Welcome back mechanically, did you know that volumetric strain determines whether a block of cement can be used to fill a gap in a wall while taking into account temperature changes in the area? This makes us wonder what is volumetric strain? Before we jump in, check out the previous part of the series to learn about what elastic constants are. Now consider a rectangular solid under stresses sigma 1, sigma 2 and sigma 3. Then epsilon 1, epsilon 2 and epsilon 3 will each denote the respective strain in their directions. The new dimensions of the rectangle become this. Volumetric strain then is defined as the increased volume of the rectangular solid upon the original volume of the solid and is denoted by theta. El strength or yield stress is the material property defined as a stress at which a material begins to deform classically, whereas yield point is the point where non-linear, that is elastic and plastic deformation begins. Prior to the yield point, the material will deform classically and will return to its original shape when the applied stress is removed. Once the yield point is passed, however, some of the fraction of the deformation will be permanent and non-reversible. The momentum cell strength is measured as the maximum stress that a material can withstand while being stretched or pulled before breaking. Some materials break very sharply without plastic deformation in what is called a brittle failure. Others, which are more ductile, including most metals, experience some plastic deformation and possibly necking before fracture. Hence, we first saw what volumetric strain is and then saw what yield stresses and finally found out what ultimate stress is.