Sharp notch deformation

The brittle-ductile transition temperature depends on the characteristics of the sample such as thickness, surface defects, and the presence of flaws or notches. Increasing the thickness of the sample favors brittle fracture a typical example is polycarbonate at room temperature. The presence of surface defects (scratches) or the introduction of flaws and notches in the sample increases Tg. A polymer that displays ductile behavior at a particular temperature can break in the brittle mode if a notch is made in it examples are PVC and nylon. This type of behavior is explained by analyzing the distribution of stresses in the zone of the notch. When a sample is subjected to a uniaxial tension, a complex state of stresses is created at the tip of the notch and the yield stress brittle behavior known as notch brittleness. Brittle behavior is favored by sharp notches and thick samples where plane strain deformation prevails over plane stress deformation. 

Since the blunt-notched specimen undergoes the same viscoelastic deformation as the sharp-notched specimen, but without crack propagation, the crack initiation time can be identified as the time at which the two curves diverge. Determination of the subsequent crack growth is a somewhat more difficult task direct optical observation was not possible, due to the wrapping bag. An indirect method based on compliance analysis was therefore adopted. 

Deformation curves in Fig. 4 clearly show that materials cannot be considered linearly elastic, and therefore Eq. 2 cannot be used directly in this form. However, by subtracting from the sharp-notched specimen curve the flexural and shear deformation contributions given by the blunt-notched specimen curve, a curve accounting for the crack length contribution to the specimen compliance can be derived from the data of Fig. 4. Eq. 2 can then be rewritten as follows ... 

Figure 10 Fillet of adhesive (A) of amine-toughened urea-formaldehyde adhesive in Southern pine flakeboard showing an arrested crack (B) after 10 vacuum-pressure soak-dry cycles. Note the plastic deformation and blunting at the rounded crack tip and the beginning of new crack growth in the sharp notch at the end of the blunt crack tip (arrow). 

The notch yield ratio, ctbh/o o.i has been accepted as a criterion for evaluating a material s ability to deform plastically in the presence of a sharp notch. This ratio was >2 (Tables III through VI) for all materials, indicating a high level of... 

Analysis of the stress field and deformation behavior in front of the tip of sharp notch or crack allows to calculate principal stresses and estimates the size of the plastic zone ahead of the crack tip. Under assumption that the material near the crack tip is an elastic-plastic continuum, the radius of the plastic zone on the crack plane can be expressed, according to Irwin (1964) ... 

Due to the weakly bonded monazite interface, the nondinear deformation is enhanced and the composites are less sensitive to the presence of sharp notches [90],... 

A very simple explanation of the effect of notching has been given by Orowan [95], For a deep, symmetrical tensile notch, the distribution of stress is identical to that for a flat frictionless punch indenting a plate under conditions of plane strain [102] (Figure 12.31). The compressive stress on the punch required to produce plastic deformation can be shown to be (2 + 7t)K, where K is the shear yield stress. For the Tresca yield criterion the value is l.Sloy and for the von Mises yield criterion the value is 2.82oy, where 0 is the tensile yield stress. Hence for an ideally deep and sharp notch in an infinite solid the plastic constraint raises the yield stress to a value of approximately 2>Oy which leads to the following classification for brittle-ductile behaviour first proposed by Orowan [95] ... 

Lately the large attention is given to macromolecular entanglements fluctuation network influence on crazes formation process in polymers [20], Much less is known about entanglements network influence on shear deformation zones (ZD) formation processes, which by their essence are crazes, without microvoids. The authors of Ref [21] fulfilled quantitative estimation of such influence on deformation processes in ZD on the example of polyarylatesulfone (PASF) samples in the form of films with a sharp notch. In this analysis two models of macromolecular entanglements network were used of binary hooking s [20] and cluster ones [22], The PASF films were prepared with the aid of nine various solvents that allows their structure wide enough variation [21]. 

The size of shear deformation local zone to a great extent defines impact toughness of HDPE samples with a sharp notch [36]. The values k ... 

The studies carried out earlier have shown that polymer film samples strength to a considerable extent is defined by growth parameters of stable crack in local deformation zone (ZD) at a notch tip [1-3], As it has been shown in Refs. [4, 5], the fiactal concept can be used successfully for the similar processes analysis. This concept is used particularly successfully for the relationships between fracture processes on different levels and subjecting fracture material microstructure derivation [5]. This problem is of the interest in one more respect. As it has been shown earlier, both amorphous polymers structure [7] and Griffith crack [4] are fractals. Therefore, the possibility to establish these objects fractal characteristics intercommunication appears. The authors of Refs. [8, 9] consider stable cracks in polyarylatesul-fone (PASF) film samples treatment as fractals and obtain intercommunication of this polymer structure characteristics with samples with sharp notch fracture parameters. 

In Ref [4] it has been shown that increase from I up to 2 by its very nature represents itself the transition from ideally sharp notch D = 1) to cavity = 2), that is, the crack with blunted tip. Thus, the physical significance of fractal model corresponds completely to the offered in Ref. [2] treatment, namely, to the mechanism of notch blunting by local plastic deformation zone. 

Metal deforms elastically at the notch if the applied normal stress (normal to the notch), a orm/ in the longitudinal y direction is relatively low. The longitudinal stress, Oy, distribution in a thin plate with a sharp notch is illustrated in Figure 3.12. A transverse elastic stress, is induced by the notch. The introduction of a, can be understood by imaging a series of small tensile... 

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