Rubber fracture behavior

Sue HJ, Garcia-Meitin El (1996) Fracture behavior of rubber-modified high-performance epoxies. In Arends CB (ed) Polymer toughening. Marcel Dekker, New York, p 131... 

The fracture behavior of toughened polymers, containing rubber or inorganic fillers, may involve several mechanisms, as schematically illustrated in Fig. 8.1 (Garg and Mai, 1988a). These include ... 

For analyzing the fracture behavior of filler clusters in strained rubbers, it is necessary to estimate the strain of the clusters in dependence of the external strain of the samples. In the case of small strains, considered above, both strain amplitudes in spatial direction n are equal (t A F i). because the stress is transmitted directly between neighboring clusters of the filler network. For strain amplitudes larger than about 1%, this is no longer the case, since a gel-sol transition of the filler network takes place with increasing strain [57, 154] and the stress of the filler clusters is transmitted by the rubber matrix. At larger strains, the local strain eAtfl of a filler cluster in a strained rubber matrix can be determined with respect to the external strain if a stress equilibrium between the strained cluster and the rubber matrix is assumed ea GpX =6rm( u)) With Eq. (29) this implies... 

This conclusion was only partly confirmed by scanning electron microscopy micrographs of RuC>4 stained surfaces taken at the crack tip of deformed specimens at 1ms-1, where the non-nucleated and /3-nucleated materials showed, respectively, a semi-brittle and semi-ductile fracture behavior. While some limited rubber cavitation was visible for both resins, crazes—and consequently matrix shearing—could not develop to a large extent whether in the PP or in the /1-PP matrix (although these structures were somewhat more pronounced in the latter case). Therefore, a question remains open was the rubber cavitation sufficient to boost the development of dissipative mechanisms in these resins ... 

Under defined conditions, the toughness is also driven by the content and spatial distribution of the -nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of the /1-phase in heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of -nucleated grades has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the /3-modification (towards the cross-hatched structure of the non-nucleated resin) which induce a higher mobility of both crystalline and amorphous phases. 

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. 

Generally, it is observed that elastomers at very low temperature become brittle and fracture with little detectable Inelastic deformation. Andrews, Reed, and co-workers (5-6) have demonstrated that prestralnlng the rubber before cooling can drastically modify its fracture behavior. In their studies and subsequent studies by others (7-8) a variety of rubbers ranging from natural rubber to silicone were prestrained ( 100% at room temperature) before reducing the temperature to -100°C or below. When further stressed at these low temperatures, the rubbers did... 

Although the toughening mechanisms may be different in RTPMMA, the transitions in fracture behavior observed here at high rates are qualitatively similar to those described by Bucknall (20) for the impact of high-impact polystyrene containing different fractions of modifier. Here, the main result is that all the transitions are shifted simultaneously when the 2 L rubber content is increased. 

Many commercial polymer blends often include an elastomer, to improve the impact strength of the blend under conditions of stress concentration (notched Izod impact strength) and to lower the ductile-brittle transition temperature of the blend. The elastomeric dispersions are judiciously employed either in the matrix phase, in the dispersed polymer phase, or in both phases, depending upon the requirement and the fracture behavior of the blend. As a general rule, the more brittle component in a given polymer blend has a greater need for rubber toughening. 

Nomura et al. (14) investigated the mechanical fracture behavior of PP/EPR blends containing additional ethylene-a-olefin copolymers (ECP). They found that the addition of a small content (8.2 wt%) of ECP did not change the rubber domain distributions in the matrix (corresponding TEM images are shown in Fig. 8.10). 

Figure 8 shows the SEM micrographs of fracture surfaces of both HX-205 and F-185 neat resins. The fracture surface of HX-205 Is very smooth. Indicative of typical brittle fracture behavior. On the other hand, F-185 has a very rough fracture surface. Indicating that the resin was highly strained before fracture occurred. There are also some craters which appear to represent the separation of spheroidal rubber domains from the matrix. 

The characteristics of fracture surfaces of F-185 neat resin and those of F-185 matrix In the composites are similar to those reported In the literature ( ). The fact that the fracture surfaces of F-185 neat resin and F-185 matrix In the composites show typical ductile fracture behavior, while the unmodified HX-205 shows brittle fracture behavior, seems to Indicate the toughening effect of F-185 as a result of Incorporation of CTBN rubber. The fracture energies of these materials are being... 

Tod Todo, M., Tahahashi, K., Beguelin, P., Kausch, H.-H. Effect of displacement rate on the mode 1 fracture behavior of rubber toughened PMMA. JSME Int. J. 42 (1999) 49-56. 

Zen Zeng, Y.-B., Zhang, M.-Z., Penc, W.-Z., Yu, Q. Microstructure, mechanical properties, and fracture behavior of liquid rubber toughened thermosets. J. Appl. Polym. Sci. 42 (1991) 1905-1910. 

OOXia Xiao, K. Q., Ye, L. Effects of rubber-rich domains and the rubber-plasticized matrix on the fracture behavior of liquid rubber-modified araldite-F epoxies. Polym. Eng. Sci. 40 (2000) 2288-2298. 

Gam Gam, K. T., Miyamoto, M., Nishimura, R., Sue, H.-J. Fracture behavior of core-shell rubber-modified clay-epoxy nanocomposites. Polym. Eng. Sci. 43 (2003) 1635-1645. 

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