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

have a relation of length to diameter of 2 : 1. When this proportion is exceeded, it can result in undesirable bending of the specimen. In the same manner of tensile strength, nominal value of compressive strength also is determined by the reason of load by cross‐sectional area (Kgf/cm²). Stress–strain curve of investigated material is determined in the same manner as established to tensile tests. Thus, the elastic modulus can also be determined by the stress–strain ratio in the elastic region [11] (as shown in Figures 1.7 and 1.8).

Figure 1.7 Direction of force is perpendicular to the object.

1.4 Flexural Strength

      The flexural strength of a material is its ability to bend before it breaks. It is obtained when the ultimate flexibility of one material is achieved before its proportional limit. This is a measure of the strength of a beam of restorative material supported at each end and subjected to a static load. Stresses on the upper surface of the beam tend to be compressive, whilst those on the lower surface are tensile. This test may be considered to combine elements of tensile and compressive testing. Flexural forces are the result of forces generated in clinical situations and the dental materials need to withstand repeated flexing, bending, and twisting. A high flexural strength is desired once these materials are under the action of chewing stress that might induce permanent deformation. To evaluate flexural strength of a dental material, bar‐shaped specimens with dimension of 25 mm in length,2 mm in width and 2 mm in height (ISO 9917 – 212) are generally used. Specimens are placed on two supports and a load is applied at the center. This test is known as three‐point bending test. The load at yield is the sample material's flexural strength that is calculated by the following formula:

      (1.1)

Figure 1.8 Schematic representation of the set‐up for compressive strength.

Figure 1.9 Schematic representation of flexural strength assessment.

      Source: Anusavice [12], Anusavice [13], International Organization for standardization [14].

where P is the ultimate load at fracture, l is the distance of the supports, b is the width of the specimen, and d is the thickness of the specimen [12–14] (as shown in Figure 1.9).

       Good to Know

      Apart from the three‐point bending test, there are four‐point bending tests and biaxial bending tests.

      The four‐point flexure test also employ specimens that are loaded symmetrically at two locations with loading rollers, and the distance between loading points is usually one‐third or one‐fourth of the support span length. In four‐point flexure test, maximum bending occurs between the loading points, whereas in three‐point flexure test, the maximum bending occurs below the loading roller.

      Bi‐axial flexure testing is a commonly used technique for the evaluation of dental ceramics. Here force is given in two axes. Bi‐axial flexure testing is independent of specimen geometry and force direction

1.5 Resistance to Fatigue

The behaviour of materials under the action of low but intermittent stresses shows the resistance to fatigue. This method permits measurement of a fatigue limit, with no fracture, at a given number of stress cycles. Compressive fatigue curves are generated when different materials are submitted to cyclic compressive stress. Tests are made with the test machine operation at a given loading frequency. The presence of defects in the microstructure of the restoration or specimen submitted to high or low stresses leads to the development of cracks. As clinical environment influences are critical factors due to the relatively low stress, these cracks will turn into fracture of the material [12] (as shown in Figures 1.10 and 1.11).

Figure 1.10 (a) Tooth sample, (b) crack propagation.

Figure 1.11 Assessment of resistance to fatigue by cyclic compression stress.

1.6 Hardness

      Major laboratory tests are performed to investigate products based on their bulk features. Hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure. The hardness of a material gives an indication of the resistance to penetration when indented by a hard asperity. The value of hardness, often referred to as the hardness number, depends on the method used for its evaluation. Generally, low values of hardness number indicate a soft material and vice versa.

Hardness measurement can be defined as macro or micro, according to the forces applied and displacements obtained. Macro means large, therefore macro hardness is a measurement of the hardness of a material when a large force of greater than 50 N is applied. Macro hardness can be regular or superficial. In regular macro hardness, it is applicable to large area with deep penetration, whereas in superficial macro hardness, it is applicable to large area with shallow penetration. Macro hardness tests can be applied with heavier loads than micro indentation tests.

      Micro hardness is a broadly used term referring to the testing of hardness involving materials by using small applied loads. A more appropriate term to describe this is micro indentation hardness testing. In this testing method, the use of a diamond indenter with a particular shape is used to make an impression called a “test load” or “applied force”, which can be at 1–1000 gf, on the material under testing. Normally, micro indentation tests involve 2 N forces, which are roughly equivalent to 200 gf. This force can produce an indentation of around 50 μm. Because of its specificity, this type of testing is applicable in cases where there is a need to watch for hardness changes on a microscopic level.

      Rockwell, Brinell and Vickers hardness tests are applied for macro hardness testing, whereas Knoop hardness and Vickers hardness tests are done for microhardness testing.

      Macro hardness testing has industrial applications such as testing hardness of steel, aluminium. Micro hardness test is applicable in dentistry for assessment of tooth samples and dental materials such as metals, ceramics and composites. Micro hardness tests are useful in giving required data when