Fracture mirror

A crack in a ceramic material upon initiation may accelerate and interact with microstructure, stress field and the generated acoustic vibrations. These interactions may lead to some fractographic features such as fracture mirror, hackle or river patterns and Wallner lines. 

Some typical features of fracture origin such as internal or surface crack are shown in Figure 2.32. The fracture mirror is a smooth surface formed when an internal crack travels radially outwards upon acceleration. Then it starts to deviate from the original plane upon reaching a critical velocity, and intersects with the microstructural features such as inclusion or precipitate. This may also occur as the direction of principal tensile... 

Characteristic strength, Sc (GPa) Fracture mirrors Pull-out length, h 1.2-3.0... 

Fig. 1.20 A schematic indicating a fracture mirror and the dimension used to predict the in situ strength.

Some fibers exhibit fracture mirrors when they fail within a composite (e.g., Nicalon). A semi-empirical calibration has been developed that relates the mirror radius, am, to the in situ fiber tensile strength, S, given by (Fig. 1.20),... 

Fig. 1.21 In situ strength distributions measured for Nicalon fibers on three CMCs, using the fracture mirror approach.

A problem in implementing the fracture mirror approach arises when a significant fraction of the fibers does not exhibit well-defined mirrors. Those fibers that do not have mirrors usually have a smooth fracture surface. It has thus been assumed that these are the weakest fibers in the distribution.61,78 The order statistics used to determine G(S) are adjusted accordingly. This assumption has not been validated. 

BS mm physical crack length augmented to account for crack tip plastic deformation (fracture mirror length)... 

With the geometrical details of the specimens, including the initial crack length a, the physical crack length augmented to account for crack tip plastic deformation (the fracture mirror length) aBs> the dynamic yield stress o-y and the dynamic flexural modulus E, the fracture mechanics parameters Ki, Ju and can be calculated [OlGre]. 

As noted in the introduction, it is important to identiiy the causes of fracture so that manufacturers, clinicians, and laboratory technicians can minimize the incidence of breakage. Identification of the mechanisms of fracture can help researchers can develop relevant testing procedures. Table 1 shows that there were multiple causes of failure including faulty design of the restoration, faulty preparation, or damage from clinician handhng or adjustments. There were no fracture mirrors, so all the fractures occurred at relatively low stress levels. 

Figure12.1 Fracturesurfaceofasilicon nitride valve fractured in a rupture test, (a) Macroscopic view the crack path is perpendicular to the loading direction (b) Fracture mirror, mist...

This type of fracture can be found in both very large and very small systems, including fractured natural rocks, in tested specimens of advanced ceramic materials, and even in broken ceramic fibers. The fracture mirror size measurements can be used for post-mortem determination of the failure stress in components [21]. 

In this chapter, a dose examination has been made of the phenomenon of fracture in ceramics. The macroscopic appearance of fracture and typical failure modes in ceramic materials has been analyzed, fracture mirrors and fracture origins have been identified, and the way in which fracture is intrinsically connected to the microstructure of a ceramic has been outlined. In particular, by detailing stress distributions it has been shown that fracture always starts at a single microstructural flaw, the stability of which can be described with simple linear elastic fracture mechanics. Notably, these features are responsible for the inherently statistical nature of failure in ceramic materials, an understanding of which can provide knowledge of the close corrdation between defect populations and fracture statistics, and of how to devdop materials parameters such as the characteristic strength. 

Fig. 6. Low-rnagnificnlion SEM photo of AI2O1/Y3AI5O12 eutectic fiber showing the overall fracture and its complexity (top left). Higher-magnification photo of the fracture mirror area and failure-initialing flaw wider bands of YjAlsOn (top right). Different morphologies have been observed along the fiber length. Two examples are shown at the bottom.

Figure 9. Ring theta specimen, bottom wail fracture. This is the same piece as above, but the origin is an etch pit on the inside surface at the base of the ring wall. A well-defined cathedral fracture mirror is centered on the origin.

Figure 12. Origin sites in two ring theta specimens. They have cathedral fracture mirrors centered on etch pit flaws, (b) is a close-up of (a), and (d) is a close-up of (c). Note how the origins are at the end ofa string of etch pits.

Fracture Mirror. A very smooth, reflective fracture surface indicating that the fracture was moving relatively slowly along a single spreading front. 

Figure 10. Matching fracture surfaces for a single SRM 2100 test specimen. The specimen was first tested by the SCF method and produced the fracture surface on the right. It is common to put the semielliptical surface flaw into the wider face. The precrack is just barely visible (arrow) at this magnification within the fracture mirror. This fracture surface has all the normal characteristics of a strength test fracture surface. One half of the broken specimen was retested in the SEPB configuration as shown on the left. The precrack extends half way into the test piece. The lower facture load for the SEPB configuration creates a much soother fracture surface.

In order to review the calculation, tests were performed with circular disks and squares made from borosilicate glass and various glass ceramic materials [3.34]. As shown in Fig. 3.24, the disks were heated in a concentric circular area and heated up to fracture. The fracture mirror showed the type of stress (radial, tangential) and the distance of the maximum tensile stress from the centre of the plates. The magnitude of the tensile stresses was determined... 

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