Presentation on theme: "Biological Materials:"— Presentation transcript:
1 Biological Materials: Chapter 9Mechanics ofBiological Materials:Stresses and Strainson the body
2 LoadsThe external forces that act on the body impose loads that affect the internal structures of the body.
3 MechanicsScience concerned with the effects of forces acting on objects (body)Rigid-body mechanicsDeformable body mechanicsFluid mechanicsRelativistic mechanicsQuantum mechanics
4 Rigid 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
5 Free 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
6 Pressure 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.
8 Pressure (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
9 Pressure or Mechanical Stress Force per unit area.P = Force / areaFor a similar forceincrease area==>decrease area==>For a similar areaincrease force==>decrease force==>
10 Bite Force Human female = 81 lbs Human male = 127 lbs Humans have 32 teeth
29 Mechanical 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.
30 Mechanical 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.
31 Stress Strain Load and Response force per unit area deformation amount of deformation divided by original length
33 Mechanical StrengthThe strength of a material has to do with the maximum stress (or strain) the material is able to withstand before failure.
34 ToughnessMechanically, toughness is the ability to absorb energy and not fail (or before failure).
35 StrainStrain is the quantification of the deformation of a material
36 Linear StrainOccurs as a result of a change in the object’s length.
37 Shear StrainOccurs with a change in orientation of adjacent molecules as a result of these molecules slipping past each other.
38 InstronMeasuring stress and strain in biological materials
39 Mechanical Properties of the Musculoskeletal System Age and activity level affect the mechanical properties of all connective tissue.
40 BoneBones are strongest in compression and weakest in shear.
41 Cartilage Three kinds: Hyaline cartilage (articular cartilage) - covers ends of long bones in joints
42 CartilageFibrous 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.
43 CartilageElastic cartilage - found in the external ear and tip of the nose.
44 CartilageCartilage is able to withstand compressive, tensile, and shear loads.Articular cartilage transmits the compressive loads from bone to bone at joints
45 Cartilage 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.
46 CartilageFunction 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.
47 Articular 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.
48 Articular 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.
49 Ligaments and TendonsLigaments, tendons, and cartilage all have similarly shaped stress-strain curves due to their collagenous composition.
50 Ligaments and TendonsUnder low stresses, these materials are pliant, but as the stresses increase past a certain threshold, they become much stiffer.
51 MuscleThe mechanical properties of muscle are not as easily examined due to its contractile ability.
52 MuscleThe ultimate stress of muscle is less that that of tendon, ligament, or bone, whereas its failure strain is much greater.