Presentation on theme: "Biological Materials:"— Presentation transcript:
1Biological Materials: Chapter 9Mechanics ofBiological Materials:Stresses and Strainson the body
2LoadsThe external forces that act on the body impose loads that affect the internal structures of the body.
3MechanicsScience concerned with the effects of forces acting on objects (body)Rigid-body mechanicsDeformable body mechanicsFluid mechanicsRelativistic mechanicsQuantum mechanics
4Rigid Body Mechanics Acceptable for analyzing gross movements Assumptionsbody does not deform by bending, stretching or compressingsegments are rigid links joined by frictionless hinges at joints
5Free body diagramFree body diagram - sketch that shows a defined system in isolation with all the force vectors acting on the systemdefined system: the body of interestvector: arrow to represent a forcelength: size of the forcetip: indicates directionlocation: point of application
6Pressure or Mechanical Stress Mechanical stress (pressure) is the internal force divided by the cross-sectional area of the surface on which the internal force acts.
8Pressure (P = F/a) Pressure - is the force per unit area. When forces are sustained by the human body, the smaller the area over which the force is distributed, the greater the likelihood of injury.Scalpel vs butter knife exampleStiletto heel vs moccasin
9Pressure or Mechanical Stress Force per unit area.P = Force / areaFor a similar forceincrease area==>decrease area==>For a similar areaincrease force==>decrease force==>
10Bite Force Human female = 81 lbs Human male = 127 lbs Humans have 32 teeth
29Mechanical loads on the human body: Bending - asymmetric loadingproduces tension on one side of the longitudinal axis and compression on the other sideAxial - directed along the longitudinal axis of a body.
30Mechanical loads Torsion Combined loading load producing twisting of a body around its longitudinal axis.Combined loadingSimultaneous action of more than one of the pure forms of loading.
31Stress Strain Load and Response force per unit area deformation amount of deformation divided by original length
33Mechanical StrengthThe strength of a material has to do with the maximum stress (or strain) the material is able to withstand before failure.
34ToughnessMechanically, toughness is the ability to absorb energy and not fail (or before failure).
35StrainStrain is the quantification of the deformation of a material
36Linear StrainOccurs as a result of a change in the object’s length.
37Shear StrainOccurs with a change in orientation of adjacent molecules as a result of these molecules slipping past each other.
38InstronMeasuring stress and strain in biological materials
39Mechanical Properties of the Musculoskeletal System Age and activity level affect the mechanical properties of all connective tissue.
40BoneBones are strongest in compression and weakest in shear.
41Cartilage Three kinds: Hyaline cartilage (articular cartilage) - covers ends of long bones in joints
42CartilageFibrous cartilage - found within some joint cavities (the menisci of the knee), the intervertebral discs, at the edges of some joint cavities, and at the insertions of tendons and ligaments into bones.
43CartilageElastic cartilage - found in the external ear and tip of the nose.
44CartilageCartilage is able to withstand compressive, tensile, and shear loads.Articular cartilage transmits the compressive loads from bone to bone at joints
45Cartilage Articular cartilage - serves two purposes: Spreads loads over a wide area so that the amount of stress at any contact point between the bones is reduced.It allows movement of the articulating bones at the joint with minimal friction and wear.
46CartilageFunction may include distribution of loads over the joint surfaces, improvement of the fit of the articulating surfaces, limitation of translation or slip of one bone with respect to another, protection of the periphery of the articulation, lubrication, and shock absorption.
47Articular Connective Tissue: Tendons - connect muscles to bones.Ligaments - connect bone to bone.Both are composed primarily of collagen and elastin fibers.Do not have the ability to contract, but they are slightly extensible.
48Articular Connective Tissue: These tissues are elastic and will return to their original length after being stretched, unless they are stretched beyond their elastic limits.Can only be fixed with surgery.
49Ligaments and TendonsLigaments, tendons, and cartilage all have similarly shaped stress-strain curves due to their collagenous composition.
50Ligaments and TendonsUnder low stresses, these materials are pliant, but as the stresses increase past a certain threshold, they become much stiffer.
51MuscleThe mechanical properties of muscle are not as easily examined due to its contractile ability.
52MuscleThe ultimate stress of muscle is less that that of tendon, ligament, or bone, whereas its failure strain is much greater.