Ductility brittle materials

To demonstrate the effectiveness of the various tests in producing a ductile-brittle transition, tests were also made on plate and welds of a typical ductile-brittle material—plain carbon steel. The steel used was SAE1034 steel of the composition shown in Table 111. 

The use of the single parameter, K, to define the stress field at the crack tip is justified for brittle materials, but its extension to ductile materials is based on the assumption that although some plasticity may occur at the tip the surrounding linear elastic stress field is the controlling parameter. 

Y = coefficient naving value in Table 10-50 for ductile ferrous materials, 0.4 for ductile nonferrous materials, and zero for brittle materials such as cast iron t,n = minimum required thickness, in, to which manufacturing tolerance must be added when specifying pipe thickness on purchase orders. [Most ASTM specifications to which mill pipe is normally obtained permit minimum wall to be 12V percent less than nominal. ASTM A155 for fusion-welded pipe permits minimum wall 0.25 mm (0.01 in) less than nominal plate thickness.] Pipe with t equal to or greater than D/6 or P/SE greater than 0.385 reqmres special consideration. 

Hydraulic piston-type compactors for collection vehicles, on-site compactors, and transfer-station compactors roll crushers used to fracture brittle materials and to crush tin and aluminum cans and other ductile materials... 

Table 4.1 Factors of safety for ductile and brittle materials and various loading conditions (values shown in brackets from 1905, without brackets from 1965) (,S = ultimate tensile strength,, Sy = yield strength)...

From this therefore it is evident that the failure stress, ductile/brittle transitions which may be observed in plastics. According to line B, as the fiaw size decreases the failure stress tends towards infinity. Clearly this is not the case and in practice what happens is that at some defect size ([Pg.132]

Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts. Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole. Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes. A possible solution is to put a relatively ductile composite material such as S-glass-epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip concept, and was addressed in Section 6.4. 

The early study of brittle failures, notably those of the Liberty ships, indicated a temperature dependence. This can be illustrated by plotting both fracture stress (of) and yield stress (Oy) against temperature (Fig. 8.81). Below a certain temperature some materials exhibit a transition from ductile to brittle fracture mode. This temperature is known as the ductile-brittle transition temperature DBTT. 

It should be recognized that tensile properties would most likely vary with a change of speed of the pulling jaws and with variation in the atmospheric conditions. Figure 2-14 shows the variation in a stress-strain curve when the speed of testing is altered also shown are the effects of temperature changes on the stress-strain curves. When the speed of pulling force is increased, the material reacts like brittle material when the temperature is increased, the material reacts like ductile material. 

The ultimate tensile strength (UTS) of a material refers to the maximum nominal stress that can be sustained by it and corresponds to the maximum load in a tension test. It is given by the stress associated with the highest point in a nominal stress-nominal stress plot. The ultimate tensile strengths of a ductile and of a brittle material are schematically illustrated in Figure 1.11. In the case of the ductile material the nominal stress decreases after reaching its maximum value because of necking. For such materials the UTS defines the onset of plastic instability. 

For brittle materials the UTS is generally used as the basis for obtaining the working stress ow = cu/nu where ou is the UTS and nu is a factor of safety. The UTS may be used as a basis for arriving at the working stress even for ductile materials in situations where some permanent deformation is acceptable. 

Figure 1.11 Depiction of ultimate tensile strengths of ductile and brittle materials.

Self-supporting disks are prepared in several stages an initial slice of material is prepared between 100 pm and 200 pm in thickness. Ductile materials may be rolled, cut with a chemical saw, or spark eroded to produce a thin slice brittle materials may be cleaved with a razor blade or cut with an ultramicrotome (see below). The 3 mm disk is then cut from the slice, using a mechanical punch for ductile materials, while brittle materials need to be spark eroded or drilled. 

What is the difference between a ductile material and a brittle material ... 

Ductile materials tend to stretch, while brittle materials do not. 

To avoid brittle fracture during operation, maintenance, transportation, erection, and testing, good design practice shall be followed in the selection of fabrication methods, welding procedures, and materials for vendor furnished steel pressure retaining parts that may be subjected to temperature below the ductile-brittle transition point. 

Apart from the above three major engineered interface concepts, the ductile coating material may also heal up the surface flaws that are often generated during the fiber manufacturing processes, and protect the brittle fiber surface during subsequent processing. 

NOTE Good design practice should be followed in the selection of fabrication methods, welding procedures, and materials for vendor-furnished steel pressureretalning parts that may be subject to temperatures below the ductile-brittle transition temperature. The published design-allowable stresses for metallic materials in internationally recognised standards such as the ASME Code and ANSI standards are based on minimum tensile properties. Some standards do not differentiate between rimmed, semi-killed, fully killed hot-rolled and normalised material, nor do they take into account whether materials were produced under fine- or course-grain practices. The vendor should exercise caution in the selection of materials intended for services between 0 °C (-20 °F) and 40 °C (100 °F). 

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