Stress

Engineering definition

Stress, a condition applied to a material that tends to deform it or rip it apart, can be calculated as the amount of force per unit area. When stress is applied in tension, it tends to elongate the member, and if the stress is high enough, it can pull the material apart. In compression, stress tends to crush a member. Both tension and compression are examples of “normal stress,” because the stress is calculated in a plane normal to the loading direction. “Normal” is the 3D equivalent to the term “perpendicular” used to describe 2D systems.

Another type of stress is shear stress. Shear stress occurs when one side of a member is pushed up and the other side down (like a scissors or shears). In this case, the thickness of the material being sheared makes a difference. In engineering, stress is a calculable parameter that accounts for specimen geometry. If stress exceeds material strength (the ability of a material to resist stress), excessive deformation (plastic) or material failure will occur.

Medical definition

For clinicians, the term “stress” is used more loosely. It is often used to describe forces on a joint or the forces on bone material that result in strengthening of the bone. Outside of biomedical research, stress levels are not calculated but instead described as low, medium, or high. The word “stress” is also used to describe stress fractures, tiny cracks in a bone often caused by overuse and repetitive activity

Confusion

For engineers, the word “stress” mean σ=F/A or τ=F/A_s. But clinicians often use the term more loosely—sometimes synonymously with “force.” A clinician often describes stress not as a calculated value but instead as simply “low” or “high.” These expressions have qualitative meaning and are not intended to be used as an exact value.

Engineers who work with clinicians would benefit from keeping in mind that obtaining exact values for parameters such as stress is difficult due to differences in individual anatomy. As a result, stress calculation would only be applicable to one person unless anatomical variability is taken into account. Additionally, conditions such as osteoporosis can degrade bone strength, making exact calculation even more complex. However, statistical distributions and factors of safety can be used to account for these difference, so in many cases engineering methods can still be applied.