Adriaan Beukers

Analysis and evaluation of bondline thickness effects on failure load in adhesively bonded structures

It is well known that adhesive joints have their optimum strength for thin bondline thicknesses (0.1-0.5 mm). The most common analytical methods used for adhesive joint analysis show an improved strength with increasing bondline thickness. This erroneous trend in prediction is investigated in this article. It is found that the through-the-thickness stress distribution in the adhesive is the main cause for the errors. The stresses, both peel and shear, at the interface between the adhesive and the adherend are found to increase, after an initial decrease in the low bondline thickness range, with increasing bondline thickness while the average stresses decrease. This trend explains the trends found in experiments. Further, as experimental results have shown, a theoretical optimum bondline thickness is found.

Implementation of bending-torsion coupling in the design of a wind-turbine rotor-blade

An investigation is performed on the implementation of bending-torsion coupling of a composite wind turbine rotor blade to provide passive pitch-control. Limited passive torsion deformation is realised with a structural coupling between flapwise bending and elastic twist of a constant speed rotor-blade. The blade and skin laminate configuration are analysed with a FEM program, in which a complete blade with spar webs is modelled. This conventional blade configuration has some disadvantages. Therefore alternative design concepts are reviewed, where the coupling plies are restricted to a load-bearing spar, while a softer skin provides for the aerodynamic shape. From additional analysis, it is found that, while for the two alternative design concepts the stress concentrations at the leading edge joint are bypassed, the bending-torsion coupling response is lower. An experiment was performed to validate the calculation methods. The experimental results show good correlation with theoretical predictions. It is recommended to investigate further the fatigue life properties of a glass/carbon hybrid FRP with off-axis fibre orientations.

Composite adhesive joints under cyclic loading

Abstract With the increasing use of adhesive bonding in structural joints in many applications, the interest in the behaviour of adhesive joints under cyclic loading has increased as well. Much work has already been performed on the analysis of adhesively bonded joints under static loading. Also the damage growth and failure mechanisms of adhesive joints under cyclic loading has been investigated, to a less extent for joints in composite structures. A major difficulty is the large amount of parameters that can be of influence on crack initiation and growth in adhesive joints. This makes it hard to characterise the debond behaviour of a joint. This article is intended to give an overview of studies performed on adhesive composite joints under cyclic loading on and to serve as a starting point for designers who need information on experimental and analytical methods of composite adhesive joints.

Experimental verification of a stress singularity model to predict the effect of bondline thickness on joint strength

A stress singularity model is used to predict joint failures in single-lap joints with varying bondline thickness. The theoretical results are compared with experimental results for verification purposes. Essentially the experimental analysis is split into two parts. The first part determines the Youngs modulus and Poissons ratio of the adhesive selected for this verification study and the second part measures the failure loads versus bondline thickness of the single-lap joint specimens. Based on the experimental data on the adhesive properties, predictions are made of the joint strength (assuming failure is in the adhesive) for varying bondline thickness. It appears that the relation between the joint strength and bondline thickness can be described with a two-parameter Weibull function. The critical stress intensity factor, or bondline toughness, is derived from the test data using an approximation formula for the change in external loading with bondline thickness. The resulting critical stress intensity factor combined with the calculated stress intensity factors gives a good prediction of the joint strength over a practical range of bondline thickness.

A stress singularity approach to failure initiation in a bonded joint with varying bondline thickness

The stress singularity at the theoretical point of maximum stress in an uncracked single lap joint is analysed by a finite element method. By treating the interface corner of a bonded joint (between adherend and adhesive) as a perfectly bonded wedge and using a fracture mechanics method, considerable advantages over other continuum mechanics approaches for investigating the bondline thickness effect on joint strength are shown. This study has essentially two aims: (i) determination of the strength of the singularity by finite element analysis and comparison with the analytical prediction of Bogy for varying bondline thickness; and (ii) determination of stress intensity factors for varying bondline thickness. Good agreement is shown between the numerically-calculated strength of the singularity with the analytical value obtained from Bogy. The calculated stress intensity, after an initial decrease in the low bondline thickness range, is found to increase with increasing bondline thickness. This agrees well with the trends predicted by experiments.

Sustainable Vacuum-Infused Thermoplastic Composites for MW-Size Wind Turbine Blades—Preliminary Design and Manufacturing Issues

This paper addresses the feasibility of using innovative vacuum infused anionic polyamide-6 (PA-6) thermoplastic composites for MW-size wind turbine blades structures. To compare the performance of this fully recyclable material against commonly used less sustainable thermoset blade materials in a baseline structural MW-size blade configuration (box-spar/skins), four different blade composite material options were investigated: Glass/epoxy, carbon/epoxy, glass/PA-6, and carbon/PA-6. Blade characteristics such as weight, costs, and natural frequencies were compared for rotor blades ranging between 32.5 and 75 m in length, designed according to both stress and tip deflection criteria. Results showed that the PA-6 blades have similar weights and natural frequencies when compared to their epoxy counterpart. For glass fiber blades, a 10% reduction in material cost can be expected when using PA-6 rather than epoxy while carbon fiber blades costs were found to be similar. Considering manufacturing, processing temperatures of PA-6 are significantly higher than for epoxy systems; however, the associated cost increase is expected to be compensated for by a reduction in infusion and curing time.

Temperature Effect on Reinforced Thermoplastic Composite Properties for Primary Aircraft Structure Applications

[Abstract] Reinforced thermoplastic composites used for primary aircraft structures are subjected to thermal effects throughout theirs lives. That is why the understanding of the temperature impact on the mechanical properties of carbon fibre reinforced plastics is very important for the choice of the appropriate plastic for aircraft design. In this study we are mainly concerned with the behaviour of different carbon weave reinforced thermoplastic composites (PPS and PEEK) to find their temperature window without loss of properties by using mechanical tests. Microscopy analysis is also performed to see the impact of the temperature on the internal structure of the composite.

Determination of Cure Dependent Properties for Curing Simulation of Thick-Walled Composites

[Abstract] Because of higher specific strength and stiffness, low weight, and good resistance to corrosion, polymer based composite materials replace more often conventional metals in high performance aerospace, maritime, and automotive structures as well as in consumer goods. Although these structures are manufactured according to high-quality standards, these are often thin-walled. On the cont rary, thick-walled composite structures are hardly used in these industries to replace meta llic equivalents. One of the reasons is that thick-walled fiber reinforced structures (i.e., str uctures with a thickness larger than 10 mm) induce larger problems on the fabrication process c ompared to thin-walled fiber reinforced structures. One of the main problems to overcome is the heat generation during the exothermic reaction, which causes not only internal stress build-up but also other structural defects. Not only this problem but many others emphasize the necessity of improving the understanding of different phenomena occurring in the manufacturing process of thickwalled structures. Since the research field is quit e extensive, the focus will be on the thermochemical behavior of thermosetting resins. In this paper a generally applicable thermochemical model is set up and cure dependent material properties for a thermosetting resin called RTM 6 are determined in order to demonstrate the applicability of the thermochemical model to simulate the curing process and t o study the thermo-chemical behavior of RTM 6. In two numerical examples, the influence of the specific heat capacity on the reaction temperature and degree of conversion and the maximum achievable thickness when constrained by the degradation temperature will be discussed.

Sound Transmission Loss Prediction of the Composite Fuselage with Different Methods

Increase of sound transmission loss(TL) of the fuselage is vital to build a comfortable cabin environment. In this paper, to find a convenient and accurate means for predicting the fuselage TL, the fuselage is modeled as a composite cylinder, and its TL is predicted with the analytical, the statistic energy analysis (SEA) and the hybrid FE&SEA method. The TL results predicted by the three methods are compared to each other and they show good agreement, but in terms of model building the SEA method is the most convenient one. Therefore, the parameters including the layup, the materials, the geometry, and the structure type are studied with the SEA method. It is observed that asymmetric laminates provide better sound insulation in general. It is further found that glass fiber laminates result in the best sound insulation as compared with graphite and aramid fiber laminates. In addition, the cylinder length has little influence on the sound insulation, while an increase of the radius considerably reduces the TL at low frequencies. Finally, by a comparison among an unstiffened laminate, a sandwich panel and a stiffened panel, the sandwich panel presents the largest TL at high frequencies and the stiffened panel demonstrates the poorest sound insulation at all frequencies.

Analysis of Conformable Pressure Vessels: Introducing the Multi-Bubble

This paper outlines the structural analysis of an articulated pressurizable structure termed the multibubble. The multibubble is a structurally efficient pressure vessel that pressurizes a volume with substantial spatial freedom. Applications are found in pressure cabins for blended-wing–body aircraft and conformable pressure vessels for liquid gases (e.g., propane) or cryogenic applications. Themultibubble in this paper can be configured in an openor closed-cell configuration and consists of cylindrical and spherical membrane elements equipped with walls and/or reinforcements at the intersections in order to ensure structural integrity. The multibubble allows pressurization loads to be carried through in-plane stresses. To solve for loads and forces in the multibubble, it is shown that the solution simply depends on pressure and geometric variables.