If failure is defined as occurring when the energy stored by a given mechanism exceeds the critical value, then failure stress is an energy level that approaches the failure threshold of a mechanism. Failure stresses are divided into seven major classifications. They are “thermal state” failure stresses, “chemical state” failure stresses, “electric-magnetic state” failure stresses, “rheological state” failure stresses, “structural state” failure stresses, “elastic state” failure stresses, and “strain state” failure stresses. They are internal to a design although they may originate externally. These stresses may be the result of force applied for primary function performance and relate to the basic physics of a design, or they may be induced from the operation of an object in a specific environment.
The forces that cause failure stresses are called “failure stressors” and are classified into two major groupings: “application” failure stressors and “environmental” failure stressors. Application failure stressors are the electrical, chemical, and physical forces internal to a design applied for primary design function performance. Environmental failure stressors are forces resulting from the environment in which a function is performed, such as temperature, pressure, penetrating radiation, physical interference, acceleration, vibration, shock, etc.
The action of failure stressors in the application area is well known to normal engineering physics. Reliability physics deals with a detailed study of the effect of environmental failure stressors on the failure stress. The sequence of stress cause and effect is of first order importance to reliability engineering and consists of the following relationship:
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Failure stressors and failure stresses are the foundation for some of the better known tools of reliability engineering, such as derating, safety factors, fail-safe design analysis, environmental damping, environmental profiles, and failure-effect analysis.


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