The value of hardness gives an approximate strength of a material, particularly its resistance to permanent deformation, indentation or scratching. The value is commonly expressed in Shore scale, after the testing device, Shore durometer. The Shore A scale is used for flexible and soft range of material. The Shore D scale is used for hard and semi-rigid materials.
In contrast to the measurement of stress to the breaking point in order to obtain the tensile strength and flexural strength, hardness measurement method is a non-destructive scheme. The testing method measures the resistance of the material against indentation force applied by a tool that is pushed down on the material's surface. The measurement result is proportional to the tensile strength and the flexural strength.