Metal Bending Assessment - Mechanical Properties of Materials Lecture

Define Strength.

the ability of a material to resist forces

• we often make choices on the strength of the materials we use of fabrication

• to compare the strengths of different materials, we must first understand stress

Define Stress.

relates to both magnitude of applied forces and amount of material resisting the force

• σ = amount of stress

• F = applied force in pounds

• A = cross-sectional area in square inches
  

Will the same amount of force applied over different ares cause radically different stresses?

yes!

What are the four types of stress?

1. tensile

2. compressive

3. shear

4. flexural

Tensile Stresses

act to pull apart an object

• occur parallel to the lines of force but perpendicular to the area in question

Compressive Stresses

act to squeeze or compress objects

• compressive stresses also occur parallel to the lines of force and perpendicular to the cross-sectional area

Shear Stress

act to scissor or shear the object, causing the planes of the material to slide over each other

• occur parallel to applied forces and cross-sectional area

Flexural Stress

bending stresses

• combination of tension and compression stresses

What is Strain?

• material subjected to sufficient stress will deform or change shape

• lengthening or shortening due to stress is called experiencing strain

ε = strain

ΔL = change in length

L0 = original length

strain is dimensionless

Tension Test

specimen size and testing conditions standardized

• how hard is it to pull apart?

What effects the shape of the stress-strain curve?

• composition

• heat treatment

• prior history of plastic deformation

• strain rate - how fast it deforms

• temperature

• state of stress

When stress is linearly proportional to strain, strain is _______.

elastic

If the strain is elastic, the stressed part will [ return, not return] to original shape?.

return

In Plastic Deformation, the material _______ deformed after the stress is removed.

remains

Hooke’s Law

σ=εE OR E =σ/ε

σ = stress in psi

ε = strain in inch/inch

E = Young’s modulus, or modulus of elasticity

• the modulus E, is the slop of the initial straight line portion of the stress-strain curve

Combines equations for stress, strain, and modulus where F, L0, and A are constants, shows the linear relationship between F and ΔL

If there is 2x force put on a material. . .

you get 2x the amount of length change

Proportional Limit

slope begins to change and linearity ends. permanent change in shape starts

Yield Point

first point where increase in strain appears without increase in stress. Not all materials have a yield point.

Tensile Strength

maximum tensile force (e) divided by original cross-sectional area

Toughness

area under the curve to the point of maximum stress (a-b-c-d-e), the ability to withstand shock loads before rupturing

Plastic or Permanent Strain (εp)

permanent strain when load removed

Thermal Stress

when a material is subjected to a change in temp, its dimensions will change. If the material is constrained by neighboring structures, stress is produced.

Plastics have a _________ coefficient of thermal expansion.

higher

Centroid

The centroid of an area is defined as the point of application of the resultant of a uniformly force acting on the area.

The center of gravity and centroid of two identically shaped objects will be the same if the density is uniform in each object.

Second Moment of Inertia (second moment of area, area moment of inertia)

is a property of a shape that is used to predict its resistance to bending and deflection.

• for a rectangle L = bh^3 /12

Stresses in Beams

there is a difference

Cantilevered Beam

One end of the beam is fixed.

Freely Supported Beam

Comparisons of Section Moduli

these are the same materials and everything, the only difference is the configuration

Most materials exhibit a linear behavior at least initially on their stress-srain curve describe by ________ Law.

Hooke’s

When subject to flexural stress the amount a beam bends is influenced by its ______.

shape