Search Results for Bolted joints

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.

A portion of a 19 mm (0.75 in.) diam structural steel bolt was found on the floor of a manufacturing shop. This shop contained an overhead crane system that ran on rails supported by girders and columns. Inspection of the crane system revealed that the bolt had come from a joint in the supporting girders and could be considered one of the principal fasteners in the track system. Analysis (visual inspection, metallographic exam, and hardness testing) supported the conclusions that fatigue induced by the overhead movement of the crane produced failure of the bolt. The bolt was deficient in strength for the cyclic applied loads in this case and probably was not tightened sufficiently. Recommendations included removing the remaining bolts in the crane support assembly and replacing them with a higher-strength, more fatigue-resistant bolt, for example, SAE grade F, 104 to 108 HRB. The bolts should be tightened according to the specifications of the manufacturer, and the system should be periodically inspected for correct tightness.

In a main range in a power station, steam was conveyed at a pressure of 645 psi, and a temperature of 454 deg C (850 deg F). Pipe diameter was 9 in. and the joints were of the bolted type in which a thin steel ring, serrated on both sides, was inserted between plain flanges. Thin jointing material was interposed between the serrated faces and the flanges. The first intimation of trouble was the onset of a high pitched noise audible over a radius of a quarter of a mile. The noise arose from violent lateral vibration of the serrated ring, which attained an amplitude and persisted for a sufficient number of cycles to produce an extensive system of fatigue cracks that resulted in partial disintegration of the ring. Microscopic examination of the material showed it to be a mild steel of satisfactory quality. The trouble was started by slight leakage, possibly resulting from a relaxation of the interfacial pressure on the joint faces, which eroded away the joint material locally at one face of the serrated ring. This reduced interfacial pressure at the opposite face of the ring, with resultant leakage and erosion of the joint material on this side.

Fretting and/or fretting corrosion fatigue have been observed on such parts as main rotor counterweight tie rods, fixed-pitch propeller blades, propeller blade clamps, pressure regulator lines, and landing gear support brackets. Microcracks started from severe corrosion pits in a failed control rotor spar tube assembly made of cadmium-plated AISI 4130 Cr-Mo alloy steel. Inadequate design was responsible for the failure. A lower tine of the main rotor blade cuff failed in fatigue. The rotor blade cuff was forged of 2014-T6 aluminum alloy. Initial stages of crack growth displayed features typical of low stress intensity fatigue of aluminum alloys. The fatigue resulted from abnormal fretting owing to inadequate torquing of the main retention bolts. Aircraft maintenance engineers and owners were advised to adhere to specifications when torquing this joint.

In a helicopter engine connecting rod, high-cycle, low-stress fatigue fractures in bolts and arms progressed about 75% across the section before the final rupture. Factors involved were insufficient specified preload, inadequate tightening during assembly, and engine overspeed. The assigned main causes were design deficiency, improper maintenance during overhaul, and abnormal service operation. The problem can be solved by proper overhauling that ensures bolted assemblies are tightened evenly and accurately, in accordance with recommended torque values. Also, the manufacturer made various modifications, such as a thicker rod, fatigue resistant bolts, and more accurate preload measurements. The configuration of these rods were changed to a tongue-and-groove design to increase service life.

Several deburring drums that fractured were filled with abrasive, water, and small parts, such as roller bearing rollers, and rotated on their axis at 36 rpm. Cracks were discovered very early in the service lives of these high-chromium white iron cast structures. All of the fractures were through bolt holes in the mounting flange. The holes had a sharp edge and exhibited uneven wear on the inside diameter. In operation, the mounting bolts were frequently found to be loose and in at least one case broken off. A 25x scanning electron microscopy (SEM) fractograph from near this fracture-initiation area showed fatigue striations. No casting or metallurgical structural defects were found that could explain the failures. This evidence supports the conclusion that cracking was a result of the stress-concentration site at the bolt holes where a fatigue-initiated fracture occurred. Recommendations included that the radii be increased at the sharp corners and that lock-wiring be used to secure against bolt loosening.

Wind turbine blades are secured by a number of high-strength bolts. The failure of one such bolt, which caused a turbine blade to detach, was investigated to determine why it fractured. Based on the results of a detailed analysis, consisting of stress calculations, chemical composition testing, metallurgical examination, mechanical property testing, and fractographic analysis, it was determined that the bolt failed by fatigue accelerated by stress concentration at low temperatures. The investigation also provided suggestions for avoiding similar failures.

A 140 ft. (42.7 m) long boom on a dragline crane used in coal strip-mining operations failed. One of the principal load-bearing longitudinal beams or chords of the trussed boom had fractured adjacent to a bolt hole at a location about halfway along the length of the boom. Over the lifetime of the crane, several repairs had been made to the boom. At least a year before the failure, a reinforcing gusset plate had been bolted and welded to this chord at this location. Stereomicroscopy revealed microcracks in the weld metal. A fatigue crack 45 mm (1.8 in.) long was observed to emanate from this microcrack. Scanning electron microscopy showed an overload crack extended across the remaining cross section of the chord. It was concluded that the presence of the bolt hole used to attach the gusset plate to the chord created a stress riser adjacent to the hole. Repeated high tensile stresses on the chord during the lifting of enormous loads initiated a fatigue crack in the weld region adjacent to the bolt hole.

A steam-pressurized Yankee dryer shell ruptured during normal operation. Cracking had occurred around much of the circumference at the drive end of the shell, which measured 3.7 m (12 ft) in diameter by 3.4 m (11 ft) long with a head bolted to each end. The crack initiated at a 90 deg corner in contact with the edge of the head. The material was a hardened gray cast iron containing 2.8% Ni and 1.2% Mo. Based on the results of visual, nondestructive, metallographic, and chemical analyses, it was concluded that failure occurred after corrosion fatigue cracking had weakened the shell. An ultrasonic examination of all Yankee dryers of the same type was recommended to look for cracking at the edge of the shell. Modification of the head-to-shell joint was recommended as well.

Two type 420 martensitic stainless steel load cell bodies, which had been installed under two of the four legs of a milk storage tank failed in service. The failure occurred near a change in section and involved fracture of the entire cross section. Examination showed a brittle fracture that was preceded by a small fatigue region. Pitting corrosion was evident at the fracture origin. The areas around the load cells had been subjected to regular washdowns using high-pressure hot water, and the pitting was attributed to crevice corrosion between the load cell and the holddown bolts. Prevention of such corrosion by the use of a flexible sealant to eliminate the crevice was recommended.

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.

During a routine inspection on an aircraft assembly line, an airframe attachment bolt was found to be broken. The bolt was one of 12 that attach the lower outboard longeron to the wing carry-through structure. Failure occurred on the right-hand forward bolt in this longeron splice attachment. The bolt was fabricated from PH13-8Mo stainless steel heat treated to have an ultimate tensile strength of 1517 to 1655 MPa (220 to 240 ksi). A water-soluble coolant was used in drilling the bolt hole where this fastener was inserted. Investigation (visual inspection, 265 SEM images, hardness testing, auger emission spectroscopy and secondary imaging spectroscopy, tensile testing, and chemical analysis) supported the conclusion that failure of the attachment bolt was caused by stress corrosion. The source of the corrosive media was the water-soluble coolant used in boring the bolt holes. Recommendations included inspecting for corrosion all the bolts that were installed using the water-soluble coolant at the spliced joint areas, rinsing all machined bolt holes with a noncorrosive agent, and installing new PH13-8Mo stainless steel bolts with a polysulfide wet sealant.

During a routine inspection on an aircraft assembly line, an airframe attachment bolt was found to be broken. The bolt was one of 12 that attach the lower outboard longeron to the wing carry-through structure. Failure occurred on the right-hand forward bolt in this longeron splice attachment. The bolt was fabricated from PH13-8Mo stainless steel heat treated to have an ultimate tensile strength of 1517 to 1655 MPa (220 to 240 ksi). A water-soluble coolant was used in drilling the bolt hole where this fastener was inserted. Investigation (visual inspection, 265 SEM images, hardness testing, auger emission spectroscopy and secondary imaging spectroscopy, tensile testing, and chemical analysis) supported the conclusion that failure of the attachment bolt was caused by stress corrosion. The source of the corrosive media was the water-soluble coolant used in boring the bolt holes. Recommendations included inspecting for corrosion all the bolts that were installed using the water-soluble coolant at the spliced joint areas, rinsing all machined bolt holes with a noncorrosive agent, and installing new PH13-8Mo stainless steel bolts with a polysulfide wet sealant.

This article first provides an overview of the types of mechanical fasteners. This is followed by sections providing information on fastener quality and counterfeit fasteners, as well as fastener loads. Then, the article discusses common causes of fastener failures, namely environmental effects, manufacturing discrepancies, improper use, or incorrect installation. Next, it describes fastener failure origins and fretting. Types of corrosion in threaded fasteners and their preventive measures are then covered. The performance of fasteners at elevated temperatures is addressed. Further, the article discusses the types of rivet, blind fastener, and pin fastener failures. Finally, it provides information on the mechanism of fastener failures in composites.

Cracking was found in the heads on large Yankee dryers, large, cylindrical, rotating, pressurized, high-temperature, cast iron pressure vessels (ASME Boiler and Pressure Vessel Code Section VIII, Rules for Construction of Pressure Vessels), used to remove moisture from sheets of tissue paper during manufacturing. The typical components consist of a cast iron shell, two cast iron concave heads, and a large cast iron internal center stay attached to journals. The heads are attached to the shell and center stay with high-strength bolts. FEA and metallurgical investigation supported the conclusion that the cracking was caused by an unexpected type of load placed on the machine, namely corrosion product buildup at the head/shell interface causing the joint to displace open. It was also found that compressive bolting loads could slightly open the head/shell interface at the periphery. Recommendations included design changes in the head/shell joint, and detailed preventive maintenance inspection procedures were also suggested.

This article discusses different types of mechanical fasteners, including threaded fasteners, rivets, blind fasteners, pin fasteners, special-purpose fasteners, and fasteners used with composite materials. It describes the origins and causes of fastener failures and with illustrative examples. Fatigue fracture in threaded fasteners and fretting in bolted machine parts are also discussed. The article provides a description of the different types of corrosion, such as atmospheric corrosion and liquid-immersion corrosion, in threaded fasteners. It also provides information on stress-corrosion cracking, hydrogen embrittlement, and liquid-metal embrittlement of bolts and nuts. The article explains the most commonly used protective metal coatings for ferrous metal fasteners. Zinc, cadmium, and aluminum are commonly used for such coatings. The article also illustrates the performance of the fasteners at elevated temperatures and concludes with a discussion on fastener failures in composites.