Ductile mechanical models

Ductile mechanical models. Stress concentrations at the base of corrosion slots or pits increase to the point of ductile formation or fracture. 

While the approach described above works reasonably well for the polyester and epoxy matrix resins which do not show extensive ductility there is clearly a need for an alternative approach when very ductile stmctural adhesives are applied. The development of extensive damage zones can be treated more efficiently using damage mechanics models and these are now being examined. 

Lemaitre J (1985), Continuous Damage Mechanics Model for Ductile Fracture. Journal of Engineering Materials and Technology, 107, 83-89. 

Dasgupta, A. and Hu, l.M. 1992b. Failure mechanism models for ductile fi acture. IEEE Trans, on Reliability... 

Elastic materials deform linearly according to Hooke s law, and as they are stretched their constituent molecules become displaced from their equilibrium positions. A simple mechanical model can be used to represent the behavior of the material as a lattice of balls connected by springs (Figure 1.18). This elastic behavior holds until the applied force is large enough that the material begins to deform irreversibly. Ductile materials exhibit a plastic deformation regime at stresses above the yield stress, whereas a brittle material will fracture at this point. 

Theoretical analyses of interfacial debonding and frictional pull-out in the fiber pull-out test were initially modeled for ductile matrices (e.g. tungsten wire-copper matrix (Kelly and Tyson, 1965, Kelly, 1966)) assuming a uniform IFSS. Based on the matrix yielding over the entire embedded fiber length, as a predominant failure mechanism at the interface region, a simple force balance shown in Fig. 4.19 gives the fiber pull-out stress, which varies directly proportionally to the cylindrical surface area of the fiber... 

The stress-strain curves for cortical bones at various strain rates are shown in Figure 5.130. The mechanical behavior is as expected from a composite of linear elastic ceramic reinforcement (HA) and a compliant, ductile polymer matrix (collagen). In fact, the tensile modulus values for bone can be modeled to within a factor of two by a rule-of-mixtures calculation on the basis of a 0.5 volume fraction HA-reinforced... 

Abstract The fracture properties and microdeformation behaviour and their correlation with structure in commercial bulk polyolefins are reviewed. Emphasis is on crack-tip deformation mechanisms and on regimes of direct practical interest, namely slow crack growth in polyethylene and high-speed ductile-brittle transitions in isotactic polypropylene. Recent fracture studies of reaction-bonded interfaces are also briefly considered, these representing promising model systems for the investigation of the relationship between the fundamental mechanisms of crack-tip deformation and fracture and molecular structure. 

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

Product particle size distributions of impact ground thermoplastics (specifically PETP and PVC) are interpreted and models describing these distributions developed. Results from multiple and single particle breakage in a hammer mill are used. The values of the model coefficients are related to the brittle-ductile transition grinding conditions and breakage mechanisms. Results are relevant to the separation of thermoplastics, as for example, is required when recycling consumer products such as bottles. 3 refs. 

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

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