A synchrotron computed tomography dataset for validation of longitudinal tensile failure models based on fibre break and cluster development

We performed in-situ tensile tests on two carbon fibre/epoxy composites with continuous scanning using synchrotron computed tomography (CT). Both composites were cross-ply laminates, and two specimens were tested for each compos- ite. The voxel size was sufficiently small to recognize individ- ual fibres and fibre breaks. For each test, 16-19 volumes were reconstructed, cropped down to the 0° plies and analysed to track fibre break and cluster development. This dataset provides the last CT volume before failure for each of the four specimens as well as the individual fibre break locations in all reconstructed volumes. These data are then plotted against predictions from six state-of-the-art strength models. The target is that these data become a benchmark for the development of new models, inspiring researchers tPostprint (published version

Enzymatic degradation of polyethylene terephthalate nanoplastics analyzed in real time by isothermal titration calorimetry

Plastics are globally used for a variety of benefits. As a consequence of poor recycling or reuse, improperly disposed plastic waste accumulates in terrestrial and aquatic ecosystems to a considerable extent. Large plastic waste items become fragmented to small particles through mechanical and (photo)chemical processes. Particles with sizes ranging from millimeter (microplastics, <5 mm) to nanometer (nanoplastics, NP, <100 nm) are apparently persistent and have adverse effects on ecosystems and human health. Current research therefore focuses on whether and to what extent microorganisms or enzymes can degrade these NP. In this study, we addressed the question of what information isothermal titration calorimetry, which tracks the heat of reaction of the chain scission of a polyester, can provide about the kinetics and completeness of the degradation process. The majority of the heat represents the cleavage energy of the ester bonds in polymer backbones providing real-time kinetic information. Calorimetry operates even in complex matrices. Using the example of the cutinase-catalyzed degradation of polyethylene terephthalate (PET) nanoparticles, we found that calorimetry (isothermal titration calorimetry-ITC) in combination with thermokinetic models is excellently suited for an in-depth analysis of the degradation processes of NP. For instance, we can separately quantify i) the enthalpy of surface adsorption ∆AdsH = 129 ± 2 kJ mol−1, ii) the enthalpy of the cleavage of the ester bonds ∆EBH = −58 ± 1.9 kJ mol−1 and the apparent equilibrium constant of the enzyme substrate complex K = 0.046 ± 0.015 g L−1. It could be determined that the heat production of PET NP degradation depends to 95% on the reaction heat and only to 5% on the adsorption heat. The fact that the percentage of cleaved ester bonds (η = 12.9 ± 2.4%) is quantifiable with the new method is of particular practical importance. The new method promises a quantification of enzymatic and microbial adsorption to NP and their degradation in mimicked real-world aquatic conditions

Multi-instrument multi-scale experimental damage mechanics for fibre reinforced composites

© Published under licence by IOP Publishing Ltd. Reliable investigation of damage in fibre reinforced composites requires concurrent in- and ex-situ application of multiple instruments at different scale: digital image correlation, acoustic emission registration, optical/electron microscopy, C-scan, X-ray imaging and micro-computed tomography. The multi-instrument experimental mechanics allows detailed damage monitoring and inspection

Void growth measurement and modelling in a thermosetting epoxy resin using SEM and tomography techniques

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Void growth measurement and modelling in a thermosetting epoxy resin using SEM and tomography techniques

Void growth and failure in an epoxy resin is investigated. Tensile tests are carried out on doublenotched round bars. Stress triaxiality in the net section is controlled by the use of two different notch root radii. SEM and computed tomography observations are conducted on regions of interest to identify voids and evaluate their growth. Failure initiation sites are analysed, revealing critical defects in the form of voids or particles. The macroscopic and microscopic experimental results are used to optimize the Gurson–Tvergaard– Needleman model. A finite element analysis is performed to study the mechanical response of the notched specimens. The peaks of the maximum principal stress are found to coincide with the locations of the failure initiation sites observed experimentally, and the model is shown to be able to handle non-uniform initial void distributions.status: Published onlin

Experimental investigation of yield and hysteresis behaviour of an epoxy resin under cyclic compression in the large deformation regime

High-performance polymers are extensively used in various applications undergoing long-term cyclic loadings. The deformation behaviour of an amorphous thermoset epoxy resin undergoing cyclic compressive loading is investigated for a range of applied deformation levels. The measurements indicate significant hysteresis upon repeated loading and unloading cycles with progressive accumulation of plastic strain. Cyclic damage leads to a reduction of the stress needed to reach the peak strain per cycle, while cyclic stiffening corresponding to an increase of elastic modulus with increasing number of cycles is observed, attributed to chain orientation effects. The dissipated energy asymptotically decreases to zero under strain-controlled cycling conditions. Interestingly, when monotonically loaded after cycling, the epoxy exhibits an increase in yield strength. This ‘re-yield’ stress level is closely related to the selected value of the peak (unloading) strain level and increases with increasing number of loading cycles.</p

Major trends in the elasto-visco-plastic behaviour of highly cross-linked epoxy resins

Highly cross-linked thermosetting polymers, widely used as matrices for advanced polymer-based fibre-reinforced composites, have suffered from a lack of in-depth mechanical characterisation. The assumption is that their overall mechanical response is inherently similar to that of high glass transition temperature (Tg) amorphous thermoplastics, except for a lower ductility and a better creep resistance [1]. However, multi-scale test strategies have increasingly been used to improve the understanding of the deformation and failure of epoxies in order to feed computational models. The main motivation driving these studies is the recognition that a micro-scale level understanding of the deformation mechanisms of these materials is necessary to accurately predict the failure of the corresponding composites structures [2]. This is particularly important for loading conditions where plastic flow within the matrix is dominant such as overall shear or creep. However, these studies mostly limit themselves to the analysis of the elastic modulus and of the yield point, highlighting the remaining knowledge gap about the post-yield visco-plastic response of epoxies. The purpose of this work is two-fold. First, in order to supplement the scarcity of accurate experimental data, we establish master trends for the entire stress-strain response of highly cross-linked epoxies, hence providing a basis for first-level modelling attempts. Additionally, we unravel trends within the stress-strain response that can be correlated to one or several physico-chemical or molecular structure parameters. For this purpose, a vast database of the elasto-visco-plastic properties resulting from tension and/or compression tests performed on seven different epoxy systems is gathered. Parameters such as rate sensitivity, softening, re-hardening and activation volumes are carefully extracted as it has been shown that they strongly affect the local fibre/matrix level stress development [3]. Among other trends, correlations between the yield and softening stresses and strains and the corresponding Tg and cross-linking densities are found for the tested resins. The re-hardening is mainly dictated by the cross-linking density, as expected