Defect-induced fracture

Much clinical and experimental experience has been obtained about the manifestation of bone diseases, especially in renal patients. Many patients with Al-induced bone disease remain asymptomatic. There are two distinct forms of Al bone disease. The most severe form is osteomalacia, with recurrent fractures and resistance to vitamin D therapy. This disease is characterized by an increase of osteoid due to a mineralization defect induced by Al that is localized at a critical site in the bone, i.e., the osteoid calcification front [250]. The adynamic bone disease is another form of Al-related bone disease, characterized by a reduced bone turnover [97]. Al can have a direct negative effect on the bone by deposition at the mineralization front, causing a defective calcification. This is due to the influence of Al on calcium-phosphorus precipitation, crystal formation and crystal growth [251]. There might also be a toxic effect on the proliferation of osteoblasts and on mature osteoblasts with a time- and dose-dependent effect on osteoblast growth and function [143]. 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

Three modes are clearly defined for crack propagation from a very thin (radius of the order of 10 gm) notch-machined in the specimen (Fig. 12.3). This notch induces a stress concentration effect, higher than those produced by all the other defects already present in the specimen, which governs the fracture initiation. For isotropic materials, mode I (the most severe) is generally used and gives the lowest value of toughness. In the case of adhesives and laminates, modes II and III are also performed. 

For a given strain rate, e, the temperature domain can be divided into two regions. At low temperatures, the fracture is brittle with a nearly constant stress, chain mobility and the fracture occurs from stress concentration at the defects, leading to the formation of holes and then crack propagation. 

This paper presents results from a study of assemblies composed of glass fibre reinforced epoxy composites. First, tests performed to produce mixed mode fracture envelopes are presented. Then results from tests on lap shear and L-stiffener specimens are given. These enabled failure mechanisms to be examined in more detail using an image analysis technique to quantify local strain fields. Finally the application of a fracture-mechanics-based analysis to predict the failure loads of top-hat stiffeners with and without implanted bond-line defects is described. Correlation between test results and predictions is reasonable, but special attention is needed to account for size effects and micro-structural variations induced by the assembly process. 

There are four principle causes for membrane failure under normal operating conditions (in other words, reduced membrane durability). These reasons are (a) hydrogen-induced embrittlement of the membrane, (b) fatigue fracture due to repetitive swelling and contraction of the membrane, (c) mismatch in the CTE of the membrane and underlying support layer, and (d) defects in the underlying support layer that cause a hole or tear to develop in the membrane. 

Another feature of the influence of structure of a solid on the intensity of adsorption-induced effects is related to the excessive free energy of defects. The example of such energy excess is the energy of grain boundaries in polycrystalline objects (see Chapter 1,2). This energy, related to the defects in the structure and stored within the object, leads to a thermodynamically more favorable development of primary cracks along such defects upon a contact of solid with adsorption-active medium. Under normal conditions the fracture of polycrystalline object takes place mainly across the body of grains, while in... 

One more feature of structural defects that determines their role in the adsorption-induced strength decrease is that in most cases the penetration of liquid phase specifically along the defects facilitates the delivery of adsorption-active medium into a pre-fracture zone, and thus allows the medium to influence the development of cracks. In this sense the role of structural defects is closely related to the role of conditions under which deformation and fracture takes place. With respect to discussed case, these are the conditions under which penetration of active medium into the zone of crack formation and development takes place. 

---