Metals Flashcards
What are the 3 main behaviours of metals?
Yield, Fracture and Fatigue
What are the 4 main metals used in construction?
Wrought Iron
Cast Iron
Steel
Prestressed Steel
What is Mild Steel?
Steel with low carbon content. It is quite ductile but has a low elastic limit so is rarely used in construction now
How do Mild Steel and High Strength Steel differ in their stress/strain graphs?
Mild: Short period of elastic behaviour up to yield stress, followed by work hardening and a small increase in stress before reaching UTS
High Strength: Long period of elastic behaviour which gradually decreases in gradient up to UTS - proof strength estimated using 0.2% strain
In what direction does steel crack under tension?
Diagonally
Define fatigue
Failure of a material due to repeated loading
How does increasing the carbon content affect a steel’s properties?
Increases strength but lowers ductility
What is wrought iron often used for and why?
Suspension bridges, because of its high ductility
What are the two primary non-ferrous metals used in construction and what are their properties?
Aluminium: Low density, high resistance to corrosion, high ductility
Copper: High electrical conductivity, resistance to chemical weathering via green patina
Why is concrete almost always reinforced with steel?
It allows for cheap yet strong constructions which can take on a greater variety of shapes.
Without reinforcement, concrete cracks under tension and hence fails immediately via brittle failure
How can reinforced concrete still crack?
If the ratio of the cross-sectional area of the steel and concrete is too low (i.e. there is not enough steel), hence the concrete takes most of the tension
How does reinforcement change the failure behaviour of concrete?
Instead of a single large crack, many smaller cracks form along the length proportional to the amount of tension in that part of the beam
What is a cheap alternative to reinforced concrete in areas with small stresses?
Leaving the area devoid of structural material
What is prestressing and why is it used?
Where the steel is stressed in tension before the structure is assembled such that the concrete around it is in compression and hence better resists tensile loading
Name and describe the two methods of prestressing
Pre-tensioning: Using external jacks to tense the steel before casting the concrete. After setting, the jacks are removed. Grooves in the steel grab and compress the concrete.
Post-tensioning: Casting the concrete with a space for the steel to be fed into. The steel is tensed as it is put in and then anchored to the ends of the beam, compressing the concrete.
What is important to consider when prestressing?
The length of the concrete beam will be decreased
Define concrete creep
The process where, under constant loading, water migrates out of the concrete such that its height decreases and width increases plastically.
The rate of creep increases over time
What other mechanical process can negate the effect of prestressing?
Steel relaxation
How can creep be accounted for?
Decreasing the water in the concrete mix design
Adding admixtures that oppose creep
How can internal metal tendons be used to resist bridge collapse?
Putting the tendons in an “M” configuration means that tensile forces from loading will lift the centre of the bridge
What is wet corrosion?
When metal is in contact with an electrolyte which ionises the metal and can form voluminous products which reduce structural integrity by creating internal stresses. This can lead to complete failure
How is reinforced concrete designed to oppose corrosion?
Concrete is designed to have a high pH, creating a passive layer around the steel which prevents corrosion
What are 3 types of corrosion?
Pitting corrosion: Corrosion localised on a specific point
Stress corrosion: Corrosion creates a crack which can then propagate due to applied stresses
Corrosion fatigue: Reduced fatigue life as a result of minor corrosions
What are 3 ways corrosion can be stayed?
Protective Coatings
Cathodic Protection: Creating an anode which has no structural purpose but is corroded instead of the structural members
Passive Coatings: Creating a thin, chemically resistant film across the metal
How is fire resistance defined as a metal property?
The time taken for the deflection of a metal member to reach a specific limit under high heat
At 400C and 700C how are the metal properties changed?
400C - Young’s Modulus and Yield Stress are greatly reduced
700C - Both the above properties have decreased by 90% each
What is intumescent paint?
Paint that expands into a low-density foam when heated, creating a heatproof barrier around the metal
