The role of notching damage on the fracture parameters of ethylene-propylene block copolymers

doi:10.1016/j.polymertesting.2010.07.003

Polymer Testing

Volume 29, Issue 7, October 2010, Pages 824-831

https://doi.org/10.1016/j.polymertesting.2010.07.003 Get rights and content

Abstract

The influence of the notching procedure on the fracture toughness measured via Elastic-Plastic Fracture Mechanics has been analyzed on four different ethylene-propylene block copolymers with two distinct dimensions, paying special attention to the morphology of the area surrounding the crack tip front. Two sharpening techniques were evaluated: the traditional steel razor blade and the femtolaser ablation process. The fracture toughness of the razor blade sharpened samples was always higher than that of the femtolaser sharpened specimens. Also, the fracture toughness of the razor blade samples was dependent on the thickness of the samples, whereas the fracture toughness of the femtolaser sharpened specimens was not influenced by the dimensions of the test specimens. The microscopic analysis of non-tested samples showed that the crack tip radii were similar for both type of sharpened samples but the damage and its extension ahead of the crack tip was dependent on the notching technique, the copolymer type and the dimensions of the analyzed specimen. The femtolaser sharpened samples presented a very tiny heat affected zone ahead of the crack tip, the size of which was independent of the copolymer type and the dimensions of the test specimen. On the other hand, the steel razor blade sharpened samples showed an area surrounding the crack tip formed by plastic deformation, the length of which increased for the smaller size of sample and for higher ethylene content in the copolymer.

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, T_g EPR, embedded in the propylene matrix, T_g 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·Δa^N, 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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Test MethodThe role of notching damage on the fracture parameters of ethylene-propylene block copolymers

Abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgements

Eng. Fract Mech.

Polym. Test.

Appl. Surf. Sci.

Test Method
The role of notching damage on the fracture parameters of ethylene-propylene block copolymers