Effect of Interface Roughness on Mechanical Strength of Adhesively Bonded CFRP Joints- Experimental and Numerical Studies

Abstract

Composite materials are well known for their superior properties such as specific strength, specific modulus, tailor-ability in different directions, fatigue resistance, durability, low weight, and low thermal expansion. These properties are desirable for applications in many industries, such as aerospace, automobile, chemical and civil structures. In all these industrial applications, structures and components consist of assemblies and subassemblies of composites. Fasteners, adhesives, welding, brazing, and soldering join these assemblies, subassemblies, and components. Because of the advantages of adhesive bonding, such as uniform stress distribution, less number of components in assembly, weight reduction etc., over other methods and the infeasibility of some methods for joining composites, the bonding of composites with adhesive is preferred. Though adhesive joints offer many advantages, the mechanical behaviour of these joints is very complex and can be affected by several factors. These factors can be listed as geometric, material, loading, environmental, and surface conditions of adherends, etc. The literature survey shows that the effect of interface roughness on the adhesively bonded joints and its evaluation is least dealt with and less understood.

Uncertainty about the strength of Adhesively Bonded Joints (ABJs) deters aircraft manufacturers from using them in primary structural components. Hence, understanding the behaviour of ABJs and evaluation of their bond strength is essential. Even a tightly controlled environment cannot guarantee perfect adhesively bonded joints. Also, ABJs are permanent joints and cannot be disassembled for inspection during the service of composite structures. Composite structures with ABJs are prone to degradation due to environmental and in-service conditions. Hence, it becomes an essential requirement that the Non-Destructive Evaluation (NDE) of ABJs be established right from the manufacturing to the end of the service. Further, reliable NDE parameters need to be identified to correlate to the strength of these joints. Existing literature reports different methods generally followed for surface preparation in ABJs. However, the evaluation of the effect of roughness on the strength and NDE parameters is yet to be established. This motivated the selection of this topic for the study.

This dissertation focuses on surface preparation and its effect on the shear strength of adhesively bonded Single Lap Joints (SLJs) in Carbon Fiber Reinforced Polymer (CFRP), their fracture properties, and the associated NDE parameters. The surface preparation was carried out using different grades of emery paper so that the interfaces of different roughness were available for bonding. The morphology of the interfaces before bonding was captured with the light interferometry Micro-System Analyzer (MSA). Then, roughness parameters were characterized by contact-based measurements. The correlations of the contact angle between the droplet of liquid and the bonding interface with varied surface roughness and the corresponding increase in area with respect to the smoothest surface were established. CFRP, one of the most preferred composite materials in the aerospace industry, has been chosen in this study.

A band of NDE techniques was utilized to evaluate the effects of surface roughness in ABJs of CFRP adherends. This included Ultrasonic Testing (UT), Infra-Red Thermography (IRT), Acoustic Wave Propagation (AWP), Acoustic Emission Testing (AET), X-ray Radiography Testing (XRT), and Digital Image Correlation (DIC).

The thesis work has clearly brought out the importance of surface preparation for adhesively bonded composite joints. Correlations of NDE parameters related to interface surface roughness in ABJs of composites have been established. Also, the effect of wettability in terms of contact angles with surface roughness and area increment of adherend at the interface has been studied. An innovative approach has been presented to identify, distinguish and analyze closely occurring multiple reflections of ultrasound waves in a complex bonded joint. This can go a long way in non-destructively evaluating the interface characteristics in bonded joints and, thereby, the bond strength. Further, using UT, NDE parameters such as reflection coefficient related to interfacial stiffness have been established. Attempts using XRT to derive an NDE parameter that can be a measure of the interface roughness were not a great success. However, it could provide some qualitative checks of the ABJs, which revealed variation in roughness, especially in coarser grades. Along the overlap length, the attenuation of acoustic waves for different interface roughness grades was studied, and the surface roughness effect on the acoustic wave attenuation was characterized. Using IRT, a unique experimental setup was presented to find the thermal conductivity of bonded composite joints, and Newton’s cooling constants were evaluated for interface characterization. Fracture properties with varied surface roughness were determined experimentally, and fracture simulations were performed with surface-to-surface interactions; the approach was successful in modelling surface roughness at interfaces as plane surfaces.

DIC was used in composite bonded joints to map the strain and stress distribution along overlap length when roughness and its orientation were varied. The surface roughness and its orientation effect on fracture properties were studied using AE monitoring during destructive shear strength tests, and AE signal parameters and amplitude distribution were correlated to orientation and strength. The online monitoring of all the destructive tests, including SLJs, Double Cantilever Beam (DCB), and End Notched Flexure (ENF) specimens, was performed using DIC and AET. The change in compliance is related to the strain energy release rate of the joint, and the compliance change in the load-displacement curve of joints with varied surface roughness was correlated to AE signal parameters such as AE energy, hits and frequency clusters. DIC could present the strain evolution in the bonded region in both fracture modes for different surface roughness.

To determine the effects of surface roughness on the fracture properties, mode-I and mode-II fracture toughness tests were carried out on ABJs with varied roughness. The energy release rates for mode-I and mode-II samples were obtained experimentally. The critical strain energy release rate was found to change with interface roughness and fracture mode. Mode-I crack resistance curves were obtained using different data reduction methods. Mode-I fracture energies increased with decrease in the surface roughness, and mode-II fracture energies increased with the increment in the surface roughness. In the FEA model, it is difficult to model micro-roughness on the adherend of mesoscale. Hence, an approach was presented to model the fracture in rough interfaces wherein modelling of joints with varied roughness was considered, and fracture properties were implemented in the commercial FEA software Abaqus. The surface-to-surface interactions were modelled for each interface. The interaction was based on the Cohesive Zone Model (CZM). Traction separation laws were derived from experimentally obtained fracture energies.

Thus, the primary objective of evaluating the effect of interface surface roughness on the strength of adhesively bonded composites has been addressed, which has yielded very significant, novel and effective outcomes. Further research along the same line can help in developing an effective and important NDE tool for health monitoring of adhesively bonded joints.