Experimental methods to capture curing induced effects in adhesive bonded joints

https://doi.org/10.1016/j.ijadhadh.2020.102735Get rights and content

Abstract

The rapidly increasing use of structural adhesives, especially in the joining of automotive body structures has motivated various investigations of the effects of adhesive curing process on joints. The automotive-grade structural adhesives require heat curing, which, in the meantime, performs thermal loading on the substrates and causes undesirable effects in the joint. For example, the curing process results in complex residual stresses in the adhesive bond which are detrimental to the performance of the adhesive bond and thereby the automobile body structure, particularly the crashworthiness. To thoroughly evaluate such effects, this paper consists of two parts. The first part presents an innovative experimental method to characterize the thermal effects of the heat curing process on a multi-material single lap shear joint using digital image correlation. The second part of the study compares the performance of residual stress-induced joints against stress-free joints under tension loading at different strain rates. The proposed experimental method and the corresponding results from this study are expected to help comprehensively understand the adhesive joining process and its potential side effects on the automobile body structure.

Introduction

Enhanced concerns associated with increasing levels of emission of local pollutants (such as SO2 and NOX) and global greenhouse gases (such as CO2) have driven the automotive industry towards producing enhanced fuel-efficient vehicles. Among various strategies, vehicle lightweighting is considered as one of the most effective and thus attractive choices. In a steel-dominant automotive body, significant weight reduction is achievable by low-density material substitution [1,2]. Nevertheless, most lightweight materials, such as aluminum and reinforced plastics, usually company with increased prices. Based on a balanced consideration, multi-material body construction is thereby frequently adopted in the automotive industry. However, this solution is strongly limited by the difficulties of multi-material joining using conventional joining techniques (like spot-welding) [3,4].

As an alternative, adhesive bonding is applicable for joining dissimilar materials, such as ferrous metals, non-ferrous metals, and fiber-reinforced plastics. Such a method not only enables high-performance and flexible joints but also eliminates the weight and cost of fasteners, such as bolts, screws, and rivets, associated with mechanical joining techniques [5,6]. Therefore, structural adhesives and sealants have been used extensively in automotive body structures and such a trend continues to grow. Nevertheless, the fact that automotive structural adhesives are single component adhesives, which need to be heat cured (except 2-component adhesives used for CFRP parts) accounts for a critical problem: the difference in the coefficients of thermal expansion (CTE) of the joined parts has significant implications on the integrity and response of the Body-in-white (BIW) to external loading, especially thermal loading.

Furthermore, as per the current trend in the automotive industry, the adhesive heat curing process is combined with the automotive paint baking process based on the considerations of manufacturing process efficiency and economy [7]. At elevated temperature in the paint baking oven, different components of the body structure expand at different rates and magnitudes depending on their different CTE and air convection properties in local areas. After the adhesive is cured at the peak temperature, it constraints the thermal contraction in the components during the cooling down phase. This (when extended to all components and different joints in a BIW) leads to distortions in the structure and, more importantly, residual stresses in the adhesive-bonded joints. It is well established that the presence of such residual stresses is detrimental to the performance of the adhesive bond and thereby the automobile body structure, particularly crashworthiness.

Several efforts have experimentally shown the effects of residual stresses in adhesively bonded joints. Reedy et al. [8] studied the effect of fabrication residual stresses on the strength of a butt joint considering the stress relaxation behavior of the adhesive at different temperatures. They concluded that the effects of the stresses diminish with time due to the relaxation behavior of the adhesives. Kim and Lee [9] found that the load-bearing capacity of an adhesive bonded joint is greatly influenced by the fabrication residual thermal stresses. Apalak et al. [10] showed that the thermal mismatch between the substrates can result in huge thermal strains and affect the adhesive-bonded joint. Yu et al. [11] studied the residual stresses due to curing shrinkage and thermal expansion in epoxy-steel bi-material beams. Kropka et al. [12] investigated the role of residual stress on joint strength on a napkin-ring joint geometry. Experimental investigations done by Teutenberg [13] indicated the effect of residual stress in a lap shear joint compared to a stress-free joint at different degrees of cure of the adhesive. The results showed a considerable decrease in the displacement across the joint before fracture, along with a decrease in maximum force. Ma et al. [14] studied the effect of several curing curves on the residual stresses generated in the high-temperature phosphate adhesive-bonded joint on a single lap joint. Lucas F. M. da Silva et al. [[15], [16], [17]] studied the thermal residual stresses generated in the joint due to non-free thermal expansion and contraction in the joint, and also due to shrinkage of adhesives caused by the curing process. They evaluated the benefits of using a dual adhesive joint design between dissimilar materials over a wide range of temperatures, to mitigate the effects of thermal mismatch in the adhesive and adherends. The thermal stresses generated in the paint baking oven due to CTE mismatch or delta-alpha problem were also studied by Dietrich [18] and Regensburger [19]. They stated that the thermally induced stresses can be reduced by reducing the oven temperature and flattening the heating and the cooling cycle.

This paper presents an innovative experimental method to capture the thermal effects on the adhesive-bonded joint and helps in quantifying the induced effects which will help in better understanding of the complex residual stresses generated due to the adhesive heat curing process. In this paper, two types of experiments and the corresponding results are discussed:

  • (i)

    First, a unique experimental approach is shown to capture the effects of heat curing on an adhesive bonded single lap shear joint during the curing process using 3D digital image correlation. The tests were conducted on two metal substrates combinations using an automotive-grade structural adhesive.

  • (ii)

    Second, the effects of the residual stresses on the strength of single-lap shear joints at different strain rates are discussed. The joints with residual stresses produced using the mentioned approach in (i) were tested in tension and the performance of the joints with residual stress was compared to stress-free joints. The highlight of this work is that the tests were performed at three different shear strain rates ranging from low (0.005/s) to high (50/s).

The experimental data generated by the discussed approach in this paper are expected to comprehensively reveal the thermal effects in the joint during the curing process. Furthermore, the test can be used for the validation of adhesive material models as it yields quantifiable parameters like thermal displacement and force (cause of residual stresses in the adhesive joint), which can be used for validating adhesive material models in a FE simulation. The adhesive material models can then be used for any application, including automotive. In addition, the effects of the residual stresses at different strain rates will help in studying the significance of the fabrication residual stresses at low to high strain rates, like in the case of a crash event.

Section snippets

Experiments to capture thermal effects during the heat curing process

The automotive paint baking cycle, which is also used for curing the adhesives, lasts approximately 30 min. When the BIW passes through the paint baking oven, the temperature of the body structure rises to the range of 160–180 °C, and then slowly falls to the ambient level [20]. The duration and temperature of the paint baking cycle are specific to each automotive manufacturer. In this work, the test aimed to reproduce the effects caused by thermal expansion of similar and dissimilar substrates

Experiments to evaluate the effects of residual stress on joint performance

In the last set of experiments, it was established that thermal expansion in the metallic substrates induces residual stress in the adhesive-bonded joint during the heat curing process. In this section, the single lap shear joints with residual stresses were pulled in tension and the results were compared to the performance of the joints without residual stresses. The joints were tested at multiple shear strain rates, ranging from a very slow rate 0.005/s to intermediate-high rate 50/s. The

Results

Force-Strain Plots: The DIC results were processed to obtain the displacement across the joint. A virtual extensometer of 30 mm was drawn with one end on each substrate in order to keep the adhesive joint within the extensometer length. Shear strain is defined as the ratio of change in the Y-length (Delta L(y)) of the extensometer to the pre-measured bond thickness. The given formula calculates the global shear strain across the adhesive joint.ShearStrain=DeltaL(y)JointThickness

The force-shear

Conclusions

In this paper, two sets of experiments and the corresponding results were discussed. The first set of experiments was an innovative approach using 3D DIC system to study the effects of thermal expansion on the adhesive joint during the heat curing process. The experimental setup allowed the curing of an adhesive bonded single lap shear joint in a specialized oven while monitoring the thermal expansion and contraction in the substrates in the heating and the cooling phase. The experimental

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