Question

Explain the significance of statistical scatter in fatigue life, especially how this may affect the use of S-N curves for engineering purposes.

Answer 1

High cycle fatigue investigation of an engineering form is conducted by finding the stress area because of cyclic stresses which happen under the particular load cases, then utilising fatigue statistics of that material to find the series of events that lead to failure. Fatigue data is generally acquired after testing samples at same amplitude stresses till failure happens. The fatigue tests are reiterated many times at various stress qualtities till an S-N curve which corresponds stress area (S) to number of cycles leading to failure (N) can be marked. Immediately after fatigue data is produced and statistical investigation is accomplished on it, the fatigue data is normally given utilising the mean curve, that shows a probability of average life, and the estimate of the standard deviation. The main basis to analyse and comprehend the way fatigue scatter is distinguished is that if the mean curve is utilised in life forecast, the cycles to failure estimated will show 50% probability of existence.

Fatigue data comprises of high amount of scatter. A test sample cycled at a provided amplitude will give a range of failure times. The normal way of considering for this scatter is to utilise a statistical method and denote the scatter utilising probability density functions.

Scatter is an intrinsic and natural feature of mechanical properties of materials. This also pertains to fatigue features. The fatigue lives of such samples under the identical fatigue load can be remarkably varied. Statical features of fatigue of arrangement and materials are well established. A scatter factor is accepted to report the scatter in the structure fatigue capacity and the particular reasonable possibility of failure.

The scatter factor denotes the difference of a variable from its mean. The fatigue scatter factor is described as the ratio of the normal divergence to mean life for a particular probability of failure. Hence, the scatter factor denotes the difference in the failure life data of a structure under stated test specification.

Hence, it is essentially significant to understand how the scatter in the fatigue data is defined, what possibility of life was deduced for the given S-N curve, and what possibility of life is recognised for the implementation.