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

  • 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.

  • 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.

  • On the buckling mechanism of imperfection sensitive monolithic thin-walled unstiffened composite cylinders – physical observations to support less conservative Knock-down Factors
    2013
    Co-Authors: Saullo Castro, Rolf Zimmermann, Saullo Arbelo, Richard Degenhardt
    Abstract:

    The need to take the imperfection sensitivity of thin-walled unstiffened cylinders into account during the early design phases motivated the development of the NASA SP-8007 guideline, which brings an empirically based lower-bound curve that gives the Knock-down Factor (KDF) as a function of the cylinder’s radius and thickness. This guideline is still used also for composites, using correction. The conservativeness of these guidelines has been proved both stochastically by Arbocz, Starnes Jr., Hilburger, et al.; and deterministically by Geier, Huhne, Degenhardt, et al. A deterministic method: single perturbation load approach; developed by Huhne, showed that after some radial load (called P1) the imperfection sensitivity decreases, i.e. the buckling load remains nearly constant even increasing the radial load. Such physical benefit is not taken into account by the NASA SP-8007 guideline. This study, in the context of DESICOS, investigates the buckling mechanism of composite benchmark cylinder, developed by Zimmermann, being extreme-case laminates with high and low imperfection sensitivity. The explanation why the buckling load is constant after P1 is given and a physical meaning for the P1 value is proposed. These findings pave the way for the development of semi-analytical approaches for calculating P1. By applying the P1 value in the analysis, one can obtain less conservative KDFs which have shown to be closer to test results, allowing cost savings for the space industry.

  • Investigations on imperfection sensitivity and deduction of improved Knock-down Factors for unstiffened CFRP cylindrical shells
    Composite Structures, 2010
    Co-Authors: Richard Degenhardt, Arne Bethge, Alexander Kling, Rolf Zimmermann, Klaus Rohwer, Jana Orf, Luise Kärger, Adriano Calvi
    Abstract:

    In the field of aerospace engineering but also in the fields of civil and mechanical engineering the industry demands for significantly reduced costs for development and operating. Reduction of structural weight at safe design is one avenue to achieve this objective. In many cases it results in thin-walled structures, which are prone to buckling if subjected to compression or shear. The presented paper is based on a recent European Space Agency (ESA) study, conducted at DLR Braunschweig, on Probabilistic Aspects of Buckling Knock-down Factors and contributes to this goal by striving for an improved buckling Knock-down Factor (the ratio of buckling loads of imperfect and perfect structures) for unstiffened CFRP cylindrical shells. Buckling tests and buckling simulations were performed to investigate the imperfection sensitivity and to validate the applied simulation methodologies. Test results as well as deterministic and probabilistic buckling simulation results are presented and compared. Finally, improved Knock-down Factors are deduced and discussed.

Shiro Kato - One of the best experts on this subject based on the ideXlab platform.

  • Buckling Load of Elliptic Paraboloidal Single Layer Reticulated Roofs with Simple Supports under Uniform Load
    International Journal of Space Structures, 2005
    Co-Authors: Shiro Kato, Yoshiro Yamauchi, Takashi Ueki, Kazune Okuhira
    Abstract:

    The present paper discusses buckling strength of elliptic paraboloidal reticular roofs under uniform load with a rectangular plan. The roofs are assumed as three way single layers with a simple support at all peripheries of a rectangular plan. The buckling analyses are performed for linear buckling, elastic buckling, and elastic-plastic buckling. The results of buckling loads are formulated based on two procedures for practical design use. One is an explicit formula for elastic buckling loads using a knock down Factor and the other one is an implicit expression for buckling loads interpreted as a column buckling strength in terms of generalized slenderness ratio.

  • Buckling Load of Elliptic Paraboloidal Single Layer Reticulated Roofs under Uniform Load
    International Journal of Space Structures, 2005
    Co-Authors: Shiro Kato, Yoshiro Yamauchi, Takashi Ueki
    Abstract:

    The present paper discusses buckling strength of elliptic paraboloidal reticular roofs under a uniform load on a rectangular plan. The roofs are assumed as three way single layers with a pin-support at all peripheries on a rectangular plan. The buckling analyses are performed for linear buckling, elastic buckling and elastic-plastic buckling. The results of buckling loads are formulated based on two procedures for practical design use. One is an explicit but approximate formula for elastic buckling loads using a knock down Factor and the other one is an implicit expression for buckling loads interpreted into a column buckling strength in terms of generalized slenderness ratio.

  • Evaluation of buckling strength of single layer reticulated shells for design practice
    Fourth International Conference on Advances in Steel Structures, 2005
    Co-Authors: Shiro Kato
    Abstract:

    Publisher Summary The chapter focuses on the roof type of a single-layer steel reticulated dome supported at peripheries, to show approximate evaluation methods for elastic-plastic buckling loads. Joint system at nodes is a rigid or almost a rigid connection. The approximate procedures cover the methods of continuum shell analogy and finite element model (FEM) linear buckling analysis with the use of column buckling concept. Both of them make use of a knock down Factor for elastic-buckling loads. Evaluation methods of buckling loads of single-layer steel reticulated domes are presented in the chapter, with an emphasis on approximate evaluation from both aspects of continuum shells and discrete beam-column elements. The methods are efficient to estimate linear, elastic-nonlinear, and elastic-plastic buckling loads under vertical loads like self-weight.

  • Elastic buckling characteristics of two-way grid shells of single layer and its application in design to evaluate the non-linear behavior and ultimate strength
    Journal of Constructional Steel Research, 2001
    Co-Authors: Tetsuo Yamashita, Shiro Kato
    Abstract:

    Abstract The present paper investigates and discusses both of the linear and elastic buckling characteristics of two-way grid shells of single layer under gravity loads focusing on the nonlinear behavior due to geometric imperfection. It is followed by a proposal of the estimation method usable in design to evaluate the nonlinear behavior and the ultimate strength of the shells based on the characteristics. The discussions are organized as follows: First, the governing concept that the elastic nonlinear behavior can be approximated based on an analogy to the axial-bending coupling of an Euler column, is presented. Second, the Knock-down Factor is analyzed in connection with imperfection magnitude and a formula to estimate the value is derived paying attention to the nonlinear amplification of the deformation due to imperfection. Finally, based on those studies, an effective method applicable in a practical engineering to evaluate ultimate buckling strength based on the modified Dunkerley equation in terms of the generalized slenderness is proposed.

Janis Varna - One of the best experts on this subject based on the ideXlab platform.

  • Master curve approach to axial stiffness calculation for non-crimp fabric biaxial composites with out-of-plane waviness
    Composites Part B: Engineering, 2014
    Co-Authors: Hana Zrida, Erik Marklund, Z. Ayadi, Janis Varna
    Abstract:

    The effect of 0 degrees-tow out-of-plane waviness on the biaxial non-crimp-fabric (NCF) composite axial stiffness is investigated. Homogenizing, the bundle mesostructure of the NCF composite is replaced by layers. Then the composite is represented by a laminate with flat layers with effective stiffness properties representing the curved 0 degrees-layer and the 90 degrees-layer with varying thickness. It is shown that the NCF composite Knock-down Factor characterizing the stiffness degradation has almost the same dependence on wave parameters as the Knock-down Factor for the curved 0 degrees-layer. Numerical analysis showed that 90 degrees-layer Knock-down Factor versus amplitude curves for different wavelength can be reduced to one master curve which can be described by a one-parameter expression with the parameter dependent on the used material. This observation is used to obtain high accuracy for analytical predictions for Knock-down Factors for cases with different wavelength and amplitudes based on two FE calculations only.

  • Master curve approach to axial stiffness calculation for non-crimp fabric biaxial composites with out-of-plane waviness
    Composites Part B: Engineering, 2014
    Co-Authors: Hana Zrida, Erik Marklund, Z. Ayadi, Janis Varna
    Abstract:

    Abstract The effect of 0°-tow out-of-plane waviness on the biaxial non-crimp-fabric (NCF) composite axial stiffness is investigated. Homogenizing, the bundle mesostructure of the NCF composite is replaced by layers. Then the composite is represented by a laminate with flat layers with effective stiffness properties representing the curved 0°-layer and the 90°-layer with varying thickness. It is shown that the NCF composite Knock-down Factor characterizing the stiffness degradation has almost the same dependence on wave parameters as the Knock-down Factor for the curved 0°-layer. Numerical analysis showed that 90°-layer Knock-down Factor versus amplitude curves for different wavelength can be reduced to one master curve which can be described by a one-parameter expression with the parameter dependent on the used material. This observation is used to obtain high accuracy for analytical predictions for Knock-down Factors for cases with different wavelength and amplitudes based on two FE calculations only.

Hana Zrida - One of the best experts on this subject based on the ideXlab platform.

  • Master curve approach to axial stiffness calculation for non-crimp fabric biaxial composites with out-of-plane waviness
    Composites Part B: Engineering, 2014
    Co-Authors: Hana Zrida, Erik Marklund, Z. Ayadi, Janis Varna
    Abstract:

    The effect of 0 degrees-tow out-of-plane waviness on the biaxial non-crimp-fabric (NCF) composite axial stiffness is investigated. Homogenizing, the bundle mesostructure of the NCF composite is replaced by layers. Then the composite is represented by a laminate with flat layers with effective stiffness properties representing the curved 0 degrees-layer and the 90 degrees-layer with varying thickness. It is shown that the NCF composite Knock-down Factor characterizing the stiffness degradation has almost the same dependence on wave parameters as the Knock-down Factor for the curved 0 degrees-layer. Numerical analysis showed that 90 degrees-layer Knock-down Factor versus amplitude curves for different wavelength can be reduced to one master curve which can be described by a one-parameter expression with the parameter dependent on the used material. This observation is used to obtain high accuracy for analytical predictions for Knock-down Factors for cases with different wavelength and amplitudes based on two FE calculations only.

  • Master curve approach to axial stiffness calculation for non-crimp fabric biaxial composites with out-of-plane waviness
    Composites Part B: Engineering, 2014
    Co-Authors: Hana Zrida, Erik Marklund, Z. Ayadi, Janis Varna
    Abstract:

    Abstract The effect of 0°-tow out-of-plane waviness on the biaxial non-crimp-fabric (NCF) composite axial stiffness is investigated. Homogenizing, the bundle mesostructure of the NCF composite is replaced by layers. Then the composite is represented by a laminate with flat layers with effective stiffness properties representing the curved 0°-layer and the 90°-layer with varying thickness. It is shown that the NCF composite Knock-down Factor characterizing the stiffness degradation has almost the same dependence on wave parameters as the Knock-down Factor for the curved 0°-layer. Numerical analysis showed that 90°-layer Knock-down Factor versus amplitude curves for different wavelength can be reduced to one master curve which can be described by a one-parameter expression with the parameter dependent on the used material. This observation is used to obtain high accuracy for analytical predictions for Knock-down Factors for cases with different wavelength and amplitudes based on two FE calculations only.

Takashi Ueki - One of the best experts on this subject based on the ideXlab platform.

  • Buckling Load of Elliptic Paraboloidal Single Layer Reticulated Roofs with Simple Supports under Uniform Load
    International Journal of Space Structures, 2005
    Co-Authors: Shiro Kato, Yoshiro Yamauchi, Takashi Ueki, Kazune Okuhira
    Abstract:

    The present paper discusses buckling strength of elliptic paraboloidal reticular roofs under uniform load with a rectangular plan. The roofs are assumed as three way single layers with a simple support at all peripheries of a rectangular plan. The buckling analyses are performed for linear buckling, elastic buckling, and elastic-plastic buckling. The results of buckling loads are formulated based on two procedures for practical design use. One is an explicit formula for elastic buckling loads using a knock down Factor and the other one is an implicit expression for buckling loads interpreted as a column buckling strength in terms of generalized slenderness ratio.

  • Buckling Load of Elliptic Paraboloidal Single Layer Reticulated Roofs under Uniform Load
    International Journal of Space Structures, 2005
    Co-Authors: Shiro Kato, Yoshiro Yamauchi, Takashi Ueki
    Abstract:

    The present paper discusses buckling strength of elliptic paraboloidal reticular roofs under a uniform load on a rectangular plan. The roofs are assumed as three way single layers with a pin-support at all peripheries on a rectangular plan. The buckling analyses are performed for linear buckling, elastic buckling and elastic-plastic buckling. The results of buckling loads are formulated based on two procedures for practical design use. One is an explicit but approximate formula for elastic buckling loads using a knock down Factor and the other one is an implicit expression for buckling loads interpreted into a column buckling strength in terms of generalized slenderness ratio.