Microscopic failure processes

The increasing use of high-performance fibrous composites in critical structural applications has led to a need to predict the lifetimes of these materials in service environments. To predict the durability of a composite in service environment requires a basic understanding of (1) the microscopic deformation and failure processes of the composite (2) the significance of the fiber, epoxy matrix and fiber-matrix interfacial region in composite performance and (3) the relations between the structure, deformation and failure processes and mechanical response of the fiber, epoxy matrix and their interface and how such relations are modified by environmental factors. 

The relevance of bulk fracture properties has therefore been considered essentially within the context of cohesive wear modes such as abrasive and fatigue wear. During abrasive wear, the initial stage is considered to be the process of contact and scratch between the polymer surface and a sharp asperity. The accumulation of the associated microscopic failure events eventually generates wear particles and gives rise to weight loss. Early approaches initiated by Ratner and co-workers [15] and Lancaster [16] attempted to correlate the abrasive wear rate with some estimate of the work to failure of the... 

Another direction is to build models which will be more realistic. In all these models described above, the exact microscopic mechanism of the failure was not considered. But it seems very likely that the exact nature of the process will influence what happens after the first failure. Yagil et al (1992, 1993) observed that after the first failure (fuse), the resistance of the sample can get decreased or increased depending on the failure process. If increase is what one expects, then the decrease means that the first failure improves the contact between the parts which melt. Thus, only by a detailed analysis of the failure process can one understand it. To come back to the dynamic problem, it is also very likely that the velocity of the failure propagation will depend on the failure mechanism. 

Morgan RJ, Pruneda CO, Steele WJ (1983) The relationship between the physical structure and the microscopic deftamation and failure process of poly(p-phenylene terephthalamide) fibers. J. Polymer Sci., Polymer Phys. Ed. 21 MSI... 

Kulkarni et al. [83] studied the failure processes occurring at the micro-scale in heterogeneous adhesives using a multi-scale cohesive scheme. They also considered failure effect on the macroscopic cohesive response. Investigating the representative volume element (RVE) size has demonstrated that for the macroscopic response to represent the loading histories, the microscopic domain width needs to be 2 or 3 times the layer thickness. Additionally, they analyzed the effect of particle size, volume fraction and particle-matrix interfacial parameters on the failure response as well as effective... 

Standardized notched impact tests such as the Izod and Charpy tests (ASTM, ISO, DIN) are the most commonly used to characterize the impact strength of plastic materials. It is very difficult to use measured data from tests using idealized laboratory specimens to predict impact behavior of end-use polymeric material. The apparent lack of good correlation between measured impact fracture energy and end-use impact resistance is due to the extreme complexity of microscopic fracture processes. In particular, the influence of specimen geometry is sometimes poorly matched with the type of failure mechanism of defects present in the actual molded part subjected to end-use impact forces. 

Fractography is an indispensable tool for extending imderstanding of how and why a material fails. On accoimt of its great depth of field, SEM is an essential item of equipment for fractiu-e siuface analysis. Scanning electron microscopic examination, together with the crack formation concepts (classification into fracture modes and description of their characteristics), provides an opportunity to reconstruct failure processes and to illustrate the limits to loading. 

The strength of the network is measured by the force required to fi-acture it, or to make it flow in the sense that a failure can be catastrophic or many microscopic failures. If the material shows some non-recoverable strain prior to failure in the form of ductile deformation, then its strength reflects contributions from the shear modulus, G, in addition to contributions from E. Because of the complexity of the failure process [823] and its dependence on the number and size of sample imperfections, in addition to the characteristic moduli, we shall... 

The second approach to fracture is different in that it treats the material as a continuum rather than as an assembly of molecules. In this case it is recognised that failure initiates at microscopic defects and the strength predictions are then made on the basis of the stress system and the energy release processes around developing cracks. From the measured strength values it is possible to estimate the size of the inherent flaws which would have caused failure at this stress level. In some cases the flaw size prediction is unrealistically large but in many cases the predicted value agrees well with the size of the defects observed, or suspected to exist in the material. 

The simplified failure envelopes are not derived from physical theories of failure in which the actual physical processes that cause failure on a microscopic level are integrated to obtain a failure theory. We, instead, deal with phenomenological theories in which we ignore the actual failure mechanisms and concentrate on the gross macroscopic events of failure. Phenomenological theories are based on curve-fitting, so they are failure criteria and not theories of any kind (the term theory implies a formal derivation process). 

Other explanations of the nature of the polymer to metal bond include mechanical adhesion due to microscopic physical interlocking of the two faces, chemical bonding due to acid/base reactions occuring at the interface, hydrogen bonding at the interface, and electrostatic forces built up between the metal face and the dielectric polymer. It is reasonable to assume that all of these kinds of interactions, to one degree or another, are needed to explain the failure of adhesion in the cathodic delamination process. 

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