Stress ductility

Modulus of elasticity (GN/m ) Poisson s Yield stress Ductile to , II-, Stress UTS brittle transition orys a isa ion relieving Hardness (VPN) Stability ... 

In industry, hydrostatic pressure testing of pipes is still widely used to assess their resistance to this type of failure. Typical results are shown in Fig. 6 [57, 58], where failure times of HDPE pipes are given as a function of the circumferential (or hoop) stress. At relatively high stresses, ductile failure is observed (stage I) although deformation is initially homogeneous, small local variations (arising from variations in the specimen thickness, for exam-... 

Upon exceeding a critical stress, ductile surfaces deform plastically. Often in nanoindentation experiments under controlled load, the indenter moves in a... 

Ductility is essential for materials that are subject to tensile and compressive stresses. Ductility is important in the construction of reactor vessels. 

Cross-linking Increase in Tg —> Increase in yield stress —> Ductile (plastic) deformation less and less competitive with brittle deformation. 

Stages 1 and 2 are governed by the tangential stress, while stage 3 is determined by normal stress. Ductile-to-brittle failure transition criteria are based on our knowledge about dislocations. They reveal the most important features and parameters that govern material susceptibility to brittleness. Using these criteria, a quantitative analysis of the main factors that affect radiation embrittlement of steel can be attempted. 

Pal] Mechanical tests Hardness, compressive yield stress, ductile-to-brittle transition temperature, steady state creep rate, and oxidation kinetics... 

As discussed in Section 2.0 (Exploration), the earth s crust is part of a dynamic system and movements within the crust are accommodated partly by rock deformation. Like any other material, rocks may react to stress with an elastic, ductile or brittle response, as described in the stress-strain diagram in Figure 5.5. 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Modified ETEE is less dense, tougher, and stiffer and exhibits a higher tensile strength and creep resistance than PTEE, PEA, or EEP resins. It is ductile, and displays in various compositions the characteristic of a nonlinear stress—strain relationship. Typical physical properties of Tef2el products are shown in Table 1 (24,25). Properties such as elongation and flex life depend on crystallinity, which is affected by the rate of crysta11i2ation values depend on fabrication conditions and melt cooling rates. 

Criteria of Elastic Failure. Of the criteria of elastic failure which have been formulated, the two most important for ductile materials are the maximum shear stress criterion and the shear strain energy criterion. According to the former criterion, from equation 7... 

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. 

A series of events can take place in response to the thermal stresses (/) plastic deformation of the ductile metal matrix (sHp, twinning, cavitation, grain boundary sliding, and/or migration) (2) cracking and failure of the brittle fiber (5) an adverse reaction at the interface and (4) failure of the fiber—matrix interface (17—20). 

This concept is explained by Figure 12 which shows the uniaxial stress— strain curve for a ductile material such as carbon steel. If the stress level is at the yield stress B or above, the problem is no longer a linear one. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

After long periods of time at operating temperature, the britde—ductile transition temperature in autoclave steels increases (13). At temperatures much above 200°C for the solutions and fiHs used in ordinary hydrothermal processes, pressures and hence stresses in autoclaves can cause faHure of metal in the brittie state. OrdinarHy, the brittie region is weH below these temperatures but careful monitoring of the brittie—ductile transition of the steel is necessary for safe autoclave use over many years. 

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