Discontinuous crack growth

Fig. 45. Discontinuous crack growth in PVC. [Elinck, J, P Bauwens, J. C., Homes, G., Int. J. Fracture Mech. 7, 277 (1971), Wolters-Noordhoff]...

As a consequence of these results the model of discontinuous crack growth originally derived by Hertzberg et al. and reproduced in Fig. 48 b must be... 

Fig. 51. Schematic representation of optical interference results of continuous craze and discontinuous crack growth in PVC...

Continuous craze growth has to be seen in relation to the discontinuous crack growth looking at a finer scale e.g. at one loading cycle then discontinuous craze growth is to be expected. 

Fig. 52. Influence of AK, on craze widths 2v at the crack tip in PVC during discontinuous crack growth...

When smooth specimens of PS are cycled at 26.7 Hz in a tensile mode at a maximum stress of 17.2 MPa discontinuous crack growth bands similar to those shown in Fig. 8 are seen in the mirror region near the fracture source. These bands are also similar in morphology to those observed in testing notched PS specimens at a minimum stress that is maintained at one-tenth the maximum stress However, DCG bands were not seen when tests are carried out under a fully reversed stress of 17.2 MPa at... 

Fracture surface morphology varies with stress amplitude and test frequency as well as with composition and molecular weight. Even under fully reversed cycling, discontinuous crack growth bands have been observed on the fatigue fracture surface in PS, SAN and ABS but they have not been seen in HIPS. 

Discontinuous crack growth bands were first observed on the fracture surface of PVC specimens subject to cyclic loading by Elinck et al. The effect of molecular weight on band stability has been studied and some of the test results for three different molecular weights are displayed in Fig. 17. As M increases and the number of entanglements per chain rises, the data show that, for any given AK value, a greater... 

BPA Bisphenol-A CTPZ Crack tip plastic zone DCG Discontinuous crack growth FCP Fatigue crack propagation... 

Under normal fatigue crack propagation (FCP) conditions, the crack advances on sach load cycle. Below a critical threshold value, no crack propagation occurs. This is denoted by the threshold value in Fig. 3. Unique to polymers, however, is a cumulative dama process at the fatigue crack tip which enables discontinuous crack growth (DCG), i.e., crack advance after a multiple number of load cycles This DCG process is sub-critical and appears below the threshold... 

The aim of this paper is to consider ESC under not only the physical but also the chemical influence of the environment. In this case the chemical reactions (for example resulting in chain scission) are important and models based on solubility parameters are not valid. Hardly any literature is available on this subject. Moskala [10] reported chemical ESC of poly(ethylene terephthalate) (PET) in an aqueous sodium hydroxide (NaOH) solution. A discontinuous crack growth with an increased crack growth rate was found upon increasing NaOH concentration. However, one major drawback is that the effect of hydrolysis without external stress was not considered. Hydrolysis on its own can potentially lead to premature failures, whether or not the material is in a stressed or unstressed condition. 

On the other hand, it is important to note that discontinuous crack growth under a static load may yield fracture surfaces, which may look quite similar to fatigue striations. To observe this phenomenon, the material has to be able to slowdown or even stop the crack propagation after the static load initiated the... 

It has been proposed that sustained load cracking of titanium alloys in aqueous salt water environments results from the diffusion of hydrogen from the environment into the crack tip region. The increased hydrogen concentration in the hydrostatic stress field ahead of e crack tip results in hydride precipitation and subsequent cleavage fracture associated with the hydrides. Discontinuous crack growth occurs each time the crack moves out of the hydride-affected region and moves into hydride-free material where it is arrested. 

Another commonly encountered feature of mode I DCB testing is the occurrence of nonsteady, discontinuous crack growth behavior. This is usually referred to as stick-slip crack... 

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