13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- cohesive fracture through the porcelain layer is in agreement with literature that crack initiation within the veneered porcelain layer is the dominant failure mode for both metal coping and all-ceramic restorations [14]. Fig. 4(b) illustrates a region of oblique fracture in porcelain that propagated towards and along the interface of the underlying zirconia structure. Further confirmation was sought by EDX analysis. It is apparent that the top left region is Si-rich (porcelain), while the bottom right region is rich in zirconium (core). (a) (b) Figure 4 Typical two fracture modes on the surface of restoration. (a) Overall micrograph of chipping failure; (b) An oblique fracture in porcelain that propagated towards and along the interface with the underlying zirconia core structure 3.4 Crack initiation and propagation Fractographic analysis was used to determine the crack propagation direction and the crack initiation location by identifying certain common features, namely “wake hackle” and “arrest lines” (identified below in Fig. 5) [15,16]. These were found at the fractured surfaces within the samples investigated in this report, and were used to trace the crack propagation back to the fracture origin. Wake hackle is generally formed when a crack passes a discontinuity, e.g. void. It is associated with the high stress field experienced by the void as the crack propagates close to it. This usually causes crack re-initiation at a point that lies slightly above or below the major crack plane. However, the fastest moving major crack eventually catches up with this new propagating micro-crack, and the latter becomes incorporated in the master crack. Meanwhile, a tell-tale surface hollow or elevated region is left behind, which is referred to as the “wake hackle”, pointing in the direction of crack propagation. Arrest line is another indicator of the crack propagation direction. It is approximately perpendicular to the general direction of propagation. Since the growing crack expands in different directions during growth, the crack origin lies behind the concave side of the crack front. Hence, the fracture origin can be traced back to the approximate center of expanding arrest lines family observed at the fracture surface [17]. Fig. 5 shows the surface of a porcelain-initiated fracture in the chipping on the surface of prosthesis #2, with crack initiated at defects in the veneer located at or close to the occlusal surface. The region indicated by the rectangular white box in Fig. 5(a) is shown at higher magnification in Fig. 5(b), where the wake hackle and arrest line features are identified. Fractographic analysis shows that crack may progress from two origins (Fig. 5(a)) lying on the left of the picture, within the porcelain veneer layer and at or close to the surface. Hence we draw the conclusion that fracture initiated close or at the veneer surface, and propagated across the unit and through the interface to cause final failure. Meanwhile, material aspects (coefficients of thermal expansion, thermal conductivity mismatch, phase transformation [18,19]) of porcelain close to the surface may give rise to residual stresses throughout the restoration that likely to promote chipping.
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