Properties of materials - tensile properties - Notes, Engineering, Semester Notes | EduRev

: Properties of materials - tensile properties - Notes, Engineering, Semester Notes | EduRev

 Page 1


1 Properties of materials – tensile properties
100-kg man
Size 12 shoes
Contact area: 200 cm
2 
Stress: 0.5 kg/cm
2 
(0.05 MPa)
50-kg lady
1.25 cm square heels
Contact area: 1.56 cm
2 
Stress: 32 kg/cm
2  
(3 MPa)
Figure 1.2  Load vs. stress for feet.
Figure 1.3  The stress–strain curve of a non-ferrous metal.
Figure 1.4  Stress–strain curves for a brittle, an elastic and a ductile 
material.
Figure 1.1  Applied forces and specimen deformations.
Torsional force twisting motion
Flexural force flexure or bending motion
Tensile force elongation (and thinning)
Specimen being loaded
Shear force distortion
Compressive force compression or crushing (and shrinkage)
Applied stress
Material strain
Material fracture
Ductility
Strength
*
Elastic region
Stress
Plastic region
High strength, brittle material
Soft, very ductile material
Strong, ductile material
Strain Strain
Stress
Figure 1.5  Elastic and plastic regions of a stress–strain curve.
Table 1.1 Desirable properties 
of dental materials.
Biocompatibility
Absence of toxicity
Esthetic appearance
Strength and durability
Low solubility
Ease of manipulation 
Long shelf life
Simple laboratory processing
Long working time
Rapid/snap set
Table 1.2 Typical mechanical properties of dental biomaterials.
Material Tensile Compressive Shear Elastic Hardness 
strength (MPa) strength (MPa) strength (MPa) modulus (GPa) (KHN)
Gold alloy 448 – – 77 22
Dental amalgam 54.7 318 188 34 110
Dentin 51.7 297 138 1.4 68
Enamel 10.3 384 90 4.6 343
Porcelain 24.8 149 111 140 460
Composite 45.5 237 – 14 –
Zn phosphate cement 8.1 117 13 13.7 40
Die stone 7.7 48 – – –
Ca(OH)
2
1.0 10.3 – – –
Glass ionomer 18 150 – 20 –
KHN, Knoop hardness number.
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COPYRIGHTED MATERIAL
Page 2


1 Properties of materials – tensile properties
100-kg man
Size 12 shoes
Contact area: 200 cm
2 
Stress: 0.5 kg/cm
2 
(0.05 MPa)
50-kg lady
1.25 cm square heels
Contact area: 1.56 cm
2 
Stress: 32 kg/cm
2  
(3 MPa)
Figure 1.2  Load vs. stress for feet.
Figure 1.3  The stress–strain curve of a non-ferrous metal.
Figure 1.4  Stress–strain curves for a brittle, an elastic and a ductile 
material.
Figure 1.1  Applied forces and specimen deformations.
Torsional force twisting motion
Flexural force flexure or bending motion
Tensile force elongation (and thinning)
Specimen being loaded
Shear force distortion
Compressive force compression or crushing (and shrinkage)
Applied stress
Material strain
Material fracture
Ductility
Strength
*
Elastic region
Stress
Plastic region
High strength, brittle material
Soft, very ductile material
Strong, ductile material
Strain Strain
Stress
Figure 1.5  Elastic and plastic regions of a stress–strain curve.
Table 1.1 Desirable properties 
of dental materials.
Biocompatibility
Absence of toxicity
Esthetic appearance
Strength and durability
Low solubility
Ease of manipulation 
Long shelf life
Simple laboratory processing
Long working time
Rapid/snap set
Table 1.2 Typical mechanical properties of dental biomaterials.
Material Tensile Compressive Shear Elastic Hardness 
strength (MPa) strength (MPa) strength (MPa) modulus (GPa) (KHN)
Gold alloy 448 – – 77 22
Dental amalgam 54.7 318 188 34 110
Dentin 51.7 297 138 1.4 68
Enamel 10.3 384 90 4.6 343
Porcelain 24.8 149 111 140 460
Composite 45.5 237 – 14 –
Zn phosphate cement 8.1 117 13 13.7 40
Die stone 7.7 48 – – –
Ca(OH)
2
1.0 10.3 – – –
Glass ionomer 18 150 – 20 –
KHN, Knoop hardness number.
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COPYRIGHTED MATERIAL
Properties of materials – tensile properties Chapter 1 3
Dental biomaterials are used in laboratory procedures and for the
restoration and replacement of teeth and bone. Material selection must
consider function, properties and associated risks, and all dental bio-
materials must satisfy certain criteria (Table 1.1).
Mechanical properties
Mechanical properties are important since teeth and restorations must
resist biting and chewing (masticatory) forces. Typical properties are
given in Table 1.2.
Biting forces
Biting forces vary with patient age and dentition, decreasing for
restored teeth and when a bridge, removable partial denture (RPD) or
complete denture is present. Effects vary with the type of applied force
and its magnitude. Types of applied force, and the resulting deforma-
tions, are shown in Figure 1.1. 
Stress
Stress, s: force per unit cross-sectional area. Strength is the stress that
causes failure. Ultimate strength is the maximum stress sustained
before failure. 
Stress, the applied force and the area over which it operates, deter-
mines the effect of the applied load. For example, a chewing force of 
72 kg (10 N) spread over a quadrant 4 cm
2
in area exerts a stress of 
18 kg/cm
2
(1.76 MPa). However, the same force on a restoration high
spot or a 1-mm
2
hard food fragment produces a stress of 7200 kg/cm
2
(706 MPa), a 400-fold increase in loading. This stress effect is one rea-
son that occlusal balancing is essential in restorative dentistry. A more
graphic example of the difference between applied force and stress is
shown in Figure 1.2. This example also clearly indicates why it is more
painful when a woman wearing high heels steps on you than when a
man does!
Proportional limit
Proportional limit is the maximum stress that the material can sustain
without deviation from a linear stress–strain proportionality.
Elastic limit is the maximum stress that can be applied without per-
manent deformation.
Yield strength, s
y
is the stress at which there is a speci?ed deviation
from proportionality of stress to strain. It is usually 0.1, 0.2 or 0.5% of
the permanent strain.
Strain
Strain, e: ratio of deformation to original length ? L / L. Strain measures
deformation at failure.  
Ductility: percentage elongation, i.e. ? L/L × 100%.
Ductile materials exhibit greater percentage elongations than brittle
materials and can withstand greater deformation before fracture. 
Burnishing index: ability of a material to be worked in the mouth or
burnished, expressed as the ratio of percentage elongation to yield
strength.
Poisson’s ratio
Poisson’s ratio, ?: ratio of lateral to axial strain under tensile loading. 
It denotes reduction in cross-section during elongation. 
Brittle materials have low ? values, i.e. little change in cross-section
with elongation, while ductile materials show a greater reduction in
cross-section, known as specimen necking.
Elastic modulus
Elastic modulus, E is the ratio of stress to strain. It is also known as
modulus of elasticity or Young’s modulus and denotes material stiff-
ness. It is determined as the slope of the elastic (linear) portion of the
stress–strain curve.
Stress–strain curves
Stress–strain curves are generated by applying a progressively
increasing tensile force while measuring applied stress and material
strain until fracture occurs. The shape of the curve indicates the proper-
ties of the material (Figures 1.3 and 1.4). Non-ferrous metals (e.g. gold
and copper) show a continuous curve to failure while ferrous materials
exhibit a ‘kink’ in the curve, known as the yield point. 
The intersection of a line parallel to the abscissa (strain) axis from the
failure point to the ordinate (stress) axis is specimen strength while the
vertical line from the failure point to the strain axis is the ductility.
High strength, brittle materials show steep stress–strain curves with
little strain at failure, e.g. ceramics. 
Strong ductile materials (e.g. metals) show moderate slopes in the
stress–strain curve but good extension until failure.
Soft ductile materials (e.g. elastomers) show long, shallow linear
stress–strain behavior followed by a sharp rise in the curve when, with
increasing applied force, the elastomer no longer extends linearly (or
elastically) and failure occurs. 
Resilience
Resilience: resistance to permanent deformation (i.e. energy required
for deformation to the proportional limit). It is given by the area under
the elastic portion of the stress–strain curve (Figure 1.5).
Toughness
Toughness: resistance to fracture (i.e. energy required to cause frac-
ture). It is given by the total area (i.e. both the elastic and plastic
regions) under the stress–strain curve (Figure 1.5).
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