Load amplitude

Load, Amplitude of Slip and the Number of Fretting Cycles... 

Constant deflection amplitude fatigue testing is probably the less demanding of the two techniques, because any decay in the modulus of elasticity of the material due to hysteretic heating would lead to lower material stress at the fixed maximum specimen deflection. In the constant load amplitude tests, maximum material stress is fixed, regardless of any decay in the modulus of elasticity of the material. 

Load amplitude One-half of the algebraic difference between the maximum and minimum loads in the load cycle. 

Loading, of fillers, 11 305-306 Loading amplitudes, fatigue and, 14 450 Loading cycles, 13 482 Loading Ratio Correlation (LRC) Method, 1 626, 628... 

Fig. 8.11 Decrease in applied load amplitude for crack initiation versus scale thickness in AfeCVSiC composites.14...

For viscoelastic materials such as polymers and many biomaterials, recently developed nanoindentation dynamic mechanical analysis (DMA) techniques enable the study of dynamic response as a function of load amplitude and frequency. The DMA stiffness mapping technique offers the unique capability of mapping storage and loss moduli of the sample... 

Figure 7.9. Constant load-amplitude fatigue crack growth curves for a mill-annealed T1-6A1-4V alloy tested in vacuum at room temperature [4],...

Danzer et al. have discussed the influence of other types of flaw populations (e.g. of bimodal distributions) on strength [I I, 12]. In these cases the Weibull modulus might depend on the applied load amplitude and on the size of the specimen. Then the determination of a design stress in the usual way may become problematic. A stress and size dependent modulus occurs for materials with an R-curve behaviour [11] and may also be caused by internal stress fields [ 11 ]. 

Equation (1) can be used to calculate the applied energy release rate, Q, for the maximum and minimum loads at each cycle. These values are plotted in Fig. 23.11b. The final step in the analysis is to obtain the incremental decrease in contact radius between cycles, and to plot this as a function of AQ, the difference in applied energy release rates between the maximum and minimum load conditions. The results of this analysis are shown in Fig. 23.12. The circular data points are associated with samples that were held at 80 °C for 5 min before cooling back to room temperature square data points were for samples held for 10 min, and triangular data points for those held for 15 min. The lowest values of AQ are obtained at the beginning of the experiment, and are determined by the load amplitude and by the contact radius that develops during the compressive portion of the experiment. For all three tests, a critical Q occurs at about... 

Young s modulus was determined during the fatigue measurements. It was interesting that Young s modulus remained constant until failure, even for load amplitudes near the fracture strain (see Fig. 9). 

The fracture strength was also determined after cyclic preloading with different load amplitudes. The fracture strength remained unaffected by cyclic preloading ... 

The extent of the load amplitude in relation to the mechanical strength of the material and... 

In a sinusoidal type of loading, the load has a wave form shown in Figure 7.1b and is characterised by the load amplitude and loading frequency. 

CEN EN 12697-26 (2012) suggests that, in order for most bituminous mixtures to determine the load amplitude, the strain should be kept at a level lower than 50 microstrain (50 X 10" m/m) to prevent fatigue damage. As for the loading frequencies and in order to allow a logarithmic presentation of isotherms, a typical set of frequencies is recommended 0.1, 0.2, 0.5, 1, 2, 5, 20 and 50 Hz. 

Three different constant stress levels of pulse form are applied to the specimens at 0.1 s loading time and 0.4 s unloading time such that the resulting strain is within the 100 to 400 pm/m range. The test is recommended to start at a loading amplitude of 250 kPa. 

In a series of fracture experiments on ceramic specimens, two important observations can be made, namely that the probabflity of failure increases with the load amplitude, and also with the size of the specimens [2-4,14]. This strength-size effect is the most prominent and relevant consequence of the statistical behavior of the strength of brittle materials. However, these observations carmot be explained in a deterministic way by using a simple model of a single crack in an elastic body rather, their interpretation requires an understanding of the behavior of many cracks distributed throughout a material. 

In Fig. 10.6 (a), the beam was repeatedly loaded at 85% static load-bearing capacity until final failure. After several cycles of loading, amplitude distributions of AE signals were measured under monotonous loading. With the increase in loading cycles, amplitude distributions shift to the... 

To characterize the cyclic crack growth resistance of both neat resins and laminates, fatigue crack propagation (FCP) experiments under Mode I conditions were performed in laboratory air (23 C/50%r.h.) using a computer controlled servohydraulic fatigue testing machine. In the case of neat resins, compact-type (CT) specimens with a width of 50 mm, a hight-to-width ratio of 0.6, and a nominal thickness of 4 mm were used. For composite laminates double cantilever beam specimens (125 x 20 x 8 inm ) were employed. FCP tests were conducted at 10 Hz. The applied waveform was sinusoidal with a constant load amplitude and a minimum-to-maximum load ratio, R, of 0.1. 

Some considerations about the tension-tension variable amplitudes employed has to be done. The minimum load amplitude was set constant for each sample, on the basis of the resistance provided by the interference contribution (Fm). The maximum amplitude was varied among different... 

The same load amplitude was employed both for the hybrid samples and the bonded-with-clearance ones. The static resistances used as reference for both the cases were calculated from the previous experiments (Gallio et al.,... 

The exploitation of adhesives with different flexibility did not provide changes in the stiffness of the hybrid systems, which seemed rather dependent on the interference. Moreover, the presence of the interference positively affected the fatigue behavior of the cylindrical joints. Hybrid joints sustained 10 fatigue cycles at load amplitudes equal to an higher percentage of then-static resistance compared to the clearance joints. 

Crooker, T. W., Bogar, F. D., and Yoder, G. R., Standard Method of Test for Constant-Load-Amplitude Fatigue Crack Growth Rates in Marine Environments, Naval Research Laboratory Report 4594, Washington, DC, 1981. 

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