Fastener Failure The primary function of a fastener is to transfer load. There are many types of fasteners. Examples of some requirements for fasteners are listed below: • •
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Higher strength Increased high temperature dependability Increased low temperature dependability Reduced cost Easier maintenance Improved corrosion resistance
The choice of a fastener is dependent dependent on the design requirements and environment in which which the fastener will be used. used. Attention to various various aspects of the fastener must must be considered. considered. Some of these are listed listed below: • • • • • •
Function of the fastener Operating environment of the fastener Type of loading on the fastener in service Thickness of materials to be joined Type of materials to be joined Configuration of the joint to be fastened
An analysis of these requirements is necessary before a suitable fastener can be chosen. The choice of the correct fastener may simply entail fulfilling a requirement for strength (static or fatigue) or for corrosion resistance. Conversely, the choice may be constrained constrained by a complex set of requirements. The scope and complexity complexity of the fastener fastener system required is frequently determined by the consequences of fastener failure. One method of guarding against failure of a new fastener in a critical application is to sufficiently sufficiently test the fasten fastener er system prior to use. A designer must be careful when extrapolate existing data to an increased size of the same fastener, because larger-diameter fasteners can have considerably lower fatigue endurance limits than smaller-diameter fasteners.
Mechanical Fasteners
Mechanical fasteners are frequently grouped as listed below: • • • • • •
Pin fasteners Threaded fasteners Rivets Blind fasteners Special purpose fasteners Fasteners for composites
Rivets, pin fasteners, and special-purpose fasteners are usually designed for permanent or semipermanent installation. Pin fasteners are fasteners are used in joints in which the load is primarily shear. Pins can be either solid or tubular. A collar is sometimes swaged or formed on the pin to secure the joint. Threaded fasteners are commonly thought of as any threaded part that, after joint assembly, may be removed without damage to the fastener or to the members being joined. Rivets are permanent one piece fasteners one end of the rivet i s mechanically upset during installation. Blind fasteners are commonly multiple part devices that can be installed in a joint that is accessible from only one side. Typically a self-contained mechanism, an explosive, or other device forms an upset on the inaccessible side when a blind fastener is installed. Special-purpose fasteners are often proprietary, such as retaining rings, latches, slotted springs, and studs. These fasteners are frequently designed to allow easy, quick removal and replacement and commonly show little or no deterioration with repeated use. Mechanical fasteners for composites are often used in combination with adhesive bonding to increase the reliability of highly stressed joints. The common pins, bolts, rivets, and blind fasteners are used with composites. However, the numerous problems that have occurred have motivated the development and testing of many special purpose fasteners. Some of the problems with fasteners for composites are listed below: •
Pullout of the fastener under load
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Drilling damage to the composite Installation damage to the composite Delamination of the composite material near the hole Differences in expansion coefficients between the composite and the fastener Galvanic corrosion between the composite and the fastener Fuel leaks around the fastener Fretting
ASTM and SAE specifications covering threaded fasteners require that the heads be marked for grade identification. Grade markings are a safety feature that provides a means to verify the selection, and use of a fastener and allows for inspection for the proper use of the fastener. The markings diminish the likelihood of selecting and using a fastener of insufficient strength. Incorrect fasteners have the potential to lead to a failure and cause damage to equipment or injury of personnel. The figure below shows the common types of failures in fasteners.
Figure 2. Failures in threaded and blind fasteners and in fastened sheet.
Failure Origins
Frequent locations for fastener failure listed below: • • •
In the head-to-shank fillet Through the first thread inside the nut The transition from the thread to the shank
Causes of Fastener Failures Some causes of fastener failure are listed below: • • • • • •
Shear Overload Fatigue Corrosion Manufacturing discrepancies Improper installation
Fastener Failure Analysis A fastener may experience either static loading or fatigue loading. Static loading may be tension, shear, bending, or torsion. These static loading conditions may occur in combination. One example of fatigue loading is vibration. In addition to overload and fatigue, some other common reasons for fastener failures include environmental issues, manufacturing discrepancies, and improper use or incorrect installation. Some common questions concerning fasteners are listed below: • • • •
How were the fasteners torqued? In what order were fasteners tightened? What is the best way to verify the torque on fasteners? How does torque value vary over time?
Fatigue is one of the most common failure modes for threaded fasteners. Fretting failures may result from small movements between adjacent surfaces. Additionally, atmospheric corrosion, liquid immersion corrosion, galvanic corrosion, crevice corrosion, stress corrosion cracking, and hydrogen damage may contribute to fastener failure Material selection, heat treatment, cutting or rolling threads, manufacturing, assembly, and design are some of the factors that effect fastener failures. Failure analysis can
determine the cause of the fastener failure and determine the primary or contributing causes of fastener failure.
