Segmentation cracks

The vertical segmentation cracks improve the in-plane strain tolerance of the TBC and are at present deliberately introduced in some of the advanced industrial coatings. Contrary, delamination is highly unfavourable since large scale spallation of the TBC would terminate the service life of the component. Accordingly, any further improvement of the plasma sprayed TBCs with respect to easier segmentation and more difficult delamination requires in-depth understanding of the fracture... 

The compliance functions (dC/da) used in the present study for segmentation crack growth in stand-alone TBCs referred to Moon et al. (4-point bending) and Knehans et al. (notched, 3-point bending). 

Resistance of the segmentation crack in the attached TBC of the composite specimen was derived from the general relationship for stress intensity... 

The two modes of macro-cracking that occur in bending tests in case of large specimen deformation with attached TBC are shown in Fig. 2. With the TBC under tension almost equidistant segmentation cracks develop (Fig. 2a). The ratio of crack distance to TBC thickness is about E k, 2.25 which is in fair agreement with... 

Micrographs of segmentation cracks in the three tested bending/material variants are shown in Fig. 3. In each case the segmentation crack exhibits a tortuous crack path. 

Figure 3. Controlled propagation of segmentation cracks in bending tests with attached (left) and stand alone TBC.

Figure 4. Influence of compressive residual stress on resistance behaviour of segmentation cracks in attached TBC.

Figure 5. R curve results for segmentation cracks in attached and stand alone TBCs. For comparison R curve of attached TBC corrected from the influence of residual stress.

Fig. 9 displays the side-face of a notched stand-alone TBC specimen before loading and after growth of the segmentation crack. Obviously the macro-crack selects and combines essentially the suitable segments from the pre-existing pattern of inter- and intra-splat cracks and thus the segmentation crack path comprises of segments of these micro-defects . ... 

Examples of microstructural crack deflection are shown in Fig. 10 for the different crack orientations. In Fig. 10a an intact spraying splat deflects a segmentation crack, in Fig. 10b the delamination crack bounces between the two interfaces with the adjacent splats, leaving a serrated fracture surface. 

Implications of cracks surface contact are observed also on a larger scale in the crack wake. The crack bridging yields formation of a new side path behind the actual tip of a segmentation crack. Obviously the tortuous crack surfaces provide sufficient contact bridging to shield the apparent crack tip, a toughening mechanism frequently observed in ceramics with R-curve behaviour . 

Figure 9. Tortuous segmentation crack path. Macro crack follows pre existing pattern of intra and inter splat micro cracks.

Figure 10. Crack deflection mechanisms in TBC. a) Segmentation crack deflects on intact spraying splat, b) Delamination crack bounces between surfaces of thin spray splat. Crack propagation from top to bottom.

However, it should be emphasised that grey scale analysis in the vicinity of segmentation cracks also revealed displacement activities caused by side cracks and crack branching, depending on the heterogeneities in the lamellar TBC microstruc-ture. 

Thus it seems premature to further quantify the measured R-curve behaviour and to describe the toughening effect in a model. Yet an important conclusion can be drawn from the present study. Tailoring plasma sprayed TBCs with low crack resistance for segmentation cracks and high delamination resistance should focus on a reduction of the density of inter-splat mieroeraeks. Variation of the spraying parameters has the potential to systematieally ehange APS-TBCs with respect to the ratio of inter- and intra-splat defects in the lamellar microstructure. 

Because of the inherent instability of PTW, two skills that riders must acquire are acute hazard perception and quick appropriate responses to emerging hazards. Small objects on the road, potholes, pavement deformations, spaces between adjoining pavement segments, cracks in asphalt and concrete, oils spills, loose gravel, and puddles that may be inconsequential to car drivers can be life-threatening hazards to the motorcyclists, and must be avoided at all costs. For example, as part of a motorcycle crash risk study conducted in Australia, Haworth et al. (1997), revisited the same sites where 206 motorcycles crashed and noticed that in 14 percent of the crashes unclean road and loose material on the road probably contributed significantly to... 

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