Browsing by Author "Boscolo, M."

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    Design and Modeling of Selective Reinforcements for Integral Aircraft Structures
    (American Inst of Aeronautics and Astronautics, 2008-09-01T00:00:00Z) Boscolo, M.; Allegri, Giuliano; Zhang, Xiang
    A numerical simulation is presented in this paper on the performance of crack retarders bonded to integral metallic structures. The work is described in two main parts. First, a novel modeling approach employing the finite element method has been developed for simulating the various failure mechanisms of a bonded structure and for predicting fatigue crack growth life. Crack growth in the substrate and the substrate/strap interface disbond failure are modeled in the framework of linear elastic fracture mechanics. A computer code interfacing with the commercial package MSC NASTRAN has been developed and validated by experimental tests. Second, the effectiveness of different strap configurations on crack growth retardation has been modeled; these include different strap materials, strap dimensions, and their locations on the substrate. The research has included two substrate materials and four strap materials, and at this stage the specimens were cured at room temperature. Strap stiffness and adhesive toughness are found to be the most influential parameters in designing crack retarders. A design tool has been developed based on the numerical simulation to achieve optimal crack retarder design in terms of prescribed fatigue life target and minimum structural weight added by the bonded reinforcement.
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    Fail-Safe Design of Integral Metallic Aircraft Structures Reinforced by Bonded Crack Retarders
    (Elsevier Science B.V., Amsterdam., 2009-01-01T00:00:00Z) Zhang, Xiang; Boscolo, M.; Figueroa-Gordon, Douglas J.; Allegri, Giuliano; Irving, Phil E.
    This paper presents an investigation on the effectiveness of crack growth retarders bonded to integral metallic structures. The study was performed by both numerical modelling and experimental tests. It focuses on aluminium alloy panels reinforced by bonded straps made of carbon-epoxy, glass-epoxy composite materials or a titanium alloy. The goal was to develop a fail-safe design for integrally stiffened skin-stringer panels applicable to aircraft wing structures. The modelling strategy and finite element models are presented and discussed. The requirements that the models should meet are also discussed. The study has focused on establishing the extent of crack retarder benefits, in terms of fatigue crack growth life improvement, by numerical simulation and experimental tests of various crack retarders. The results of predicted fatigue crack growth retardation have been validated by tests of laboratory samples. This study concludes that by bonding discrete straps to an integral structure, the fatigue crack growth life can be significantly improved.
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    Finite element analysis of bonded crack retarders for integral aircraft structures
    (Cranfield University, 2009-04) Boscolo, M.; Zhang, Xiang
    Trends in aircraft design and manufacture are towards the reduction of manufacturing cost and structural weight while maintaining high level of safety. These reductions can be achieved by using integral structures. However, integral structures lack redundant structural members, hence fail safety is not guaranteed. Bonded selective reinforcements (straps) can obviate this problem and improve the damage tolerance capability of integral structures, although increase the design di±culties. The objective of this research is to develop an effective analysis method to predict the fatigue crack growth (FCG) life of integral structures reinforced by bonded crack retarders, determine the effectiveness of the reinforcements, and assess the important strap design parameters. The main mechanisms that influence the crack propagation have been identified, modelled, and discussed. When a crack propagates in the panel skin, bonded straps delay the fracture growth by exerting bridging forces at the crack tip. Nevertheless damage also affects the strap due to the stiffness mismatch and high stress concentration, and the strap/substrate interface is affected by a progressive delamination that advances together with the substrate crack and limits the strap bridging action. Tensile thermal residual stresses (TRS) in the cracked substrate, caused by the adhesive cure process, act to open the crack and hence increase the growth rate. Last but not least, secondary bending caused by the non-symmetric configurations induces a stress gradient along the crack front. This reduces the effectiveness of the bridging action and causes a curved crack front. An enhanced 2D FE modelling technique that takes into account of these mechanisms and their interactions has been developed and implemented in a computerprogram that interfaces the commercial code NASTRAN. This program is used to calculate the stress intensity factors and the FCG life of bonded strap reinforced integral structures. This modelling technique has been validated for a wide range of test samples in terms of TRS and their redistribution with crack propagation, disbond areas, and FCG lives. The FCG life of a large scale integral skin-stringer panel reinforced by various bonded straps has also been predicted and compared with the experiments. Numerical predictions have shown good agreement with the experimental measurements. Parametric studies have been conducted to understand the effectiveness of different strap configurations on crack growth retardation; these include different strap materials, strap dimensions and locations on the substrate. A design tool has been developed aimed at achieving optimal crack retarder design in terms of prescribed fatigue life target and minimum structural weight. In conclusion, a novel modelling tool has been developed, the effectiveness of bonded straps in retarding fatigue crack growth has been demonstrated and, following the parametric analysis, the most important parameters in the design of bonded straps have been identified.
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    Life extension techniques for aircraft structures-Extending durability and promoting damage tolerance through bonded crack retarders
    (2011-12-31T00:00:00Z) Irving, Phil E.; Zhang, Xiang; Doucet, J.; Figueroa-Gordon, Douglas J.; Boscolo, M.; Heinimann, M.; Shepherd, G.; Fitzpatrick, M. E.; Liljedahl, D.; Jerzy, Lomorowski
    This paper explores the viability of the bonded crack retarder concept as a device for life extension of damage tolerant aircraft structures. Fatigue crack growth behaviour in metallic substrates with bonded straps has been determined. SENT and M(T) test coupons and large scale skin-stringer panels were tested at constant and variable amplitude loads. The strap materials were glass fibre polymer composites, GLARE, AA7085 and Ti-6Al-4V. Comprehensive measurements were made of residual stress fields in coupons and panels. A finite element model to predict retardation effects was developed. Compared to the test result, predicted crack growth life had an error range of -29% to 61%. Mechanisms and failure modes in the bonded strap reinforced structures have been identified. The strap locally reduces substrate stresses and bridges the crack faces, inhibiting crack opening and reducing crack growth rates. In the absence of residual stress, global stiffness ratio accounts for effects of both strap modulus and strap cross section area. In elevated temperature cure adhesives, retardation performance was best in aluminium and GLARE strap materials, which have the closest thermal expansion coefficient to the substrate. Strap materials of high stiffness and dissimilar thermal expansion coefficient such as titanium had poor retardation characteristics.
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    A modelling technique for calculating stress intensity factors for structures reinforced by bonded straps. Part I: Mechanisms and formulation
    (Elsevier Science B.V., Amsterdam., 2010-04-01T00:00:00Z) Boscolo, M.; Zhang, X.
    This paper describes a 2D FE modelling technique for predicting fatigue crack growth life of integral structures reinforced by bonded straps. This kind of design offers a solution to the intrinsic lack of damage tolerance of integral structures. Due to the multiple and complex failure mechanisms of bonded structures, a comprehensive modelling technique is needed to evaluate important design parameters. In this Part I of a two-part paper, the actions and mechanisms involved in a bonded structure are discussed first, followed by presenting the modelling approaches to simulate each mechanism. Delamination or disbond of the strap from the substrate is modelled by computing the strain energy release rate on the disbond front and applying a fracture mechanics criterion. Thermal residual stresses arising from the adhesive curing process and their redistribution with the substrate crack growth are calculated and taken into account in the crack growth analysis. Secondary bending effect caused by the un-symmetric geometry of one-sided strap is also modelled. In the classic linear elastic fracture mechanics, a non-dimensional stress intensity factor, i.e. the geometry factor β, depends only on the sample’s geometry. This β factor cannot be found for this kind of bonded structures, since the magnitude of disbond is related to the applied stress and the disbond size modifies the geometry of the structure. Moreover, secondary bending effect is geometric nonlinear thus the stress intensity factor cannot be normalised by the applied stress. For these reasons an alternative technique has been developed, which requires calculating the stress intensity factors at both the maximum and minimum applied stresses for each crack length. This analysis technique is implemented in a computer program that interfaces with the NASTRAN commercial code to compute the fatigue crack growth life of strap reinforced structu
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    A modelling technique for calculating stress intensity factors for structures reinforced by bonded straps. Part II: Validation
    (Elsevier Science B.V., Amsterdam., 2010-04-01T00:00:00Z) Boscolo, M.; Zhang, X.
    In this second part of the two-part paper validation of the 2D FE modelling technique described in the first part is presented for a range of test configurations. Each mechanism that influences crack growth behaviour of strap reinforced structures is modelled for different substrate geometries, strap materials and dimensions in order to test the accuracy and robustness of the methodology. First, calculated through-thickness strain energy release rate distribution is compared with the result of a 3D FE model to validate this 2D model. Second, calculated disbond areas, thermal residual stresses and their redistribution with crack propagation are validated against experimental measurements. Third, influence of geometric nonlinearity and the need to use the alternate analysis method described in part I are demonstrated by examples, and errors generated by not following this analysis rule are given. Finally, using the 2D model calculated stress intensity factors, fatigue crack growth rates and lives are predicted for different specimens, strap materials and applied stress levels and are compared with the experimental tests. Good or acceptable agreement has been achieved for each case.