Delamination mechanism

Altus, E. and Ishai, O. (1990). The effect of soft interleaved layers on the combined transverse cracking/ delamination mechanisms in composite laminates. Composites Sci. Technoi. 39, 13-27. 

Locus of failure studies 75 80) on metal/epoxy joints that had been exposed to water indicate that corrosion of the metal substrate does not occur until after interfacial failure has occurred. This suggests that corrosion itself does not play a primary role in the loss of adhesion strength mechanism of metal/epoxy joints, but rather is a post-failure phenomenon. However, for the case of metal/epoxy protective coating systems, Leidheiser and coworkers 88-91 -92) and Dickie and coworkers 5 87-89-90> have proposed that localized corrosion processes are part of an important delamination mechanism. 

Fig. 10. Schematic diagram of the corrosion-induced delamination mechanism for a steel/ epoxy coating system...

Regarding the delamination mechanism, FEM analysis was conducted for the bilayer TBC. TBC was applied on one of the end faces of a 17 mm cylindrical sample, and the sample was heat-treated at 980lC. Assuming the condition after being cooled down to room temperature from heat-treated temperature as the initial condition, the state of the sample heated with the burner was analyzed. Figure 6 shows the analytical conditions. For data of the material s physical properties, values in literature were used[2][3]. 

Figure 7. Illustration of delamination mechanism at a corrosion site on painted steel.

For nongalvanized steel, cosmetic corrosion generally involves a cathodic delamination mechanism the surface under the paint becomes cathodic and the surface exposed in the hole becomes anodic. To slow down or prevent atmospheric corrosion, it is therefore important that the siuface treatment be a good cathodic inhibitor in the finished product. The phosphate layer increases corrosion resistance by limiting the available free surface for the cathodic reaction. In general, the activity of the free surface is further reduced by passivating posttreatments or by the deposition of amorphous phosphate films between the crystals. 

The cathodic delamination mechanism is discussed in detail in Sect. 5.4.3.1.2 while the FFC on iron is discussed together with FFC on aluminum in Sect. S.4.3.3 because of the similar mechanisms. 

A. Leng, H. Stieckel, and M. Stratmann, The delamination of polymeric coatings from steel. Part 1. Calibration of the Kelvin probe and basic delamination mechanisms, Corros. Sci., 41, 547-578 (1999). 

A number of explanations have been put forward for delamination mechanism whereby the alkaline environment under the film affects the integrity of the metal-polymer interface, or perhaps more properly the interface between the oxide and the polymer. Koehler [94] showed that this form of failure only occurs when there are alkali metal cations available in the environment to act as counter ions to the cathodically generated OH ions. 

Obviously, the interface can be strongly polarized by the defect, which will result in different delamination mechanisms. 

O Brien, T.K., Characterization of Delamination Onset and Growth in a Composite Laminate in Damage in Composite Materials, ASTM STP 775, p. 140-167,1982 Poursartip, A. Ashby, M. F., Beaumont P.W.R., The Fatigue Damage Mechanics of Fibrous Laminates in Proceedings of the European Workshop on Nondestructive Evaluation of Polymers and Polymer Matrix Composites, Polymer NDE (edited by Khg. Ashbee), Technomic Publishing, p. 250-260, 1984... 

Nondestmctive testing (qv) can iaclude any test that does not damage the plastic piece beyond its iatended use, such as visual and, ia some cases, mechanical tests. However, the term is normally used to describe x-ray, auclear source, ultrasonics, atomic emission, as well as some optical and infrared techniques for polymers. Nondestmctive testing is used to determine cracks, voids, inclusions, delamination, contamination, lack of cure, anisotropy, residual stresses, and defective bonds or welds in materials. 

Marshall, D.B. and Evans, A.G., Measurement of adherence of residually stressed thin-films by indentation. I. Mechanics of interface delamination. J. Appl. Phys., 56(10), 2632-2638 (1984). 

Repair. Repairs for damaged bonded structure can be either mechanical or adhesively bonded. Mechanical repairs are metallic doublers on one or both sides of a damaged component, held on by fasteners. The fasteners transfer the load through the doubler around the damaged site and restore part functionality. Although common for metal-to-metal bonded structure, mechanical repair of sandwich structure is rare because of the risk of further delamination. Unless the doubler and fasteners are perfectly sealed, water can travel into the honeycomb core eventually causing freeze-thaw damage and delamination. 

Local repair of delamination originally caused by non-durable surface treatment is only temporarily successful at best. The surface treatment on the unrepaired portion of the assembly remains susceptible to attack and the area of delamination will likely continue to grow once the assembly is put back into service and exposed to moist conditions. Replacement or complete remanufacture of the component is the only way to permanently address this type of damage. However, time-limited repairs using bonded or mechanical methods can be used to extend the life of the component until a major overhaul is scheduled. In some cases such as widespread disbond of fuselage doublers, mechanical repairs (rivets and fastened doublers) and continued inspection are used to extend the life of the skin indefinitely because of the high cost of replacement. 

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