The word stress has many meanings depending upon the context in which it is used. However, in mechanics it has a precise meaning. Stress is a measure of force intensity in a material and may be described in terms of a force per unit area (e.g. pounds per square inch or PSI). It is important to differentiate stress from strain, which is a measure of the deformation of a material resulting from stress. Stress and strain are separate, but physically coupled entities, which are related mathematically. Anytime an object is subjected to stress, it changes its physical dimensions. The amount of this change depends on the properties of the material. For example, a rubber band will deform more than steel wire under the same level of stress because steel is stiffer than rubber. Engineers spend a lot of time testing materials and developing mathematical methods to understand the effects of stress. A well-engineered device will not break under the levels of stress for which it is designed. However, when an object breaks, it is because it was subjected to too much stress. When a part breaks due to overstressing, engineers say that it has “failed”. It has failed to withstand the levels of stress that it was subjected to. Things that can fail from overstress range from automotive parts to parts of the human body – and in some cases both. For example a severe side-impact auto collision may result in the failure of a support beam in the car’s door, which in turn may result in the failure of the driver’s pelvic bones. These kinds of mechanical stress can then lead to psychological stress!
In addition to discrete mechanical definitions of stress, scientists often consider variables referred to as physiological stresses. When a person is exposed to physical or emotional stress it can excite the sympathetic nervous system. This phenomenon is referred to as the sympathetic stress response, the "fight or flight reaction" or an “alarm reaction”. During a stressful situation, sensory signals from peripheral nerve receptors send signals to the spinal cord and brain stem or hypothalamus, which may elicit reflex responses back to peripheral organs or tissues to regulate their activities. If fear, rage or severe pain activates the hypothalamus, mass sympathetic discharge enables the body to perform vigorous muscle activity. Blood flow and arterial pressure are increased in active organs and muscles, while blood flow to less-needed tissues is decreased, conserving energy for strenuous activity. Increased cellular metabolism, increased blood glucose and increased conversion of glucose to lactic acid in the muscles contribute to increased muscle strength and energy enabling vigorous physical activity [i.e. fight or run]. Other types of physiological stress include low oxygen states (e.g. hypoxia and asphyxia) due to suffocation or drowning, a lack of oxygen reaching vital organs as a result of a severe bleeding (hemorrhage), reduced blood flow (ischemia), or a blocked artery (embolism, stroke or heart attack). Environmental extremes such as heat (hyperthermia), cold (hypothermia), low or high ambient pressure (hypobaria and hyperbaria) are additional physiological stressors that can result in pathology, injury or death.
For more information on stress, see our article titled, STRESS in The Wiley Encyclopedia of Biomedical Engineering…and if you have a really big legal case, don’t stress, call a Forensic Consultant!
This article written by Jack Debes, Ph.D. and Lori Wickham, Ph.D. appeared in the May 3, 2006 edition of The San Diego Daily Transcript as part of the Forensic Consultants Association Newsletter. Dr. Debes and Dr. Wickham are members of John Fiske Brown Associates, www.fiskebrown.com., San Diego’s most experienced forensic science and engineering group.
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