12.3 Stress, strain, and elastic modulus  (Page 4/26)

Objects can often experience both compressive stress and tensile stress simultaneously [link] . One example is a long shelf loaded with heavy books that sags between the end supports under the weight of the books. The top surface of the shelf is in compressive stress and the bottom surface of the shelf is in tensile stress. Similarly, long and heavy beams sag under their own weight. In modern building construction, such bending strains can be almost eliminated with the use of I-beams [link] .

Bulk stress, strain, and modulus

When you dive into water, you feel a force pressing on every part of your body from all directions. What you are experiencing then is bulk stress, or in other words, pressure    . Bulk stress always tends to decrease the volume enclosed by the surface of a submerged object. The forces of this “squeezing” are always perpendicular to the submerged surface [link] . The effect of these forces is to decrease the volume of the submerged object by an amount $\text{Δ}V$ compared with the volume $V_0$ of the object in the absence of bulk stress. This kind of deformation is called bulk strain and is described by a change in volume relative to the original volume: