Loading and Material Properties

Flashcards based on lecture notes about loading and material properties.

What is the technical definition of 'Load'?

Force or forces applied to a structure.

How do tissues respond to loading?

The tissues respond to the loading or lack of loading.

Which tissues are affected by loading?

Bones, capsule, ligaments, muscles, tendons, cartilage.

Why manipulate loads during rehabilitation?

To optimize function and structure.

What is deformation?

Changes in length, shape, etc. when a load is applied.

What is the toe region in a load-deformation curve?

Initial amount of load needed to ‘take up the slack’ of the tissue.

What is elasticity?

Ability to return to original state following deformation.

What do the X and Y axes represent in a Load-Deformation Curve?

Y-axis – Apply a load, X-axis – Amount of deformation due to the load.

What is the yield point?

End of elastic region; will not return to original state immediately after load is removed.

What is plasticity?

Permanent deformation occurs after the load is removed.

What happens during plasticity?

Loading great enough that permanent deformation occurs after the load is removed.

What is the ultimate failure point?

When loading in plastic region continues creating overt failure.

What are some types of failures at the ultimate failure point?

Rupture, Avulsion, Fracture.

Define Rupture in the context of load-deformation.

Rupture - within structure of connective tissue.

Define Avulsion in the context of load-deformation.

At tendon/ligament attachment to bone.

Define Fracture in the context of load-deformation.

Within bony tissue.

What is a load-deformation curve used for?

Determine strength and stiffness of whole structure.

What properties does a load-deformation curve examine?

Elasticity, plasticity, ultimate strength and stiffness.

What does a load-deformation curve tell us about the structure?

Amount of energy storage before failure.

What is stress?

Measure of a load in an object, expressed in terms of force per unit area.

What is strain?

Percent change in length or cross-section of material.

What does the stress-strain curve represent?

Amount of deformation of a sample vs. the load per unit area of the sample.

What is strength (of a material)?

Load sustained before failure, deformation sustained before failure, energy it can store before failure.

What is stiffness?

Resistance offered to external loads by a specimen or structure as it deforms.

How is stiffness indicated on a load-deformation curve?

Indicated by slope of load-deformation curve in elastic region.

What is modulus of elasticity (Young’s modulus)?

Value obtained by dividing the stress at any point in the elastic region of the stress-strain curve by the strain at that point.

What does Young's Modulus help examine?

Helps to examines stiffness of a material.

What is the relationship between Stiffness and Compliance?

Higher stiffness, lower compliance and vice-versa.

Give example value ranges of Young's Modulus for common orthopedic materials.

Stainless steel: 200, Titanium: 100, Cortical bone: 7-21, Cement: 2.5-3.5, Cancellous bone: 0.7-4.9.

What factors influence the type of stress and strain that develop in human tissues?

The material, type of load applied, point at which the load is applied, the direction and magnitude of the load, the rate of loading, duration of loading.

What can stress-strain curves be used to compare?

Strength properties of one material with that of another material and The same tissue under different conditions.

What is viscosity?

Material's resistance to flow.

What properties does a Viscoelastic Material exhibit?

Elasticity + viscosity.

Example of Viscoelastic Materials?

Connective tissues.

What is Creep?

Constant low loading over extended period that produces a slow deformation of soft tissues.

Give clinical examples of Creep, as a Friend.

Prevent foot drop during prolonged bed rest, weight-bearing training using tilt table, dynamic splints.

Give clinical examples of Creep, as a Foe.

Re-loading too soon.

What is stress-relaxation?

Ability of tissues to need gradually less stress (force) to maintain the same elongation over time.

Give clinical examples of Stress Relaxation.

Muscle stretching, joint mobilization, static progressive splint.

What is Hysteresis?

Loss of energy demonstrated by a viscoelastic material.

How does Hysteresis affect energy?

Difference between energy expended when loaded and energy regained when unloaded.

How can one determine if there is High or Low Hysteresis of a material?

If there is little-to-no loss then the material has low hysteresis, high loss → high hysteresis.

What is Strain-rate sensitivity?

Most tissues behave differently if loaded rapidly or slowly.

What happens when the load is applied rapidly?

The tissue is stiffer and a larger peak force can be applied to the tissue.

How to increase the length of a connective tissue structure with minimal risk of injury?

Apply a load slowly to the maximum tolerable level and then maintained while creep occurs.

What is Isotropic?

Properties do not vary regardless of where force is applied.

Give an example of an Isotropic Material.

Glass.

What is Anisotropic?

Behavior exhibited by a structure whose strength and elasticity vary when loaded in different directions.

List the loading modes?

Compression, Tension, Bending, Shear, Torsion, Combined loading.

Give an example of Anisotropic Material.

Bones