Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 114

The mechanical behavior of a material is commonly determined by taking nominally identical specimens and deforming them in a mechanical testing machine according to a standard procedure. This type of experiment commonly gives data for the applied load and the deformation of the specimen. Load is actually a force and these two terms are often used interchangeably. Force, which has the unit newton (N), is commonly converted to a stress by dividing it by the area of the specimen over which it acts. This means that stress has units of N/m², also called pascal (Pa). The stresses encountered in testing of materials are often quite high and, thus, units of MPa (10⁶ Pa) or GPa (10⁹ Pa) are often used.

The measured deformation of a specimen in response to a stress, such as its elongation, is normalized to the appropriate dimension of the specimen, such as its original length, thus converting it to a strain, expressed as a fraction or percent (that is, strain is unitless). Conversion of load and deformation to stress and strain, respectively, serves to normalize the data. This compensates for dimensional changes of the specimen during testing and the use of different specimen dimensions in different testing laboratories. A common way to show the mechanical response of a material is a diagram in which the measured stress (Y‐axis) is plotted versus the measured strain (X‐axis) to give what is called a stress versus strain curve, sometimes written stress–strain curve for short. However, in some experiments, it may also be required to show a plot of the applied force versus the deformation.

In mechanical testing, the load can be applied in a few different ways, giving data for the mechanical response of a material under these different loading modes. These loading modes include uniaxial tension and compression, shear, torsion, and bending.