Mechanical Properties of Solids

What are different types of loads and how do materials respond to them?

• static loads: loads that remain constant over time

• dynamic loads: loads that change over time, such as impact

• environmental loads: loads caused by environmental factors like temperature, humidity, or corrosion

How do materials respond to Static Loads?

• materials experience Elastic Deformation or Plastic Deformation if the load exceeds the yield strength

• Very high static Loads can cause fracture

• ex. a building column supporting a roof

How do materials respond to Dynamic Loads?

• can cause fatigue, leading to fracture at lower stress levels than static loads

• ex. car wheels hitting bumps

How do materials respond to Environmental Loads?

• thermal expansion can cause internal stresses, and corrosion can weaken materials

• ex. rust weakening a bridge

What is Tensile Testing?

• mechanical test that measures how a material responds to pulling or stretching forces

• sample is pulled until it breaks, while force and elongation are recorded

What is tensile testing used for?

to determine:

• yield strength (when permanent deformation starts)

• ultimate tensile strength (UTS) (maximum stress before breaking)

• Elongation/strain (how much it stretches/ductility)

• Modulus of Elasticity (stiffness of the material)

What does a tensile stress strain curve look like for metals?

what does a tensile stress-strain curve look like for elastomer?

what does a tensile stress-strain curve look like for ceramics, glasses and concrete?

What is engineering stress?

the applied force divided by the original cross-sectional area of a matieral

σ = F/Ao

What is engineering strain?

the change in length of a material divided by its original length

ε = ΔL/Lo

What is yield strength?

the stress at which a material begins to deform permanentl

What are elastic properties obtained from the tensile test?

• Young’s Modulus (E) E = S/e

• Poisson’s ratio (v) v = -e_lateral/e_longitudinal

• Modulus of Resilience (Er) Er = (1/2)(yield strength)(strain at yielding)

what is tensile toughness?

the energy absorbed by a material prior to fracture

the are under the stress-strain curve

what is ductility?

the ability of a material to be permanently deformed without breaking when a force is applied

How does temperature affect the mechanical behaviour of materials?

High Temperatures:

• decreases yield strength and UTS → material is softer and more ductile

• increases plastic deformation

Lower Temperatures:

• increases strength but decreases ductility → materials become more brittle

• can lead to fracture at a lower stress under impact or dynamic loads

Dislocations:

• higher temp → dislocations move more easily → easier plastic deformation

• lower temp → dislocations move less → materials resist deformation

What is the Bend Test for Brittle Materials?

a mechanical test used to measure the flexural strength of brittle materials

what is flexural strength or modulus or rupture?

the stress requires to fracture a specimen during a bend test

what is the flexural modulus?

the modulus of elasticity calculated from the bend test results

What is Hardness?

resistance to penetration by sharp objects

• macro-hardness: hardness measured using loads >2 N

• micro-hardness: hardness measured using loads <2 N

• nano-hardness: hardness of materials measured at 1-10m, length using very small forces (~100μN)

What is an Impact Test?

measures the how much a material can absorb a sudden load without breaking

What is Impact Toughness?

the energy absorbed by a material during fracture during the impact test

what are the properties obtained from the impact test?

ductile to brittle transition temperature (DBTT): the temperature below which a material behaves in a brittle manner in an impact test