Bonded Joint

Question 1

What are several of the advantages of Adhesive Bonds?

Answer 1

More even load distribution; More resistant to flex, fatigue, and vibrational
stresses; Negligible weight penalty; Bonds and seal the joints; Smooth contour
and no change in part dimension; Less expensive

Question 2

What are several of the disadvantages of Adhesive Bonds?

Answer 2

Requires surface preparation; Heat and pressure may be required during bonding;
Long cure time may be needed; Health and safety issues; Inspection is difficult;
More trained labor; Does not allow for repeated assembly and disassembly

Question 3

Simply, What is creep? What is fatigue?

Answer 3

Creep - continued strain at constant load.

Fatigue – strain from cycled stress loading and unloading

Question 4

What are the two main types of bonding in composites?

Answer 4

Cohesive and Adhesive

Question 5

What is the first requirement for an adhesive to be effective as glue? How would
you measure this experimentally? Explain clearly

Answer 5

The first requirement is that the adhesive should wet the surface; this requires that
the adhesive have favorable physical interaction with the surface. An indication
of this is the contact angle that the adhesive makes with the surface. To test this a
drop of adhesive is placed on the surface, if the adhesive does not like the surface,
it will bead up on the surface; takes a spherical shape. If it likes the surface, it will
spread on it.
Next, the adhesive must have the ability to withstand the required stresses and
withstand in-service environmental conditions.

Question 6

In a lap joint under tension, where is the shear stress the highest?

Answer 6

At the edges, where the two parts are bonded

Question 7

Explain the goals of what one is trying to achieve during surface preparation for
an adhesively bonded joint? This comes directly from the chemistry that should
occur at the interface.

Answer 7

i. Create more porosity to enhance mechanical interlocking. This is done by
making the surface more rough (sanding, filing).
ii. Cleaning the surface from loose particles and debris. This is done to
ensure that the mechanical interlocking would hold and not fail.
iii. Cleaning the surface with solvent. This is done to make sure that the
adhesive molecules come in contact with the surface, that they have a
chance to either physically interact or diffuse into the matrix.
iv. Applying chemicals to the surface to enhance its interaction with the
adhesive. i.e. a chemical applied to enhance the wetting ability of the
adhesive (to introduce oxygen-containing groups on their or cause some
sort of etching by chemical reaction).

Question 8

What thickness of adhesive should be used to bond to pieces together?

Answer 8

Depending on material and manufacturer, it is best to maintain a bond thickness
from 0.004 in. - 0.008 in. (Strong Co.) or from 0.005 in. - 0.015 in. (Hercules)
To maintain thickness, use bond wires (or fishing string), glass beads, or scrim
cloth,.