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Abstract Thin-ply composites represent a promising approach to further improve the performance of carbon fibre composite structures thanks to their ability to delay the onset of matrix cracking and delamination up to the point of fibre dominated failure. However, this increased strength comes with a more brittle failure response which raises concerns on damage tolerance. Thus a careful material optimization is needed to address this trade-off. In this work, eight different formulations of thin-ply composites ranging from low modulus to high modulus carbon fibres are evaluated to understand the effects of the fibre and matrix constituents on the onset of damage and strength in unnotched tensile (UNT) tests of quasi isotropic laminates for ply thicknesses between 300 and 30 microns. The obtained experimental data are combined in master curve diagrams for simplified material selection process. It is observed that certain thin-ply composites with a ply thickness t < 134 μm can reach UNT strength corresponding to or approaching the ultimate strain of the fibres as well as UNT stress at onset of damage as high as 92% of the latter. Based on this knowledge, a novel aerospace grade toughened thin-ply composite system is developed which can reach a quasi-isotropic UNT strength above 1 GPa (>95% of the fibre strain). The newly developed composite is further optimized to improve damage tolerance by toughening the resin and selected interfaces. The effect of those modifications on damage tolerance are evaluated through compression strength after impact (CAI) tests and open hole tensile tests (OHT). It is found that an optimized interlayer toughened thin-ply composite based on 68 microns plies of intermediate modulus fibre can reach both outstanding strength properties with comparable or better CAI and OHT strength compared to current aerospace grade composites. [ABSTRACT FROM AUTHOR]

Fiber bridging is one of the main toughening mechanisms in mode I interlaminar and intralaminar fracture of laminated composites. Intact fibers exert closing forces on both faces of the crack, restraining crack propagation. An effective identification procedure is required to characterize bridging tractions and to develop accurate prediction models. The method proposed in this work is based on the resistance (R)-curve and assumes fiber bridging tractions decreasing non-linearly with respect to the crack opening displacement, following a parametric function. The distribution parameters are identified by a fixed-point iterative procedure where the energy release rate computed in a numerical model is matched with the values obtained experimentally in two points of the R-curve. The method is applied and validated through three cases, from low bridging intensity in thin-ply delamination to high bridging intensity in intralaminar fracture. [ABSTRACT FROM AUTHOR]

Three-dimensional finite element simulations of nano-porous silver structures are performed to understand the correlation between the porous morphology and the mechanical behavior. The nanostructures have been obtained from ptychographic X-ray computed tomography. The simulations allow distinguishing between the interplay and role of the ligament size, the pore morphology and the porosity, and therefore provide a better comprehension of the experimental observations. We show that the proposed model has a predictive character for mechanical behavior of nano-porous silver. [ABSTRACT FROM AUTHOR]

Fatigue delamination growth in composites is accompanied by large scale bridging (LSB) that yields important toughening effects. However, the extent of this mechanism depends on the laminate geometry rendering its modeling a challenging task. This work presents a combined experimental/numerical study on characterization of specimen thickness dependence of LSB in fatigue delamination. Double cantilever beam specimens of different thicknesses ( h = 2, 4 and 8 mm), equipped with arrays of multiplexed fiber Bragg grating sensors, are subjected to mode I fatigue loads. Measured strain data with the sensors are employed to identify the bridging tractions and subsequently compute the energy release rate (ERR) due to the bridging as well as the ERR at the crack tip. The obtained results confirm that fatigue delamination growth strongly depends on the specimen geometry when LSB prevails. It is shown that both the extent of bridging and critical ERR at failure increase by increasing the specimen thickness while the maximum bridging traction at the crack tip is found independent of the specimen geometry. The identified traction-separation relations serve to establish a power correlation, between the crack growth rate and ERR at the crack tip which is independent of the specimen thickness. [ABSTRACT FROM AUTHOR]

Large scale bridging is an important toughening mechanism in composite laminates that depends on the specimen geometry as well as the constituent materials. In this work, an iterative approach is applied to identify the effect of specimen thickness on traction–separation behavior in the bridging zone. Double cantilever beam specimens (DCB) with embedded arrays of wavelength-multiplexed fiber Bragg grating (FBG) sensors are subjected to monotonic mode I fracture loading. As processed specimens with thicknesses h = 2, 4, 8 and 10 mm as well as specimens of h = 4 mm milled down from the 8 and 10 mm are tested. Non-homogeneous strain distributions in the vicinity of the interlaminar crack plane are locally monitored by means of embedded FBGs. The measured strain data are used to quantify the bridging tractions associated with each specimen thickness and consequently the energy release rate (ERR) due to the bridging. The results show that the bridging zone length and the ERR at the plateau level increase with specimen thickness while the results for all specimens with h = 4 are the same thus excluding any processing effects. Moreover, the maximum stress of traction-opening in the bridging zone and crack opening displacement at the end of the zone are independent of thickness. In contrast, the rate of tractions decay depends on the specimen thickness in that it decreases with thickness scaling. Thus the so-called bridging law is not a material parameter. The identified traction–separation relations are employed in a cohesive zone model to predict fracture of DCB specimens with different thicknesses. [ABSTRACT FROM AUTHOR]

Thin-ply composites are rapidly gaining interest in the composite industry, not only because of the larger design possibilities that they offer, but also because of positive size effects that have been shown to improve performance in various loading conditions [1] . In this work, carbon fiber–epoxy composites of different ply thicknesses (30–300 g/m 2 fiber areal weight) were produced from the same batch of Toray M40JB fiber and NorthTPT TP80ep matrix to study the influence of ply thickness on the ultimate strength and on the onset of damage in lamina, laminates and components. Uniaxial tension, open-hole compression and open-hole tensile fatigue tests on quasi isotropic [45°/90°/−45°/0°] ns laminates showed very significant improvements regarding the on-set of damage, and in some cases ultimate strength, when decreasing the ply thickness . These performance improvements are the result of major changes in the damage progression and failure modes of the laminates caused by a systematic delay or near suppression of transverse cracking and delamination growth in thin-ply composites. On the component level, thin-ply laminates enabled a marked improvement for bolted-joint bearing, especially in hot–wet conditions. Under impact, the 30 μm thin ply laminate exhibited a quasi-brittle failure with extensive translaminar cracking while a ply thickness of 100 μm was found as optimum to minimize the projected damage area. Ply thickness scaling of transverse and in-plane shear strength was identified based on classical laminate theory and unnotched tensile tests on quasi-isotropic specimens. The empirical scaling was found to follow a linear trend over a range of ply thicknesses from 30 to 250 μm. Due to the near suppression of delamination, the strength of thin-ply composites could then be modeled more effectively than thick ply composites using classical laminate theory or standard multilayer shell modeling. [ABSTRACT FROM AUTHOR]

Abstract: Interfacial debonding is one of the most critical damage modes that threatens the structural performance of sandwich composites. In this work, integrated multiplexed fiber Bragg grating (FBG) sensors of 1mm gauge length are used for detection and characterization of crack propagation along the face–core interface. Single cantilever beam specimens incorporating arrays of FBG sensors are subjected to monotonic and fatigue debonding tests. The experimental configurations are simulated by finite element method in order to verify the experimental procedures and analyze the strain data corresponding to the location of the embedded FBGs. It is demonstrated that the embedded FBGs can successfully reveal the strain field imposed by the interface crack into the sandwich face. The good agreement between the experimental results and concurrent numerical simulations suggests that such data can be used for monitoring of interfacial fatigue and fracture in sandwich structures. [Copyright &y& Elsevier]

Abstract: Fiber bridging is regularly encountered in mode I delamination tests of unidirectional fiber reinforced composites. However, characterization of the bridging tractions is rather difficult. One way to indirectly evaluate the bridging traction distribution is to embed a fiber Bragg grating (FBG) sensor close to the crack tip and to measure the distributed strain along this FBG. The strain measurements from the FBG sensor are used to characterize the fiber bridging tractions by an identification method. In this work, the sensor is embedded in a unidirectional carbon/epoxy composite. Firstly, it is treated as an inclusion near the crack plane and a numerical analysis is performed to study its effect on the measured strain field and energy release rate. The results demonstrate that the sensor, located at about two fiber diameters from the crack plane, has a negligible effect on the fracture process. Secondly, among the identified linear, bilinear, and exponential bridging traction distributions, the exponential one is found to be a suitable model. Characterization of the bridging tractions allows to calculate the energy release due to the bridging fibers which is similar to the difference between the initiation energy release and the propagation value . The results also agree with the bridging tractions evaluated from the conventional energy release rate – crack opening displacement method. [Copyright &y& Elsevier]

Abstract: Bridging by intact fibers in composite materials is one of the most important toughening mechanisms. However, a direct experimental assessment of its contribution is not easy to achieve. In this work a semi-experimental method is proposed to quantify its contribution to fracture of unidirectional carbon fiber/epoxy double cantilever beam (DCB) specimens in mode I delamination under monotonic and 1Hz fatigue loads. In each specimen, an embedded optical fiber with an array of eight wavelength-multiplexed fiber Bragg gratings is used to measure local strains close to the crack plane. The measured strain distribution serves in an inverse identification procedure to determine the tractions in the bridging zone in monotonic and fatigue loads. These tractions are used to calculate the energy release rate (ERR) associated with bridging fibers. The results indicate that the ERR due to bridging is about 40% higher in fatigue. The bridging tractions are further included in a cohesive element model which allows to predict precisely the complete load displacement curve of monotonic DCB tests. Using the principle of superposition and the identified tractions, the total stress intensity factor (SIF) is calculated. The results show that the SIF, at initiation, is very close to the one calculated at crack propagation and bridging by intact fibers is responsible for the entire increase in toughness seen in the DCB specimens used herein. [Copyright &y& Elsevier]

Abstract: This study demonstrates the feasibility of using fibre Bragg grating (FBG) sensors to monitor the effects of hygrothermal ageing on the axial strains in a cylindrical epoxy specimen with a centrally located optical glass fibre. A 24mm long FBG, inscribed on the fibre, is used to monitor the moisture absorption in the epoxy, immersed in distilled water at 50°C for 2330h. Hygromechanical simulations support the experimental strain data when the first part of the absorption process is neglected or when a variable coefficient of moisture expansion is used. Also, the effects of using average moisture or when it is distributed according to Fick’s law are negligible. The FBG response also indicates the appearance of progressive debonding after sufficient exposure to moisture. [Copyright &y& Elsevier]

The durability and reliability of lead-free solder joints depends on a large number of factors, like geometry, processing parameters, microstructure and thermomechanical loads. In this work, the nature and influence of the plastic constraints in the solder due to joining partners have been studied by parametric finite element simulation of solder joints with different dimensions. The apparent hardening due to plastic constraints has been shown to strongly depend on the solder gap to thickness ratio with an inversely proportional evolution. Due to interaction of several parameters, the macroscopic stress–strain constitutive law of lead-free solder materials should be determined in the most realistic conditions. In order to identify the elasto-plastic constitutive law of Sn–Ag–Cu solders, a sub-micron resolution Digital Image Correlation technique has been developed to measure the evolution of strain in solder joints during a tensile test. Experimental results of the stress–strain response of Sn–Ag–Cu solder joints have been determined for several solder gaps. The measured load–displacement curves have been used in an inverse numerical identification procedure to determine the constitutive elasto-plastic behaviour of the solder material. The effects of geometrical constraints in a real solder joint with heterogeneous stress and strain fields are then studied by comparing the apparent (constrained) and constitutive (non-constrained) stress–strain relationships. Once the size dependant constraining effects have been removed from the stress–strain relationship, the scale effects can be studied separately by comparing the constitutive elasto-plastic parameters of joints with a variable thickness. Experimental stress–strain curves (constrained and unconstrained) for Sn–4.0Ag–0.5Cu solder in joints of 0.25–2.4 mm gap are presented and the constraining and the size effects are discussed. [ABSTRACT FROM AUTHOR]

In this paper the attention has been focused on the evaluation of how friction can influence Four-point End-Notched Flexure test in carbon fibre-epoxy composite materials. The starting point has been the hysteresis loop in the experimentally obtained Load versus Displacement curve due to an unloading-loading cycle. Different locations for friction have been considered. Numerical simulations have been performed by using Finite Elements and an experimental test campaign has been also carried out to validate the model and to optimize friction coefficients through the comparison with test results. The outcomes of the numerical analysis have given useful indications. Firstly, the comparison of the simulated maximum load and dissipated energy with those coming from the experimental tests has given good results. Secondly, it has been found that the contact between pins and specimens is the most significant location for friction. [ABSTRACT FROM AUTHOR]

Identification of traction–separation relations is at the forefront of damage and fracture mechanics of composite laminates. Such relations are of particular interest in fracture of laminates consisting of layers of different fiber orientations. In this study, an iterative method based on internal strain measurements and parametric numerical modeling is employed to identify a traction–separation relation in quasi-static mode I dominated delamination of a cross-ply carbon fiber epoxy laminate. The results demonstrate that crack propagation is accompanied by a large bridging zone which is smaller than the zone in a uniaxial composite specimen of the same materials and linear dimensions. While the initiation value of the energy release rate (ERR) is the same, the ERR at steady state propagation and the maximum stress in the bridging zone are respectively 1.7 and 3.1 times larger than the corresponding values of the unidirectional specimen. The obtained traction–separation relation is employed in a cohesive zone model to predict the loading response and crack growth. The adopted approach is a step towards a better understanding of delamination in cross-ply composites. [ABSTRACT FROM AUTHOR]

Abstract: The hydrothermal ageing of glass/epoxy interface is investigated using an experimental–numerical approach on cylindrical epoxy specimens with centrally located optical fibers. A 24mm long Bragg grating sensor is inscribed on the optical fiber and used to monitor strains along the fiber, due to processing and subsequent ageing in water at 50°C. The distributed strains are used to: (a) evaluate the residual strain field developed during processing, employing a parametric finite element identification scheme, (b) monitor the evolution of the moisture induced strains during ageing using linear and non-linear responses for the epoxy recorded experimentally, (c) track debond growth at the interface, generated during ageing, by adopting a concentration dependent cohesive finite element model. Good agreement is found between experimental data and simulations until 47days of immersion (or 63% of saturation). Afterwards, the model is not quantitatively accurate but indicates well the trend of the experimental data. [Copyright &y& Elsevier]