Fastener Failure

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Transcript of Fastener Failure

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    Fastener Failure

    The primary function of a fastener is to transfer

    load. There are many types of fasteners.Examples of some requirements for fasteners arelisted below:

    Higher strength Increased high temperature

    dependability Increased low temperature dependability Reduced cost Easier maintenance Improved corrosion resistance

    The choice of a fastener is dependent on the design requirements andenvironment in which the fastener will be used. Attention to various aspectsof the fastener must be considered. Some of these are 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 canbe chosen. The choice of the correct fastener may simply entail fulfilling arequirement for strength (static or fatigue) or for corrosionresistance. Conversely, the choice may be constrained by a complex set ofrequirements. The scope and complexity of the fastener system required isfrequently determined by the consequences of fastener failure.

    One method of guarding against failure of a new fastener in a criticalapplication is to sufficiently test the fastener system prior to use. A designermust be careful when extrapolate existing data to an increased size of thesame fastener, because larger-diameter fasteners can have considerablylower fatigue endurance limits than smaller-diameter fasteners.

    Mechanical Fasteners

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    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 forpermanent or semipermanent installation.

    Pin fasteners are fasteners are used in joints in which the load is primarilyshear. 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, afterjoint assembly, may be removed without damage to the fastener or to themembers being joined.

    Rivets are permanent one piece fasteners one end of the rivet is mechanicallyupset 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-containedmechanism, an explosive, or other device forms an upset on the inaccessibleside 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 designedto allow easy, quick removal and replacement and commonly show little or nodeterioration with repeated use.

    Mechanical fasteners for composites are often used in combination with

    adhesive bonding to increase the reliability of highly stressed joints. Thecommon pins, bolts, rivets, and blind fasteners are used withcomposites. However, the numerous problems that have occurred havemotivated 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 theheads be marked for grade identification. Grade markings are a safety featurethat provides a means to verify the selection, and use of a fastener and allowsfor inspection for the proper use of the fastener. The markings diminish thelikelihood of selecting and using a fastener of insufficient strength. Incorrect

    fasteners have the potential to lead to a failure and cause damage toequipment 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

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    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. Staticloading may be tension, shear, bending, or torsion. These static loadingconditions may occur in combination. One example of fatigue loading isvibration. In addition to overload and fatigue, some other common reasons forfastener 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. Frettingfailures may result from small movements between adjacent surfaces. Additionally,atmospheric corrosion, liquid immersion corrosion, galvanic corrosion, crevicecorrosion, stress corrosion cracking, and hydrogen damage may contribute to fastenerfailure

    Material selection, heat treatment, cutting or rolling threads, manufacturing, assembly,and design are some of the factors that effect fastener failures. Failure analysis can

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    determine the cause of the fastener failure and determine the primary or contributingcauses of fastener failure.