MECHANICAL PROPERTIES OF DENTAL MATERIALS By Dr Khawaja Rashid Hassan Assistant Professor RAWAL INSTITUTE OF HEALTH SCIENCES RAWAL COLLEGE OF DENTISTRY ISLAMABAD

MECHANICAL PROPERTIES OF DENTAL MATERIALS Defined by the laws of mechanics. The physical science that deals with energy and forces and their effects on the bodies. Mechanical properties need to be considered collectively. Intended application of a material is important.

MECHANICAL PROPERTIES OF DENTAL MATERIALS Failure or success potential of any prosthesis / restoration is dependent upon the mechanical properties of the material. The material response may be, Elastic …. reversible on force removal. Plastic …… Irreversible / non-elastic. Mechanical properties are expressed in terms of stress and/or strain.

MASTICATORY FORCES Tooth Average force (N) Second molar 800 First molar 390 Bicuspids 288 Cupids 208 Incisors 155 Occlusal forces applied by adult dentition is greatest in posterior region. In growing children there is an average annual increase in force of 22 N. Denture wearers only apply 40% of the forces given in table.

STRESS When a force acts on the body, a resistance is developed to the external force applied. This internal reaction is equal in magnitude/intensity and opposite in direction to the applied force and is called as “STRESS” Denoted by “S” or “σ” Designated as force per unit area (σ=N/m²) Pascal = 1 N / m². Commonly stress is reported in terms of megaPascals.

STRAIN Relative deformation of an object that is subjected to stress. It is change in length per unit length. It may be elastic, plastic or both elastic and plastic. It is denoted by “ε” Designated as ∆L / L.

TYPES OF FORCES APPLIED Generally, the force applied may be Axial (tensile or compressive) Shear (sliding, rubbing) Bending (bending movement) Tortional (twisting movement)

TYPES OF FORCES APPLIED Tension results when a body is subjected to two sets of forces directed away from each other in a straight line. Force is directed away from the objcet. Compression results when the body is subjected to two sets of forces directed towards each other in a straight line.

TYPES OF FORCES APPLIED TENSION COMPRESSION

TYPES OF FORCES APPLIED Shear is a result of two sets of forces directed parallel to each other , but not along the same straight line. Torsion results from the twisting of the body. Bending results by applying bending movement.

TYPES OF STRESSES 3 simple types. 1. TENSILE STRESS: causes the body to stretch or elongate. Tensile stress is always accompanied by tensile strain. 2. COMPRESSIVE STRESS: causes the body to shorten or compress. Compressive 3. SHEAR STRESS: resist the sliding or twisting of one portion of the body over another.

TYPES OF FORCES APPLIED Complex stresses FLEXURAL STRESS: Also called as bending stress. Produced by bending forces over the dental appliance. Application of shear force may produce elastic shear strain or plastic shear strain.

Hooke's Law Hooke's Law states that "within the limits of elasticity the strain produced by a stress (of any one kind) is proportional to the stress". The stress at which a material ceases to obey Hooke's Law is known as the limit of proportionality.

Hooke's Law Hooke's law can be expressed by the formula stress / strain = a constant. The value of the constant depends on the material and the type of stress. For tensile and compressive forces it is called Young's modulus, E; for shearing forces, the shear modulus, S; and, for forces affecting the volume of the object, the bulk modulus, K.

PROPORTIONAL LIMIT It is the maximum stress at which the stress is equivalent/proportional to strain and above this limit the plastic deformation of a material occurs. The material may be subjected to any type of applied force.

STENGTH Strength is the maximum stress that a material can withstand without sustaining a specific amount of plastic strain. OR Stress at the point of fracture.

STRENGTH PROPERTIES ULTIMATE TENSILE STENGTH : Simply called as TENSILE STRENGTH. It is defined as the Tensile stress at the point of fracture. YIELD STRENGTH : It is the stress at which a test specimen exhibits a specific amount of plastic strain. Used in the conditions when proportional limit cannot be determined with accuracy.

STRENGTH PROPERTIES SHEAR STRENGTH: Maximum shear stress at the point of fracture. FLEXURAL STRENGTH: Defined as “force per unit area at the point of fracture of a specimen that is subjected to flexural loading” Also called as “BENDING STRENGTH” or “MODULUS OF RUPTURE”

STRENGTH PROPERTIES FATIGUE STRENGTH: Determined by subjecting a material to cyclic stress of maximum known value and determining the number of cycles required to cause failure of the material. Maximum service stress (endurance limit) can be maintained without failure over an infinite number of cycles. Endurance limit is lower for materials with brittle and rough surface.

STRENGTH PROPERTIES FATIGUE STRENGTH: Dental restorative materials may exhibit static fatigue failure or dynamic fatigue failure. Depends upon the nature of loading or residual stress situations. Failure begins as a flaw that propagates till the catastrophic fracture occurs.

STRENGTH PROPERTIES IMPACT STRENGTH: Impact is the reaction of a stationary object to a collusion with a moving body. Impact strength is defined as energy required to fracture a material under an impact force. The energy units are joules.

ELASTIC MODULUS Also called as modulus of elasticity or Young’s modulus. It is the relative stiffness or rigidity of a material. Measured by the slope of the elastic region of the stress strain curve. If a tensile or compressive stress (below the proportional limit) is divided by corresponding strain value, a constant of proportionality will be obtained.

ELASTIC MODULUS Unaffected by the amount of elastic or plastic stress induced in the material. Independent of ductility of a material. The lower the strain for a given stress, greater will be the elastic modulus. E.g. two wires of same shape and size. Polyether impression materials. Unit is Giganewtons/m² (GPa).

FIRST MONTHLY CLASS TEST THEORY PAPER 3RD MAY 2012 (THURSDAY) LECTURE TIMING VIVA 4TH MAY 2012 (FRIDAY) TUTORIAL TIMINGS TOPICS: INTRODUCTION TO DENTAL MATERIALS SELECTION & EVALUATION OF DENTAL MATERIALS. BIOCOMPATIBILITY OF DENTAL MATERIALS. PHYSICAL PROPERTIES OF DENTAL MATERIALS. MACHANICAL PROPERTIES OF DENTAL MATERIALS

STRESS-STRAIN CURVE For materials in which strain is independent of the length of time that a load is applied “ STRESS STRAIN CURVES“ are important.

ANALYSIS FOR A STRESS STRAIN CURVE STIFFNESS & FLEXIBILITY If longitudinal portion of the curve is closer to the long axis the material is stiff & not flexible. If it is away from the long axis the material is flexible.

ANALYSIS FOR A STRESS STRAIN CURVE TOUGHNESS & BRITTLENESS If material fractures after a long concave portion of the curve, it donates that the material is tough & ductile. If elastic portion of the curve is minimal, it shows the brittleness of the material.

ANALYSIS FOR A STRESS STRAIN CURVE STRNGTH & WEAKNESS If longitudinal portion of curve is longer, means that the material is strong. If longitudinal portion is short the material is weak. HENCE FROM THE ANALYSIS OF THE STRESS STRAIN CURVE IT IS POSSIBLE TO HAVE AN IDEA ABOUT THE PROPERTIES OF A MATERIAL.

STRAIN TIME CURVES For materials in which the strain is dependent upon the time for which the load is being applied “STRAIN TIME CURVES” are mor useful in explaining the properties of a material than stress strain curves. Examples: Alginate & rubber base impression materials, dental amalgam & human dentin.

STRESS STRAIN CURVES

STRESS STRAIN CURVES

Dynamic Young’s Modulus Can be measured by dynamic method. Ultrasonic longitudinal and transverse wave transducers and appropriate receivers are used. The velocity of sound wave and density of material are used to calculate elastic modulus.

RESILIENCE The amount of elastic energy per unit volume released when the stress is removed. With increase in interatomic spacing the internal energy increases. Until the stress is lower than proportional limit, the energy is called as RELILIENCE.

TOUGHNESS Amount of elastic and plastic deformation energy required to fracture a material. Measured by the area under the elastic region of the stress strain curve. Toughness increases with increase in strength and ductility. Tough materials are generally strong. Resistance of a brittle material to propagation of flaws under an applied stress (FRACTURE TOUGHNESS)

DUCTILITY and MALLEABILITY Ability of a material to deform plastically under a tensile stress before fracture. e.g. metal drawn readily into long thin wires. MALLEABILITY: The ability of a material to sustain plastic deformation, without fracture under compression.

DUCTILITY and MALLEABILITY Gold is the most ductile and malleable pure metal, followed by silver. Platinum is ranked third in ductility. Copper ranks third in malleability.

HARDNESS In mineralogy, relative hardness of a substance is based upon its ability to resist scratching. In metallurgy and mostly in all other disciplines, hardness is defined as resistance to indentation. Designated as KNOOP HARDNESS NUMBER. BRINELL HARDNESS NUMBER. VICKERS HARDNESS NUMBER. ROCKWELL HARDNESS NUMBER.

TERMS TO REMEMBER Shapes produced by indentors On materials KNOOP HARDNESS TEST VICKERS HARDNESS TEST BRINELL & ROCKWELL HARDNESS TEST

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