Biomedical Engineering Reference
In-Depth Information
situation as possible. The cyclic loading profile is devised. This need not be a sine wave;
it can be an exact replication of the loading cycle. Running the test system for a stipulated
number of cycles tests the life cycle. Time can be accelerated by selecting higher frequencies;
but not so high that the test becomes unrealistic due to, say, heat generation or material
nonlinearities. Figure 9.3 illustrates two typical cyclic testing machines. Figure 9.3(a) is a low
cycle machine (up to about 2 cycles per second); Figure 9.3(b) is a hydraulic system and is
high frequency (up to 100,000 cycles per second). Static Loading
The testing machines described in the previous section are normally able to provide static
compressive and tensile loads too, as illustrated by the device in Figure 9.4 . Humidity and Temperature
Will your device be subject to various levels of humidity? Will it purely be used in a wet
environment? Is there likelihood of water absorption? If so, how long does it take to be
detrimental? Is there any chance of corrosion? Can corrosion and cyclic loading combine to
create a corrosion-fatigue environment?
What is the normal limit of temperature? If your device were to run its full design life at this
temperature would it last?
Figure 9.3
Typical cyclic loading machines: (a) electromechanical; (b) hydraulic. (Courtesy Staffordshire University)
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