Dental Material Testing Notes

Dental Material Testing

Niyom Thamrongananskul PhD.
Department of Prosthodontics
Faculty of Dentistry
Chulalongkorn University

Properties of Materials

1. Mechanical Properties

2. Physical Properties

Mechanical Properties

สมบัติทางกล (Mechanical Properties)

Key Definitions

• Stress = ความเค้น

• Strain = ความเครียด

• Tension = การดึง

• Shear = การเฉือน

• Compression = การกดอัด

• Modulus = มอดุลัส

• Strength = ความแข็งแรง

• Hardness = ความแข็ง

• Plastic deformation = การเสียรูปพลาสติก

Stress

ความเค้น (stress) หมายถึงแรงต้านทานภายในเนื้อวัสดุ ที่เกิดขึ้นเพื่อรักษาสภาพเดิมเมื่อมีแรงภายนอกมากระทำ เพื่อไม่ให้เกิดการเปลี่ยนแปลงรูปร่างไปจากเดิม ซึ่งแบ่งตามรูปแบบของแรงที่กระทำ จะทำให้เกิดความเค้นได้ 3 ประเภท คือ

• ความเค้นดึง (tensile stress)

• ความเค้นอัด (compressive stress)

• ความเค้นเฉือน (shear stress)

Types of Stress

• Tension

• Shear

• Compression

Greek Alphabet

• Αα Alpha

• Ββ Beta

• Γγ Gamma

• Δδ Delta

• Εε Epsilon

• Ζζ Zeta

• Ηη Eta

• Θθ Theta

• Ιι Iota

• Κκ Kappa

• Λλ Lambda

• Μμ Mu

• Νν Nu

• Ξξ Xi

• Οο Omicron

• Ππ Pi

• Ρρ Rho

• Σσς Sigma

• Ττ Tau

• Υυ Upsilon

• Φφ Phi

• Χχ Chi

• Ψψ Psi

• Ωω Omega

Tension Test

$\sigma = \frac{F}{A<em>o}$ $\varepsilon = \frac{L</em>i - L<em>o}{L</em>o}$

• $\sigma$ = Engineering stress

• $\varepsilon$ = Engineering strain

• $F$ = instantaneous force (N)

• $A_o$ = original cross-section (m2)

• $L_i$ = instantaneous length

• $L_o$ = original length

Compression Test

$\sigma = \frac{F}{A<em>o}$ $\varepsilon = \frac{L</em>o - L<em>i}{L</em>o}$

• $\sigma$ = Engineering stress

• $\varepsilon$ = Engineering strain

• $F$ = instantaneous force (N)

• $A_o$ = original cross-section (m2)

• $L_i$ = instantaneous length

• $L_o$ = original length

Shear Test

$\tau = \frac{F<em>{//}}{A</em>o}$
$\gamma = tan \theta$

• $\tau$ = Engineering stress

• $\gamma$ = Engineering strain

• $F_{//}$= instantaneous force // (N)

• $A_o$ = surface area (m2)

$tan \Theta = \frac{ข้าม}{ชิด} = \frac{Sin \Theta}{Cos \Theta}$

Engineering Strain

$\gamma = tan \theta = \frac{x}{h}$

Universal Testing Machine

A machine used for compression and tension testing.

Instron Universal Testing Machine

Used for testing compression, stress, and strain in materials like concrete cylinders.

Tension/Tensile Test

Testing reinforcing steel (rebar).

Extensometer

A strain measurement device used to measure the extension of a material under load.

Stress-Strain Diagram

Shows linear elastic deformation for loading and unloading cycles in tension.

Hooke’s Law: $\sigma = E \varepsilon$

Where:

• $\sigma$ = Stress

• $\varepsilon$ = Strain

• $E$ = Modulus of elasticity or Young’s modulus (Pa or psi)

Units

• Psi (Pound per Square Inch)

• Pa (Pascal): 1 Pa = 1 N/m2

Engineering Stress and True Stress

Engineering stress: $\sigma = \frac{P}{A_o}$
Where:

• $P$ is the load on the specimen

• $A_o$ is the original cross-sectional area.

True stress is the load divided by the true area, which continues to be smaller due to the tensile load. True stress continues to increase to the point of fracture, while engineering stress decreases due to the increasing load and constant cross-sectional area.

Engineering Strain

$\varepsilon = \frac{l - l<em>0}{l</em>0}$

Where:

• $l$ is the gauge length at a given load

• $l_0$ is the original gauge length with zero load.

Modulus of Elasticity (Young’s Modulus)

$E = \frac{\sigma}{\varepsilon} = \frac{stress}{strain}$

Material Stiffness

Modulus of elasticity or Young’s modulus is a measure of material stiffness (given by the slope of the stress-strain curve).

• Modulus of elasticity is determined by the binding forces between atoms (structure insensitive property).

• Cannot change E, but can improve by forming composites.

• Only slightly affected by alloying addition, heat treatment, or cold work.

• Temperature dependence.

Modulus of Elasticity at Different Temperatures (GPa)

Young’s Modulus

Relates to Stiffness, Rigidity, and Deflection. Higher E indicates less deflection.

Stiffness

A material with high stiffness has a high modulus of elasticity and resists deformation more effectively than a material with low stiffness.

Ductile vs. Brittle Materials

A is a ductile material, and B is a brittle material.

Stress-Strain Curve

• A: Proportional Limit

• B: Elastic Limit

• C: Yield Point

• D: Ultimate Tensile Strength

• E: Fracture or Breaking

Hooke's Law

$\sigma = E \varepsilon$

• A: Proportional Limit

• B: Elastic Limit

• C: Yield Point

• D: Ultimate Tensile Strength

• E: Fracture

Yield Point

• Yield point = จุดคราก = จุดจ านน

• C upper, C lower

Yield Phenomenon

Yield refers to the phenomenon that the stress does not increase, and the strain increases.

Elastic and Plastic Behavior

• O-B: Elastic Behavior

• B-E: Plastic Behavior

Plastic Property

Plastic (adj) ที่สามารถปั้นได้ Synonym: Moldable

In materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces.

Cold Working

Hot Working vs. Cold Working

1. Hot Working: Process where metal is mechanically worked at a temperature higher than the recrystallization temperature but below the melting point. Examples include forging and hot rolling.

2. Cold Working: Process of deforming a metal plastically at a temperature low enough to prevent recrystallization.

Cold Working Examples

1. Bending

2. Drawing

3. Cold Rolling

4. Extruding

5. Squeezing

6. Shear Spinning

7. Straight Bending

8. Deep Drawing

Drawing

Deep Drawing

Stress-Strain Curve and Cold Working

The material behaves elastically from O to B and plastically from B to E. Cold working involves deformation in the plastic region.

Stress-Strain Curves for Different Materials

Graphs showing stress-strain curves for various materials like steel, aluminum, titanium, and magnesium alloys.

Yield Point Determination

For materials like aluminum and copper that do not exhibit a clear yield point, the yield strength is determined using the 0.2% offset method.

Proof Stress

Proof Stress (Offset strain is typically 0.2% or 0.002) is the stress at which 0.2% plastic deformation occurs.

Engineering Stress vs. True Stress

True stress accounts for the change in cross-sectional area as loads are applied. As strain increases and the cross-sectional area decreases, true stress becomes much larger than engineering stress.

Strain Hardening

Also known as work hardening. When a material undergoes permanent deformation, dislocations occur, making further atomic plane movement difficult.

Stress-Strain Relationship and Young’s Modulus

Slope represents Young's Modulus.

Stiffness

A material with high stiffness has a high modulus of elasticity and resists deformation more effectively than a material with low stiffness.

Flexible Thermoplastic Denture Base Materials

Polyamides or Nylon (Du Pont, 1938)

Material Properties

Matching material properties (Hardness and Toughness) to application:

a. Hard and Brittle
b. Hard and Tough
c. Soft and Tough
d. Hard and Strong

Key Definitions

Students should be able to define:

• Hook’s Law

• Proportional Limit

• Elastic Limit

• Yield Point (จุดคราก)

• Ultimate Tensile Strength

• Fracture

• Elastic Behavior

• Plastic Behavior

• Plastic Deformation

• Permanent Deformation

Ductility

Ductility is a measure of the degree of plastic deformation sustained at fracture. A material with little or no plastic deformation at fracture is considered brittle.

Ductility vs. Brittleness

A comparison of stress-strain curves for ductile and brittle materials.

Toughness

Toughness is the ability of a material to absorb energy without fracturing. It is related to both strength and ductility.

Modulus of Toughness

Can be estimated from the area under the stress-strain curve obtained from a tensile test. It represents the energy per unit volume required to cause fracture.

Impact Test

Measures the amount of energy a material can absorb when subjected to a sudden impact.

Impact Strength

Impact Testing Methods

1. Charpy Impact Test

2. Izod Impact Test

Charpy Impact Test

Izod Impact Test

Flexural Strength

Also known as Bend Strength, Fracture Strength, or Modulus of Rupture.

Flexural Test

Applied loads “bend” the sample.

Three-Point Bending Test

Also known as a Transverse Test.

Flexural Properties

Flexural Strength

The ability of a material to resist bending forces.

Flexural Tests

3-point bending and 4-point bending.

Compression and Tension

Mechanical Behavior

1. Creep

2. Stress Relaxation

3. Recovery

Creep

Creep is the time-dependent deformation of a material under constant stress, typically at elevated temperatures, even below the elastic limit.

Stress Relaxation

Stress relaxation is the decrease in stress over time under conditions of constant strain.

Tensile Elastic Recovery

Diametral Tensile Strength Test (Brazil Test)

Tensile Strength of Amalgam

The tensile strength of amalgam measured after 15 minutes.

Stress Distribution in Diametral Compression

Key Terms

• Stress = ความเค้น

• Strain = ความเครียด

• Tension = การดึง

• Shear = การเฉือน

• Modulus = มอดูลัส

• Compression = การกดอัด

• Strength = ความแข็งแรง

• Hardness = ความแข็ง

• Plastic Deformation = การเสียรูปพลาสติก

• A: Proportional Limit

• B: Elastic Limit

• C: Yield Point

• D: Ultimate Tensile Strength

• E: Fracture or Breaking

• Hook's law

• Proportional Limit

• Elastic Limit

• Yield Point (จุดคราก)

• Ultimate tensile strength Fracture

• Elastic Behavior

• Plastic Behavior

• Plastic Deformation

• Permanent Deformation

Mechanical Behavior

1. Creep

2. Stress relaxation

3. Recovery