Slip-stick propagation

Crack propagation observed in epoxies has also been described as smooth crack growth and slip stick growth. Smooth crack growth is more often observed at high strain rates and lower temperatures. Slip stick crack propagation is indicative of... 

Upon unloading the joint, the force-displacement trace not returning to the origin may indicate that plastic deformation of the substrates has occurred see ISO (2009) for full procedures. Such an occurrence would invalidate the values of Qc determined. The presence of stick-slip crack propagation in the continuously loaded BS or ISO methods requires some special considerations to be taken into account. Again, fiill details are provided in ISO (2009). Under these conditions, steady-state propagation values of Qc are not determined, instead values at crack initiation and crack arrest are obtained. 

Fig. 2 a and b. Schematic load versus displacement curves for crack propagation in epoxy polymers obtained using a double torsion specimen a Stable continuous propagation, b Unstable stick/slip propagation... 

LEFM, for fractures occurring in the elastic domain, leads us to characterize the materials using critical characteristics for crack initiation (or arrest in the case of stick-slip propagation). 

At a constant temperature, the increase of e involves an increase of crack propagation is generally of the stick-slip type (see Fig. 12.4), at low e, and becomes brittle-unstable when e increases (Galy et al., 1994). KIca (for arrest) remains at an almost constant value (Kinloch, 1985). 

At a constant strain rate, the crack propagation is brittle-stable at low temperatures and becomes of the stick-slip type at high temperatures, with an increase of KIci (Kinloch, 1985). 

The changes in crack propagation types (from stable to stick-slip) are associated with the crack blunting mechanism, which is favored by high temperatures and low strain rates, conditions that decrease general trends cannot be extended to very high strain rates because a transition from isothermal to adiabatic conditions may... 

One of the most curious aspects of crack growth in most epoxies is the apparently unstable manner by which propagation occurs, even over wide ranges of temperature and test rate. This behavior is commonly referred to as stick-slip , and is characterized by the crack growing in a series of discrete, unstable jumps. Even some of the earliest works on epoxy fracture report this mode of crack growth. The suspected origins of stick-slip fracture behavior in epoxies is discussed in a subsequent section. Unhke epoxies, thermoplastic polymers, such as poly(methyl methacrylate) and polystyrene, are characterized by stable, continuous crack growth. This mode of fracture sometimes can be observed in epoxies, in particular, when they are tested at fast rates and/or low temperatures. 

Both stable and stick-slip modes of crack propagation are illustrated in Fig. 9, which shows typical load-displacement traces from a testing machine at a constant... 

Unstable delamination growth (stick-slip) has been observed to coincide with jumps from one side of the specimen mid-plane to the other during saw-tooth delamination propagation in the cross-ply laminates. 

Studies of the dynamic crack propagation behaviour of commercial RTPMMA materials [52,53] showed that crack propagation occurred by a stick-slip process. The toughening was attributed to crack-tip blunting by shear yielding. Unfortunately, the use of thin bhmtly-notched specimens means that the stress state in the samples cannot be specified with certain. ... 

Zone 2A - It is characterized by brittle zone with rougher surface compared to PP. Parabolic ridges are seen in the micrographs at farther edge. These ridges show the stick slip zone here the crack propagation is slowed down [102]. 

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