Badania numeryczne i doświadczalne stateczności i nośności kompozytowych słupów cienkościennych poddanych ściskaniu.

W rozprawie zajmowano się zagadnieniem stateczności i procesu zniszczenia ściskanych cienkościennych słupów kompozytowych o przekrojach otwartych. Do opisu zagadnienia stateczności zastosowano badania doświadczalne prowadzone na rzeczywistych konstrukcjach kompozytowych oraz metody obliczeniowe z wykorzystaniem metody analityczno-numerycznej (MAN) oraz metody elementów skończonych (MES). W ramach badań doświadczalnych na wykonanych techniką autoklawową kompozytowych słupach o przekrojach ceowych i omegowych prowadzono analizę stanu krytycznego, pokrytycznego oraz fazy zniszczenia konstrukcji. Rejestracji wyników badań dokonywano z wykorzystaniem metod tensometrii oporowej, laserowego pomiaru ugięć, szybkiej kamery i metody emisji akustycznej. Dodatkowo prowadzono ocenę jakości wytworzonych struktur kompozytowych z zastosowaniem metod nieniszczących NDT, mikrotomografii rentgenowskiej oraz mikroskopii optycznej. Zastosowane techniki umożliwiały lokalizację wewnętrznych wad wytworzonego materiału (porowatości, delaminacji), jak również identyfikację charakteru zjawiska zniszczenia struktury materiału kompozytowego po przeprowadzonych badaniach niszczących. Badania symulacyjne prowadzono z wykorzystaniem dwóch technik obliczeniowych: metody analityczno-numerycznej oraz metody elementów skończonych. Otrzymano dobrą zgodność jakościową i ilościową wyników obliczeń z wynikami badań doświadczalnych, w szczególności dla stanu krytycznego i pokrytycznego. Potwierdziło to adekwatność opracowanych modeli numerycznych do analizy zagadnienia stateczności badanych konstrukcji kompozytowych. Stan krytyczny opisano, rozwiązując zagadnienie własne. Stany pokrytyczne stanowiące nieliniowe zagadnienie stateczności rozwiązano z wykorzystaniem metody analityczno-numerycznej bazującej na asymptotycznej teorii układów zachowawczych Koitera oraz metody elementów skończonych z wykorzystaniem metody przyrostowo-iteracyjnej Newtona-Raphsona oraz strategii korekcyjnej opartej na kontroli długości łuku w sformułowaniu metody Riksa. Analizę zniszczenia badanych konstrukcji przeprowadzono z wykorzystaniem naprężeniowych kryteriów zniszczenia kompozytu: kryterium maksymalnych naprężeń, kryterium Tsai-Hill’a, kryterium Tsai-Wu oraz kryterium Azzi-Tsai-Hilla. Wykazano jednoznaczność oraz akceptowalną przydatność zastosowanych kryteriów do opisu fazy zniszczenia ściskanych słupów kompozytowych, weryfikując otrzymane wyniki badaniami doświadczalnymi.Na podstawie przeprowadzonych analiz zauważono istotny wpływ układu włókien kompozytu na wartość obciążenia krytycznego i niszczącego. Dotyczy to również otrzymywanych postaci lokalnego wyboczenia ścian słupów, dla których w zależności od zastosowanej konfiguracji kompozytu otrzymywano inne liczby półfal w kierunku wzdłużnym charakteryzujących proces utraty stateczności badanych konstrukcji. Prezentowana praca składa się z jedenastu rozdziałów. Rozdział pierwszy stanowi wprowadzenie w tematykę zagadnienia stateczności oraz zniszczenia cienkościennych struktur kompozytowych. W rozdziale drugim omówiono najważniejsze pozycje literaturowe dotyczące tematyki niniejszej pracy. Najważniejsze cele oraz tezę prezentowanej pracy sformułowano w rozdziale trzecim. Rozdział czwarty zawiera podstawowe równania stateczności oraz właściwości mechaniczne cienkościennych płyt kompozytowych. W rozdziale piątym omówiono wybrane kryteria zniszczenia kompozytów. W rozdziale szóstym przedstawiono metodykę rozwiązania zagadnienia stateczności metodą analityczno-numeryczną (MAN) oraz metodą elementów skończonych (MES). Rozdział siódmy zawiera opis przedmiotu badań, metod wytwarzania i oceny właściwości wytworzonych struktur kompozytowych. W rozdziale ósmym przedstawiono wyniki badan doświadczalnych stanu krytycznego. Rozdział dziewiąty zawiera wyniki analizy analityczno-numerycznej i numerycznej w kontekście wyników badań doświadczalnych. W rozdziale dziesiątym przedstawiono wyniki analizy zniszczenia badanych konstrukcji cienkościennych. Wnioski końcowe pracy zawarto w rozdziale jedenastym.The thesis deals with of a problem of stability, as well as damage processes taking place in thin-walled composite columns having open cross-sections. In order to describe the stability phenomena, both experimental tests, lead on the real composite structures and computational methods, exploiting an analyticalnumerical method, as well as the Finite Element Method (FEM) were performed. Within the framework of experiment a critical state analysis, as well as a postcritical one, comprising the structure’s failure phase were conducted on an Ω-type and a C-type channel section composite columns manufactured with the autoclaving technique. The data acquisition process was performed with an electrical strain gage technique, a laser sensor deflection measurement, a high speed camera and an acoustic emission technique. In addition, the columns’ quality check was performed using non-destructive techniques (NDT), an X-ray microtomography and an optical microscopy. The exploited techniques enabled a location of internal flaws (porosity, delamination) of the manufactured composite material, as well as an identification of damage mechanisms of the structure after performing destructive tests. The numerical simulations were lead with the two computational techniques: the analytical-numerical one and the Finite Element Method. A good agreement of the computational and the experimental results was gained, both in qualitative and in quantitative sense, especially for the critical and the postcritical state. This confirmed the aptitude of the elaborated numerical models to be used in the analysis of stability of the examined structures. The critical state was defined by solving the eigenproblem. The postcritical state, being a nonlinear problem of stability was solved by taking advantage of both analytical-numerical method and the Finite Element Method. The former based on the Koiter’s asymptotic theory of conservative systems, whereas the latter exploited the incremental-iterative Newton-Raphson method accompanied by a corrective strategy based on the arc length control in the formulation of the Riks method. The failure analysis of the examined structures was performed using several stress criteria for damage in composites: the maximum stress criterion, as well as the Tsai-Hill, the Tsai-Wu and the Azzi-Tsai-Hill criterion. An acceptable suitability of the applied criteria for the preliminary description of the failure phase of the compressed composite columns was proved by a verification of the numerically gained results with the experimental ones. On the basis of the performed analyses a considerable influence of the composites’ fibre direction sequence on the value of critical load, the failure load and the postcritical stiffness of the structure was observed. This applied also tothe received modes of local buckling of the columns’ walls, for which different half-wave numbers were gained in longitudinal direction depending on the composite sequence. The number half-waves characterized the process of stability loss of the examined structures. The performed research enabled an elaboration of a procedure of complex stability and load capacity analysis of thin-walled composite columns having complicated shape of its cross-section. The performed multidisciplinary study within the above described domain enabled sufficient identification and interpretation of the composite structures’ damage processes. The presented thesis comprises eleven chapters. The first one is an introduction to the subject of stability and damage problems of thin-walled composite structures. In chapter two, the most important literature items, concerning the subject area of the thesis were discussed. Main goals of the thesis were formulated in chapter three. Chapter four contains basic equations of stability of thin-walled composite plates. In chapter five, chosen criteria of composite failure were talked over. In chapter six, the procedure of solving the stability problem with the analytical-numerical method and with the Finite Element Method was presented. Chapter seven contains a description of the research objects, manufacturing methods and ways of determination of the fabricated composite structures’ properties. In chapter eight, the critical state’s experimental results were presented. Chapter nine contains the results of the analytical-numerical, as well as the numerical analysis within a context of experimental results. In chapter ten, the results of damage analysis of the examined thin-walled structures were collected. The conclusions arising from the thesis were presented in chapter eleven

STABILITY AND FAILURE OF THIN-WALLED COMPOSITE STRUCTURES WITH A SQUARE CROSS-SECTION

This paper is devoted to the analysis of the stability and load-carrying capacity of thin-walled composite profiles in compression. The specimens reflect elements made of carbon fibre reinforced laminate (CFRP). Thin-walled columns with a square crosssection were made from 4 layers of composite in 3 different combinations of layer arrangements. Advanced numerical analyses have been carried out. In the first stage of the study, a buckling analysis of the structure was performed. In further numerical simulations, two advanced models were used simultaneously: the Progressive Failure Analysis (PFA) and the Cohesive Zone Model (CZM). The results showed significant differences between the critical load values for each layer configuration. The forms of buckling and the areas of damage initiation and evolution were also dependent on the applied layup

Short cervix in twin pregnancies: current state of knowledge and the proposed scheme of treatment

Short cervical length (SCL) should be defined as cervical length (CL) less than 25 mm between 18 and 22 weeks of gestation. This definition of SCL is fully applicable for singleton pregnancies but is not entirely correct for twin pregnancies. So far there are no explicit guidelines on the treatment of twin pregnancy with SCL. The use of progesterone in the treatment of SCL and preterm birth (PTB) prophylaxis is one of the interventions recommended by the Polish Ministry of Health for cervical shortening in singleton pregnancies. In twin pregnancies attention should be paid to the potential benefits of using vaginal progesterone in reduction of neonatal mortality and incidence of neonatal complications in a group of patients with twin pregnancies and CL less than 25 mm or below the 10th percentile for the gestational age, measured between 18 and 22 weeks of gestation. It is still difficult to identify the benefits of using pessaries in the prevention of PTB in twin pregnancies. The usage of pessaries appears to be beneficial only in selected subpopulations of patients with asymptomatic CL less than 25 mm or 10th percentile for gestational age. The use of cervical cerclage in PTB prevention in twin pregnancies is limited to cases where the external cervical dilation is >1 cm an

Stability Problems of Comperssed Thin-Walled Structures

The object of this study is a thin-walled columns made of carbon-epoxy composite with open top-hat cross-section. The material used was a composite of epoxy matrix reinforced with carbon fiber (system HexPly M12, Hexcel). The M12 system is used above all in aircraft structures. It exhibits high fatigue durability and good maintenance properties at relatively low specific gravity. The research was lead as the FEM numerical analyses and experimental tests in buckling and post-buckling state. The numerical tool used was the ABAQUS software

The Influence of Composite Lay-Up on the Stability of a Structure with Closed Section

This paper presents a study on the stability of a composite structure with a closed section. The research was carried out solely based on numerical simulations using the finite element method. ABAQUS software was used to conduct numerical simulations, which allowed to determine the values of critical loads, as well as the corresponding forms of buckling. As part of the research, the influence of composite material arrangement of layers on the stability of the structure was analyzed. The study was conducted on a composite material, which is a carbon-epoxy composite (CFRP). The obtained research results will be verified in subsequent studies by the results of experimental tests

POSTBUCKLING BEHAVIOUR OF LAMINATED PLATES WITH A CUT-OUT

The subject of research is the numerical and experimental analysis of a thin-walled plate with the cut-out, made of the laminate and subjected to the axial compression. In order to ensure the stable plate work in the postcritical range, it need to force its work by higher flexural-torsional form of buckling. The scope of the research included the nonlinear numerical analysis with Finite Element Method (FEM) and experimental verification of calculation results. For the composite material in numerical calculations, was defined the model of orthotropic material in the flat state of tension. For the mapping of the plate element was applied coating finite elements of type SHELL with shape function of the second order to allowing the mapping of the composite structure for the element thickness. The experimental research was carried out in a special grips providing the articulated support of the upper and lower edges of the plate. The deformation registration of compression structures were carried out with resistance extensometry. This allowed to define of postcritical equilibrium paths of real structure, showing the dependence of the force to displacement. The instrument used was a numerical program ABAQUS®

Experimental and Numerical Analysis of Post-Critical States and Damage of Thin-Walled Channel Section Columns Made of Carbon/Epoxy Composite

The paper presents results of a numerical and experimental analysis of post-critical state of a thin-walled channel-section simply supported beam under axial compression. The beam was made of carbon-epoxy symmetrical composite prepared with a pre-preg technology using 8 layers of unidirectional band. The material used was a composite of epoxy matrix reinforced with carbon fiber (system HexPly M12, Hexcel). The M12 system is used above all in aircraft structures. It exhibits high fatigue durability and good maintenance properties at relatively low specific gravity. The research was lead as the FEM numerical analyses and experimental tests in post-buckling and limit states, as well. Experimental studies were conducted to confirm results obtained from numerical calculations, which was performed using the ABAQUS® software