Stable crack propagation

Elastic energy release due to subcritical crack growth is one recognized source of structure-related AE within its acknowledged lunitations, AEBIL provides a viable means of early on-line deteetion and localization of stable crack propagation. 

Various types of crack propagation, namely unstable, partially stable, and fully stable can be observed during SENB tests [85,131]. Solvent-modified epoxy networks prepared via CIPS with 13-16 wt % and 22 wt % cyclohexane show unstable crack propagation. For solvent-concentrations of 18 wt % and 20 wt % cyclohexane, a partially stable crack propagation is observed. The amount of energy consumption upon crack propagation is only 10% and 14% respectively for these two compositions. The change in the fracture behavior indicates that the... 

It is worth noting that most of the fracture characteristics reported in this paper correspond to stable crack propagation. 

A ductile behavior accounting for intense shear yielding. It is characterized by stable crack propagation, entire stress-whitening of the fracture surface and is accompanied by more or less pronounced shear lips. 

For AK = 0.7 MPa. s/m (Fig. 23b), after the injection of IPA crack propagation speeds up twice at point B and point D. In between, the increase in a IW is negligibly small. The stable crack propagation after point E is again slower than in air. Failure occurs at about 4 x 105 cycles. 

Fig. 17 Contact mechanics analysis of Herztian cracks within brittle materials.a Schematic description of a Hertzian cone crack induced under normal indentation by a rigid sphere, b Reduced plot of JC-field as function of cone crack length and for increasing loads pf < p// < pm during sphere-on-flat normal indentation of brittle materials. Arrowed segments denote stage of stable ring crack extension from Cf to cc (initiation), then unstable to ci at P = P,n (cone-crack pop-in) (From [67]). Branches (1) and (3) correspond to unstable crack propagation (dK/dc > 0), branches (2) and (4) to stable crack propagation (dK/dc < 0)...

It is worth noting that this semi-ductile behavior has been found in other polymers Newmann and Williams [4] observed stable crack propagation before brittle fracture in ABS over the temperature range from —40 to 0°C Bernal and Frontini also observed this type of behavior in a rubber-modified thermoplastic at room temperature [12]. 

Figure 9a shows a typical load-loadline displacement curve of a nylon compact tensiiin specimen. The slight deviation from linear elastic behaviour prior to fracture relates to the presence of a plastic deformation zone confined near the pre-crack tip (Fig. 9b). This stable crack propagation domain is adjoining a wide hackle zone (rapid crack growth domain) characteristic of a brittle fracture.

Whatever the concentration of fibres and the state of fibre/matrix interface, stable crack propagation is obtained up to total failure. With type A interface, increasing the fibres concentration results in a loss of resilience by simultaneous reduction of the peak load and the ultimate displacement. 

The presence of the rubber considerably enhances the crack initiation toughness, especially at the lower fibre concentrations, and increases noticeably the reliability owing to the stable crack propagation thus induced. 

FIGURE 16.10 Illustration of unstable and stable crack propagation for a brittle material in tension (T) and compression (C), respectiveiy. Stabie crack propagation wiii iead to crushing. 

Testing of Plastics - Instrumented Charpy impact test Procedure for determining the crack resistance behaviour using the instrumented impact test Part I Determination of characteristic fracture mechanics parameters for resistance against unstable crack propagation Part II Determination of characteristic fracture mechanics parameters for resistance against stable crack propagation... 

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