Material Properties -

Basic concepts

Load
- Force applied to the surface or internal cross-section of a material
- Measured as Newtons
Stress
- A measure of the impact of an external force on the internal structure
- Force divided by cross-sectional area = N/m^2
Strain
- Is the measure of deformation
- Change of length (distance) / original length (distance) = no units
Young’s modulus
- Characteristic of a material within its elastic deformation range,
- Describes the change in length per unit strain
Hook’s Law
- The law governing spring or elastic behaviour within the elastic range
- The degree of deformation of a spring is proportional to the force applied
- Fs = kx
- Where Fs = force applied to spring, k = spring constant, and x = spring stretch or compression
Elastic deformation
- Deformation in response to a force which remains within the elastic zone, which will then return to original shape / length when the force is removed
Plastic deformation
- Permanent deformation of a material, without complete structural failure
Toughness
Creep
- The progressive deformation of a material (in particular metals) in response to a force being applied for a prolonged period
Load relaxation
Hysteresis

Elastic zone
- Range of stress/strain within which materials will return to original dimensions
Yield point
- Transition point where elastic deformation transitions to plastic deformation
Yield strength
Breaking point
Ultimate strength
Isotropic material
- Response of material to load is the same regardless of direction of force applied
- Will be a homogenous / crystalline structure such as metal or ceramic
Anisotropic material
- Response of material to load depends on direction of force applied
- Will typically be fibrous material (eg ligaments / tendons) or a composite material (eg bone, carbon fibre)

Ceramic
- Isotropic material – response to force is the same in all directions
- Highest Young’s modulus of this list – most stress (N/m^2) required to produce a given amount of strain (fractional change of length)
- Brittle material – Has minimal capacity for plastic deformation before ultimate failure
Cobalt-chrome
- Isotropic material – stress/strain response is symmetrical in all directions.
- Second-highest Young’s modulus of listed materials
Stainless steel
- Isotropic material
- Ductile material – able to absorb significant amount of plastic deformation prior to absolute failure
Titanium
- Isotropic material
- Young’s modulus closest to cortical bone of all metals
Cortical bone
- Anisotropic material – response to force depends on direction of applied force, by virtue of its composite nature of fibres and crystals
PMMA
- Isotropic, brittle material
- Youngs modulus intermediate between cortical and cancellous bone
Polyethylene
Cancellous bone
- Anisotropic material – response to applied force depends on direction of application
Tendon / ligament
- Anisotropic material – response to force depends to direction of applied force depending on
Cartilage

Sean Griffiths, WH Resident, 2020