Notched Tensile and Impact Fracture

In Chapter 8 tensile and impact behavior of unnotched specimens have been discussed. The principal changes in behavior to be expected from the presence of a notch would have to be due to the changed state and intensity of stress in the crack tip zone, to the essential confinement of fracture development to a limited region, and to the increase in rate of local deformation. An analytical description of these effects has been attempted in terms of linear elastic fracture mechanics (9 I). Neglecting the geometrical correction term the uniaxial brittle strength of a cracked thick plate is derived from Eq. (9.10) as  

The notch sensitivity of polymeric materials has been comprehensively investigated by Takano and Nielsen [211]. The authors define a notch sensitivity factor k of the yield or breaking strength of tensile bars as 

From their measurements a sequence of ks(s) values can be established  

PMMA (1.64), Kraton -l 101 ( 1.69), SAN (1.62), and PPO (1.48). There are, how-ever, quite a number of (ductile) materials with ks(s) values smaller than one, notably HDPE (0. 8), PTMT (0.96), PTFE (0.94), PA 6 (0.93), and Hytrel 4055. Values below unity of k ( i) are more numerous than those of kg(s). 

Notch sensitivity factors for energy, kj, show a somewhat different order. With some rare exceptions (PP + 20% glass, PE + 40% glass) kj values are always larger than unity. The factor kj(s) reaches values of 625 (semi-transparent PETP), 241 (PSU), 61 (PA 66 + 0.6% H2O), 48 (PP and PMMA), and 16 to 17 (mineral fiUed PA 66, PC). For kj( i) the authors determine 81 (PA 66 + 0.6% H2O), 34 (PPO), 29 (semi-transparent PETP), 19 (PP), and 14 (annealed HDPE and transparent PETP). All other kj values measured are smaller than 13. 

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