The local excitation is not detected at other positions of the medium instantaneously, as some time would be necessary for the disturbance to propagate from its source to other parts of the medium. This simple fact constitutes a general basis for the interesting subject of “wave propagation.” Well-cited examples of wave propagation in different media include, for instance, the transmission of sound in air, the propagation of a seismic disturbance in the earth, the transmission of radio waves, among others. In the particular case, when the suddenly applied disturbance is mechanical, e.g., an impact force, the resulting waves in the medium are due to mechanical stress effects and, thus, these waves are referred to as “mechanical stress waves” or simply “stress waves.” Our attention in this text is restricted to the study of the propagation of stress waves in engineering materials.

A mechanical wave is a wave that is not capable of transmitting its energy through a vacuum. Mechanical waves require a medium in order to transport their energy from one location to another. A sound wave is an example of a mechanical wave. Sound waves are incapable of traveling through a vacuum.