Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 6M

Year: May 2014, Dec 2014

Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and generally increases as they near their melting point. The rate of deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade.

Stages in Creep

In the initial stage, or primary creep, the strain rate is relatively high, but slows with increasing time. This is due to work hardening. The strain rate eventually reaches a minimum and becomes near constant. This is due to the balance between work hardening and annealing (thermal softening). This stage is known as secondary or steady-state creep. This stage is the most understood. The characterized "creep strain rate" typically refers to the rate in this secondary stage. Stress dependence of this rate depends on the creep mechanism. In tertiary creep, the strain rate exponentially increases with stress because of necking phenomena.

Andrade’s analysis of classical creep curve

Andrade considered that the constant stress steep curve represent the superimposition of two separate creep processes which occur immediate after sudden strain and results from applying the load. The first component of creep curve is ‘transient creep’ with a creep rate decreasing with time added to this is a constant rate ‘viscous creep’

Andrade found that the Creep curve would be represented by following empirical equation

$$l = l_0(1 + βt^{1/2}) e^{κt}$$

• Creep is the time-dependent plastic strain at constant stress and temperature
• Steady-state creep-rate (εD s or simply εD)

• Temperature and Stress Dependencies total creep curve : ε = εo + εp + εs

  εo = instantaneous strain at loading (elastic, inelastic and plastic)

  εs = steady-state creep strain (constant-rate viscous creep ) = εDst

  εp = primary or transient creep : Andrade-β flow (or 1/3 rd law) : βt1/3 primary or transient creep

$\text{Andrade}-β \text{flow} (or 1/3rd\ \ \text{law}): εp = βt^{1/2}⇔ \text{problem as} \ \ t→0$

A method is described whereby the constants in Andrade's formula, l = l0(1 + βt1/2) eκt, for the flow of metals under constant stress, can be rapidly deduced from experimental results by direct reading from a system employing sliding templates of calculated shape. The general equation to which the method is applicable is deduced.