Selecting Materials under Dynamic Loading

SELECTING MATERIALS UNDER DYNAMIC LOADING Hysteresis Measurement Gives Dynamic Characteristics 

One objective of the dynamic testing of materials and components, in addition to determining the number of cycles to break, is to establish the causes and pattern of failure. Normally, in such tests the stress and strain (extension) are measured and an elastic modulus derived from these values. However, this can be only the initial step in the direction of developing a coniplete description of the failure pattern. This analysis is based on the work by F. Orth and G. W. Ehrenstein Kassel [300]. 

If the dynamic behavior of a material or a component must be guaranteed, the material and its processing conditions are normally subject to continuous checking to ensure that the properties do not vary. The better way, of monitoring the dynamic properties directly, is confronted with the current high cost of testing. The hysteresis measurement procedure offers the possibility of obtaining the maximum of information on the dynamic behavior of a material or a component in a very short time. 

To determine the values of material characteristics, the stress and extension signals are plotted against each other. Damping (mechanical loss) in a material causes a phase shift between strain and stress. When the two signals are superimposed, a hysteresis diagram is obtained on which four different characteristic quantities can be defined stresses, strains, elastic moduli, and mechanical losses. 

Special significance attaches to the intersection of the middle curve with the mean value of stress, 

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Orth, F., et al., Selection of Materials Under Dynamic Loading, Kunststoffe, Aug. 1989. 

Deflection temperature under load (DTUL) measures the temperature at which a specimen of a certain geometry deflects a fixed amount under a very specific set of conditions. However, it is often used in material selection as a measure of the maximum continuous use temperature for that material. The development of dynamic mechanical analysis (DMA) has shown that traditional DTUL test results often give a false measure of the thermal performance of polymeric materials. By measuring the elastic modulus versus temperature by DMA the thermal profile of any polymer can be obtained and a more realistic assessment of the elevated temperature performance can be obtained. New techniques were also presented for testing adhesive bond strength tests for piping systems. The technique developed utilized lap-shear plaques to predia performance in the pipe joint systems. Results indicate extreme sensitivity to minor variations in preparation. 

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