Scaling of fracture response in Over-height Compact Tension tests

An experimental investigation into in-plane scaled Over-height Compact Tension (OCT) [45/90/−45/0]_4s carbon/epoxy laminates was carried out to study the scaling of fracture response. The dimensions of the baseline specimens were scaled up and down by a factor of 2. Interrupted tests were carried out for specimens of each size in which the tests were stopped after certain load drops in order to study the failure mechanisms. X-ray Computed Tomography (CT) scanning was applied after the interrupted tests to examine the damage development and its effect on the fracture response. The test results showed that the scaling of the initial propagation of fracture follows Linear Elastic Fracture Mechanics (LEFM), but the development of the damage process zone differs with specimen sizes. The OCT specimens were found to be not large enough to generate a self-similar damage zone during propagation, and so no conclusions could be drawn regarding the R-curve effect.     

Sea-water effects on foam-cored composite sandwich lay-ups

This article concerns the effects of sea-water on foam cored composite sandwich structures under long-term exposure. Special attention is focused on sea-water induced damage in foam materials, weight gains and expansional strains, as well as on possible degradation in the properties of foam materials due to such extended exposure. In addition, sea-water effects on the fracture behavior of foam materials and on face/core interfacial debonding fracture are investigated experimentally and interpreted by means of computational fracture mechanics.     

The effect of PVDF nanofibers on mode-I fracture toughness of composite materials

In this study, the fracture behavior of carbon/epoxy laminates interleaved by polyvinylidene fluoride (PVDF) nanofibers is investigated. For this aim, a mode-I fracture test is conducted on virgin and modified laminates. Unlike the results of other studies, it is shown that PVDF nanofibers can increase mode-I fracture toughness (G_I) noticeably in a specific situation. The results show that G_I is enhanced about 43% and 36% in initiation and propagation stages of the fracture, respectively, using PVDF nanofibers. The morphology of the fractured surface is also presented for investigating the mechanism of toughening.     

Fiber–matrix adhesion from the single-fiber composite test: nucleation of interfacial debonding

The level of fiber–matrix interfacial adhesion in composites is traditionally evaluated by means of a stress-based parameter. Recently, it was suggested that an interfacial energy parameter might constitute a valid alternative. From an overview of the literature regarding the single-fiber composite fragmentation test, it appears that energy-based approaches have already been proposed in the past, but were either not successful, or not fully developed. Our recent energy balance scheme, proposed for the analysis of the initial interface debonding which occurs at fiber breaks during a fragmentation test, is presented and expanded here. The effects of thermal residual stress in the fiber, and of friction in the debonded area, are now incorporated in the energy balance model. We use a different shear-lag parameter proposed by Nayfeh, rather than the commonly used Cox parameter. New, extensive single-fiber fragmentation data regarding the interface crack initiation regime is presented, using sized and unsized E-glass fibers embedded in UV-curable or epoxy polymers. Some data for unsized carbon in epoxy is also presented. Fiber fragmentation is forced to take place entirely in the linear elastic region of the stress–strain curve, by means of pre-stressed single fibers. The importance of this procedure is discussed. Future work will focus on the interface crack propagation regime.     

Experimental bond tests on masonry panels strengthened by FRP

The results of an experimental campaign on bond between Glass Fiber Reinforced Polymer (GFRP) sheets and single clay brick or masonry panel is presented. Four different types of clay bricks (new and ancient) are considered, where the difference between bricks is not only due to their mechanical properties but also to their surface texture. Another focus point of the experimental campaign is the effect of mortar joints on the GFRP-masonry panel bond. Moreover, the effects of different surface preparations on the debonding load were investigated, concerning both bricks and masonry panels. A total number of 38 specimens was tested and results in terms of debonding force, strain along the GFRP and failure modes are here reported. The experimental results were also compared to design formula proposed by the new version of Italian Guidelines. Furthermore, in order to numerically describe the bond behaviour of the specimens tested, non-linear interface laws were calibrated starting from the debonding load and the measured strains along the GFRP for various loading levels.     

A data reduction method based on the J-integral to obtain the interlaminar fracture toughness in a mode II end-loaded split (ELS) test

Various difficulties arise in the data reduction of the end-loaded split (ELS) test. On one hand, a small Fracture Process Zone (FPZ) at the crack front is assumed in the existing mode II end-loaded split test methodologies based on Linear Elastic Fracture Mechanics (LEFM). However, mode II fracture has been reported to involve large FPZ and a fuzzy crack tip. Furthermore, the ELS test, is usually affected by geometrical non-linearities.This work proposes a closed-form solution based on the J-integral to determine the interlaminar fracture toughness in an ELS test. This solution avoids the need to measure the crack length, and is applicable when a large FPZ is present, as occurs in adhesive bonded joints between CFRP. In addition, because the ELS test involves large vertical deflections, a correction of the formulation for large displacements has been implemented.This new methodology has been compared to other methods available in the literature based on LEFM by means of an experimental campaign of delamination tests using unidirectional CFRP specimens in order to make a first validation of the method.     

Mode-I,mode-II and mixed-mode I+II fracture behavior of composite bonded joints: Experimental characterization and numerical simulation

The fracture behavior of composite bonded joints subjected to mode-I, mode-II and mixed-mode I + II loading conditions was characterized by mechanical testing and numerical simulation. The composite adherents were bonded using two different epoxy adhesives; namely, the EA 9695 film adhesive and the mixed EA 9395-EA 9396 paste adhesive. The fracture toughness of the joints was evaluated in terms of the critical energy release rate. Mode-I tests were conducted using the double-cantilever beam specimen, mode-II tests using the end-notch flexure specimen and mixed-mode tests (three mixity ratios) using a combination of the two aforementioned specimens. The fracture behavior of the bonded joints was also simulated using the cohesive zone modeling method aiming to evaluate the method and point out its strengths and weaknesses. The simulations were performed using the explicit FE code LS-DYNA. The experimental results show a considerable scatter which is common for fracture toughness tests. The joints attained with the film adhesive have much larger fracture toughness (by 30–60%) than the joints with the paste adhesive, which exhibited a rather brittle behavior. The simulation results revealed that the cohesive zone modeling method performs well for mode-I load-cases while for mode-II and mixed-mode load-cases, modifications of the input parameters and the traction-separation law are needed in order for the method to effectively simulate the fracture behavior of the joints.     

Embedded flaws for crack path control in composite laminates

An experimental investigation was conducted on using small flaws purposefully introduced into composite laminates to control growth of interlaminar cracks and through-thickness crack branching. Mode I crack growth specimens were used to study branching through 0°, 90° and 45° plies. The results showed that crack growth through 0° plies could be promoted by a ply gap, but this was not as controllable as combining a ply gap with a pre-crack to create a “crack branch flaw”. Crack branching through 45° plies could be controlled using crack branch flaws, and also promoted controllably using ply gaps. Crack branching through 90° plies was seen without any flaws, but was better controlled with embedded delaminations. Using these outcomes, crack branching through two quasi-isotropic laminates was demonstrated. The results have application to improved damage tolerance and fracture toughness, by taking advantage of high toughness crack growth mechanisms.     

Mode I and Mode II interlaminar fracture toughness of composite laminates interleaved with electrospun nanofibre veils

In this study, the effects of interleaved nanofibre veils on the Mode I and Mode II interlaminar fracture toughness (ILFT) of autoclave cured unidirectional carbon/epoxy composite laminates were investigated. Various electrospun nanofibre veils consisting of a range of different polymer types, fibre diameters and veil architectures were placed in the laminate mid-planes, which were subsequently subjected to double cantilever beam and end-notch flexure tests. It was found that the polymer type and veil areal weight were the most important factors contributing to laminate performance. A 4.5 g/m² PA66 veil provided the best all-round performance with fracture toughness improvements of 156% and 69% for Mode I and Mode II, respectively.     

Wide-shallow beams with and without steel fibres: A peculiar behaviour in shear and flexure

This paper reports an experimental campaign on Reinforced Concrete (RC) Wide-Shallow Beams (WSBs) with or without fibres, tested under shear and flexure. A wide-shallow beam is a rather frequent structure in residential buildings in Southern Europe (as in Italy and in Spain). In order to study both the shear and flexural behaviour of WSBs and evaluate the possibility of substituting the minimum conventional transverse reinforcement required by Eurocode 2 with steel fibres, full-scale beams have been tested. Specimens, all 250 mm deep, had two different widths, fibre contents and also, minimum amount of classical shear reinforcement. Results evidenced that a relatively low volume fraction of fibres can significantly increase shear bearing capacity and beam ductility. Moreover, WSBs did not show the typical brittle failure in shear, even without any shear reinforcement, as the effect of fibres was more prominent than in deep beams. Peculiarities of WSBs were evidenced in terms of enhancements both in shear and in flexure. Experimental results have been evaluated in terms of strength, ductility, post-cracking stiffness, shear and flexural cracking, collapse mechanism and fibre effect.     

Strain energy release rate determination of prescribed cracks in adhesively-bonded single-lap composite joints with thick bondlines

An analytical model for determining the strain energy release rate due to a prescribed crack in an adhesively-bonded, single-lap composite joint with thick bondlines and subjected to axial tension is presented. An existing analytical model for determining the adhesive stresses within the joint is used as the foundation for the strain energy release rate calculation. In the stress model, the governing equations of displacements within the adherends are formulated using the first-order laminated plate theory. In order to simulate the thick bondlines, the field equations of the adhesive are formulated using the linear elastic theory to allow non-uniform stress distributions through the thickness. Based on the adhesive stress distributions, the equivalent crack tip forces are obtained and the strain energy release rate due to the crack extension is determined by using the virtual crack closure technique (VCCT). The specimen geometry of ASTM D3165 standard test is followed in the derivation. The system of second-order differential equations is solved to provide the adherend and adhesive stresses using the symbolic computational tool, Maple 7. Finite element analyses using J-integral as well as VCCT are performed to verify the developed analytical model. Finite element analyses are conducted using the commercial finite element analysis software ABAQUS™. The strain energy release rates determined using the analytical method correlate well with the results from the finite element analyses. It can be seen that the same prescribed crack has a higher strain energy release rate for the joints with thicker bondlines. This explains the reason that joints with thick bondlines tend to have a lower load carrying capacity.     

Mechanical hinge system for delamination tests in beam-type composite specimens

During the experimental study of composite delaminations external loads are usually applied by means of steel or aluminium parts bonded to the surface of beam-type specimens. The bonded joints between the metallic parts and the composite specimen might fail, especially when the tests are carried out under extreme temperatures or fatigue conditions. In addition, the point of application of the external load does not coincide with the neutral axis of the specimen beam, inducing non-linear effects that can lead, for example, to incorrect estimations of fracture toughness. In this paper, the relative importance of the non-linear effects in delamination tests is evaluated and the corresponding correction factors discussed. Next, the design of an improved mechanical hinge that avoids non-linear effects, eliminates bonded joints and can be adapted to different specimen thicknesses is introduced.     

Chemical functionalization of composite surfaces for improved structural bonded repairs

In terms of lightweight design, aerodynamics and structural integrity, bonded repairs represent the preferred approach for repairing composite structures in aircraft applications. In this work the influence of crucial surface parameters including roughness, polarity and chemical composition on the performance of bonded repairs is studied. Besides mechanical and physical interactions, the study aims at the surface modification of carbon-fiber reinforced polymers (CFRP) to tailor chemical interactions with the adhesive. Reactive epoxy and mercapto derivatives are attached onto the CFRP surface by a 2-step functionalization route to ensure optimized adhesion and covalent bonding to epoxy-based adhesives. The performance of bonded coupon joints is determined by single lap shear tests (tensile-shear loading) and fracture mechanical tests (mode I loading). The results give evidence that chemical interactions play a key role in the quality of bonded repair systems. By controlling the chemical surface properties improved bond strength, homogenous crack growth and cohesive failure patterns are achieved.     

A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

The characterisation of mixed-mode fracture toughness and fatigue delamination growth in fibre-reinforced composites is crucial for assessing the integrity of structural elements in service. An asymmetric cut-ply coupon (ACP) loaded in four-point bending is here proposed to carry out the aforementioned characterisations. Analytical expressions of the energy release rate and mode-mixity for the ACP are derived and validated by means of finite element analysis. A fracture toughness and fatigue characterisation of the carbon/epoxy material IM7/8552 is carried out via ACP specimens. It is proved that the material data obtained from ACP specimens match those generated using ASTM standard mixed-mode bending (MMB) coupons. The main reason for the introduction of the ACP test resides in its applicability to characterisation scenarios where measuring the delamination length with optical means, as required for MMB coupons, is difficult. Such scenarios include the investigation of static and fatigue delamination growth at low and high temperatures, which requires the usage of environmental chambers. This poses significant constraints in terms of volume available for the test rigs, and, most importantly, limitations on visual access to observe delamination propagation. However, the manufacturing of ACP coupons is more complex than for MMB specimens and the testing requires several additional precautions that are here discussed in detail.     

Side Clamped Beam (SCB) hinge system for delamination tests in beam-type composite specimens

The experimental study of composite delaminations and adhesive joints is usually carried out using beam-type specimens with bonded metallic fixtures to transfer the applied load. However, bonded joints between the composite specimen and the metallic parts might fail, especially under extreme temperature or fatigue test conditions. Moreover, the quality and alignment of the bonding procedure highly depends on the operator skills and is time consuming. In this paper the design of an improved mechanical hinge is introduced. The new hinge eliminates bonded joints by mechanically clamping the sides of the specimen and avoids the misalignment effects between specimen and fixture. The new system has proven to be easy to use, fast and reliable.     

Influence of concrete matrix and type of fiber on the shear behavior of self-compacting fiber reinforced concrete beams

Steel fibers are known to improve shear behavior. The Design Codes (Eurocode 2 (EC2), Spanish EHE-08, Model Code 2010 and RILEM approach) have developed formulas to calculate the fiber contribution to shear, mainly focused on standard FRCs, i.e. medium strength concretes with a low content of normal strength steel fibers. However, in real applications other combinations are possible, such as high or medium strength concretes with high strength steel fibers of different lengths and geometry. An experimental program consisting of 12 self-compacting fiber reinforced concrete (SCFRC) I-type beams was carried out. All the beams had the same geometry and fiber content (50 kg/m³), and they were made with two different concrete compressive strength values and five different types of steel fibers and were tested for shear. The main conclusions reached were that the type of fiber substantially affects shear behavior, even when the Design Code formulas indicate similar contributions. The combination of high strength concrete matrixes with low strength fibers does not seem to be efficient. Also, the use of high residual flexural tensile strength values (e.g. f_R3 or f_R4) does not appear to be the most accurate reference value to calculate the beam shear strength in these cases. The present Design Codes consider standard FRCs, but their formulas should be revised for concretes with fibers of different strengths, slenderness and geometry, since these properties substantially affect shear behavior.     

Effect of mono and composite coating on dynamic fracture toughness of metals at different temperatures

It is well known that during the operating condition of any metallic structural system the dynamic crack growth speed is in the order of 1–2 km/s. Industrial finishes like coating which form the integral part of manufacturing is adopted to improve fracture toughness of metals. These coated samples coated with thin films are mechanically tested by Charpy V-notch impact tester for estimating dynamic fracture toughness. Coatings improve the wear and corrosion resistance of materials; they tend to reduce the strength of materials, because of the increased residual stresses due to the coating process. Defects cannot be precluded from these coated and treated components; strength of those components in the presence of these defects can be analyzed by fracture mechanics approach. An attempt has been made to analyze the effectiveness of coating methods like electroplating, PVD (Physical Vapour Deposition), coating thickness and the service temperature on the fracture behaviour of metals. Experiments have been carried out on EN8 steel and aluminium for different temperatures and the later samples were corroded for 2400 h and tested for corrosion resistance. The specimen preparation and experimentations were carried out according to the ASTM standard E-23. Finite element analysis was done by FRANC 2D (Fracture Analysis Code) for estimating the stress intensity factor at different crack lengths along with influence of temperature and corrosion. PVD coated samples of Al–N (aluminium nitride) and nano-crystalline layer of Ti–Al–N (titanium aluminium nitride) showed improved dynamic fracture toughness properties. The same set of samples showed decrease in stress intensity factors and excellent corrosion resistance compared to conventional Ni (nickel) and Cr (chromium) coated samples. Mechanical behaviour of selected metals under heat affected zone is of also discussed in this paper, the study aims at both coated and uncoated cases. Performances of metals in cryogenic condition are also paid attention in this paper.     

Quasi-static and high strain rate response of aluminum matrix syntactic foams under compression

Aluminum alloy matrix syntactic foams were produced by inert gas pressure infiltration. Four different alloys and ceramic hollow spheres were applied as matrix and filler material, respectively. The effects of the chemical composition of the matrix and the different heat-treatments are reported at different strain-rates and in compressive loadings. The higher strain rates were performed in a Split-Hopkinson pressure bar system. The results show that, the characteristic properties of the materials strongly depends on the chemical composition of the matrix and its heat-treatment condition. The compressive strength of the investigated foams showed a limited sensitivity to the strain rate, its effect was more pronounced in the case of the structural stiffness and fracture strain. The failure modes of the foams have explicit differences showing barreling and shearing in the case of quasi-static and high strain rate compression respectively.     

Damage sensing of adhesively-bonded hybrid composite/steel joints using carbon nanotubes

Carbon nanotube networks have been used previously for in situ sensing of matrix damage in fiber-reinforced composites. In this research, the ability of carbon nanotube networks to sense and distinguish different types of damage in adhesively-bonded hybrid composite-to-metal joints is evaluated. Toward this end, conductive networks of carbon nanotubes are introduced to the composite substrate as well as the epoxy adhesive. By altering the geometry and chemically treating the steel substrate surface, different failure mechanisms of the single-lap shear joints are achieved. It is demonstrated that these failure mechanisms each possess a distinct resistance response, therefore proving the ability to not only sense failure in situ, but also to distinguish the extent and nature of damage which occurs.     

Measuring the rate-dependent mode I fracture toughness of composites – A review

Composite materials are often subjected to mechanical impact causing delamination. For quasi-static loading, measuring the mode I fracture toughness has been standardized. However, for high-rate loading, additional challenges arise. Consequently, no standard test has yet been defined for measuring the mode I fracture toughness under high rates of loading. This article therefore reviews candidate tests for measuring the high-rate mode I fracture toughness. Strength and weaknesses of different specimen designs and test setups are shown. Different approaches to measuring crack growth and loads are presented. The different approaches are compared and recommendations are provided for measuring the mode I fracture toughness of composites under high rates of loading.