High strain rate compression of epoxy micropillars

https://doi.org/10.1016/j.eml.2020.100905Get rights and content

Abstract

The study of the high strain rate mechanical behaviour of materials using micropillar compression tests has been hindered so far due to the lack of suitable instrumentation. In the present study, a novel high strain rate micropillar compression set-up is introduced. The high speed tests are carried out with the impact configuration of a pendulum-based nanoindentation device that was instrumented with force-sensing capability by means of a piezoelectric load cell. This is necessary due to the considerable inertia effects that make the actual applied force on the sample much higher than the actuated one. The proposed novel technique was successfully applied to study the mechanical behaviour of an epoxy resin over a wide range of strain rates. The results reveal a significant strain rate sensitivity that correlates well with that obtained for the same material at the macroscale, validating the novel set-up.

Introduction

The mechanical characterization of materials at the micro- and nano-scales is becoming an important discipline in engineering and science. This is driven, partly, by recent advances in fields like microelectromechanical systems (MEMS) and advanced coatings, and the need to understand the physics behind the size-dependent deformation of materials. And also, by the need to develop micromechanical tests that allow determining the mechanical properties of the constituents in bulk composite materials, specially towards incorporating them into multiscale models of composite behaviour [1]. Nanoindentation has been traditionally the main tool for nano-/micromechanical characterization of materials, not only for the determination of hardness and elastic modulus [2], but also for extracting the plastic constitutive behaviour [3], [4]. However, the determination of the constitutive behaviour has proven to be especially challenging due to the large strain gradients and complex stress states that develop in the material during indentation. This issue was overcome with the pioneering work of Uchic et al. who demonstrated the possibility of fabricating micron-size pillars in a material surface that could be tested with a conventional nanoindentation system [5], [6]. This way, the stress and strain are uniform along the tested material and the uniaxial constitutive behaviour can be directly probed, without the need of a complex inverse analysis. Since then, the micropillar compression technique has been applied to study the mechanical behaviour of all classes of materials: from the study of size-dependent deformation in metals [7], [8], ceramics [9], [10] and polymers [11], [12], [13], to the calibration of micromechanical models of polycrystalline metals [14] and fibre-reinforced polymer composites [15], [16], [17].

Micropillar compression tests have been also performed over a wide range on environments and loading conditions, following new developments in nanoindentation instrumentation, such as high temperature and varying strain rates [18], [19]. A large number of studies have dealt with the study of high temperature deformation [20], [21], and more recently with material behaviour under cryogenic conditions [22]. In terms of loading conditions, micropillar compression tests have been performed under conditions of high cycle fatigue [23] and creep [24]. An area that has received less attention is the study of high strain rate behaviour, due to the absence of suitable instrumentation for obtaining reliable data. Understanding how materials deform under conditions of high strain rates is of great importance to model machining and forming processes, and materials subjected to impact events. Furthermore, the high strain rate characterization at the microscale allows extending the in situ calibration of micromechanical models of materials, like crystal plasticity models, to higher strain rates. Recently, Guillonneau et al. [25] demonstrated the capabilities of a novel nanoindentation device to perform high strain rate micropillar compression, and applied the technique to study the rate dependent deformation behaviour of nanocrystalline nickel. The technique was later expanded to the study of ceramics materials [26] and polymers [27]. The high strain rate characterization of polymers is of particular importance because these materials can be highly rate sensitive [28]. New developments in this area will open the door to the calibration of micromechanical models of the impact behaviour of fibre-reinforced polymer composites, incorporating strain rate dependent behaviour, a topic that has remained elusive so far [15].

In the present study, a new set-up for performing high strain rate micropillar compression tests that offers reliable data is presented. The set-up is based on impact indentation and it uses a pendulum-based nanoindentation device modified to measure force via a piezoelectric sensor. Micropillar compression tests over a wide range of strain rates were performed on an epoxy resin typically used as matrix material in fibre-reinforced polymer composites. Results were validated against macroscopic mechanical characterization results obtained in the same material, using a range of devices, including a electromechanical testing machine, a hydraulic testing device and a split-Hopkinson pressure bar (SHPB). Unlike the macroscopic tests, that require access to bulk resin specimens, the impact micropillar compression test allows the in situ testing of the resin matrix within the real composite material, so this test unravels the capability of calibrating composite micromechanical models at high strain rates.

Section snippets

Material and specimen preparation

The tested material was an aerospace-grade Hexcel 8552® epoxy resin, used as matrix material in fibre-reinforced composite materials. The resin in fresh state was heated up to 100 °C for 30 min and speed-mixed at 2300 rpm for 1 min to eliminate entrapped air. It was then poured into open moulds and cured in an autoclave for 135 min at 180 °C with a heating/cooling rate of 2 °C/min and a pressure of 7 bar. The autoclave process ensured that porosity was kept to a minimum.

Specimens for

Results and discussion

Fig. 2 plots the stress–strain curves and strain rates for the micropillar in all loading conditions and for all repeats. The strain rate achieved when using the impact configuration, of the order of 10–100 s−1, was about four orders of magnitude larger than the strain rate obtained for the low speed cases. Besides, the strain rate was fairly constant throughout the strain range of interest. Even higher strain rates can potentially be achieved in the impact configuration by impacting the pillar

Conclusions

A novel test set-up for high strain rate micropillar compression testing was proposed and applied to the study of the mechanical behaviour of an epoxy resin over a wide range of strain rates. The high strain rate test is based on impact loading, which is performed on a pendulum-based nanoindenter modified to measure the actual force imposed on the sample via a piezoelectric force sensor. The micropillar compression tests on the epoxy resin revealed a significant strain rate sensitivity of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement Nº 722096, DYNACOMP project.

References (33)

  • DaoM. et al.

    Acta Mater.

    (2001)
  • DeanJ. et al.

    Mech. Mater.

    (2017)
  • DimidukD.M. et al.

    Acta Mater.

    (2005)
  • MaaßR. et al.

    Acta Mater.

    (2009)
  • GuruprasadT.S. et al.

    Polymer

    (2016)
  • CruzadoA. et al.

    Acta Mater.

    (2015)
  • GonzálezC. et al.

    Prog. Mater. Sci.

    (2017)
  • KorteS. et al.

    Scr. Mater.

    (2009)
  • LupinacciA. et al.

    Acta Mater.

    (2014)
  • GuillonneauG. et al.

    Mater. Des.

    (2018)
  • Rueda-RuizM. et al.

    Mater. Des.

    (2020)
  • SneddonI.N.

    Internat. J. Engrg. Sci.

    (1965)
  • ZhangH. et al.

    Scr. Mater.

    (2006)
  • LLorcaJ. et al.

    Adv. Mater.

    (2011)
  • OliverW.C. et al.

    J. Mater. Res.

    (1992)
  • UchicM.D. et al.

    Science

    (2004)
  • Cited by (8)

    • Effects of loading rate and temperature on crushing behaviors of 3D printed multi-cell composite tubes

      2023, Thin-Walled Structures
      Citation Excerpt :

      The results showed that the CFRP tubes had better energy absorption performance in repeated impact tests. Due to the wide application of thin-walled structures, their mechanical properties, energy absorption and failure modes were characterized under different loading conditions and environments [18–20]. It was reported that the mechanical properties of composite materials were sensitive to strain rate and temperature [21–23], and the dependencies were dominated by the behavior of matrix materials [24].

    View all citing articles on Scopus
    View full text