Interphase fatigue

What of the corrosion resistance of new turbine-blade alloys like DS eutectics Well, an alloy like NiaAl-NisNb loses 0.05 mm of metal from its surface in 48 hours at the anticipated operating temperature of 1155°C for such alloys. This is obviously not a good performance, and coatings will be required before these materials are suitable for application. At lower oxidation rates, a more insidious effect takes place - preferential attack of one of the phases, with penetration along interphase boundaries. Obviously this type of attack, occurring under a break in the coating, can easily lead to fatigue failure and raises another problem in the use of DS eutectics. 

Structural applications of composite materials require not only acceptable static mechanical properites but the ability to withstand the generation and propagation of cracks without premature failre. For example, impact resistance, fracture toughness and fatigue resistance are desireable composite properties. Fiber-matrix structure at the interphase can affect the values attainable for these properties. 

The interphase provided by the adhesion promoter may be hard or soft and could affect the mechanical properties. A soft interphase, for example, can significantly improve fatigue and other properties. A soft interphase will reduce stress concentrations. A rigid interphase improves stress transfer of resin to the filler or adherend and improves interfacial shear strength. Adhesion promoters generally increase adhesion between the resin matrix and substrate, thus raising the fracture energy required to initiate a crack. 

Altstadt V (1997) Fatigue crack propagation in homopolymers and blends with high and low interphase strength. In European Conference on Macromolecular Physics -Surfaces and Interfaces in Polymers and Composites, Lausanne, Switzerland... 

The properties of filled materials are eritieally dependent on the interphase between the filler and the matrix polymer. The type of interphase depends on the character of the interaction which may be either a physical force or a chemical reaction. Both types of interaction contribute to the reinforcement of polymeric materials. Formation of chemical bonds in filled materials generates much of their physical properties. An interfacial bond improves interlaminar adhesion, delamination resistance, fatigue resistance, and corrosion resistance. These properties must be considered in the design of filled materials, composites, and in tailoring the properties of the final product. Other consequences of filler reactivity can be explained based on the properties of monodisperse inorganic materials having small particle sizes. The controlled shape, size and functional group distribution of these materials develop a controlled, ordered structure in the material. The filler surface acts as a template for interface formation which allows the reactivity of the filler surface to come into play. Here are examples ... 

Attention is now given to the fatigue performance of these systems as influenced by the local property changes/gradients. As these materials will eventually make their way into structures that experience time-varying loads, questions must also be addressed about how a material s durability is affected by the character of the interphase. 

This chapter will now outline two recent pieces of work that have been performed by the authors in the area of fatigue performance of composites with modified interphases. 

As with the work of Swain [7] and Subramanian [9], the S-N curve for the 8 series shows substantial differences between the performance of the A and O fiber sized composites. Reviewing the S-N curve for the 8 series compression-compression, R = 10 fatigue, a best fit line through each of the data sets assists in delineation of the difference in performance of the A and O interphases, as shown in Fig. 11.1. The apphed cyclic stress levels are presented as a percentage of their ultimate notched compression strength, as reported in Table 11.1. The 810-A composite exhibited a slightly lower rate of fife reduction with increasing load level. However, all lifetimes... 

Figure 11.1 Applied gross compression stress (as a percentage of the ultimate notched [O/eoigs compression strength) versus the cycles (log of n), for the 810-A and O materials under / = 10 fatigue. , epoxy interphase , PVP interphase...

In summary, the authors conclude that it is not the LS splitting which controls the difference in lives between the A and O interphase materials, but the inherent characteristics of the material which dominate their fatigue response. In addition the authors are of the opinion that it is not merely the inherent strength of the material that influenced the durability of these varied interphase systems but, more exactly, the toughness or damage tolerance of the material. 

R E Swain III, The role of the fiber/matrix interphase in the static and fatigue behavior of polymeric composite laminates, Dissertation, Department of Engineering Science and Mechanics, Virginia Polytechnic Institute State University, February, 1992. 

Fig. 8. Transmission electron micrograph of a cross-section of the interphase region of a PAA-preireated joint from a dry cyclic-fatigue test 110. ...

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