Abbott Aerospace – Analysis Method

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

Adhesive Joints – Failure Modes

Abbott Aerospace – Analysis Method

XL-VIKINGDisplay Your Math in Excel

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

Adhesive Joints – Failure Modes

On test and in service; the desired failure mode is that the first ply (or more) should separate from the surface of the adherend.

Single Adhesive Lap Joint Desired Failure Modes (NASA-CR-112236, 1973)

Abbott Aerospace – Analysis Method

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

Adhesive Joints – Failure Modes

The joint geometry affects the shear and peel stress in the joint; both of which peak at the edge of the adhesive.

Single Shear Joint Shear Stress Distribution (NASA-TN-D-7855, 1975)

Single Shear Joint Peel Stress Distribution (NASA-TN-D-7855, 1975)

Abbott Aerospace – Analysis Method

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

Adhesive Joints – Failure Modes

(NASA-CR-112236, 1973) gives the following example of a way to reduce the peak peelstresses in a single lap shear adhesive joint.

Abbott Aerospace – Analysis Method

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

Adhesive Joints – Failure Modes

It is not practical to increase the thickness as shown in the figure above, but the inclusion of a chamfer feature at the free edge of the adherend can increase apparent joint shear strength by 15% or more.

Experience has shown that a simple conservative apparent lap shear analysis has sufficient accuracy for most structural application if the joint conforms to preferred design guidelines.

In our experience, a suitable preliminary working value for the apparent shear strength of an adhesive joint at worst case environment condition is 1000psi. This value must be confirmed by test before being used in a critical application in service.