Mechanical Properties of Materials Lecture Notes PDF

Biophysics dept. Mechanical properties of materials Dr.Rehab Abdel Sattar LECTURE OUTLINE Bone modeling and remodeling. Mechanical stresses. Elastic moduli. General stress- strain curve. Definitions of some mechanical features of materials. LECTURE ILOs Stude...

Biophysics dept. Mechanical properties of materials Dr.Rehab Abdel Sattar LECTURE OUTLINE Bone modeling and remodeling. Mechanical stresses. Elastic moduli. General stress- strain curve. Definitions of some mechanical features of materials. LECTURE ILOs Student will be able to understand the difference between bone modeling and remodeling. Student will be able to analyze the different types of mechanical stresses. Student will be able to understand elastic moduli. Student will be able to analyze the general stress-strain curve. Student will be able to understand the different terms of mechanical properties of materials. Student will be able to apply the mechanical properties of Bones Bone is an important and dynamic living tissue. Its mechanical functions are to support and protect other body tissues and to act as a system of rigid levers that can be manipulated by the attached muscles. Bone’s strength and resistance to fracture depend on its material composition and organizational structure. Bones The properties of mandibular and maxillary bone are central to dental implant success. Bone is a specialized connective tissue consisting of an organic matrix permeated by a poorly crystallized calcium hydroxyapatite. The mineral composition of bone differs from that of other dental hard tissues and bone has a greater organic content. Bone modeling and remodeling According to Wolff’s law, the densities and, to a much lesser extent, the shapes and sizes of the bones of a given human being are a function of the magnitude and direction of the mechanical stresses that act on the bones. Dynamic mechanical loading causes bones to deform or strain, with larger loads producing higher levels of strain. These strains are translated into changes in bone shape and strength through a process known as remodeling. Remodeling involves resorption of fatigue-damaged older bone and subsequent formation of new bone.  Remodeling can occur in either “conservation mode,” with no change in bone mass, or “disuse mode,” with a net loss of bone mass characterized by an enlarged marrow cavity and thinned cortex. Bone modeling is the term given to formation of new bone that is not preceded by resorption, and is the process by which immature bones grow. Fractures A fracture is a disruption in the continuity of a bone. The nature of a fracture depends on the direction, magnitude, loading rate, and duration of the mechanical load sustained. Fractures are classified as simple when the bone ends remain within the surrounding soft tissues and compound when one or both bone ends protrude from the skin. Mechanical stress The most important properties of dental materials is their ability to withstand various mechanical stresses placed on them during their use as restoration, appliance, impression, …etc. Materials can be classified into: (i)Elastic: This is the material which returns back to its original shape and volume when the applied stress is removed. (ii)Plastic: This is the material which does not return back to its original shape and volume when the applied stress is removed. Stress: This is the value of the “applied force per unit area of the surface” of the material (F/A). The unit of stress is N/m2. Strain: This is the value of the “relative deformation that occurs to the material as a result of the applied stress”. Strain has no unit, it is just a ratio. Mechanical stresses Compressive stress, tensile stress, and shear stress point to the direction of the acting stress of the squeezing force. Stresses definitions 1.Tensile stress: When a long bar is subjected to a tension force along its axis; perpendicular to its cross-section, it will be applied by a tensile stress that equals the “perpendicular force per unit cross-sectional area”. 2.Compression stress: When a long bar is subjected to a compression force along its axis; perpendicular to its cross-section, it will be applied by a compression stress that equals the “perpendicular force per unit cross- sectional area”, 3.Shearing stress: When the material is subjected to a tangential force on its surface; it will be applied by a shear stress that is equal to the “tangential force per unit surface area”. Torsion occurs when a structure is caused to twist about its longitudinal axis, typically when one end of the structure is fixed. Elastic moduli Elastic modulus is the ratio between the stress acting on the body and the resulting strain; it is one of the physical properties of the material. Types of the elastic modulus are (stress/strain): 1. Young's modulus: Young's modulus is the ratio between the longitudinal stress and the longitudinal strain; Y= ϭ = F/A N/m2 ϵ Δl/ l0 Δl : change in length , l0: original length Shear modulus 2. Shear modulus: Shear modulus is the ratio between the shear stress and the shear strain: G = F/A = N/m2 θ Bulk modulus 3.Bulk modulus: Bulk modulus is the ratio between the compressive stress and the volume strain: B = F/A ΔV/V0 Δ V: change in volume. V0 : original volume. General stress-strain curve  The limit of proportionality: It is the maximum stress for a linear proportionality between the stress and the strain of the material. Ultimate strength (break point): It is the maximum stress that a material can withstand before rupture. Some mechanical definitions of materials Ductility: This is the ability of a material to undergo plastic deformation when the material is under tension. In other words, the ability of the substance to be drawn into wires under tension without fracture, like the copper. Maleability: This is the ability of a material to undergo plastic deformation under compression. In other words; it is the ability of the material to be hammered to thin sheets without fracture.  Resilience: This is the amount of energy per unit volume required to deform the material to the elastic limit. It is measured by the area under stress-strain graph in the elastic (linear) portion. Toughness: This is the energy per unit volume required to deform the material to the point of fracture. Fatigue: This is the deformation produced due to application of cyclic stress. This property is important for certain types of dental restoration subjected to alternating forces during mastication,  Hardness: This is the ability of the surface of a material to resist penetration by a point under a specific load. It is the resistance of a material against penetration. Creepness: It is a time dependent plastic straining of a material under the effect of a static load. It usually occurs at temperature near the softening point of the material ~ 0.4 Tm. Creepness is useful in describing the flow of set amalgam sample under the applied load. Mechanical properties of human tissues: Biological tissues are a collection of specialized cells that work together to perform a specific function in an organism. The mechanical properties of biological tissue refer to their physical characteristics and behaviors under mechanical forces. These properties include elasticity, stiffness, strength, toughness, viscoelasticity, and resilience, which determine how tissues respond to stress, strain, compression, tension, and deformation. The mechanical properties of soft tissues play a key role in studying human injuries and their mitigation strategies.  The lack of knowledge on a majority of human tissues inhibit their study for applications ranging from surgical planning, ballistic testing, implantable medical device development, and the assessment of traumatic injuries. Elasticity: The elasticity of tissue refers to the tissue’s ability to deform under an applied force and return to its original shape after removal. Elastic tissues have a high capacity to stretch and recoil. Ex: Elastic fibers are the important resilient component of mammalian connective tissue, and their presence is necessary for the proper structure and function of the cardiovascular, pulmonary, and intestinal systems. Stiffness: Stiffness represents tissue resistance to deformation. Stiffer tissues require higher forces to produce  Toughness: Toughness describes the ability of a tissue to absorb energy and resist fracture. Tough tissues can withstand significant deformation and absorb impact without breaking. Ex. The hard tissues of humans are bone, tooth enamel, dentin, and cementum..Viscoelasticity: Viscoelasticity combines the properties of both viscosity and elasticity. Viscoelastic tissues exhibit time- dependent behavior, meaning their response to stress or strain is influenced by the rate and duration of the applied force. Ex. Ligaments , tendons, muscles and Skin.. Resilience: Resilience refers to the ability of a tissue to recover its original shape and function after deformation or  Ex. Connective tissues ( fibulins Protein).  Mechanical properties of oral tissues: Your teeth are composed of four dental tissues. Three of them—enamel, dentin and cementum—are hard tissues. The fourth tissue—pulp, or the center of the tooth that contains nerves, blood vessels and connective tissue—is a soft, or non-calcified, tissue.  Mechanical properties of bones: Elasticity Bone mineral is a ceramic material and exhibits normal Hookean elastic behaviour, i.e. a linear stress-strain relationship.. Tendons (attaching muscle to bone) and ligaments (attaching bone to bone) have somewhat unique behavior under stress. Functionally, tendons and ligaments must stretch easily at first to allow for flexibility,but then resist significant stretching under large stress to prevent hyper-extension and dislocation injuries.  https://www.google.com/search?sa=X&tbm=bks&sxs rf=ALiCzsbFuwIiXLrbaMOdAaMyDhCDXmrnaA:166368 2327248&tbm=bks&q=inauthor:%22Donald+Mackay %22&ved=2ahUKEwjEr4nRw6P6AhUC_4UKHRZvBcA Q9Ah6BAgEEAY&biw=360&bih=718&dpr=3. https://books.google.com.eg/books?id=2mGC832OcdoC& printsec=frontcover&dq=mechanical+properties+of+mat erials&hl=en&sa=X&ved=2ahUKEwjar56jxaP6AhUDXRoK HQcVBrIQ6AF6BAgIEAM. Joshua Pelleg · 2012 · Preview · https://books.google.com/books?id=UaXpAgAAQBAJ&print sec=frontcover&dq=mechanical+properties+of+human+ tissues&hl=en&newbks=1&newbks_redir=0&source=gb_ mobile_search&sa=X&ved=2ahUKEwj8hN2dkMqIAxWdUa