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Basics in dental material MCQs 2



 

Basics in dental material ; Mechanical properties as stress , flexibility , stiffness , ductility , malleability ,  toughness , brittleness , resilience and hardness l  MCQs in dental material for dental students


Basics in dental material ; Mechanical properties as stress , flexibility , stiffness , ductility , malleability ,  toughness , brittleness , resilience and hardness l  MCQs in dental material for dental students



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Important key points in Basics of dental material 

Heat Of Vaporization:

  • It is the amount of energy needed for transformation of a boiling liquid into vapor.
  • For example, 540 cal. of heat is required to vaporize lgm. of water at 100°C and 1 atmosphere pressure.

Latent Heat Of Fusion:

  • It is the amount of energy released (in form of heat) when a liquid freezes.
  • When 1 gm. of water freezes, 80 cal. of heat is released.

Melting Temperature:

  • It is the amount of energy required for transformation of solids (such as metals) into liquid state.

Sublimation:

  • It is the process in which some solids transform directly to a gas phase.

Ionic bond:

  • Eg.: Gypsum, phosphate cements.

Covalent bond:

  • Eg.: Dental resins.

Metallic bond:

  • Eg.: Pure gold.

Hydrogen bond:

  • Eg.: H2O.

Vanderwalls forces: 

  • Are characterized by physical forces

Glass Transition Temperature

  • The temperature at which there is an abrupt increase in the thermal expansion coefficient, indicating increased molecular mobility is called the glass transition temperature and it is characteristic of the particular glassy structure.

The force of attraction:

  • Between like molecules is called cohesion. The force of attraction between unlike molecu les is adhesion
  • The material or film added to produce the adhesion is known as the adhesive, and the material to which it is applied is called the adherend.

The capacity for the adhesion

  • Is directly related to the surface energy of the material.
  • Because of the higher surface energy of many restorative materials compared with that of the tooth, there is a greater tendency for the surface and margins of the restoration to accumulate debris with resultant high incidence of marginal caries.

Hue:

  • It is the specific color produced by a specific wavelength of light.
  • It describes the dominant color of an object, for example, red, green or blue.

Value (Brilliance)

  • Is the lightness or darkness of an object.
  • Lighter shaded objects are with highest value and dark shaded objects are with lower value.

Saturation (Chroma)

  • Is the amount of color per unit area of an object. Chroma represents the degree of  saturation of a particular hue. For example, some teeth appear more yellow than others.

Translucency:

  • Is property of an object that permits the passage of light through it but does not give any distinguishable image.

Metamerism:

  • Objects that appear to be color matched under one type of light may appear different under another light source. This phenomenon is called metamerism.

Fluorescence of dental material :

  • The energy that a tooth absorbs is converted into light with longer wavelengths, and becomes a light source. This phenomenon is called fluorescence. Fluorescence makes a definite contribution to the brightness and vital appearance of a human tooth

Benzold-brucke Effect:

  • Change in color/ hue due to change in brightness or light intensity.

Rheology in dental material:

  • The study of matter flow characteristics is called rheology. The resistance to motion is called viscosity.

Newtonian dental fluid material:

  • It is ideal fluid with constant viscosity and demonstrates a shear stress that is proportional to the strain rate.
  • Newtonian exhibits a constant slope of shear stress plotted against strain rate i.e., the plot is a straight line.

Pseudoplastic dental material:

  • The viscosity decreases with increasing shear rate.

Dilatant dental liquid material :

  • These liquids become more rigid with increasing shear rate i.e., as the rate of deformation increases, these liquids become more viscous.

Thixotropic dental material:

  • A liquid, which becomes less viscous and more fluid under pressure is referred to as thixotropic.
  • Dental prophylaxis pastes, plaster, resin cements and some impression materials are thixotropic.

The hydrophobic dental materials:  

  • (polysulphide, addition and condensation silicones) have a water contact angle of 95 degrees whereas hydrophilic materials (Alginate, Agar, and Polyether) will have a water contact angle of 30-35 degrees

Stress:

  • Stress is the force per unit area. It is the internal force exerted throughout a unit of area that resists an external force applied for the area.

1. Tensile stress:

  • Tends to stretch or elongate a body.

2. Compressive stress :

  • Tends to compress a body.

3. Shear stress:

  • Resists the sliding of one portion of a body over another.

Strain:

  • It is expressed as change in length per unit length of body when a stress is applied.

Poisson's Ratio:

  • When a object is subjected to a tensile or compressive stress.
  • There is simultaneous axial and lateral strain.
  • Within the elastic range, the ratio of lateral to the axial strain is called poisson's ratio. For an ideal isotropic material of constant volume. the ratio is 0.5.

Elastic Limit:

  • It is the maximum stress that a material can withstand without permanent deformation.

Proportional Limit:

  • It is the greatest stress that may be produced in a material such that the stress is directly proportional to strain.
  • For a material to satisfy HOOKE's law the elastic stress must be proportional to the elastic strain. Hooke's law is applicable to all orthodontic wires except super elastic A-NiTi wires.

Elastic / Young's Modulus / Modulus 0f Elasticity:

  • It describes the relative stiffness or rigidity of a material. It is the ratio of stresses to strain within the proportional limit.

The shear modulus:

  • Can be calculated from the formula Where, G - Shear modulus; E - Elastic modulus Thus, the shear modulus is always 38% of elastic modulus.

Yield Strength:

  • It is the strength at which a material begins to function in a plastic manner. It is defined as the stress at which a material exhibits a limiting deviation from proportionality of stress to stress.

Resilience: (Energy)

  • Resilience is basically an express of energy. It is defined as the amount of energy absorbed by a structure when it is stressed not to exceed its proportional limit.

Toughness of dental material:

  • It is the amount of energy required to fracture a material and it is a measure of the resistance to fracture.
  • Toughness is directly related to strength and ductility of the material.

Brittleness of dental material:

  • Is opposite to that of toughness. It is defined as the inability of a material to sustain plastic deformation before the fracture occurs.

Ductility of dental material:

  • It is the ability of the material to withstand permanent deformation under a tensile load without rupture.

Malleability of dental material:

  • It is the ability of the material to withstand permanent deformation under a compressive load without rupture.

Flexibility of dental material: 

  • Is defined as the strain that occurs when the material is stressed to its proportional limit.

 


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