Your understanding is right Nick.

There are varying methods for determining the fatigue behaviour of dental composites.

If you use a constant force then you are measuring the fatigue resistance at that load/force...a kinda go/no go method.

Staircase method as you describe is to determine the fatigue endurance. Step wise decrement in force is used in case no failure for a certain number of fatigue cycles. There is a ISO standard for performing fatigue test ( I do not recall the number...will check it out). You may have a lookk at the ISO site.

There are also reports that study the crack growth behaviour using bend geometry. Prof. Ritchie Group (Google for Ritchie Group) from US have done work on this.

Basically, you need a small loading frame (typically with a 200-500N load cell capacity) and appropriate loading fixtures to perform these tests.

You may look at Instron's website (www.instron.uk) for some of the specialised loading frames for bio-engineering materials.

We have two such loading frames from Instron (though we do not deal with extensive bio material testing as such) in our lab. We use them for testing very small volume of metallic materials. Nevertheless, you can use the same loading frame for the kind of tests you need if proper fixtures are either procured/locally fabricated.

Since you are from UK, you may contact Instron. Have a look at the ISO standard to get some idea of what/which fatigue method is essential.

Hope the info is useful to you.

I can give you some general advice based on a fair amount of experience with fatigue testing of medical devices:

-Before you start you will want to define what your goal is. "Measuring the fatigue" is a little vague. Are you interested in determining the endurance limit of the material? Stress-Life or Strain-Life? Do you think the mean stress is important? If so, what mean stress would you want to use for your testing (think about the service conditions of the part) ? Do you want to vary both the mean and the amplitude to create a Goodman Diagram?

-Regarding test setup, the most important thing is that you should be able to easily understand the relationship between the load that you apply and the stress in the part. You will use the calculated stress to construct your S/N plot or staircase plot or whatever you choose. If you can't easily determine the stress in the part you will need FEA.

-As for the testing: Assuming you want the endurance limit of the material at a single mean strain I'd recommend running a few screening tests to give you a ballpark idea where the endurance limit might be. Think about what you would consider a run-out. Take a look at published data from similar materials and determine the ratio between the endurance limit and the UTS. If you know the UTS of your test material - apply the same ratio and run a few one-off tests there. If the specimen breaks fast - decrease the load; if the specimen runs out - increase the load. Try to confirm that you are in the ballpark. You could continue like this but if each test takes a week or so then you are looking at a long long project.

What most people do once they are in the ballpark is set up a standard S/N test at a known mean stress. Provided you have multiple testers or can test multiple specimens on a single tester, this method will finish much faster. There is some art to choosing the various increments and levels of stress amplitude and it will depend somewhat on your application. You will want to think about randomization and grouping in order to minimize the effects of uncontrolled variables.

At the end you should have a reasonable estimation of the the endurance limit (or fatigue distribution if that is what you are interested in) created within a reasonable timeframe. Refer to ASTM E739-91 for additional advice. Good luck!

Hi,

I can give you some general advice based a fair amount of experience with fatigue testing of samples in different fatigue test such as pure bending (Moor's Beam), three point bending test, 4 points bending test and fatigue test under random vibration loading, so you know that in the first level you should define your goal of experiment? what do you want to obtain ? fatigue limit, cycle of fracture, endurance limit or observe types of damage in composite material.

I have a experimental study on unidirectional composite by tree point bending fatigue test and obtain S-N curve, and you can find ISO standards for fatigue testing and all details in related it such as provide samples (dimension and other things)

Good Luck.

Hi,

I can give you some general advice based a fair amount of experience with fatigue testing of samples in different fatigue test such as pure bending (Moor's Beam), three point bending test, 4 points bending test and fatigue test under random vibration loading, so you know that in the first level you should define your goal of experiment? what do you want to obtain ? fatigue limit, cycle of fracture, endurance limit or observe types of damage in composite material.

I have a experimental study on unidirectional composite by tree point bending fatigue test and obtain S-N curve, and you can find ISO standards for fatigue testing and all details in related it such as provide samples (dimension and other things)

Good Luck.

Hi,

The methodology you are talking about, was carried out by one of my colleagues during his PhD.

Please go through the following paper, you will find the information of your interest.

Curtis A, Palin WM, Fleming GJ, Shortall AC, Marquis PM (2009). The mechanical properties of nanofilled resin-based composites: The impact of dry and wet cyclic pre-loading on bi-axial flexure strength. Dental Materials, 25: 188-97.

The authors in this study supported the results using Weibull Statistics! However, it is a big question, whether Weibull Statistics is applicable with Resin-based dental composites or not?

You can also take a look at the following paper and may find it interesting.

Dent Mater. 2009 Jul;25(7):909-16. doi: 10.1016/j.dental.2009.01.097. Epub 2009 Feb 23. Mechanistic aspects of fatigue crack growth behavior in resin based dental restorative composites. Shah MB, Ferracane JL, Kruzic JJ.

Best Wishes,

It very much depends on what it is that you are trying to determine. Are you after S-N curves or are you after the crack growth curve for the material? If the later then it is essential that you do not use the E647 load reduction technique as that can often lead to erroneous crack growth curves. If you are using a non standard rig then to determine the crack growth rates you will need to determine the K versus a relationship for your test geometry. If you are attempting to determine the S-N curve for the material that is a different mater. However, in that case you are best to test at a fixed load to avoid problems similar to that mentioned above and you will need to account for the biaxial stress state.

If the object is to determine the endurance stress then remember that you can get any almost value that you want. It is a strong function of the surface finish and the proceedure used to manufacture the specimen.

Best of luck.

I do agree with all the comments posted above, in general. At the same time, I do not understand why there should be so much of confusion on which method of fatigue evaluation is required.

There is an ISO standard (ISO 14801:2007 Dentistry -- Implants -- Dynamic fatigue test for endosseous dental implants) that specifies what and how to do. I am sure sufficient water would have flown in developing this standard. There is no point in re-inventing the wheel.

If you want to develope a totally non-standard fatigue evaluation method, then it is a different issue.

But, if it is evaluating the performence of a material and bench marking, it is always better you follow a "standard" evaluation procedure as given above.

I think, the ISO 14801 : 2007 given above is for metallic implants not for polymeric based composites.

Not sure...unless I read the standard and I do not have copy.

But it seems to be generic to both metals & non-metals.

Copying the abstract of the standard below...if you can make out let me know"

"ISO 14801:2007 specifies a method of fatigue testing of single post endosseous dental implants of the transmucosal type and their premanufactured prosthetic components. It is most useful for comparing endosseous dental implants of different designs or sizes.

While it simulates the functional loading of an endosseous dental implant body and its premanufactured prosthetic components under “worst case” conditions, ISO 14801:2007 is not applicable for predicting the in vivo performance of an endosseous dental implant or prosthesis, particularly if multiple endosseous dental implants are used for a prosthesis."

Following ASTM standards may help you:

ASTM F2118-10

ASTM C1361-10

ASTM C1360-10

Thanks very much to everyone for their input and sorry for my late response. This has raised some useful questions which I shall look into further.

Dear Nick,

Tthe following specification would be helpful for u.

ISO standard (ISO 14801:2007 Dentistry -- Implants -- Dynamic fatigue test for endosseous dental implants).

Rama Krishna Alla

Nick J Walters ...Good question… Please check references below.. hope it will be helpful for you…

Regards…

https://www.researchgate.net/publication/7287463_Fatigue_of_packable_dental_composites

http://www.jopdentonline.org/doi/pdf/10.2341/09-337-L