Sagar, please can you specify the material in play?

 For steel please look at the following link:

http://www.dtic.mil/dtic/tr/fulltext/u2/a285301.pdf

http://onlinelibrary.wiley.com/doi/10.1111/j.1460-2695.2009.01350.x/abstract

http://www.gruppofrattura.it/ocs/index.php/ICF/icf13/paper/view/11235/10614

Thus, looking at the literature there are still controversies about the effect of fatigue on metals . In particular the potential difference of fatigue strengths due to huge frequency gap between ultrasonic method (usually carried out at 20 kHz) and servo-hydraulic one (usually in the range of 1~100 Hz) is still unclear and

needs to be studied for a various kind of metallic materials.

In general at low stresses (corresponding to the "service" stresses) the effect vanishes, while frequency seems important at higher stresses.

Concerning composites recent research programs have indicated that the frequency of cyclic loading has a significant effect on the fatigue life of certain composite materials for given lay-ups. In some cases, reduction in loading frequency has resulted in a proportional reduction in fatigue life.

In composites the lay-up sequence is very important. For instance, lay-ups having 0° plies were found to have a lesser, but still significant, effect of frequency on fatigue life. Life for these specimens increased by factors of 3 to 4 when load frequencies were increased by a factor of 100. Preliminary analyses indicate that the sensitivity of fatigue life to load frequency appears related to the matrix stress state. Fatigue life sensitivity was correlated with the ratio of matrix shear stress to applied axial stress.

Please, see the following link: http://www.astm.org/DIGITAL_LIBRARY/STP/PAGES/STP31817S.htm

Anyway, in composites it is the stochastic nature of fatigue life much more important than the effect of frequency. In composites, fatigue occurs by accumulation of diffuse damage (rather than the propagation of a single crack as in metals) having different origin and location and length scale (matrix crack, fiber/matrik debonding, interlaminar crack propagation and so on...)

Thus tests must be conducted on statistical basis.

Finally, to be honest, a test campaign for composites, taking into account simultaneously for the effect of fatigue and the stochastic nature of fatigue, is impossible to be realized.

Thanks a lot Alberto for the answer.

Yes, I've gone through the links previously and yes as you said, considering the literature, the affect was still unclear to me. Sorry, forgot to mention that I'm referring to ferrous materials, typically low carbon steels and for servo hydraulic equipment. We did carry out few tests in our laboratory on automobile components where the loads were kept constant and loading frequency was varied. The parts were strain gauged and strain was used to study the effect and as per our experiments, there is no noticeable difference in strains in a frequency range of 0.1 to 35 Hz.

This becomes a very much important question in deciding to what extent the frequency can be increased to reduce test time.

The higher the frequency, the better fatigue life will be obtained. But to reduce the testing time, you can choose 35 HZ.

Joining to this discussion a bit (!!) late. Nevertheless, as pointed out Mr. Alberto, the answer is not straight forward. Yes, in general the frequency can affect fatigue behaviour of a material but then to produce a reasonable influence in fatigue life the frequency needs to changed in order of magnitudes (from Hz to kHz to MHz). In metals, it is generally believed that variation of frequency within  the same order will not produce a noticeable difference in the fatigue life. Therefore no wonder that you did not observe appreciable variation in fatigue life for tests conducted at 0.1Hz to 35Hz. If you suspect any change in material condition due to heat generation during high frequency (as pointed out by Ignor in case of austenitic stainless steel which undergo phase transformation during deformation, which in turn depends upon the temperature of deformation or  certain materials (eg plastic) that are sensitive to temperature) then the test frequency can play a significant role. Another parameter that you should consider is  corrosion. If you suspect corrosion-fatigue interaction, then small variations in test frequency would certainly play a role.

Ideal situation to avoid all these confusions is to run the fatigue test at the service frequency of the component. But image if you are dealing with an aero engine where the service frequency will be of the order of giga Hz. In how many places do you have the facility to run such high frequency fatigue tests? You cannot run these tests with conventional servo-hydraulic systems. Do a simple arithmetic to calculate how long would it take to complete 10^9 cycles (giga cycle) at the conventional frequency of 30-40 Hz.  You require special ultrasonic horne system to run such tests within a reasonable time frame.  As a matter of fact the giga cycle fatigue is fast picking up (read/google on giga cycle fatigue) and is expected to provide a paradigm shift in our understanding of fatigue. Two things that are certainly going to change on our current understanding on fatigue are: (i) fatigue is a surface phenomena and fatigue crack always initiates from the surface and (ii) every material has a fatigue limit or endurance limit - a stress level below which fatigue cracks do not initiate. Start reading with the works of Bathias.

Said that there are also parallel argument on the effect of  heat generated during such high frequency tests.....hope you now understand the confusion on frequency effect on fatigue and where do we stand with respect to solving the fatigue problems. But, I can assure you for the case that you have in hand, my explanation in the first para holds good. 

How to select a correct frequency to obtain good results in the testing of Titanium Grade-2 fatigue using Nano Utm ( BISS - Plug & Play ).If any suggestions please let me know

Thank You in Advance

Madhukar Samatham You would generally run the test at as high a frequency as you can, while still meeting your target loads. The frequency itself (within reason) will not have an effect on life.

https://re-test.com/fatigue-analysis-consulting

Dear Sagar,

It can influence life time to increase or decrease both or no effect also.

Fatigue life is dependent on the cycle history of the loading magnitude since crack initiation requires a larger stress than crack propagation. The fatigue life of the component can be determined by the strain, stress, or energy approach. Fatigue is a very complex process affected by many factors.

Fatigue life is defined as the number of loading (stress) cycles of a specified character that a specimen sustains before failure of a specified nature occurs. The number of cycles is related to engine speed. It can be converted to equivalent durability hours. Fatigue life is affected by cyclic stresses, residual stresses, material properties, internal defects, grain size, temperature, design geometry, surface quality, oxidation, corrosion, etc. The fatigue life of a component under the following different fatigue mechanisms can be ranked from low to high as: thermal shock, high temperature LCF, low temperature LCF, and HCF. In the assessment of the risk of fatigue failure, it may be assumed that the component is safe for an infinite number of cycles if it does not show failures after more than ten million cycles.

· In general the frequency can affect fatigue behaviour of a material but then to produce a reasonable influence in fatigue life the frequency needs to changed in order of magnitudes (from Hz to kHz to MHz).

· One author reports, Increasing test frequency has been to both increase and decrease lifetimes, as well as have negligible effects on the fatigue lives of materials.

· Some author says Fatigue results indicated that some niobium and tantalum alloys showed prolonged lifetimes and increased mean endurance limits at a higher loading frequency of 20 kHz than at 100 Hz.

· The experimental results showed no effect of frequency on the fatigue behavior at room temperature, which is expected in some type of alloy, and result yields confidence in the reliability of the servohydraulic fatigue testing system.

· Some author results indicate that the fatigue life is shortened in some materials at the ultrasonic frequency; several papers report an enhancement for other materials.

· However, there is almost no information about the influence of frequency on the fatigue life of 316 LN SS in mercury, which is a central question for the SNS target container.

· If I see to the temperature, the frequency effect on the fatigue life largely contributed by the self-rising of specimen temperature during the high-frequency fatigue process. With a comparable specimen temperature, fatigue lives at different frequencies are comparable, indicating a minimal strain rate effect.

Just aside of this above all, just think materials goes under complete reversible, half full cycle, fos is low, fos is high and under HCF ...you will get your answer by damage mechanics.

So conclusion is: it can be decreased, increased or sometime there is no influence.

Hope content is helpful for you.

Ashish