Unstable fracture

The authors studied the glassy fracture behavior of the homologous series of DGEBA/DDS networks listed in Table 2. The fracture specimen employed was the double torsion test piece. Fracture data were collected over the temperature range Tg — 120 to Tg — 20 K, and all testing was performed at a single slow crosshead rate of 0.05 cm/min. This test rate was chosen because it minimized hysteretic effects and made all the networks fracture unstably over most of the temperatures investigated. 

When the material exhibits a ductile behavior in impact fracture, unstable fracture does not occur. The crack propagation is generally completely stable. 

Vertical Type C Cl complete Cl.l iliac fracture Unstable (complete rupture of both... 

If we then introduce a flaw into the system, by poking a pin into the inflated balloon, the balloon will explode, and all this energy will be released. The membrane fails by fast fracture, even though well below its yield strength. But if we introduce a flaw of the same dimensions into a system with less energy in it, as when we poke our pin into a partially inflated balloon, the flaw is stable and fast fracture does not occur. Finally, if we blow up the punctured balloon progressively, we eventually reach a pressure at which it suddenly bursts. In other words, we have arrived at a critical balloon pressure at which our pin-sized flaw is just unstable, and fast fracture just occurs. Why is this ... 

In Chapter 13 we showed that, if a material contains a crack, and is sufficiently stressed, the crack becomes unstable and grows - at up to the speed of sound in the material -to cause catastrophically rapid fracture, or fast fracture at a stress less than the yield stress. We were able to quantify this phenomenon and obtained a relationship for the onset of fast fracture... 

Acute coronary syndromes most often result from a physical disruption of the fibrous cap, either frank cap fracture or superficial endothelial erosion, allowing the blood to make contact with the thrombogenic material in the lipid core or the subendothelial region of the intima. This contact initiates the formation of a thrombus, which can lead to a sudden and dramatic blockade of blood flow through the affected artery. If the thrombus is nonocclusive or transient, it may either be clinically silent or manifest as symptoms characteristic of unstable angina. Importantly, if collateral vessels have previously formed, for example, due to chronic ischemia produced by multi vessel disease, even total occlusion of one coronary artery may not lead to an acute myocardial infarction. 

The other mechanism responsible for unstable spinning is the mechanical resistance of a viscoelastic material to rapid deformation. It is a well-known fact that increased yam breaks can be a consequence of spinning speed, which relates to prolonged relaxation time and therefore to break. The fracture can be observed at a maximum extent of deformation under distortion of the covalent bonds. 

In a first testing series, the fracture behavior of the neat, fully crosslinked epoxy network was studied. A fully unstable crack propagation behavior was observed and the critical stress intensity factor, Kj (0.82 MPaxm ), and the critical energy release rate, Gj (0.28 kj/m ), were determined [87]. These are typical values for highly crosslinked epoxy networks prepared with DGEBPA and aromatic or cycloaliphatic diamines. 

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... 

Kait is the so-called fracture toughness parameter. It was first discussed by Griffith [A. A. Griffith (1920)] and describes the mechanical equilibrium of the crack, but not the thermodynamic equilibrium of the unstable crystal. Rewriting the criterion given by Eqn. (14.32) in terms of Eqn. (14.30) one finds... 

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