsequent solution treatment. Recommendations included bending the connector tubes in the T4 temper as early as possible after being quenched from the solution temperature. The tubes should be stored in dry ice after the quench until bending can be done. The tubes should be aged immediately after being formed. Flattening and slow-bend tests should be specified to ensure that the connector tubes had satisfactory ductility.

An SB407 alloy 800H tube failed at a 100 deg bend shortly after startup of a new steam superheater. Three bends failed and one bend remote from the failure area was examined. Visual examination showed that the fracture started on the outside surface along the inside radius of the bend and propagated in a brittle, intergranular fashion. Chemical analysis revealed that lead contamination was a significant factor in the failure and phosphorus may have contributed. The localized nature of the cracks and minimum secondary cracking suggested a distinct, synergistic effect of applied tensile stress with the contamination. Stress analysis found that stress alone was not enough to cause failure; however the operating stresses in the 100 deg bends were higher than at most other locations in the superheater Reduced creep ductility may be another possible cause of failure. Remedial actions included reducing the tube temperature, replacing the Schedule 40 100 deg bends with Schedule 80 pipe, and solution annealing the pipe after bending.

A stepped drive axle (hardened and tempered resulfurized 4150 steel forging) used in a high-speed electric overhead crane (rated at 6800 kg, or 7 tons, and handling about 220 lifts/day with each lift averaging 3625 to 5440 kg, or 4 to 6 tons) broke after 15 months of service. Visual examination of the fracture surface revealed three fracture regions. The primary fracture occurred approximately 50 mm (2 in.) from the driven end of the large-diam keywayed section on the stepped axle and approximately 38 mm (1 in.) from one end of the keyway where the crane wheel was keyed to the axle. Macroscopic, microscopic, and chemical examination revealed composition that was basically within the normal range for 4150 steel. This evidence supports the conclusion that cracking initiated at a location approximately opposite the keyway, and final fracture was due to mixed ductile and brittle fracture. Axial shift of the crane wheel during operation, because of insufficient interference fit, was the major cause of fatigue cracking. Recommendations included redesigning the axle to increase the critical diameter from 140 to 150 mm (5.5 to 6 in.) and to add a narrow shoulder to keep the drive wheel from shifting during operation.

Drive cables from a rubber processing machine were failing in less than 8 h of operation, the expected service life being much greater than 100 h. Comparison cables were tested to failure under known stress conditions, including tensile overload, torsional loading, reversed bending alternating stress, and buckling (compressive) cyclic loading. The mode of failure was found to be reversed bending fatigue caused by drive cables moving over guide pulleys of small radii. Modifications of the machinery and drive cable system were suggested.

Silver solid-state bonded components containing uranium failed under zero or low applied load several years after manufacture. The final operation in their manufacture was a proof loading that applied a sustained tensile stress to the bond, which all components passed. The components comprised circular cylinders fabricated by plating a thin layer of silver on each of the contact surfaces (uranium and stainless steel) and pressing the parts together at elevated temperature to solid-state bond the two silver surfaces. The manufacturing process produced a high level of residual stress at the bond. The failures appeared to be predominantly located between the silver layer and the uranium substrate. Normal fracture location of specimens taken from similar components was at the silver/silver bond interface. Laboratory testing revealed that the uranium/silver joint was susceptible to premature failure by stress-corrosion cracking under sustained loading if the atmosphere was saturated with water vapor.

Galvanized A36 steel unsleeved shear-type anchor bolts failed during installation. The galvanized steel bolts were approximately 18 mm (0.7 in.) in diameter with a 90 deg bend between the long and short legs. As-fractured, sawcut, and unfractured specimens were examined. Failure analysis revealed dark thumbnail regions at the fracture origins and a very narrow and uniform shear lip. The thumbnail region exhibited zinc deposits with no apparent fracture detail, indicating preexisting cracks that had occurred before galvanizing. The balance of the fracture exhibited a transgranular mode with cleavage and ductile, dimpled shear. Hardness values as high as 35 HRC were measured in the bend area. The as-galvanized bolts fractured in a brittle manner. Failure was attributed to improper bending of the bolts, which provided a severely cold-worked bend area susceptible to strain-age embrittlement.

Valve seats fractured during testing and during service. The seats were machined from grade 11L17 steel and were surface hardened by carburization. Investigation (visual inspection, hardness testing, 59x SEM images, and 2% nital etched 15x cross sections) supported the conclusion that the fracture occurred via brittle overload, which was predominantly intergranular. The amount of bending evidence and the directionality of the core overload fracture features suggest that the applied stresses were not purely axial, as would be anticipated in this application. The level of retained austenite in the hardened case layer likely contributed to the failure.

Superelastic nitinol wires that fractured under various conditions were examined under a scanning electron microscope in order to characterize the fracture surfaces, produce reference data, and compare the findings with prior published work. The study revealed that nitinol fracture modes and morphologies are generally consistent with those of ductile metals, such as austenitic stainless steel, with one exception: Nitinol exhibits a unique damage mechanism under high bending strain, where damage occurs at the compression side of tight bends or kinks while the tensile side is unaffected. The damage begins as slip line formation due to plastic deformation, which progresses to cracking at high strain levels. The cracks appear to initiate from slip lines and extend in shear (mode II) manner.

A 20 ton polar crane motor fell during a 3400 kg (7500 lb) lift, narrowly missing personnel working beneath the crane. Witnesses reported that the motor fall was preceded by a falling oil mass, and it was believed that the motor was intact prior to impact. The maintenance history of the crane showed that the motor had been removed, repaired, and reinstalled 2 years prior to the failure. Observations of oil leakage were noted yearly up to the failure. The motor casing was held onto the adapter plate by eight 14-20 UNC x 25 mm (1 in.) long hex socket cap screws. Examination of the motor adapter plate, motor casing shards (aluminum), the gear side of the motor housing, and seven fractured cap screws (ASTM A574) showed that the motor casing was intact at the time of “uncontrolled descent” and that the screws had failed by high nominal stress reverse bending load fatigue, which was probably the result of insufficient torque on the bolts.

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 turbine vane made of cast cobalt-base alloy AMS 5382 (Stellite 31; composition: Co-25.5Cr-10.5Ni-7.5W) was returned from service after an undetermined number of service hours because of crack indications on the airfoil sections. This alloy is cast by the precision investment method. Analysis (visual inspection, 100x/500x metallographic examination of sections etched with a mixture of ferric chloride, hydrochloric acid, and methanol, and bend tests) supported the conclusions that cracking of the airfoil sections was caused by thermal fatigue and was contributed to by low ductility due to age hardening, subsurface oxidation related to intragranular carbides, and high residual tensile macrostresses. No further conclusions could be drawn because of the lack of detailed service history, and no recommendations were made.