Impact fracture modes

Heterogeneous compatible blends of preformed elastomers and brittle plastics are also an important route for the development of blends of enhanced performance with respect to crack or impact resistance. Polycarbonate blends with preformed rubber particles of different sizes have been used to provide an insight into the impact properties and the fracture modes of these toughened materials. Izod impact strength of the blends having 5-7.5 wt% of rubber particles exhibits best overall product performance over a wide range temperature (RT to -40°C) [151-154]. 

Impact modifiers for PET are generally elastomeric compounds that increase impact strength and elongation while usually decreasing modulus. An effective way to enhance the impact strength and to induce a brittle/ductile transition of the fracture mode, is by the dispersion of a rubber phase within the PET matrix. The... 

The temperature dependence of the absorbed energy for standard specimens is shown in Figure 1. Impact behaviors and fracture modes of edge notched specimens depending on temperature were reported elsewhere (9, 10). In edge notched specimens of unidirectional roving reinforced material (GC 68 wt % ), cloth laminates, and mat reinforced material the fracture mode changed with temperature all materials re-... 

We can predict the impact behavior, namely impact strength and fracture mode, over a wide range of temperatures independent of specimen size by using standard specimens. 

For this semi-ductile fracture mode, a method using two material parameters has been proposed to characterize the impact resistance of the polymer [11], The method takes account of an average value of the fracture energy during the stable propagation of the crack, and also the fracture energy at instability. In this case, the total energy absorbed by the sample to break, U, has been shown... 

Residual stresses and molecular orientation play an important role in the toughness enhancement of cold-worked plastics, because toughness is primarily based on the mechanics of craze formation and shear band (crazes and flaws) formation. The shear bands determine the fracture mode and toughness of a polymer when subjected to impact loads. The amount of energy dissipated will depend on whether the material surrounding the... 

The friermal decohesion model for impact and dynamic fracture in thermoplastics asserts that fracture resistance is not constant, and is strongly supported by experimental data. It also emphasises that the nature of resistance to impact britfle fracture is profoundly different from that to other brittle fracture modes. The model can be used, however, to construct an alternative scheme for transferring data between impact configurations. This scheme has been demonstrated for a practical impact failure problem. 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

In many cases, a product fails when the material begins to yield plastically. In a few cases, one may tolerate a small dimensional change and permit a static load that exceeds the yield strength. Actual fracture at the ultimate strength of the material would then constitute failure. The criterion for failure may be based on normal or shear stress in either case. Impact, creep and fatigue failures are the most common mode of failures. Other modes of failure include excessive elastic deflection or buckling. The actual failure mechanism may be quite complicated each failure theory is only an attempt to explain the failure mechanism for a given class of materials. In each case a safety factor is employed to eliminate failure. 

Fig. 8.10. Correlation of residual compression-after-impact (CAI) strength with composite mode II interlaminar fracture toughness, Gji. After Masters (1987a).

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