Mechanical Properties and Types of Stress

Mechanical Properties

• Force is the action applied to an object to change its position of rest or motion
• Force is defined by four characteristics: speed, magnitude, point of application, and direction.
• Units for force include N, kg, and lb.
• Stress is the internal reaction of a structure to an external applied load.
• External load and internal stress are equal in magnitude and opposite in direction.
• Stress (σ) = force/area
• Units for stress include MN/m² and MPa (M=1000).
• Types of stress include compressive, tensile, and shear stress.

Types of Stress

• Compressive Stress: The body is subjected to two sets of forces directed toward each other on the same straight line.
• Tensile Stress: The body is subjected to two sets of forces directed away from each other on the same straight line.
• Shear Stress: The body is subjected to two sets of parallel forces directed toward or away from each other.

Complex Stress

• If a wire is stretched, the observed stress is tensile, but the cross-section decreases, indicating the presence of compressive stresses.

Strain

• Strain (ε) is the change in length per unit length.
• Strain (ε) = (L final - L original) / L original
• Strain units are dimensionless
• Types of strain include elastic strain (temporary) and plastic strain (permanent).

Stress-Strain Relationship

• The stress-strain relationship of a dental material is studied by applying a load, measuring the deformation, and calculating stress and strain.

       Hook's Law: Stress is directly proportional to strain until the proportional limit.
  

• A graph (stress-strain curve) illustrates the relationship between stress and strain, showing various limits (proportional limit, elastic limit, yield strength, ultimate strength, and fracture strength).

Properties Derived from the Stress-Strain Curve

• Proportional Limit (PL): The maximum stress a material can withstand without deviating from the stress-strain proportionality.
• Elastic Limit (EL): The maximum stress a material can withstand without permanent deformation.
• Yield Strength (σy): The stress at which the material begins to deform plastically.
• Tensile Strength (UTS or σu): The maximum stress the material can withstand before fracture.
• Fracture Strength (σf): The stress at which the material fractures.
• Order of rigidity (stiffness) of materials: A>B>C. Order of Flexibility is C>B>A. Order of ductility is A>B>C.

Modulus of Elasticity

• Modulus of elasticity (E) is the constant of proportionality between stress and strain.
• It measures the stiffness or rigidity of a material.
• E=σ/ε
• Units are N/m², MN/m² (MPa), or kg/cm².
• Stiffness depends on the strength of interatomic bonds within a material, which is affected by material composition. (e.g., elastomers and polymers have low modulus, while metals and ceramics have high modulus).

Flexibility

• Flexibility describes the strain that occurs in a material when stress reaches the elastic limit.
• Measured in dimensionless units.
• Materials with higher flexibility exhibit greater strain under the same stress.

Ductility and Malleability

• Ductility is a material's ability to deform under tensile force without fracturing.
• Malleability is a material's ability to deform under compressive force without fracturing.
• Both ductility and malleability are represented on a stress–strain curve by the plastic strain portion; the materials with higher plastic strain are more ductile.

Brittleness

• Brittleness describes a material's characteristic of exhibiting little to no plastic deformation before fracture.
• Brittle materials fracture at or near their proportional limit.
• Brittleness is characterized by low toughness and low percentage elongation; also brittle materials are weaker in tension compared to compression.

Fracture Toughness

• Fracture toughness measures the resistance of a material to crack propagation.
• Brittle materials exhibit lower fracture toughness than ductile materials.
• Adding certain materials such as zirconia or fillers to a material will deflect or obliterate the crack, increasing the material's fracture toughness

Resilience and Toughness

• Resilience is the amount of energy absorbed by a material when stressed to the proportional limit.
• Toughness is the total energy absorbed by a material before fracturing.
• It's the area under the stress-strain curve.

Other Mechanical Tests

• Diametral Compression Test: Used to determine the tensile strength of brittle materials.
• Transverse Strength Test (3-Point Bending): Used to determine the flexural strength of materials. Load is placed in the centre of a supported beam to measure the materials ability to withstand bending.

Fatigue

• Fatigue occurs when a material fails under repeated cyclic loading. This repeated stress results in cracks which grow and eventually cause failure.
• Failures depend on the magnitude of the load and number of loading repetitions.
• Cyclic loading can cause crack propagation, resulting in fracture.

Impact Strength Test

• Measures energy required to fracture a material under sudden force.
• Two main types: Charpy and Izod.
• High-impact acrylic resins have been developed for dental materials.

Surface Mechanical Properties

• Hardness measures a material's resistance to permanent indentation, penetration, or scratching.
• Methods include Brinell, Knoop, Vickers, Rockwell, and Shore A. Hardness values are numerically represented; smaller indentations lead to higher hardness values.
• Clinical significance of surface hardness: avoid scratching teeth or restorations by utilizing materials with different hardness properties.

Wear

• Wear is the loss of material arising from repetitive mechanical action.
• Physiological (normal mastication), pathological (bruxism), and mechanical (improper brushing) are the primary causes of wear.

Rheological Properties

• Rheology is the science of flow and deformation of matter.
• Dentists manipulate materials that flow or deform under stress.

Viscoelasticity

• Viscoelasticity describes materials that exhibit characteristics of both elastic solids and viscous fluids.
• Viscoelastic materials are strain-rate dependent.
• A rapid rate of loading results in less permanent deformation

Creep

• Creep is time-dependent permanent deformation at stresses below the proportional limit and near the softening point of a material.
• Not important for metals/ceramics but critical for many polymers/waxes

This quiz covers the fundamental concepts of mechanical properties of materials, focusing on force and stress. You will explore different types of stress including compressive, tensile, and shear stress, as well as the relationship between external loads and internal stresses. Test your understanding of these essential principles in mechanics.