Abbott Aerospace – Analysis Method

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Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Fastener Stiffness

Abbott Aerospace – Analysis Method

XL-VIKINGDisplay Your Math in Excel

Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Fastener Stiffness

Fastener shear stiffness or fastener joint stiffness’ are often used in bolt groupmodels and finite element models. It is important to note that at best, fastenerstiffness values are approximations. The stiffness of a joint and each fastener withinthe joint depends on many factors including – the condition of the joint surfaces, theclamp-up force in the joint, the clearance of the fasteners in the fastener holes, thematerial of the fastener, the material of the sheet, the thickness of the sheets, thesize of the fastener, the fastener head size, the nut type, whether washers are used,etc.

It is recommended that a relatively simple approach is used.(NACA-TN-1051, 1946) provides a rational method and experimental derivation for

the critical parameters.

Abbott Aerospace – Analysis Method

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Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Fastener Stiffness

The flexibility of a single fastener is given by the following expression:

𝐶 =8

𝑡𝑎𝑣 ∙ 𝐸𝑏𝑏∙ 𝐵1 ∙

𝑡𝑎𝑣𝐷

2

∙ 𝐵2 +𝑡𝑎𝑣𝐷

2

+ 𝐵3

Where:𝒕𝒂𝒗 Average joint plate thickness, in (see below)𝑬𝒃𝒃 Young’s modulus of the fastener, psi (see below)𝑫 Diameter of fastener, in𝑩𝟏 Joint parameter 1 (see table below)𝑩𝟐 Joint parameter 2 (see table below)𝑩𝟑 Joint parameter 3 (see table below)

Case Plate Strap Fastener B1 B2 B3

1 Aluminum Aluminum Aluminum 0.130 2.120 1.000

2 Steel Steel Steel 0.130 2.120 1.000

3 Aluminum Aluminum Steel 0.130 2.120 1.870

4 Aluminum Steel Steel 0.130 2.120 1.430

5 Aluminum Steel Aluminum 0.130 2.120 0.840

Abbott Aerospace – Analysis Method

XL-VIKINGDisplay Your Math in Excel

Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Fastener Stiffness

Average Joint Plate Thickness for a single shear joint:

𝑡𝑎𝑣 =𝑡1 + 𝑡22

Average Joint Plate Thickness for a Double Shear Joint:

𝑡𝑎𝑣 =2 ∙ 𝑡1 + 𝑡2

2

You can download a spreadsheet for this method at the link below:

AA-SM-028-001 Fastener Flexibility - NACA Method