Material characteristics fractures

On one hand, inherent flaws or perturbations in a fracturing body, which are the sites of internal fracture nucleation, have been recognized as important in determining characteristic fracture spacing and, consequently, the nominal fragment size in a fracture event. Theoretical work based on a physical description of these material imperfections has been actively pursued (Curran et al., 1977 Grady and Kipp, 1980). 

Although epoxies dominate the thermoset fracture literature, work has been reported on other systems, e.g., polyester resins, phenol-formaldehyde compounds, peroxide cured polystyrene, and highly crosslinked polyurethanes. In general, these materials exhibit fracture behaviors similar to epoxies, and suggest that thermosets, as a class of materials, display characteristic crack growth properties. 

The geometry-independent material characteristic Gic as a function of the initial crack opening displacement (ICOD) is introduced. Typical values of the delamination fracture toughness of 3D glass and carbon composites are presented. 

The delamination properties of laminated composites are very important material characteristics. The fracture toughness of regular laminated composites has rather low values (the critical delamination energy release rate G c in mode I usually does not exceed 0.2-0.4 kJ/m ). One way of improving the delamination toughness consists in introducing a small amount of fibers in the thickness direction of plates. 

The additional parameter — the crack opening displacement at the initial precrack (ICOD) — was measured to obtain the bridging law, which is a material characteristic independent of the specimen geometry. It is recommended to use the graphs Gic (ICOD) instead of the traditional / -curves Gic(Afl) for characterizing the delamination fracture resistance of composites with extensive bridging. 

We must recognize, however, that a tear that begins to propagate from an initial cut or flaw will soon develop a characteristic tip radius r of its own, independent of the sharpness of the initiating stress raiser (Thomas, 1958). It is therefore more appropriate to treat the product r as a characteristic fracture property of the material. Indeed, Irwin (1948, 1957) proposed that fracture occurs for different shapes of test piece and under varied loading conditions at a characteristic value of a stress intensity factor, Kc, defined as... 

Laser emission efficiency has a strong dependence on the distribution of the de-excitation processes. It has been proved that proper management of the pump and laser mode volumes is crucial to achieve high CW laser efficiency, reduce the parasitic luminescence, and decrease the heat generation, which can result in the loss of excitation by ASE, distortion of the beam, and mechanical fracture. These properties are also important for the design of cooling systems and evaluation of material characteristics, e.g., emission quantum efficiency from thermal lensing data [1, 11]. 

The energetic criterion, according to which the crack development into a mechanically solicited composite releases a certain amount of elastic energy that is stocked in material and can be evaluated. When its level attains a critical value, G, the cracks will propagate until the material s fracture. The critical energy of propagation represents an intricate characteristic of material, and in the case of brittle fracture, the Griffith criterion can be ex-... 

Cracks may be characterized by their width, length and pattern. The opening of a crack is always related to the creation of new free surface in the material, where free means that the surface is not loaded by any stress. An amount of energy is required to create a unit area of a new free surface of a crack. It is considered to be an important material characteristic and is called specific fracture energy, which is difficult to directly measure, but may be calculated in an approximate way when the amount of released energy and crack dimensions are estimated and considered to be known. 

Probably the idea to use a dimensionless magnitude called the fracture index as a possible material characteristic was first published by Henager (1978). The test beam standardized by ASTM of cross-section 102x102 mm and of 305 mm span was used. It was loaded in its centre up to the total deflection of 1.9 mm. The toughness index (TT) was calculated as the ratio of area A+B to area A as it is shown in Figure 10.28. For plain concrete or mortar the value of TI is 1.0 or a little more, and for strongly reinforced fibre composite it may reach values of 30 or even 46. 

Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. They occur when melt flow fronts collide in a mold cavity. Material characteristics can affect the knitting of the melt fracture. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of part strength. 

Thus the FCGR becomes infinite as K ax approaches the toughness of the material Kc, which actually means final fracture, Stage III of fatigue. Note that in the Foreman Eq. (10.13) A and m are material characteristic whose value is different from C and n of the Paris law (10.10). Weertman [11] proposed an alternative equation for Regions II and III... 

Failure in ceramics and refractories was discussed in Section I. Here, we shall consider the mechanical fracture resulting from the application of a load. Because ceramics are brittle materials, the fracture is fast. Fast fracture is a characteristic of brittleness. This type of fracture is sudden and occurs without any warning such as plastic deformation. 

---