Deformation and failure modes

In Fig. 26, we schematically illustrate four stages of failure in epoxies under an increasing tensile load. In each stage we document the craze/crack structure, the stress at the craze/crack surface and the resultant fracture topography. 

In addition to the fracture topography observations in the slower crack propagation regions of epoxies a number of additional observations also indicate permanent molecular flow can occur in amine-cured epoxy networks in the glassy state. 

Transmission electron microscopy (TEM) and birefringence studies of strained and/ or fractured epoxies have revealed more direct experimental evidence that molecular flow can occur in these glasses. Films of DGEBA-DETA ( 11 wt.- % DETA) epoxies, 1 pm thick, were strained directly in the electron microscope and the deformation processes were observed in bright-field TEM 73 110). Coarse craze fibrils yielded in-homogeneously by a process that involved the movement of indeformable 6-9 tan diameter, highly crosslinked molecular domains past one another. The material between such domains yielded and became thinner as plastic flow occurred. 

Failure stage Coarse craze Slow crack growth Slow crack growth Fast unstable 

Rough initiation Smooth mirror Rough parabola 

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Making a comprehensive analysis from the landslide slope s topography features, lithology, soil structure, deformation and failure characteristics, the landslide s deformation and failure mode is excavated the foot of the slope and then the slope lose support—surface precipitation and water pipe leakage due to slope body weight gain, strength... 

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

In general, the use of FE signals accompanying the deformation and fracture of composites offer elucidation of failure mechanisms and details of the sequence of events leading upto catastrophic failure. The extent of interfacial failure and fiber pull-out are also potential parameters that can be determined. FE can assist in the interpretation of AE and also provide an independent probe of the micro-events occurring prior to failure. FE has been shown to be sensitive to the locus of fracture and efforts are underway to relate emission intensity to fracture mechanics parameters such as fracture toughness (Gjp). Considerable work still remains to fully utilize FE to study the early stages or fracture and failure modes in composites. 

The principal physical structural parameters that control the modes of deformation and failure and mechanical response of epoxies are (1) macroscopic inhomogenieties such as microvoids or concentrations of unreacted monomer, (2) the glassy-state free volume and (3) the crosslinked network structure characteristics. 

It is important to operate the fuel cell at different compression pressures in order to determine the correct compression pressure for a DL material. If the applied compression pressures are too high, the DLs may deform, both the porosity and permeability of the DL decrease, and the probability of failure modes increases significantly. On the other hand, if the pressures are too low, then gas leaks and serious contact resistance between the components of the cell may be present. Various studies have been presented in which the compression pressure of the fuel cell is varied in order to observe how the cell s performance is affected [25,183,252]. In general, there is an optimal compression pressure range in which the cell s performance is the highest however, this depends on the DL material and on the MPL thickness (see Figure 4.21). 

Fracto-emission (FE) is the emission of particles (electrons, positive ions, and neutral species) and photons, when a material is stressed to failure. In this paper, we examine various FE signals accompanying the deformation and fracture of fiber-reinforced and alumina-filled epoxy, and relate them to the locus and mode of fracture. The intensities are orders of magnitude greater than those observed from the fracture of neat fibers and resins. This difference is attributed to the intense charge separation that accompanies the separation of dissimilar materials (interfacial failure) when a composite fractures. 

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