Crack Theory + Ceramics

Question 1

What is Crack Theory

Answer 1

• The explanation of why the practical, mechanical strength of a material is lower than its theoretical mechanical strength when in use.
• This was because of irregularities in the surface that cause stress concentrations and then cracks.

Question 2

Why do some steels crack more easily than others?

Answer 2

• If the microstructure of a steel has smaller grain size, it will crack more easily
• This is because a straighter line can be formed through the grain boundaries which allows for cracks to propagate.
• These are called slip planes, the smaller the grain size, the more slip planes there will be.

Question 3

What is the Critical Crack Length

Answer 3

• The maximum length a crack can reach before failure occurs

- The longer the critical crack length of a material, the less likely it is to fail

Question 4

What are some methods to eliminate cracks from forming

Answer 4

• Design the item without sharp corners as they concentrate stress at a point
• Place an interface within a material to block the cracks from passing the critical length.
• Use materials with a high critical crack length

Question 5

What are the mechanical properties of stone

Answer 5

• Weak in tension but strong in compression
• Brittle/Low toughness
• Brittleness makes it favourable for chisellng

Question 6

What are the mechanical properties of glass

Answer 6

• Extremely brittle

- Weak in tension but strong in compression

Question 7

How is toughened glass made

Answer 7

• The glass pane is heated then the outer surfaces are cooled quickly by blasts of cold air
• As the interior cools it contracts and places the outer surface into compression
• This compressive force must now be overcome before tension failure can occur which makes the glass stronger

Question 8

What are the mechanical properties of cement

Answer 8

• Weak in tension but strong in compression
• Brittle/Low toughness
• Can be made into any shape through casting

Question 9

What is concrete made of

Answer 9

• Cement, sand and aggregate

- The sand fills in the gaps between the aggregate and the cement acts as the binder that holds everything together

Question 10

Properties of concrete

Answer 10

• Similar to cement but stronger
• Fireproof
• Does not corrode

Question 11

How does reinforced concrete improve concrete’s inherent weakness in tension

Answer 11

• Concrete is reinforced with rods or steel mesh that will resist tensile loads

Question 12

How does Pre-Stressed Reinforced Concrete work

Answer 12

• Pre-Stressed (pre-tensioned) Reinforced Concrete involves casting concrete over steel rods that have already been tensioned.
• Once the concrete solidifies, the cables are released which leaves the beam in a compressive state

Question 13

How does Post-Stressed Reinforced Concrete work

Answer 13

• Post-Stressed (post-tensioned) Reinforced Concrete involves casting concrete over tubes.
• After the concrete sets, wires are pulled through the tubes and anchored to plates at one end and tensioned at the other
• The tensioning tends to compress the concrete slab and place it into a compressive state

Question 14

What happens when the reinforced steel within the concrete corrodes?

Answer 14

• Spalling (Concrete Cancer) occurs which causes expansion of the steel and cause cracks to form

Question 15

What is Asphalt made up of

Answer 15

• Hard Aggregate

- Bitumen

Question 16

What are the properties of Asphalt?

Answer 16

• Tough and crack resistant
• Hard-wearing
• Deals well with slight movements better than concrete

Question 17

What are the advantages of Laminated Glass

Answer 17

• Shatter-resistant

- Does not break into shards after impact