Notch Sensitive Materials

There has been a conspicuous number of in-service failures of pressure vessels built of certain steels having h her strengths obtained by quenching and tempering. Most of these failures originated in geometric details that had been used for decades w ith conventional steels without similar difficulties. 

It was fairly evident from a study of the service failures that these steels were abnormally sensitive to stress-raising geometries, but a quantitative assessment of this characteristic was lacking. So long as this condition prevailed, any incentive to use the higher strength Q T steels was overshadowed by the possibility of a major pressure vessel failure in refinery service. 

Finally, a criterion was discovered. It was the notched beam ductility test, which seemed to pinpoint the abnormal behavior and to discriminate between acceptable and unacceptable materials of this type. 

This test has been adopted as a supplementary requirement for evaluating higher strength Q T steels. The acceptance criterion used with this test is the minimum deflection at a peak load of 0.15 inch. So far, all currently produced Q T steels evaluated by this test have been acceptable. Most steel producers have been quite willing — and, in fact, anxious — to apply this test to their steels. 

A further supplemental requirement having to do with ductility is that the elongation over a 2-inch gage length as determined by the conventional ASTM tensUe test shall be at least 16% for plates and 14% for forgings. 

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Likewise, dead sharp corners or notches subjected to tensile loads during impact may decrease the impact resistance of a product by acting as stress concentrators, whereas generous radii in these areas may distribute the tensile load and enhance the impact resistance. This point is particularly important for products comprised of materials whose intrinsic impact resistance is a strong function of a notch radius. Such notch sensitive materials are characterized by an impact resistance that decreases drastically with notch... 

The radius of the notch is quite important, particularly for plastics. For example, polyvinyl chloride (PVC) is a notch-sensitive material. If the notch is blunt (2-mm radius), the impact strength is higher than that for ABS. If the notch is sharp (0.25-mm radius), the impact strength of PVC is lower than that for ABS. Other polymers that are notch brittle are high-density polyethylene (HDPE), polypropylene (PP), polyethylene teraphthalate (PET), dry polyamides (PAs), and acetals. 

Notches act as stress raisers and redistribute the applied stress so as to favor brittle fracture over plastic flow. Some polymers are much more notch sensitive than others, but the brittleness temperature depends in general on the test specimen width and notch radius. Polymers with low Poisson ratios tend to be notch sensitive. Comparisons of impact strengths of unnotched and notched specimens are often used as indicators ofthe relative danger of service failures with complicated articles made from notch sensitive materials. 

Equipment Design Handbook Notch Sensitive Materials... 

Coupons must be identifiable before and after testing. Most simply, the coupons can be identified by a mark made directly onto the sample. However, with some notch-sensitive materials such as stainless steels that may have a tendency to favor localized corrosion over uniform corrosion, such a mark can modify the sample corrosion behavior. For the most sensitive materials, even a tag attached to the sample can affect the corrosion. In these cases, the materials should be identified by location within a test environment. Of course, the larger the test sample, the less of an influence some small mark will have on the overall results. 

The heliarc butt-weld tensile data of Fig. 4 and Table III again show the typical increase in tensile strength between 70 and-320 F, followed by a decrease and wide scatter between -320 and -423 F. The scatter in tensile data at -423 F are typical of many notch-sensitive materials. The annealed material present in the welded joint has been shown to be even more prone to occur in the austenite-martensite reaction due to the smaller number of defects present in its structure (the defects presumably block atomic shear movements which result in the martensite transformation) [2]. 

In other areas, the minimization of surface texture irregularities is employed to avoid initiating a crack, which would then propagate through the entire part in a notch-sensitive material such as PVC as shown in Fig. 10.1. Textured surfaces such as simulated wood-grained embossing or a traction print can be made as shallow as possible. The downside of this strategy is the softened texture may look less like real wood or have worse wet traction properties. 

In analyzing these data it is necessary to go beyond the curves and determine the expected behavior of the material with respect to notch sensitivity. Problems with notch sensitivity can often be corrected by modifying the processing steps and/or heat treatment. 

Graphs such as Fig. 2.79 also give a convenient representation of the notch sensitivity of materials. For example it may be seen that sharp notches are clearly detrimental to all the materials tested and should be avoided in any good design. However, it is also apparent that the benefit derived from using generously rounded comers is much less for ABS than it is for materials such as nylon or PVC. 

Endo, K., Komai, K. and Nakamuro, N., Estimation of corrosion fatigue strength from corrosion resistance and notch sensitivity of the materials . Bull. Jap. Soc. Mech. Eng., 13, 837-46 (1970)... 

The Izod impact test may indicate the need to avoid inside sharp corners on parts made of such materials. For example, nylon and acetal-type plastics, which in molded products are among the toughest materials, are notch-sensitive and register relatively low values on the notched Izod impact test. 

Polysulfone It is a high performance amorphous plastic that is tough, highly heat resistant, strong and stiff. Products are transparent and slightly clouded amber in color. Material exhibits notch sensitivity and is attacked by ketones, esters, and aromatic hydrocarbons. Other similar types in this group include polyethersulfone, polyphenyl-sulfone, and polyarylsulfone. Use includes medical equipment, solar-heating applications and other performance applications where flame retardance, autoclavability and transparency are needed. 

PC/PE. In the case of PC/PE, plane strain alone does not produce significant changes in the yield stress and the deformation behavior. Its yield locus in the tension-tension quadrant is therefore either very nearly a quarter circle or similar to a Tresca locus. The exact shape of the locus can be determined only by much more elaborate biaxial tests. This material is not very notch sensitive compared with PC. The energy to break in a notched Izod impact test is 15 ft-lb/inch for Vs-inch thick bars and 11 ft-lb/inch for 4-inch bars whereas for PC the latter figure is about 2 ft-lb/inch. This reduction in notch sensitivity over pure PC appears to be related to the material s ability to void internally, probably relieving the plane strain. 

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