Failure modes optical microscopy

The SEM photomicrographs in general supported the failure mode assignments based on optical microscopy. Howeverr detailed features of the Ti 6-4 adherend surfaces are clearly delineated in the SEM photomicrographs. 

The reflection technique is more limited in resolution and magnification than is transmission. Reflection optical microscopy of adhered surfaces is often revealing, but the very limited depth of field is a severe handicap. Nevertheless, examination of surfaces after testing or failure is a simple, but important, first step in determining locus of failure (see Stress distribution mode of failure). Surface reflectivity of specimens is often improved by deposition of a layer or metal. The reflection microscope is especially useful when operated in dark-field mode, where topographical differences are accentuated. 

In the third case, where the failure and fracture occur outside the laboratory, microscopy is often used in a forensic manner to determine the failure mode and its point of initiation. Microanalysis may be used to pinpoint a specific cause and assign responsibility for the failure. Here the special features of specimen preparation relate to the nature of the work and optical photography may be required to document the relation of the specific SEM samples to the original object. 

Various techniques such as optical and electron microscopy, IR-ATR ESCA and SIMS have been used for failure mode analysis in polymeric adhesive joints. However, these techniques are not amenable for in situ, non-destructive examination of the failure surfaces as failure occurs. In order to develop in situ techniques, UV reflection and fluorescence methods are evaluated, as they can be applied via optical fibres to the actual failure surface. Failure surfaces of two types of adhesive joints are examined by UV reflection and fluorescence, complemented by FT-IR ATR studies. The first joint studied is PU on epoxy-coated steel. The second is PETP/PE/PETP. 8 refs. 

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