Test MethodThe role of notching damage on the fracture parameters of ethylene-propylene block copolymers
Introduction
Recently, some works have appeared in the literature reporting the relevance of the notch sharpening technique when evaluating the fracture toughness of polymeric materials [1], [2], [3], [4]. It is a well-known fact that the quality of the notch is of deciding importance to determine well established fracture toughness values using Linear Elastic Fracture Mechanics (LEFM), J–R resistance curves using Elastic-Plastic Fracture Mechanics (EPFM) or Essential Work of Fracture (EWF) parameters using Post Yielding Fracture Mechanics (PYFM). Therefore, the notch sharpening procedure is crucial and tackled by both ESIS (European Structural Integrity Society) [5], [6], [7] and ASTM International (Previously American Society for Testing and Materials) [8], [9], [10].
Martínez et al. [1], [2] and Salazar et al. [3], [4] have focused their research on the influence of the notch sharpening technique on the fracture parameters determined under LEFM [3], [4], EPFM [3], [4] and PYFM [1], [2], [4] conditions. The polymeric material utilized for their investigation was an ethylene-propylene block copolymer. The sharpening procedure was performed using steel razor blades and a femtosecond pulsed laser. The femtosecond pulsed laser ablation is characterized by very rapid creation of vapor and plasma phases, negligible heat conduction and the absence of a liquid phase [11], [12]. Hence, this technique can remove the material of the notch tip by ablating it with almost no heat dissipation, preventing melting and thermal deformation of the surrounding area. These works proved that, independently of the fracture mechanics conditions, the fracture toughness of the specimens sharpened through femtosecond laser ablation showed lower values compared to those obtained on the samples sharpened using a steel razor blade. These differences could range from ∼10% up to 90% under LEFM and PYFM conditions, respectively. The damage produced ahead of the crack tip through plastic deformation in the steel razor blade sharpened samples seemed to be the reason of this increase in the fracture toughness.
To date, the reports found in the literature concerning the influence of the notch sharpening technique on the fracture parameters have been focused on the experimental notching procedure, regardless of the type of polymeric material. Also, they do not quantify the damage facing the crack front. For these reasons, the objective of the present work is to evaluate if the differences found in the fracture parameters between femtolaser and sharp razor blade notched specimens are held independently of the morphological and structural properties of the ethylene-propylene block copolymer. Hence, four ethylene-propylene block copolymers with different structural properties have been chosen and the fracture parameters have been determined under EPFM conditions. Subsequently, the fracture toughness values are discussed in the light of the specimen thickness, the size and type of damage ahead of the crack tip and the structural parameters of the different copolymers.
Section snippets
Materials
The materials under study were four commercial grade ethylene-propylene block copolymers, EPBCs, supplied by Repsol in the form of pellets. The bulk specimens for fracture characterization were prepared by injection moulding.
The basic characteristics such as the ethylene content and the isotactic index determined from Nuclear Magnetic Resonance (NMR), the glass transition temperatures corresponding to the elastomeric particles, Tg EPR, embedded in the propylene matrix, Tg PP, measured via
Results and discussion
The mechanical response of the four copolymers at 23 °C and at a cross-head speed of 1 mm/min was highly non-linear and the multiple specimen method was applied to determine the J-crack growth resistance curves [6]. Fig. 1, Fig. 2 combine the R-curves of the S-Type and F-Type specimens with 6.35 and 9 mm in thickness, respectively. These plots also include the fit of the J-crack growth resistance curve to the power law J = C·ΔaN, with N ≤ 1. Independently of the copolymer type and specimen
Conclusions
The influence of the notching procedure on the fracture parameters obtained via EPFM has been analyzed in four different ethylene-propylene block copolymers with two distinct dimensions. The crack growth initiation energy of the specimens sharpened via traditional steel razor blade was always higher than that of the samples sharpened via the femtolaser ablation technique. Furthermore, for the razor blade samples, the fracture toughness was dependent on the thickness of the samples, being lower
Acknowledgements
Authors are indebted to Ministerio de Educación of Spain for their financial support through the project MAT2009-14294, to REPSOL YPF for the materials supply and to PhD Pablo Moreno from the Universidad de Salamanca for the femtolaser notching of the specimens.
A. Segovia thanks the National Council of Science and Technology (CONACYT) of Mexico, for the support of a doctoral research scholarship.
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2015, Engineering Fracture MechanicsCitation Excerpt :Moreover, these investigations indicate not only that the sharpening method influences significantly the fracture parameters of thermoplastic polymers, but also that these differences were more noticeable under elastic–plastic and fully plastic conditions. For multiphase semicrystalline ethylene-propylene copolymers, the differences were approximately ∼10% under Linear Elastic Fracture Mechanics (LEFM) conditions [2,4,6], of ∼25–75% under elastic–plastic conditions [2–5] and up to ∼90% for fully plastic conditions [1,4]. For the amorphous polycarbonate, the differences reached ∼40% under LEFM conditions [8] and up to ∼400% under elastic–plastic situations [5].
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2013, Engineering Fracture MechanicsCitation Excerpt :Independently of the specimen’s dimensions, the resistance curve of F-Type is below the one of S-Type. This tendency was also observed in another copolymer similar to the one under study in the present work except for slight differences in the molecular weight [7,8]. Table 3 presents the fracture toughness, J0.2 values, obtained from the resistance curves of S-Type and F-Type specimens with 6.35 mm and 9 mm in thickness.
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2013, Polymer TestingCitation Excerpt :Recently, users of ISO 13586 [2] have noticed that the repeatability and reproducibility of the results depend strongly on the quality of the sharpened notches. Several works have been found in the literature highlighting the role of the notch sharpening technique on the fracture parameters of different type of polymers such as semicrystalline, heterogeneous ethylene-propylene block copolymers [11–13] and amorphous polycarbonate [14]. In all these studies, the values of the fracture toughness obtained using different approaches were determined from specimens sharpened via two procedures: a non- contact technique based on femtosecond laser ablation and the traditional contact technique using a razor blade.