Plastic deformation compression tests

Machine components ate commonly subjected to loads, and hence stresses, which vary over time. The response of materials to such loading is usually examined by a fatigue test. The cylinder, loaded elastically to a level below that for plastic deformation, is rotated. Thus the axial stress at all locations on the surface alternates between a maximum tensile value and a maximum compressive value. The cylinder is rotated until fracture occurs, or until a large number of cycles is attained, eg, lO. The test is then repeated at a different maximum stress level. The results ate presented as a plot of maximum stress, C, versus number of cycles to fracture. For many steels, there is a maximum stress level below which fracture does not occur called the... 

Plastic deformation is commonly measured by measuring the strain as a function of time at a constant load and temperature. The data is usually plotted as strain versus time. Deformation strain can be measured under many possible loading configurations. Because of problems associated with the preparation and gripping of tensile specimens, plastic deformation data are often collected using bend and compression tests. 

The test can provide compressive stress, compressive yield, and modulus. Many plastics do not show a true compressive modulus of elasticity. When loaded in compression, they display a deformation, but show almost no elastic portion on a stress-strain curve those types of materials should be compressed with light loads. The data are derived in the same manner as in the tensile test. Compression test specimen usually requires careful edge loading of the test specimens otherwise the edges tend to flour/spread out resulting in inacturate test result readings (2-19). 

Microindentation hardness normally is measured by static penetration of the specimen with a standard indenter at a known force. After loading with a sharp indenter a residual surface impression is left on the flat test specimen. An adequate measure of the material hardness may be computed by dividing the peak contact load, P, by the projected area of impression1. The hardness, so defined, may be considered as an indicator of the irreversible deformation processes which characterize the material. The strain boundaries for plastic deformation, below the indenter are sensibly dependent, as we shall show below, on microstructural factors (crystal size and perfection, degree of crystallinity, etc). Indentation during a hardness test deforms only a small volumen element of the specimen (V 1011 nm3) (non destructive test). The rest acts as a constraint. Thus the contact stress between the indenter and the specimen is much greater than the compressive yield stress of the specimen (a factor of 3 higher). 

FCP tests of PS samples parallel and perpendicular to the injection molding direction were performed in comparison to material that was compression-molded from granules [76]. To minimize the effect of a plastic deformation possibly induced by precracking with a razor blade, the precrack was extended for a minimum of 2 mm by fatigue loading before onset of the experiment. 

Figure 14.5b represents the uniaxial compression test, which uses samples with cylindrical or rectangular cross section. The stress and strain are defined in an analogous way to that of the tensile test. This test overcomes the disadvantages mentioned in relation to a tensile test. The stress is compressive, and consequently there is no possibility of the brittle fracture observed in tensile deformation. Plastic yield can even be seen in thermostable materials, which, under other conditions, can be brittle. In addition, the determination of the yield stress is made under conditions of stable deformation since there is no geometrical reason for the formation of a neck such as occurs in tension. A problem that can arise in this test concerns the diameter/height ratio of the sample. If this ratio is too large friction between plates and sample will introduce a constraint, and if it is very small... 

In comparison the refractory intermetallic Ti5 Si3 compound is brittle at deformation temperatures below 950 °C and no macroscopic ductility has been observed under compressive load. At the test temperature of 1000 °C a remarkable high flow stress of about 1050 MPa was achieved. Crack initiation revealed after plastic deformation of Spi = 1.5%. 

The yield stress, i.e. the 0.2% proof stress of Al3Nb in air, is shown in Fig. 19 as a function of temperature above 850 °C. Between 600°C and 800 °C, Al3Nb fails catastrophically in air tests because of grain boundary oxidation, which is known as a pest phenomenon. Below 500 °C, the fracture strain in compression is smaller than 0.2 %, whereas plastic deformation in bending has only been observed above 1050°C. The plane strain fracture toughness is only about 2 MN/m at room temperature, as was also found by others (Schneibel etal., 1988). 

Usually we refer to ductile fracture, underlining the role of plastic deformation which precedes failure of the part. The fracture occurs on the surfaces, for instance, in tensile test or in the inner (central bursting in extrusion). Shear stress could play a relevant role as it occurs in tension or compression tests in which the failure follows the surfaces characterized by the larger shear stress (Fig. 11). 

The compression properties characterize the strength, deformation and stiffness behaviour of plastics under quasi-static uniaxial compression load conditions. Generally, for these tests commercial universal test systems with different load capacity are used. The valid and common used standard for the compression test of plastics is the ISO 604 (2002) Plastics - Determination of compressive properties. The data collected include also values determined according DIN 53454 and DIN 53457 as well as ASTM D 695 (Fig. 4.33). The specimen of preference exhibits dimensions of 50 x 10x4 mm for the determination of modulus of elasticity and 10 x 10x4 mm for the investigation of the other compressive properties (Fig. 4.34). 

This test method is not recommended for use in organic or friable soils. This test method may not be applicable for soft, highly plastic, non-cohesive, saturated or other soils that are easily deformed, compress during sampling or may not be retained in the drive cylinder. 

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