Critical crack opening displacement

In the traditional Dugdale model [56], a = Oy and the familiar result is obtained, Gic = cTySc- In the EPZ model, cr exceeds maximum stresses cr in the deformation zone... 

The boundary of the proposed deformation zone is shown in Fig. 7.3 according to Eq. (7.2). The displacement at the crack tip at x = 0 is called crack opening displacement 5 or critical crack opening displacement 8C if the crack is going to propagate. 

A further use for this model is in providing a fracture criterion. Many processes in polymers are governed by strain rather than stress criteria as might be expected from consideration of molecular measurements, and for fracture both indirect and direct measurements confirm that a critical crack opening displacement (COD),... 

To obtain an upper limit for Q0 Oprii is set equal to the failure stress Ojlhrij = Z-efffl, of the continuum fibril structure which gives a failure criterion for the critical crack opening displacement <5 = Sc at the crack tip. Here Z(, is the number of effective connector strands per nominal unit craze area can be less than Z due to the scission of chains during formation of the craze fibrils. 

Polymer Network A/E = 1 [105] Glass transition temperature T/C Molecular mass of strands (experimental) Mc/kg moF1 Critical stress intensity factor (crack initiation) KIC/MPa ]/m Yield stress, taken from Fig. 9 [110] cry/MPa Half crack opening displacement w = Kfc/2E[Pg.347]

The presented results and the additional information taken from various references indicate the direct relevance of the size of the network strands for the crack opening displacement and consequently for the toughness of the polymer. In polymers under load, the molecular chains at the tip of the crack break after the deformation zone ahead of the crack has grown to a critical width 5C, that is the crack opening displacement. This value 5C is proportional to the length of the molecular strands of the network and is linked in this way to the molecular structure of the polymer. However, the molecular mechanism for chain breakage in the deformation zone is not known at present. 

Wells [11] first proposed the crack opening displacement as a measure of fracture toughness for tough metals such as the structural steels where LEFM is not valid. The concept became a standard in 1979 in the form of British Standard (BS) 5762 [12], which is now-superseded by BS 7448 Part 1. 1991. Method of determination of Afi. critical CTOD and critical J values of metallic materials [13], More recently. ASTM E1290-9,3, Crack-tip opening displacement (CTOD) fracture toughness measurement [14] was issued. It is stated that this method may be used to characterize the toughness of materials that arc (a) too ductile or fail the size conditions to be tested for or (b) likely to produce unstable crack extension that would invalidate Ji tests in accordance with the requirements of ASTM E81.3 [15]. 

The linear-slope CDCB specimens shown in Figs. 13 and 14 were used for mode-I fracture tests on FRP-wood bonded interfaces under both dry and wet conditions. A typical specimen under mode-I fracture load is shown in Fig. 15 for phenolic FRP wood interface under dry conditions, and the corresponding critical tip load versus crack opening displacement is plotted in Fig. 16. As an illustration, the fracture surfaces of epoxy FRP-wood with HMR and RF primers on wood substrates are shown in Figs. 17 and 18,... 

If the crack opening displacement exceeds a certain critical value Sc, being characteristic of the material, the crack keeps on growing. It follows for the criterion of stress at break at a plane state of stress ... 

Fracture mechanics is a study of the strength of structures which include flaws such as cracks and voids where stresses can be said to be singular. Fracture mechanics applies criteria to assess whether the conditions are such that failure will occur at these points, one such criterion being the critical energy release rate, G. Other criteria are also available, including crack opening displacement measurements, the J integral, and stress intensity measurements. 

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