172 sustainability tensile stress

Zirconium resists attack by nitric acid at concentrations up to 70 wt % and up to 250°C. Above concentrations of 70 wt %, zirconium is susceptible to stress-corrosion cracking in welds and points of high sustained tensile stress (29). Otherwise, zirconium is resistant to nitric acid concentrations of 70—98 wt % up to the boiling point. 

It is worthy of note that the simultaneous presence of a sustained tensile stress and a corrodent with a sensitised steel may result in rapid cracking... 

Fig. 7.91 Sustained tensile-stress failure time for 76 mm (3 in.) plate of 7075-T651 aluminum alloy. Shaded bands indicate combinations of stress and time known to produce SCC in specimens intermittently immersed in 3.5% NaCI solution. Point A is the minimum yield strength in the long transverse direction for plate 76 mm (3 in.) thick. Source Ref 98...

What are the elongational viscosity and elongational strength of the liquid polymer Note that in the polyethylene film unit the tubular film exists as a liquid as it emerges from the shaping unit. It must here sustain tensile stresses and the questions of interest concern the liquid response to these tensile stresses. ... 

If the liquid film, with free surfaces, can sustain tensile stresses, it then follows that tensile (as well as shear) stresses may be generated in polymer liquids flowing in closed vessels. What effect will this have on polymer flow ... 

Note that normal liquids cannot sustain tensile stresses, so this question cannot arise for them. It is for this reason that the reader may not have considered the question of tensile stress in liquids before. 

From the ASTM definition, stress corrosion cracking is the simultaneous action of a corrosive environment and a sustained tensile stress promoting rupture" [54]. Both SCC and deallo)dng often occur in the same environment. By the 1940s, deaUoying was considered to be involved in the initiation of SCC [57]. 

C-ring tests indicate that zirconium will have a long life in concentrated acids when they are stressed below the yield point. Cracking can be prevented by avoiding high, sustained tensile stresses or by applying other preventive measures. 

The mechanical response of fibrils in the direction of the applied stress has been determined for polycarbonate [83] and polystyrene [120]. Figure 9.12 shows a stress-strain curve of craze material obtained by Kambour and Kopp [83] for PC. It may be noted that the craze material can sustain tensile stresses only slightly less than the yield stress Up of the bulk material. The strains, however, are enormously greater in the craze - between 40 and 140% as compared to the bulk yield strain of about 2%. 

SCC is a time-dependant intergranular fracture mode (in aluminum alloys) that requires the combined presence of a susceptible alloy, a sustained tensile stress and a corrosive environment. 

Stress Relieving. Residual stresses are induced in aluminum products when they are solution heat treated and quenched. Figure 5(a) shows the typical distribution and magnitude of residual stresses in thick high-strength material of constant cross section. Quenching places the surfaces in compression and the center in tension. If the compressive surface stresses are not disturbed by subsequent fabrication practices, the surface has an enhanced resistance to SCC because a sustained tensile stress is necessary to initiate and propagate this type of conosion. 

D Low. Failure due to SCC is anticqiated in any rqrplication involving sustained tensile stress in the digitated test diiectioiL This rating is currently designated only for the short-transverse direction in cotain products. 

Stress-corrosion cracking of susceptible high-strength aluminum alloys can occur in moist air, seawater, and potable waters, and it varies with the alloy and temper and the magnitude of sustained tensile stress. Chloride solutions are generally favored for laboratory tests of both smooth and precracked specimens because sodium chloride is widely distribu in nature, and the test results are potentiaily relatable to stress-corrosion behavior in natural environments. 

Zirconium is used in nitric acid service for cooler condensers, tail gas preheaters and reboilers. It rivals tantalum in its corrosion resistance to nitric acid at all concentrations up to the boiling point. Its resistance extends up to 230°C and 65 wt %. However it is susceptible to stress corrosion cracking, which can be prevented by avoiding high, sustained tensile stresses104. 

We can understand how the observed threshold effect may arise as follows. Movement of the tie molecules in and out of the lamellae is a thermally activated rate process, and some chains will always be entering at the same time as others are leaving. The effect of the tensile stress is to bias the movement towards a greater rate of leaving. Only when a sufficient stress is applied to shift the equilibrium, such that the reduced number of tie molecules is unable to sustain the stress without breaking, will breakdown ensue. 

---