Stress Types, Equations, and Material Properties in Engineering

What are the three common states of stress?

Compression, tensile, and shear stress

Which one is tensile stress?

which is shear stress

Which one shows compression stress

What is the equation for stress?

σ=F/Ao

What does a tensile testing machine look like?

What is the equation for shear stress?

𝜏 = F/Ao

What is the unit of stress in the metric system?

MPa (megapascals), which is equal to 10^6 Pa or 10^6 N/m².

What is Poisson's ratio and its typical value for polymers?

𝑣, unitless, and typically around 0.4 for polymers.

How is Poisson's ratio defined mathematically?

𝑣 = -transverse strain / longitudinal strain.

what is shear strain denoted as?

γ (gamma)

What does the shear strain (γ) represent?

Shear strain is defined as γ = Δx/y = Tan Θ.

What is the modulus of elasticity (E)?

measures a material's resistance to elastic deformation.

What are other names for the modulus of elasticity (E)?

Young's modulus and elastic modulus

What units is E in?

GPa or MSI where 1 GPa = 10^9 Pa

What is Hooke's Law?

stress (σ) is proportional to strain (ε) in the elastic region: σ = E ε.

What is yield strength?

the stress level at which noticeable plastic deformation occurs.

What is tensile strength (TS)?

The maximum stress that a material can withstand on the stress-strain curve.

Define ductility in materials.

the amount of plastic deformation a material can undergo before failure.

What does toughness measure?

the energy absorbed by a material before fracture.

What happens to PMMA properties when temperature decreases?

Lower temperatures increase the strength and decrease ductility, making it more brittle.

What does the relaxation modulus (Er) indicate?

how a material's modulus decreases with temperature above its glass transition temperature (Tg).

What is the significance of the glass transition temperature (Tg) for polymers?

it’s the temperature at which a polymer transitions from a hard, glassy state to a softer, more rubbery state.

What are hardness tests used for?

Hardness tests measure a material's resistance to surface plastic deformation, such as dents or scratches.

What is the relationship between stiffness and modulus of elasticity?

Higher modulus of elasticity (E) indicates a stiffer material that deforms less under stress.

What is the Elastic Shear Modulus denoted as and what is it for?

Denoted as G, it's used for torsion tests (twisting)

What is the formula for elastic shear modulus (G)?

The elastic shear modulus is defined as τ = G γ, where τ is shear stress and γ is shear strain.

What does the term 'plastic deformation' refer to?

permanent deformation that occurs beyond the elastic limit of a material.

Is plastic deformation linear or non-linear on a stress-strain curve

non-linear part

What is the significance of the elastic (linear) portion of the stress-strain curve?

the material returns to its original shape after the load is removed.

What is εZ?

a measure of deformation in the z direction under tensile stress.

What is yield strength denoted as?

σy

What does yield strength represent?

When there is noticable plastic deformation (non-linear bend)

Where does fracture strength occur on a stress vs. strain graph?

Very end

What is tensile strength on a stress vs. strain graph?

max stress on stress-strain curve

Define ductility

Amount of plastic deformation at failure

Which material has low ductility and which one has high ductility

what is toughness?

amount of energy absorbed by material before fracture

Where is toughness on a stress vs. strain graph?

area under the curve

what are the units of toughness?

enery/unit volume

draw a three point bend (flex) test

What materials are tested in a 3 point bend test?

polymers/composites

what is a hardness test?

measure resistance to surface plastic deformation (dent or scratch)

what does high hardness mean?

high resistance to dent (scratch) from compressive load

What is a kind of hardness test?

Rockwell

Draw bending (flexural) force

Draw torsion force

Draw an Izod (impact test)

What is ultimate strength?

The max amount of stress a material can withstand before necking