Test MethodFast screening of the fatigue properties of thermoplastics reinforced with short carbon fibers based on thermal measurements
Introduction
Short-fiber-reinforced thermoplastics (SFRT) provide a major opportunity to obtain lightweight parts at a reasonable cost and have been widely investigated [[1], [2], [3], [4], [5], [6]]. Over the last few years, this kind of material has been used for structural components in the automotive industry to replace die casting aluminum or stamped steel parts [[7], [8], [9], [10], [11]]. The possible use of these materials for aeronautic applications is also under consideration. The main reasons are the reasonable properties of these materials, especially when using a thermostable matrix, and the use of the injection molding process which allows both a cost reduction and the definition of complex parts with stiffeners and ribs, to circumvent the loss in rigidity compared to metallic solutions. Fatigue campaigns are usually very time consuming, especially for polymeric materials, as the test frequency cannot be very high to avoid any bias brought about by the heat build-up induced under cyclic stressing. As these materials also provide high versatility in fiber content, grades and suppliers, a screening tool, able to discriminate good candidates for fatigue applications quickly prior to a full fatigue campaign, would be a crucial asset.
One of the experimental tools to do so could be based on the analysis of the heat build-up response. Since the pioneer studies [12,13], this technique has become of considerable interest over the last few years on metallic materials for empirical approaches [[14], [15], [16], [17]] or through constitutive modeling [[18], [19], [20]]. This is explained mainly by the considerable development of thermography devices and analysis [16,21,22]. The extension of the technique to other materials has also been tested [17] and is now applied, following different approaches, to rubbers [[23], [24], [25]], continuous fiber composites [26,27] and short fiber reinforced plastics [11,28,29]. All these approaches use the evolution of the sample temperature but can be quite different. Some of them rely on the temperature only, which is quite limited as temperature is dependent on the frequency, on the sample geometry and on the thermal boundary conditions. Therefore, these analyses give no access to an intrinsic parameter. Most of the protocols now seek the dissipated energy associated with the temperature variations. This evaluation is not trivial and requires solving the heat equation according to a wide range of hypotheses, both on the temporal aspects (adiabatic or stabilized configurations) and on the spatial distribution of the dissipation sources. This evolution of the dissipated energy versus the amplitude of oscillation (strain or stress, depending on the materials tested) is then generated and is called the heat build-up curve. The following step is to relate it to the ”fatigue properties”, which could cover a pretty wide range of ambitions.
The most basic is to evaluate an unlimited fatigue lifetime or at least a lifetime high enough compared to the range of cycles aimed at by the application. This analysis uses the change of shape of the heat build-up curve to identify this threshold. A more interesting objective is to predict the fatigue mean curve. This approach relies on the use of an energy based fatigue criterion and could be determined either empirically [23,25,29], with the help of a constitutive model [18,30], or using additional microstructural measurements [31,32]. In all cases, it is necessary to check if the method can be applied, because a main difference of this approach compared to classic fatigue tests is that it relates the response over a few initial cycles to the fatigue failure occurring after several days or weeks of testing. It is, therefore, necessary to check that the constitutive response under fatigue conditions reaches a stationary state within this limited number of cycles [33,34] before relating the cyclic thermomechanical response under several loading amplitudes to the fatigue lifetime. An even more desirable objective is to predict the full fatigue curve including the fatigue scattering from the heat build-up curve. This has been proven to be possible on several metallic materials [[18], [19], [20]], by relating the dissipated energy to the damage mechanisms throughout a constitutive modeling analysis, including a probabilistic failure criterion. This last approach is clearly the most appealing but it faces several difficulties for short fiber reinforced composites. The main difficulty is the extremely numerous dissipation phenomena possibly involved in the material response under fatigue loading, ranging from visco-elasticity to damage and plasticity [[35], [36], [37], [38]]. Despite numerous studies [6,39,40], relating the macroscopic dissipation to the basic fatigue mechanisms remains an open issue. A second issue is that predicting a relevant local dissipation, and thus reliable fatigue predictions, requires a very accurate evaluation of the hysteretic loop, which clearly remains difficult for these materials [41,42]. A last point is that an extensive modeling approach usually also requires an extensive testing campaign, involving a long preliminary study for each compound tested. This could not be integrated easily into a fast testing approach.
This study focuses on a thermostable matrix reinforced with short carbon fibers and aims at three main objectives. The first is to check if the hypotheses required to apply the approach is valid for these materials, both from a measurement point of view (adiabaticity, spatial distribution of the dissipation sources) and from a criterion point of view (evaluation possible over a limited amount of cycles). The second is to check the accuracy of the technique to predict the mean fatigue curve of these materials. The last is to challenge its capacity to be used as a fast screening tool to discriminate quickly several possible materials, for different grades, suppliers and carbon fiber amount. This study provides two main original aspects compared to the team's previous studies. The first one is the application of the heat build-up approach to these materials that have not been investigated yet. The second is the use of a specific identification of the temperature variation, giving access to both dissipation and thermoelastic couplings, for each pixel of the measured area.
A first section presents the material investigated and the sample geometry. In the second section, the experimental protocols used for the fatigue tests, the thermal characterization and the determination of the heat build-up curve are presented. The third section presents the results obtained both for the fatigue and the heat build-up tests. The fourth section provides an analysis of the results obtained and discusses the objectives of the paper, i.e. the consistency of the hypotheses and the capacity of the approach to predict the fatigue properties accurately, and to discriminate quickly the fatigue properties of several close materials.
Section snippets
Material and samples
Four different materials were tested, with different kinds of polymer matrix and amounts of carbon fibers (referenced as A, B, C, D). All the polymer matrices were thermostable materials sourced from several suppliers and are either PEEK or PAEK matrix reinforced with 30%–40% (in weight) of short carbon fibers. Specific details are not given here due to confidentiality restrictions. It is important to underline that all these materials are similar and are potential candidates for industrial
Evolution of the mechanical data during the fatigue tests
The evolutions of the cyclic mechanical features were recorded during the fatigue tests. The secant modulus, the hysteretic loop and the residual strain (i.e. the strain at the end of each cycle) were computed from the nominal strain and stress derived from the extensometer and the load cell. Typical curves of these evolutions are presented in Fig. 5. These evolutions illustrate that the hysteresis remained stable over the full fatigue test and that the value evaluated at the beginning of the
Validation of the hypothesis for the heat build-up curves
The objective of this section is to sum-up and check the hypotheses required to deduce the fatigue properties from the heat build-up curve. A first set of hypotheses is related to the evaluation of the dissipated energy. In this case, a protocol valid for adiabatic conditions was applied on the additional assumption of constant dissipation per cycle. Both conditions were validated in Section 2.4.2. Moreover, the energy evaluated seems representative of the whole section of the sample. Finally,
Conclusions
This paper focused on thermostable matrices reinforced with short carbon fibers and aimed at three main objectives. The first was to highlight and investigate if a heat build-up approach can be applied to these materials. This was checked both from a measurement point of view (adiabaticity, spatial distribution of the dissipation sources) and from a criterion point of view (evaluation possible over a limited amount of cycles). Compared to previous papers on other materials published by the
Acknowledgements
The authors would like to thank the French ANRT Agency for its financial support (CIFRE n° 2014/1081).
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