Fatigue constant-amplitude loading

Another fatigue testing related consideration is that in an ideal test dK/da = 0, in other words a constant amplitude loading is maintained. A real structure, however, experiences a spectrum of stresses in service, which can result in overloads. A detailed analysis of the subject of variable amplitude loading and the associated concepts are covered by Anderson [16]. 

Although the stiffness reduction curves offer a reasonable basis on which to establish damage curves for constant amplitude loading at different stress levels, these damage curves may not reflect all physical effects which are important for the development of fatigue damage. That, however, would mean that the development of realistic cumulative damage laws for composite materials would require considerable further research and development work. 

Constant amplitude loading A type of loading in fatigue in which all peak and valley loads are equal. Fatigue life The number of loading cycles that a specimen can sustain before failure. 

Fatigue tests were performed under load control mode on a Schenck horizontal fatigue testing machine with hydraulic grips and a maximum load capacity of 25 kN. Tension-tension constant amplitude fatigue tests were carried out at three stress levels 60% a , 70% Cu and 80% a at two different stress ratios R = 0.1 and R = 0.5. The test frequency was kept constant (f = 3 Hz) for all the tests. 

In order to accurately model the fatigue behavior of rubber, fatigue analysis methods must account for various effects observed for rubber during constant amplitude testing. Effects associated with load level, 7 -ratio (ratio of minimum to maximum loading level), and crack closure are presented in this section. 

Harbour, R.J., Fatemi, A., and Mars, W.V., Fatigue crack growth of filled rubber under constant and variable amplitude loading conditions. Fat Fract. Eng. Mat. Struct, 30, 640, 2007. 

The SN-curve is a line in the Oamp-N-plane (N is a measure of number of cycles to failure, S is a general symbol referring to stress, strain, load, displacement etc. In this case, S denotes aamp). When constant-amplitude fatigue data are available for the material, an SN-curve can be found using regression analysis of the test data. Usually, it is appropriate to... 

Fatigue crack propagation tests were performed on 73-nmi x 73-mm (2.9 in. x 2.9 in.) compact-tension specimens that had been machined from the plaques and then precracked. An electrohydrau-lic closed-loop test machine was used to produce a constant-amplitude, 10-Hz sinusoidal load. Host of the tests were performed in duplicate at room temperature in laboratory air at an average ambient RH of A0%. In view of the slow rate of moisture equilibration in air at room temperature (15), the difference between the ambient RH and that of the nylon being tested was presumed to be unimportant within the time period of the test (3-5 hr). 

Crack extension caused by constant amplitude fatigue loading and expressed in terms of crack extension per cycle of fatigue, da/dN [Borders et al., 1946]. 

For the case of a cycling load of constant amplitude, the subcritical fatigue crack growth rate (FCGR) of a material can be estimated by the Paris [ ] equation... 

Fatigue results in a brittle-appearing fracture, with no gross deformation at the fracture. On a macroscopic scale the fracture surface is usually normal to the direction of the principal tensile stress. The crack ejctension depends upon material, amplitudes of alternating stress, frequency of stress, temperature of operation. The process of fatigue failure under a load of a constant amplitude typically includes several stages ... 

ASTM D638-95, Standard Test Method for Tensile Properties of Plastics (Philadelphia, 1996) ASTM D648-82, Deflection Temperature of Plastics Under Flexural Load (Philadelphia, 1988) ASTM D671-93, Flexural Fatigue of Plastics by Constant Amplitude of Force (Philadelphia, 1993)... 

Fatigue (Structural) The progressive localized permanent structural change that occurs in a material subjected to constant or variable amplitude loads at stresses having a maximum value less than the ultimate strength of the material. 

Under conditions of free corrosion and constant amplitude (S-N curves), a tolerable stress amplitude results about 20% lower than in air. With cathodic polarisation, the air exposure values are generally achieved once again. In tests with exposure to random stress loads as well, cathodic polarisation extends the fatigue life. 

Fatigue crack growth The rate of crack growth under constant amplitude fatigue loading (da/dN). 

Note that it is not done, at this time, any hypothesis about the time history of load spectra application that can be arbitrary. Fatigue failure occurs when this summation reaches the value of the critical damage D r for the particular material. Consider, for example, a block of three-step spectra of constant amplitude stress S > S2> Sj. Values of the respective cycle ratios n/V,-, with i = 1, 2, 3, can be deducted from the S-N diagram of the material, as shown in Fig. 8.16. Fatigue failure occurs when, as said, the total damage introduced by the three load spectra reaches the value of the critical damage... 

In general, under a system of a given number of load spectra each of constant amplitude 5, failure will occur by fatigue when it is... 

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