Stress analysis
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Stress analysis is an engineering discipline that determines the stress in materials and structures subjected to static or dynamic forces or loads (see statics and dynamics) (alternately, in linear elastic systems, strain can be used in place of stress).
The aim of the analysis is usually to determine whether the element or collection of elements, usually referred to as a structure, can safely withstand the specified forces. This is achieved when the determined stress from the applied force(s) is less than the ultimate tensile strength, ultimate compressive strength or fatigue strength the material is known to be able to withstand, though ordinarily a factor of Safety is applied in design.
The factor of safety is a design requirement for the structure based on the uncertainty in loads, material strength (yield and ultimate), and consequences of failure. Often a separate factor of safety is applied to the yield strength and to the ultimate strength. The factor of safety on yield strength is to prevent detrimental deformations and the factor of safety on ultimate strength is to prevent collapse. The factor of safety is used to calculate a maximum allowable stress.
Factor of Safety = Ultimate Tensile Strength/Maximum allowable stress
When performing stress analysis, a Design Factor is calculated to compare with the required factor of safety. The factor of safety is a design requirement given to the stress analyst. The Analyst calculates the design factor. Margin of safety is another way to express the design factor.
Design Factor = Ultimate Tensile Strength / Maximum Calculated Tensile Stress
A key part of analysis involves determining the type of loads acting on a structure, including tension, compression, shear, torsion, bending, or combinations of such loads.
Sometimes the term stress analysis is applied to mathematical or computational methods applied to structures that do not yet exist, such as a proposed aerodynamic structure, or to large structures such as a building, a machine, a reactor vessel or a piping system.
A stress analysis can also be made by actually applying the force(s) to an existing element or structure and then determining the resulting stress using sensors, but in this case the process would more properly be known as testing (destructive or non-destructive). In this case special equipment, such as a wind tunnel, or various hydraulic mechanisms, or simply weights are used to apply the static or dynamic loading.
When forces are applied, or expected to be applied, repeatedly, nearly all materials will rupture or fail at a lower stress than they would otherwise. The analysis to determine stresses under these cyclic loading conditions is termed fatigue analysis and is most often applied to aerodynamic structural systems.
[edit] Load transfer
The evaluation of loads and stresses within structures is directed to finding the load transfer path. Loads will be transferred by physical contact between the various component parts and within structures. The load transfer may be identified visually, or by simple logic for simple structures. For more complex structures, more complex methods such as theoretical solid mechanics or by numerical methods may be required. Numerical methods include Direct Stiffness Method which is also referred to as the Finite element method.
The object is to determine the critical stresses in each part, and compare them to the strength of the material (see Strength of materials).
For parts that have broken in service, a Forensic engineering or failure analysis is performed to identify weakness, where broken parts are analysed for the cause or causes of failure. The method seeks to identify the weakest component in the load path. If this is the part which actually failed, then it may corroborate independent evidence of the failure. If not, then another explanation has to be sought, such as a defective part with a lower tensile strength than it should for example.
