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Richard Degenhardt - One of the best experts on this subject based on the ideXlab platform.

  • Investigation on the Geometric Imperfections driven Local Buckling Onset in Composite Conical Shells
    Applied Composite Materials, 2016
    Co-Authors: Maria Francesca Di Pasqua, Mariano A. Arbelo, Regina Khakimova, Saullo G. P. Castro, Aniello Riccio, Antonio Raimondo, Richard Degenhardt
    Abstract:

    Buckling is a critical failure phenomenon for structures, and represents a threat for thin shells subjected to compressive forces. The global buckling Load, for a conical structure, depends on the geometry and material properties of the shell, on the stacking sequence, on the type of applied Load and on the initial geometric imperfections. Geometric imperfections, occurring inevitably during manufacturing and assembly of thin-walled composite structures, produce a reduction in the carrying Load capability with respect to the design value. This is the reason why investigating these defects is of major concern in order to avoid over-conservative design structures. In this paper, the buckling behavior a conical structure with 45° semi-vertical angle is numerically investigated. The initial imperfections are taken into account by using different strategies. At first, the Single Perturbation Load Approach (SPLA), which accounts for defects in the form of a lateral Load, normal to the surface, has been adopted. Then, the actual measured defects have been applied to the structure by using the Real Measured Mid-Surface Imperfections (MSI) approach. Investigations on cylindrical shells using the first strategy have already shown the occurrence of a particular phenomenon called “local snap-through”, which represents a preliminary loss of stiffness. In order to better understand this phenomenon for conical shells, both the aforementioned techniques have been used to provide an exhaustive overview of the imperfections sensitiveness in conical composite shells. This study is related to part of the work performed in the frame of the European Union (EU) project DESICOS.

  • Buckling of axially compressed CFRP truncated cones with additional lateral Load: Experimental and numerical investigation
    Composite Structures, 2016
    Co-Authors: Regina Khakimova, Rolf Zimmermann, Dirk Wilckens, Klaus Rohwer, Richard Degenhardt
    Abstract:

    Abstract Truncated thin-walled conical shells are often used as transition parts between cylinders of different diameters. Parts of space launcher transport systems are one example for the application of conical shells. Buckling of such thin-walled imperfection sensitive structures is a very important phenomenon to be considered during their design phase. Existing design guidelines, NASA SP-8007 for cylinders and NASA SP-8019 for cones, dated from the late 1960’s are currently used in the aerospace industry and employ conservative lower-bound knock-down factors. These empirically based lower-bound methods do not include important mechanical properties of laminated composite materials, such as the stacking sequence. New design approaches that allow taking full advantage of composite materials and take into account specific manufacturing methods are therefore required. The Single Perturbation Load Approach (SPLA) is an alternative proposal for a deterministic procedure for the design of thin-walled cylinders and cones under axial compression that accounts for geometric imperfections. The study deals with the buckling experiments on axially compressed, unstiffened carbon fiber-reinforced polymer (CFRP) truncated cones with an additional lateral Load, performed by DLR for validation of the SPLA applied to this type of structure. Three geometrically identical cones with different layup were designed, manufactured and tested. During testing a digital image correlation system was employed and Load-shortening data is extracted. The experimental results are compared with the FEA results.

  • New design scenario for future composite launcher structures
    2016
    Co-Authors: Richard Degenhardt
    Abstract:

    The Space industry demand for lighter and cheaper launcher transport systems. The running EU project DESICOS (New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures, cf. [1]), which started in February 2012, contributes to these aims by a new design procedure for imperfection sensitive composite launcher structures, exploiting the worst imperfection approach efficiently by implementation of the Single Perturbation Load Approach [2]. Currently, imperfection sensitive shell structures prone to buckling are commonly designed according the NASA SP 8007 guideline using the conservative lower bound curve. The guideline dates from 1968, and the structural behaviour of composite material is not considered appropriately, in particular since buckling Load and imperfection sensitivity of shells made from such materials substantially depend on the lay-up design. This is not considered in the NASA SP 8007, which allows designing only so called "black metal" structures. Here is a high need for a new precise and efficient design approach for imperfection sensitive composite structures which allows significant reduction of structural weight and design cost. For most relevant architectures of cylindrical and conical launcher structures (monolithic, sandwich - without and with holes) DESICOS investigates a combined methodology from the Single Perturbation Load Approach and a Specific Stochastic Approach which guarantees an effective and robust design. A recent investigation demonstrated, that an axially Loaded unstiffened cylinder, which is disturbed by a large enough single Perturbation Load, is leading directly to the design buckling Load 45% higher compared with the respective NASA SP 8007 design [3]. Within DESICOS the new methods will be further developed, validated by tests and summarized in a handbook for the design of imperfection sensitive composite structures. The potential will be demonstrated within different industrially driven use cases. This paper deals with the objectives of the DESICOS project, describes the line of actions of the new approach, and specifies the theoretical and experimental project work already done and still to be carried out in order to meet the objectives.

  • The Influence of Geometrical Parameters on the Buckling Behavior of Conical Shell by the Single Perturbation Load Approach
    Applied Composite Materials, 2015
    Co-Authors: Maria Francesca Di Pasqua, Mariano A. Arbelo, Regina Khakimova, Saullo G. P. Castro, Aniello Riccio, Richard Degenhardt
    Abstract:

    Since the development of the first theories to predict the buckling induced by axial compression in shells sensitive to imperfections, a significant discrepancy between theoretical and experimental results has been observed. Donnell and Koiter are among the first authors demonstrating, for these structures, the relevant influence of the geometrical imperfections on the reduction of the buckling Load. Currently, the preliminary design of imperfections sensitive shell structures used in space applications is carried out according to the NASA SP-8007guideline. However, several studies have proven that this guideline leads to over-conservative design configurations when considering the geometrical and material imperfections existing in real cones. Since the pioneer work of Arbocz, alternative methods have been investigated to overcome this issue. Among the different approaches, in this paper, the Single Perturbation Load Approach (SPLA), originally developed byHühne as a deterministic way to calculate the knock-down factor of imperfection sensitive shells, is further studied. Indeed, a numerical investigation about the application of the SPLA to the simulation of the mechanical behavior of imperfection sensitive composite conical structures under axial compression is presented. This study is related to part of the work performed in the frame of the European Union (EU) project DESICOS.

  • Semi-analytical tools for the Single Perturbation Load Approach using the Ritz Method
    2014
    Co-Authors: Saullo Castro, Christian Mittelstedt, Mariano A. Arbelo, Regina Khakimova, Richard Degenhardt
    Abstract:

    Many structural components or launcher structures can be modelled using a conical or cylindrical-like set of shell equations. The current guidelines used at the preliminary design phase are perceived as conservative and do not take into account properly the stacking sequence of laminated shells. New guidelines has been developed for example by the NASA project Shell Buckling Knock-Down Factor (SBKDF) and by the European consortium DESICOS. One of the approaches currently investigated is the Single Perturbation Load Approach (SPLA), originally proposed by Christian Huhne (2008) which is based on the phenomena that the axially compressed shell buckling Load gets nearly constant after a threshold of geometric imperfection caused by a lateral Load, obtaining a convenient design Load. A disadvantage of this method is that it requires many non-linear computations in order to find this Load threshold. This study presents the development of semi-analytical tools that allow a fast prediction of the design Load using the SPLA, with the aim to overcome the computational disadvantage of this method.

Babak Bazrgari - One of the best experts on this subject based on the ideXlab platform.

Rolf Zimmermann - One of the best experts on this subject based on the ideXlab platform.

  • Buckling of axially compressed CFRP truncated cones with additional lateral Load: Experimental and numerical investigation
    Composite Structures, 2016
    Co-Authors: Regina Khakimova, Rolf Zimmermann, Dirk Wilckens, Klaus Rohwer, Richard Degenhardt
    Abstract:

    Abstract Truncated thin-walled conical shells are often used as transition parts between cylinders of different diameters. Parts of space launcher transport systems are one example for the application of conical shells. Buckling of such thin-walled imperfection sensitive structures is a very important phenomenon to be considered during their design phase. Existing design guidelines, NASA SP-8007 for cylinders and NASA SP-8019 for cones, dated from the late 1960’s are currently used in the aerospace industry and employ conservative lower-bound knock-down factors. These empirically based lower-bound methods do not include important mechanical properties of laminated composite materials, such as the stacking sequence. New design approaches that allow taking full advantage of composite materials and take into account specific manufacturing methods are therefore required. The Single Perturbation Load Approach (SPLA) is an alternative proposal for a deterministic procedure for the design of thin-walled cylinders and cones under axial compression that accounts for geometric imperfections. The study deals with the buckling experiments on axially compressed, unstiffened carbon fiber-reinforced polymer (CFRP) truncated cones with an additional lateral Load, performed by DLR for validation of the SPLA applied to this type of structure. Three geometrically identical cones with different layup were designed, manufactured and tested. During testing a digital image correlation system was employed and Load-shortening data is extracted. The experimental results are compared with the FEA results.

  • An empirical formula for the design Load obtained by use of Single Perturbation Load Approach
    2014
    Co-Authors: Regina Khakimova, Mariano A. Arbelo, Rolf Zimmermann, Saullo G. P. Castro, Richard Degenhardt
    Abstract:

    The stability of shell structures has been an object of studies for more than a century. Thin walled cylindrical and conical structures are widely used in aerospace, offshore, marine, civil and other industries. The importance of taking into account geometric imperfections for cylindrical and conical thin-walled structures in buckling had been already recognized a long time ago. In spite of a multitude of publications on buckling of imperfect shells, such structures are still today generally designed at the preliminary design phase according to the NASA SP-8007 [1] for cylinders and the NASA SP-8019 [2] for truncated cones. Both guidelines date from 1960’s and they are based on a lower bound curve which does not consider important mechanical characteristics of laminated composite shells, such as the stacking sequence, thus producing configurations that are over-conservative or even non-conservative for some cases. Koiter in 1945 [3] was the first who theoretically demonstrated the already experimentally observed imperfection sensitivity that affects the buckling behavior of thin-walled structures. Nowadays, with the everyday increasing computational power, it becomes easier to consider imperfections in numerical simulations. However, in the early design stage the real geometric imperfection pattern of a new type of structure is not available. The Single Perturbation Load Approach (SPLA), a design method developed by Huhne [4], is a deterministic approach where a lateral Load is applied prior to the axial compression (Figure 1), stimulating a single dimple. At this dimple the buckling process will start and a single buckle is produced, which will then propagate until the structure collapses. This paper presents investigations to develop an empirical formula for the design Load for isotropic conical structures obtained by use of Single Perturbation Load Approach. The study is part of the running European Union (EU) project DESICOS (New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures, cf. [5]), which contributes to lighter and cheaper structures by a new design procedure for imperfection sensitive composite launcher structures, combining probabilistic and deterministic approaches.

  • The single Perturbation Load approach applied to imperfection sensitive conical composite structures
    Thin-Walled Structures, 2014
    Co-Authors: Regina Khakimova, Rolf Zimmermann, Mariano A. Arbelo, Saullo G. P. Castro, Christopher J. Warren, Richard Degenhardt
    Abstract:

    Abstract The importance of taking into account geometric imperfections for cylindrical and conical thin-walled structures prone to buckling had been already recognized by the first authors dealing with new formulations. Nowadays, the analysts still use empirically based lower-bound methods such as the NASA SP-8007 guideline to calculate the required knock-down factors (KDFs), which does include important mechanical properties of laminated composite materials, such as the stacking sequence. New design approaches that allow taking full advantage of composite materials are required. The single Perturbation Load approach (SPLA), a new deterministic approach first proposed by Huhne, will be investigated with unstiffened composite conical structures varying the geometry, lamina and layup. The SPLA׳s capability for predicting KDF is compared with the NASA approach. The SPLA was applied to the geometrically perfect structures and to the structure with geometric imperfections of two types, mid-surface imperfections and thickness imperfections. The study contributes to the European Union (EU) project DESICOS, whose aim is to develop less conservative design guidelines for imperfection sensitive thin-walled structures.

  • Numerical characterization of imperfection sensitive composite structures
    Composite Structures, 2014
    Co-Authors: Mariano A. Arbelo, Richard Degenhardt, Saullo G. P. Castro, Rolf Zimmermann
    Abstract:

    Currently, imperfection sensitive shell structures prone to buckling are designed according to the NASA SP-8007 guideline, from 1968, using its conservative lower bound curve. In this guideline the structural behavior of composite materials is not appropriately considered, since the imperfection sensitivity and the buckling Load of shells made of such materials depend on the lay-up design. In this context a numerical investigation about the different methodologies to characterize the behavior of imperfection sensitive composite structures subjected to compressive Loads up to buckling is presented in this paper. A comparative study is addressed between a new methodology, called “Single Perturbation Load Approach”, adopted by the European project DESICOS, and some classical approaches such as non-linear analyses considering geometric and thickness imperfection obtained from real measurements. An extension of the Single Perturbation Load Approach called “Multiple Perturbation Load Approach” is also introduced in this paper to investigate if one Perturbation Load is enough to create the worst geometrical imperfection case. The aim of this work is to validate these numerical methodologies with experimental results and point out their limitation, advantage and disadvantage, to calculate less conservative knock-down factors than the obtained with the NASA SP-8007 guideline for unstiffened composite cylinders.

  • Geometric imperfections and lower-bound methods used to calculate knock-down factors for axially compressed composite cylindrical shells
    Thin-Walled Structures, 2014
    Co-Authors: Saullo G. P. Castro, Rolf Zimmermann, Mariano A. Arbelo, Regina Khakimova, Mark W. Hilburger, Richard Degenhardt
    Abstract:

    The important role of geometric imperfections on the decrease of the buckling Load for thin-walled cylinders had been recognized already by the first authors investigating the theoretical approaches on this topic. However, there are currently no closed-form solutions to take imperfections into account already during the early design phases, forcing the analysts to use lower-bound methods to calculate the required knock-down factors (KDF). Lower-bound methods such as the empirical NASA SP-8007 guideline are commonly used in the aerospace and space industries, while the approaches based on the Reduced Stiffness Method (RSM) have been used mostly in the civil engineering field. Since 1970s a considerable number of experimental and numerical investigations have been conducted to develop new stochastic and deterministic methods for calculating less conservative KDFs. Among the deterministic approaches, the single Perturbation Load approach (SPLA), proposed by Huhne, will be further investigated for axially compressed fiber composite cylindrical shells and compared with four other methods commonly used to create geometric imperfections: linear buckling mode-shaped, geometric dimples, axisymmetric imperfections and measured geometric imperfections from test articles. The finite element method using static analysis with artificial damping is used to simulate the displacement controlled compression tests up to the post-buckled range of Loading. The implementation of each method is explained in details and the different KDFs obtained are compared. The study is part of the European Union (EU) project DESICOS, whose aim is to combine stochastic and deterministic approaches to develop less conservative guidelines for the design of imperfection sensitive structures.

Mariano A. Arbelo - One of the best experts on this subject based on the ideXlab platform.

  • Investigation on the Geometric Imperfections driven Local Buckling Onset in Composite Conical Shells
    Applied Composite Materials, 2016
    Co-Authors: Maria Francesca Di Pasqua, Mariano A. Arbelo, Regina Khakimova, Saullo G. P. Castro, Aniello Riccio, Antonio Raimondo, Richard Degenhardt
    Abstract:

    Buckling is a critical failure phenomenon for structures, and represents a threat for thin shells subjected to compressive forces. The global buckling Load, for a conical structure, depends on the geometry and material properties of the shell, on the stacking sequence, on the type of applied Load and on the initial geometric imperfections. Geometric imperfections, occurring inevitably during manufacturing and assembly of thin-walled composite structures, produce a reduction in the carrying Load capability with respect to the design value. This is the reason why investigating these defects is of major concern in order to avoid over-conservative design structures. In this paper, the buckling behavior a conical structure with 45° semi-vertical angle is numerically investigated. The initial imperfections are taken into account by using different strategies. At first, the Single Perturbation Load Approach (SPLA), which accounts for defects in the form of a lateral Load, normal to the surface, has been adopted. Then, the actual measured defects have been applied to the structure by using the Real Measured Mid-Surface Imperfections (MSI) approach. Investigations on cylindrical shells using the first strategy have already shown the occurrence of a particular phenomenon called “local snap-through”, which represents a preliminary loss of stiffness. In order to better understand this phenomenon for conical shells, both the aforementioned techniques have been used to provide an exhaustive overview of the imperfections sensitiveness in conical composite shells. This study is related to part of the work performed in the frame of the European Union (EU) project DESICOS.

  • The Influence of Geometrical Parameters on the Buckling Behavior of Conical Shell by the Single Perturbation Load Approach
    Applied Composite Materials, 2015
    Co-Authors: Maria Francesca Di Pasqua, Mariano A. Arbelo, Regina Khakimova, Saullo G. P. Castro, Aniello Riccio, Richard Degenhardt
    Abstract:

    Since the development of the first theories to predict the buckling induced by axial compression in shells sensitive to imperfections, a significant discrepancy between theoretical and experimental results has been observed. Donnell and Koiter are among the first authors demonstrating, for these structures, the relevant influence of the geometrical imperfections on the reduction of the buckling Load. Currently, the preliminary design of imperfections sensitive shell structures used in space applications is carried out according to the NASA SP-8007guideline. However, several studies have proven that this guideline leads to over-conservative design configurations when considering the geometrical and material imperfections existing in real cones. Since the pioneer work of Arbocz, alternative methods have been investigated to overcome this issue. Among the different approaches, in this paper, the Single Perturbation Load Approach (SPLA), originally developed byHühne as a deterministic way to calculate the knock-down factor of imperfection sensitive shells, is further studied. Indeed, a numerical investigation about the application of the SPLA to the simulation of the mechanical behavior of imperfection sensitive composite conical structures under axial compression is presented. This study is related to part of the work performed in the frame of the European Union (EU) project DESICOS.

  • Semi-analytical tools for the Single Perturbation Load Approach using the Ritz Method
    2014
    Co-Authors: Saullo Castro, Christian Mittelstedt, Mariano A. Arbelo, Regina Khakimova, Richard Degenhardt
    Abstract:

    Many structural components or launcher structures can be modelled using a conical or cylindrical-like set of shell equations. The current guidelines used at the preliminary design phase are perceived as conservative and do not take into account properly the stacking sequence of laminated shells. New guidelines has been developed for example by the NASA project Shell Buckling Knock-Down Factor (SBKDF) and by the European consortium DESICOS. One of the approaches currently investigated is the Single Perturbation Load Approach (SPLA), originally proposed by Christian Huhne (2008) which is based on the phenomena that the axially compressed shell buckling Load gets nearly constant after a threshold of geometric imperfection caused by a lateral Load, obtaining a convenient design Load. A disadvantage of this method is that it requires many non-linear computations in order to find this Load threshold. This study presents the development of semi-analytical tools that allow a fast prediction of the design Load using the SPLA, with the aim to overcome the computational disadvantage of this method.

  • An empirical formula for the design Load obtained by use of Single Perturbation Load Approach
    2014
    Co-Authors: Regina Khakimova, Mariano A. Arbelo, Rolf Zimmermann, Saullo G. P. Castro, Richard Degenhardt
    Abstract:

    The stability of shell structures has been an object of studies for more than a century. Thin walled cylindrical and conical structures are widely used in aerospace, offshore, marine, civil and other industries. The importance of taking into account geometric imperfections for cylindrical and conical thin-walled structures in buckling had been already recognized a long time ago. In spite of a multitude of publications on buckling of imperfect shells, such structures are still today generally designed at the preliminary design phase according to the NASA SP-8007 [1] for cylinders and the NASA SP-8019 [2] for truncated cones. Both guidelines date from 1960’s and they are based on a lower bound curve which does not consider important mechanical characteristics of laminated composite shells, such as the stacking sequence, thus producing configurations that are over-conservative or even non-conservative for some cases. Koiter in 1945 [3] was the first who theoretically demonstrated the already experimentally observed imperfection sensitivity that affects the buckling behavior of thin-walled structures. Nowadays, with the everyday increasing computational power, it becomes easier to consider imperfections in numerical simulations. However, in the early design stage the real geometric imperfection pattern of a new type of structure is not available. The Single Perturbation Load Approach (SPLA), a design method developed by Huhne [4], is a deterministic approach where a lateral Load is applied prior to the axial compression (Figure 1), stimulating a single dimple. At this dimple the buckling process will start and a single buckle is produced, which will then propagate until the structure collapses. This paper presents investigations to develop an empirical formula for the design Load for isotropic conical structures obtained by use of Single Perturbation Load Approach. The study is part of the running European Union (EU) project DESICOS (New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures, cf. [5]), which contributes to lighter and cheaper structures by a new design procedure for imperfection sensitive composite launcher structures, combining probabilistic and deterministic approaches.

  • Evaluation of multiple Perturbation Load approach for experimental characterization of design buckling Load on cylindrical shells
    2014
    Co-Authors: Kaspars Kalnins, Mariano A. Arbelo, Oļģerts Ozoliņš, Richard Degenhardt
    Abstract:

    This article presents a study of more tailored approach to robust design a composite shell structures. The thin walled shell structures are basically main Load frames of the launcher, airframe etc. structures. As the thickness to radius ratio is considerably high this cause a large uncertainties on prediction of the Load carrying properties. Geometrical/thickness etc. imperfection which could be considered as manufacturing signature has not been considered so far as a design criteria. The current design approach for shell structures prone to buckling should be designed according to the NASA SP-8007 guideline [1], from 1968, using its conservative lower bound curve. In this guideline the structural behavior of composite materials is not appropriately considered, since the imperfection sensitivity and the buckling Load of shells made of such materials depend on the lay-up design and anisotropic material behavior. In this context, currently several European projects have been working to find more reliable methodologies for design and optimize structures prone to buckling using advanced composite materials and fabrication process. For instance a new methodology to predict the knock-down factor of unstiffened cylinders, called “Single Perturbation Load Approach”, developed by Huhne et al [2], uses the influence of radially applied Load on the buckling Load as an indication of imperfection sensitivity. In this context, a current experimental study presents the effects on the buckling Load of the “Multiple Perturbation Load Approach” (MPLA) against the SPLA on a carbon fiber composite cylindrical shell with a radius over thickness ratio of 400. A test series of three fully instrumented (Figure 1.) unidirectional tape pre-preg CFRP cylinders has been prototyped and tested to evaluate the MPLA approach.

Ali Shahvarpour - One of the best experts on this subject based on the ideXlab platform.