Short-term creep and low cycle fatigue unified criterion for a hybridised composite material

https://doi.org/10.1016/j.ijfatigue.2021.106571Get rights and content

Highlights

  • An original hybrid material combining short and continuous fibres is presented.

  • An unified fatigue criterion based on steady state creep strain rate is proposed.

  • The unified criterion shows a link between fatigue and creep phenomena.

  • The Monkman–Grant relationship is verified both for creep and low cycle fatigue.

Abstract

The objective of this work is to determine a low cycle fatigue criterion for a thermoplastic matrix composite reinforced with both short and continuous fibres. The proposed criterion is based on the steady state creep strain rate and unifies the results for different fibre orientations, stress ratio, frequency and water content. The criterion is applicable in low cycle fatigue and creep. By means of this criterion, the Monkman–Grant relationship is verified both for creep and low cycle fatigue and offers interesting perspectives for numerical approaches.

Introduction

Thermoplastic matrix composites are increasingly used for transport and mobility applications because they offer an interesting potential for weight reduction by replacing metal parts. Short and long fibres reinforced thermoplastic composite materials are now used in structural parts such as clutch pedals [1], engine mounts [2] and fuel rails [3]. The addition of continuous reinforcements using the DynaFib [4] process further improves the mechanical properties and thus the weight reduction potential of these composite materials. Hybridisation of short fibres with continuous fibres in the same material brings innovative design solutions. In the material under consideration, a continuous fibre reinforced thermoplastic (PA6GF60) is overmoulded with a short fibre reinforced thermoplastic (PA66GF50). The prescribed hybridisation does not use fibres of different natures, as is generally the case for composites, but relies on the use of two lengths and two diameters of reinforcement through the thickness of the material. In the following we will use the term hybrid material to refer to the continuous UD ply covered on both sides with SFRT plies, i.e. the short fibre reinforced thermoplastic plies.

In order to better assess the lifetime of the technical parts made from this hybrid material, it is essential to use reliable representative mechanical models to meet all relevant criteria such as part stiffness, strength, durability, impact, noise, vibration and harshness, creep deformation [5]. This work focuses on the Low Cycle Fatigue (LCF) of the hybrid material. The hydrothermal environmental variables, as well as the microstructure induced by the manufacturing process are important parameters to take into account [6], [7], [8], [9]. This paper aims to define a LCF criterion for the whole hybrid material as well as for the SFRT separately while providing a relationship with the creep loading. This work is part of a more global study, which consists of experimentally identifying the damage mechanisms in the LCF domain, both on macroscopic and microscopic scales.

The fatigue response of polyamide 6.6 (PA66) SFRT materials has been studied mainly in the field of High Cycle Fatigue Domain (HCF), beyond 103 cycles. For HCF, several authors have shown that the cyclic creep strain rate criterion seems to be relevant to unify fatigue curves of such materials. [10] showed a linear correlation between the fatigue life and the cyclic creep strain rate. [11] investigated the influence of frequency on the fatigue behaviour of a polyamide 6 (PA6) SFRT. They expressed the relationship between temperature rise, applied stress, and cyclic creep strain rate in terms of a parameter derived from the Larson-Miller [12] steady creep parameter. [13] showed that cyclic creep strain rate unifies tests at different temperatures for a PA66 reinforced with 30 wt% short glass fibres. [9] was able to unify the fatigue results on a PA66GF30 at 50 % RH for load ratios ranging from 0.1 to 0.7 with cyclic creep strain rate as criterion. [14] proposed two cyclic creep strain rate based criteria; one of the two introducing anelastic energy, to better take into account mean stress effect and unify fatigue life predictions within scatter bands of factor two. Finally, [15] evaluated the accuracy of many of the literature criteria on a large database that took into account the influence of orientation (0°, 45° and 90°), stress ratio (−0.5 to 0.7) and the position of the test temperature in relation to the glass transition temperature (Tg). They showed that criteria based on cyclic creep strain rate gave very good results to unify fatigue tests except for negative stress ratios. Finally, they proposed two unified criteria based on cyclic creep strain rate and mean stress in order to unify the whole test database (over 90 % of the database was unified within a scatter band of three).

All these studies have been carried out mainly in the field of HCF with materials with free faces. The first purpose of this paper is to unify results in the LCF domain for SFRT with a criterion based on cyclic creep strain rate. In the hybrid material under study, the UD reinforcement constrains the SFRT at the interface between the two materials (Fig. 1). Crack growth mechanisms observation of the hybrid material, at different scales, allowed the determination of a damage chronology that highlights the main influence of the SFRT in the failure. Fig. 1(a) presents the crack formation on the SFRT surface during the last fatigue cycle of the hybrid material, 2.7 ms before the strength loss. On most of the fatigue specimens, a crack appeared in the SFRT shortly before the failure, this crack seemed to propagate towards the face with the UD reinforcement and then led to the failure of the material. These observations were also made on a microscopic scale using fast X-ray tomography. Fig. 1(b) illustrates the cracks in SFRT on a scan performed during the last cycle. This information was obtained in a specific campaign carried out with synchrotron radiation. The local approach with micro-CT observation is outside the scope of this article which focuses on the global approach. This results shows that SFRT has a major role in the failure of hybrid material. This leads to the second purpose of this paper: apply this criterion to the hybrid material studied, consisting of an SFRT constrained by a UD reinforcement.

Concerning creep loading, links between the minimum or steady state creep strain rate and time to failure (tf) are usually represented in terms of Monkman–Grant relationships [16], [17]. The creep strain rate was first introduced as criterion to predict creep failure times for metals. It has been demonstrated by several authors [18], [19] that this empirical relationship is a powerful and reliable way to predict the long-term material lifetime from short-term tests over a wide range of temperatures and stress fields. Despite its simplicity it has succeeded to predict failure times, with some accuracy [20], for a wide range of materials, including also polymers [21], [22] or adhesive anchor systems [23]. The last objective is to relate fatigue to creep through this criterion base on creep strain rate.

In order to achieve these objectives, the paper is organised as follows: the first section describes the methodology for determining the criterion in the fields of fatigue and creep. The second section presents the material and the specimens used in the papers. In the third and last section the experimental approach and results are discussed. In this section, the influence of different parameters, such as microstructure and moisture absorption on LCF and creep results is highlighted. Then, in the last section, the cyclic creep strain rate criterion is evaluated in the LCF domain, first on the SFRT, then on the whole hybrid material. Finally, the cyclic creep strain rate is related to time instead of the number of cycles to failure in the case of fatigue loading in order to unify the results obtained in fatigue and creep.

Section snippets

Methodology

In the present work, all the scheduled tests were load controlled. This kind of loading is specific for creep tests for which the material response is generally analysed in terms of creep displacement or creep deformation/strain. For fatigue tests with an imposed load, a similar analysis is attempted here, by contrast with many results in the literature, with stress approach [13], [24] or energetic approach [25], [26]. In the case of load imposed tests, the strain is the dual variable of the

Material

The UD reinforcement is a PA6 reinforced by 60 wt% of continuous glass fibres with a diameter of about 20 µm; this material is referred to as UD PA6. The wrapping material is a PA66 matrix reinforced by 50 wt% of short glass fibres with an average length and diameter of about, respectively, 250 µm and 11 µm; this material is named as SFRT.

The hybrid material plates were processed, with the same mould, in three steps:

  • a three plies lay-up of UD PA6 was manufactured using an automated fibre placement

Experimental procedure

This section deals with fatigue and creep tests carried out on both SFRT and the hybrid material. As the tests were under load control, the focus will be on the cyclic creep strain rate. All the tests were performed on an INSTRON 8801 servo-hydraulic fatigue machine. The material response was then analysed by measuring the extension of the specimens by using an extensometer with a gauge length of 12.5 mm. In some cases this measurement was duplicated with strain gauges or Digital Image

Discussion: proposition for creep/fatigue criterion

The objectives of the criterion are to unify:

  • Fatigue results for SFRT and hybrid material;

  • Fatigue and creep SFRT result.

The principle of determining the criterion is set out in Section 2.

Conclusions

Creep mean strain rate criterion has been evaluated on a wide range of SFRT LCF and creep tests. LCF results have been obtained for a wide range of load ratios (from 0 to 0.8), frequency (0.5 Hz to 4 Hz), three fibre orientations (0°, 45° and 90°) and three water content conditions (0 % RH, 50 % RH and 80 % RH) at 23 °Celsius. Creep results have been collected for a range three fibre orientations (0°, 45° and 90°) and three water content conditions (0 % RH, 50 % RH and 80 % RH) at 23 °Celsius. LCF hybrid

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 authors gratefully acknowledge ContiTech AVS France for supporting this work. The authors wish to thank Yvon Tirel and Gilles Lucas for their help in moulding the plates and developing the process, and Romain Ricordel, Zohra Announe and Aginath Selliah for their help during LCF and creep testing. Finally SG gratefully acknowledges Susan Cameron for her proofreading.

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