Mechanical Properties of Dental Materials PDF

Presented by: Dr. Eman Mohamed Mohamady lecturer of Dental Biomaterials  Mechanical properties of materials are sets of physical properties, that describe the behavior of materials under a force or a load.  One of the most important properties of dental materials of dental mat...

Presented by: Dr. Eman Mohamed Mohamady lecturer of Dental Biomaterials  Mechanical properties of materials are sets of physical properties, that describe the behavior of materials under a force or a load.  One of the most important properties of dental materials of dental materials is the ability to withstand the various mechanical forces placed on them during use as restoration, impression, models, appliances and tools  Is the force per unit area induced in a body in response to some externally applied force  σ = F/ A  Units of stress: Pa = N/m2 MPa = MN/m2 Ib/in2 Factors affecting stress: 1. Force ➔ varies directly 2. Area ➔ varies inversely 1. Tensile stresses: Two sets of forces acting away from each other in the same straight line and in the same plane. Tend to elongate the body 2. Compressive stresses: Two sets of forces acting toward each other in the same straight line and in the same plane. Tend to shorten the body. 3. Shear stresses: Two sets of forces acting toward or away from each other but not in the same straight line or plane. Tend to sliding the body.  Results when all types of stresses develop in a structure as in dental restoration  Importance of stress in dentistry  Dental restorations are subjected to extremely great stresses because the area over which the forces are applied is extremely small.  The forces applied to a dental restoration are (complex stresses).  Is the measurement of the resistance of the material to the external applied force.  The deformation or distortion produced as a result of the stress produced within the material.  Defined as: Change in length ∆ L (Lo -Lf) per unit original length.  ∊= ∆ L / Lo  Has no unites.  Each type of stress is capable of producing a corresponding type of strain i.e tensile, compressive, shear and complex.  These strain are either: 1. Elastic strain (temporary)(elastic deformation) The body return to the original size and shape with removal of stresses. 2. plastic strain (permanent)(plastic deformation) : body demonstrated permanent amount of deformation with removal of stresses.  Elastic strain plastic strain A  1-Proportional limit: P.L (A) Is the maximum stress the material can withstand without deviation from the law of proportionality between stress and strain.  D C E A B 2- Elastic limit : E.L (B) The maximum stress the materials can withstand without permanent deformation.  Elastic limit and proportional limit represent the same value A B 2- Yield strength: Y.P ( C) The stress the materials begins to function in a plastic manner. D The stress at which the material exhibit deviation from the proportionality between stress and strain. B c A 1. The material should withstand high stresses while in function without permanent deformation. 2. The permanent deformation of restoration even without fracture is consider a functional failure. 3. During adjustment of the restoration the stress applied should be greater than the yield strength to produce permanent deformation ( burnishing)  4-Ultimate strength U.S. (D) Is the maximum stress the material can withstand before fracture. G.R D E Yield strength is of greater importance than ultimate strength.  5-Frecture strength U.S. (E) Is the stress at which the material fracture. D E Stress terms Ultimate strength Yield strength Elastic limit Proportional limit Fracture strength  It is the constant of proportionality between stress and strain within the elastic region.  Represent the slope of the elastic portion of stress– strain curve.  Denoted by “E“.  E= σ/ ∊  Units kg/cm² or Mpa or Ib/in².  Represents the stiffness or rigidity of the material within the elastic region.  Stiffness= rigidity Is the resistance of the martials to elastic deformation.  Clinical importance:  Even stress distribution over the area to which the load is applied and it is important in: 1. Long span bridge. 2. Denture base materials have high E and can be used in thinner cross sections without fear of uneven stress distribution e.g. cobalt chromium denture base material compared to gold alloys. 1- Flexibility: ❑ The material show high elastic strain with moderate or low stress. ❑ It is the strain when the material is stressed to the P.L.  Clinical importance: 1. In elastic impression material to be easily removed from the patient mouth. 2. In orthodontic wire and endodontic files to be easily bend elastically with little stresses. 2- Ductility and malleability: ❑ Isthe ability of the material to plastically deformed without fracture. ❑ Ductility: is the ability of the material to be drown into wire under tension ❑ Malleability: is the ability of the material to hammered into sheets under compression.  Clinical importance: Ability of the material to be easily burnished 4- brittleness:  The material shows no or little plastic deformation.  The material fracture near or at P.L  Brittle materials are week in tension than compression.  Importance: Brittle materials should be designed to receive compressive loads during function and minimize tensile load.  1- Resilience:  The amount of absorbed energy by the material when it is stressed to the P.L.  Amount of energy needed to deform the material elastically.  Clinical importance: 1- Evaluation of orthodontic Wires to know the amount of work expected from a particular spring to move the teeth. 2- Resilient denture base material  2- Toughness: ❑ The amount of energy absorbed by the material up to the point of fracture. ❑ Energy needed to fracture the material. ❑ Represent by the area under the elastic and plastic portion of the curve.