Extensometers (extensiometers) are used to indicate the deformation of material while it is subjected to stress. Extensometers are often based on strain gauge technology. Laser extensometer measures strain by scanning across stripes painted on the sample. As the sample is stressed, the position of the stripes change, which affects the reflected laser. A tape extensometer uses a tape stretched between two reference points on large structures such as tunnel walls and bridge supports to detect changes in the distance between the two reference points. Borehole extensometers measure movement of soil and rock along the axis of a borehole. A rod extensometer is a borehole extensometer that uses rods to connect anchors in the hole with a reference head at the top of the hole. Material stress has two primary components: direct stress and shear stress. Direct stress changes the volume of the material. Shear stress deforms the material without changing its volume. The modulus of elasticity (also known as Young's modulus or elastic modulus) is a measure of the stiffness of a given material. Fatigue testing determines the relationship between stress and the number of times it can be applied before causing failure. Testing machines are used for applying cyclically varying stresses and cover tension, compression, torsion and bending or a combination of these stresses.