Bio-inspired hierarchical design of composite T-joints with improved structural properties

The biological principle of hierarchical (multi-scale level) design was used at the structural and laminate levels to design a novel carbon/epoxy T-joint with improved structural properties for potential use in light-weight aircraft structures. The bio-inspired structural modification mimics tree branch–trunk joints by embedding the stiffener flange into skin plies. This design concept results in increased fracture toughness due to crack branching and deflection. Simultaneously, bio-inspired ply angle optimisation was used to mimic the tailored arrangement of cellulose micro-fibrils observed in the wood cells contained within tree branch joints. The optimisation procedure minimises the interlaminar stress concentration in the T-joint radius bend and increases strength while maintaining similar global laminate stiffness properties. The hierarchical joint resulted in a significantly improved tensile strength compared to a conventionally designed T-joint. The new design additionally exhibited higher absorbed strain energy to failure load for bending and tension loading. Additionally, the hierarchical T-joint had a significantly reduced critical joint cross-sectional area (weight) due to the embedded design.     

Effects of mixed-mode ratio and step-shaped micro pattern surface on crack-propagation resistance of carbon-fiber-reinforced plastic/adhesive interface

This study investigated the effects of the mixed-mode ratio of applied loads (G_II/G) and aspect ratio A of step-shaped micro patterns on the crack-propagation resistance of a carbon-fiber-reinforced plastic (CFRP)/adhesive interface fabricated by in-mold surface modification. Experiments showed that the fracture behaviors change and that the apparent mixed mode fracture toughness G_C increases with G_II/G and A. We used the Benzeggagh–Kenane (B–K) failure criterion for the mixed-mode fracture toughness considering the transition of the failure mode of the step-shaped micro patterns. The B–K criterion agreed well with the improvement of G_C due to an increase in G_II/G for various fixed values of A. We clarified the relationship between the aspect ratio A and the parameter η, which is required to describe the B–K criterion, and therefore, η can be estimated from A. Consequently, it was verified that G_C of the CFRP/adhesive interface with step-shaped micro patterns can be predicted for arbitrary G_II/G and A values by substituting the η–A relationship in the B–K criterion.     

Failure analysis of adhesively bonded composite joints under transverse impact and different temperatures

In this study, load-carrying capacity of a single-lap joint bonded by an adhesive was found by experimental method. Glass fiber–epoxy composites were used as adherends. They were manufactured by using vacuum assisted resin infusion method (VARIM). Loctite 9466 A&B2 was used as an adhesive material. In this experimental study, two different surfaces (rough and smooth surface), four different temperatures (−20, 23, 50, and 80 °C) and four different impact energies (5, 10, 15, and 20 J) were considered. The result shows that when the temperature increases or decreases, adhesive joint losses its adhesion and rough surfaces provide high strength according to smooth surfaces. Load-carrying capacity decrease at 5, 10 and 15 J; however, it increases for 20 J.     

Determination of cohesive laws of composite bonded joints under mode II loading

In this work, mode II cohesive laws of carbon–epoxy composite bonded joints were obtained using the direct method applied to the end notched flexure (ENF) test. The direct method is based on the differentiation of the relation between the evolution of the fracture energy (J_II) and the crack tip opening displacement in mode II (CTOD_II) during the test. A data reduction scheme based on equivalent crack length concept was used to obtain the evolution of the fracture energy during the test. The method allows overcoming problems related to identification of crack tip in mode II tests and the presence of a non-negligible fracture process zone (FPZ), which both difficult the right estimate of J_II. The digital image correlation technique (DIC) was used to monitor the CTOD_II, which was synchronized with the load–displacement data. A trapezoidal cohesive law was fitted to the experimental one in order to perform numerical simulations using finite element analysis. The main goal was to validate all the procedure used to get the cohesive laws. The good agreement obtained between the numerical and experimental load-CTOD_II curves and between the cohesive laws demonstrates the adequacy of the proposed procedure concerning the evaluation of the composite bonded joints cohesive laws under mode II loading.     

End geometry and pin-hole effects on axially loaded adhesively bonded composite joints

An experimental investigation was performed to analyze the potential impacts of varying joint region geometries and adhesive filled pin holes on adhesively bonded composite structures. Tapers, especially half-length ones are observed to provide an anticipated progress in single lap joints. Besides, scarf joints with aligned adherends in the same plane exhibited enhanced stiffness and strength in consideration of single lap joints. In terms of the stiffness and strength, thickening of adherends was also found to be impressively efficient on composite single lap joints as well as scarf joints. Contrary to the expectation of that the hardened adhesive previously filled into the holes during adhesion would create a pin effect in load bearing, holey specimens exhibited poor performance and induced degradation in joint quality.     

Modification of surface functionality of multi-walled carbon nanotubes on fracture toughness of basalt fiber-reinforced composites

In this work, we studied the influence of surface functionality of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of basalt fiber-reinforced composites. Acid and base values of the MWCNTs were determined by Boehm's titration technique. The surface properties of the MWCNTs were determined FT-IR, and XPS. The mechanical properties of the composites were assessed by measuring the interlaminar shear stress, fracture toughness, fracture energy, and impact strength. The chemical treatments led to a change of the surface characteristics of the MWCNTs and of the mechanical interfacial properties of MWCNTs/basalt fibers/epoxy composites. Especially the acid-treated MWCNTs/basalt fibers/epoxy composites had improved mechanical properties compared to the base-treated and non-treated MWCNTs/basalt fibers/epoxy composites. These results can probably be attributed to the improved interfacial bonding strength resulting from the improved dispersion and interfacial adhesion between the epoxy resin and the MWCNTs.     

Strain energy release rate in shaft-loaded blister tests for composite repairs on steel

The design of composite repairs of corroded oil and gas pipelines must take into account the strength of the interface adhesion between composite and metal. A shaft-loaded blister test is a common method to measure interface fracture toughness and energy release rate. The study aimed on evaluating shaft-loaded blister tests as replacements for more complex pressure blister tests. Specimens investigated were thick fibre-reinforced plates bonded on metal disks as substrates containing a circular through-hole defect. This paper presents the influence of different punch head geometries on the resulting energy release rates and compares the results with blister tests using fluid pressure. Test and simulation results are presented and analytical solutions were derived and evaluated to establish best fitting formulations. It was shown, that significant variations between the different means of loading exist.     

Plasma electrolytic oxidation of aluminium networks to form a metal-cored ceramic composite hybrid material

A commercially-available low density aluminium network material (Duocel™) has been processed by plasma electrolytic oxidation to produce a ceramic hybrid material comprising an assembly of ceramic struts with metallic cores. The architecture and microstructure of this material were studied using X-ray tomography, scanning electron microscopy and densitometry. Conversion fractions were determined from mass gains and by image analysis of cross-sections, and the ceramic density was evaluated by hydrostatic weighing. Tensile and compressive testing of the hybrid material was used to study the toughness, as a function of the conversion fraction. Such material retains some of the beneficial mechanical properties of a metal (ductility and toughness), while also exhibiting a low overall density and a high specific surface area of ceramic. It can thus be considered as a highly permeable ceramic scaffold, with a relatively high toughness.     

N2 plasma-assisted grafting of fluorinated chains onto partially cured epoxy resins

Two routes for the grafting of fluorinated molecules to an epoxy resin were studied. The first one deals with the grafting of the liquid-state resin whereas the second one is focused on the grafting onto the solid-state resin. These grafting reactions were shown to be similar as studied through FTIR and XPS spectroscopies. However, it appears that the grafting onto the solid-state resin is limited by the curing advancement. N₂ plasma-activation was used to solve this drawback and enhanced the grafting yield. This grafting improvement was mainly explained in terms of the surface wetting improvement and the attachment of nitrogen containing groups at the surface of the treated resin.     

Comparison between rectangular and trapezoidal bonded composite repairs in aircraft structures: A numerical analysis

The optimization of the patch shape of bonded composite repair in aircraft structures is a good way to improve the repair performance. In this study, the three-dimensional finite element method is used to compare the repair performance of patches with rectangular and trapezoidal shapes in aircraft structures. The comparison is done by analysing the stress intensity factor (SIF) at the tip of repaired crack and the distribution of the adhesive stresses for the two patch shapes. The obtained results show that, when the crack length is ranged from 5 to 20 mm, the trapezoidal shape presents lower stress intensity factor at the crack tip, which is beneficial for the fatigue life and lower adhesives stresses, which is beneficial for the repair durability. These advantages disappear when the crack length reaches the value of 40 mm. It is also shown that the use of the trapezoidal shape reduce the mass of the patch, which can reduce the repair cost.     

A numerical methodology for optimizing the geometry of composite structural parts with regard to strength

The increased use of composite materials in lightweight structures has generated the need for optimizing the geometry of composite structural parts with regard to strength, weight and cost. Most existing optimization methodologies focus on weight and cost mainly due to the difficulties in predicting strength of composite materials. In this paper, a numerical methodology for optimizing the geometry of composite structural parts with regard to strength by maintaining the initial weight is proposed. The methodology is a combination of the optimization module of the ANSYS FE code and a progressive damage modeling module. Both modules and the interface between them were programmed using the ANSYS programming language, thus enabling the implementation of the methodology in a single step. The parametric design language involves two verifications tests: one of the progressive damage model against experiments and one of the global optimization methodology performed by comparing the strength of the initial and the optimum geometry. There were made two applications of the numerical optimization methodology, both on H-shaped adhesively bonded joints subjected to quasi-static load. In the first application, the H-shaped joining profile was made from non-crimp fabric composite material while in the second from a novel fully interlaced 3D woven composite material. In the optimization of the joint’s geometry, failure in the composite material as well as debonding between the assembled parts was considered. For both cases, the optimization led to a considerable increase in joint’s strength.     

Optimisation of steel–composite connections for structural marine applications

Parametric variation and optimisation using genetic algorithms employing single and multi-objective functions are proposed for the optimisation of a structural steel/composite connection. The joint in marine applications is the connection between the steel hull and the composite superstructure of a naval vessel. A baseline joint is defined and all parametric variations and optimised joints are compared to this. The parametric results provided design curves of the joint performance determined from the weight, Von Mises stress in the adhesive and the global stiffness indicating performance sensitivity to specific changes in the joint geometry.

The results indicated that the parametric variations can lead to an improvement in the performance but high levels of human interaction are required to make a combined improvement to the performance. The use of genetic algorithms provided an efficient method of searching the design space for an optimal joint. The single objective function provides an excellent reduction in the weight and maintaining or improving the performance of the joint to in-plane compressive loading. The use of the multi-objective function whereby a weighting was applied to the weight, stress and stiffness performance criteria proved extremely successful in further optimising the joint. The use of genetic algorithms has been demonstrated to efficiently search the complex design space of a structural connection and the use of multi-objective functions as the most effective selection method.     

Analysis of adhesively bonded CFRE composite scarf joints modified with MWCNTs

The main objective of the present work is to improve the performance of bonded joints in carbon fiber composite structures through introducing Multi-Walled Carbon Nanotubes (MWCNTs) into Epocast 50-A1/946 epoxy, which was primarily developed for joining and repairing of composite aircraft structures. Results from tension characterizations of structural adhesive joints (SAJs) with different scarf angles (5–45°) showed improvement up to 40% compared to neat epoxy (NE)–SAJs. Special attention was considered to investigate the performance of SAJs with 5° scarf angle under different environments. The tensile strength and stiffness of both NE-SAJs and MWCNT/E-SAJs were dramatically decreased at elevated temperature. Water absorption showed a marginal drop of about 2.0% in the tensile strength of the moist SAJs compared to the dry one. Cracks initiation and propagation were detected effectively using instrumented-SAJs with eight strain gauges. The experimental results agree well with the predicted using three-dimensional finite element analysis model.     

Influence of electroless nickel-plating on fracture toughness of pitch-based carbon fibre reinforced composites

Nickel-Pitch-based carbon fibres (Ni-PFs) were prepared by electroless nickel-plating to enhance fracture toughness of Ni-PFs reinforced epoxy matrix composites (Ni-PFs/epoxy). The surface properties of Ni-PFs were determined by scanning electron microscopy (SEM), X-ray photoelectron spectrometry (XPS), and X-ray diffraction (XRD). The fracture toughness of the Ni-PFs/epoxy was assessed by critical stress intensity factor (K_IC) and critical strain energy release rate (G_IC). The fracture toughness of Ni-PFs/epoxy was enhanced compared to those of PFs/epoxy. These results were attributed to the increase of the degree of adhesion at interfaces between Ni-PFs and matrix resins in the composites.     

Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: State of the art

Over the past three decades, the strengthening and repair of existing civil engineering structures using FRP laminates has attracted a great deal of attention. With the advances in polymer science, adhesive bonding has become a common joining technology in these applications. Despite numerous studies that address the short-term behaviour of adhesively bonded FRP/steel joints, uncertainty with respect to long-term performance still remains. This knowledge gap is regarded as a critical barrier, hindering the widespread application of FRPs to strengthen and retrofit steel structures. This paper presents the state of the art in terms of the durability of FRP/steel joints used in civil engineering applications. Important influential factors relating to the durability of adhesively bonded joints are reviewed and different damage mechanisms are discussed. Moreover, related investigations of the combined environmental durability of these joints are critically reviewed and the findings are presented. The paper concludes with a discussion to motivate future research topics, while it is emphasised that the generalisation of the available results is questionable.     

Damage mode characterization of mechanically fastened composite joints using carbon nanotube networks

The practice of upgrading metal parts with composites in large structures has led to an increased use of composite joints, particularly mechanical fastenings, due to ease of assembly and replacement. A drawback of mechanical joints is that damage is difficult to detect visually. In this research, an embedded carbon nanotube network has been used to modify the conductivity of bolted composite joints. In situ electrical resistance measurements in conductive composites have potential to provide quantitative evidence of damage as well as insight into the type of damage which occurs during tensile loading. We demonstrate that the electrical resistance of a bolted composite joint is more sensitive to certain modes of damage (e.g., matrix cracking and delamination) than others (bearing and shear-out), due mostly to the varying amount of void space created, thus proving the potential of an embedded carbon nanotube network in the health monitoring of mechanically fastened cross-ply composites.     

Experimental and numerical study on the effect of humidity-temperature cycling on structural multi-material adhesive joints

The following paper describes investigations on the impact of harsh environment on shear and tensile strength of multi-material adhesive joints. The samples were made from carbon fiber – epoxy composites, aluminum and two types of advanced steels: abrasion resistant and high-strength. In order to assess the suitability of structural bonding for this sort of applications, it was decided to test two different epoxy-based adhesives, designed for moderate and elevated operating temperatures. The harmful conditions were simulated by means of humidity-temperature cycling tests, according to the SAE standard. The obtained results revealed that even moderately harsh humidity-temperature loads can cause debonding of the joints, even if no external forces are applied. In order to gain insight into this phenomenon, a series of finite element analyses was performed, simulating the exposure of the samples to the chosen environmental conditions. Based on these studies, the temperature expansion coefficient was identified as the crucial factor for the performance of the joints made from dissimilar materials. The results of the described experiments, confirmed by numerical calculations, constitute a guideline for multi-material structural design, supporting this constantly growing branch of modern engineering with a relevant input.     

An experimental analysis of the fracture behavior of composite bonded joints in terms of cohesive laws

Modeling adhesive joints by means of cohesive models relies on the definition of cohesive laws. Although cohesive laws are known to be dependent on the loading mode, there is a lack of experimental evidences to describe this dependence. At the same time, the adherend and adhesive thicknesses are known to affect the fracture toughness of the bond, but their effect on the cohesive law has not been clarified. In this work, an experimental characterization of an epoxy adhesive is presented. The effect that the mode mixity has on the bond toughness and its cohesive law is compared against the effect of the adhesive and adherend thicknesses. The impact of these two latest parameters is shown to be minor if compared to the influence of the mode mixity, which mainly defines the cohesive law shape. Finally, the implications of these experimental findings on the numerical simulation of adhesive joints are discussed.     

Fabrication and characterization of poly-d-l-lactide/nano-hydroxyapatite composite scaffolds with poly (ethylene glycol) coating and dexamethasone releasing

The control of pore size and structure, drug release capacity, and biodegradation of scaffolds is of importance for bone tissue engineering. In this study, a technique combining polymer coagulation, cold compression molding, salt particulate leaching and drug coating method was developed to fabricate poly (ethylene glycol)/dexamethasone coated porous poly-d-l-lactide/nano-hydroxyapatite (PDLLA/nano-HAp) scaffolds. These scaffolds possess homogenous pore networks with high porosity (66-82%) and controllable pore size (200-300 μm). The compressive moduli and strength of the scaffolds after incorporation of nano-HAp were improved by 50% and 20%, respectively. The surface hydrophilicity of the scaffold was significantly improved by poly (ethylene glycol)/dexamethasone coating and nano-HAp addition, leading to a higher initial drug loading amount. The results showed that the drug release behavior of the scaffolds after 35-day immersion in water could be adjusted by varying the porosity level and by incorporation of 20 wt.% of nano-HAp.     

Micrographic studies on adhesively bonded scarf repairs to thick composite aircraft structure

The hard-patch approach to scarf repairs involves adhesively bonding a pre-formed patch into the scarf cavity. This approach has several potential advantages compared with the conventional soft-patch approach, which involves forming the patch from pre-preg and co-bonding it with the adhesive during cure of the patch directly in the repair cavity.Two methods for producing the hard-patch were investigated. The first was the moulded approach where the patch was laid up in a mould and cured prior to bonding in the repair cavity. The development and implementation of the moulded hard-patch repair technique on an F/A-18 horizontal stabiliser is described. The second approach involves machining the patch from a composite panel using digitised data obtained from the use of surface profiling equipment to capture the scarf cavity surface. Micrographic techniques were used to assess critical features of the bond-line produced from the different techniques. The results are compared with microscopic studies from a second F/A-18 horizontal stabiliser that was repaired much earlier using the soft-patch approach. Each repair is assessed in terms of the consolidation of plies along the bond-line and the conformity of the patch to the repair cavity as well as adhesive uniformity and porosity.