Lower yield stress

Fig. 8.18 Effects of grain size on lower yield stress, 5% flow stress and stress-corrosion fracture stress for 0.08%C steel in 8 n Ca(N03)2 (after Henthorne and Parkins )...

At higher temperatures or lower strain rates, the stress-strain curve of the same material may exhibit a more gradual initial slope and a lower yield stress, as well as the drastic deviation from initial linearity and the higher failure stain characteristic of a ductile material. 

Harden s (27) market survey of the growth of polyolefin foams production and sales shows that 114 x 10 kg of PE was used to make PE foam in 2001. The growth rate for the next 6 years was predicted as 5-6% per year, due to recovery in the US economy and to penetration of the automotive sector. In North America, 50% of the demand was for uncrosslinked foam, 24% for crosslinked PE foams, 15% for EPP, 6% for PP foams, 3% for EVA foams and 2% for polyethylene bead (EPE) foam. As protective packaging is the largest PE foam use sector, PE foam competes with a number of other packaging materials. Substitution of bead foam products (EPP, EPE, ARCEL copolymer) by extruded non-crosslinked PE foams, produced by the metallocene process was expected on the grounds of reduced costs. Compared with EPS foams the polyolefin foams have a lower yield stress for a given density. Compared with PU foams, the upper use temperature of polyolefin foams tends to be lower. Eor both these reasons, these foams are likely to coexist. 

The pressure exerted by the rotating extrusion heads on the screen as product is being pushed out in an A-R screw extruder (25) is shown in Figure 10. Each spike in the profile represents one revolution of the extrusion head while the frequency represents screw speed. Based on the shear stress developed during extrusion, wet mixtures of MCC-lactose had a lower yield stress in the radial screw extruder (130kN/m ) than in the ram... 

Stress at craze II initiation Lower yield stress Area... 

The bend tests also show that the yield stress of the quaternary Ti-lOAl-X-(5 or 7.5)B (at.%) alloys is quite high, and that the hypo-eutectic alloys have a lower yield stress but their plasticity is much larger. 

The yield stress is not a clear-cut material property, since its magnitude generally depends on time scale, often markedly so. If the applied stress lasts longer, a lower yield stress is commonly observed. In other words, the Deborah number determines whether or not yielding occurs at a given stress. 

A lower yield stress. As mentioned, a low yield stress leads to a relatively thick zone around the crack where yielding occurs, hence to blunting of the crack tip, hence to decreased stress concentration. 

For some metals, notably steels, there is an abrupt break in the stress-strain plot at the upper yield point (Figure 10.18). This is followed by continued deformation at a lower yield stress, at the lower yield point, before the curve rises again. Between the upper and lower yield points, deformation occurs in localised regions that have the form of bands, rather than across the specimen in a uniform manner. The reason for this is that the dislocations, which would move during plastic deformation, are pinned in the steel, mainly by the interstitial carbon atoms present. At the upper yield stress, these become mobile and, once released, they can move and multiply at a lower stress value. This is analogous to sticking and slipping when a body over-... 

The optimum foam is one where the loading line meets the intersection of the dangerous areas for head injury and foam bottoming out (Fig. 14.15). Lower yield stress foams bottom out before the load reaches 10 kN. If the foam has a too high yield stress, the force reaches 10 kN before the foam bottoms out. 

Wagstrom et al. 2005) analyzed a finite element model of the front structure of the vehicle and then-results indicate that lower mean deceleration can be accomphshed by lower-yield-stress material but it is necessary to avoid stiff engine-firewall contact because it can cause hi mean deceleration. 

From the tensile tests presented, it appeared that the nanocomposites made by alkyl silane-functionalized sepiolite give the best mechanical performances, in particular for what concerns the yield stress. In fact, the sepiolite surface fimctionalization by silane is a reactive treatment, which decreases the interparticle aggregation (improved dispersion) and, at the same time, increases the matrix-filler interactions. The addition of fimc-tionalized polymers is, instead, a nonreactive surface treatment. It leads to a decrease of the particle-particle interaction but can also reduce the matrix-particle interaction, which leads to lower yield stress and ultimate tensile stress. 

A comparison of the yield stress as a function of temperature for different flexibilized epoxy resin systems is given in Fig. 6. The crystalline EP exhibit lower yield stresses than the conventional flexibilized EP. The crystalline system, based on sebacic acid even shows ductile behaviour at -80 C, whereas the adipic acid system reaches its critical yield stress limit at -70°C, and brittle failure at lower temperatures is found. Measurements of Hartwig also showed brittle fracture at cryogenic temperatures. But there also seems to be no evidence of ductile behaviour in the case of the sebacic acid polyester segments. The lowest yield stress at -100°C is exhibited by the rubberlike polymer based on sebacic acid polyester. Here too, the adipic acid polyester shows higher yield stresses, but achieves ductile behaviour at -100°C. The more rapid increase in yield stress is due to the absence of crystallinity. These results show that good low temperature flexibility may be... 

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