Stiffener Runout Research Articles

Effect of bimodularity and thermomechanical stresses from composite curing on mixed-mode fracture behavior of functionally graded skin-stiffener runout

ABSTRACT The present work illustrates the influence of bimodulus polarity and curing stress on interface delamination in skin stiffener runout. In contrast to unimodular assumptions, the bimodulus skin stiffener’s fracture behavior is much more unsymmetrical depending on the sequential thermo-mechanical loading. The numerical study has been done to demonstrate debonding and delamination with the help of the Virtual Crack Closure Technique on compliant runout configuration under varying bimodular ratios. The modified design of the runout has been displayed, and different design configurations have been compared to determine the merits of the design of skin-stiffener runout: a baseline configuration, a tip-tapered configuration, and notch-tapered configuration with the functionally graded bimodular property. A three-dimensional finite element-based analysis has been performed on these configurations to evaluate the strength and damage tolerance. Also, the Strain Energy Release Rate has been assessed for non-identical loading and boundary conditions. It has been found that bimodularity significantly reduces failure growth. This analysis aims to decelerate the interfacial fracture extension rate by introducing functionally graded bimodularity in the structure.

A floating connector element formulation for multi-level modelling of composite structures

Design and optimisation of large structures, including the positioning of lower-level components, typically require extensive user involvement and sequential mechanical analysis/optimisation iterations. This paper presents an original method that enables adaptive positioning of lower-level models (such as components) within higher level-models (such as large structures), and that achieves a combined mechanical/optimisation problem for the design of structures with various hierarchical levels (such as the positioning of stiffeners within a wingbox). As the position of the lower-level model evolves, our proposed method does not require re-generating of the geometry, remeshing or modifying the stiffness matrix of the elements corresponding to the various hierarchical levels. Instead, we achieve the adaptive positioning via an original concept that we propose here: Floating Connector (FC) elements. In this paper, we validate the FC elements against reference purely-mechanical solutions, show that they can be combined with gradient-descent method and genetic algorithms, and that they can be applied to optimise the positioning of a stiffener runout taking into account a debonding manufacturing defect.

Damage tolerance of composite runout panels under tensile loading

Abstract Stiffener runout regions are of vital importance for the integrity of modern aerostructures due to the abrupt change of the load path. This investigation deals with the structural analysis in terms of damage tolerance of different runout conceptions from experimental and numerical points of view. The specimens are subjected to tensile loading up to structural failure. The experiments revealed that, for each of the configurations here considered, damage is initiated around the runout region at the skin-stringer joint, with a strong dependency upon the specific design, and progresses along the longitudinal path of the joint. Numerically, these panels are analyzed by means of global-local methodologies incorporating interlaminar failure modeling capabilities at local level. For this purpose two numerical strategies based on interface elements formulations are employed: (1) standard Cohesive Zone Models, and (2) the new enhanced three-dimensional version of the Linear Elastic Brittle Interface Model. Computational damage predictions are compared with experimental data, showing the applicability for complex structures of the analysis tools here presented.

Experimental study on compressive behavior of GFRP stiffened panels using digital image correlation

Stiffened composite panels are primary structural components in ship, aircraft and aerospace vehicles. The main objective of this study is to investigate the behavior of glass fiber reinforced polymer (GFRP) composite panels with stiffener runout condition and different stiffener configurations such as blade, T and hat under axial compression. A special fixture was designed and fabricated for the compressive testing of GFRP composite stiffened panels. Fabricated specimens were tested until failure and the effect of different stiffener configurations on the load–deformation behavior is compared. Digital image correlation (DIC) was used to measure the deformations and strains on the GFRP composite panels. Test results indicated that T-stiffeners performed well with higher failure load and stiffness as compared to other stiffeners. DIC results show that the presence of stiffeners reduced the out-of-plane displacement in the stiffened panels. As the load level increases, damage initiates and propagates. The surface strain and displacement are sensitive to damages and it can alter their distribution. This could be captured using DIC technique as the damage distribution takes place. The T-stiffeners were found to be effective in resisting bending strains and produced more uniform distribution of compressive strain compared to other stiffener configuration.

Impact damage identification in composite skin-stiffener structures based on modal curvatures

The feasibility of a vibration-based damage identification approach for impact damage in two advanced composite skin-stiffener structures is investigated in this paper. Mode shape curvatures combined with the modal strain energy damage index (MSE-DI) algorithm are utilized to identify the damage. Special attention is paid to the effective application of this vibration-based methodology by investigating the relation between the damage location, the structural design, and the dynamic behavior. The performance of a 1D and 2D formulation of the MSE-DI algorithm is compared for several damage scenarios. Experiments demonstrated the capabilities of the MSE-DI algorithm to detect, localize, and roughly quantify the size of barely visible impact damage in advanced composite structures. It is concluded that the method is particularly effective for health monitoring of skin-stiffener connections. The most effective results were obtained by considering the 1D formulation in the direction of the stiffeners for the stiffener mid-section and perpendicular to the stiffeners for the stiffener run-out. The method remained inconclusive in the case of pure skin-related damage. The results obtained in this study contribute to the development of guidelines for vibration-based structural health monitoring of advanced composite skin-stiffener structures. Copyright © 2015 John Wiley & Sons, Ltd.

Investigating the use of compliant webs in the damage-tolerant design of stiffener run-outs

In this work, the use of a compliant web design for improved damage tolerance in stiffener run-outs is investigated. Firstly, a numerical study that incorporates the possibility of debonding and delamination (using VCCT) is used to select a favourable compliant run-out configuration. Then, three different configurations are compared to establish the merits of the compliant design: a baseline configuration, a configuration with optimised tapering and the selected compliant configuration. The performance of these configurations, in terms of strength and damage tolerance, was compared numerically using a parametric finite element analysis. The energy release rates for debonding and delamination, for different crack lengths across the specimen width, were used for this comparison. The three configurations were subsequently manufactured and tested. In order to monitor the failure process, acoustic emission (AE) equipment was used and proved valuable in the detection and analysis of failure. The predicted failure loads, based on the energy release rates, showed good agreement with the experiments, particularly when the distribution of energy release rate across the width of the specimen was taken into account. As predicted numerically, the compliant configuration failed by debonding and showed improved damage tolerance compared to the baseline and tapered stiffener run-outs.

Compression Failure Analysis of Adhesively Stiffened Composite Panels

Experimental and numerical investigations of adhesively stiffened composite panels were performed to discuss the load-carrying capacity of different design at stiffener run-out. The compression experiment revealed that the failure was initiated at the edge of the run-out and propagated across the skin-stiffener interface. A fracture mechanical approach to utilize cohesive finite element was adopted in the numerical analysis of these specimens. It was shown that the finite element analysis using solid element and cohesive element was able to qualitatively predict the crack growth characteristics for each specimen

Design of composite stiffener run-outs for damage tolerance

A major concern in stiffener run-out regions, where the stiffener is terminated due to a cut-out, intersecting rib, or some other structural feature which interrupts the load path, is the relatively weak skin–stiffener interface in the absence of mechanical fasteners. More damage tolerant stiffener run-outs are clearly required and these are investigated in this paper. Using a parametric finite element analysis, the run-out region was optimised for stable debonding crack growth. The modified run-out, as well as a baseline configuration, were manufactured and tested. Damage initiation and propagation was investigated in detail using state-of-the-art monitoring equipment including Acoustic Emission and Digital Image Correlation. As expected, the baseline configuration failed catastrophically. The modified run-out showed improved crack-growth stability, but subsequent delamination failure in the stiffener promptly led to catastrophic failure.

Numerical Analysis of Stiffener Runout Sections

The recent trend of incorporating more composite material in primary aircraft structures has highlighted the vulnerability of stiffened aerostructures to through-thickness stresses, which may lead to delamination and debonding at the skin–stiffener interface, leading to collapse. Stiffener runout regions are particularly susceptible to this problem and cannot be avoided due to the necessity to terminate stiffeners at rib intersections or at cutouts, interrupting the stiffener load path. In this paper, experimental tests relating to two different stiffener runout specimens are presented and the failure modes of both specimens are discussed in detail. A thinner-skinned specimen showed sudden and unstable crack propagation, while a thicker-skinned specimen showed initially unstable but subsequent stable crack growth. Detailed finite element models of the two specimens are developed, and it is shown how such models can explain and predict the behaviour and failure mode of stiffener runouts. The models contain continuum shell elements to model the skin and stiffener, while cohesive elements using a traction-separation law are placed at the skin–stiffener interface to effectively model the debonding which promotes structural failure.

Fracture mechanisms and failure processes at stiffener run-outs in polymer matrix composite stiffened elements

Abstract This paper describes the fractographic analyses of three stringer run-out designs which had been loaded to failure in tension. The main aims of the investigation were to deduce the failure processes in the elements, and to characterise the effect of local geometry of the stringer run-out on the failure process. The analysis showed that the critical failure mechanism in the elements was the development of +45° ply splitting at the skin surface, initially under mode I dominated intralaminar fracture. However, as these splits grew beneath the stringer foot, the mode II component increased. This led to mixed-mode delamination growth, extending parallel to the +45° ply, at the skin/adhesive interface. Subsequently, the delamination migrated through the skin via ply splits, ultimately reaching the interface between the second and third (−45°/0°) plies, in which it remained until catastrophic failure. Extending and tapering the stringer foot led to shifting of the site of the +45° ply splitting; this was attributed to in-plane tensile stresses in the skin being inhibited in the modified designs. The reduced out-of-plane support on the stiffener foot in the modified designs led to an increase in the mode I component at the delamination from the stringer tip. It also led to an increase in the degree of multi-plane delamination growth. Based on the fractographic observations, recommendations for modelling the elements were suggested.

The Behavior of Compressively Loaded Stiffener Runout Specimens – Part I: Experiments

The development of the next generation of civil and military transport aircraft will inevitably see an increased use of advanced carbon fibre composite material in the primary structure if performance targets are to be met. One concern in this development is the vulnerability of co-cured and co-bonded stiffened structures to through-thickness stresses at the skin–stiffener interfaces, particularly in stiffener runout regions. These regions are a consequence of the requirement to terminate stiffeners at cutouts, rib intersections, or other structural features which interrupt the stiffener load path. This work presents the results of an experimental programme investigating the failure of thick-sectioned stiffener runout specimens loaded in uniaxial compression. For all tests, failure initiated at the edge of the runout and propagated across the skin–stiffener interface. It was found that the failure load of each specimen was greatly influenced by intentional changes in the geometric features of these specimens. High frictional forces at the edge of the runout were also deduced from a fractographic analysis, indicating a predominantly Mode II initial failure mode.

Adhesively bonded joints under cyclic loading spectra

Abstract Current designs which involve the use of composite materials in primary aircraft structures are often conservative. This, in turn, significantly lowers the weight advantage that composites have over established metallic airframe materials. Strain restrictions are often applied because the failure mechanism(s) in (fibre) composite joints and stiffener runouts where the stress state is often complex, are not fully understood. Nevertheless, from the airworthiness perspective it is essential that both the static strength and the fatigue behaviour of the components subjected to complex multiaxial stress conditions are both understood and predicted. This topic is extremely complex, and numerous criteria ranging from the purely empirical to the theoretical have been proposed. In both cases, it is necessary to know the localised stress–strain history.One common design methodology is to keep the stresses so low that fatigue will not be an issue. However, this can lead to an overly conservative design. On the other hand, while a detailed (nonlinear) finite element analysis can be performed it is often both resource‐intensive and time‐consuming. The present paper shows that Glinka's hypothesis can be used in order to calculate the localised stresses and strains for a bonded joint subjected to cyclic loading.This is a new result and has not previously been noted. It has the potential to extend the Hart‐Smith design methodology to the adhesively bonded joints in order to encompass durability considerations. This formulation also raises the possibility of enabling the degree of conservatism inherent in traditional joint design to be relaxed provided that failure occurs in the adhesive.This paper also addresses the problem of variable adhesive thickness. We show that while variable adhesive thickness can change the stress and the energy fields, the peak in the strain energy density is relatively insensitive to the stress–strain relationship for the adhesive and that Glinka's hypothesis still appears to be true. This means that, for the present class of problems, even if there is variability in the thickness of the adhesive bond the energy field and, hence, the strength of the joint can be estimated from a purely linear elastic analysis of the joint, provided that failure occurs in the adhesive.

Shape optimization of critical stiffener runouts for F-111 airframe life extension

ABSTRACT Optimal rework shapes for the most critical stiffener runout fatigue locations in the F-111 wing pivot fitting have been determined using a recently developed finite-element-based gradient-less shape optimization procedure. The resulting precise free-form shapes render the local notch stress distributions near uniform and typically provide a 30–40% reduction in peak elastic stresses as compared to the current rework shapes that exist for aircraft in service with the Royal Australian Air Force. The present numerical results are also consistent with recent preliminary experimental results, and a significant program of further validation testing is envisaged. Hence it is expected that the stress reductions predicted in the present work will be sufficient to provide a basis for extending inspection intervals by at least a factor of two; from 500 to 1000 h. Implementation of such an extension to the F-111 fleet in service with the Royal Australian Air Force would provide a very significant maintenance cost saving. The anticipated reduction in local crack growth rates would also allow achievement of the planned withdrawal date for the aircraft.

Failure of thick-skinned stiffener runout sections loaded in uniaxial compression

Recent efforts towards the development of the next generation of large civil and military transport aircraft within the European community have provided new impetus for investigating the potential use of composite material in the primary structure. One concern in this development is the vulnerability of co-cured stiffened structures to through-thickness stresses at the skin–stiffener interfaces particularly in stiffener runout regions. These regions are an inevitable consequence of the requirement to terminate stiffeners at cutouts, rib intersections or other structural features which interrupt the stiffener load path. In this respect, thicker-skinned components are more vulnerable than thin-skinned ones. This work presents an experimental and numerical study of the failure of thick-sectioned stiffener runout specimens loaded in uniaxial compression. The experiments revealed that failure was initiated at the edge of the runout and propagated across the skin–stiffener interface. High frictional forces at the edge of the runout were also deduced from a fractographic analysis and it is postulated that these forces may enhance the fracture toughness of the specimens. Finite element analysis using an efficient thick-shell element and the Virtual Crack Closure Technique was able to qualitatively predict the crack growth characteristics for each specimen.