Properties of materials Flashcards
Types of Stress Shear
forces directed parallel to each other not along the same straight line
Types of Stress
Tension
elongation forces are directed away from each other in the same straight line, molecules resist being pulled apart
Types of Stress Compression
shrinkage forces directed toward each other on the same straight line molecules resist being forced more closely together NOTE: different than Shear in that it IS on the same straight line as opposed to NPT
Stress
Ration of force per area
Types of Stress Torsion
Twisting forces
Types of Stress Bending
results from a bending moment
Types of Stress
Shear Tension Torsion Bending Compression
Strain
Deformation caused by stress measured as a % of Deformation = deformation/length
Proportional limit
The greatest stress sustained without deviation from the linear proportionality of the stress and strain.
In Proportional limit below A is_______ and is called______
reversible strain occurs Elastic Region exception superelastic materials
In proportional limit Above A is ___________and is called the ________
irreversible or permanent strain plastic region exception superelastic materials
Elastic limit
The greatest stress sustained without permanent deformation Note: for lineraly elastic materials the PROPORTIONAL LIMIT AND ELASTIC LIMIT THE SAME STRESS WITHIN THE STRUCTURE. exception are the superelastic materials
Elastic deformation
Non permanent
Plastic deformation
permanent
Yield strength
The stress at which the material begins to function in a plastic manner. Limited permanent strain has occurred .2%
Yield strength is slightly______ than the proportional limit because it includes a slight amount of permanent deformation
Higher
Permanent deformation can be good or bad in the oral cavity in what two ways
- Destructively- fillings crowns bridges (causes improper occlusion and marginal breakdown) 2. Constructively- orthodontic wires
Ultimate strength
Greatest stress occurs at C, the spot on the graph right before it begins to decrease the stress.
Ultimate tensile strength or UTS is the
Max stress that a material can withstand in tension
Ultimate compression strength UCS is the
Max stress a material can withstand in compression
Fracture strength Sf
stress at which a brittle material fractures at D, the decreasing portion of the graph NOTE: a material does not always fracture at the point of greatest stress C.
Why might a material not fracture at the point of greatest stress C.
It may elongate excessively reducing the correctional area (necking) causes a reduction in stress
Dental alloys and ceramics subjected to tension have similar
Ultimate strength and Fracture strength (Sf)
Elongation
deformation that results fro the application of tensile force. Will indicate the workability of the alloy. Expressed as a %
What does 0-A and A-D represent in regards to elongation
- below proportional limit- deformation not permanent 2.permanent deformation- plastic elongation
Elastic Modules (Young’s Modulus)
Elastic Modules (E)- the measure of elasticity represents the stiffness of a material within the elastic region. the slope of S/S in the elastic portion.
Elastic Modules- what is responsible for the property of elasticity
interatomic/intermolecular forces are responsible for the property of elasticity. The stronger the forces the more stiff or rigid the material. NOT AS RIGID A SLOPE E=stress/strain
Elastic modulus
the higher the value of E the more stiff or rigid the material is. example are Metals and Ceramics have a HIGH E Elastomers and polymers have a low E
Resilience
resistance of a material to permanent deformation, area under the elastic portion.
Toughness
resistance of a material to fracture, indicates the amount of energy needed to cause fracture. Area under both the plastic and elastic area
Combination of yield strength-ultimate strength- strain. increasing any of these also increases
toughness
Fracture toughness Fracture mechanics are characterized as
the behavior of materials with cracks or flaws defects generally weaken a material, occur in more bridle materials glass diamonds sheet rock enamal
General properties of stress/strain curves
Tensile properties of brittle materials
Diametral compression test is used when
measureing brittle materials tensile properties
examples include amalgam, cements ceramics plaster nad stone
Compressive properties
Brittle materials such as Amalgam, resin composites, cements, plaster, investments
will have both elasti and plastic properties but the plastic response is small
Fatigue Strength
the stress at which a material fails under repeated loading depends on teh Magnitude of the load and the Number of loads
Endurance Limit
A type of Fatigue Strength
stress at which the material can be loaded an infinite number of times without failing
Viscoelasticity
Rate of loading
important for many materials (alginate elastomeric impression materials, waxes, amalgam, polymers.
increases values for E, PL and US
Viscoelasticity
Materials with mechanical properties dependent on loading rate are termed
Viscoelastic
Materials with mechanical properties independent of loading rate are termed
Elastic
Viscosity
The resistance of fluid to flow
time and temperature dependant
Viscoelastic materials Stress Relaxation
Reduction in stress in a material subjected to constant strain ortho bands for example
Viscoelastic material CREEP
Increase in strain in material under constant stress (amalgam)
Tear Strength
Resistance to tearing forces, important for polymers in thin sections, impression materials in the sulcus, soft liners, depnds on rate of loading rapid loading = higher values.
