Internal geometry of structurally stitched NCF preforms

Internal geometry of a textile reinforcement is an important factor of the reinforcement performance during the composite manufacturing and service life. In this article, generalized geometrical models of structural stitching loops are presented for the sewing, tufting, and dual-needle stitching methods. The term 'structural' presumes here that the stitching yarn does not only consolidate the plies (as the non-structural one does) but forms also a through-the-thickness (3D) reinforcement. The models account for the general features of the yarn loop geometry and are believed to allow for enough precise modelling on the meso-scale (textile unit cell) level. The modelling approach is validated with experimental data

A statistical treatment of the loss of stiffness during cyclic loading for short fiber reinforced injection molded composites

Injection molded short fiber reinforced composites (SFRC) have different local fiber orientation distribution (FOD) at every point. SN curves of short fiber reinforced composites are known to depend on the fiber orientation distribution. Such materials also suffer from continuous loss of stiffness during cyclic loading. It is not known whether the loss of stiffness is different for SFRC with different FOD. A statistical analysis of the loss of stiffness curves is presented in this paper. Tension-tension fatigue experiments are performed and loss of stiffness is collected for every data point in the SN curve. A systematic method for comparing the loss of stiffness is developed. It is concluded that the difference in loss of stiffness curves for coupons of SFRC with different FOD is not statistically significant. (C) 2016 Elsevier Ltd. All rights reserved

Do high frequency acoustic emission events always represent fibre failure in CFRP laminates?

© 2017 Elsevier Ltd When damage in carbon fibre reinforced composites (CFRP) is monitored by acoustic emission (AE), it is a common belief that high frequency AE events originate from fibre failure. This shows that this statement may not correspond to the reality, and matrix cracks can emit high frequency AE signals. Quasi-static tension of [−45 2 /0 2 /+45 2 /90 2 ] s laminates was monitored by AE, Digital Image Correlation (DIC) on the surface of the sample and in-situ optical microscopy on the sample's polished edge. Unsupervised k-means clustering algorithm was applied to the AE results. By comparison with the direct DIC and microscopic observations, the AE cluster with high frequency and low amplitude was found to correspond to directly observed matrix cracks

Finite element modelling of inter-ply delamination and intra-yarn cracking in textile laminates

The aim of the current study is to demonstrate the effect of inter-ply delamination on stiffness degradation of multi-ply woven composites. Such a demonstration becomes possible due to new technique of modelling textile laminates. It is based on set of boundary value problems for unit cell of a single ply, where boundary conditions imitate interaction with the other plies. Once these problems are solved, local stress distribution and stiffness of the laminate are determined analytically as function of number of the plies and local stress/strain fields obtained in these problems. Hence, it opens the road for an efficient modelling of delamination, which is described as gradual reduction of plies in the laminate

A feasibility study of the Master SN curve approach for short fiber reinforced composites

Short fiber reinforced composite (SFRC) materials have a different fiber orientation distribution (FOD) at every point. The fatigue properties of SFRC are known to depend on the FOD. The Master SN curve (MSNC) method for predicting an SN curve for a given FOD based on the known SN curve for the reference FOD is used to predict the local SN curve of a SFRC component by relating the damage at the microscopic level to the macroscopic fatigue properties. A simplified version of MSNC method, which needs even less experimental input, uses an assumption of constant SN curve slope is also presented in this paper. The paper validates both variants of the MSNC method on three sets of experimental data on fatigue of short fiber composites and analyses their accuracy. It is demonstrated that the MSNC approach needs only one SN curve as input with no specific requirements to the fiber orientation of the test coupon. Test coupons could have either uniform fiber orientation in the thickness or a "skin core" orientation variation. (C) 2016 Elsevier Ltd. All rights reserved

Multi-instrument in-situ damage monitoring in quasi-isotropic CFRP laminates under tension

© 2018 Digital Image Correlation monitoring of the surface strains, microscopic in-situ observations of the micro-damage on the specimen edge and Acoustic Emission (AE) are utilized simultaneously during tension tests of quasi-isotropic carbon fibre reinforced polymer composites. It is found that the cluster analysis for characteristic parameters of AE events (the main being the signal amplitude and frequency) does not unambiguously identify the type of damage which causes the event. With optical instruments, it is observed that the signatures of AE events depend on the position of the ply where damage happens and on the ply orientation (90° vs 45°). Robust evidences for the variations in AE characteristics of damage modes in different lay-ups are presented. AE events, originated from surface cracks, have high amplitude and low frequency, whereas AE events, originated from transverse cracks in the inner plies, have low amplitude and high frequency characteristics. Any conclusion for fibre breaks are not reached in this study. Therefore, measurements in this study rather point out that the AE events, which could be interpreted as fibre breaks because of their high frequency characteristics, as optical observations prove, correspond to other damage types in quasi-isotropic laminates

Pseudo-grain discretization and full Mori Tanaka formulation for random heterogeneous media : predictive abilities for stresses in individual inclusions and the matrix

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Strain mapping at the micro-scale in hierarchical polymer composites with aligned carbon nanotube grafted fibers

For the first time, micro-scale digital image correlation (μDIC) is investigated for measurement of strain fields in hierarchical fiber-reinforced composites. The methodology is developed on an exemplary alumina fiber/epoxy composite laminate with aligned carbon nanotubes (A-CNTs) grown on fibers. Utilizing environmental scanning electron microscopy and nano-scale random speckle patterns, sufficient precision is achieved to detect the influence of the A-CNTs on the deformation field around the fibers. Debonded regions at the fiber/matrix interface with openings as small as 35 nm could be detected. μDIC could identify the propagation of the debonded region based on the non-linear increase of the opening. The image correlation uncertainty in the displacement analysis is estimated to be below 5 nm. The experimental results are validated by computational analysis performed on the region of interest. For this, an advanced model with two scales of reinforcement (microscopic fibers and nanotubes) and boundary conditions taken from the experiment is used. As verified by the model, A-CNTs are found to constrain matrix deformation in their longitudinal direction. Keywords: Digital image correlation (DIC); Scanning electron microscopy (SEM); Carbon nanotubes; Mechanical properties; Finite element analysis (FEA

Simplified and complete phase-field fracture formulations for heterogeneous materials and their solution using a Fast Fourier Transform based numerical method

This paper focuses on the numerical implementation of phase-field models of fracture using the Fast Fourier Transform based numerical method. Recent developments in that field rely on the separate solution of a coupled problem where the mechanical equilibrium problem is solved first, and then the phase-field evolution equation. The latter involves a diffusion term which has been simplified in previous works relying on the Fast Fourier Transform based numerical method. This simplification has theoretically no effect for homogeneous materials, but might influence predictions significantly for heterogeneous materials where fracture properties vary between the different components. In this paper, the influence of this simplification is assessed and a complete formulation is proposed as well as a novel implementation of this formulation using the Fast Fourier Transform based numerical method. The assessment relies on simulations with a material containing two components, one of them being defined as unbreakable by using higher fracture properties. Using the simplified formulation, the presence of an artificial diffusion of damage between the two components is evidenced, and non-zero damage values are observed in the unbreakable component. Although the complete formulation leads to an increase of the number of iterations to solve the phase-field evolution equation, it suppresses completely the diffusion of damage towards the unbreakable component. The two formulations, in fact, lead to identical results when the fracture properties are homogeneous, but the results diverge both in terms of local fracture patterns and global stress–strain relations when the fracture properties contrast increases. This difference is also more pronounced when the regularization length introduced by the phase-field model increases.</p