Gradients of cyclic indentation mechanical properties in PR520 epoxy and its 3D carbon fiber composite induced by aging at 150 °C

https://doi.org/10.1016/j.polymdegradstab.2021.109720Get rights and content

Highlights

  • Cyclic indentation tests allowed to evaluate the effects of thermal aging on local elastic, plastic and viscoelastic properties of PR520 epoxy resin and its composite.

  • Thermal aging affects the local plastic and elastic behavior, while viscoelastic response does not change between virgin and aged samples.

  • Property gradient profiles measured through the thickness of resin and composite samples aged in the same conditions are well superposed.

Abstract

A cyclic indentation test was carried out on PR520 epoxy resin and its 3D interlock carbon fiber reinforced composite in order to study the effects of thermo-oxidative aging on the material local properties. Thermal aging has been performed in air at 150 °C, which is close to the service temperature in warm zones of aircrafts, for different durations. Two parameters have been considered in this study: the reload indentation modulus, representative of the elastic response, and the ratio between irreversible and total indentation energy, representative of plastic (first cycle) and viscoelastic response. The analysis of cyclic indentation curves shows that elastic and plastic behavior are affected by thermal aging, while viscoelastic behavior is not. Moreover, a gradient of local properties has been measured through the thickness of aged samples. The affected zone extends of about 150 µm from the surface exposed to the oxidative environment, while the properties of the sample core are similar to those of the virgin material. Property gradient profiles measured on resin and composite aged in the same conditions are well superposed.

Introduction

The growing use of polymer-matrix composites in the “warm zones” of aircraft structures leads to the necessity to understand the degradation phenomena due to their exposure at elevated temperatures. In these “warm” zones, the temperature range is between 150 and 300 °C that, together with the presence of oxygen, create environmental conditions conducive to degradation phenomena of the polymer matrix over long time of exposure. For temperatures lower than glass transition temperature, the polymer degradation is most often due to pure thermal oxidation, while for temperatures higher than the glass transition, secondary anaerobic degradation phenomena (thermolysis) could also occur. In the specific case of thermo-oxidative aging, an evolution of local mechanical properties occurs due to the change in macromolecular structure caused by the interaction with oxygen promoted by high temperatures [1]. The visual effect of thermal oxidation of polymers is the change of surface color [2], [3], [4], [5], [6], [7], [8], [9]. In particular, the thermally aged sample appears darker than the virgin one and this change is more significant as the aging conditions are severer. This darker layer, which corresponds to the oxidised zone as opposed to the non-oxidised polymer core, has a thickness of several hundreds of micrometres at most [9]. Because of this small size of the affected zone, instrumented indentation turns out to be the most suitable experimental technique to measure possible gradients of mechanical property in the oxidised layer. In its classical form, instrumented indentation consists in driving a small hard tip of several microns, usually made of diamond, which shape is known at nanometric scale, into the surface of the material under either load or displacement control. The force and displacement data are recorded during the test and analysed to obtain mechanical properties of the tested material. The most often, indentation protocol is composed by loading and unloading, sometimes completed with a dwell at the maximum load. The application of the Oliver and Pharr analysis method [10] allows to determine two mechanical properties of the material: the Elastic Indentation Modulus E* and the Hardness H.

The instrumented indentation test has been used in numerous previous studies [3], [4], [5], [6], [7], [8] to trace the evolution of elastic indentation modulus from the external surface to the core of thermally aged polymer samples, highlighting the presence of a gradient through the sample thickness. In most of the cases, an increase of indentation modulus is observed near the surface directly exposed to the oxidative environment, where it reaches its maximum value. Then, the indentation modulus decreases in the oxidized layer and reaches a value close to that of the virgin material at the sample core. Even if the Oliver and Pharr analysis method is widely used in the literature to derive elastic indentation modulus and hardness of polymers, many studies have highlighted the difficulties that arise from its application on load-displacement curves resulting from indentation of polymeric surfaces [11], [12], [13], [14], [15], [16]. In fact, the hypothesis of elastic unloading, on which the Oliver and Pharr analysis method is based, turns out to be too strong on unloading curves of polymers, in which creep phenomena occurs because of their viscoelastic nature. Moreover, time independent properties like elastic modulus and hardness are not sufficient to fully describe the complex time dependent behavior of polymers. Therefore, in this work, a cyclic indentation testing method developed previously for polymers (see [17]) has been used to characterize the environmental effects, not only on elastic, but also on plastic and viscoelastic local behavior.

The aim of this study is to characterize the gradients of mechanical properties of PR520 epoxy resin used in aeronautical application due to thermal aging at 150 °C, which is higher than in the previous studies and closer to the glass transition of this polymer. Moreover, the effect of thermal aging on the local time dependent properties of the same material as a matrix of a 3D interlock carbon fiber reinforced composite aged in the same conditions has been studied as well. This paper is organised as follows. After the presentation of the materials and the cyclic indentation method, the results are grouped into three parts. Firstly, the indentation loops of the virgin resin and its composite and their shift by material aging are presented and discussed. Secondly, the mechanical properties from the tests on the exposed surface are obtained and presented. Finally, the gradients of cyclic mechanical properties in the thickness of polymer and composite samples are plotted and discussed.

Section snippets

Materials

The material used in this study is the PR520 epoxy resin, a thermoset polymer used as matrix of carbon fiber reinforced composites, commonly employed in the aerospace industry. Neat epoxy resin was provided by Cytec Engineered Materials in the form of 1 cm thick plates, from which several samples with a thickness of 3.5 mm were cut transversally.

The composite material is a 3D interlock reinforced with carbon fibers and manufactured through Resin Transfer Molding process. It was provided by

Indentation loops of virgin and aged resin and composite

In this Section, the cyclic indentation curves will be presented and discussed. Firstly, the cyclic load-displacement response obtained on virgin PR520 is presented in Fig. 4. The first cycle is very large since it encloses plastic and viscoelastic dissipation followed by a significant hysteresis decrease in second cycle and a progressive thinning of the loops. This behavior indicates that the viscoelastic dissipation tends to vanish with time during the test. Moreover, this curve shows that

Conclusions

In this work, the effects of thermal aging in air at 150C on elastic, plastic and viscoelastic local properties of PR520 epoxy resin and composite have been studied through cyclic indentation tests. It has been shown that the kinetics of evolution of the reload indentation modulus (Erel*) and the energy ratio (η) with cycles is similar for virgin and thermally aged samples and that the amount of energy dissipated to produce time-dependent strain (the energy ratio from the second cycle) is not

CRediT authorship contribution statement

M. Pecora: Investigation, Formal analysis, Writing – original draft. O. Smerdova: Conceptualization, Writing – review & editing. M. Gigliotti: Supervision, Writing – review & editing.

Declaration of Competing Interest

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that

Acknowledgments

This work pertains to the domain of research of the French Government program “Investissements d'Avenir” (LABEX INTERACTIFS, reference ANR-11-LABX-0017–01; EQUIPEX GAP, reference ANR-11-EQPX-0018, EUR INTREE, reference ANR-18- EURE-0010). The authors thank the Poitou-Charentes Region for funding the PhD scholarship of Marina Pecora and SAFRAN-SNECMA (now SAFRAN Aircraft Engines) for providing the composite material. Pprime Institute gratefully acknowledges “Contrat de Plan Etat - Région

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