Solid body fracture mechanism

Percussion is a repetitive solid body impact, such as experienced by print hammers in high-speed electromechanical applications and high asperities of the surfaces in a gas bearing. Repeated impacts result in progressive loss of solid material. Percussive wear occurs by hybrid wear mechanisms, which combine several of the following mechanisms adhesive, abrasive, surface fatigue, fracture, and tribochemical wear.75... 

The second approach, due to lrwin is to characterise the stress field surrounding a crack in a stressed body by a stress-field parameter K (the stress intensity factor ). Fracture is then supposed to occur when K achieves a critical value K - Although, like Griffith s equation, this formulation of fracture mechanics is based on the assumptions of linear elasticity, it is found to work quite effectively provided that inelastic deformations are limited to a small zone around the crack tip. Like, however, the critical parameter remains an empirical quantity it cannot be predicted or related explicitly to the hysical properties of the solid. Like,, K. is time and temperature de ndent. 

Under the conditions of mechanical solicitation, the tensions are mainly concentrated on the structural faults, therefore in the transition and surface layers. With other words, the most dangerous zone of a solid body is located just at its surface, where the mechano-cracking and fracture processes can be activated with higher rate as in body s volume. 

Over the years, numerous versatile studies in the area of surface damageability have been carried out and significant progress has been achieved in the elucidation of the mechanisms and trends involved in fracture and stability loss in solid bodies of any nature and structure. Critical and priority studies were carried out by Shchukin in collaboration with Savenko, Kochanova, and Kuchumova [44-46,59,68,70]. 

As it has been noted in chapter five, the local plasticity zone type defines the fracture type if a craze forms at critical defect tip, then polymer failed quasibrittle and if deformation zone (ZD) or local shear yielding zone ( shear lips ) - then quasiductile [13]. The inelastic deformation mechanism change is considered as brittle-ductile transition [14]. The treatment of the indicated tiarrsition will be considered below within the frameworks of both cluster model and solid body synergetics. 

The scope of the series covers the entire spectrum of solid mechanics. Thus it includes the foundation of mechanics variational formulations computational mechanics statics, kinematics and dynamics of rigid and elastic bodies vibrations of solids and structures dynamical systems and chaos the theories of elasticity, plasticity and viscoelasticity composite materials rods, beams, shells and membranes structural control and stability soils, rocks and geomechanics fracture tribology experimental mechanics biomechanics and machine design. 

This approach is the most useful for engineering purposes since it expresses fracture events in terms of equations containing measurable parameters such as stress, strain and linear dimensions. It treats a body as a mechanical continuum rather than an assembly of atoms or molecules. However, our discussion can begin with the atomic assembly as the following argument will show. If a solid is subjected to a uniform tensile stress, its interatomic bonds will deform until the forces of atomic cohesion balance the applied forces. Interatomic potential energies have the form shown in Fig. 1 and consequently the interatomic force, whidi is the differential of energy with respect to linear separation, must pass throt a maximum value at the point of inflection, P in Fig. 1. 

Contact mechanics is both an old and a modern field. Its classical domains of application are adhesion, friction, and fracture. Clearly, the relevance of the field for technical devices is enormous. Systematic strategies to control friction and adhesion between solid surfaces have been known since the stone age [1]. In modern times, the ground for systematic studies was laid in 1881 by Hertz in his seminal paper on the contact between soHd elastic bodies [2]. Hertz considers a sphere-plate contact. Solving the equations of continuum elasticity, he finds that the vertical force, F , is proportional to where S is the indentation. The sphere-plate contact forms a nonlinear spring with a differential spring constant k = dF/dS oc The nonhnearity occurs because there is a concentration of stress at the point of contact. Such stress concentrations - and the ensuing mechanical nonhnearities - are typical of contact mechanics. 

Simultaneously, processes of plastic deformation, fracture and interactions with the environment, and counterbody can occur. The latter ones have been studied by mechanical engineers and tribologists, but the processes of phase transformations at the sharp contact have been investigated for only a few materials (primarily, semiconductors) and the data obtained so far can only be considered preliminary. One of the reasons for the lack of information may be the fact that the problem is at the interface between at least three scientific fields, that is, materials science, mechanics, and solid state physics. Thus, an interdisciplinary approach is required to solve this problem and understand how and why a nonhydrostatic (shear) stress in the two-body contact can drive phase transformations in materials. 

Mercury porosimetry (or intrusion) Measurement of the specific porous volume and of the pore size distribution function by applying a continuous increasing pressure oti liquid mercury such that an immersed or submerged porous solid is penetrated by mercury. If the porous body can withstand the pressure without fracture the Washburn equation, relating capillary pressure to capiUaiy diameter allows converting the pressure penetration curves into a size distribution curve. If a sample is contracted without mercury intrusion, a specific mechanical model based on the buckling theory must be used... 

Adhesion is the interaction that develops between two dissimilar bodies when they are contacted. Adhesion is thus a multidisciplinary science dealing with the chemistry and physics of surfaces and interfaces as well as the mechanics of deformation and fracture of adhesive joints. In this overview, these various aspects of adhesion are discussed. We begin by describing the general types of adhesive bonds. This is followed by sections on solid surfaces and their characterization, interfacial properties, surface treatment, and finally a discussion of the mechanics of adhesive joints. 

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