7.9Fastener Tensile Failure

ECSS-E-HB-32-23A, Space Engineering — Threaded Fasteners Handbook

Last Updated 29 Oct 2026

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7.9.1 External Vs Overall Load

The calculations of the MoS for axial loads depend on how the joint transmits externally applied shear loads. Two modes of shear load transfer are included in these guidelines, ‘Friction Grip’ and ‘Bearing’ (see Section 9).

It is important to check the failure criterion for ‘overall load’ in the fastener for both types of joint. The overall load, Ftot , is the sum of the maximum preload and the part of the externally applied axial load that is seen by the fastener, i.e.,

F_{tot} = F_{v\ max} + \Delta F_{b,A} \tag{7.9.1}

Failure criteria for ‘external load’ are only relevant when there is gapping (i.e. zero clamping force between the flanges), which leads to the fastener transmitting the full external axial load (i.e. Fb =FA).

Friction grip joints generally do not normally need assessment of this criterion since they are intended to transmit shear forces in case of minimum compression forces and therefore they are more critical in the slipping failure mode.

Table 7-3 shows which axial strength criteria be checked for each type of joint.

Table 7-3Criteria for Axial Load Analysis

Fastener Loading	Case	Joint Type	Check to be performed
Overall load (F_tot)	No Gapping	Friction Grip	Yes
Overall load (F_tot)	No Gapping	Bearing	Yes
External load (F_A)	Gapping	Friction Grip	No
External load (F_A)	Gapping	Bearing	Yes

The following Section presents the equations for the relevant MoS.

7.9.2 Margin of Safety on Fastener Failure

The margins of safety for external load criteria are all based on the fastener material’s yield strength, σ y , and ultimate strength, σ ult .

The margins of safety for the overall load, $F_{tot}$ , are,

MoS_{tot,y} = \frac{A_{s} \sigma_{y}}{F_{V,\max} + \Phi_{n} F_{A} sf_{y}} - 1 \text{ (for yield failure)} \tag{7.9.2}

MoS_{tot,ult} = \frac{A_{S} \sigma_{ult}}{F_{V,\max} + \Phi_{n} F_{A} sf_{ult}} - 1 \text{ (for ultimate failure)} \tag{7.9.3}

If gapping is tolerable, the margins of safety on fastener failure, under the external load, $F_{A}$ , become:

MoS_{A,y} = \frac{A_{S} \sigma_{y}}{F_{A} s f_{y}} - 1 \text{ (for yield failure)} \tag{7.9.4}

MoS_{A,ult} = \frac{A_{S} \sigma_{ult}}{F_{A} s f_{ult}} - 1 \text{ (for ultimate failure)} \tag{7.9.5}

Equations [7.9.2] and [7.9.3] express the force increment seen by the fastener in terms of the external axial load, and include the appropriate loading plane factor and safety factors. The safety factors are only applied to the external axial load, FA , since the uncertainty in preload is already factored into FV,max .