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Engineering | Finite Element Analysis | Stress

Value Design

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Structures and products may fail in-service prior to it's expected life. A service environment containing dynamic or vibration characteristics has the potential for causing premature failure. Stress-life fatigue procedures may be applied to the analysis of structures or products to determine cycles to failure. Strain-life fatigue procedures may be applied to the analysis of structures or products to determine cycles to crack initiation. Linear elastic fracture mechanics (LEFM) may be applied to the analysis of structures or products to determine the life expectancy from crack initiation to final fracture.

Components may also fail prematurely if they are subject to significant loads over long periods of time. Creep is the slow change in dimensions of a material from prolonged exposure to stress.

The interaction of creep & stress rupture with cyclic stressing and the fatigue process is not yet understood, but is of great importance in many high performance engineering applications.

Creep

Creep is effectively a pre-failure of the material. Creep may occur under the following conditions:

1) Under a fixed level of continuous force, deformation increases with the passage of time and the deformation is not completely recovered even if the force is removed. This is known as creep deformation.

2) When a fixed amount of deformation is maintained for a long time, the resistance to load decreases with the passage of time. This is known as stress relaxation.

3) If the loading time is further extended, rupture occurs. This is known as creep rupture.

Creep strains are of significant importance are not usually encountered until the operating temperatures reach a range of approximately 35% to 70% of the melting point on a scale of absolute temperature (Kelvin). The phenomenon of creep is manifested by a time-dependent deformation under a constant strain. The material develops creep strains which increase with duration of loading. The constitutive law of creep is usually defines the rate of creep as a function of stresses and total creep strains. The result is usually a system of first order differential equations with non-linear coefficients. This system is usually solved using an iterative procedure such as Newton-Raphson.