An adaptive stochastic multi-scale method for cohesive fracture modelling of quasi-brittle heterogeneous materials under uniaxial tension

An adaptive stochastic multi-scale method is developed for cohesive fracture modelling of quasi-brittle heterogeneous materials under uniaxial tension. In this method, a macro-domain is first discretised into a number of non-overlapping meso-scale elements (MeEs) each of which containing detailed micro-scale finite element meshes. Potential discrete cracks in the MeEs are modelled by pre-inserted cohesive interface elements (CIEs). Nonlinear simulations are conducted for the MeEs to obtain the crack patterns under different boundary conditions. The macro-domain with the same number of overlapped, adaptively size-increasing MeEs are then simulated, until the potential cracks seamlessly cross the boundaries of adjacent MeEs. The resultant cracks, after being filtered by a new Bayesian inference algorithm to remove spurious cracks wherever necessary, are then integrated as CIEs into a final anisotropic macro-model for global mechanical responses. A two-dimensional example of carbon fibre reinforced polymers was modelled under two types of uniaxial tension boundaries. The developed method predicted crack patterns and load-displacement curves in excellent agreement with those from a full micro-scale simulation, but consuming considerably less computation time of the latter

Multiscale image-based modelling of damage and fracture in carbon fibre reinforced polymer composites

This paper is the first to predict and then validate the overall stress-strain curve and the damage sequence comprising matrix cracking, interface debonding and fibre fracture against X-ray Computed Tomography (CT) observations for a multidirectional laminate. Until recently, numerical modelling of multi-directional multi-ply composites required idealised continuum mechanics models or idealised unit cell approaches (or homogenisation method) that cannot reliably capture property variations and the complex sequence of damage events that occur upon tensile loading. Here a multiscale 3D image-based model is used to simulate stochastic crack growth in a double-notch (-45°/90°/+45°/0°/-45°/90°/+45°/0°) s carbon fibre reinforced polymer (CFRP) composite specimen subjected to tensile loading monitored by time-lapse X-ray CT. The data integration approach involves: (1) parallel simulations of meso-scale elements (MeEs) for each ply for which the orientation of the individual fibres has been extracted from an X-ray CT image, (2) local hierarchical coupling of the MeEs into a macro-scale mechanical model of the test piece, and (3) the use of a random variation in material properties where microstructural details are not revealed by the X-ray CT characterisation method. Cohesive interface elements (CIEs) are used at both scales to predict the accumulation of interface damage and crack growth. The fibre-level modelling captures the detailed damage sequence and crack morphology including fibre/matrix debonding, sliding, matrix cracking and fibre fracture events. The multiscale model is validated by comparison with the measured tensile loading curve and the damage evolution recorded by the X-ray CT. This approach can reduce the reliance of certification on extensive heirarchical structural testing schemes from test-piece to full-scale component. </p

Experimental investigation of the performance of demountable composite beam shear connectors at ambient &amp; elevated temperatures

This paper presents the results of an experimental programme to investigate the performance of a new composite shear connector that can be used to build deconstructable composite floors. A total of 15 push-out tests were conducted to quantify the effects of different design parameters, including different nominal temperature levels (20oC, 100oC, 300oC, 450 oC, 600oC), with the steel decking perpendicular to the steel section, two different shear stud lengths and two different grades of concrete (C40/50 and C20/25). The tests were mainly carried out under steady-state condition where the specimen temperatures were increased to the desired target and the load was then applied until failure of the specimen. A few transient tests were also carried out in which a load equal to the load carrying capacity of the steady-state test at 600oC was applied followed by increasing the specimen temperature until failure. The main failure mode at temperatures not exceeding 400oC was initiated by diagonal fracture of concrete in the trough of the steel sheeting, leading to concrete sliding along the steel sheeting and shear stud fracture at the point of complete failure of the specimen. At elevated temperatures above 400oC, the failure mode changed to shear stud fracture. The Eurocode 4 calculation equations were shown to produce shear connector resistances to be in reasonable agreement with the steady state test results at both ambient and elevated temperatures, even though the Eurocode equations were developed for welded shear studs. However, these equations gave calculated shear connector resistances higher than the transient state test results. All of the tests indicate that the demountable shear studs have sufficient deformation capacity (&gt;6mm) to enable plastic distribution of forces in the shear studs in composite beams, as suggested in Eurocode EN 1994-1-1, however, shear connector slips at high temperatures (just over 6mm) were considerably lower than those at ambient and low temperatures (&gt;20mm)