Machine Elements: Elasticity, Ductility, and Material Properties Part 2

What does the modulus of elasticity (E) represent?

The modulus of elasticity indicates the stiffness of a material or its resistance to deformation.

How is the modulus of elasticity determined from a stress-strain diagram?

It is the slope of the straight-line portion of the stress-strain diagram.

Define ductility in materials.

Ductility is the degree to which a material will deform before ultimate fracture.

What is the opposite of ductility?

Brittleness.

Why are ductile materials preferred in machine members?

Ductile materials allow for easy detection of impending failure and are less likely to fail suddenly.

What is the usual measure of ductility?

The percent elongation of the material after fracture in a standard tensile test.

What is a standard tensile test?

a controlled, destructive mechanical procedure where a precisely shaped material sample is pulled along its axis at a steady rate until it fractures

How is percent elongation calculated?

Percent elongation is the difference between the final length and the original length divided by the original length, expressed as a percentage.

What percent elongation indicates a ductile material?

A material is considered ductile if its percent elongation is greater than 5%, with >12% being advisable for repeated loads.

What does shear strength refer to?

the maximum resistance a material, component, or soil can withstand against structural failure or sliding when parallel, opposing forces push or pull on opposite sides.

Define Poisson's ratio.

a material property that measures how much a material expands or contracts sideways when you pull or push it lengthwise

What does the modulus of elasticity in shear (G) indicate?

It indicates a material's stiffness under shear loading and resistance to shear deformation.

What is the coefficient of thermal expansion?

It measures the change in length of a material subjected to a change in temperature.

How does temperature affect the size of materials?

Most materials expand with heat and contract when cooled.

What is the significance of the thermal expansion coefficient?

measures how much a material changes its size, area, or volume when its temperature changes

Why is it important for materials in a composite to have similar thermal expansion coefficients?

to prevent internal stress, cracking, and structural failure when temperatures change.

What is fatigue strength?

the highest stress level a material can handle for a set number of load cycles before breaking under repeated, cyclic bending, pulling, or pushing

How is fatigue strength determined?

through experimental cyclic testing of material specimens to create S-N curves (stress versus number of cycles to failure)

Define toughness in materials.

Toughness is the ability of a material to absorb applied energy without failure.

What is impact energy?

Impact energy is the amount of energy required to break a specimen made from a material.

Why is the geometry of a test sample important in toughness testing?

it controls the stress state, constraint, and triaxiality at the crack tip, ensuring reproducible and meaningful material data

Can impact energy values be used quantitatively in design calculations?

No, impact energy values measured from tests like Charpy V-notch cannot be used directly in quantitative design calculations for stress or load-bearing capacity.