Embrittlement relationship

Relatively small changes in comonomer content can result in significant changes in physical or chemical properties. Polymer resin manufacturers exploit such relationships to control the properties of their products. The composition of a copolymer controls properties such as stiffness, heat distortion temperature, printability, and solvent resistance. For example, polypropylene homopolymer is brittle at temperatures below approximately 0 °C however, when a few percent ethylene is incorporated into the polymer backbone, the embrittlement temperature of the resulting copolymer is reduced by 20 °C or more. 

The relationship of brittle fracture to plastic deformation has, of course, been elaborated in various ways with the aid of dislocation theory, e.g. nucleation of microcracks has been discussed in terms of piling-up of dislocations [124]. Davies [145] has shown that embrittlement requires the presence of islands of martensite (about 1 pm in size) and has suggested that cracks are initiated in the martensite or at the martensite-ferrite interface. 

To extend this model to the prediction of embrittlement time, one needs first to identify the causal chain leading to embrittlement, second to express mathematical relationships corresponding to the elementary links of the chain, and third to define pertinent end life criteria. According to previous studies (2,3,4), two causal chains are possible in the case under study (Figure 1). 

The second condition to validate the scheme B is that embrittlement must correspond to a critical morphological state that is the only approach to explain its sudden character. The extensive and careful work of Kennedy et al. (//) on relationships between fracture behavior, molar mass and lamellar morphology, shows that this condition is fulfilled in the case of PE. Comparing various samples of different molar masses with different thermal histories, they found that the thickness of the amorphous layer (la) separating two adjacent lamellae is the key parameter (Fig. 6). As a matter of fact, there is a critical value lac of the order of 6-7 nm. For la > lac the samples are always ductile whatever their molar mass, whereas for U < laC the samples are consistently brittle. As a result, lac appears to be independent of the molar mass. Indeed, there is a specific molar mass, probably close to 70 kg.mof for PE below which crystallization is so fast that it is impossible to have la values higher than lac whatever the processing conditions. 

The time to embrittlement of polyolefins during photo-oxidation is not directly related to initiation carbonyl concentration (9-11). This is exemplified for HOPE in Figure 3 and a similar relationship exists for LDPE and... 

Figure 3, Relationship between carbonyl content and embrittlement time of...

The relationship between ductility and strain rate under conditions conducive to hydrogen embrittlement is also shown schematically in Fig. 7.81. Under these conditions, the controlling factor is the absorption of hydrogen resulting from the reduction of hydrogen ions. The slower the strain rate, the longer the time for absorption of hydrogen,... 

Structural metals become more susceptible to hydrogen embrittlement as the materials are exposed to higher gas pressures. Thermodynamic equilibrium between hydrogen gas and dissolved atomic hydrogen is expressed by the general form of Sievert s Law, i.e. C = Sf [16]. This relationship shows that as fugacity (pressure) increases, the quantity of atomic hydrogen dissolved in the material increases consequently, embrittlement becomes more severe. 

Figure 18. Relationship between peroxide and carbonyl concentrations and embrittlement time for polypropylene. (—CD—) peroxide formation, open chamber (— —) carbonyl formation, open chamber (—A—) peroxide formation, closed chamber (— —) carbonyl formation, closed chamber.

The direct effect of accelerated anodic dissolution at the crack front is most important at low stress amplitudes, while reduced lifetime due to hydrogen embrittlement is associated with high stresses, particularly for higher-strength materials. There is, however, a certain relationship between the mechanisms, since accelerated anodic dissolution leads to an acidic local environment and thereby significant hydrogen development within the crack. 

Odette G R, Lombrozo P M and Wullaert R A (1985), Relationship between irradiation hardening and embrittlement of pressure vessel steels, pp. 840-860 in Effects of Radiation on Materials Twelfth International Symposium, ASTM STP870, F A Garner and J S Perrin, eds, American Society for Testing and Materials, Philadelphia, PA. 

---