Dear Lazuardi:

The entire fatigue crack growth curve - da/dN vs. deltaK spans many decades of growth rates from 10-11 m/cycle to 10-3 m/cycle., from the fatigue threshold (which is of order 1 to 10 MPa/m in most materials) to the fracture toughness (which could be anything from 1 to 100 or more MPa/m). Whatever your material, try to estimate what your range of stress intensities will be - over what deltaKs that you're interested in - and choose a deltaK in the middle of this range to start your first test. You will need to use the relevant K solutions to determine what load this represents for the geometry of your sample and your starting crack length. I would do these initial tests under constant load range cycling (at fixed R ratio, of course). If you want faster growth rates, increase this initial deltaK (and hence starting load). If you're interested in lower near-threshold growth rates, then you'll need to load-shed as described in the standard.

Since you're using UHMWPE, try to find some da/dN vs. deltaK data in the literature so that you have a good idea what range of stress intensities that are appropriate for your material, and then simply find the right K solution for your specimen geometry and calculate the required loads to give you these K values.

I am attaching a paper where I did this for high-strength steels.

I hope that this helps.

ROR

Thank you Prof. Ritchie. Is fatigue crack propagation testing performed on stresses under the yield strength or under the UTS of the material? Thank you

Dear Lazuardi:

To measure fatigue crack propagation rates, it is customary to use a notched/precracked sample, such as a single-edged notched or compact-tension specimen, and to analyze the results using a linear-elastic fracture mechanics (LEFM) approach of relating the growth rates da/dN to the stress-intensity range (deltaK), or sometimes the maximum stress intensity. Although there are alternative nonlinear elastic fracture mechanics approaches based on da/dN vs. deltaJ, they are rarely used.  Accordingly, if you use the standard LEFM da/dN vs. deltaK approach, the sample must be nominally elastic and the applied (global) loads/stresses would  not exceed yield.  This doesn't mean, of course, that the local stresses at the crack tip, which cause the fatigue crack to grow, are elastic; on the contrary, they will exceed the yield stress and depending upon factors such as the strain-hardening exponent and the crack-tip constraint, the local crack-tip maximum stress can reach some 3 to 5 times the flow (yield) stress (some people think even higher, if you invoke the notion of strain-gradient plasticity).

This was the dilemma when fatigue failures were first recognized in the 1800s, primarily from the fatigue fracture of railway axles.  One of the first major accidents was in Versailles in 1842 when such an axle suddenly fractured, the train derailed and some 60 or more people were reportedly killed.  The axles had been designed such that the in service stresses did not exceed yield (obviously) and so how could the axle fail? The issue was of course that the global stresses were elastic but because of stress concentrations from notches, radiused sections or pre-existing cracks, the local stresses can definitely exceed the yield strength which leads to local regions of plasticity which are the source of the initiation of fatigue cracks.

I hope that this helps answer your question.

ROR

Thank you

The ASTM standard for FCGR is good but the British standard, BS ISO 12108, provides more details on how to calculate forces based on sample geometry and testing method i.e. SENB, CT etc.  You should be able to get a copy through your university library for free.

The standard details the calculation of stress intensity factor in Section 6.4 and provides a procedure for fatigue precracking in Section 7 along with how to carry out subsequent FCGR measurements for K-increasing or K-decreasing procedures.

I agree with you about E647.  It expects a level of prior knowledge.

Hope this helps.

Dear Lazuardi - Rob has given the answers Just one note. using da/dN curves you can determined the delta K, the stress intensity amplitude for a given R-ratio, and then calculate both sigma max and amplitude required for your tests for that R-ratio. From the compliance relations of the compact tension specimen that you can get from ASTM standards, you can calculate the max Load and the amplitude. which you need to apply for your test  

One cannot measure the force; it has to be calculated from K value and the Elastic modulus. Hence you question reduces to how to calculate the stresses or loads required for the specimen to obtain a particular value of Delta K needed for the crack growth in that material, for a compact tension specimen. These can be done using compliance relation for the compact tension specimen given by ASTM. Best wishes.

https://www.researchgate.net/publication/301348952_Licensed_Under_Creative_Commons_Attribution_CC_BY_Investigation_on_Mode_-_I_Fracture_Parameters_Using_Steel_Fibers_in_High_Strength_Concrete

Article Licensed Under Creative Commons Attribution CC BY Investigat...

Dear Lazuardi this deprnds on the value of yield point of the alloy to be tested.

Dear Lazuardi

by simply performing FEA on the geometry of your favored specimen under unit load and comparing the K value with the data given in the following article you can have an estimation of the applied load.

Article Peak stress intensity dictates fatigue crack propagation in UHMWPE

Good luck