Tissue Mechanics Flashcards
Mechanical stress
-the internal force divided by the cross sectional area of the surface on which the internal force acts
Tensile stress
-axial stress that occurs at the analysis plane as a result of a force or load that tends to pull apart the molecules bonding the object together at that plane
Compressive stress
-the axial stress that results when a load tends to push or squash the molecules of a material more tightly together at the analysis plane
Shear stress
-a transverse stress that acts parallel to the analysis plane as a result of forces acting parallel to this plane
Bending
-an example of a more complex load in which there are multiple stresses that occur across the analysis plane
What is the pro of an object with greater depth/diameter
-if 2 objects have the same cross sectional area but one of them has a greater depth, the object with the greater depth can withstand greater bending loads because they have a greater moment arm (from the center)
Torsion
-rotational stress
-object with a greater diameter can withstand greater torsional loads because the counteracting shear stresses are lower because they have a larger moment arm
Combined loads
-bones and cartilage often encounter a combination of loading configurations, as opposed to a single load such as a rope or cable
Mechanical strain
-quantification of the deformation of a material
Linear strain
-change in length (as a result of tensile or compressive stress)
Shear strain
-change in orientation of adjacent molecules (as a result of these molecules slipping past each other due to shear stress)
Elastic modulus
-the ratio of stress to strain
-shown graphically as the slope of the stress-strain curve
Elastic behaviour
-stretches under a tensile load, but returns to original shape when load is removed
Linear elastic behaviour
-as stress increases, the strain increases by proportional amount
Plastic behaviour
-permanent deformation of the object occurs under a load
Material strength
-has to do with maximum stress the material is able to withstand before failure
-there are several qualifications of strength
Yield point
-point on the stress-strain curve where further stress will cause permanent deformation
Yield strength
-stress at the elastic limit of a materials stress-strain curve
Ultimate strength
-maximum stress the material is capable of withstanding
Failure strength
-stress where failure actually occurs
-stress corresponding to the endpoint of the stress-strain curve
-failure in this sense means breakage or rupture
Ductile materials
-have large failure strains
Brittle materials
-have small failure strains
Hard materials
-have large failure stresses
Soft materials
-have small failure stresses
Toughness
-ability to absorb energy
-a material is tougher if more energy is required to break it
-estimation of the toughness of a material is given by the area under the curve
Viscoelastic materials
-any material that exhibits both viscous and elastic characteristics
Viscoelastic properties
-strain-rate dependency
-stress-relaxation
-creep
-hysteresis
Strain-rate dependency
-the rate at which you deform/strain a tissue will affect the stress it feels
-faster loading rate will create more stress than a slower loading rate
Stress-relaxation
-when a tissue is stretched and maintained at a constant length, the stress within the tissue will reduce over time
-eventually the stress will reach an equilibrium
Creep
-if a constant load is applied to a tissue, it will slowly continue to deform and eventually reach an equilibrium length
Hysteresis
-the loading and unloading stress-strain curves of viscoelastic materials will differ
-this is energy lost due to heat
Active elements of musculoskeletal system
-muscle tissues
Passive elements of musculoskeletal system
-connective tissues
Connective tissue
-composed of living cells and collagen, elastin, ground substance, minerals, and water
Isotropic materials
-have the same mechanical properties in every direction
Anisotropic materials
-have different mechanical properties depending on the direction of the load
2 primary factors affecting mechanical properties of tissues
-activity
-age
What type of stress are bones strongest and weakest in
-strongest in compression
-weakest in shear
Types of cartilage
-hyaline cartilage
-fibrous cartilage
-elastic cartilage
Tendons and ligaments
-ligaments have more elastin than tendons
