Effect of Cyclic Frequency

The occurrence of thermal failures in a plastic depends not only on the cyclic frequency and applied stress level but also on the thermal and damping characteristics of the material. For example, polycarbonate has very little 

75 Typical fatigue behaviour of acetal at several frequencies 

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Figure 10 illustrates the minimal effect of loading frequency on CF crack formation and early growth for blunt notched steel specimens in aqueous chloride solution at free corrosion 46]. These data are notable for the rare combination of low f and high cyclic lives (175 days were required to obtain 3 x 10 cycles at 12 cpm), and for the lack of a frequency effect on the cycles required to produce 1 mm of CF cracking. This result indicates that one or more of the early... 

The effect of cyclic stresses on the durability of aluminum-epoxide joints has been observed by Briskham and Smith (2000), using a range of surface treatments. Joints were immersed in water at 55°C with average stress levels of approximately 0.15 or 1.2 MPa and a frequency of 2 Hz. The best performing treatment was phosphoric acid anodization and treatment with an amino.silane coupling agent was the poorest, which perhaps was not expected as this performed better than all the other methods with unstressed joints. Joints with phosphoric acid anodization consistently failed in a cohesive manner, while all the other methods showed some interfadal failure. 

Nagatomi, J., B. P. Arulanandam et al. 2001. Frequency- and duration-dependent effects of cyclic pressure on select bone cell functions. Tissue Eng 7(6) 717-28. 

Assuming that the cyclic waveform used in the previous section was sinusoidal then the effect of using a square wave is to reduce, at any frequency, the level of stress amplitude at which thermal softening failures start to occur. This is because there is a greater energy dissipation per cycle when a square wave is used. If a ramp waveform is applied, then there is less energy dissipation per cycle and so higher stresses are possible before thermal runaway occurs. 

Solid metal electrodes are usually polished mechanically and are sometimes etched with nitric acid or aqua regia. Purification of platinum group metal electrodes is effectively achieved also by means of high-frequency plasma treatment. However, electrochemical preparation of the electrode immediately prior to the measurement is generally most effective. The simplest procedure is to polarize the electrode with a series of cyclic voltammetric pulses in the potential range from the formation of the oxide layer (or from the evolution of molecular oxygen) to the potential of hydrogen evolution (Fig. 5.18F). 

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