Engineering
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Mechanical hysteresis
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Mechanical hysteresis
Hysteresis is a delay of the effect when the forces acting upon a body are changed (as if from viscosity or internal friction), or lagging in the values of resulting magnetization in a magnetic material (as iron) due to a changing magnetizing force.
Hysteresis is a phenomenon whereby the value assumed by a quantity dependent on others is determined not only by the instantaneous values of the latter but also by the values they had previously assumed; that is, in other words, hysteresis is the characteristic of a system of reacting late to the applied stresses and depending on the previous state.
Ductile materials plasticize with a corresponding kinematic-type work hardening. Assuming that a body is subjected to a uniaxial tensile greater than the yield stress, from the time the material is subjected to unloading and then a compressive stress, the new compressive yield stress will have a lower value.
In performing a calibration, hysteresis is the characteristic of an instrument to provide different values for the same measuring point when it is measured in an increasing and decreasing manner. Typically, in calibration standards, hysteresis is defined as the maximum difference between the values read on an instrument for the same point in the two up and down cycles. In the calibration of a pressure gauge, for example, the difference in values between rising and falling pressure cycles is an indication of the elastic capacity of the instrument’s sensing element.
The deformation of many materials can exhibit hysteresis. For example, plastic materials, after being deformed by a stress, do not return to their initial configuration when the stress is removed, but exhibit residual deformation. For example, this is the phenomenon for which plastic seals in joints degrade after a certain time, and it is also the reason why pressure changes in the human body increase the risk of circulatory diseases such as ischemia or aneurysms.
Even in the mechanics of porous media the same phenomenon is present, since the porosity of a soil depends both on the tensional state to which it is subjected, but also on its past tensional history, in particular the maximum tension to which it has been subjected in its tensional history. If the maximum tension is greater than the current one, the soil is called over-consolidated, and has a lower porosity than the theoretical one expected due to the load acting on it; on the contrary, the soil is called normal-consolidated.
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