Interfacial fracture, locus

The results are plotted in Fig. 14. The upper two lines refer to the A fiber and the lower two lines to the HM fiber. For both fibers, the addition and removal of surface chemical groups did not produce reversible interfacial behavior. The untreated fiber surfaces produced results that could not be duplicated when the surface groups were removed. Microtoming of single fiber specimens pinpointed changes in the locus of interfacial fracture that were relatable to the interphase conditions caused by the surface treatment. 

Identification of Locus of Adhesion Failure. To clarify whether the disruption is cohesive failure of the lacquer or interfacial failure between the substrate and lacquer, the lacquer and metal sides of the fracture surface were both measured by XPS. The results are shown in Fig. 4. For the purpose of comparison, UVC lacquer coated and DOS oiled nickel-plated sheets are shown in the top and bottom of the diagram, respectively. The contribution at ca. 286.5 eV of the lacquer surface is attributed to carbon singly bonded to oxygen... 

On the interfacial lacquer surface, the concentration of the ester group decreased toward the outside (the fracture interface), suggesting the diffusion of DOS into the lacquer film. SEM observation of the fracture surface of metal side is shown in Fig. 6. It can be seen that some lacquer remains as an island state. The apparent disagreement with XPS data seems to be due to the presence of invisible lacquer by this magnification (x 550). According to XPS results and SEM observation the locus of failure may be schematically represented as shown in Fig. 7. 

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. 

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. 

Bond failure may occur at any of the locations indicated in Fig. 1. Visual determination of the locus of failure is possible only if failure has occurred in the relatively thick polymer layer, leaving continuous layers of material on both sides of the fracture. The appearance of a metallic-appearing fracture surface is not definite proof of interfacial failure since the coupling agent, polymer films, or oxide layers may be so thin that they are not detectable visually. Surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and contact angle measurements are appropriate to determine the nature of the failure surfaces scanning electron microscopy (SEM) may also be helpful if the failed surface can be identified. 

As an illustration, consider Fig. 1, which shows four different failure modes for aluminum adherends bonded with a model epoxy adhesive. Simply by altering the loading mode, one can obtain cohesive failures, apparent (visually) interfacial failures, failures that oscillate within the adhesive layer, and alternating failures that jump back and forth from one interface to the other. Predicted theoretically, this dramatic variation in locus of failure was experimentally achieved not by altering the substrates or the surface preparation but simply by changing the loading conditions for these beam-type fracture specimens. 

The above comments are seen to be reinforced by observations on the failure path in joints before and after environmental attack. The locus of joint failure of adhesive joints when initially prepared is usually by cohesive fracture in the adhesive layer, or possibly in the substrate materials. However, a classic symptom of environmental attack is that, after such attack, the joints exhibit some degree of apparently interfacial failure between the substrate (or primer) and the substrate. The extent of such apparently interfacial attack increases with time of exposure to the hostile environment. In many instances environmental attack is not accompanied by gross corrosion and the substrates appear clean and in a pristine condition, whilst in other instances the substrates may be heavily corroded. However, as will be shown later, first appearances may be deceptive. For example, to determine whether the failure path is truly at the interface, or whether it is in the oxide layer, or in a boundary layer of the adhesive or primer (if present), requires the use of modern surface analytical methods one cannot rely simply upon a visual assessment. Also, the presence of corrosion on the failed surfaces does not necessarily imply that it was a key aspect in the mechanism of environmental attack. In many instances, corrosion only occurs once the intrinsic adhesion forces at the adhesive/substrate interface, or the oxide layer itself, have failed due the ingressing liquid the substrate surface is now exposed and a liquid electrolyte is present so that post-failure corrosion of the substrate may now result. 

Another basic major advantage is that the cyclic-fatigue fracture-mechanics data may be gathered in a relatively short time-period, but may be applied to other designs of bonded joints and components, whose lifetime may then be predicted over a far longer time-span. Obviously, the fracture-mechanics tests need to be conducted under similar test conditions and environments as the joints, or components, whose service-life is to be predicted. This is important since the fracture-mechanics test specimens do need to exhibit a similar mechanism and locus of failure (e.g. cohesively through the adhesive layer, or interfacially between the adhesive and substrate, or through the oxide layer on the metallic substrate, etc.) as observed in the joints, or components, whose lifetime is to be ranked and predicted. 

Thus, a visual assessment of the fracture surfaces of a joint which has been subjected to environmental attack does not indicate exactly the locus of failure and techniques such as Auger and X-ray photoelectron spectroscopy are needed to identify precisely the locus of failure. However, it does indicate that we should concentrate our attention on the interfacial regions if we are to understand and prevent such environmental failure of adhesive joints. 

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