Burak Gozluklu

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.

Design Concept of a CFRP External Trailing Edge for Ailerons

The most recent civil aircraft dominantly use composite structures such as in the ailerons. However, airworthiness authorities raise concerns about lightning strike damage and repairability if the aileron is made of composite material. Today, aerodynamic profiles of aircraft wings become more complex and challenging for designers who need adequate space to fit conventional “torque box” designs inside the wing surfaces. The aero-surfaces may become too shallow and curved in the trailing edge side of the wing which is highly exposed to edge impact and lightning strike damages. This paper presents a new design concept for Carbon Fiber Reinforced Plastic (CFRP) External Trailing Edge (CETE) structure for the trailing edge of the ailerons. CETE is attached to the main aileron torque box where the main load carrying composite spar, ribs and panels are located. The design objective of the CETE combines various important features such as better strength characteristics against lightning strike and edge impact, and easier repairability with a lighter aileron. This paper also discusses recurring and non-recurring costs and monetary benefits of the new design concept. The CETE concept is as simple as creating a secondary torque box on the trailing edge side of the aileron with two C-section parts; inner and outer parts of CETE. The inner part of CETE provides a secondary spar to the aileron to sustain the main torque box force flows and support the trailing edge panels. The main structural feature of CETE is to form a low loaded zone by its outer part which is located at the outermost region of the trailing edge where mechanical edge impact and lightning strike damages are frequently encountered. It is revealed during the lightning strike tests that, the resulting damages can be catastrophic and located at the trailing edge line where the metallic strips are located. In case of lightning strike damage on the CETE, the aileron is able to carry loads since the flows in the main torque box is minimally disturbed as the inner part of CETE is still intact while the outer part is damaged. Similarly, the damage after edge impact is trapped at the outer section of the CETE where the loads are minimized by CETE design. In case of a larger damage, CETE can be replaced easily instead of replacing the whole aileron which is cost effective in long term although the initial costs seem higher than the conventional designs.Copyright

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.

Interaction mechanism of crushing of tubes and honeycomb under axial loading

ABSTRACT This paper presents a simple novel crash energy absorbing system made of plain aluminium thin-walled tubes and honeycomb. The proposed tube/honeycomb system is characterized by slightly shorter tubes inserted into the cells of the honeycomb with close fit. The individual tube and honeycomb structures are numerically modelled under low-speed vertical impact loading and compared with experiments conducted in this study. Next, three representative triangular sections, which involve the symmetric boundary conditions of each cell group, are employed to model the tube/honeycomb system. Load-displacement curves and deformation patterns are compared with the experiments. The proposed honeycomb/tube system is found to produce higher energy absorption capacities for both per unit mass and volume, compared to an ordinary superposition of the constituents. Almost a perfect crushing load plateau is achieved without a peak force. These enhancements are associated with the spontaneous pre-deformation of the tubes pushed by the collapsed honeycomb walls. This interaction also enables synchronised deformation throughout the crash. Finally, a parametric study is performed, based on the height difference between the tubes and honeycomb, which is found to be the determiner of the performance for the proposed system.

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.

Dynamic delamination in curved composite laminates under quasi-static loading

In the wind energy industry, new advances in composite manufacturing technology and high demand for lightweight structures are fostering the use of composite laminates in a wide variety of shapes as primary load carrying elements. However, once a moderately thick laminate takes highly curved shape, such as an L-shape, Interlaminar Normal Stresses (ILNS) are induced together with typical Interlaminar Shear Stresses (ILSS) on the interfaces between the laminas. The development of ILNS promotes mode-I type of delamination propagation in the curved part of the L-shaped structure, which is a problem that has recently raised to the forefront in in-service new composite wind turbines. Delamination propagation in L-shaped laminates can be highly dynamic even though the loading is quasistatic. An experimental study to investigate dynamic delamination under quasi-static loading is carried out using a million fps high speed camera. Simulations of the experiments are conducted with a bilinear cohesive zone model implemented in user subroutine of the commercial FEA code ABAQUS/explicit. The experiments were conducted on a 12-layered woven L-shaped CFRP laminates subjected to shear loading perpendicular to the arm of the specimen with a free-sliding fixture to match the boundary conditions used in the FEA. A single delamination is found to initiate at the 5th interface during a single drop in the load. The delamination is then observed to propagate to the arms at intersonic speed of 2200m/s. The results obtained using cohesive zone models in the numerical simulations were found to be in good agreement with experimental results in terms of load displacement behavior and delamination history.

Modeling of Dynamic Delamination in L-Shaped Composite Brackets

One of the widely used geometrically complex parts in recent civil passenger aircrafts is the L-shaped composite brackets connecting ribs to skins. Due to the sharp curved geometry, interlaminar opening stresses are induced and delamination occurs under considerable mode-mixities at the corner. Dynamic phenomena during delamination initiation and propagation of L-shaped beams are investigated using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods (CZM). In ABAQUS a sequential explicit analysis followed by static (implicit) solution is used where the solution duration is considerably reduced. The thickness of the specimens is varied from 1.0 mm to 4.0 mm while the inner radius is kept same. Loading is applied parallel to one of the arms quasi-statically. Even though the crack is at the very middle of the specimen, this specific loading type yields variable traction fields and mode-mixities in the two sides of the crack in which delamination occurs under shear stress dominated loading on one crack tip and opening stress dominated loading on the other. It is observed that the delamination propagation is highly dynamic even though the loading is quasi-static. The speed of the delamination under shear dominated loading at one side can reach 800 m/s and under normal stress dominated loading is 50 m/s in dedicated thickness levels. In addition, moving elasto-dynamic radial compressive waves along the interface are observed. An important observation for design applications, a typical solution of adding more plies to the laminate might yield failure transition to a secondary crack nucleating at the arm and propagating towards the center crack.Copyright

Experimental and Computational Investigation of Debonding at the Interface of Two L-Shaped Polycarbonate Plates

L-shaped composite structures are increasingly replacing metal ribs and box structures especially in recent civil aerospace structures and wind turbine blades. Delamination of these L-shaped composite laminates occur by interlaminar opening stresses at the curved region under perpendicular loading to one arm. In this study, delamination at the curved region is studied experimentally and computationally by using a simplified model material consisting of two L-shaped polycarbonate plates bonded together. The effect of precrack size at the curved region on the initiation and propagation of delamination under loading perpendicular to one of the arms is studied. Experimental observations are carried out using photoelasticity to visualize the isochromatic stress patterns in the model. Finite element analysis (ABAQUS) is carried out using static (implicit) analysis during elastic loading followed by dynamic (explicit) analysis during crack growth in conjunction with a bilinear cohesive zone model. For precracks smaller than 40mm, a sudden drop in load-displacement plot is observed corresponding to dynamic crack growth at 80 m/s crack tip speed from the corner to the arms. Afterwards, a generally monotonic increase in load-displacement behavior is observed albeit with a lower stiffness corresponding to continued stable crack growth. For larger pre-cracks, load-displacement increases monotonically even after crack initiates with stable stick-slip type crack growth. These two regimes of crack growth are consistent with the observations of Wimmer et al (2009) on cross-ply composite L-beams.Copyright

Failure Mode Transition During Delamination of Thick Unidirectional L-Shaped Composite Laminates

Curved composite laminates such as L-beams are frequently used in wind turbine blade structures such as spars and ribs. It is widely assumed that delamination initiates at the curved region of the L-shaped laminate leading to loss of loading carrying capacity. However, as shown in this paper, under certain conditions a second failure mode in thick L-shaped laminates is observed in which a secondary crack initiates at the arm region. Delamination in L-shaped laminates is modeled using a sequential analysis with implicit analysis followed by explicit dynamic (explicit) finite element analysis in conjunction with cohesive zone methods. The 2-D model consists of 24 plies of unidirectional CFRP laminate with an initial crack at the center of curved region. Loading is applied parallel to one arm quasi-statically and the observed delamination occurs dynamically. For thin laminates and larger precracks, delamination starts from the initial crack and propagates towards the arms. For thicker L-shaped laminates and smaller precracks at the center of curved region, formation of secondary crack in the arm region is observed. Therefore the size of the initial crack as a function of thickness at the center of the curved region mainly decides the failure mode.Copyright