Brittleness changes

Exposure of the molded HDPE articles to sunlight and air also attacks the polymer over time, especially at wavelengths less than 400 nm. Photooxidation resembles thermooxidation in that it is a complex chain of radical transformations. Such exterior aging of the polymer results in development of surface cracks, brittleness, changes in color, and a deterioration of mechanical and dielectrical properties. Photooxidation degradation is prevented by small amounts of light stabilizers, such as 2-4% carbon black or, for colorless articles, esters of salicylic acid or derivatives of benzotriazole or benzophone and others in the 0.1-0.5% range. 

It is very important, from one hand, to accept a hypothesis about the material fracture properties before physical model building because general view of TF is going to change depending on mechanical model (brittle, elasto-plastic, visco-elasto-plastic, ete.) of the material. From the other hand, it is necessary to keep in mind that the material response to loads or actions is different depending on the accepted mechanical model because rheological properties of the material determine type of response in time. The most remarkable difference can be observed between brittle materials and materials with explicit plastic properties. 

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). 

The Fe—Co alloys exist ia the fee stmeture above 912—986°C to ca 70 wt % Co. Below this temperature range, the stmeture changes to bcc. At ca 50 wt % Co, the material further orders to the CsCl-type B2 stmeture below about 730°C and becomes very brittle. The addition of V ia Permeadur retards the rate of orderiag and imparts substantial ductiHty to the adoy, although quenching is necessary. Vanadium addition also iacreases the resistivity, eg, from 7—26 fifl-cm usiag a 2% addition. 

T and are the glass-transition temperatures in K of the homopolymers and are the weight fractions of the comonomers (49). Because the glass-transition temperature is directly related to many other material properties, changes in T by copolymerization cause changes in other properties too. Polymer properties that depend on the glass-transition temperature include physical state, rate of thermal expansion, thermal properties, torsional modulus, refractive index, dissipation factor, brittle impact resistance, flow and heat distortion properties, and minimum film-forming temperature of polymer latex... 

Because oxides are usually quite brittle at the temperatures encountered on a turbine blade surface, they can crack, especially when the temperature of the blade changes and differential thermal contraction and expansion stresses are set up between alloy and oxide. These can act as ideal nucleation centres for thermal fatigue cracks and, because oxide layers in nickel alloys are stuck well to the underlying alloy (they would be useless if they were not), the crack can spread into the alloy itself (Fig. 22.3). The properties of the oxide film are thus very important in affecting the fatigue properties of the whole component. 

The cellulose fiber in paper is attacked and weakened by sulfur dioxide. Paper made before about 1750 is not significantly affected by sulfur dioxide (11). At about that time, the manufacture of paper changed to a chemical treatment process that broke down the wood fiber more rapidly. It is thought that this process introduces trace quantities of metals, which catalyze the conversion of sulfur dioxide to sulfuric add. Sulfuric acid causes the paper to become brittle and more subject to cracking and tearing. New papers have become available to minimize the interaction with SO2. 

Cross-linking which may lead to hardening, brittleness and changes in solubility. 

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