Fracture composite

SSIMS has been used in the TOP SSIMS imaging mode to study very thin layers of organic materials [3.32-3.36], polymeric insulating materials [3.37], and carbon fiber and composite fracture surfaces [3.38]. In these studies a spatial resolution of ca. 80 nm in mass-resolved images was achieved. 

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 cross-ply laminates, the stress-strain behavior is slightly nonlinear, as illustrated in Figure 5.123. The stress-strain behavior of a unidirectional lamina along the fiber axis is shown in the top curve, while the stress-strain behavior for transverse loading is illustrated in the bottom curve. The stress-strain curve of the cross-ply composite, in the middle, exhibits a knee, indicated by strength ajc, which corresponds to the rupture of the fibers in the 90° ply. The 0° ply then bears the load, until it too ruptures at a composite fracture strength of ct/. 

The effect of the interphase on composite fracture can best be shown by hypothetic-ally separating the fracture process into its component parts to determine which are interphase dependent 79 "83). 

The conclusion to be made from this work is that the fiber and its properties as well as the epoxy matrix and its properties are the same for all three cases. Only the interphase has been altered. Strength of materials fracture models would not predict a difference in fracture toughness and yet experimentally alteration of a 200 nm interphase zone changes the composite fracture properties dramatically. 

A second method consists in observing the composite fracture surfaces. For example, in the case of MWNTs in a poly (hydroxy-aminoether) matrix, C. Bower et al. (70) observed a lot of pulled out nanotubes and concluded that the load transfer from polymer to nanotube was not sufficient to fracture the nanotubes. In the same time, a lot of kinked MWNTs were observed, which were believed to be plastically deformed. 

Laterally injected dikes of distinct composition Fractured and tilted dikes of distinct composition... 

From these results, it is clearly shown that particle size is one of the major parameters controlling the CaC03/PP composite fracture toughness. Surface treatment contributes to better particle dispersion, and also to lower particle - matrix interaction [24]. 

Effect of Diamine Spacer Length on Composite Fracture Toughness... 

Figure 4. SEM micrographs of epoxy/Al20 composite fracture surface (a) D230 AA18 50% and (b) crack pinning tails in DGEBA/D230 with 10% AA10 AI2O3.

Scanning Electron Microscopy (SEM) with magnification capability of up to 20,000X was used for the study of the polymer composite fracture surfaces. The freshly fractured surfaces were coated with a 250 thick Au-Pd conducting layer. 

Figure 9 Composite fracture toughness test modes (a = Mode I, b=Mode II).

In some cases, a multifilamentary NbsSn conductor appears undamaged at strains far greater than those that degrade L. In other cases, the entire composite fractures at 0.6 to 1.5% strain. The volume percentage (v/o) of the brittle NbaSn phase in multifilamentary NbsSn superconductors is the major factor in determining the ductility of the overall composite, according to McDougall [ ] and Old and Charlesworth [%... 

Hristov V. N. Krumova M. Vasileva St. Michler G.H. Modified polypropylene wood flour composites. Fracture deformation and mechanical properties. Journal of Applied Polymer Science, vol.92, No.2, (2004), pp. 1286-1292... 

Aside from the work performed at NASA Ames, recent work by Levine et al., at NASA Glenn, has been conducted to improve the oxidation resistance of monolithic materials and to develop fiber reinforced UHTC materials to improve composite fracture toughness and impart a level of graceful failure in materials. 

FIGURE 26. Almax/Al203 composite fracture surface at RT. 

---