Composites Science and Technology 189 (2020) 108007 Available online 16 January 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech Strain rate dependent damage evolution in long glass fiber reinforced polypropylene J. Lienhard ∗ , D. Discher, J. Hohe Fraunhofer-Institute for Mechanics of Materials IWM, Germany ARTICLE INFO Keywords: LFT Strain rate Infrared Damage detection Field correlation ABSTRACT Strain rate dependent characterizations of glass fiber reinforced thermoplastic (LFT) under different multiaxi- alities show an increasing fracture strain and higher energy absorption capacity if the loading rate rises. The present paper gives a clue for the underlying micro-thermo-mechanical mechanisms of this effect. The method of correlating experimental field information of strain and heat generation provides data for advanced analysis. Strain and heat distribution of the deformation zone as well as a hot-spot occurrence display give hints on expanded damage zones at high strain rates. Quasi-static and dynamic interrupted tensile tests provided data to investigate the damage evolution. Scanning electron microscopic (SEM) images show differences in the area between fiber and matrix depending on the strain rate. Based on SEM images and correlated and analyzed field data a model representation was established that presents, in agreement with the literature, a perception of the damage mechanisms in the interface and its consequences for global deformation. 1. Introduction Strain rate dependent material investigations with composites were conducted by Welsh and Harding, Lienhard and Böhme, Kander and Siegmann, Friedrich et al. Fitoussi et al. Chen et al. and Fan et al. [1–8]. These studies with different composites show higher strengths with increasing strain rate. Fiber-reinforced materials show in many studies also higher fracture strain at higher strain rates. In [9,10], an injection-molded LFT plate material was characterized at different multiaxialities depending on the strain rate. For all testing types rising fracture strain was observed with increasing strain rate. Lienhard and Schulenberg [9] show a decrease in local strain loc as global engineer- ing strain eng. increases. These findings demand on an explanation of the underlying mechanism of the strain rate dependent damage. Some knowledge of damage mechanism in composites and their thermal relation can be found in the literature. In publications by Kander and Stiegman [3] was pointed out on an improved fiber-matrix adhesion with increased strain rate. Fitoussi calls the microstructure of LFT and a changed stress state in the interface layer as a possible reason for higher energy absorption at a higher strain rate [5]. Bartus named the damage mechanisms, fiber breakage, fiber delamination, fiber pull- out and matrix damage in LFT [11]. Fitoussi introduced a strain rate dependent local damage criterion on the number of microcracks in the vicinity of the fibers, observed in a scanning electron microscope (SEM). The higher deformed boundary layers at higher strain rates shall block the crack propagation from the fiber towards the matrix. ∗ Corresponding author. E-mail address: [email protected] (J. Lienhard). The fibers, the boundary layers and the matrix material can bee seen as a three- or four-phase system. The interface layer between the fiber and the matrix is crucial in composite materials for the deformation and damage behavior [12]. Investigations by Wang [13,14] provide infor- mation on the nature of the interface between fiber and thermoplastic matrix. The fiber surface forms the nucleus for a crystalline, spherulitic structure near the fibers. Bartus claimed, that damage initiates from the fibers [11]. The distribution of this damage is influenced by the interface layer. Important for these damage effects in LFT is the development of heat in the vicinity of the fibers (induced by the fiber or interface layer or both) and their effects on the environment. Two important material parameters in pure thermoplastic matrix material, the start of flow and hardening as well as their strain rate dependence, correlate directly with thermal energy. According to Hoy et al. yield stress and hardening are determined by the same physical processes [15]. The dissolution and reconnection of secondary valence bonds between the molecular chains at the atomic level and the texture change of the crystals at the microscopic level are the decisive processes of thermal energy release [15]. Experimental investigations of Rittel support the mentioned investigations [16]. There, the thermal energy increases from the yield stress and stagnates from the beginning of hardening. For Oleinik and Strobl as well, the sliding of crystal blocks is responsible for the heat release [17,18]. After fragmentation of spherulitic crystal https://doi.org/10.1016/j.compscitech.2020.108007 Received 13 September 2019; Received in revised form 3 January 2020; Accepted 12 January 2020
Strain rate dependent damage evolution in long glass fiber reinforced polypropylene
Jun 16, 2023
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