Subject stress-strain diagram

The strength properties of solids are most simply illustrated by the stress-strain diagram, which describes the behaviour of homogeneous brittle and ductile specimens of uniform cross section subjected to uniaxial tension (see Fig. 13.60). Within the linear region the strain is proportional to the stress and the deformation is reversible. If the material fails and ruptures at a certain tension and a certain small elongation it is called brittle. If permanent or plastic deformation sets in after elastic deformation at some critical stress, the material is called ductile. 

In conclusion regarding this subject, it can be stated that creep data and a stress-strain diagram indicate whether plain plastic properties can lead to practical product dimensions or whether a RP has to be substituted to keep the design within the desired proportions. For long-term product use under continuous load, plastic materials have to be considered with much greater care than would be the case with metals. 

In determining creep properties, a series of specimens are subjected to static loads at different levels and their increase in strain over time is measured. Data may be either presented directly as creep strain on a logarithmic time scale, as creep modulus c(t)= isochronous stress—strain diagrams where the relation between stress on the axis of ordinates over strain on the axis of abscissa is presented for different time levels. The schematic approach to converting creep data into an isochronous stress-strain diagram is illustrated in Fig. 34.7. 

Dynamic mechanical analyzers can be divided into resonant and defined frequency instruments. The torsion pendulum just described is, for example, a resonant instrument. The schematic of a defined-frequency instrument is shown in Fig. 4.155. The basic elements are the force generator and the strain meter. Signals of both are collected by the module CPU, the central processing unit, and transmitted to the computer for data evaluation. The diagram is drawn after a commercial DMA which was produced by Seiko. At the bottom of Fig. 4.155, a typical sample behavior for a DMA experiment is sketched. An applied sinusoidal stress, o, is followed with a phase lag, 6, by the strain, e. The analysis of such data in terms of the dynamic moduli (stress-strain ratios, see Fig. 4.143) at different frequencies and temperature is the subject of DMA. 

Permanent structural changes that occur in a material subjected to fluctuating stress and strain, which cause decay of mechanical properties. See S-N diagram. The ability of a material to plastically deform before fracturing in constant strain amplitude and low-cycle fatigue tests. See S-N diagram. ... 

If the bonding is located on a section subjected to frequent shear strains that can cause slanting cracks, the tensile stress exerted on the composite at a distance a should be calculated after having transferred the diagram of the flexural moment, dimension aj. This transfer should take place in the direction leading to an increase of the absolute value of the flexural moment. 

The problem with the semi-solid, self-bodied emulsion systems has been to measure their consistency. Measurements from continuous shear measurements, as they are the result of structural breakdown in the systems under study, have to be treated with some degree of caution. In order to obtain a true measure of consistency or body the systems should be tested in their native state, this requiring a method of measurement which does not disrupt the structures in the emulsion. Thus so-called creep measurements may be applied in which the emulsions are subject to only relatively minor deformities. In creep a shear is quickly imposed on the sample and maintained at a constant level the time-dependent strain or compliance response to this steady stress provides the creep curve. A recovery curve is obtained on removal of the stress, a typical diagram showing the profile for creep and recovery is given in Fig. 8.38. 

---