Effects of plasma treatments of polypropylene adhesive joints used in the automotive industry

Plasma treatment has been used in recent years to activate the surfaces of adhesive substrates and thus as an adhesion promoter between adhesive and substrates. The use of plasma treatments is widely adopted in the automotive industries especially for polymers that present low surface energy, such as polypropylene. In this work, polypropylene substrates used in the automotive industries have been treated with two different techniques: vacuum and atmospheric plasma. Then, polyurethane and methacrylate adhesives have been used to bond single lap joints (SLJs). Typically, these two adhesives cannot bond polypropylene substrates without surface treatments. An experimental plan has been designed to investigate the process parameters that can increase the functional polar groups (FPGs) maximizing the adhesion strength. Besides the types of plasma, two different gas carriers (air and nitrogen) and different treatment times have been investigated. The substrates, treated and not treated, have been assessed through scanning electron microscopy, energy-dispersive X-ray analysis, and Fourier-transform infrared spectroscopy to quantitatively assess the increment of FPGs after the different treatments. The experimental plan shows that the atmospheric plasma can improve the surface of the substrates by using a smaller time. Mechanical tests on SLJs show that methacrylate and polyurethane cannot bond polypropylene substrates without the plasma treatment. On the other hand, the treated substrates can form a strong bonding with the adhesive since all SLJs exhibit a substrate failure. Mechanical tests have been also carried out after three different aging cycles showing that the adopted plasma treatment is not affected by the aging cycles.

Failure analysis of simple overlap bonding joints and numerical investigation of the adhered tip geometry effect on the joint strength

This study was carried out in two stages. In the first step, a numerical study was performed to verify the previous experimental study. In accordance with the previous experimental study data, single lap joints models were created using the ANSYS finite element analysis program. Then, nonlinear stress and failure analyses were performed by applying the failure loads obtained in the experimental study. The maximum stress theory was used to find finite element failure loads of the single lap joints models. As a result of the finite element analysis, an approximate 80 % agreement was found between experimental and numerical results. In the second step of the study, in order to increase the bond strength, different overlap end geometry models were produced and peel and shear stresses in the adhesive layer were compared according to the reference model. As a result of the analyses, significant strength increases were calculated according to the reference model. The strength increase in model 3 and model 5 was found to be 80 % and 67 %, respectively, relative to the reference model.

Effect of preload and shim types on the mechanical properties of composite-aluminium bolted joints

The influence of both preload and the presence of shim types on the mechanical properties of composite-aluminium single-bolt, single-lap joints were studied in this paper. The load-displacement curve and surface strain field of joints in different shim types and preloads were obtained through tensile experiments. A progressive damage model was established using the UMAT subroutine in ABAQUS. A hybrid failure criterion and a linear continuous degradation model were used to describe the progressive damage of composite laminates. The results show that for joints with no shim and for those with various types of shims, the tensile stiffness, peak load and initial damage load could be reduced when the preload is insufficient or too large. Compared with joints with no shims or with peelable fibreglass shims, joints with liquid shims required a larger preload to achieve the best mechanical properties. As the proportion of peelable fibreglass shim increased, the tensile stiffness and peak load continued to increase in joints with a mixed shim of liquid and peelable fiberglass shim. Shims can serve as tension bearings, but have little effect on the initiation and development of bearing failure.

DYNAMIC PERFORMANCE OF ADHESIVELY BONDED SINGLE LAP JOINTS WITH DIFFERENT FIBER ANGLE ORIENTATIONS OF ADHERENDS

This work aims to investigate the dynamic response of the adhesive bonding of Single Lap Joints (SLJs) using a free vibration technique. For this purpose, the joints with fixed-end conditions were subjected to the vibration test, and the results were compared with the numerical ones which were obtained from the Finite Element Method (FEM) via the ANSYS package program. The materials used in this study are an adhesive film, AF163 2K produced by 3M, and adherends, manufactured from a glass reinforced polymer matrix composite, produced by Hexcel. While four different adherends with different fiber orientations were used, the thickness of the adhesive layer in bonded region was kept constant, 0.2 mm. In doing so, the main concentration was given to the adherends as the energy dissipation was believed to come mainly from them. The main objective was to get high damping values without compromising any decrease in the structural performance of the joints. The experimental natural frequency, flexural rigidity and damping values of the joints were obtained as a parameter of the different adherend types. The results were also validated using numerical modal analysis.

ENHANCING SINGLE-LAP COMPOSITE JOINTS LIFE ESTIMATION BY GRAPHENE ADDITION: AN HYBRID APPROACH ANALYSIS

The hybrid formulation based on experimental data and the finite element procedure seems to be able to predict crack initiation under fatigue conditions for single lap joints aged by UV exposure. The UV exposure decreased stiffness and strength. The addition of graphene seems to postpone such ageing. However, by considering that cracks initiates when the (TS/σVM)~1. “Soft” adhesives can reach longer fatigue life at low loadings, while “stiff” adhesives have its fatigue life drastically decreased but at higher loadings. Stiff adhesives are more suitable for low cycle fatigue conditions, while for high cycle fatigue the option is a soft adhesive.