Fracture toughness of interfaces

Qc critical strain energy release rate or fracture toughness of interface... 

Fig. 32. Maximum achievable fracture toughness of interfaces between A and B polymers reinforced with block copolymers or end-grafted chains as a function of the degree of polymerization N of the reinforcing block. (A) PS-b-PMMA between PPO and PMMA ( ) dPS-COOH chains in a HIPS matrix grafted on an epoxy interface ( ) dPS-COOH chains in a PS matrix grafted at an epoxy interface (O) PS-b-PVP chains at the interface between PS and PVP. Data from [22,36,38,40]...

Fig. 37. Comparison of the fracture toughness of interfaces between PS and PVP reinforced with a 800-870 dPS-PVP diblock ( ) and with a 580-1620-580 PVP-fo-dPS-fo-PVP triblock ( ). While is quite similar for both block copolymers at low coverage, the triblock is much more effective at higher levels of coverage as explained in the text. Data from [33,46]...

Fig. 44. Fracture toughness of interfaces between PS and PVP reinforced by a thick layer of a VSj-r-VVP1 random copolymer. The pronounced maximum as a function of/reflects the situation where Xrcp-PS = Xrcp-PVP Data from [75]...

It should be noted, however, that in a later publication [94], Lee and Char pointed out that SMA is significantly more brittle than PS and this could also explain the lower fracture toughness of interfaces where a thick layer of nearly pure SMA is present close to the interface. 

Fig. 51. Fracture toughness of interfaces between poly(2-vinylpyridine) and poly(styrene) containing a small amount of sulfonated PS. Qc is plotted as a function of the mole fraction of PS which is functionalized. Data from [95]...

We have shown that the fracture toughness of interfaces between polymers is dependent on the molecular structure at the interface as well as on the bulk properties of the polymers on either side of the interface. This relationship is now relatively well established for glassy polymers and the main results are summarized in Figs. 53 and 54, as well as in Sects 3.2-3.5. However, these results should be used with caution when the polymers on either side of the interface are rubbery or semicrystalline. The stress-transfer mechanisms, and in particular the role of the entanglements, will be very different from those observed for the glassy polymers and only preliminary data are currently available on those systems. In principle, fracture mechanisms maps analogous to those depicted in Figs. 53 and 54 could be drawn for these systems but the relevant parameters are not yet as clearly identified. 

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. 

Tsai, H.C., Arocho, A.M. and Cause, L.W. (1990). Prediction of fiber-matrix interphase properties and their influence on interface stress, displacement and fracture toughness of composite materials. Mater. Sci. Eng. A126, 295-304. 

IMPROVEMENT OF TRANSVERSE FRACTURE TOUGHNESS WITH INTERFACE CONTROL... 

Chapter 7. Improvement of transverse fracture toughness with interface control... 

Further details of the above techniques to improve the transverse fracture toughness of composites by controlled interfaces of various nature and modifying materials are discussed in the following sections. The effectiveness of these toughening methods on transverse fracture toughness and strength in relation to the controls are summarized in Table 7.1. 

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