# Environmental effects on bolted composite joints subjected to preload

Ivanna Pivdiablyk, Patrick Rozycki, Laurent Gornet, Frédéric Jacquemin, Stéphane Auger

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Ivanna Pivdiablyk, Patrick Rozycki, Laurent Gornet, Frédéric Jacquemin, Stéphane Auger. Environmental effects on bolted composite joints subjected to preload. ICCM22 - 22nd International Conference on Composite Materials 2019, Aug 2019, Melbourne, Australia. hal-02443124

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# ENVIRONMENTAL EFFECTS ON BOLTED COMPOSITE JOINTS
SUBJECTED TO PRELOAD

I. Pivdiablyk<fnref n="1" />, P. Rozycki<fnref n="1" />, L. Gornet<fnref n="1" />, F. Jacquemin<fnref n="2" /> and S. Auger<fnref n="3" />

**Keywords:** thermoplastic composite, environmental impact, bolted joint, preload

## ABSTRACT

Bolted joints are among most commonly used mechanical connection types in various domains. Application of bolt preload favours “filtering” external forces acting on a structural unit. Nevertheless, this preloaded state tends to loosening over time. Being slightly noticeable in metallic joints, the loss is multiplied by numerous parameters in case of composite ones and can reach nearly 30% within two weeks. The influence of some parameters, like specimen thickness and moisture content, is studied experimentally. The experimental campaign is completed by a numerical analysis of the composite material properties with an application of a homogenisation methodology and finite element method. Experimentally computed matrix properties and reconstructed relative volume elements of a fibre bundle and composite serve as a basis for the evaluation of out-of-plane and in-plane mechanical parameters. The latter are validated by the experimental in-plane properties of the composite material. As demonstrated, the reduction in out-of-plane elastic modulus leads to the increase of part compliance, causing the disjunction of assembled parts due to the higher losses of preloading force.

## 1 INTRODUCTION

A necessity to employ different materials in a structure, containing numerous parts, leads to the use of various connection types that are to be safe. They can be either demountable or permanent; hence, one is preferred to another or they are combined according to an application field, a structural material, external loadings, etc. Regardless of brand-new and promising joints dedicated to composite materials, the bolted ones still remain among the most commonly used connection types. Such assemblies possess both advantages and disadvantages. Material disarrangement and damage due to a clearance hole, bearing damage in a zone “material-nut” represent some problematics of bolted joints related to composite materials. What is more, the creep damage may occur resulting from a prolonged loading. However, the possibilities to maintain good performance over time and to filter external forces are not neglected, thus the bolted connections are widely used to fasten the composite materials.

A design of a bolted joint is directly related to the properties of assembled parts and bolt, applied forces, etc. A European standard NF E 25030-2 [1] specifies the integral computation of such connection, providing a complete mechanical behaviour and a final product design. The methodology, firstly introduced in 1977 and then revised in 1986, 2001, 2015, facilitates the comprehension of numerous parameters, used for the proper design, whereas the computation is based on the mechanical characteristics of the elementary constituents of a joint. In order to facilitate both, design and computation of metallic connections, the CETIM developed and commercialised a software CETIM-Cobra, which is based on the standard. It is thus adopted by enterprises, specialising in aeronautical, automobile, railways, oil and gas domains. Nevertheless, the standard, hence the software, is yet limited to the use of metallic materials in terms of homogeneous structure and elastic properties.

<footnote n="1">¹ Research Institute in Civil and Mechanical Engineering, Ecole Centrale Nantes, Nantes, France, [https://www.ec-nantes.fr/](https://www.ec-nantes.fr/), [ivanna.pivdiablyk@ec-nantes.fr](mailto:ivanna.pivdiablyk@ec-nantes.fr), [patrick.rozycki@ec-nantes.fr](mailto:patrick.rozycki@ec-nantes.fr), [laurent.gornet@ec-nantes.fr](mailto:laurent.gornet@ec-nantes.fr)</footnote>

<footnote n="2">² Research Institute in Civil and Mechanical Engineering, Université de Nantes, Saint-Nazaire, France, [https://www.univ-nantes.fr/](https://www.univ-nantes.fr/), [frederic.jacquemin@univ-nantes.fr](mailto:frederic.jacquemin@univ-nantes.fr)</footnote>

<footnote n="3">³ Technical Centre for Mechanical Industry, Nantes, France, [https://www.cetim.fr/](https://www.cetim.fr/), [Stephane.Auger@cetim.fr](mailto:Stephane.Auger@cetim.fr)</footnote>
Resulting from the increasing application of composite materials, the dimensioning norms are to be extended and adapted for them as well.

Despite some mentioned drawbacks, a relevant benefit, differing bolted from the other mechanical joints, lies in the capability to apply a preloading force that would delay the separation of assembled parts. The preload is dependent on such parameters as the part and bolt compliances and load introduction factor [1,2]. However, the applied preload has a tendency to decrease over time. For instance, the loss can reach 4–7,5% in case of metallic assembly with applied coatings [3]. Higher preload loss is expected in joints with the use of some composite materials. It is related to the weak resistance to creep over time, which can be enhanced by an impact of environmental conditions.

Globally, the effect of temperature and humidity results in a decrease of important mechanical characteristics of glass fibre reinforced polyamide 6 in elastic and inelastic regions as reported by numerous authors [4–6]. Such high sensitivity is related to the matrix that possesses amide groups. The latter has a significant impact on the glass transition temperature $T_g$. A brief overview of the neat resin properties is introduced in [7]. Consequently, physical and mechanical properties of the composite tend to degrade over time, leading to the poor out-of-plane resistance as well [8–12]. This directly affects the durability of preloaded bolted joints [2].

Evaluation of the joint capacity to sustain a preload is done through an experimental campaign. It is of a great interest and importance to analyse the effects of part geometry and environmental conditions in more details. The purpose of variation of specimen thickness and quantity is to reveal an impact of changing mechanical properties on the magnitude of preload loosening. Hence, the specimens of 2 mm, 4 mm and 6 mm are loaded and placed in a controlled environment of +23°C and RH 50%/85% for the preload recording over seven – ten days. This study is therefore supplemented by the numerical modelling of a Representative Volume Element (RVE) of the composite material. Its out-of-plane resistance is computed with the use of a homogenisation technique for a two-fibre model and the RVE as a function of environmental conditions, including the ones from the experimental campaign. This estimation would allow us to relate the preload losses to the actual mechanical properties of the composite material.

## 2 MATERIALS AND EQUIPMENT

The composite material, used in the project, is a polyamide 6 reinforced with continuous glass fibres from Lanxess. The fabric is of twill weave 2×2 with a thickness per layer of 0.5 mm and a fibre volume content 47%. Delivered in form of panels, so the thicknesses are 2 mm, 4 mm and 6 mm, it was cut with a water jet according to required geometrical configurations. “Washer-type” specimens with external and internal diameters of 36 mm and 13 mm respectively are used for the experimental part, aimed at analysing the loss of preload. In order to ensure the required moisture content, the specimens are initially subjected to the desiccation and the consequent humid ageing according to the proposed experimental protocol in [7].

Bolts of a diameter 12 mm, provided by Bumax manufacturer, possess a high resistance to a great range of temperatures. They are instrumented with two diametrically opposed uniaxial strain gauges in order to track occurring elongation when applying preload. The calibration of each bolt is performed to eliminate the residual stresses and ensure the proper response of gauges. It also allows the direct conversion of axial deformation into applied force, where the latter is used to maintain the control over experiments. The nuts and washers have the external diameter 18 mm.

## 3 NUMERICAL MODELING OF A COMPOSITE MATERIAL

This section is dedicated to the numerical study of the composite material that is conducted in parallel with the joint testing. The aim is to supplement the experimental results, presented in the section 4, in order to have a better comprehension of the mechanical behaviour of the material and its joint when in service.

### 3.1 Research problem

Material mechanical out-of-plane properties play a crucial role in joint dimensioning. A while ago,
it was not of concern to researches and engineers due to the homogeneous properties of metals. However, more attention is devoted to this issue nowadays as a result of extensive application of composite materials. Experimental inaccessibility of certain out-of-plane parameters, like $E_3$, $G_{13}$ or $G_{23}$, is one of the current challenges; even if one carries out such experiments, the probability of induced errors and, thus, the misleading statements is high. To partially resolve this challenge, a numerical simulation with the use of a homogenisation technique is proposed here to identify the missing parameters. It consists in computing the homogenised properties of the RVE using the finite element method FEM via Cast3M – CEA. In terms of the initial input parameters, these calculations are partially based on the experimentally obtained mechanical properties of a neat resin PA6 that is used as a matrix in the composite (Fig. 1). This campaign allows us to calculate the Young's modulus $E_3$ and shear moduli as a function of some environmental parameters of the RVE. The results are then validated by the comparison with the experimental in-plane tensile properties, introduced in [7].

The neat PA6 resin is tested according to the same environmental conditions as mentioned by authors in the article [13], hence at five temperatures, -40°C, -10°C, +23°C, +40°C, +80°C, and at three levels of Relative Humidity RH as 0%, 50% and 85% with the initial conditioning according to the developed protocol, presented in [7].

<figure><img src="image_2.png" /><figcaption>Figure 1: Input: tensile modulus of PA6 as a function of temperature and RH.</figcaption></figure>

## 3.2 Reconstruction of RVE

The geometry of the RVE of the woven composite material is created using the free software TexGen according to the local observations of the composite material. The microscopic analysis is performed using the images, taken with Scanning Electron Microscope SEM at several magnitudes (Fig. 2 and 3). They are then processed with the ImageJ software – a free platform for the image analysis (see [13] for more explanations).

<figure><img src="image_3.png" /><figcaption>Figure 2: SEM image of a bundle at ×900 magnitude.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3: SEM image of a composite cross-section at ×30 magnitude.</figcaption></figure>
The real fibre content in bundles is thus evaluated: $v_f = 72.4 \pm 5\%$. Then, along with obtained bundle dimension, such as width, thickness and spacing, the RVE is reconstructed. Selected values are the average of forty measurements. The model is thus used for Cast3M computations.

## 3.3 Homogenisation

A two-scale homogenisation approach is applied for more efficient computation. The first homogenisation is conducted on the micro-scale (fibres and matrix), while the second one is on the meso-scale (bundles and matrix).

In order to realise a thorough numerical characterisation of the woven RVE (Fig. 5), bundle mechanical properties are required. A bundle, seen as a combination of fibres linked to each other with the matrix, is impossible to characterise experimentally and get necessary parameters. Thus, it is proposed to apply the numerical homogenisation method by using the manufacturer's characteristics for the fibres and the experimentally obtained resin properties (Fig. 1). For that, the widely used two-fibre model (Fig. 4) is applied in accordance with the calculated fibre content on the different scales (section 3.2). Consequently, a range of fibre bundle mechanical properties is computed as a function of environmental conditions, introduced together with the resin.

<figure><img src="image_5.png" /><figcaption>Figure 4: Homogenisation of a bundle.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>a)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>b)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>c)</figcaption></figure>

<caption>Figure 5: Overview of the RVE homogenisation. The notations stand for: a) fabric with homogenised bundles; b) matrix and c) layer of the composite, consisting of the fabric and the matrix.</caption>

Resulting from the homogenisation at the microscopic level, the bundle and resin properties are used for the homogenisation of the woven RVE. This model is initially reconstructed in accordance
with the experimental geometrical characteristics of the composite material (section 3.2); the consequent slight adjustment is necessary in order to ensure the global fibre content of 47%. The RVE is, therefore, created under some hypothesis: 1) the perfect theoretical geometry; 2) perfect contact between fibres/matrix and bundles/matrix; 3) homogeneous moisture content in the matrix and bundles; 4) the matrix and the experimental resin PA6 are equivalent. The numerical simulations allow identifying the homogenised out-of-plane properties of the composite material, required for computations of bolted joint parameters.

## 3.4 Results and Discussions

The composite material – PA6 GF – is known for its hydrophilic properties due to the present amide groups in the matrix. Mechanical parameters, determined from the thorough experimental campaign by authors, are presented in [7,13] and Fig. 6 – 7. They certainly vary when exposed to different temperatures, but also the coupled effect “temperature – humidity” appears due to the changing glass transition temperature $T_g$, which is in agreement with [10,14,15].

The numerical simulation of the RVE, aiming at identifying out-of-plane properties, demonstrates the correspondence between the in-plane numerical and experimental results, presented in Fig. 6 – 7. Comparing $G_{12}$ of RH 50% and 85% at -10°C and +23°C, it is evident that the latter is slightly lower. One of the reasons is its proximity to the $T_g$ that is around -1°C for RH 85% and +23°C for RH 50% [7]. Such variation is caused due to the presence of moisture in the composite material.

The validation of numerically estimated in-plane shear modulus allows us to estimate the elastic modulus $E_{33}$ of the woven material, which is difficult to access experimentally.

<figure><img src="image_9.png" /><figcaption>Figure 6: Numerical and experimental results PA6 GF at RH 50% and four temperatures.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 7: Numerical and experimental results PA6 GF at RH 85% and four temperatures.</figcaption></figure>

Computed out-of-plane elastic modulus is demonstrated as a function of environmental conditions in Fig. 8. It is important to note that it has a tendency to decrease with the rise of temperature. Regarding the impact of moisture content, the lower is the RH level, the higher are the properties. Hence, the elastic modulus affects the bolt preload and, thus, the joint behaviour through the part compliance as presented in the following equations:

$$ \delta_p = \frac{1}{S_{eq}} \cdot \sum_{i=1}^{n} \frac{L_{pi}}{E_{pi}}; \qquad (1) $$

where $\delta_p$ stands for the part compliance, $E_{pi}$ is the out-of-plane modulus, $S_{eq}$ is the equivalent compressed section and $L_{pi}$ is the part thickness;

$$ F_0 = \frac{\delta_p}{\delta_p + \delta_b} F_{ext}; \qquad (2) $$

where $F_0$ stands for the preload, $\delta_b$ is a bolt compliance and $F_{ext}$ is the acting external forces [1,2]. As a consequence, the material rigidity has a significant influence on the preload level that delays the interface opening of tightened parts. What is more, the preloaded bolt has a tendency to loosening over
time due to creep behaviour of the composite parts. As demonstrated in the following section, it might be amplified by the environmental conditions.

<figure><img src="image_11.png" /><figcaption>Figure 8: Numerical results of $E_{33}$ as a function of environmental conditions.</figcaption></figure>

# 4 EXPERIMENTAL STUDY OF PRETIGHTENED BOLTED JOINTS

The loss of mechanical properties of assembled parts has an unfavourable effect on durability of a bolted joint. As shown in the section 3, the mechanical behaviour of the studied composite is indeed dependent on the environmental conditions that have a harmful impact; hence, the elastic parameters undergo an important decrease. The sudden drop of mechanical properties and the increase of creep deformation emphasize the viscous nature of the thermoplastic material at particular environmental conditions [9,16,17]. This leads to the decrease of capacity to sustain a high level of preload, also required to ensure fatigue endurance of fasteners [1]. Hence, an experimental campaign is launched in order to evaluate the preload losses over time when joints are exposed to regulated environmental conditions.

## 4.1 Methods

Circular-shaped specimens of 2 mm, 4 mm and 6 mm thickness are conditioned following the methodology, described by authors in [7]. Almost every experimental configuration consists of joining two specimens of the same thickness, so the final thickness of assembled parts is 4 mm, 8 mm and 12 mm. Two more configurations are used: one specimen of 4 mm and a few metallic specimens. The external diameter of 36 mm provides a complete inclusion of compressed zone [2], theoretically presented in the form of cone with the border's slope of 45° in case of metals [1]. The experimental configuration is demonstrated in Fig. 9. It is repeated three times; thus the total number of simultaneously running tests is 16, so the last bolt remains free for the temperature compensation. The operating length of bolt is 42 mm.

<figure><img src="image_12.png" /><figcaption>Figure 9: Design of a bolted joint by CETIM Cobra 6. The thin composite parts (2 mm + 2 mm) are of blue and orange colours.</figcaption></figure>
A torque, equivalent to compression of 35 kN, is applied to fifteen joints. This loading plays a role of pretightening. The approach of the load computation is presented in [7]. It consists in the loading till the appearance of bearing damage on the composite specimens caused by the metallic washer. Then the experimental facility is placed into a climatic chamber Binder KMF-115, regulated in temperature and humidity. This part of experiment runs for seven – ten days with the continuous data recording of bolts elongation. The subsequent reloading to 35 kN is then applied. The second part of the test lasts equally for seven – ten days.

## 4.2 Results and Discussions

The intermediate results of the experimental campaign are demonstrated in Fig. 10 and 11. It is important to mention that the applied reload results in the lower loss of bolt elongation over the same period of time as compared to the initial loading. This is valid for all specimen configurations, including the metallic specimens. Regarding the humidity impact, the pretightening relaxation is globally higher at RH 85% in comparison to RH 50% for all specimens. With respect to the specimen thickness, the losses are smaller for the thinner specimens. The presence of the joint plane also results in the higher losses of bolt elongation as it is shown for the 4 mm specimen and two specimens of 2 mm thickness. Nevertheless, there are some undesirable effects that occurred during the loading. Hence, the sliding between the assembled parts and the bolt torsion could have an impact on the results. A proper evaluation of probable induced errors is under consideration.

To conclude, this study facilitates the evaluation of the bolt elongation as a function of specimen geometries and environmental conditions. It will contribute to the proper design of bolted connections via a commercial software CETIM-Cobra.

<figure><img src="image_13.png" /><figcaption>Figure 10: Loss of bolt elongation at +23°C and RH 50%</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Figure 11: Loss of bolt elongation at +23°C and RH 85%</figcaption></figure>

## 5. Conclusion

Environmental parameters have the significant impact on durability of the composite material polyamide 6 reinforced with glass fibres due to its hygroscopicity and temperature dependence. The rigorous RVE modeling and its numerical simulation, validated with the experimental in-plane shear modulus, show the impact of the moisture content and temperature on the out-of-plane elastic modulus, which tends to decrease with the rise of both. This results in the increase of composite compliance, thus, as demonstrated by authors, the mechanical behaviour of the bolted joints as well as the applied preload is affected. The loss of bolt elongation is demonstrated too be dependent not only on the environmental conditions, but the thickness of assembled parts and the presence of joint plane as well. However, the second applied pretightening leads to the reduction in the preload losses.

Thereafter, for short-term perspectives, extra analyses of the plastic deformation in composite parts caused by the metallic washer could serve to evaluate the limitations of preload to avoid material damage if required. Regarding long-term perspectives, an impact of higher/lower and cyclic loading on tightened bolted composite joints could be explored. What is more, the variation of part and washer geometries might widen the database of CETIM Cobra, used for the joint dimensioning.
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