Mechanical Properties
Dental Amalgam Tensile / Compressive Strength
48-69 and 310-483...similar to enamel which is important!
Resilience
Ability of a material to store (absorb) energy. Energy stored best in elastic region. Clinical Significance: the resilience has a particular importance in the evaluation of orthodontic wires, for the expected amount of work from a spring to move a tooth. Acrylic denture teeth: they are more resilient than porcelain so, they absorb most of the masticatory forces & transmit less force to the ridge.
Structure of Matter
All matter consists of some distribution of atoms or molecules in a solid: the atoms, head together by forces that are mainly electrical, are located at specific positions with respect to one another and vibrate about those positions at low temperatures the vibrating motion is slight and the atoms can be considered essentially fixed as energy is added to material amplitude of the vibrations increases a vibrating atom can be viewed as being bound in its equilibrium position by springs attached to neighboring atoms. a collection of such atoms and imaginary springs is shown.
Correlation between hardness and tensile strength
Both hardness and tensile strength are indicators of a metal's resistance to plastic deformation. For cast iron, steel and brass, the two are roughly proportional.
Clinical Significance of Modulus
Denture base should be constructed from a rigid material for two reasons: Load distribution/ to be used in thin sections without bending.
Clinical Significance of Hardness
Denture-wearing patients must take care not to be aggressive during the cleaning of their dentures by using brushes with hard bristles. hardness is an important property consider for model and die materials on which crown and bridge wax patterns are made, because a soft surface may become scratched, affecting the accuracy of final restoration.
Stress-Strain Diagram
Elastic Region: slope= young's elastic modulus yield strength plastic region: ultimate tensile strength/ strain hardening/ fracture
Polymer response to stress
In an undeformed thermoplastic polymer tensile sample, the polymer chains are first randomly oriented. When a stress is applied, a neck develops as chains become aligned locally. The neck continues to grow until the chains in the entire gage length have aligned. This increase strength of polymer.
Toughness
It is the energy required to stress the material to the point of fracture. It is represented by the area under the elastic and plastic portion of the stress-strain curve. Therefore, toughness of a material is ability to absorb energy. The toughest materials are those with high proportional limits and good ductility. However, two highly different materials can have same toughness. Defined as energy required to fracture a material. A "tough material" has strength and ductility ! Higher toughness: metal. Low toughness: ceramics (VERY BRITTLE) brittle fracture: elastic energy ductile fracture: requires elastic and plastic energy.
Clinical Significance
It is very important for dental restorations to have high yield strength than ultimate strength, so that during mastication no permanent deformation takes place after load distribution. Average occlusal forces for fully dentate patients: anterior region (150 newtons) , posterior region (500 newtons). Maximal occlusal forces: up to 35000 N!
Range
Maximum elastic strain. Defined as distance the wire will bend elastically before permanent deformation occurs.
Region Between M and F
Metals: occurs the noticeable necking starts. Ceramics: occurs when crack propagation starts. Polymers: occurs when polymer backbones are aligned and about to break.
Elastic Properties of Materials
Poisson's ratio: when a metal is strained in one direction, there are corresponding strains in all other directions. For a uniaxial tension strain, the lateral strains are constrictive. Conversely, for a uniaxial compressive strain, the lateral strains are expansive. I.e.; the lateral strains are opposite in sign to the axial strain. The ratio of lateral to axial strains is known as Poisson's ratio.
Poisson's ratio
Ratio of lateral to axial strain within the elastic range. The increase in length of a material under tension is associated with a decrease in cross-sectional area. The increase in length is known as axial strain and decrease in cross-sectional area is known as lateral strain.
Mechanical Properties Slope
Slope of Stress strain plot (which is proportional to elastic modulus) depends on bond strength of material.
Stress Systems
Tensile/ compressive/ shear
Brittle
a material that suffers very little plastic deformation before fracture.
Fracture toughness
ability of material to resist fracture through its resistance to crack propagation. In general, high fracture toughness indicates good resistance to crack propagation.
Malleability
ability of material to withstand rupture under compression, as in hammering or rolling into a sheet. most malleable metal is GOLD, then SILVER, PLATINUM, COPPER.
Elastic Deformation
all materials deform reversibly on application of low loads atoms are first displaced from their equilibrium position by applied energy. Then they return to original position once energy is removed. Do not move their position relative to one another.
Formability
amount of permanent deformation that a wire can withstand before failing. it represents the amount of permanent bending the wire will tolerate (while being formed into a clinically useful spring) before it breaks.
Hardness
defined as ability of material to resist scratching, indentation or penetration. it is a surface property not related directly to any other mechanical property i.e. strong or stiff materials are not necessarily hard. hardness can't be seen or calculated from stress-strain curve but only by using one of the following: brine, know, vickers, rockwell and shore A hardness tests.
Creep
defined as time dependent plastic deformation that occurs in a completely set material like solid metal subjected to seal load below its Elastic Limit or proportional limit (same thing) increase in strain in a material under constant stress seen in amalgams different types of creep
Mechanical Properties
defined by law of mechanics that is the physical science dealing with forces that acts on bodies and the resultant motion, deformation or stress that those bodies experience...
Liquid
definite volume/ no definite shape
Solid
definite volume/ shape
Knoop hardness test
diamond indenting tool is used. for both hard and soft materials.
Springback
difference between a given deflection (activation) and the residual deformation after unloading to 0 gm-mm.
Fatigue
fatigue is progressive and localized structural damage that occurs when material is subjected to CYCLIC LOADING. Fatigue Life, is the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. Surface fatigue: surface fatigue is a process why which the surface of material is weakened by cyclic loading. fatigue wear: is produced when wear particles are detached by cyclic crack growth of micro cracks on the surface. These micro cracks are either superficial cracks or subsurface cracks. Associated with poor surface finish. Steel has better fatigue than aluminum.
Strain Hardening
if the material is loaded again from point 4, the curve will follow back to point 3 with he same elastic modulus. the material now has a higher yield strength of point 4. raising the yield strength by permanently straining the material is called strain hardening.
Fracture
if the material is stretched beyond point 3, the stress decreases as necking and non-uniform deformation occur. fracture will finally occur at point 5.
Hardening
increase in yield strength due to plastic deformation
Elastic Region (Point 1-2)
material will return to its original shape after material is unloaded (like a rubber band) the stress is linearly proportional to the stain in this region. Point 2: yield strength: a point where permanent deformation occurs (if it is passed, the material will no longer return to its original length)
Impact Strength
materials such as glasses, ceramics, cements and amalgam have low resistance to breakage when a load is applied by impact. (dynamic load) the impact strength is defined as energy required to fracture a material under impact force.
Strain measurement
measure of the amount of deformation that takes place when a force is exerted. change in length per unit length when stress is applied; the change in length/ original length.
Stress Measurement
measure of the force acting per unit of x-sectional area. internal resistance to applied external force. Stress= force/area
Ductility
measure of the plastic deformation that has been sustained at fracture may be expressed as percent elongation or percent reduction in area.
Hardness measurement
measurement of a material's resistance to localized plastic deformation ( a small dent or scratch) qualitative hardness techniques have been developed where a small indenter is forced into surface of a material. the depth or size of the indentation is measured, and corresponds to a hardness number. the softer the material, the larger and deeper the indentation (and lower hardness number). ENAMEL IS HARDNEST ACCORDING TO KNOOP.
gas
no definite volume or shape
Plastic Deformation
plastic means permanent material deformation is irreversible occurs on application of higher loads atoms are permanently displaced to new positions by applied force inter-atomic distances are kept same overall CHANGE IN SHAPE
What is Brittleness?
relative inability of material to deform plastically before it fractures. if a material showed no or very little plastic deformation on application of load it is described as being brittle, a brittle material fractures at or near its proportional limit (PL) . brittle materials are weak in tension; for example, dental amalgam has compressive strength which is nearly six times higher than its tensile strength.
Large Hardness
resistance to plastic deformation or cracking in compression better wear properties
Flexural Strength
schematic for a 3-point bending test. able to measure the stress-strain behavior and flexural strength of brittle ceramics. flexural strength (modulus of rupture or bend strength) is stress at fracture.
Strength of materials
the capacity of materials to withstand stress (the internal force exerted by one part of an elastic body upon an adjoining part) and strain (the deformation or change in dimension occasioned by stress) HOOKES LAW: strain is proportional to stress.
Tensile Strength
the largest value of stress on the diagram is called tensile strength or ultimate tensile strength it is the maximum stress which the material can support without breaking all deformation up to the maximum stress is uniform throughout the tensile sample. however, at max stress, a small constriction or neck begins to form. subsequent deformation will be confined to this neck area. fracture strength corresponds to stress at fracture.
Elongation and Compression
the percent of elongation and compression are measures of ductility and malleability, respectively. these two properties indicate the amount of plastic deformation that occur before the material fractures, which indicates the BRITTLENESS OF THE MATERIAL.
Viscoelastcitiy
time dependent deformation strain lags behind applied stress...seen in polymers
Resilience and toughness
two similar materials can have approximately same resilience, but material B is considerably tougher. Toughness increase with increase in strength and ductility.
Structure of Matter part 2
we can picture applied external forces as compressing these tiny internal springs when the external forces are removed, the solid tends to return to its original shape and size. consequently, a solid is said to have elasticity. an understanding of the fundamental properties of these different states of matter is important in all the sciences, in engineering and in medicine. forces put stresses on solids, and stresses can strain, deform and break those solids, whether they are steel beams or bones.
