MECHENG236 - Adhesives

Question 1

Advantages and disadvantages of adhesive bonding

Answer 1

Advantages * Sealing as well as jointing. * Uniform distribution of mechanical stress over the joint. * Good vibration damping. * Low weight penalty. * Reduced part count. * Can reduce assembly time. * Can improve fatigue and corrosion resistance. * Reduced stress concentration (no bolt/rivet holes). * Can join very dissimilar materials. * Joints may be electrically/thermally conductive or insulating
Disadvantages * Disassembly is difficult/impossible. * Good surface preparation required. * Accurate assembly may be required (jigs, fixtures). * Non-destructive testing (NDT) difficult. * Possible sensitivity to environment (both during assembly and in use) and service temperature. * Can be affected by age. * Low creep strength.

Question 2

Factors to consider between welding, mechanical fasteners, and adhesives

Answer 2

• Cost. * Is skilled labour required? * Is machinery required? * Heat and distortion. * Heat affected zone (HAZ). * On-site work. * Electrical conductivity/insulation

Question 3

Types of Adhesives. May be classified by

Answer 3

Chemistry *Epoxies, polyurethanes, polyimides, etc.
Form *Paste, liquid, film, pellets, tape.
Type *Hot melt, reactive hot melt, thermosetting, pressure sensitive, contact.
Load-carrying capability *Structural, semi-structural, non-structural

Question 4

How can the stress distribution in adhesive joints be modified?

Answer 4

• Increase the adhesive thickness. * Select adhesive with different modulus. * Increase joint length. * Modify adhered geometry.

Question 5

What is ageing of an adhesive or adhesive bond

Answer 5

The ageing of an adhesive or adhesive bond is defined as the change in the quality of adhesive bonding as a function of time and the external effects that provoke the change.

Question 6

Factors causing ageing of an adhesive or adhesive bond

Answer 6

There are three groups of external factors that cause the ageing phenomenon both within the adhesive itself (cohesion)and the junction of the adhesive and the substrate (adhesion).
1. Mechanical effects: * Static loads * Dynamic loads * Cyclic loads * Impact Loads
2.Chemical Effects: * Effect of humidity and water condensation * Effect of atmospheric oxygen * Concentrations of gases, solvents, salts etc. in the environment
3. Physical Effects: * Temperature * Ultraviolet Radiation

Question 7

How to prevent effects of ageing and validate design with adhesives

Answer 7

Use a series of accelerated ageing tests