Fracture toughness of polymers in the ductile‐to‐brittle transition region: Statistical approach and lower bound determination

Abstract

A method available in literature was adapted and proposed for treating scatter and nonlinearity effects in fracture toughness of polymers in the ductile‐to‐brittle transition regime. The materials used were polypropylene homopolymer (PPH) and a polypropylene‐elastomeric polyolefin blend (PPH/POes 20 wt %), at room temperature and at 20‐mm/min test rate. Under such conditions, the fracture toughness presents a large scatter and a mean value can not be used as a design parameter because it leads to toughness overestimation. Then, there is a need to find a threshold of toughness, as a safe characteristic value for design. The toughness was evaluated by using the J‐integral approach. Large sets of specimens, 53 samples per each material, were tested with the purpose to reveal a reliable tendency in fracture behavior. As the toughness was considered nonuniform throughout the material, a weakest link model was assumed, and then results were analyzed statistically by means of a three‐parameter Weibull model (3P‐W). The PPH responded well to this 3P‐W model, whereas some deviations from the original model were observed in the PPH/POes blend. However, lower‐bound toughness values could be determined for both materials by censoring nonvalid data (Δa > 0.1b0). From an engineering point of view, the results are very encouraging, since this methodology allows to obtain a threshold of fracture toughness from a given population, that is suitable to characterize the material fracture toughness at a given temperature and strain rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3674–3684, 2005

Morphology−performance relationship of polypropylene−nanoclay composites processed by shear controlled injection moulding

“…The differences obtained for each condition are typical of semicrystalline polymer injected parts influenced by microstructural parameters, which include crystalline structure, degree of crystallinity, supermolecular structure or skin−core configuration . In addition, this kind of material is considered to be in the ductile to brittle transition regime which implies large scatter due to dissimilar contributions of plastic flow and stable crack growth in different specimens of the same set. In fact, when J values were assessed as J c taking into account all samples, the scatter is actually inadmissible in some conditions.…”

Determination of fracture toughness of propylene polymers at different operating temperatures

“…This theory states: (1) fracture toughness is variable, differing throughout the given material and particularly along the specimen crack front, (2) the fracture toughness of any specimen is governed by the point or region having the lowest toughness along the crack front. The experimental results were fitted by a three-parameter Weibull model (3P-W) given by the following expression [34,35]:…”

“…The fracture toughness data were highly scattered and showed a strong sensitivity to notch [19][20][21][22][23][24][25][26][27][28][29][30][31] and geometry dimensions [4,5,28] and strain rate [5,12,15,16,30,31]. In addition, some [4,12,19,33,34,35]. This so-called ductile to brittle transition regime behaviour complicated the application of the LEFM methodologies and was explained in terms of the nearness of the testing temperature to the T g of the PP homopolymer [1].…”

“…However, Santarelli and Frontini [28] and Cocco et al [34,35] demonstrated that a statistical approach based on the weakest-link model such as the three-parameter Weibull model could be successfully applied to characterize the fracture behaviour of polymers in the ductile to brittle region without the need of lowering temperature. From the statistical parameters, a toughness threshold can be determined and Cocco et al demonstrated that this parameter is an actual representative material toughness [34,35]. The copolymers, especially the block copolymers, have been extensively analyzed under the combination of high rate and low temperatures.…”

Polypropylene/Clay Nanocomposites Produced by Shear Controlled Orientation in Injection Moulding: Deformation and Fracture Properties

“…The differences observed are typical of semicrystalline polymers injection mouldings that are influenced by microstructural parameters, which include crystalline structure, degree of crystallinity, supermolecular structure, and skin-core configuration [34]. In addition, PP systems are considered to be in the ductile-to-brittle transition regime at room temperature, which implies a larger scatter [35] due to dissimilar contributions of the plastics melt flow and the stable crack growth seen in different specimens of the same set. The addition of nanoclay improved the fracture performance of the SCORIM PP/nanoclay samples thanks to the differences induced by the fillers in the microstructure: as we stated previously [16], the fillers refine the crystal structure and orient the α-PP crystals thus promoting epitaxial growth of the γ-phase, which improves toughness [18].…”