Material Science: Adhesives and Testing

Adhesive Properties

• In a brittle adhesive, the ultimate tensile strength is much lower than the yield strength.
• The ultimate tensile strength is not always the same as the yield strength.

Testing Adhesives

• The single lap shear test is a popular method for testing adhesively bonded joints because it is cheap and simple to perform.
• The test does not accurately represent the true shear strength of the adhesive, and the adhesive is not subjected to uniform stress across the joint.
• Bond line thickness and load rates can affect the results of the single lap shear test.

Fracture Types

• Based on a stress/strain curve, brittle fractures can be identified.
• Brittle fractures are different from ductile and plastic fractures.

Failure Modes

• Failure modes can be categorized as cohesive, inter-facial, mixed-mode, or brittle.
• Adhesive, cohesive, and substrate failures are different types of failure modes.
• Mixed-mode failure is a type of failure mode.

Causes of Adhesive Failure

• Improper preparation of the substrate surface can lead to adhesive failure.
• Excessive bond-line thickness and defects in the adhesive layer can also cause adhesive failure.
• Consistent bond-line thickness is not a cause of adhesive failure.

Thermoplastic Adhesives

• Thermoplastic adhesives are polymer-based adhesives that are applied in a molten state and cure as they cool.
• They can be reversible upon re-heating.

Particle Fillers

• Particle fillers are often used to reduce costs and increase tack.
• They can also reduce viscosity, but this is not their primary purpose.

Additives

• Plasticizers are substances added to adhesives to improve flexibility, reduce brittleness, and decrease viscosity.
• Wetting agents, thickeners, and hardeners are different types of additives.

Material Properties

• Young's modulus is a measure of stress to recoverable strain of a material.
• Poisson's ratio is a different material property.

Material Laws

• Hooke's Law states that the stress imposed on a material, within its elastic limit, is proportional to the strain produced.
• Young's Law, Ohm's Law, and Brinell's Law are different laws.