Tiêu đề class for sdm-converted.pdf ✅

Ductility—Relative ability of a material to elongate plastically under a tensile stress. This property is reported quantitatively as percent elongation. Brittleness—Relative inability of a material to deform plastically before it fractures. Flexural strength—Force per unit area at the instant of fracture in a test specimen subjected to flexural loading. Malleability—Ability to be hammered or compressed plastically into thin sheets without fracture. Stress—Force per unit area within a structure subjected to a force or pressure. Strength— (1) Maximum stress that a structure can withstand without sustaining a specific amount of plastic strain (yield strength); (2) stress at the point of fracture (ultimate strength). Tensile stress—Ratio of tensile force to the original cross-sectional area perpendicular to the direction of applied force. Tensile strength (ultimate tensile strength)—Tensile stress at the instant of fracture. There are few pure tensile stress situations in dentistry. However, a tensile stress can be generated when structures are flexed. The deformation of a bridge is a example of these complex stress situations. In fixed prosthodontics clinics, a sticky candy (e.g., Jujube, a sticky/gummy candy) can be used to remove crowns by means of a tensile force when patients try to open their mouths after the candy has mechanically bonded to opposing teeth or crowns. Compressive stress—Compressive force per unit area perpendicular to the direction of applied force. When a body is placed under a load that tends to compress or shorten it, the internal resistance to such a load is called a compressive stress. A compressive stress is associated with a compressive strain. To calculate compressive stress, the applied force is divided by the cross-sectional area perpendicular to the axis of the applied force. SHEAR STRESS - This type of stress tends to resist the sliding or twisting of one portion of a body over another. Shear stress can also be produced by a twisting or torsional action on a material. For example, if a force is applied along the surface of tooth enamel by a sharp-edged instrument parallel to the interface between the enamel and an orthodontic bracket, the bracket may debond by shear stress failure of the resin luting agent. Shear stress is calculated by dividing the force by the area parallel to the force direction. Elastic strain—Amount of deformation that is recovered instantaneously when an externally applied force or pressure is reduced or eliminated. Elastic modulus (also modulus of elasticity and Young’s modulus)—Stiffness of a material that is calculated as the ratio of elastic stress to elastic strain. Flexural strength (bending strength or modulus of rupture)—Force per unit area at the instant of fracture in a test specimen subjected to flexural loading. Flexural stress (bending stress)—Force per unit area of a material that is subjected to flexural loading. when a patient bites into an object, the anterior teeth receive forces that are at an angle to their long axes, thereby creating flexural stresses within the teeth.
When one chews a hard food particle against a ceramic crown, the atomic structure of the crown is slightly deformed elastically by the force of mastication. If only elastic deformation occurs, the surface of the crown will recover completely. when the force is eliminated. Elastic stresses in materials do not cause permanent (irreversible) deformation. On the other hand, stresses greater than the proportional limit cause permanent deformation and, if high enough, may cause fracture. For brittle materials that exhibit only elastic deformation and do not plastically deform, stresses at or slightly above the maximal elastic stress (proportional limit) result in fracture. These mechanical properties of brittle dental materials are important for the dentist to understand in designing a restoration or making adjustments to a prosthesis. Based on Newton’s third law of motion (i.e., for every action there is an equal and opposite reaction), when an external force acts on a solid, a reaction occurs to oppose this force which is equal in magnitude but opposite in direction to the external force. The stress produced within the solid material is equal to the applied force divided by the area over which it acts. A tensile force produces tensile stress, a compressive force produces compressive stress, and a shear force produces shear stress. A bending force can produce all three types of stresses, but in most cases, fracture occurs because of the tensile stress component. In this situation, the tensile and compressive stresses are principal axial stresses, whereas the shear stress represents a combination of tensile and compressive components.