An investigation of in-plane dynamic behavior of adhesively-bonded composite joints under dynamic compression at high strain rate
Introduction
As the role of structural adhesives becomes more pronounced in composites used in technological applications such as aeronautics, military and naval etc, it has become necessary to understand the dynamic behavior of adhesively bonded joints. To design structures subjected to high impact loads in such applications, the knowledge of dynamic properties is required for developing constitutive models of the tested materials [1], [2]. In addition, the link between the dynamic behavior and the dynamic damage evolution in adhesively bonded joints at high strain rates, is still far from being fully understood. Thus, it is significant to achieve an accurate description of damage under dynamic loading and effect of strain rate on the behavior of these materials for safety and design considerations. The split Hopkinson bar (SHPB) has been widely used to determine the dynamic properties of materials at high strain rates [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Significant efforts have been made to investigate the effect of high strain rate on the dynamic behaviour of polymers adhesives and composites laminates using the Split Hopkinson bar under various loading conditions [13], [14], [15], [16]. Hosur et al. [17] investigated the dynamic response of unidirectional carbon/epoxy composite material subjected to in-plane dynamic compression tests using SHPB with different strain rates of 82, 164 and 817 s−1. They compared the dynamic results with static compression tests and observed that strength and stiffness of the material are higher in dynamic compression than in static compression test. Kumar et al. [13] studied the dynamic compressive response of unidirectional glass/epoxy composite at various off-axis angles in the transverse and logitidunal direction.
They demonstrated in their study that the longitudinal compressive strength and failure strain increase however, the longitudinal modulus decreases with increasing strain rate. El-Habak et al. [14] examined the mechanical behaviour of woven glass fibre-reinforced composites at strain rates from 102 to 103 s−1 for three types of matrix (polyester, vinylester and epoxy). They concluded that the vinyester matrix allows to obtain the highest strength. Tarfaoui et al. [5] studied the strain rate effect on the dynamic behaviour of glass fibre reinforced polymer under in-plane and out-of-plane dynamic compression tests using SHPB varying the fibres orientation and the loading conditions. The results showed that the dynamic strength of the material depends strongly on fibre orientation and impact pressure for in-plane tests and the material is significantly sensitive to fibre orientation for out-of-plane tests at the same impact pressure. Arbaoui et al. [9], [10] studied the dynamic compressive behaviour of [0/90°]26 glass/epoxy laminates in-plane and out-of-plane directions using (SHPB). They concluded that the dynamic properties are strain rate sensitive and the material is more resistant in the case of out-of-plane loading as compared to the in-plane loading case. Li et al. [18] studied the thermodynamic behaviour of unidirectional carbon/epoxy laminate subjected to in-plane and out-plane dynamic compression and tensile tests at different strain rates using (SHPB). It was concluded that the damage modes are responsible for heat dissipation in the material for both dynamic tests.
It is apparent from the available literature that little research has been carried out to study the dynamic behaviour of the adhesively bonded joints in comparison with the specimens made entirely in polymeric adhesives [19], [20], [21], [22], [23], [24]. Although, obtaining reliable data regarding these adhesively bonded joints is complex and depends upon the combination of different materials but in general, most of the research is carried out to investigate either the adhesive formulation, or to study the effect of the strain rate and temperature on adhesive under dynamic impact using various simple ad hoc tests. For example, L. Goglio et al. [20] studied the effect of the strain rate on the dynamic tensile and compression strength of specimens made of adhesive «epoxy» using the SHPB. Their results showed that the adhesive strength increases substantially by increasing the strain-rate. Takeshi Iwamoto et al. [21] investigated the effect of strain rate on the compressive stress-strain behaviour of epoxy resin adhesive using SHPB. They concluded that in the elastic deformation, a semi-logarithmic approach could be established to link the peak stress in the low strain rate and nonlinear laws in high strain rates. To explained the non-linear behaviour, the authors have defined rheological models taking into account the plastic deformation of the glassy epoxy polymer. Sharon et al. [24] studied the effects of loading rate and temperature on the mechanical properties of four structural adhesives. They demonstrated that the yield stress and modulus decreased with temperature, while increase in loading rate resulted in an increase in yield stress with negligible influence on the modulus. Calberger et al. [19] studied the effect of the temperature and the strain rate on the cohesive properties of an epoxy adhesive. Their results showed a negligible effect of temperature on the fracture thoroughness from −40 °C to 80 °C (the glass transition temperature of the epoxy adhesive investigated is +90 °C) but the peak stress in peel loading decreased significantly with increasing temperature in this range and both cohesive parameters increased with increasing the strain rate. Srivastava et al. [25] investigated the effects of temperature and strain rate on the bond strength of lap joints in Ti-6Al-4 V and C/C-SiC composite. They concluded that the bond strength decreases by 40–50% at 300 °C, while it increases with the strain rate. So, it is clear that adhesive materials are high strain rate sensitive. Therefore, it can be expected that the dynamic response of adhesive joints would be strongly dependent on the strain rate under impact conditions.
In this study the effect of the strain rate on the dynamic behaviour of adhesively-bonded composite joints subjected to in-plane loading using SHPB is investigated. Compression tests were performed at four different impact pressures (1, 2, 3 and 4 bar). High speed camera was used to draw up the history of the dynamic damage in the specimens in response to the strain rate evolution. The failure modes of adhesively-bonded composite joints have been determined and also discussed. To summarize the experimental results, empirical models depending upon the strain rate effect, have been established. These empirical models will play a useful role in developing damage models for design optimization of adhesively-bonded composite structures.
Section snippets
Materials
The material used in this study is the one taken directly from a real superstructure of Fregate. It consists of a 45° Bi-axial fiber-glass mat of 0.286 mm thickness in a Polyester resin matrix. In adhesive bonding, a NORPOL Polyvinylester of 1 mm thickness has been used to assemble the composite substrate. The composite mechanical properties are given in Table 1, Table 2, Table 3. For these dynamic compression tests, samples with 13 mm × 13 mm × 9 mm for in-plane tests are considered, as
Strain rate dependencies of mechanical properties
Fig. 3 shows typical signals for the above mentioned dynamic compressive tests at 1 bar and 4 bar. The incident and transmitted waves are compressive while the reflected wave is tensile. It can be seen that the respective incident, reflected and transmitted wave heights depend significantly on the striker bar velocity. As expected [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], the second peak of the reflected pulse and the short duration of the transmitted pulse, show the presence of
Conclusion
In the course of this study, an experimental approach was implemented with an aim of investigating the in-plane dynamic mechanical behavior of an adhesively bonded composite joints. Samples were tested at four different impact pressures of 1, 2, 3, 4 bar and strain rates of 210, 312, 528, 713 s−1 respectively with SHPB apparatus. It has been concluded that materials have shown a strength dependency on strain rate and there is considerable increase in dynamic parameters with the increase in the
Acknowledgments
This work was funded by DGA (Direction générale de l'armement – Ministry of Defense, France), MRIS project: Study of the thermomechanical behavior of composites under dynamic loading. The Authors of this paper gratefully acknowledge the financial support of the DGA, France. Acknowledgments have also addressed to Pr. Bruno Mortaigne from DGA.
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