Kaspars Kalnins

Surrogate models for optimum design of stiffened composite shells

An optimization procedure is developed for the design of composite stiffened shells subjected to buckling and post-buckling constraints. The optimization method is based on building surrogate models employing the experimental design and response surface methodology. A combined data set consisting of test results of stiffened shells and numerical data obtained by finite element simulation is used for building the surrogate models. These models are used for sensitivity analysis, evaluation of the weight saving parameters and for design optimization of stiffened composite panels under axial compression loading. It was shown that employing the surrogate models satisfactory accuracy can be achieved to describe the post-buckling behavior of the stiffened panels and to use these models in design optimization.

METAMODELING METHODOLOGY FOR POSTBUCKLING SIMULATION OF DAMAGED COMPOSITE STIFFENED STRUCTURES WITH PHYSICAL VALIDATION

A metamodeling methodology has been proposed for postbuckling simulation of stiffened composite structures with integrated degradation scenarios. The presence of artificial damage in between the outer skin and stiffeners has been simulated as softening of the material properties in predetermined regions in the structure. The proposed methodology for the fast design procedure of axially or torsionally loaded stiffened composite structures is based on response surface methodology (RSM) and design and analysis of computer experiments (DACE). Numerical analyses have been parametrically sampled by means of ANSYS/LS-DYNA probabilistic design toolbox extracting the load-shortening response curves in the preselected domain of interest. These response curves have been simplified using piece-wise linear approximation identifying the buckling and postbuckling stiffness ratios along with the values of the skin and the stiffener buckling loads. Three stiffened panel designs and a closed box structure with preselected damage scenarios have been elaborated and validated with the tests performed within the COCOMAT project. The resulting design procedure provides a time effective design tool for preliminary study and for elaboration of the optimum design guidelines of composite stiffened structures with material degradation restraints.

Experimental Nondestructive Test for Estimation of Buckling Load on Unstiffened Cylindrical Shells Using Vibration Correlation Technique

Nondestructive methods, to calculate the buckling load of imperfection sensitive thin-walled structures, such as large-scale aerospace structures, are one of the most important techniques for the evaluation of new structures and validation of numerical models. The vibration correlation technique (VCT) allows determining the buckling load for several types of structures without reaching the instability point, but this technique is still under development for thin-walled plates and shells. This paper presents and discusses an experimental verification of a novel approach using vibration correlation technique for the prediction of realistic buckling loads of unstiffened cylindrical shells loaded under axial compression. Four different test structures were manufactured and loaded up to buckling: two composite laminated cylindrical shells and two stainless steel cylinders. In order to characterize a relationship with the applied load, the first natural frequency of vibration and mode shape is measured during testing using a 3D laser scanner. The proposed vibration correlation technique allows one to predict the experimental buckling load with a very good approximation without actually reaching the instability point. Additional experimental tests and numerical models are currently under development to further validate the proposed approach for composite and metallic conical structures.

Metamodels for Optimum Design of Laser Welded Sandwich Structures

All-metal sandwich panels, made by a process of laser welding faceplates to core-stiffeners, show advanced cost/weight properties compared with the conventional structural applications of stiffened plates. However, optimal design of these advanced structures requires a fast simulation procedure that should have the same level of reliability compared to finite element calculations and natural tests, while being more time effective and less complex. It was shown that different polynomial functions together with design of computer experiments can contribute to such an aim by providing simple however reliable metamodels. The validation procedure indicated an average of 10% relative root mean square error prediction accuracy, and due to this precision the procedure is capable to be used for further (cost/weight) design optimisation together with structural sizing studies and parametric sensitivity analysis.

Material degradation assessment for stiffened composite shells using metamodelling approach.

The intense interest coming from the aerospace industry indicates the need of safe exploitation of composite materials in stiffened shell structures. Since stiffened shells are far most consumed structural component, it is important to study the behaviour of material degradation to evaluate the safe design guidelines. Moreover, current numerical procedures cannot simulate the collapse of stiffened shells with sufficient reliability and efficiency, leading to over-conservative designs. One can assume that great potential exist for future increase of effectiveness of stiffened composite structures by allowing of post-buckling of skin to occur during the exploitation [1].

Applicability of the Vibration Correlation Technique for Estimation of the Buckling Load in Axial Compression of Cylindrical Isotropic Shells with and without Circular Cutouts

Applicability of the vibration correlation technique (VCT) for nondestructive evaluation of the axial buckling load is considered. Thin-walled cylindrical shells with and without circular cutouts have been produced by adhesive overlap bonding from a sheet of aluminium alloy. Both mid-surface and bond-line imperfections of initial shell geometry have been characterized by a laser scanner. Vibration response of shells under axial compression has been monitored to experimentally determine the variation of the first eigenfrequency as a function of applied load. It is demonstrated that VCT provides reliable estimate of buckling load when structure has been loaded up to at least 60% of the critical load. This applies to uncut structures where global failure mode is governing collapse of the structure. By contrast, a local buckling in the vicinity of a cutout could not be predicted by VCT means. Nevertheless, it has been demonstrated that certain reinforcement around cutout may enable the global failure mode and corresponding reliability of VCT estimation.

Test results on the stability and vibrations of composite shells

Abstract This chapter presents experimental results for tests being performed on various thin-walled structures (shells and cones) manufactured from either laminated composite sandwich/materials or isotropic materials (steel, aluminum). All the specimens were under axial compression and were tested till their complete collapse.

Flexural behavior of sandwich panels with cellular wood, plywood stiffener/foam and thermoplastic composite core

A series of experimental tests have been carried out on three types of novel sandwich panels mainly designed for application in lightweight mobile housing. Two types of the panels are manufactured entirely from wood-based materials while the third one presents a combination of plywood for surfaces and corrugated thermoplastic composite as a core part. All sandwich panels are designed to allow rapid one-shot manufacturing. Mechanical performance has been evaluated in four-point bending comparing the data to the reference plywood board. Additionally, finite element simulations were performed to evaluate global behavior, stress distribution and provide the basis for a reliable design tool. Obtained results show sufficient mechanical characteristics suitable for floor and wall units. Compared to a solid plywood board, sandwich alternative can reach up to 42% higher specific stiffness, at the same time maintaining sufficient strength characteristics.

Metamodeling approach for analysis of post-buckling in composite panels with structural degradation

The intense interest coming from the aerospace industry indicates the need of safe exploitation of composite materials within post-buckling region. Since stiffened curved panels are by far the most consumed structural component, it is important to study the behavior of structural degradation to evaluate the safe design guidelines. An assessment of structural degradation in terms of stiffness reduction in the skin stringer zone is carried out to estimate the degradation influence on the postbuckling stiffness of the axially loaded stiffened panels. The presented procedure is based on the building of metamodels employing experimental design and response surface methodology. Metamodels are built using stiffened shell geometrical variables adding structural degradation variables such as degradation region length ratio and material elastic property reduction coefficient. The numerical responses, obtained from explicit FEM simulations of composite stiffened shells subjected to buckling and post-buckling, are used for the building of metamodels. The resulting design procedure provides an effective analysis tool for the safe exploitation of composite stiffened panels under axial compression.