Dental Biomaterials I - Mechanical Properties

Dental Biomaterials I - Mechanical Properties

• Mechanical properties are a group of properties that describe how materials react to forces.
• Restorative materials must withstand forces during fabrication and mastication.
• This chapter covers elastic, plastic, and viscoelastic deformation, plus mechanical quantities like force, stress, strain, strength, toughness, hardness, friction, and wear. These are considered in the context of how dental materials perform in the oral environment.

I - Force

• Force is the external action that produces or tends to produce motion of a body
• Units (kg, lb, N) - SI unit is Newton (N)
• A force is defined by magnitude (amount), direction of application, and point of application.
• Forces can be static or dynamic.

II - Stress

• Stress is the internal reaction to an external force. It's equal in intensity and opposite in direction to the external force.
• Stress is calculated as force per unit area (σ = F/A)
• Common units include Kg/cm², lb/in², N/mm² (MPa), and N/m² (MN/m²) - MPa being common.

Types of Stress

• Compressive stress: Two forces acting towards each other, shortening the material.
• Tensile stress: Two forces acting away from each other, stretching or elongating the material.
• Shear stress: Two forces acting parallel to each other but not in the same line, causing atoms to slide against each other.

III - Deformation and Strain

• Strain is the change in length per unit length of a body when a stress is applied.
• Strain is unitless (ε = ΔL/L)
• Strain can be elastic (temporary) or plastic (permanent)

IV - Stress-Strain Curve

• This curve plots stress against strain, showing how a material responds to increasing load.
• It's measured using a Universal Testing Machine.

False and True Curves

• False/Engineering Curve: The cross-sectional area of the specimen remains constant.
• True Curve: The cross-sectional area of the specimen changes during the test.

Relation Between Stress and Strain (Hook's Law)

• Stress is directly proportional to strain up to the proportional limit.
• Hook's Law describes this linear relationship.

Terms of the Stress-Strain Curve

• Proportional Limit (P.L.): The maximum stress a material can withstand without deviation from proportionality between stress and strain.
• Elastic Limit (E.L.): The maximum stress a material can sustain without permanent deformation. Normally very close to the proportional limit.
• Elastic, and plastic regions are the portions of the stress strain curve associated with the proportional limit and elastic limit.
• Terms such as ultimate tensile strength, yield strength, and fracture strength are relevant to the stress strain curve.

Yield Strength (YS)

• Yield strength is the stress level where the material begins to deform.
• This property is important in dental restorative materials to minimize permanent deformation under normal function pressures.
• Bridge materials should have a yield strength higher than the expected chewing force.

Ultimate Strength

• The maximum stress a material can withstand before failure.
• Yield strength is usually more critical than the ultimate strength because anything beyond that will cause permanent deformation.

Fracture Strength

• The stress level where the material fractures.

Elastic Modulus (Young's Modulus)

• Measures a material's stiffness or rigidity within its elastic range.
• The ratio of stress to strain.
• Common units are MPa or GPa

Elastic Modulus (Young's Modulus) - Material Values

• Several Types of dental materials have different elastic moduli and elastic strength values.
• Examples include: Enamel, Dentin, Gold Alloys, etc.
• Elastomers and polymers have low elastic modulus values.

Elastic Modulus (Young's Modulus) - Significance

• The stronger the interatomic or intermolecular forces, the higher the elastic modulus and the stiffer the material.
• The property generally depends on material composition, not heat treatment or mechanical processing.
• The rigidity of connectors in dentures affect the stability of the whole design. Stiffer alloys allow for thinner components.

II - Strain Terms

• Flexibility: The ease in which a material or structure bends, with slight stress and large elastic strain.
• Opposite to stiff or rigid materials.
• Important in impression materials due to ease of removal from the mouth.

Ductility and Malleability

• Ductility is a material's ability to deform permanently under tensile force without breaking.
• Malleability is a material's ability to deform permanently under compressive force without breaking (hammered or rolled into thin sheets).

Brittleness

• Materials that fracture at or near their proportional limit without significant deformation.
• Many dental materials (porcelain, cements, amalgam) are brittle.
• Dental amalgam has a high compressive strength compared to its tensile strength

III - Energy terms (Resilience and Toughness)

• Resilience: The energy needed to permanently deform a material to the proportional limit.
• Toughness: The total energy absorbed by the material before fracturing.
• Acrylic resin often is more resilient than porcelain, absorbing more energy and transmitting less to the jaw bone.

Resilient and Tough Materials

• Resilient Material: High ability to absorb energy without permanent deformation.
• Tough Material: High ability to absorb energy without fracturing.

Summary of Stress-Strain Curve Terms

• Stiff/rigid materials have high strength/low ductility, and high toughness.
• Flexible materials have low strength/high ductility, and low toughness

Different Stress-Strain Graphs

• Various types of stress-strain graphs illustrate different material behaviors.

Evaluation Sheets

• Evaluation sheets address specific definitions and applications of mechanical properties.

Diametral Compression Test

• Used to measure the tensile strength of brittle dental materials (cements, amalgam, investments).

Transverse Bending Test

• Measures the flexural strength of materials loaded in a three-point bending configuration.
• The top of the beam experiences compressive stress, the bottom tensile stress, and shear stress at the supports.

Fatigue

• Failure of a structure subjected to repeated loading below its proportional limit. It causes alternating/cyclic stresses.
• The S-N curve plots stress against number of cycles to fracture.
• Fatigue limit/endurance limit is the stress that can be repeatedly applied—without fracture. It is determined from the S-N curve.

Importance of Mechanical Properties in Dental Materials

• Understanding and applying mechanical properties help in choosing the right materials for specific dental applications.
• Designing functional and durable dental restorations.

Impact Strength

• Measures a material's resistance to fracture caused by sudden impact.
• Important for complete dentures to prevent catastrophic failure (falling).

Fracture Toughness

• Measure of a material's resistance to catastrophic crack propagation.
• Presence of filler particles, such as glass or zirconia, typically increases fracture toughness

Surface Mechanical Properties (Hardness, Wear, and Friction)

• Hardness: Resistance to penetration, indentation, or scratching—measured using different methods.
• Wear: Material loss due to mechanical action. This can be undesirable except finishing and polishing processes, where it's desirable.
• Friction: Resistance to motion of one material over another—caused by molecular bonding at surfaces. Useful for dental implants, to reduce motion with surrounding tissues.
• Denture-wearing patients must be cautioned on proper brushing techniques. Hard brushes can lower surface hardness and cause wear.