Edgars Eglītis

Experimental and numerical study on buckling of axially compressed composite cylinders

Experimental and numerical study on buckling of axially compressed composite cylinders Thin shells are lightweight, efficient structures that can support very high buckling loads. However, unlike columns and plates, shells usually have a very unstable post-buckling behaviour that strongly influences their buckling characteristics. Axially compressed cylinder may be one of the last classical problems in structural mechanics for which it remains difficult to obtain close agreement between careful experiments and the best predictions from numerical modelling, and therefore much more research is needed on this subject. Within this investigation, 18 glass fibre composite shells with wall thickness of 1.1 mm, 0.3 m and 0.5 m diameters and various lengths have been produced along with flat specimens for determination of elastic properties of the material. The cylinders were repeatedly loaded until post-buckling and load-shortening curves and pictures of buckling mode shapes have been registered. Finite element models of the specimens have been developed in ABAQUS finite element package and non-linear explicit dynamic analyses have been performed for comparison with experimental and analytical results. Gradually improving the finite element model and adding artificial initial imperfections resulted in good agreement between the experimental and numerically obtained critical loads. The buckling modes observed during the experiments are in good agreement with the results of finite element simulations as well.

Optimal design of composite upper covers of lateral wings with the effect of rib attachment to stiffener webs

The present investigation is devoted to the development of new optimal design concepts that exploit the full potential of advanced composite materials in the upper covers of aircraft lateral wings. A finite-element simulation of three-rib-bay laminated composite panels with T-stiffeners and a stiffener pitch of 200 mm is carried out using ANSYS to investigate the effect of rib attachment to stiffener webs on the performance of stiffened panels in terms of their buckling behavior and in relation to skin and stiffener lay-ups, stiffener height, and root width. Due to the large dimension of numerical problems to be solved, an optimization methodology is developed employing the method of experimental design and the response surface technique. Minimal-weight optimization problems were solved for four load levels with account of manufacturing, repairability, and damage tolerance requirements. The optimal results were verified successfully by using the ANSYS and ABAQUS shared-node models.

Experimental evaluation of damage influence on buckling performance of stiffened CFRP shells

Experimental evaluation of damage influence on buckling performance of stiffened CFRP shells Delamination type failures are often observed in carbon-epoxy composites, where catastrophic failure is generally preceded by constituent level damage accumulation. Out-of-plane loading such as internal pressure in a composite fuselage or out-of-plane deformations in compression-loaded post-buckled panel may lead to debonding of the frame or stiffener from the panel. On other hand, numerous investigations show the ability of stiffened composite structures to work in the post-buckling region, which is considered unsafe in the conventional design procedures. Experimental evaluation of damage influence on post-buckling performance of stiffened shells is essential for development of safe design guidelines that would allow exploitation of these structures in the post-buckling region. The results of this investigation show that a damaged stiffened composite shell can be loaded up to 200% of skin buckling load without any propagation of delaminations. This can serve as the base for more extended studies on the subject, including numerical modelling and improvement of the existing design guidelines.