Cyclic Fatigue Testing
Joint Stiffness
Useful Definitions
References
The fatigue properties of a bonded joint are a function of the joint geometry and adhesives, and therefore cannot be determined from the intrinsic properties of the adhesive. It is therefore necessary to conduct cyclic fatigue tests on representative joints to those to be used in service. Fatigue testing consists of applying a specified mean load (which may be zero) and an alternating load to the bonded joint (or structure) – see Figure 1 for the nomenclature for stress parameters for constant amplitude cyclic loading. The number of cycles required to produce failure (fatigue life) is recorded. Generally, the test is repeated with identical specimens and various fluctuating loads. For joint characterisation purposes it is recommended that specimens are mechanically loaded at each of five stress levels (i.e. 80%, 70%, 55%, 40% and 25% of the quasi-static strength of the joint). Loading modes include tension, compression, shear (torsion) and flexure. Loads can be applied in a single direction (e.g. tension-tension) or reverse direction (tension-compression) defined by the stress ratio (see Figure 1).
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Minimum stress, sMIN Maximum stress, sMAX Stress range, D s = sMAX - sMIN Stress amplitude, sA = D s/2 = ( sMAX - sMIN)/2 Mean stress,sMEAN = ( sMAX + sMIN)/2 Stress ratio, R = sMIN/ sMAX R = –1 for fully reversed loading R = 0 for zero-tension fatigue, and R = 1 for a static load |
Figure 1: Nomenclature for stress parameters for constant amplitude cyclic loading
Fatigue data is generally plotted in the form of an S-N diagram,
which is a plot of the number of cycles required to cause failure Nf
in a specimen against the amplitude of the applied stress. For
inter-comparative purposes, fatigue strength data (Figure 2) are normalised
with respect to the ultimate static strength
Po
of
identically conditioned specimens measured at an equivalent loading rate to
the test frequency. The uncertainty in life expectancy at any stress
level is large (typically an order of magnitude).
Figure 2: Normalised S-N curve for mild steel/epoxy single-lap joints
Fatigue data are normally obtained at the highest frequency possible in order to minimise the duration of tests. Restrictions on test frequency can arise from test equipment limitations (response time), time dependent processes and hysteretic (self-generated) heating. Hysteretic heating, which increases with increasing load and frequency, can result in thermal softening of the adhesive, adversely affecting the fatigue performance of composite joint. Reliable data can be obtained at high frequencies provided the stress levels are low. Test frequencies of the order of 10 to 30 Hz can result in substantial heating, particularly in the grip regions. The upper frequency limit will be dependent upon the thermal conductivity of the adherend/adhesive system, mode of loading and specimen size. Trials may be necessary to determine the upper frequency limit. The results presented in Figure 2 show no evidence of a self-heating effect, and hence the merging of fatigue results for 5Hz and 25 Hz.
It is recommended that the temperature rise of the material surface be kept to a minimum. It may be necessary to stop testing to allow the specimen to cool. Alternatively, the test could be carried out in an environmental cabinet with a thermocouple attached to the specimen surface for monitoring and controlling the temperature of the test specimen, although the cooling rate may be too slow to be practical. Thermal imaging equipment can be used to monitor surface temperature, although the latter is beyond the budget of most industrial facilities. The temperature resolution is ~1 °C for the two methods.
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