Fracture Energy Of Adhesive Joints Research Articles

Increase of shear fracture energy of adhesive joints using double-double laminates

ABSTRACT Double – Double (DD) is a new family of laminates that holds the potential to completely overhaul the design of composite structures. These laminates are not only lighter and more robust than the classical QUAD laminates, but they also accelerate manufacturing processes and reduce costs. However, the application of DD laminates as substrates in adhesive joints has not been studied so far. In this work, end-notched flexure (ENF) tests with composite substrates are performed in both QUAD and DD configurations. The main goal is to evaluate the maximum load and shear fracture energy for both laminates. Additionally, the displacement fields are measured through the digital image correlation (DIC) analysis. Experimental results showed that DD laminates can significantly improve the shear fracture energy of adhesive joints and also delay the crack propagation over the test. The findings presented herein could play a crucial role in driving significant alterations in the design procedures of adhesive joints with composite materials, highlighting the DD advantages.

Effects of low cycle impact fatigue on the residual mode II fracture energy of adhesively bonded joints

Cyclic loading exerts a profound influence on the durability of adhesively bonded joints, particularly under cyclic impact loads. Under normal cyclic loading, adhesive joints typically reach their fatigue limit when subjected to approximately 50% of their maximum static strength. However, the endurance limit is considerably lower for cyclic impact loading, sometimes even falling below 5% of the impact strength. This paper investigates the effects of low-energy cyclic impacts on the fracture energy of an end-notched flexure (ENF) joint bonded with an epoxy-based adhesive. The study includes an analysis of the static fracture energy of the ENF joints prior to subjecting them to repeated impact cycles. Additionally, certain joints were subjected to repeated impact cycles until complete crack propagation occurred. To evaluate the residual critical fracture energy of these joints, certain cyclic impact tests were interrupted, and the joints were subsequently subjected to quasi-static loading conditions. The results reveal the remarkable sensitivity of the fracture energy of adhesive joints to the impact energy level, challenging the assumption of infinite life under cyclic impacts often made in conventional fatigue tests. Stress concentration caused by cyclic impact stress waves leads to a higher density of cracks at the specimen edges. The findings are supported by numerical analysis conducted in this study. These results underscore the importance of comprehensive inspections for bonded structures exposed to low-energy cyclic impacts, as impact fatigue can significantly diminish joint strength. The fracture energy decreases as the number of impact cycles increases, indicating a cumulative effect. The findings contribute to a better understanding of the mode II fracture behavior of adhesively bonded joints under repeated impact loads and provide insights for safe design and inspection practices.

Mode I Fracture Energy of Adhesive Joints Bonded with Adhesiveswith Different Characteristics Under Quasi-static and Impact Loading

Mode I Fracture Energy of Adhesive Joints Bonded with Adhesiveswith Different Characteristics Under Quasi-static and Impact Loading

Analysis of the metal adhesively bonded double cantilever beam specimen

The double cantilever beam specimen is currently standardized for measuring the mode I fracture energy of adhesive joints. In addition, it has been increasingly employed to evaluate the adhesive traction-separation law by the direct method, which involves crack tip separation measurements. The three-dimensional finite element analyses here conducted showed that significant anticlastic deformations of the metal adherends compromise the accuracy of the direct method in the elastic domain. It was also seen that the adherend plane stress and adhesive uni-axial strain hypotheses are adequate for the typical specimen geometries. Finally, the new elastic crack length correction derived from a beam model can be used to predict accurately the initial specimen compliance, to obtain conservative fracture energy values and to gain additional insight into the adhesive fracture behaviour.

The use of ECT and 6PBP tests to evaluate fracture behavior of adhesively bonded steel/epoxy joints under Mode-III and Mixed Mode III/II

Evaluation of fracture energy of adhesive joints under mode-III and mixed-mode III/II is a key issue when failure analyses have to be performed. It is thus useful to determine fracture toughness under mode-III and characterization of crack front behavior of bonded joints under mixed-mode III/II loading conditions, which were the overall goal of this paper. For this purpose, edge crack torsion (ECT) and six-point bending plate (6PBP) tests were employed using steel/epoxy adhesive joint. The experimental tests were performed under displacement control and three types of configurations were tested to achieve different mixed-mode ratios. Test results showed considerable linearity before the maximum load point both under pure mode III and mixed-mode III/II; also, failure examination indicated that dominate failure under tearing mode was adhesive/adherend interface, which will be discussed in detail. The experimental tests were numerically simulated and virtual crack closure technique (VCCT) was employed to reproduce behavior of crack front propagation. Both experimental compliance method and finite element analysis proved applicable for the fracture mechanism of adhesive assemblies under mode-III and mixed-mode III/II.

Pure mode II fracture characterization of composite bonded joints

A new data reduction scheme is proposed for measuring the critical fracture energy of adhesive joints under pure mode II loading using the End Notched Flexure test. The method is based on the crack equivalent concept and does not require crack length monitoring during propagation, which is very difficult to perform accurately in these tests. The proposed methodology also accounts for the energy dissipated at the Fracture Process Zone which is not negligible when ductile adhesives are used. Experimental tests and numerical analyses using a trapezoidal cohesive mixed-mode damage model demonstrated the good performance of the new method, namely when compared to classical data reduction schemes. An inverse method was used to determine the cohesive properties, fitting the numerical and experimental load–displacement curves. Excellent agreement between the numerical and experimental R-curves was achieved demonstrating the effectiveness of the proposed method.

Crack equivalent concept applied to the fracture characterization of bonded joints under pure mode I loading

In this work, an accurate and suitable data reduction scheme is developed to measure the fracture energy of adhesive joints under pure mode I loading. The method is based on the crack equivalent concept and is applied to the double cantilever beam specimen. Using the proposed methodology it is not necessary to measure the crack length during propagation, which can introduce non-negligible errors on the fracture energy measurements. Moreover, it accounts for the fracture process zone effects which can be significant when ductile adhesives are used. The new method was compared to classical data reduction schemes and was validated numerically using a trapezoidal mixed-mode cohesive damage model. The fracture characterization in mode I using a developed trapezoidal cohesive damage model is performed by an inverse method. Excellent agreement between the numerical and experimental R-curves was achieved demonstrating the adequacy of the proposed method.

Characterization of the mode I fracture energy of adhesive joints

This investigation was aimed at improving the calculation of the Mode I fracture energy, G IC, of adhesive joints by incorporating the elasticity of the adhesive layer. It was also aimed at proposing ways to improve the calculation of G IC over the existing standard for the measurement of that material property. Our experiments were performed with a variety of aluminum specimens bonded with a large number of different adhesives in various thicknesses. All specimens were tested under mode I loading. In calculating G IC, it is common to neglect the properties of the adhesive layer, especially when the compliance of the system is considered. The validity of this attitude was tested in the present study, and it was found to be in accord with the experimental results but only for joints bonded with a relatively thin layer of adhesive. A method for improving the calculation of the fracture energy of standard specimens, using a theoretical model for the compliance is proposed. This method comprises all the adhesive parameters and is appropriate for linear elastic joints.

Evaluation of Mode-II Fracture Energy of Adhesive Joints with Different Bond Thickness

Abstract A study on the mode-II edge-sliding fracture behaviour of aluminium-adhesive joints was carried out. Compact pure shear (CPS) adhesive joints of different bond thickness were produced using a rubber-modified epoxy resin as the adhesive. An analytical model was developed to calculate the stress distribution along the bond line of the joint. A crack-closure technique was used to evaluate the mode-II strain energy release rate. G II, as a function of the adhesive bond thickness. The results indicated that for a given applied load, G II increased gradually with the bond thickness. A finite element model (FEM) was also developed to evaluate the stress state along the bond line and the strain energy release rate of the CPS specimens. Consistent results were obtained between the theoretical model and finite element analysis. Scanning electron micrographs of the fracture surface illustrated a mainly interfacial fracture path between the adherends and the adhesive for all adhesive joint specimens. The critical fracture load increased very rapidly with bond thickness in the range 0.02 mm to 0.1 mm but remained constant thereafter. However, the mode-II critical fracture energy rose more gradually as the bond thickness was increased.