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SUMMER 2018

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Distributor's Link Magazine Summer 2018 / Vol 41 No3

114 THE DISTRIBUTOR’S

114 THE DISTRIBUTOR’S LINK MICHAEL MOWINS FASTENER HEAD STRENGTH WHITE PAPER from page 36 The Problem During recent reviews of qualification test data for Aerospace series - Screws, 100° countersunk head, six lobe recess, threaded to head, in titanium alloy TI-P64001,anodized, MoS2 coated - Classification: 900 MPa at ambient temperature)/ 350° C it was found that a batch of M5 diameter parts failed to meet the required tensile test limits with lower than required failure levels in the head to shank juncture. Through investigation it was found that the prior approval parts had previously been qualified by analogy with a different standard, Aerospace series - Screws, pan head, six lobe recess, coarse tolerance normal shank, medium length thread, in titanium alloy, anodized, MoS2 lubricated - 1100 MPa at ambient temperature)/315 °C which has a different head style (pan head), a different shank configuration (coarse tolerance normal shank) and higher material strength (1100 MPa). Additionally, because of the different head style and shank configuration the pan head part has a much different Head to Shank Juncture geometry than the 100° countersunk head that is threaded to the head. Given the differences between these designs and materials what method or data should be used to qualify similar or dis-similar parts by analogy or to evaluate the potential tensile strength capability of a given head to shank geometry? A poorly designed head and recess configuration can result in fasteners prematurely failing to meet tensile strength requirements with lower than required failure levels in the head to shank juncture. This white paper discusses what method or data should be used to ensure that the strength around this area is maintained such that the fastener is capable of meeting the design tensile strength requirements. Defining The Variables For the purpose of this exercise we will assume a basic level of understanding of the tensile strength of various materials and how it affects the mechanical properties that a specific design can achieve. The variables in the head to shank juncture area of a fastener that influence tensile strength capability can then be limited to the effective geometry of the head to shank interface while the tensile strength of the threaded portion of the fastener can be based on the effective tensile stress area of the thread itself . We must define the diameter to be used to calculate the effective tensile stress area of the portion of the fastener as it transitions from a threaded area to an unthreaded area. That “effective diameter” is different for a given thread geometry (a 10-32UNJF for example) as it transitions to an unthreaded shank (a bolt with a grip length defined by an unthreaded portion) as opposed to when it transitions from a fully threaded part (a screw that is threaded to the head) to the underside of the head of the fastener. In a bolt, the thread transitions to a full body diameter (0.190” for our 10-32 UNJF example) and the result is that the weakest tensile stress area occurs at the thread pitch diameter one or two threads below the thread to full body transition. The thread pitch diameter is often equivalent to the blank diameter (the diameter of the unthreaded fastener before the thread profile is rolled onto the base material) that the fastener manufacturer uses during the production process. In the case of a fastener that is threaded to the head the minimum tensile stress area often occurs in the area between the last full thread and the bottom of the head or the thread to head transition zone. Here the effective tensile stress diameter may be the same as the thread pitch diameter (0.1658”min for our 10-32UNJF example) or it may be slightly smaller if this area is fillet rolled to relieve stress built up during the manufacturing process. This is significant because the difference in the effective tensile stress area in the head to shank transition will be different for a full body bolt than for a threaded to the head screw due to this dimensional difference at the bottom of the head between 0.190” for the bolt and the smaller 0.1658” for the screw. The stress cone, or the effective cross sectional area between the shank to head transition and the bottom to the internal recess, is calculated using either the larger diameter of the bolt transition or the smaller diameter of the fully threaded transition. CONTINUED ON PAGE 172

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