Mechanics of Biologic Materials

Question 1

Internal Loading

Answer 1

External (mechanical) loads (forces) deform internal structures in addition to producing movement
Ligament
Tendon
Cartilage
Bone
Muscle
Material properties describe the responses of these internal structures to the mechanical loads

Question 2

Mechanical Loading Types

Answer 2

Uniaxial
Compression
Tension
Shear
Combined
Bending
Torsion
Mechanical loads create mechanical stress

Question 3

Stress

Answer 3

Force (internal) per unit area over which the force acts (σ)
σ = force / surface area

Units: N/m2 or Pascal
Force (N)
Surface area (m2)

Similar to pressure, but multidirectional
Able to compare elements of different shapes and sizes

Question 4

Three Principal Types of Mechanical Stress

Answer 4

Tension
Compression
Shear

Question 5

Tension

Answer 5

Pulling or stretching force directed axially through a body
Acts ⊥ to analysis plane (axial or normal stress)
Analysis plane: cuts structure ⊥ through longest dimension

Question 6

Compression

Answer 6

Pressing or squeezing force directed axially through a body

Acts ⊥ to analysis plane (axial or normal stress)

Question 7

Shear

Answer 7

Force directed parallel to a surface
Causes a sliding of surfaces in opposite directions
Acts // to analysis plane (transverse stress)

Question 8

Combined Loads

Answer 8

More common during functional activities

Cause:
Multiple stresses at analysis plane
Different magnitudes of stress at analysis plane

Types:
Bending
Torsion

Question 9

What is Bending?

Answer 9

Loads applied to structure causing it to bend about a neutral axis
Compression & Tension develop on opposite sides of axis
Shear developed along axis

Question 10

What is Torsion?

Answer 10

Load producing twisting of a body around its longitudinal axis
Shear stress occurs parallel to axis (greatest magnitude)
Compression & tension occur diagonal to axis

Question 11

Strain

Answer 11

Mechanical stresses causes strain (ε)
Quantification of the deformation of a material
Excessive deformation (strain) causes injury

Deformation of a material with external forces
Linear strain due to compressive or tensile loads
Standardized as % total length

Question 12

Assessing Biomechanical Properties of Tissue

Answer 12

Apply a known load and measure the resulting deformation

Load – Deformation Curve

Question 13

Load-Deformation Curve

Answer 13

Strength (resistance to deformation / failure)
Yield strength
Ultimate strength
Failure strength
Total deformation before failure point
Energy stored before failure point
Area under the curve

Stiffness (k = F / x) or (k = load / deformation)
Slope of the elastic region
↑ slope = ↑ stiffness

Question 14

Load-Deformation Curve: Limitation

Answer 14

Can’t compare materials of different size & shape

Question 15

Load-Deformation Curve: Fix

Answer 15

Stress – Strain Curve
Load per unit area (stress) vs. Amount of deformation as a % of change in length (strain)
Stress = force / surface area
Strain = ∆ length / original length
Standardizes for size & shape of different materials