Crack front

A problem arises in using platelet reinforcements if their naturally mechanically weak crystallographic direction is aligned perpendicular to the crack front. The platelets easily fracture in this orientation. Further research is needed to grow platelets with favorable crystallographic orientations. 

Rather than bearing an infinite stress at the crack tip, yielding occurs resulting in a volume of inelastically deformed material along the crack front called the process zone, as shown in Fig. 2. The size of the inelastic zone, r j , under a monotonic tensile stress, o , can be approximated by substituting o = Oj into eq. 2 for the horizontal plane, 0 = 0... 

Hence, when Kj 2 2-/2Ki a particle ahead of the crack front is certain to fracture. 

This coitesponds to the condition K, < K, < 2V2K,<. when the probability of a particle failing in the row ahead of the crack front is given by equation (39). 

Double torsion test specimens take the form of rectangular plates with a sharp groove cut down the centre to eliminate crack shape corrections. An initiating notch is cut into one end of each specimen (Hill Wilson, 1988) and the specimen is then tested on two parallel rollers. A load is applied at a constant rate across the slot by two small balls. In essence the test piece is subjected to a four-point bend test and the crack is propagated along the groove. The crack front is found to be curved. 

Owing to hydrogen embrittlement, the mechanical properties of metallic and nonmetal-lic materials of containment systems may degrade and fail resulting in leaks. Hydrogen embrittlement depends on many factors such as environmental temperature and pressure, purity of metal, concentration and exposure time to hydrogen, stress state, physical and mechanical properties, microstructure, surface conditions, and the nature of the crack front of material [23]. 

Fig. 13. Optical micrograph of fra-ture surface of glass-filled epoxy polymers showing the crack front pinned between glass particles 22) (Arrow indicates direction of crack growth)...

Fig. 12a and b. Pre-crack fronts of fracture toughness specimens a 10-2330-20F and b 15-1500-70F. Ellipses highlight torn rubber domain. Arrows indicate direction of crack propagation. Original SEM magnification, 300 x... 

The second noteworthy morphological feature is presented in Fig. 12b. This micrograph depicts the pre-crack front of 15-1500-70F, which had a value significantly above that of the control, as shown in Fig. 11 a. The holes may be examples of the dilatation effect observed in CTBN-modified epoxies l9,22> in which rubber particles dilate in mutually perpendicular directions under the application of a triaxial stress and then collapse into spherical cavities following fracture. Dilatation requires a mismatch in coefficients of thermal expansion of resin and rubber 11. This effect will therefore be most striking when the elastomeric phase is homogeneous, as is apparently the case here. 

Study of the CT fracture surfaces by SEM reinforced earlier observations of the predominant features of fracture. The fracture surface of the unmodified epoxy was essentially featureless as before a thin pre-crack front extended to a very smooth fast crack region. On the surface of the 10-2330-20F specimen observed there were found nodular 20-40 pm particles. In the pre-crack front, such particles exhibited irregular tears when crossed with a microcrack but otherwise remained undisturbed and encircled by an extremely fine border. In the fast crack region, slightly larger particles appeared to be more severely torn. Those small particles which did exist... 

Fig. 14. Pre-crack fronts of resin modified with 10% of ATBN and CTBN elastomers. At top, low magnification micrographs of the four indicated materials. At bottom, high magnification micrographs of small particles in 10C-3880-I7AN and 10A-1750-18AN. Original SEM magnifications, 300 x and 10000 x...

Substantial work on the application of fracture mechanics techniques to plastics has occurred since the 1970s (215—222). This is based on eadier work on inorganic glasses, which showed that failure stress is proportional to the square root of the energy required to create the new surfaces as a crack grows and inversely with the square root of the crack size (223). For the use of linear elastic fracture mechanics in plastics, certain assumptions must be met (224) (/) the material is lineady elastic (2) the flaws within the material are sharp and (5) plane strain conditions apply in the crack front region. 

Displacement of the crack front—from the start of crack formation until final destruction—takes place at a variable rate. For the crack to overcome impediments (such as macromolecules, chain bundles, super-molecular structure formations, inclusions, and micropores), it needs varying time lengths, and the fissure perimeter takes on a sinuous form. The limit between different formations on the entire perimeter of the crack front is evidently determined by the equilibrium set up between elastic mechanical forces and bond forces displacement of the crack front results from this equilibrium. 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

The particles, arranged in lines, act as obstacles for the crack front, in the same way as a line of trees constitutes a good protection against the wind. The crack front has to bow locally between particles in order to pass through the line of particles, slowing down the propagation rate. 

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