Load-Cycling Experiments

The lifetimes Tc, tl, te, and tal characterize the macroscopic stress fatigue, the nanoscopic long period fatigue, the nanoscopic lamella-extension fatigue, and the nanoscopic structure-heterogeneity fatigue, respectively. Data in brackets are only based on a part of the curve (cf. Fig. 5.19) 

Layer-extension reduction starts after 4 min at e 0.06 and cr 27 MPa for all studied materials. Thus the lag-time of lamellae disruption is not changed in the MMT-containing materials with respect to pure PP. 

In the present load-cycling study, macroscopically the peak stress is well above the transition threshold. Thus in pure PP and in the blend many crystallites break. During the elongational branch of the cycles pieces of lamellae are moved apart. They recombine in the relaxational branch of the cycle. In the well-bonded composite the 

MMT bears part of the load, thus saving the polypropylene lamellae from breaking. On the other hand, the fraction of the load that is exerted on the polypropylene is low enough to guarantee a dominance of strain-induced crystallization over lamella disruption. 

Compared to sample PP the behavior of the MMT materials is characterized by 4 features. First, they show higher heterogeneity. Second, they respond from the beginning by a high-amplitude oscillation. Third, the phase of the oscillation is inverted with respect to the stimuius e(f) (cf. arrows in Fig. 5.18). Thus the distances between the crystallites become more uniform in each straining branch and relax into a more inhomogeneous state. A possible explanation is based on the assumption of a considerable fraction of undersized, premature crystallites that are too weak to 

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Slow load-cycling experiments are performed because at the moment (CCD detector at A2, HASYLAB) the lowest required exposure time for obtaining low-noise SAXS data is ca. 20 s. In a real fatigue test the exposure time would be below l(X)ms. 

Fig. 3.2 Accuracy of the correlation method is demonstrated by the macroscopic elongation s t) determined in a load-cycling experiment...

Table 5.1 Fatigue in a slow load-cycling experiment (cycle time 4 min) estimated from exponential fits...

In all tests the macroscopic response a (i) to the applied signal (i) is rather simple. The monotonous branches of the saw-tooth function (f) are immediately responded by monotonous branches of <7(f). Thus little phase-shift is observed in these low-frequency load-cycling experiments. 

Fatigue test at a synchrotron. Presently the minimum X-ray exposure required to record good anisotropic scattering patterns is still too long to monitor practical fatigue tests. With advanced technique and respective synchrotron beamlines this will become possible in the future. Nevertheless, as we have tried to demonstrate, even slow load-cycling experiments can shed some light on the mechanisms of structure response of polymeric composites under dynamic load. 

Neumann, T. Fennel, W., Rremp, C., 2002. Experimental simulations with an ecosystem model of the Baltic Sea a nutrient load reduction experiment. Global Biogeochemical Cycles, 16, 7-1-7-19. 

Table 8.25 gives some data from repeated loading unloading experiments on sisal, abaca, and henequen [75]. After five cycles, sisal shows the smallest amount of permanent set and hysterisis loss while retaining significant levels of tenacity. Abaca was next best in this respect. 

The permanent axial strain is measured during load cycles and the strain rate is calculated (microstrain/cycle). Initially, the sample experiences high strain, as the material is seated at the beginning of the test. Then, the strain rate settles down to a constant value. Later, as the material becomes unstable, the strain rate increases again. The flow number is defined as the number of load cycles corresponding to the minimum permanent strain rate. A higher flow number indicates a longer time for the material to become unstable and therefore a more rut-resistant bituminous mixture. 

The load ratio and the calm ratio were estimated in the test of the dismantled girder. Results are shown in Fig. 13.10. It is clearly observed that with the increase in loading cycles, plotted points move from the sound zone to the serious damaged zone. The zones classified are not yet clearly described with threshold values, because these values are to be specified from experiments data of structural members. In the case of the present test, the loading cycle up to 200 kN is to be one threshold value. Thus the applicability of the recommended practice (NDIS 2421) is confirmed. Data... 

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