Embrittlement impact testing

Low-temperature embrittlement occurs in carbon and low-alloy steels at temperatures below their brittle-ductile transition temperature range. The effect is reversible when the alloy is heated above the transition range, ductility is restored. This embrittlement is avoided by following the Charpy impact test requirements of the relevant engineering codes. The need to test depends primarily on the material, its thickness and the minimum design temperature. 

Duplex stainless steels are susceptible to 885°F (475°C) embrittlement and to sigma-phase formation, and they are usually not selected for temperatures above 650°F (345°C). Because of their ferrite content, they are susceptible to low-temperature embrittlement. However, the duplex stainless steels tend to have relatively low brittle-ductile transition temperatures. The engineering codes typically require the duplex stainless steels to be qualified for low-temperature service by impact testing. They can be susceptible to hydrogen embrittlement, but are less susceptible than are the ferritic and martensitic stainless steels. 

The ability of a material to resist deformation and fracture in the presence of a notch. The most common toughness test is the Charpy impact test which, because of its high speed of straining, is normally unable to detect the presence of hydrogen embrittlement in ferritic steels. 

Within FP-5 project FRAME work will be done to improve the assessment of the most important parameter used to measure the embrittlement conditions of the RPV. Currently this is done through indirect measurements in a rather conservative way (the so-called reference temperature methodology, which makes use of Charpy-V notch impact testing). It is difficult to estimate in a quantitative way the conservatism of this methodology. Therefore the work proposed will focus on the development of a method which allows to measure directly the fracture toughness. This should result in a better and more accurate estimation of the embrittlement conditions of the RPV material. 

KWO] Chaipy impact testing, mechanical tests (on the Instron machine) Tempered martensite embrittlement (TME), impact toughness, ultimate tensile strength... 

The commercial value of PC-siloxane resins is based primarily on the enhanced impact strength these copolymers exhibit compared to PC homopolymers under severe (i.e., embrittling) end-use conditions. Frequently, the superiority of the PC-siloxane resins is demonstrated by impact testing at low temperatures. Figure 14.11 displays Izod impact data measured at room temperature and -30°C that clearly demonstrate this performance advantage. 

The specimens are shown in Fig. 1. In each case, the tests were run over the temperature range from 75 to -320 F. The tensile tests evaluated the effect of only one embrittling service condition—low temperatures. The unnotched tensile impact tests added a second embrittling condition—high strain rate. The notched tensile impact tests and the Charpy keyhole impact tests added the third embrittling service condition—stress concentration. 

NOTCHED TENSILE IMPACT TESTS. The results of the notched tensile impact tests are shown in Fig. 7. In these tests all three embrittling service conditions were combined, i.e., low temperatures, high rate of loading, and stress concentration. 

CHARPY KEYHOLE IMPACT TESTS. The results of the Charpy keyhole impact tests are shown in Fig. 8. This test is also a severe test, combining all three of the embrittling service conditions. It is one of the laboratory tests most likely to produce a ductile—brittle transition in a material that is susceptible to brittle fracture. 

The widespread use of Izod and Charpy impact tests to evaluate plastics is, to an unprejudiced eye, rather difficult to justify. Many structural polymers us in load-bearing applications do show a range of fracture behaviour from ductile to brittle . Most thermoplastics can show either kind of behaviour, and may suffer an abrupt tough-to-brittle transition with any of a number of parameters — one of which is the rate of loading at a notch. In order to select a polymer for a specific application it may be important to know its sensitivity to this kind of impact embrittlement. However, it is difficult to see how one might learn this fiem conventional impact strength data. 

There is some uncertainty as to whether zinc embrittles aluminum. However, indium severely embrittles aluminum. Alkali metals, sodium, and lidiium also are known to embrittle aluminum. Aluminum alloys containing either lead, cadmium, or bismuth inclusions embrittle when impact-tested near the melting point of these inclusions the sevoity of embrittlement increases from lead to cadmium to bismuth. 

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

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