Composites + Tensile test

Question 1

How was the composites made for the lab?

Answer 1

VART (Vaccum Assisted Resin Transfer).
1. Fibers are placed in a mold
2. Resin is injected
3. Vaccum helps distribute the resin evenly

Question 2

Why is the matrix needed if the fibers are strong?

Answer 2

Small diameter fibers are very strong, however they buckle easily. Therefore a supporting matrix is needed.

Question 3

What are unidirectional plies?

Answer 3

• All fibers are parallel
• They are strong and stiff in fiber direction, but weak perpendicular to fibers

Question 4

what is an orthotropic material?

Answer 4

Different properties in different directions

Question 5

What are the elastic constants of unidirectional plies? And factors influence the different constants?

Answer 5

• 4 elastic constants: E_1,E_2, v_12, G_12
• E_1is fiber dominated, increases with fiber volume fraction
• Rest is matrix dominated

Question 6

Why is grip tape needed during a tensile test of a laminate?

Answer 6

• Provides a secure grip, preventing slip
• Avoids stress concentrations and helps distribute the stress more evenly
• Protect from damage or deformation that could occur if the grips directly contact the material

Question 7

How does a strain gauge work?

Answer 7

It converts applied force to an electrical signal that can be measures. They have small metal wires that expand with the material, changing the resistance through the material.

Question 8

What are the failure mechanisms on a ply level?

Answer 8

• Fibre failure
• Matrix cracking
• Delamination

Question 9

During a tensile test of [0,90,90,0] laminate, what is the specific role of the 0-degree fibers and the 90-degree fibers?

Answer 9

The 0-degree fibres are primarily responsible for carrying the axial tensile load, while the 90-degree fibres provide transverse stiffness and prevent delamination.

Question 10

When does fiber failure occur?

Answer 10

When the applied load exceeds the strength of the individual fibers. This failure can manifest as fractures, splitting, or other forms of damage to the fibers.

Question 11

What is matrix cracking, and howis it manifested in the stress-strain curve?

Answer 11

• Formation of microscopic cracks within the matrix
• A turn in the curve during the linear region can represent start of matrix failure.
• When the matrix is saturated with cracks, the curve is linear again with some stiffness compromised.
• After the matrix crack, the stresses are distributed mainly among the fibres.

Question 12

What is delamination?

Answer 12

• A mode of failure where there is a seperation or debonding between the layers in a composite

Question 13

What are the five ply strenght properties?

Answer 13

1. Tensile Strength in 1 direction (fiber direction)
2. Tensile Strength in 2 direction (perpendic. to fibers)
3. Compressive Strength in 1 direction (fiber direction)
4. Compressive Strength in 2 direction (perp. to fibers)
5. In-Plane Shear Strength in 1-2 direction