In this blog article, we introduce you to one of our research projects in which we are active: FAnTeStick. You will get an overview of the project, the motivation behind it as well as the challenges, technical developments and the insights gained so far. In this and future articles on the FAnTeStick project, we will show you how the failure and fatigue behaviour of bonded fibre composite components can be predicted using targeted testing and calculation methods. Based on these results, durable, sustainable composite structures can be developed.
Introduction & Motivation
An important aspect in component development is the evaluation of the connection between components to ensure their structural integrity and thus the reliability of the overall system. To achieve a reliable and strong connection between fibre composite components, two joining techniques are usually used: conventional mechanical fasteners (e.g. screws and rivets) and/or adhesive bonding. The latter offers numerous, promising advantages, such as lower stress concentrations in the joining area, the possibility of joining different materials, flexibility in the design as well as high damage tolerance and good fatigue behaviour.
However, bonded joints usually contain micro-cracks and isolated voids (air pockets) in the adhesive layer, which inevitably occur during the manufacturing process or assembly of the joint. Since the presence of such defects can compromise the strength of the loaded joint, it is necessary to evaluate the fracture and fatigue behaviour as well as the failure mechanisms to ensure safe design of bonded structures.
The FAnTeStick project is being carried out in cooperation with our research partner, the Leibniz Institute for Composite Materials (IVW) in Kaiserslautern. The aim is to develop a practical calculation method for predicting the service life of bonded joints in fibre composite structures. The analysis of the bonded joint is carried out with the help of analytical and numerical approaches developed within the project, as well as with data from experimental tests carried out in parallel.
Fracture mechanics calculation of bonded joints
Conventional methods for designing and analysing a component work with stress or strain-based strength concepts. In this approach, a material failure is defined when the occurring stress or strain exceeds the material’s ultimate/yield stress or ultimate strain, which is defined based on the chosen failure criterion. The manufacturing-related flaws in adhesive layers lead to the formation and propagation of cracks. Such cracks lead to local stress concentrations, which can result in material failure before the expected nominal strength limit determined by conventional testing.
Fracture mechanics is used to understand and predict the failure of materials due to these flaws. Fracture mechanics is a field of engineering and materials science that studies the behaviour of cracks and defects in materials under external loads. The concept of strain energy release rate (SERR) is used for the assessment as it is a measure of the energy required to propagate a crack. The SERR represents the energy lost per unit area of a newly formed fracture surface during the fracture process.
Fracture mechanics is an effective tool for characterising the failure of monolithic materials as well as bonded joints. To evaluate the fracture behaviour, the bonded structures are subjected to a wide range of tensile (mode I) and shear stresses (mode II) as well as mixed stresses (mode I+II). Therefore, it is essential to experimentally characterise the entire range of tests in order to obtain the so-called fracture envelope.
In addition to the experimental evaluation, a numerical calculation model was created using the finite element method (FEM) to simulate the fracture behaviour. Finite element analysis offers a cost-effective, virtual solution for fracture mechanics evaluation that reduces the cost and time required for further experimental investigations.
For the simulation of crack initiation and propagation under mechanical loading, several numerical approaches have been developed within the framework of FEM. Among the available approaches, the Virtual Crack Closure Technique (VCCT) originally proposed by Rybicki and Kanninen  was selected as it is an already well researched method for simulating crack growth at interfaces. VCCT is based on linear elastic fracture mechanics (LEFM) and assumes that the strain energy required to enlarge a crack by a given amount is equal to the strain energy required to close it to its original length.
The sample preparation and the tests were carried out by the IVW in Kaiserslautern. So far, the quasi-static tests for Mode I, Mixed Mode and Mode II loads have been evaluated. Figure 1 shows the Double Cantilever Beam (DCB) test for the evaluation of Fracture Mode I.
The DCB test shows that the crack spreads in the middle of the adhesive layer, which means a cohesive failure. This fracture behaviour is exactly what we expected in the observation. Therefore, it is possible to determine the SERR for the bonded joint under peel load (mode I) with this test.
Considering the same boundary conditions as in the experimental test, the numerical simulation was performed using the VCCT approach and the result is shown in Figure 2.
It can be seen that the simulation and test results show very similar fracture behaviour. For further illustration, the load-displacement curves are plotted in Figure 3.
The simulation with the VCCT approach has shown that the fracture behaviour, i.e. crack initiation and propagation, can be predicted with high accuracy compared to the experimental results. Therefore, the numerical simulation is validated for fracture mode I. The same procedure was carried out for mixed mode (I + II) and mode II, and good results were also obtained.
Experimental tests were carried out, the results of which were used as input for the numerical simulation. After the evaluation and validation of the simulation results, a fracture envelope curve was created. With the calculation method used in this project, it is therefore possible to predict the failure limits for the entire load range of mode I, mixed mode I+II and mode II.
In the current work package, we are looking at assessing the development of fatigue damage, i.e. the process by which damage accumulates and progresses over time in the bonded joint subjected to cyclic loading. This is crucial for the assessment of fatigue life.
A phenomenological approach is used, based on the stiffness degradation of the bonded structure. Through the knowledge gained in the research project, ranging from experimental methods to numerical simulations, we can help you improve your product design. Improved failure and fatigue prediction leads to more safety and longer life of your bonded joints. This, in turn, ensures lower repair and maintenance costs and and increases the reliability of your products and the satisfaction of your customers. satisfaction of your customers.
As an alternative approach to the VCCT method, additional investigations based on cohesive zone models (CZM) are now being analysed to further improve the modelling of the fracture behaviour. Here, stresses in the area of the crack tip are calculated via so-called interface or cohesive zone elements.
In the next article on the FAnTeStick project, you will learn more about the evaluation of cyclically stressed bonded joints and their fatigue behaviour. Stay tuned!