Modes I and II fatigue induced delamination in co-cured composite structural joints (original) (raw)
Related papers
IOP Conference Series: Materials Science and Engineering, 2018
This paper analyses the Mode II fatigue delamination growth onset for secondary bonded joints compared to co-cured joints. The materials used were composed by two carbon fiber reinforced sub-laminates joined by the secondary bonded and co-cured (without adhesive) methods. Mode II fatigue tests were performed using three-point bending End Notched Flexure test setup. The tests were performed under displacement control and a sinusoidal displacement applied at a frequency of 5 Hz with a Rd = 0.1, that mean, δmin = 0.1 δmax. The main objective of this study was to obtain the strain energy release rate (SEER) versus number of cycles (Nf) in order to evaluate the effect of the adhesive on fatigue life of bonded joints. A Closed form solution for 3-ENF setup was used in order to define the displacement amplitudes that were applied in the fatigue test. The results show that the use of adhesive causes a reduction on Mode II fatigue delamination growth onset SERR (Gth) for secondary-bonded compared to cocured joints. Finally, a scanning electron microscopy (SEM) was used to analyze the fracture surfaces of the Mode II secondary bonded specimens. The fractography images show no crack growth for the specimens tested below or at Gth loading levels, which indicated that the value corresponds to the threshold. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Frequency effect on delamination fatigue crack grows behavior were investigated for carbon fiber/epoxy laminates. Classical 10Hz tests were performed on servo-hydraulic machine on double cantilever beam specimens for the mode I and three-point end notched flexure specimens for the mode II. High frequency fatigue tests were carried out with the help of a shaker at 100Hz for the mode I and 260Hz / 400Hz for the mode II. By comparing the results on T700/M21 specimens, significant effect of the frequency were found out. By increasing the resonance frequency during tests, the propagation threshold decrease for low number of cycles. Those results were discussed based on fractographic observations.
Engineering Fracture Mechanics, 2017
Due to the lack of fundamental knowledge of the physics behind delamination growth, certification authorities currently require that composite structures in aircraft are designed such that any delamination will not grow. This usually leads to an overdesign of the structure, hampering weight reductions. In real structures, delaminations tend to grow under a mix of modes I and II. Although some studies have tried to assess mixed-mode fatigue delamination, little progress was made in understanding the physics behind the problem. Therefore, this work scrutinizes mixed-mode fatigue delamination growth and examines experimentally the damage mechanisms that lead to fracture. To this aim, mixed-mode delamination fatigue tests were performed at different mode mixities, displacement ratios and maximum displacements. Selected fracture surfaces were analysed after the tests in a Scanning Electron Microscope to gain insight on the damage mechanisms. The physical Strain Energy Release Rate G* was used as the similitude parameter, enabling the characterization of fatigue mixed-mode delamination propagation. The results obtained show no displacement ratio or maximum displacement dependence. Furthermore, the energy dissipated per area of crack created is approximately constant for a given mode mixity. However, the analyses of the fracture surfaces and the correlation of the damage features with energy dissipation indicate that different damage mechanisms that might be activated under different loading parameters cause the resistance to delamination to change under a given loading mode.
Fatigue and residual strength of composite aircraft structures
1999
This thesis consists of six papers within the area of composite fatigue and residual strength in aircraft structures. The dissertation is a mixture of experimental, numerical and theoretical work with focus on some structural hot-spots as defined in the certification process. Basic fracture mechanical properties were investigated, both numerically and experimentally for the standard DCB-and ENF-specimens, with respect to different ply interfaces and materials. Toughness measurements on specimens with non-zero interfaces have a considerable higher toughness than specimens with zero interfaces, due to the fact that the initial crack deviates from the original symmetrical crack plane. A good prediction between FE-calculations and beam data analysis methods could be obtained if the non-homogenous characteristics of specimens with non-zero interfaces was taken into account. The cyclic delamination growth rates increased rapidly with increased ∆G, raising doubts about the structural appli...
Simulation of Fatigue Delamination Growth in Composites with Different Mode Mixtures
Journal of Composite Materials, 2007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights An efficient method for modelling delamination under fatigue is proposed. The method is based on contact interaction of two layers modeled by shell elements. A method of the durability prediction using cohesive contact elements is developed. The algorithm gives an instrument for solving high-cycle fatigue problems. A coarse mesh is sufficient to obtain acceptable accuracy of results.
Composite bonded joints under mode I fatigue loading
International Journal of Adhesion and Adhesives, 2011
Experimental investigation on fatigue behavior of carbon-epoxy composite bonded joints under mode I loading was performed in this work. The objective is to evaluate the performance of different data reduction schemes to obtain the energy release rate (G I ) in the fatigue crack growth (FCG) rate using double cantilever beam (DCB) specimens. This law relates the evolution of the crack along time as a function of the energy release rate (G I ) and is generally composed of three different regions: damage nucleation, stable propagation and abrupt final failure. The second phase corresponding to stable propagation leads to a linear trend on the Paris law representation (log-log scale) and must be well characterized to define the fatigue behavior of the structure. During fatigue tests the classical methods require rigorous monitoring of the crack length during its propagation, which is cumbersome and not easy to perform in some materials. In this work, an alternative data reduction scheme based on specimen compliance and crack equivalent concept is proposed to overcome this difficulty. The results provided by the proposed method, namely Compliance Based Beam Method (CBBM), are compared to the ones obtained from the polynomial and Beam on Elastic Foundation Method (BEFM), both of which require crack monitoring. The first is a compliance calibration method that fits a third-order polynomial curve to the experimental results (compliance (C) versus crack length (a)). The second one uses the beam theory to establish the C ¼ f(a) relationship taking into account the properties of the adhesive. One additional advantage can be pointed to the proposed CBBM relative to the other classical methods. In fact, the equivalent crack is related to the specimen compliance, thus taking into account the influence of fracture process zone on specimen behavior. This issue is particularly important when adhesives with some ductility are being characterized in fatigue tests.
Delamination growth in composite plates under compressive fatigue loads
Composites Science and Technology, 2006
Under the patronage of The Seminar The application of composite materials in structural industrial components of large volume production is strongly growing year by year due to both increase in the demand and continuous improvements in the manufacturing processes associated to the reduction of the production cost. This highlights the need of reliable tools for an effective and safe design of composite components. For the complete and proper exploitation of composite materials further efforts are therefore required and the seminar aims to give a contribution in this direction, representing first of all a forum for researchers, scientist, engineers and designers, both of academic and industrial field, for exchanging ideas, proposing new solutions or even to put on the table questions still open. The development of design procedures, the integration between testing, design and process are therefore among the subjects expected to be discussed during the seminar. The seminar main topics are: • Mechanics of materials and mechanical properties • NDT methods • Damage characterization and analysis
Mixed-mode crack growth in bonded composite joints under standard and impact-fatigue loading
Journal of Materials Science, 2008
Carbon fibre reinforced polymers (CFRPs) are now well established in many high-performance applications and look set to see increased usage in the future, especially if lower cost manufacturing and solutions to certain technical issues, such as poor out-of-plane strength, can be achieved. A significant question when manufacturing with CFRP is the best joining technique to use, with adhesive bonding and mechanical fastening currently the two most popular methods. It is a common view that mechanical fastening is preferred for thicker sections and adhesive bonding for thinner ones; however, advances in the technology and better understanding of ways to design joints have lead to increasing consideration of adhesive bonding for traditionally mechanically fastened joints. In high-performance applications fatigue loading is likely and in some cases repetitive low-energy impacts, or impact fatigue, can appear in the load spectrum. This article looks at mixed-mode crack growth in epoxy bonded CFRP joints in standard and impact fatigue. It is shown that the backface strain technique can be used to monitor cracking in lap-strap joints (LSJs) and piezo strain gauges can be used to measure the strain response of impacted samples. It is seen that there is significant variation in the failure modes seen in the samples and that the crack propagation rate is highly dependent on the fracture mode. Furthermore, it is found that the crack propagation rate is higher in impact fatigue than in standard fatigue even when the maximum load is significantly lower.
The concept of fatigue fracture toughness in fatigue delamination growth behavior
2015
This paper provides a study on mode I fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface is observed to be rougher for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.
Engineering Fracture Mechanics, 2013
An overview is given of the development of methods for the prediction of fatigue driven delamination growth over the past 40 years. Four categories of methods are identified: stress/strain-based models, fracture mechanics based models, cohesive-zone models, and models using the extended finite element method. It is highlighted that most models are phenomenological, based on the observed macro-scale behaviour of test specimens. It is suggested that a more physics based approach, focusing on elucidating the mechanisms involved, is needed to come to a full understanding of the problem of delamination growth.