Failure polymers

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. 

In an initially ductile polymer, failure properties (ultimate elongation, fracture toughness, impact resistance) decrease rapidly during a chain-scission aging process, whereas elastic and yield properties are practically unaffected at the embrittlement point. 

The 20th Europhysics Conference on Macromola ular Physics in Lausanne (Sept. 88), devoted to Physical Mechanisms in Polymers Failure offered the chance of bringing together practically all of the authors of this volume with each other, with other leading experts, and with an interested audience this volume might therefore also be considered as partial Proc ings of that Conference. 

We have chosen to study polymer failure In a framework which assumes that failure is the result of a cumulative damage process. When the damage reaches a critical value, then failure occurs. If the rate of damage accumulation is a function of only the current stress and not of stress rate or of the current state of damage, the simplest form of this concept is valid. 

The oriented film specimens were mounted on aluminum frames and exposed on an Atlas Weather-Ometer, Model 65WR. An 18 minute spray cycle together with an 102 minute cycle at 55% relative humidity and approximately 65°C was used. At regular intervals, the test specimens were removed from exposure and their tensile strength measured on an Instron Model 1102. A decrease in tensile strength, expressed as tenacity, over the tensile strength of the same formulation before exposure, is a measure of the deterioration of the physical properties of the polymer. "Failure" in this test is defined as a loss of 50% or more of the initial sample tenacity. 

FIGURE 42 Role of tie molecules in the polymer failure model. 

Review of Some Important Facts about Polymer Failure, and Crazing. 

Using loss in reflectance as an indication of mirror failure and change in tensile properties as an Indication of polymer failure, we can rank the polymer/mirror assemblies studied for overall performance. In Table I we rank all the assemblies in terms of poor, intermediate, and good performance. FEK-244 or PMMA(3M)/Al/adhesive is the most durable of the polymer/mirror combinations studied. Poor performers warrant no further study. Intermediate cases show some promise and could possibly be improved with modification of the polymer/mirror assembly. Both physical and chemical failure have been observed in the pol3raier-... 

Let us consider one more feature of polymer failure and the effect of MDI thereon. The termination of molecular chains causes formation of submicrocracks and cavities in the direction perpendicular to the tension in the pol5rmer. The concentration of the submicrocracks increases the effect of the load over time, which asymptoticaUy... 

Shear Fracture. Polymer failure is initiated when critical shear stresses are exceeded. Stretching at the separation faces ultimately leads to lips and peaks that on two mating fracture surfaces point toward each other. Fracture due to shear failure can only be distinguished from that due to tear fracture on two mating fractme smTaces. The separating mechanism involved is that of normal stress fractme and tear fractme, since gliding along defined crystal planes as in metals cannot occur (Fig. 5). 

Polymer failure takes place at stress levels much lower than expected on the basis of bond strength. Theoretical strength values are actually orders of magnitude greater than experimentally observed. 

The strain-invariant polymer failure model (SIFT) permits the superposition of separate analyses for shear and induced peel loads. There is no interaction between the two failure mechanisms. While the need for this has been minimized through sound design practice (gentle tapering of the ends of the adherends to minimize the induced peel stresses, as explained later), the new model puts this technique on a secure scientific foundation and also accommodates any applied transverse shear loads. 

The following is a compilation of general comments about the use of polymers in industrial tribological applications. Polymer failure in tribological applications is often catastrophic, occurring in a very short timescale of the order of minutes or hours. 

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