Morphologies, failure

Failure Morphologies. Ductile failure of notched polycarbonate specimens has long been recognized to occur with shear yielding from the notch tip (6). This occurs for the block polymers for all rates of test. Hull and Owen (5) recently reported from micrographic studies of impact fracture surfaces that the brittle failure of polycarbonate involves the formation and breakdown of a craze at the notch tip. The ductile-... 

Figure 9. Failure morphology of BPFC-DMS block polymer...

Apparently this model in its present form does not predict a fibrillated fracture morphology caused by shear failure, as has been clearly observed for the PpPTA, PBO and PBT fibers. Knoff noticed the similarity between the tensile failure morphology of PpPTA fibers and that of a uniaxially oriented fiber-reinforced composite. The latter fails in tension via matrix shear failure initiated at the fiber ends. This made him to conclude that, if the shear forces at a discontinuity exceed the shear strength of the bond between the fibrils, the fiber tensile strength should be proportional to the fiber shear strength [182]. 

Fig. 7.102 Change of delamination failure morphologies [32]. With kind permission of Professor Yamazaki for the editorial office...

The finished packaged product is shipped by various methods to distribution centers and care centers. During shipping, the carton undergoes random vibration, which causes the folds and creases to flex repeatedly. While this was observed to lead to some deformation and deterioration of the film surface at the tip of a crease, flexing of the creases alone did not lead to panel leak formation, except under very extreme vibration conditions. Under those conditions, however, the failure morphology was very different from the panel leaks observed in the field. Again, crease formation and/or vibration (either alone or in combination) did not lead to panel leak formation. 

Passivation at the metal/active mass interface, or of the active mass itself can also lead to failure. Detrimental changes in the morphology of the active mass and microstructural changes in the grid material can also occur. 

In high-pressure boilers, there are three types of on-load corrosion acidic chloride, neutral chloride/dissolved oxygen, and caustic attack. The first and second (once it becomes established) are brittle and thick-walled and are accompanied by hydrogen damage which can lead to failure within a few hundred hours. Caustic attack tends to produce a gouged appearance of the metal due to extensive wastage. The morphology is fairly characteristic of the failure type. Acidic chloride forms hard, laminated oxide, whilst, with caustic attack, the oxide is often soft, and, as it is easily removed, may be absent. 

Since most polymers, including elastomers, are immiscible with each other, their blends undergo phase separation with poor adhesion between the matrix and dispersed phase. The properties of such blends are often poorer than the individual components. At the same time, it is often desired to combine the process and performance characteristics of two or more polymers, to develop industrially useful products. This is accomplished by compatibilizing the blend, either by adding a third component, called compatibilizer, or by chemically or mechanically enhancing the interaction of the two-component polymers. The ultimate objective is to develop a morphology that will allow smooth stress transfer from one phase to the other and allow the product to resist failure under multiple stresses. In case of elastomer blends, compatibilization is especially useful to aid uniform distribution of fillers, curatives, and plasticizers to obtain a morphologically and mechanically sound product. Compatibilization of elastomeric blends is accomplished in two ways, mechanically and chemically. 

Having said this, it was felt therefore that there is a need for a book addressing analysis and characterisation of polymers from the point of view of what we wish to call the primary analytical question. Many excellent textbooks and reference works exist which address one or more individual analytical techniques, see, for example, references [1-10]. These books form the basis of the knowledge of the technique expert. They also contain many excellent and varied examples on successful applications of analytical techniques to polymer analysis and characterisation. There are also books which address the multitude of analytical techniques applied in polymer analysis, see, for example, references [11-24], However, a synthetic chemist may wish to know the constitution of his/her polymer chain, a material scientist may want to find out the reasons why a fabricated sample had failed. What technique is best or optimal to study chain constitution will depend on the situation. Polymer failure may result from morphological features, which needs to be avoided, a contaminant, a surface property degradation, etc. When a sample has been processed, e.g., a film blown, molecular orientation may be the key parameter to be studied. A formulation scientist may wish to know why an additive from a different supplier performs differently. It is from such points of view that polymer analysis and characterisation is addressed in this book. 

From the above discussion, it is clear that the stabilization or failure of graphite electrodes depends on a delicate balance between passivation phenomena (due to the formation of highly cohesive and adhesive surface films) and a buildup of internal pressure due to the reduction of solution species inside crevices in the graphite particles. This delicate balance can be attenuated by both solution composition (EC-DMC vs. EC-PC or PC, etc.) and the morphology of the graphite particles (i.e. the structure of the edge planes and the presence of crevices). 

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