Demirkan Coker

Experimental Observations of Dynamic Delamination in Curved [0] and [0/90] Composite Laminates

Curved composite parts are increasingly replacing metal ribs and box structures in recent civil aerospace structures and wind turbine blades. Delamination of L-shaped composite laminates occurs by interlaminar opening stresses in addition to the interlaminar shear stresses at the curved region. An experimental setup is designed to investigate dynamic delamination in L-shaped composite brackets under quasi static shear loading. The materials are unidirectional [0]17and cross-ply [0/90]17 epoxy/graphite composite laminates. The load displacement curves are recorded and subsequent dynamic delamination is captured with a million fps high speed camera. The failed specimens are analyzed under a microscope. It is seen that layup differences change the failure mechanism in composites. Multiple delaminations in one load drop are observed in failure of unidirectional laminate whereas sequential delamination at each discrete load drop is seen in cross ply laminates. In the [0] laminate single delamination in the center ply followed by symmetric delamination nucleations around the two crack tips are observed. In the 0/90 cross-ply laminate, multiple load drops are recorded and delaminations start near the inner radius by peeling of 0/90 plies sequentially at each load drop. In both layups, first time observation of intersonic delamination speeds up to 2,200 m/s are made.

Delamination-Debond Behaviour of Composite T- Joints in Wind Turbine Blades

Wind turbine industry utilizes composite materials in turbine blade structural designs because of their high strength/stiffness to weight ratio. T-joint is one of the design configurations of composite wind turbine blades. T-joints consist of a skin panel and a stiffener co-bonded or co-cured together with a filler material between them. T-joints are prone to delaminations between skin/stiffener plies and debonds between skin-stiffener-filler interfaces. In this study, delamination/debond behavior of a co-bonded composite T-joint is investigated under 0° pull load condition by 2D finite element method. Using Abaqus® commercial FE software, zero-thickness cohesive elements are used to simulate delamination/debond in ply interfaces and bonding lines. Pulling load at 0° is applied and load-displacement behavior and failure scenario are observed. The failure sequence consists of debonding of filler/stringer interface during one load drop followed by a second drop in which the 2nd filler/stringer debonds, filler/skin debonding and skin delamination leading to total loss of load carrying capacity. This type of failure initiation has been observed widely in the literature. When the debond strength is increased 30%, failure pattern is found to change in addition to increasing the load capacity by 200% before total loss of loading carrying capacity occurs. Failure initiation and propagation behavior, initial and max failure loads and stress fields are affected by the property change. In all cases mixed-mode crack tip loading is observed in the failure initiation and propagation stages. In this paper, the detailed delamination/debonding history in T-joints is predicted with cohesive elements for the first time.

Experimental investigation of CNT effect on curved beam strength and interlaminar fracture toughness of CFRP laminates

High mechanical properties and light weight structures of composite materials and advances in manufacturing processes have increased the use of composite materials in the aerospace and wind energy industries as a primary load carrying structures in complex shapes. However, use of composite materials in complex geometries such as L-shaped laminates creates weakness at the radius which causes delamination. Carbon nanotubes (CNTs) is preferred as a toughening materials in composite matrices due to their high mechanical properties and aspect ratios. However, effect of CNTs on curved beam strength (CBS) is not investigated in literature comprehensively. The objective of this study is to investigate the effect of CNT on Mode I and Mode II fracture toughness and CBS. L-shaped beams are fabric carbon/epoxy composite laminates manufactured by hand layup technique. Curved beam composite laminates were subjected to four point bending loading according to ASTM D6415/D6415M-06a. Double cantilever beam (DCB) tests and end notch flexure (ENF) tests were conducted to determine mode-I and mode-II fracture toughness, respectively. Preliminary results show that 3% CNT addition to the resin increased the mode-I fracture toughness by %25 and mode-II fracture toughness by %10 compared to base laminates. In contrast, no effect on curved beam strength was found.

Experimental Investigation of Strength of Curved Beam by Thin Ply Non-Crimp Fabric Laminates

Resistance against delamination failure and through the thickness tensile properties of curved carbon fiber reinforced plastics composites are investigated experimentally by conducting the curved beam strength tests. Effect of novel material thin ply non crimp fabric (NCF) architecture on delamination resistance of carbon fiber reinforced composites are investigated and compare with that of standard UD layups. In order to determine through the thickness tensile properties of curved carbon fiber composites, standard test method is carried out, namely four-point bending tests. The dynamic delamination propagation and failure sequences under curved beam bending is captured using Photron© Fastcam SA5 ultra high speed system. For the non-crimp fabric configuration an increase in the curved beam strength is observed in comparison with [0] and [0/45/-45/0] laminates by unidirectional (UD) tape material. For the UD tape, the initial defects caused by the out-of-autoclave manufacturing process is found to be the potential failure sites. The test results and observations suggest that thin-ply NCF is much less vulnerable to the existence of manufacturing voids in contrast to standard thickness UD tape. Finally, TPNCF is shown to have superior properties in regard to delamination resistance and curved-beam strength.

Role of Laminate Thickness on Sequential Dynamic Delamination of Curved [90/0] CFRP Composite Laminates

In aerospace industry, high demand for the lightweight structures are fostering the use of carbon fiber reinforced polymer composites in a wide variety of shapes, as primary load carrying elements. However, once a composite laminate takes a highly curved shape, such as an L-shape, interlaminar stresses augmented in the curved region cause highly dynamic delamination nucleation and propagation. This paper provides experimental observations of dynamic delamination failure in cross-plied L-shaped composite laminates under quasi-static shear loading for varying laminate thickness. In the experiments, load-displacement curves are recorded and dynamic delamination events areas captured using a million fps high speed camera. In our previous work, two distinct types of failure modes have been identified depending on the laminate layup: (i) formation of multiple delaminations leading two single load drop in its load-displacement curve during the failure of unidirectional laminates, [0]17, and (ii) formation of sequential delaminations associated with each discrete load drop in its load-displacement curve were during the failure of cross-ply laminates, [90/0]17. Accordingly this current study shows that formation of sequential delaminations is independent from the laminate thickness.

Development of Artificial Neural Network Based Design Tool for Aircraft Engine Bolted Flange Connection Subject to Combined Axial and Moment Load

................................................................................................................ v ÖZ .............................................................................................................................. vii ACKNOWLEDGMENTS............................................................................................ x TABLE OF CONTENTS ............................................................................................ xi LIST OF TABLES .................................................................................................... xiii LIST OF FIGURES ................................................................................................... xv CHAPTERS ................................................................................................................. 1 1.INTRODUCTION ........................................................................................ 1 1.1. Scope of the Thesis ........................................................................ 6 2.FINITE ELEMENT ANALYSIS OF BOLTED FLANGE CONNECTIONS ............................................................................................. 9 2.1. Finite Element Modeling and Geometry ........................................ 9 2.1.1. The analysis geometry ................................................ 9 2.1.2. Finite element modeling ........................................... 14 2.1.3. Loads and boundary conditions ................................ 20 2.2. The Application of Bending Moment .......................................... 22 2.3. Application of the Shear Force .................................................... 30 2.4. The Effect of the Shear Force ...................................................... 36 3.PARAMETRICAL ANALYSES AND DATABASE CONSTRUCTION 45 3.1. The Selection of Input Parameters ............................................... 45 3.2. Database Generation .................................................................... 47 3.3. The Parametric Finite Element Analysis Results ......................... 54 4.SETUP OF ARTIFICIAL NEURAL NETWORK AND RESULTS ......... 59 4.1. The Modeling of Artificial Neural Network ................................ 59 4.2. The Training Results of the Artificial Neural Network ............... 61 xii 4.3.Comparison of Artificial Neural Network and Finite Element Analysis Results ................................................................................. 65 4.4.The Graphical User Interface of the Bolted Flange Design Tool . 70 5.CONCLUSION ........................................................................................... 73 5.1.Future Work .................................................................................. 76 REFERENCES ........................................................................................................... 77 APPENDICES ............................................................................................................ 81 A. JUSTIFICATION OF EQUATION FOR AXIAL FORCE DUE TO MOMENT ..................................................................................................... 81 B. ARTIFICIAL NEURAL NETWORK TRAINING CODE ..................... 83 C. CREATING THE GRAPHICAL USER INTERFACE ........................... 85 D. BOLTED FLANGE DESIGN TOOL GRAPHICAL USER INTERFACE MAIN CODE ................................................................................................ 91 E. BOLTED FLANGE DESIGN TOOL GRAPHICAL USER INTERFACE USER MANUEL ........................................................................................ 101 F. THEORETICAL BOLT STRESS CALCULATION CODE ESDU ... 103

Experimental and Computational Investigation of Out-of-Plane Low Velocity Impact Behavior of CFRP Composite Plates

Strength of composite materials under transverse loading has remained a major weakness despite numerous advancements in composite technologies. Most frequent and critical result of this characteristic is internal delamination damage, which is undetectable and lead to major strength reduction in the structure. This condition is usually encountered in low-velocity impact situations which frequently occur during the maintenance of aircraft. Past studies have successfully developed experimental and analysis methods for accurately predicting impact force history and damage footprint based on the comparison with post-impact results. However, there is almost no experimental work on the progression sequence of damage during impact in the literature. This paper focuses on experimental and computational investigation of the damage initiation and growth process during low-velocity impact of [07/904] s and [907/04] s cross-ply CFRP laminates. In the experiments, through-the-thickness direction is tracked using ultra-high speed camera and DIC technique to record damage progression and dynamic strain fields. In the numerical part of the study 3-D explicit, finite element analysis is conducted to model matrix crack initiation and propagation. The finite element results are then compared with experiments in terms of failure modes and sequence.

Experimental Investigation of the Effect of CNT Addition on the Strength of CFRP Curved Composite Beams

Carbon nanotubes (CNT) have been attracting attention as a toughening material in composite matrix due to their excellent mechanical properties. However, superior properties of CNTs have not yet been realized in the strengthening of composites against fracture. This study focuses on investigating the effect of CNT variation in the epoxy resin on the strength of curved composite beams. Specimens are [0/90] fabric carbon/epoxy composite laminates manufactured by hand layup technique 3 % wt CNT fractions in the epoxy resin. Curved beam composite laminates were subjected to four point bending loading according to ASTM D6415/D6415M–06a and the load displacement plot is recorded. Digital Image Correlation technique is used to obtain deformation field in the laminate at the curved region just before delamination failure initiates. A high speed camera at 28,000 fps was used to capture the deformation sequence after initiation of failure. For the CNT added laminate, both CBS and failure load is found to decrease with the load-displacement behavior found to change from single load drop to multiple load drops. In addition, delamination is found to be constrained to the curved region for the CNT added laminate in contrast to the base laminate where delamination extends to the arms.

Development of Bolted Flange Design Tool Based on Finite Element Analysis and Artificial Neural Network

In bolted flange connections, commonly utilized in aircraft engine designs, structural integrity and minimization of the weight are achieved by the optimum combination of the design parameters utilizing the outcome of many structural analyses. Bolt size, the number of bolts, bolt locations, casing thickness, flange thickness, bolt preload, and axial external force are some of the critical design parameters in bolted flange connections. Theoretical analysis and finite element analysis (FEA) are two main approaches to perform structural analysis of bolted flange connections. Theoretical approaches require the simplification of the geometry and are generally oversafe. In contrast, finite element analysis is more reliable but at the cost of high computational power. In this paper, a methodology is developed for iterative analyses of bolted flange that utilizes artificial neural network approximation of a database formed with more than ten thousand non-linear analyses with contact algorithm. In the design tool, a structural analysis database is created by taking permutations of the parametric variables. The number of intervals for each variable in the upper and lower range of the variables is determined with the parameters correlation study in which the significance of parameters are evaluated. The prediction of the ANN based design tool is then compared with FEA results and the theoretical approach of ESDU. The results show excellent agreement of the ANN based design tool with the actual non-linear finite element analysis results within the training limits of the ANN.Copyright

Investigation of Crack Growth Along Curved Interfaces in L-shaped Composite and Polymers

Delamination in unidirectional L-shaped composite laminates is modeled with two L-shaped polycarbonate plates bonded to each other where the effect of pre-crack length on the stability of the crack growth is investigated experimentally and computationally. In the experimental study, a unique testing fixture with a sliding platform is designed to create a pure vertical displacement to one of the arms. The full-field technique of photoelasticity is used in order to visualize isochromatic fringe pattern around the crack tip located at the bonded interface of the L-shaped polycarbonate plates. In the computational study, debonding at the interface of L-shaped plates is modeled using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods. In numerical analysis, pure vertical displacement is applied to one of the arms to reflect the same loading condition as the experiment. Experimental and finite element analysis results are in agreement in terms of load–displacement behavior and stress distribution, which indicate a successful use of cohesive zone method in modeling of crack growth. Stable and unstable crack growth regimes, depending on the precrack length, are identified in agreement with energy release rate calculations. The crack growth regimes are also consistent with unstable crack growth observed in L-shaped unidirectional composite laminates.