Adhesive joints

Question 1

What is the definition of adhesive joining?

Answer 1

Two (dissimilar) solid components are held together by contact with adhesive such that mechanical force is transferred across interfaces

Question 2

What are the possible bonding mechanisms in adhesive joints, and what enhances them?

Answer 2

1. Secondary bonding forces (dipoles, electrostatic attraction), enhanced by surface cleanliness
2. Mechanical interlocking (from surface roughening and pores for capillary action) improves strength due to increased effective contact surface area + abrading reveals clean and fresh/reactive surface
3. Primary bonds, enhanced by using a coupling agent/primer

Question 3

What are the main challanges of adhesive joining?

Answer 3

• Complicated and sensitive manufacturing process (surface preparation, cleaning, etc)
• Sensitivity of the bondline to the working enviroment (temperature, diffusion of moisture)
• uncertainty in long-term properties (creep, fatigue)
• disassembly

Question 4

What needs to be considered when selecting a structural adhesive bonding method?

Answer 4

• Nature of the adherends (must bond to the surface)
• Range of application temperatures
• Strength + long term properties/enviromental performance
• Working conditions during installation

Question 5

How much of the surface does the adhesive interact with?

Answer 5

A very thin layer of molecules at the very to of the substrate (ca. 10 molecules thick, dimensions in (tens of) [nm]). This is what the adhesive interacts and forms bonds with

Question 6

What is the most important thing to control for achieving predictable and consistent joint performance?

Answer 6

The surface quality

Question 7

What is the most accessible way to characterize the surface quality?

Answer 7

Contact angle based methods. It shows how strongly this specific surface attracts the specific liquid.

Question 8

What is the purpose of surface preparation?

Answer 8

Aims to make the chemistry and the composition of surface molecules most suitable for bonding. The molecules become reactive and eager to bond with the adhesive

Question 9

What are the possible curing methods for fixing the adhesive?

Answer 9

• Pressure under force control (glueline settles) to get uniform thickness and help with removal of air and volatiles.
• Vaccuum to draw away air
• Temperature in the oven or autoclave

Question 10

If we have a relatively low-strength adhesive, connected to a much stronger substrate, what needs to be considered?

Answer 10

• Bonded area must be much larger than the crosssection of the substrates. As large as possible!
• Joint should preferably be loaded in compression or shear, avoid cleavage, peel, tension.
• Joint design aims to minimize stress concentrations

Question 11

Which type of joint stresses are the most desirable and which is to be avoided?

Answer 11

Desired: Compression and shear
Avoid: Peel, tension and cleavage

Question 12

What are some ways to allivate the tension in adherent joints?

Answer 12

• Tapering (overlap ends more flexible, to reduce stress concentrations)
• fix the ends

Question 13

How does the failure begin in the glueline?

Answer 13

• A series of microcracks develop
• Failure starts at the end of the overlap, moved towards the centre
• Microcracks precede the macrocrack

Question 14

What are the failure types in adhesive joints?

Answer 14

• Adhesive failure – interface failure between adhesive and one of the adherends. One surface looks clean, the other covered with adhesive
• Cohesive failure - failure inside the adhesive layer, both surfaces remain covered with adhesive. This is the ideal

Question 15

What are some pros of adhesive joints?

Answer 15

• A large stress bearing area where stresses are uniformly distributed
• Good strength to weight ratio
• The adhesive can have other functions (insulating, seal ot gases/liquids, dampen)