Mechanical Properties (1) - DBM701 2024-2025 Dental Biomaterials PDF

DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Mechanical properties Mechanical properties are a group of physical properties that describe the behavior of materials under force or load. Mechanic...

DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Mechanical properties Mechanical properties are a group of physical properties that describe the behavior of materials under force or load. Mechanical properties of orthodontic materials are important because these materials are subjected to forces in service i.e. during fabrication or function. In general, the behaviour of a solid under load is determined by the nature and/or strength of its interatomic and intermolecular bonds. 1. Force: Force is an action which, when applied to a body, produces or tends to produce a change in the body’s position, movement and/or shape. Characteristics of force: A force can be defined in terms of: a. Magnitude: The units of force are Newton (N) or Pound (lb) where 1 lb = 4.4 N b. Speed: According to its speed, force can be static or dynamic (Fig.1). Figure (1): Static and dynamic forces. c. Point of application: When two forces applied on one body act on the same line, whether directed towards each other or away from each other, this set of forces gives a resultant normal force. On the other hand, when two forces applied on one body act on two parallel lines (not on the same line), whether directed towards or away from each other, this set of forces gives a resultant tangential force (Fig. 2). d. Direction: When two forces, applied on one body, are acting on the same line and directed away from each other, the resultant force is tensile while if the two forces are directed towards each other, the resultant force is compressive (Fig. 2). 1  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Figure (2): Effect of: (A) Point of application and (B) Direction of force 2. Stress: Stress is defined as the internal reaction to an external force and is equal in intensity and opposite in direction to the applied external force. Both the applied force and the internal resistance are distributed over the same given area of the body over which the force is applied. The stress (σ) is calculated as the force per unit area. force F stress = or  = area A From the above equation, we see that the stress in a structure varies directly with the external force and inversely with the area over which it is applied. The most convenient way to measure the stress is to measure the external force applied to the area, then calculate the stress from the previous equation. Units: The units of stress are Pascal (Pa = N/m2), Megapascal (MPa = 2 1,000,000 Pa) and lb/in. Types: There are different types of stresses: (Fig. 3) 1. Normal or Axial:  Tensile stress (t ),  Compressive stress (c ), 2. Tangential:  Shear stress (). 2  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Figure (3): Types of stresses Tensile stress (t ): Tensile stress is induced in a body loaded by tensile force i.e. two sets of axial forces acting on the same line directed away from each other. Molecules making up the body must resist being pulled apart. Compressive stress (c ): Compressive stress is induced in a body loaded by compressive force i.e. two sets of axial forces acting on the same line directed towards each other. Molecules making up the body must resist being forced more closely together. Shear stress (): Shear is the result of two sets of forces being directed towards each other or opposite each other but not on the same straight line i.e. parallel to each other (such that the distance between them will be close to, but not equal to, zero). Molecules of such a body must resist sliding past one another. e.g. scissors are often called shears. 3  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Complex stresses In reality, there is no pure tensile or pure compressive stress. When a compressive force is applied to a body, compressive stress is generated along the line of action of the force which tends to shorten the body. Simultaneously, tensile stresses are generated in planes perpendicular to the line of action of the force which tends to increase the cross sectional area of the body. Shear stresses are also generated in some areas of the body as can be seen in (Fig. 3) N.B.: Under certain loading designs, other types of stresses are generated like torsional stress and transverse stress. 3. Deformation and strain: Deformation: is the distortion produced as a result of the stress induced within the material. In case of axial stress, the deformation can be defined as the change in length (Fig. 4). It is calculated as: Figure (4): Deformation Strain: is defined as the change in length per unit length and is calculated as: deformation strain = or original length mm/mm or dimensionless 4  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Types of strain: Each type of stress is capable of producing a corresponding type of strain in the body e.g. tensile stress produces strain in the form of elongation while compressive stress produces strain in the form of reduction in length (shortening). Any type of those strains can be either elastic or plastic (Fig. 5A&B). The differences between elastic and plastic strains can be summarized in the following table: Elastic strain Plastic strain It is the strain that recovers upon It is the strain that does not recover unloading. upon unloading and is therefore permanent. Associated with stretching but not Interatomic bonds are broken and breakage of the chemical bonds new bonds are formed, thus atoms between the atoms in the solid. change their neighbors. Figure (5.A) : Mechanism of elastic deformation (on atomic level)| Figure (5.B): Mechanism of plastic deformation (on atomic level) 5  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly 4. Stress-strain curve: The relationship between stress and strain is often used to characterize the mechanical properties of materials. Such data are generally obtained using a mechanical testing machine which enables strain to be measured as a function of stress and recorded automatically (Fig. 6). Figure (6): Mechanical testing machine One of the commonly used tests for evaluating the mechanical properties of dental materials is the tensile test in which a rod sample of the material is grabbed from its two ends then pulled to failure in a relatively short time at a constant rate. The change in length is measured by an external extensometer attached to the sample. The induced tensile stress (calculated by dividing force over area) and the corresponding strain (calculated as change in length per unit length) are plotted graphically with the stress represented on the vertical axis and the strain plotted on the horizontal axis. Form the obtained stress-strain curve (Fig. 7), several mechanical properties of the material can be identified: 6  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Figure (7): stress-strain Curve a. Proportional limit: In the early (low strain) portion of the curve, many materials obey Hooke’s law which states that “Up to a certain stress value, stress is proportional to strain”. This means that up to this stress value, when the stress is doubled, the strain is doubled and when the stress is tripled, the strain is tripled and so on. As stress is gradually increased and once a specific stress value called “proportional limit” is exceeded, many materials eventually deviate from this linear proportionality. Thus, the proportional limit can be defined as “the greatest stress the material can withstand without deviation from Hooke’s law or from the law of proportionality between stress and strain”. b. Elastic limit: If a relatively small tensile stress is induced in the rod specimen, the specimen will return to its original length when the load is removed. If the load is increased progressively in small increments and then released after each increase in stress, a stress value will be reached at which the specimen does not return to its original length when unloaded. At this point, the material has been stressed beyond its elastic limit. The elastic limit is defined as “the greatest stress to which a material can be subjected such that it returns to its original dimensions when the force is released” i.e. it is “the greatest stress the material can withstand without 7  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly a resulting permanent deformation”. Therefore, the elastic limit describes the elastic behaviour of the material. For most materials, the proportional limit and the elastic limit represent the same value, however, they differ in their fundamental concept. c. Yield strength: The yield strength lies on the stress-strain curve just after the elastic limit. Once the elastic limit is exceeded, permanent strain occurs in the material. Since it is often difficult to pinpoint the exact stress at which plastic deformation begins, the yield strength is defined as “the stress at which a limited specified amount of permanent strain has occurred in the material”. The amount of permanent strain is arbitrarily selected, may be 0.1%, 0.2% or 0.5% of permanent strain. This selected amount of permanent strain is referred to as percent offset. When comparing the strengths of two materials, it is not correct to compare the yield strength at 0.5% offset for one material with the yield strength at 0.1% offset for the other material. The selected percent offset should be the same for both materials to ensure standardization. Clinical significance: During construction of an appliance from a wire, the applied force should induce a stress greater than the yield strength of the wire’s material to allow permanent shaping. During adjustment of a restoration, the applied force should induce a stress greater than the yield strength of the material to produce permanent deformation. During function, the restoration should not be subjected to stresses above the yield strength, otherwise, permanent deformation would occur and the restoration will no more be fitting the purpose in spite of the fact that it did not break. This is considered as a functional failure. d. Ultimate strength and fracture strength: If you carry on stressing the material beyond the yield strength, the sample will eventually fracture (fail). The ultimate strength is “the maximum stress the material can withstand before fracture”. It can be ultimate tensile, compressive or shear strength depending on the mode of loading. The ultimate strength of a material can be determined by drawing a horizontal line from the highest point on the stress-strain curve to the stress axis. The stress where this line 8  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly intersects with the stress axis is called the ultimate strength. The ultimate tensile strength is sometimes just called the tensile strength and the ultimate compressive strength is also called the compressive strength. The fracture strength is “the stress at which fracture of the material occurs”. After the maximum stress is reached, the material begins to elongate excessively and the stress calculated from the force and original cross- sectional area drops before final fracture occurs. The curve obtained by calculations based on the original cross-sectional area is called “engineering stress-strain curve”. If the decrease in the cross-sectional area of the material is taken in consideration, the true stress-strain curve will be obtained. In the true stress-strain curve, the facture strength would be higher than in the engineering curve (Fig. 8). Although the true curve represents the situation more accurately, the engineering curve is more commonly used. Figure (8): (A): True versus engineering stress-strain curve. (B): Localized reduction in cross-sectional area (necking) e. Modulus of elasticity (Young’s Modulus) “E”: The modulus of elasticity or Young’s modulus denoted by “E”, is the constant of proportionality between stress and strain and represents the slope of the elastic portion of the stress-strain curve. It can be calculated from the following relation:  E= kg/cm2 or MPa or lb/in2  9  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly The modulus of elasticity represents the stiffness of the material within the elastic range. Materials with high modulus are called “rigid” or “stiff” while those of low modulus are described as “flexible”. The modulus of elasticity of a material does not change either tested under compression or tension (Fig. 9). Figure (9): Same slope of the elastic part of both S-S curves of one material tested under tension and compression. The elastic modulus depends on the interatomic or intermolecular forces of the material. The stronger the basic attraction forces, the more difficult is the stretching of bonds, thus, the greater is the value of the modulus of elasticity. Materials such as rubbers and plastics have low values for the modulus of elasticity and are readily deformed elastically, whereas many metals and alloys have much higher values. Representative values of the modulus of elasticity of some materials are shown in the following table: Material Elastic modulus (GPa) Stainless steel 179 Cobalt-chromium-nickel 184 Ni-Ti 41 Beta-titanium 72 Enamel 84.1 Dentin 18.3 Composite resin 16.6 Clinical significance: 1. Materials with high modulus of elasticity allow an even stress distribution over the area to which the load is applied. This is an important feature in denture base materials. Denture materials with high modulus of elasticity can be used in thinner section without fear of uneven stress distribution. 10  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly 2. Bridges, particularly long span bridges, should be fabricated from a material that has high modulus of elasticity to allow even stress distribution, and to avoid bending. 3. The stiffness of the orthodontic wire determines its ability to deliver a suitable force that allows tooth movement during orthodontic treatment. Stiff wires (like stainless steel) can deliver higher forces that allow a more rapid tooth movement, while more flexible wires (like Ni-Ti) apply smaller forces which bring about slower tooth movements. f. Flexibility: The maximal flexibility is defined as “the strain that occurs when the material is stressed to its proportional limit” (Fig. 10). Figure (10): Maximum flexibility It is computed from the following relation: Where, εm = maximum flexibility PL = proportional limit E = modulus of elasticity Clinical significance: The maximum flexibility of impression materials should be high to allow removal of the impression materials from sever undercuts without plastic deformation. Orthodontic wires should have high maximum flexibility to allow the wire to undergo large deflections without permanent deformation. A 11  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly closely related term commonly used by orthodontists to describe this property is the “springback potential”. The springback potential is calculated as the ratio between yield strength and modulus of elasticity. It is an indication of the springness of the wire. g. Brittleness: If a material demonstrates no or very little plastic deformation on load application, it is described as being brittle. Thus, a brittle material fractures at or very near its proportional limit (Fig. 14A). This fracture occurs by crack propagation and the fracture surface is characterized by granules (Fig. 15A). In general, brittle materials are weaker in tension than in compression. This is attributed to the fact that brittle materials inherently contain flaws or cracks. When subjected to tensile loading, the resultant tensile stresses tend to further open these cracks leading to crack propagation. On the other hand, when a brittle material is subjected to compressive loading, the generated compressive stress tends to close the cracks (Fig. 11). Examples of brittle materials used in dentistry are dental amalgam, dental ceramics and dental cements. N.B.: A brittle material does not necessarily lack strength e.g. several ceramics have high strength but are still brittle. Figure (11): Compression tends to close cracks while tension tends to open them further h. Malleability and ductility: These are properties of metals and alloys. They indicate the workability of the metal or alloy. The malleability of a material is its ability to be hammered into thin sheets without fracturing i.e. to be plastically deformed under compressive force (Fig. 12). It is related to the burnishability of the material. 12  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Figure (12): Malleable material hammered into sheets On the other hand, the ductility of the material is its ability to be drawn into wires without fracture i.e. to be plastically deformed under tensile force (Fig. 13). Figure (13): Ductile material drawn into wires Fracture of ductile materials when subjected to tensile loading is preceded by localized reduction in cross-sectional area at the fracture site, a phenomenon called “necking” (Fig. 14B & 15B). i. Percent elongation: The deformation that results from the application of a tensile force is elongation. The ability of an alloy to permanently elongate under tensile loading gives an indication of the workability of such alloy. The percent elongation represents the maximum amount of permanent deformation a material can exhibit and it can be calculated as: % Elongation = (increase in length / original length) x 100 N.B.: In this equation, the increase in length is measured at the point of fracture. A material, like many dental gold alloys, that exhibits a 20% total elongation at the time of fracture has increased one fifth of its length. Such alloys are described to be ductile, whereas, a material with only 1% elongation would be considered brittle e.g. Nickel-chromium alloy. 13  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Comparison between brittle and ductile materials Brittle material Ductile material Exhibits no or little plastic Exhibits a large amount of plastic deformation i.e. fractures at or near deformation the proportional limit Has low % elongation Has high % elongation Figure (14A): Stress-strain curve of a brittle Figure (14B): Stress-strain curve of a material ductile material Fractures by crack propagation Fractures by necking Figure (15A): Fracture of a brittle material Figure (15B): Fracture of a ductile material Brittle fracture requires energy to Ductile fracture requires not only separate atoms and to expose new the energy to separate atoms, but surfaces along the fracture path. much additional energy to deform the material plastically before fracture. Examples: Dental amalgam, cements Examples: Most metals and alloys and ceramics j. Resilience: Resilience represents “the amount of energy needed to deform the material to its proportional limit”. It is represented by the area under the straight portion of the stress-strain curve i.e. the area of the shaded triangle in (Fig. 16). 14  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Figure (16): The area under the straight part of the stress-strain curve representing resilience Resilience is also called the stored energy because when the material is stressed to its proportional limit, the absorbed energy remains stored in the material as long as it is loaded. Once the load is removed, this energy is released causing complete recovery of the elastically deformed material. Clinical significance: Resilience is an important requirement of orthodontic wires because when the wire is elastically bent, its stored energy can be released over the required time to move teeth gradually. The greater the resilience of the wire, the more the energy it can temporarily store then release during the orthodontic treatment. l. Toughness: Toughness represents “the energy required to stress the material to the point of fracture”. It is represented by the area under both the elastic and plastic portions of the stress-strain curve (Fig. 17). The toughest materials are those with high proportional limit and good ductility. Thus, brittle materials tend to be not tough. Figure (17): The area under both elastic and plastic parts of the stress-strain curve represents toughness 15  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Fracture Toughness When cracks are present in a material, less force is needed to cause fracture because the stresses tend to be intensified at the cracks’ tips. This phenomenon is called “stress concentration”. This is especially true with brittle materials (like ceramics) while with ductile materials (like metals), cracks are not as dangerous. The resistance of the material to catastrophic propagation of cracks and flaws under an induced stress is called fracture toughness. Accordingly, brittle materials have low fracture toughness while ductile materials have high fracture toughness. A commonly used strengthening mechanism is to incorporate fillers into the material. The fillers deflect the crack or absorbs its energy, thus, more energy will be needed to propagate the crack leading to higher fracture toughness. Interpretation of the stress-strain curve: Several properties of the material can be determined from its stress-strain curve: The slope of the elastic part of the curve determines whether the material is stiff or flexible. A material with a high slope is described as stiff and that with low slope is flexible. The value of the proportional limit determines whether the material is weak or strong. The amount of plastic deformation (i.e. the length of the plastic part of the curve) determines whether the material is brittle or (ductile or malleable) The area under the elastic part of the curve determines whether the material is resilient or not. The area under both elastic and plastic parts of the curve determines whether the material is tough or not (Fig. 18). 16  DBM701- Dental Biomaterials (2024-2025) Mechanical Properties (1) - Prof. Gihan Waly Figure (18) Stress-strain curves for materials with various combinations of properties 17