Manual of Laboratory Testing Methods for Dental Restorative Materials. Paromita Mazumdar

speed of 0.125 mm/minute. Fracture of the specimen is identified by a sudden drop in load during the test. Visual examination of the fractured parts is performed to ensure that the fracture plane is through the notch, and that it is perpendicular to the vertical and horizontal planes through the center of the specimens.

      The fracture toughness was then calculated by:

      (1.2)

      The variables are defined as: Kic is the stress intensity factor, P is the load at fracture, L is the span, distance between the supports, w is the width of the specimen, b is the thickness of the specimen, and a is the crack length.

      Good to Know

      Cross‐head speed is also known as deformation rate. It is measured in mm/min. Every material has different average cross‐head speeds recommended by ISO 4049. For e.g. resin‐based composites have a speed of (0.75 ± 0.25 mm/min).

1.9 Nanoindentation

      With nanoscience gaining popularity, nanoindenters have advantages over traditional mechanical testing by providing both elastic modulus and hardness data of the tested samples. Nanoindentation is conducted with a calibrated Berkovich diamond indenter tip. A Berkovich tip is a three‐sided pyramidal indenter.

      During the nanoindentation process, a calibrated indenter tip approaches the surface of the sample. The force–displacement data is used to determine the point of contact. After the sample is contacted, the force is linearly increased and the tip indents into the surface of the sample. A short dwell time occurs at the maximum force and then the sample is unloaded. At the initial point of unloading, the stiffness is measured.

1.10 Bond Strength

      Longevity of a restoration is predicted to some extent by its adhesive ability, and this in turn can be measured by bond strength testing. An ideal bond strength test should be accurate, clinically reliable and less technique‐sensitive. It should involve the use of relatively unsophisticated and inexpensive test protocols. Static tests are categorized into macro‐tests where the bond area is >3 mm² and micro‐tests with <3 mm² bond area [15].

      1.10.1 Macro‐Test Methods

      The macro‐bond strength can be measured in shear, tensile or using a push‐out protocol.

      1.10.1.1 Macro‐Shear (SBS) Test

      In a shear bond test, two materials are connected via an adhesive agent and loaded in shear until fracture occurs.

      1.10.1.2 Macro‐Tensile (TBS) Test

      In a tensile bond test, load will be exerted on either sides of the test specimen. The specimen can be held by active or passive gripping methods. Active gripping method involves mechanical fastening of specimen to gripping device, such as glue or clamps, whereas in passive gripping method, specimen is placed in a testing device without the aid of glue or mechanical gripping. It can be used to measure, for instance, the bond strength of cements to hard materials such as ceramics and metal alloys [16].

      1.10.1.3 Push‐Out (PO) Test

      In this method, load is applied through a plunger mounted in the UTM. The plunger must provide near‐complete coverage of the testing material without touching the substrate. This method is useful to test adhesion of root canal sealers and retention of posts luted in root canals [17].

      1.10.2 Micro‐Test Methods

      1.10.2.1 Micro Shear Test

      Shear bond strength (SBS) testing with bonded cross‐sectional areas of 3 mm² or less is referred to as “micro” SBS. It permits efficient screening of adhesive systems, regional and depth profiling of a variety of substrates and conservation of teeth. Micro‐shear tests remain an extremely useful test for those substrates with properties such as glass ionomers or enamel that make them particularly susceptible to the specimen preparation effects and testing conditions of μTBS testing [18]^.

      1.10.2.2 Micro Tensile Test

      Micro‐tensile bond test (μTBS) allowed additional research designs that the “macro” tests did not, such as the elimination of tooth dependency through balanced designs. However, there is a difficulty in measuring bond strengths lower than 5 MPa [19].

      1.10.2.3 Micro Push‐Out Bond Strength

      Micro‐push‐out test is a modification of push‐out test where the specimen thickness is less than or equal to 1 mm² [15]^.

      Key Points

      1 Tensile strength, diametral compression test, compressive strength, flexural strength, resistance to fatigue, hardness, elastic modulus, fracture toughness are the various mechanical properties that are measured in the field of dental material sciences.

      2 Universal Testing Machine (UTM) is used for most of the tests with the only difference being that the sample preparations differ for each test. Hardness testing depends upon the material to be tested.

      3 The samples are prepared as per ISO standards to get optimum results. Also, the area which is subjected to the force differs due to the size of the stick/blade.

      4 Axial forces in a straight line and in opposite directions result in tension in the body of a material, and the resistance of the material to this load is called tensile strength. Nominal value of tensile strength is determined by the equation of load and cross‐sectional area (Kgf/cm2).

      5 The diametral tensile strength (DTS) test is useful for materials where information regarding stretching or elongation resistance is required and the failure has to occur along the center of the specimen.

      6 Compressive strength is used to test materials which are subjected to occlusally directed forces in the oral cavity.

      7 Flexural forces are the forces that generate repeated flexing, bending and twisting of the materials.

      8 Resistance to fatigue is the measurement of a fatigue limit, with no fracture, at a given number stress cycles.

      9 Hardness of a material gives an indication of the resistance to penetration when indented by a hard asperity.

      10 Types of hardness: macro hardness and micro hardness.

      11 Macro hardness has force application greater than 50 N, and micro hardness has force application less than 10 N. Macro hardness test is done using Vickers, Rockwell and Brinell hardness tester, whereas micro hardness test is done using Vickers and Knoop hardness tester.

References

      1 1 Albers, H.F. (1996). Tooth‐Colored Restoratives, 8e. Santa Rosa: Alto Books.

      2 2 Craig, R.G. (1997). Mechanical Properties. Restorative Dental Materials, 10e, 56–103. St. Louis: Mosby.

      3 3 Craig, R.G. (1997). Optical, Thermal, and Electrical Properties. Restorative Dental Materials, 10e, 30–55. St. Louis: Mosby.

      4 4 Vieira, D.F. (1965). Propriedades mecânicas. Propriedades dos materiais odontológicos, 31–89. São Paulo, Universidade de São Paulo.

      5 5 Vieira, D.F. (1965). Propriedades ou constantes físicas. Propriedades dos materiais odontológicos, 92–105. São Paulo: Universidade de São Paulo.

      6 6 Wang,