Engineering | Finite Element Analysis | Stress
© Copyright Value Design Consulting Ltd
Value Design
Consulting
Finite Element creep analyses can be carried out with time or strain hardening laws. Indeed, some packages facilitate the creation of special creep laws as defined by the user. FE packages also have the capability of automatically switching from explicit to implicit time stepping when the explicit time step is restricted by numerical stability considerations, thus providing for efficient solution of long-time creep problems.
Fatigue
Cyclic or repeated loading can cause failure at lower stresses than static loading.
This aspect is central to fatigue performance. Fatigue can be described as a progressive
failure phenomenon that proceeds by the initiation and propagation of cracks to an
unstable size.
High frequencies with low amplitudes are characteristic of noise and
vibration studies while the low frequencies with moderate amplitudes represent classical
fatigue. Finally, low frequency with high amplitude is typical of impact fatigue.
Test specimens are tested in a chosen mode -- tensile or flexural -- for thousands
or millions of cycles. The yield stress for a given number of cycles is termed the
fatigue strength. The fatigue life of a part is the number of cycles to failure at
a given fluctuating load. S-N data can be used reliably for design only if the test
conditions for generating S-N data match the service conditions for the component.
The most critical choice for tests is between load controlled or displacement controlled
cyclic loading. Other test variables are temperature, mean stress, amplitude of fluctuation
and frequency. Elevated temperatures hasten failure
FEA can predict fatigue stresses.
However, stresses do not allow life predictions unless the fatigue characteristics
of the material are known. Unfortunately, there are significant problems in determining
fatigue characteristics. Fatigue can be affected by the frequency of vibration, so
that conventional (low frequency) handbook data may not predict the fatigue at ultrasonic
frequencies. Even where it might be reliable, low frequency data is usually too limited
to provide life predictions at ultrasonic frequencies. For example, low frequency
tests are often stopped at 500 million cycles, which represent only seven hours of
continuous ultrasonics at 20 kHz.
Fatigue
Navigate the knowledge base
