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

to deformation, and after this point, it will result in its rupture. In this point ultimate strength value is obtained. Toughness is the resistance of a material to fracture and corresponds to the amount of energy required to cause it [6]. All these concepts can be applied in clinical situations as many complex forces occur in the oral cavity and tend to deform the material (tensile, compressive, shear, bending forces), the knowledge and interpretation of how these materials behave under such forces are important to understand the performance of the material. Thus the various mechanical parameters that are evaluated for the suitability of a product for any application in the field of conservative dentistry are as follows:

      Good to Know

      The Universal Testing Machine is named so as it is capable of testing compression, tension, bending and flexion.

      It works by measuring the stress–strain relationship of each material.

      Tensile strength, diametral compression test, compressive strength, flexural strength, resistance to fatigue, hardness, elastic modulus, fracture toughness and bond strength.

Different equipment are used for testing different mechanical properties. Each of these tests may be conducted using either tooth or materials or both (in case of bonded structures). All these tests require variations in the assembly. Most of the tests except hardness require the use of Universal Testing Machine (UTM) where the sample preparations differ for each test. For instance, if tensile testing is done on a sample, the shape of the sample is dumb bell shaped; it is designed so that the specimen can be gripped at each end and stretched. For compressive strength testing, cylindrical‐shaped specimens are tested. Bar‐shaped specimens are used for flexural strength testing (as shown in Figures 1.1 and 1.2).

Figure 1.1 Universal testing machine.

Figure 1.2 Tooth sample testing.

1.1 Tensile Strength

When a body is subjected to axial forces in a straight line and in opposite directions, it results in tension. The resistance of the material to this load is called tensile strength. The length alteration that results from the application of a tensile force on a body before its rupture is defined as elongation. Nominal value of tensile strength is determined by the equation of load and cross‐sectional area (Kgf/cm²). Values of stress–strain determine a curve, characterizing the performance of the material under tensile test. From this curve, elastic modulus, ultimate tensile strength, resilience and toughness of such product can be registered [7]. Tensile testing is normally applied to materials which are placed under loading that is generally applied in different directions, as the opposing cusps move over the restoration surface. Loads that stretch or elongate a material cause tensile stresses (as shown in Figures 1.3–1.5).

Figure 1.3 Tensile strength assessment.

      Figure 1.4 Tensile strength assessment.

Figure 1.5 1, No load; 2, uniform elongation; 3, maximum load; 4, necking.

       Good to Know

      For a DTS test, the fracture line doesn't always occur along the central line, thus giving false results and making the test inaccurate.

      The diametral tensile strength (DTS) test is useful for materials that exhibit very limited plastic deformation and where information regarding stretching or elongation resistance is required. The DTS is a property described by American Dental Association (ADA)/American National Standards Institute (ANSI) Specification 27 for characterizing dental composite restoratives (DCR). It represents the minimal stress that a body will withstand without rupture when tensile loads are applied. The DTS test is considered useful because masticatory forces are frequently applied obliquely and tend to create tensile stress. A UTM is generally used to determine the tensile strength of a material. Materials which plastically deform would produce erroneous DTS values and also would be expected to display strain rate sensitivity. Composite resins are subjected to complex intraoral forces during mastication and parafunctional habits. Of the three tests used in this study to replicate intraoral forces, diametral tensile is the most difficult to interpret. Failure must occur in the center of the specimen along the diameter due to tensile forces if the diametral test is to yield useful results. The method was used to evaluate the influence of different cross‐head speeds on DTS of a resin composite material (Tetric N‐Ceram) by Anubhav Sood et al. in 2015 where they found that the cross‐head speed variations did not have a significant effect on the DTS of the resin composite [8].

1.2 Diametral Compression Test

Rupture under low tension characterizes fragile materials, susceptible to brittleness. In these cases, tensile strength is not indicated to evaluate material reaction, because of the low cohesive condition. An alternative method of tensile strength is calculated by compressive testing. It is a relatively simple and reproducible test. It is defined as diametral compression test for tension or indirect tension. Disk sample is necessary to conduct this test, where it is compressed diametrically introducing tensile stress in the material in the plane of the force application by the test. This is calculated by the formula: 2 P/π × D × T, where: P = load applied, D = diameter of the disk, T = thickness of the disk, π = constant [9, 10] (as shown in Figure 1.6).

Figure 1.6 Diametral compressive strength assessment.

      Source: Cefaly [9], Cattani‐Lorente [10].

1.3 Compressive Strength

Compressive testing is normally applied to materials that are expected to be placed in situations of occlusal loading. Since most of mastication forces are compressive in nature, it is important to investigate materials under this condition. To test compressive strength of a material, two axial sets of force are applied to a sample in an opposite direction, in order to approximate the molecular structure of the material. Here, cylindrical‐shaped specimens