Material properties impact fracture

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]. 

Analysis of these effects is difficult and time consuming. Much recent work has utilized two-dimensional, finite-difference computer codes which require as input extensive material properties, e.g., yield and failure criteria, and constitutive laws. These codes solve the equations of motion for boundary conditions corresponding to given impact geometry and velocities. They have been widely and successfully used to predict the response of metals to high rate impact (2), but extension of this technique to polymeric materials has not been totally successful, partly because of the necessity to incorporate rate effects into the material properties. In this work we examined the strain rate and temperature sensitivity of the yield and fracture behavior of a series of rubber-modified acrylic materials. These materials have commercial and military importance for impact protection since as much as a twofold improvement in high rate impact resistance can be achieved with the proper rubber content. The objective of the study was to develop rate-sensitive yield and failure criteria in a form which could be incorporated into the computer codes. Other material properties (such as the influence of a hydrostatic pressure component on yield and failure and the relaxation spectra necessary to define viscoelastic wave propagation) are necssary before the material description is complete, but these areas will be left for later papers. 

Previous work pursued the model analytically, for a linearly elastic [5] (or, later, non-linearly elastic [6]) material with constant thermal properties. The analytical model explained several measured fracture properties of thermoplastics the magnitude of impact fracture toughness and its dependence on impact speed [7] and the absolute magnitude of resistance to rapid crack propagation [8]. Recent results have shown that the impact fracture properties of some amorphous and crosslinked polymers show the same rate dependence [11],... 

ANTHONY G. EVANS is the Gordon McKay Professor of Materials Engineering at Harvard University. His research interests include the mechanical properties of brittle materials particularly the fracture of ceramics under conditions of impact thermal and mechanical stress and failure prediction based on nondestructive evaluation. He is a recipient of the American Ceramic Society s Ross ColEn Purdy Award and has authored and co-authored several publications. Dr. Evans is a member of the National Materials Advisory Board and has served on several National Research Council committees. Dr. Evans was elected to the National Academy of Engineering in 1997 for his contributions to the development and understanding of structural materials. 

Criteria based solely on material testing requirements. These are usually intended to demonstrate that some material property (e.g. impact energy) has been shown by previous experience or by full scale demonstration prototype tests to give satisfactory performance, or may be correlated to fracture toughness to provide adequate margin against brittle fracture. 

Fracture Toughness from Flexed-Beam Impact Tests. As stated above, the so-called impact toughness values obtained from Izod and Charpy tests are not material properties because they depend on specimen thickness, notch depth, notch radius, and other factors imrelated to material properties. These... 

In Fig. 10.8, the dependence of Gj on is presented, corresponding to the Eq. (10.14), for HDPE and PS. Let us note, that within the frameworks of linear fracture mechanics the value Gj is assumed as material property, that is, at fixed testing temperature and strain rate it should be constant and independent on a value [23]. However, in impact tests HDPE and PC samples the value G changes in 2 3 times. This is usually explained by the... 

Figure 3 shows the stress contour of a medial transversal impact load on tibia by considering the elastic isotropic material properties for tibia. The maximum stress of 176 MPa compared with the bending ultimate stress of the cortical bone, i.e. 140 MPa [8], shows that fracture will occur in the mid-shaft of tibia and near the dorsal edge. 

It is known that bone is a semi-brittle material and will experience fracture with forces greater than its ultimate strength. Based on the maximum amount of stress, the viscoelastic behavior of tibia showed a tendency to propagate the fracture lines and also higher stresses were reached compared to the isotropic and transversely isotropic property cases of tibia during the impact cycle. This indicates the dependency of a viscoelastic material to the time and also the strain rate, as expected. Also, the observed reduction in the maximum amount of stress after removing the impact load can be seen as a consequence of stress relaxation in tibia. The minimum stress was found to be when a transversely isotropic material property was considered for tibia. 

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