On the influence of mechanical loadings on the porosities of structural epoxy adhesives joints by means of in-situ X-ray microtomography

https://doi.org/10.1016/j.ijadhadh.2020.102568Get rights and content

Abstract

Structural bonding is a beneficial technique extensively used in numerous industrial fields. This technique is however prone to structural defects such as pores, which are created during the mixing of the adhesive and during the shaping of the joint. Depending on their characteristics, these pores are likely to influence the mechanical behaviour of adhesively bonded joints, as they induce local decreases in the cross-section of the bonds and they may also create threatening stress concentrations. It is also fair to assume that the characteristics of the pores within an adhesive joint are subject to changes when the assemblies are submitted to external loads. In order to investigate these changes, adhesively bonded samples were made using two different bicomponent epoxy structural adhesives. These samples were placed inside an X-ray tomograph, containing a tensile machine. In-situ X-ray tomography measurements were made simultaneously with the application of a tensile load on the samples. It was therefore possible to characterise the porosity states of each sample under mechanical loading, and to compute various quantities (porosity volumetric ratio, the pores number, equivalent diameters distributions, etc.). It was found that the pores in the joints are impacted by the increasing mechanical stress, resulting in pore nucleation, pore growth and coalescence. Moreover, the present study shows that this microstructural behaviour cannot be generalised, as different adhesives may display different properties.

Introduction

A wide range of industrial fields nowadays use structural bonding for their applications, such as aeronautics, automotive or renewable energies. This extensive use is explained by the many advantages adhesive bonding features as opposed to bolting or riveting: multimaterials assembly capabilities, decrease in weight, preserved structure integrity, etc. Unfortunately, this technique also has disadvantages [1]: the quality of the bond highly depends on the bonding process (surface treatment, curing, etc.) [2], the mechanical behaviour of structural adhesives is non-linear and difficult to model accurately, and bonding defects are very likely to happen during the shaping of the joint. These imperfections, often unavoidable, include the presence of pores within the material. These pores, created during the mixing of the adhesive components and during the shaping of the adhesive joint, can be a threat to the good mechanical strength of the bond: they damage the integrity of the material, they decrease the cross-section of the joint, and they can induce unwanted stress concentrations. These pores being structural defects inside the adhesive joint, it is fair to hypothesise that they could have an influence on the mechanical properties of a bonded assembly. To validate this assertion, it is however essential to be able to detect these pores inside an adhesive joint.

X-ray microtomography is a fairly popular solution to detect and visualise such entities located in a bulk of a medium. This technique is increasingly used in materials science due to its various advantages as it is detailed by Buffière et al. [3]: it is non-destructive, the measurements are three-dimensional, it allows the visualisation of the internal structure of non-transparent media, etc. Moreover, depending on the tomograph, the resolution of the measurements can be lesser than 1 μm, which is a quite attractive feature for damage and defects detection. That is why this tool has been used in the past on a variety of materials for similar purposes such as alloyed metals and composite materials. For instance, Liu and Bathias studied the effects of the presence of pores in a Aluminium alloy reinforced composite in terms of tensile and fatigue properties [4]. In a similar fashion, Breunig et al. managed to detect fibre fracture and interface debonding in a SiC/Al MMC material under wedge and 3-points bending loadings using X-ray tomography [5]. Polymeric materials were also studied using such a technique: in particular Garcea et al. succeeded in visualising cracks in a polymer composite [6], in spite of the many experimental issues specific to X-ray measurements performed on these materials. Notably, the authors used the reconstructed volumes to build a Finite Element Model, so as to be able to predict both damage initiation and propagation in the material. As far as damage characterisation is concerned, in-situ microtomography measurements are particularly useful, as they enable the tracking of both the appearance and the evolution of damage-related phenomena under mechanical stress (crack propagation, pores coalescence, delamination, etc.), as shown in numerous studies carried on by the teams of Maire, Adrien and Buffière [[7], [8], [9], [10], [11]]. Most of these reference works take advantage of in-situ X-ray tomography to characterise materials under various types of loadings: metals under tensile loading [10], polymeric syntactic foams under compression [11], metal matrix composites loaded in tension [8], etc.

On the subject of polymers, a few studies may be found in the literature. A commonly found topic is the fabrication of polymeric structures by means of additive manufacturing processes. These techniques tend to generate voids in the resulting materials, which are easily studied using X-ray microtomography. For instance, a paper released by Pavan et al. in 2016 characterised the porous network of laser-sintered polyamide structures, for different sizes [12]. It was found that the size of the structure had a significant influence the characteristics of the voids created during the process. More recently, Wang et al. proposed a micromechanical model in order to characterise the mechanical behaviour of 3D-printed polymers [13]. Nonetheless, few studies may be found specifically on adhesives, and even more so regarding adhesively bonded assemblies. This is probably explained by the a priori low risk of pore creation for these materials when compared to additive manufactured polymers. X-ray tomography has been fairly recently applied to the field of structural bonding, but mainly to characterise the interfaces between adhesives and adherend. For example, Schwarzkopf [14] used tomography measurements coupled with simulations to build a micromechanical model of adhesive-wood interfaces. McKinley et al. [15] took advantage of this experimental technique to characterise the bonding process and the penetration of the adhesive in the fibrous structure of wooden pieces. Virtually no attention has been given to the precise study of the microstructure of adhesive joints, whereas it may be an important factor to describe macroscopic phenomena, such as crack propagation. It should however be reminded that microtomography is not the only experimental technique able to quantify the porous state of polymers, or materials in general. For instance, one can use gas sorption and Hg injection, as it has been done by Rohr et al. [16] for porous resins derived from acrylate monomers. Such techniques are able to provide valuable data for extremely small pores, which cannot be visualised through X-ray tomography. However, it should be noted that less extensive knowledge regarding the geometry of the pores is obtained using these techniques, and they may not be suited to the study of bonded assemblies for in-situ testings, for example.

In this paper, the authors characterise the effect of an out-of-plane tension stress on the detectable pores included in adhesives joints using in-situ X-ray microtomography measurements. This is achieved on bonded assemblies, using two bicomponent epoxy adhesives. This characterisation is performed for various values of the applied load, in order to track diverse porosity-related quantities, such as the number, the volume fraction, the diameters distribution etc. A discussion on the results is finally proposed, so as to explain the highlighted phenomena. It should be reminded that this aim of this study is not the absolute characterisation of the porous network of these materials, but the detection of phenomena achievable with state-of-the-art laboratory tomography on adhesively bonded assemblies.

Section snippets

Design

The samples used in this study are butt-joint samples, bonded using a structural bicomponent epoxy adhesive. The dimensions of the samples are kept relatively small, in order to fit in the tomograph used for the tests campaign. As such, they are designed to feature a 6 × 6 mm2 bonded surface (Fig. 1a). These samples are waterjet cut from aluminium 2017A Scarf samples to form a rake-shaped pattern as shown in Fig. 1b. Each specimen is then to be cut from its Scarf base after the curing (Fig. 1

X-ray tomography principle

The interested reader may find detailed information on this particular matter in Ref. [17].

Microtomography is a non-destructive, three-dimensional imaging technology originally developed and used for medical applications [3,18]. As such, it quickly became of interest for materials science, as it allows researchers to access data from the bulk of a non-transparent material. Moreover, these data are three-dimensional, with a resolution down to 1 μm per pixel [3].

The technique relies upon the

Porosity volumetric ratio and number of pores

As these tomographic measurements were performed for various loads applied to the samples, it is possible to apply the analysis technique presented above to each dataset, in order to track the evolution of some characteristics that may be extracted from the segmented volumes. In a first stage, the following quantities shall be investigated: the porosity ratio η (Equation (1)), and the number of pores in the adhesive joints.η=100VporiphasesViwhere Vpor is the volume of the pores and Vi is the

Conclusion

Samples were bonded using structural epoxy adhesives in order to perform in-situ microtomography measurements under mechanical loads. Two adhesives were studied under out-of-plane tensile loadings, to investigate the influence of the applied load on the pores inside the joint. Using a specially designed segmentation tool, the tomography reconstructed volumes were segmented into their different constitutive phases. The isolated pores were firstly characterised using global quantities over the

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