Controlled fracture

Meitl, M. A. et al. 2007. Stress focusing for controlled fracture in microelectro-mechanical systems. Appl. Phys. Lett. 90 083110. 

Materials Description. Three CIBA-GEIGY epoxy/hardener systems were studied Araldite 6010/906, Araldite 6010/HY 917 and Araldite 6010/972 with stoichiometries 100/80, 100/80 and 100/27, respectively. Araldite 6010 was a DGEBA epoxy resin. The hardeners 906, HY 917 and 972 were, respectively, methyl nadic anhydride (MNA), methyltetrahydro phthalic anhydride (MTPHA) and methylene dianiline (MDA). These systems were investigated previously for the matrix controlled fracture in composites (6-8). The curing cycles used can be found in (6). The ideal chemical structures of the systems are shown in Table I. Neat resins were thoroughly degassed and cast into 1.27 cm thick plates for preparation of test specimens. 

Fig. 2 shows a horizontal cross-section through NGFs coupled shear flow test (CSFT) cell. This biaxial cell allows fractured samples (14 x 12 x 5 cm) to be displaced by a maximum 8 mm under controlled fracture normal stresses and temperatures up to 80°C. Variation in fracture aperture (dilation) is measured by a total of four displacement gages, whereas shear displacement is measured by a total of two gages. The flow measurements, as illustrated in Fig. 2, measure the total flow in the direction of the fracture plane and do not differentiate between fracture and matrix flow. To quantify the matrix flow component, tests on non-fractured samples with the same principal geometry as the fractured tests and under the same stress conditions, were conducted in the CSFT cell. 

Based on the results presented, the following factors controlling fracture flow and bulk flow have been identified ... 

Through disk-bend testing on a series of ZrOj/Ni composite specimens fabricated by powder processing, we have examined the fracture behavior of ceramic/metal composites under an equibiaxial plane-stress loading, and derived, by making a micromechanical analysis of elastoplastic stress states, a brittle phase-controlled fracture criterion of the form, ( )max const., in terms of the equivalent normal stress a. This criterion is conceptually simple and quite useful particularly for our micromechanics-based approach to the FGM architecture. 

The fact that polished fibers, and fresh fibers, were stronger suggested that the defects were predominately surface Haws (scratches, etc.), and confirmed the concept of defect-controlled fracture. 

Brittle fracture is used for shaping and machining ceramics after they have been fired. Ceramics can be modified to make them machinable this is controlled fracture and is the approach we adopt with machinable glass-ceramics such as Macor (Chapter 26). Of course, many ceramics already are machinable and can be shaped into intricate and beautiful forms as illustrated in the carved marble sculpture shown in Figure 18.1. 

Molecular-scission-controlled fracture of oriented semi-crystalline poiymers... 

Healing efficiency was also measured by carefully controlled fracture experiments for both the virgin and the healed materials using a well established protocol [5]. 

G. Demazeau, New Synthetic Mica-like Materials for Controlling Fracture in Ceramic Matrix Composites, Mat. Tech., 1043-58(1995). 

FIGURE 6. Fracture mechanisms in porous matrix CMCs as a response of the non-isotropic microstiuctures left matrix-controlled delamination right fiber-controlled fracture. 

Associated with the development of the Master Curve concept, which is discussed in Section 10.3.4, studies have concluded that it is possible to develop correlations describing a relationship between crack initiation and arrest toughness (Wallin, 2001). These studies have focused on clarifying which elements of the Master Curve approach should be modified for assessing crack arrest and finding possible correlations between initiation and arrest parameters. Due to different mechanisms and differences in factors controlling fracture initiation and arrest events (e.g. the local properties are crucial for crack initiation, but not so critical for crack arrest), the weakest link theory applied in the Master Curve approach is not directly suitable for crack arrest. 

S. J. Glass, E. K. Beauchamp, C. S. Newton, R. G. Stone, W. N. Sullivan, R. T. Reese, S. D. Nieolaysen, and R. J. Kipp, Controlled Fracture of Ion Exehanged Glass Rupture Disk, Sandia National Laboratories Report, SAND2000 0828 (2000)... 

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