Uncertainty quantification for advanced progressive damage models for composites by means of efficient emulators and bootstrapping

A methodology was developed and validated to quantify the uncertainty for advanced progressive damage models for composites. It relies on a pragmatic approach entailing the definition of efficient emulators, the use of state-of-the-art computational models, and the employment of bootstrapping statistic techniques. The proposed methodology was calibrated on numerical results obtained running a limited amount of virtual experiments (five for each configuration) of unnotched and open-hole specimens in tension and compression. The structural strength was taken as the quantity of interest, and a methodology was proposed and validated to determine its distribution and associated statistics

On the statistics of transverse permeability of randomly distributed fibers

An RVE–based stochastic numerical model is used to calculate the permeability of randomly generated porous media at different values of the fiber volume fraction for the case of transverse flow in a unidirectional ply. Analysis of the numerical results shows that the permeability is not normally distributed. With the aim of proposing a new understanding on this particular topic, permeability data are fitted using both a mixture model and a unimodal distribution. Our findings suggest that permeability can be fitted well using a mixture model based on the lognormal and power law distributions. In case of a unimodal distribution, it is found, using the maximum-likelihood estimation method (MLE), that the generalized extreme value (GEV) distribution represents the best fit. Finally, an expression of the permeability as a function of the fiber volume fraction based on the GEV distribution is discussed in light of the previous results.</p

Measurement of the compressive crack resistance curve of composites using the size effect law

This paper presents a new methodology to measure the compressive crack resistance curve of the longitudinal plies of carbon-epoxy laminates. The methodology is based on three main steps: the first one corresponds to the determination of the energy release rate of cross-ply laminates with two edge cracks using a parametric finite element model. The energy release rate is used in the definition of a relation between the crack resistance curve and the size effect. Finally, experimental tests are performed in scaled double-edge notched specimens to quantify the size effect law, thus proving the last piece of information required to define the crack resistance curve. The full crack resistance curve is obtained for the IM7-8552 carbon epoxy composite material. The methodology proposed in this paper overcomes the inherent limitations of the existing test methods, and it serves as the basis for the identification of cohesive laws used in some analysis models

High strain rate characterisation of intralaminar fracture toughness of GFRPs for longitudinal tension and compression failure

The elastic parameters, strengths, and intralaminar fracture toughness are determined for an E-Glass polymer composite material system, statically and at high strain rate, adapting methodologies previously developed by the authors for different carbon composites. Dynamic experiments are conducted using tension and compression Split-Hopkinson Bars (SHBs). A unique set of experimental parameters is obtained, and reported together with the experimental set-up, in order to ensure reproducibility. While in-plane elastic and strength properties were obtained by testing one specimen geometry, intralaminar fracture properties required the testing of different sized notched specimens with scaled geometries. This allowed the use of the size-effect method for the determination of the dynamic R-curve. When comparing these results with those previously obtained for a carbon/epoxy material system, it is observed that the dynamic fracture toughness exhibits a much more significant increase in both tension and compression. The obtained results permit the identification of the softening law at different strain rates, allowing its use in any analytical or numerical strength predictive method

Resistance welding of carbon fibre reinforced PEKK by means of CNT webs

Single lap shear specimens were manufactured using resistance welding of carbon fibre reinforced substrates by means of CNT web-based heating elements. Heating elements were manufactured by embedding the CNT web layers between layers of PEKK/glass fibre and connecting them to copper electrodes. An experimental campaign explored their electrothermal behaviour influencing the welding process. Single lap shear specimens were then welded at a pressure of 0.05 MPa and different levels of power and duration. An optimum bond was obtained with a specific power of 80–90 kW/m2 and a time of 150 s, achieving a shear strength of 30 MPa. Post-mortem analysis revealed that fracture propagated within the substrates. This work represents a further step in the integration of CNT web-based heating elements in an industrial welding process

Tensile and impact properties of melt-blended nylon 6/ethylene-octene copolymer/graphene oxide nanocomposites.

The addition of stiff nanoparticles to a polymer matrix usually proves beneficial for the enhancement in stiffness and strength, however, the impact strength is usually lowered. Conversely, the use of elastomeric additives can enhance the toughness and impact strength but causes a reduction in overall stiffness and strength. To take advantage of the desirable effects of both additives, they may be simultaneously added to the host matrix. Graphene oxide (GO), along with a thermoplastic elastomer ethylene-octene copolymer (EOC), was chosen to be added to nylon 6 for the current investigation. Maleated EOC (EOC-g-MA) was used as a compatibilizer for this study. 3 wt% GO nanoparticles, 20 wt% ethylene-octene copolymer (EOC) and 3 wt% EOC-g-MA were added to nylon 6 to prepare the nylon 6/EOC/GO blend-based nanocomposites. A high shear rate screw running at 300 rpm was used for melt-blending with a twin-screw extruder. Increased stiffness and tensile strength were observed by the addition of GO nanoparticles while elongation at break, toughness and impact strength were lowered by the addition of GO. The addition of EOC and EOC-g-MA enhanced the elongation at break, toughness and impact strength. However, the stiffness and strength of nylon 6/EOC blend was lower than that of the neat nylon 6. The addition of GO nanoparticles and EOC to neat nylon 6 caused a reduction in its impact strength. However, simultaneous addition of EOC and EOC-g-MA to nylon 6 caused a significant increase in the impact strength compared to neat nylon 6 and yielded a nylon 6/EOC/EOC-g-MA bend with the highest impact strength. The addition of GO nanoparticles to this blend, however, again caused a significant reduction in the impact strength. Nylon 6/EOC/EOC-g-MA blend showed the highest toughness and impact strength. Simultaneous addition of EOC and GO helped achieve a balanced stiffness and toughness

Measurement of resistance curves in the longitudinal failure of composites using digital image correlation

This paper presents a new methodology to measure the crack resistance curves associated with fiber-dominated failure modes in polymer–matrix composites. The crack resistance curves not only characterize the fracture toughness of the material, but are also the basis for the identification of the parameters of the softening laws used in the numerical simulation of fracture in composite materials. The proposed method is based on the identification of the crack tip location using Digital Image Correlation and the calculation of the J-integral directly from the test data using a simple expression derived for cross-ply composite laminates. It is shown that the results obtained using the proposed methodology yield crack resistance curves similar to those obtained using Finite Element based methods for compact tension carbon–epoxy specimens. However, it is also shown that, while the Digital Image Correlation based technique mitigates the problems resulting from Finite Element based data reduction schemes applied to compact compression tests, the delamination that accompanies the propagation of a kink-band renders compact compression test specimens unsuitable to measure resistance curves associated with fiber kinking

Three-dimensional failure criteria for fiber-reinforced laminates

This paper proposes a new fully three-dimensional failure criteria for polymer composites reinforced by unidirectional fibers. Existing failure criteria based on three-dimensional stress states are revisited and their limitations and pitfalls are identified. A new set of failure criteria for both longitudinal and transverse failure mechanisms where the effect of ply thickness on the material strength is accounted for is proposed. The accuracy of the failure criteria is assessed by comparing the analytical predictions with existing experimental data obtained under multiaxial stress states. A good agreement between the predictions and experimental data is generally observed

On the mechanical properties of melt-blended nylon 6/ethylene-octene copolymer/graphene nanoplatelet nanocomposites.

Ethylene-octene copolymer (EOC) with a loading level of 20 wt%, maleated EOC (EOC-g-MA) with a loading level of 3 wt% and graphene nanoplatelets (GnPs) at four different loading levels, i.e., 3 wt%, 5 wt%, 10 wt% and 15 wt% were added to nylon 6 to prepare nanocomposites using a twin-screw extruder with a high shear rate screw running at 300 rpm. Increased stiffness was observed with the addition of GnPs while tensile strength of nanocomposites was only slightly influenced. Addition of GnPs into nylon 6 and nylon 6/EOC blend caused either a reduction in the Charpy impact strength or it remained unaffected. Similarly, the Izod impact strength of compatibilized nylon 6/EOC blend increased while that of nylon 6/EOC blend-based nanocomposites decreased. An increase was observed in the compressive Izod impact strength of compatibilized nylon 6/EOC blend. Addition of GnPs to nylon 6/EOC blend caused an increase in the fracture toughness due to their influence on the morphology and fracture mechanisms. This study shows that simultaneous addition of high surface area GnPs and an impact modifier to neat nylon 6 can help achieve enhancement and tailoring of stiffness and toughness