Mechanical Properties of Materials PDF

Mechanical properties of materials By Dr. Reham Mohammed Abdallah professor of dental biomaterials Items to be covered Introduction on mechanical properties Force Stress & types Strain Stress-strain curve Stress terms  Proportional limit  Elastic limit  Yield...

Mechanical properties of materials By Dr. Reham Mohammed Abdallah professor of dental biomaterials Items to be covered Introduction on mechanical properties Force Stress & types Strain Stress-strain curve Stress terms  Proportional limit  Elastic limit  Yield stress Ultimate strength  Fracture strength Strain terms  Flexibility  Ductility Energy terms  Resilience  Toughness  Mechanical properties are important in understanding and predicting a material's behavior under load. Quantities of force, stress, strain, strength, toughness, hardness, friction and wear can help to  Identify the properties of a material (polymer, ceramic and metal).  Understand reasons of failure. Select and design of dental restorations and appliances. Standardization of laboratory tests is essential to control quality and permit comparison of results between investigators. Force One body interacting with another generates force. Forces may be applied through actual contact of the bodies or at a distance (e.g., gravity). The International System Unit of force (SI unit) is the Newton (N). Stress  When a force acts on a body tending to produce deformation, a resistance is developed to this external force application.  Definition: It is the Internal resistance to the externally applied force.  It is denoted by (σ)  Stress (σ)= Force/Area (fig.1).  Pascal = 1 N / m².  Commonly stress is reported in terms of mega Pascals (MPA). MPA=106 Pascal. Fig.1:Stress measurements Types of stresses 1. Axial stresses Compressive stress Compression results when the body is subjected to two sets of forces directed towards each other in the same straight line. (fig.2) Tensile stress Tension results in a body when it is subjected to two sets of forces directed away from each other in the same straight line. (fig.2) Fig.2.Types of axial stresses Axial Compressive Tensile 2. Non axial stresses Shear stress Shear is the result of two sets of forces directed towards each other but not in the same straight line.(fig.3) Torsion It results from the twisting of the body. (fig.3) Bending It results by applying bending movement.(fig.3)  Fig.3.Types of non axial stresses Non Axial Shear Torsion Bending Strain Definition: It is the change in length per unit length. It represents the relative deformation of an object that is subjected to stress. It may be elastic, plastic or both elastic and plastic. It is denoted by “ε” Designated as ∆L / L. So, it is unitless. Strain(ε)= Deformation/Original length  When the force is applied, the rod's length changes from its original length L0, to the extended length L1. The resulted strain, ɛ, is given by ɛ = (L1- L0)/ L0 (fig.4) Fig.4:strain In an object subjected to stress Stress-Strain Relationship If a bar of material is subjected to an applied force, F, the magnitude of the stress and the resulting deformation (ɛ) can be measured. This is done with tensile, compressive or shear loading of samples using universal testing machine (Fig.5). Graph representing stress (or load) and strain (elongation) can be obtained (Fig.6). Fig.5: Universal testing machine Fig.6: Stress-strain curve for a material subjected to tensile stress. Stress-Strain Behavior (types of strain) 1. Elastic deformation Reversible: When the stress is removed, the material returns to the dimension it had before the loading. 2. Plastic deformation Irreversible: When the stress is removed, the material does not return to its previous dimension. I. Stress terms 1.Proportional Limit It is the maximum stress up to which, the stress is linearly proportional to strain. (fig.6)Point A A material with high value of proportional limit can withstand greater stress without permanent deformation. Significance Dental restorations should be constructed from materials with a high proportional limit. Any dental restoration that is permanently deformed through the forces of mastication is usually a functional failure to some degree.  For example, a fixed partial denture that is permanently deformed by excessive occlusal forces would exhibit altered occlusal contacts. 2. Elastic Limit Maximum stress a material can withstand without undergoing permanent deformation. It describes the elastic behavior of the material. (fig.6)Point B  The elastic and proportional limits have nearly the same values, as they represent the same phenomena. 3. Yield Stress or Proof stress It is the stress at which materials start to show permanent deformation or function in a plastic manner.(fig.6) Point C Significance Permanent deformation and stresses in excess of the elastic limit are desirable when shaping an orthodontic arch wire or adjusting a clasp on a removable partial denture.  It is important for the material to exhibit high value of yield strength to prevent the plastic deformation.For example, the crown and bridge restorations show permanent deformation on exposure to the occlusal forces in case of low yield strength. 4. Ultimate(Tensile or compressive) Strength or stress Maximum stress that the material can withstand before failure (fracture) under tension or compression respectively. The material could not withstand any more stresses, as it will fracture. (fig.6) Point D  The yield strength is often of greater importance than ultimate strength in design and material selection because it is an estimation of when a material will start to deform permanently. 5. Fracture strength or stress It is the strength at which the material fractures. (fig.6) Point F II. Strain terms 1. Flexibility The maximum flexibility is defined as the strain occurring when the material is stressed to its elastic limit. Significance A larger strain or deformation with slight stresses is an important consideration in orthodontic appliances.  Impression materials should have large flexibility or elastic deformation to withdraw through severe undercuts without permanent deformation. 2. Ductility The amount of plastic strain produced in the specimen before fracture.  Or the ability of a material to be drawn and shaped into wire by means of tension. When tensile forces are applied, the wire is formed by permanent deformation.  However, malleability of a substance represents its ability to be plastically deformed under compression (hammered or rolled into thin sheets) without fracturing. Significance Orthodontic appliances could be bent and partial denture clasps could be adjusted when they have high ductility. Brittleness :  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. Ductile material Brittle material 1) Is the ability of a 1) brittle material Material to withstand fractures at or near Plastic deformation its proportional limit. Under tensile stress Without fracture. Fracture occur far Fracture occur at or Away from P.L near P.L Elastic Modulus (E) It is the constant of proportionality between stress and strain. It represents the slope of the elastic portion of the stress – strain curve. It is a measure of rigidity or stiffness  Materials with higher Young’s modulus value are said to be stiffer or more rigid than those of low Young’s modulus values because they require much more stresses to produce the same amount of strain.  It is measured by stress/strain  It is measured by GPA=109 Pascal. Elastic Modulus of material (A) is higher than that of material (B) Significance 1. Oral appliances and restorative materials should have high proportional limit and elastic modulus to resist forces of mastication and permanent deformation. 2. In orthodontic, when rapid large forces are needed to be applied to cause a tooth to move, a high stiffness wire is used. However, flexible wires are used when a low force is needed for slow movement of the tooth. 3. Rigidity of a denture base is required to allow distribution of the force on the whole structure. Also, it is important to allow it to be used in a thin section. III. Energy terms 1. Resilience Resilience is the resistance of a material to permanent deformation. It indicates the amount of energy necessary to deform the material to the proportional limit.  Resilience is therefore measured by the area under the elastic portion of the stress strain curve. Significance 1. Resilience has particular importance in the evaluation of resilient-denture lining materials, tissue conditioners and maxillofacial materials. 2. It is very important property for orthodontic wire as the amount of expected force applied on the teeth is very important. 2. Toughness Toughness, which is the resistance of a material to fracture, is an indication of the amount of energy necessary to cause fracture. It is represented by the area under the elastic and plastic portions of a stress-strain curve. Fracture toughness It is defined as the ability of the material to resist fracture through the resistance to crack propagation. Significance Addition of zirconia, alumina and leucite to dental porcelain to resist crack propagation and increase the fracture toughness.  ANALYSIS FOR A STRESS STRAIN CURVE STIFFNESS & FLEXIBILITY 1) If longitudinal portion of the curve is closer to the long axis, the material is stiff & not flexible. 2) If it is away from the long axis the material is flexible.  TOUGHNESS & BRITTLENESS 1) If material fractures after a long concave portion of the curve, it donates that the material is tough & ductile. 2) If elastic portion of the curve is minimal, it shows the brittleness of the material.  STRENGTH & WEAKNESS If longitudinal portion of curve is long, it means that the material is strong. If longitudinal portion is short, it means that the material is weak.  HENCE FROM THE ANALYSIS OF THE STRESS STRAIN CURVE, IT IS POSSIBLE TO HAVE AN IDEA ABOUT THE PROPERTIES OF A MATERIAL. Comparison of materials properties