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RESEARCH PAPERS

Clamp Load Loss due to Fastener Elongation Beyond its Elastic Limit

[+] Author and Article Information
Sayed A. Nassar, Payam H. Matin

Fastening and Joining Research Institute, Department of Mechanical Engineering, Oakland University, Rochester, Michigan 48309

J. Pressure Vessel Technol 128(3), 379-387 (Aug 05, 2005) (9 pages) doi:10.1115/1.2217971 History: Received June 23, 2004; Revised August 05, 2005

The amount of clamp load due to an externally applied separating force is determined for a boiled assembly in which the fastener is elongated past its proportional limit, while the clamped joint remained within its elastic range. After the initial tightening of the fastener, the joint is subsequently subjected to a tensile separating force, which further increases the fastener tensile stress into the nonlinear range. Such separating force will simultaneously reduce the clamping force in the bolted joint. Upon the removal of the separating service load, the bolted joint system reaches a new equilibrium point between the fastener tension and the joint clamping force. At the new equilibrium point, the fastener tension is reduced from its value at initial assembly, due to the plastic elongation of the fastener. The reduction in fastener tension translates into a partial—yet permanent—loss of the clamping load that may lead to joint leakage, loosening, or fatigue failure. A nonlinear model is established in order to describe the fastener behavior past the proportional limit of its material, and to determine the clamp load loss due to the permanent set in the fastener after the separating force has been removed. Two fastener materials with significantly different rates of strain hardening are used for modeling the behavior of the bolted joint system. The effect of three nondimensional variables on the amount of clamp load loss is investigated. The first variable is the stiffness ratio of the joint and the fastener. The second is the ratio of initial fastener tension to the fastener elastic limit, and the third variable is the ratio of the separating force to the force that causes joint separation to start. Analytical results are presented for a range of stiffness ratios that simulates both soft and hard joint applications. Experimental verification of the analytical results is presented.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 5

Discretized model of the fastener behavior beyond yield

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Figure 6

(a) Fastener tension versus elongation for Class 8.8 fastener (7). (b) Fastener tension versus elongation for austenitic stainless steel fastener.

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Figure 7

Effect of the separating force on clamp load loss for Class 8.8 fastener (7)

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Figure 8

(a) Effect of the stiffness ratio on clamp load loss for Class 8.8 fastener with Fe∕Femax=0.5. (b) Effect of the stiffness ratio on clamp load loss for Class 8.8 fastener with Fe∕Femax=1.

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Figure 10

(a) Effect of the stiffness ratio on clamp load loss for an austenitic stainless steel fastener with Fe∕Femax=0.5. (b) Effect of the stiffness ratio on clamp load loss for an austenitic stainless steel fastener with Fe∕Femax=1.

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Figure 11

Bolted joint system designed for the experimental verification

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Figure 12

(a) Experimental setup showing a tensile test machine to provide separating force. (b) Separating force applied by the crossheads of the tensile test machine. (c) Separating force (close-up photo).

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Figure 13

Theoretical and experimental clamp load loss versus separating force

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Figure 2

Effect of external separating force in the elastic range

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Figure 3

Fluctuations in Fb and Fc corresponding to fluctuations in Fe(5)

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Figure 4

Nonlinear joint diagram

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Figure 9

(a) Effect of the separating force on clamp load loss for an austenitic stainless steel fastener with Kc∕Kb=1. (b) Effect of the separating force on clamp load loss for an austenitic stainless steel fastener with Kc∕Kb=2. (c) Effect of the separating force on clamp load loss for an austenitic stainless steel fastener with Kc∕Kb=4.

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Figure 1

Bolted joint model

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