Top-hat Stiffeners Research Articles

Failure study of compressed thin-walled composite columns with top-hat cross-section

This paper presents considerations on the failure of axially compressed thin-walled composite structures with open sections (using an top-hat section structure). The analyzed columns were made of composite material — carbon fiber reinforced polymer (CFRP) using the autoclave technique. The study was conducted in a comprehensive approach using both experimental methods and advanced numerical simulations based on the finite element method (in ABAQUS® program). Investigations were performed up to the total failure of the structure, while considering the phenomena directly contributing to the failure (damage initiation, damage evolution, delamination). During the conducted experimental studies, both the postbuckling equilibrium paths obtained from the universal testing machine and the acoustic emission signals were registered. Regarding the simultaneously conducted numerical simulations, two independent advanced damage models (progressive failure analysis — PFA and cohesive zone model — CZM) were used, allowing for a thorough analysis of the complex mechanism of the composite material failure phenomenon. Both numerical and experimental results show high agreement. This paper provides a more comprehensive understanding how failure occurs such that the design of the analyzed composite sections. The most important aspect of this paper is the potential to enable to understand the failure of the top-hat stiffeners during post buckling such that made it possible to utilize the stable, undamaged region of the post-buckling region. In addition, the paper conducts an advanced study based on the simultaneous use of two independent damage models, allowing in particular for a thorough analysis of the damage with delamination phenomenon detection, on structures with a complex shape of the end cross-sections of the structure.

Research on structural optimization method of FRP fishing vessel based on artificial bee colony algorithm

Due to the increase of the operating cost and material cost, weight reduction has attracted much attention among ship designers and ship owners. Ship structure optimization remains a very complicated problem due to the feasible combinations of structural members with different sections are numerous. In the present work, based on artificial bee colony (ABC) algorithm, we propose a structural design optimization method of fiber reinforced plastic (FRP) ship structure. The objective of the proposed optimization method is to minimize the weight of FRP ship structure under a group of constraints. In order to deal with parametric finite element analysis (FEA) of the whole FRP ship structure, the modeling work is simplified by using beam element instead of shell element to simulate top hat stiffener. To evaluate the applicability of the proposed method, it was applied to a problem for finding optimal structure of a 32.98 m FRP fishing vessel. The result shows that the developed program can decrease the structural weight by about 8.31%. Thus, this study will be able to contribute to make energy saving and environment-friendly ship.

Damage detection in bent plates using shear horizontal guided waves

Study of the interaction of shear horizontal guided mode with defects in the bend region of an isotropic top hat stiffener is presented. Compared with the SH0 wave in a plate, the shear mode in the bend is dispersive and its wavefield characteristics are affected by the curvature of the bend. The scattering studies showed that the sensitivity of the wave to outer surface cracks in the bend increases with increasing frequency compared to inner surface cracks. Further numerical simulations demonstrated that the shear mode is sensitive to the delamination in the bend due to non-zero transverse shear stress. Results of finite element modeling were validated by experiments and reasonably good agreements were obtained.

The application of reliability methods in the design of tophat stiffened composite panels under in-plane loading

Composite materials have been widely used in modern engineering fields such as aircraft, space and marine structures due to their high strength-to-weight and stiffness-to-weight ratios. However, structural efficiency gained through the adoption of composite materials can only be guaranteed by understanding the influence of production upon as-designed performance. In particular, topologies that are challenging to production including panels stiffened with pi or tophat stiffeners dominate many engineering applications and often observe complex loading. The design of stiffened composite panels against buckling is a key point of composite structures. While a growing number of studies are related to the reliability analysis of composites few of these relate to the local analysis of more complicated structures. Furthermore for the assessment of these structures in a design environment it is important to have models that allow the rapid assessment of the reliability of these local structures. This paper explores the use of a stochastic approach to the design of stiffened composite panels for which typical applications can be found in composite ship structures. A parametric study is conducted using Navier grillage theory and First-order Reliability Methods to investigate any detectable trend in the safety index with various design parameters. Finally, recommendations are made to provide guidance on applications.

Structural Strength Analysis of FRP Yacht by FEM

This paper discusses the laminated structure and sandwich structure by finite element modeling, the process of finite element modeling of composite panel with top-hat stiffeners and finite element analysis of the whole hull. The result shows that the method and steps of modeling FRP yacht based on FEM to directly calculate the hull structural strength are instructive.

Delamination failure of composite top-hat stiffeners using finite element analysis

The paper presents the finite element analysis of top-hat stiffeners (transversely loaded keel support structure) under a static load where splitting–delamination is the primary mode of failure. A set of experiments is conducted on the composite top-hat stiffeners with three different lay-up arrangements. The experimental study, together with the finite element modelling to capture the delamination failure, is presented. Fracture initiation is predicted using an interlaminar-strength-based criterion. A fracture-mechanics-based formulation is used to predict delamination propagation by connecting the composite layers with two-dimensional and three-dimensional interface elements using the commercially available MSC Marc Mentat 2008 r1 finite element software package. Reasonable agreement between the experiments and finite element analysis was achieved. The reserve strength identified by the experimental testing is successfully modelled. The factors affecting the fibre–matrix interface strength are studied. The failure location and mode for fracture initiation and propagation are determined.

Failure monitoring of E-glass/vinylester composites using fiber grating acoustic sensor

This paper reports an application of an optical fiber sensor in a continuous and in situ failure testing of an E-glass/vinylester top hat stiffener (THS). The sensor head was constructed from a compact phase-shifted fiber Bragg grating (PS-FBG). The narrow transmission channel of the PS-FBG is highly sensitive to small perturbation, hence suitable to be used in acoustic emission (AE) assessment technique. The progressive failure of THS was tested under transverse loading to experimentally simulate the actual loading in practice. Our experimental tests have demonstrated, in good agreement with the commercial piezoelectric sensors, that the important failures information of the THS was successfully recorded by the simple intensity-type PS-FBG sensor.

Acoustic emission techniques for failure characterisation in composite top-hat stiffeners

Modern day aerospace, automotive, marine, mechanical and civil structures rely on the advanced composites for their added benefits over conventional metallic structures. The complex damage process in composites involves various failure modes such as matrix cracking, fibre-matrix debonding, fibre fracture and delamination. This paper presents an acoustic emission technique for the failure characterisation of top-hat stiffener specimens using conventional piezoelectric acoustic emission sensors and modern fibre Bragg Gratings. Strain insensitive fibre Bragg Grating sensors are used in this experimental investigation to compare the performance of piezoelectric sensors for composite structures. This work has contributed to the development of an acoustic emission sensing system based on fibre Bragg Gratings. Main objectives of this work are to compare the sensing characteristics simultaneously in a composite structure with a surface-bonded acoustic emission–fibre Bragg Grating sensor and acoustic emission–piezoelectric sensor. The feasibility of the system is demonstrated in typical applications of in-situ structural health monitoring based on acoustic emission techniques.

Top hat stiffeners: A study on keel failures

The need to assess the structural capability of curved marine composite structures that support the keel of high performance yachts has increased due to the rise in the number of accidents and human casualties. The top hat stiffener is the basic structural member of the yacht to sustain the tensile and bending load of the keel. This paper characterises the mechanical strength of E-glass/vinylester composite top hat stiffeners subjected to transverse loading. The out-of-plane load transfer was studied and failure modes were identified for the three different composite layups which were tested to failure. Experimental results obtained from mechanical testing were compared with FEA predictions, based on the quadratic interlaminar stress criterion. Secondary reserve strength from the unidirectional fibres was identified and the potential to use this secondary strength was further investigated.

Damage mechanics of top-hat stiffeners used in FRP ship construction

This paper is concerned with the assessment of damage tolerance of a top-hat stiffener to plate connection in FRP marine structures. The subject is addressed in two parallel schemes, using stress-based and fracture-dependent criteria. Numerical modelling is used to determine the internal load transfer characteristics and failure mechanisms in top-hat stiffeners under typical loadings seen in practice. The finite element models are benchmarked against published test result, which include phenomena such as delaminations. The models are then extended to include crack elements, which are employed to calculate the strain energy release rates in the form of G values. The result from this modelling is compared against typical experimentally derived data pertaining to G cr values for the materials in question. Finally, an attempt is made to compare the results of the studies using the two approaches and to judge the overlap.

5453226 Curing of compsite materials using extended heat transfer models

To accelerate research on out-of-plane joints in fibre-reinforced plastic (FRP) structures, the work that has been done in the past is reviewed in this paper. Areas covered include the connections between two orthogonal plate elements and top-hat stiffeners attached to plates. Both single skin and sandwich topologies are included. Specific attention is focused on design synthesis considerations, structural static response analysis, response under impulsive loading, and creep and fatigue characteristics. The final section outlines key areas for future research to improve our understanding of the mechanical behaviour of out-of-plane joints.

Examination of key aspects defining the performance characteristics of out-of-plane joints in FRP marine structures

To accelerate research on out-of-plane joints in fibre-reinforced plastic (FRP) structures, the work that has been done in the past is reviewed in this paper. Areas covered include the connections between two orthogonal plate elements and top-hat stiffeners attached to plates. Both single skin and sandwich topologies are included. Specific attention is focused on design synthesis considerations, structural static response analysis, response under impulsive loading, and creep and fatigue characteristics. The final section outlines key areas for future research to improve our understanding of the mechanical behaviour of out-of-plane joints.

An investigation into the performance characteristics of top-hat stiffener to shell plating joints

This paper is concerned with the design of top-hat stiffener to shell plating joints such as those typically found in fibre-reinforced plastic (FRP) ships and boats. The background to the problem is presented, as characterised by a review of past work and current design/production procedures. From these considerations influential geometric variables are defined, and a ‘new’ design of stiffener joint is developed. Finite element modelling is used to compare the behaviour of this joint with typical current practice. Load transfer paths and failure mechanisms are determined. Finally the results of a study are presented in which the key design parameters are systematically varied to highlight their influence.

Design of joints and attachments in FRP ships' structures

This paper examines the problems of forming efficient joints between the major structural components of fibre-reinforced plastic (FRP) ships and boats. Joint types addressed by the paper are concerned primarily with stiffened single-skin construction ships, with regard to 1. (i) tee-joints between panels, for example between watertight bulkheads and shell plating; 2. (ii) attachment of top-hat stiffeners to plating. The development of numerical models to asses failure modes is discussed and the results of finite element analysis and validation testing of a range of alternative configurations is described. Optimised joint designs capable of achieving higher strength at reduced weight are presented. These new concepts would also facilitate the use of pre-formed stiffeners and mechanised production equipment for rapid rates of joint formation and therefore they offer the possibility of significantly reduced labour hours and ship build cost.

A simplified method for ultimate load prediction of all-steel sandwich panels

This paper is concerned with the ultimate load prediction of a sandwich panel manufactured from a steel plate assembly. The panel essentially consists of two facing plates, spot welded onto a corrugated steel core. All the plates of a panel have identical thickness. Three groups of panels with varying plate thicknesses were examined. The typical panel considered in this study had plan dimensions of 2·1 m × 1·0 m with a core consisting of top-hat stiffeners of depth 60 mm, placed side by side. The panel was simply supported across its y-direction boundaries and subjected to uniform lateral loading over its entire surface. The plastic hinge theory is used to evaluate the ultimate collapse load of the panel. In this respect, two approaches are compared: one based on fullsection properties and the other based on the effective width concept. It is shown that the effective section gives more reliable collapse-load predictions than the full section. Theoretical collapse-load predictions agree very well with the experimental collapse load for a series of 13 panels. To accentuate the salient features of the technique, a numerical example is presented.

Computational procedures for a matrix analysis of the stability and vibration of thin flat-walled structures in compression

This paper is concerned with computational problems arising in the application of a previously published matrix analysis of the stability and vibration of structures consisting entirely of a series of thin flat rectangular plates connected together along longitudinal edges. The theory requires that the conditions at the ends of the structure permit a mode which varies sinusoidally in the longitudinal direction, and it is then possible to consider each individual flat plate as one element having four degrees of freedom at each of its two longitudinal edges. The corresponding stiffness matrices are essentially exact within the spirit of thin plate theory. The main computational problem is that of testing whether or not, for a specified wavelength of buckle, any chosen value of the compressive stress is less than or greater than the lowest buckling stress of the structure. This problem is discussed in detail, and a systematic test procedure is derived. Examples of panels with integral unflanged stiffeners, bonded Z-section stiffeners, bonded top-hat stiffeners and corrugated-core sandwich panels are discussed.