Towards understanding the influence of porosity on mechanical and fracture behaviour of quasi-brittle materials:experiments and modelling

In this work, porosity-property relationships of quasi-brittle materials are explored through a combined experimental and numerical approach. In the experimental part, hemihyrate gypsum plaster powder (CaSO 4 ⋅1/2H 2 O CaSO4⋅1/2H2O) and expanded spherical polystyrene beads (1.5–2.0 mm dia.) have been mixed to form a model material with controlled additions of porosity. The expanded polystyrene beads represent pores within the bulk due to their light weight and low strength compared with plaster. Varying the addition of infill allows the production of a material with different percentages of porosity: 0, 10, 20, 30 and 31 vol%. The size and location of these pores have been characterised by 3D X-ray computed tomography. Beams of the size of 20×20×150 20×20×150 mm were cast and loaded under four-point bending to obtain the mechanical characteristics of each porosity level. The elastic modulus and flexural strength are found to decrease with increased porosity. Fractography studies have been undertaken to identify the role of the pores on the fracture path. Based on the known porosity, a 3D model of each microstructure has been built and the deformation and fracture was computed using a lattice-based multi-scale finite element model. This model predicted similar trends as the experimental results and was able to quantify the fractured sites. The results from this model material experimental data and the lattice model predictions are discussed with respect to the role of porosity on the deformation and fracture of quasi-brittle materials

Towards understanding the influence of porosity on mechanical and fracture behaviour of quasi-brittle materials:experiments and modelling

In this work, porosity-property relationships of quasi-brittle materials are explored through a combined experimental and numerical approach. In the experimental part, hemihyrate gypsum plaster powder (CaSO 4 ⋅1/2H 2 O CaSO4⋅1/2H2O) and expanded spherical polystyrene beads (1.5–2.0 mm dia.) have been mixed to form a model material with controlled additions of porosity. The expanded polystyrene beads represent pores within the bulk due to their light weight and low strength compared with plaster. Varying the addition of infill allows the production of a material with different percentages of porosity: 0, 10, 20, 30 and 31 vol%. The size and location of these pores have been characterised by 3D X-ray computed tomography. Beams of the size of 20×20×150 20×20×150 mm were cast and loaded under four-point bending to obtain the mechanical characteristics of each porosity level. The elastic modulus and flexural strength are found to decrease with increased porosity. Fractography studies have been undertaken to identify the role of the pores on the fracture path. Based on the known porosity, a 3D model of each microstructure has been built and the deformation and fracture was computed using a lattice-based multi-scale finite element model. This model predicted similar trends as the experimental results and was able to quantify the fractured sites. The results from this model material experimental data and the lattice model predictions are discussed with respect to the role of porosity on the deformation and fracture of quasi-brittle materials

Fatigue damage assessment of uni-directional non-crimp fabric reinforced polyester composite using X-ray computed tomography

In this study, the progression of tension-tension fatigue (R ¼ 0.1) damage in a uni-directional (UD) composite made from a non-crimp glass fibre fabric used for wind turbine blades is investigated using multi-scale 3D X-ray computed tomography (CT). Initially, a representative volume is examined at one specific damage level. UD fibre fractures are only observed close to the supporting thin transverse backing layers. Furthermore, UD fibre fractures are only observed at locations where backing fibre bundles intersect one another and are at the same time locally close to a UD bundle. In addition, to study the progression of damage as a function of stiffness degradation at higher resolution four samples are subjected to different numbers of cycles before examination by CT. One sample is examined during the initial stiffness drop, two samples during stable stiffness degradation, and one close to final failure. Damage is observed to occur as chains of individual fibre breaks or clusters of fibre fractures rather than large fracture planes. Our work indicates how fracture of UD fibres initiates from intersecting ±80 degrees backing bundles extending progressively further into the UD layer. The fibre fracture zone becomes more diffuse further from the backing layer. Our work supports a scheme explaining stiffness degradation in terms of UD fibre damage accumulation and demonstrates the importance of 3D and ideally time-lapse imaging studies

X-ray tomography imaging of sandwich panels made of ramie fibre and carbon fibre

This paper uses X-ray tomography to investigate the microstructure of two novel eco-friendly bio-composites that could be used in aircraft secondary structures. One sample each of two sandwich panels are studied. The two samples both have a ramie fibre honeycomb core. One is covered with sheets made of ramie fibre and the other is covered with sheets made of carbon fibre. With the results of the scanning, this paper aims to discover the critical features which may affect the mechanical properties and multifunctional performance of the composites. The scanning consists of two steps; the first involves imaging the whole sandwich panels, and the second provides a higher resolution region of interest scan of 2*2 cm2 strips. In each step of the scanning, samples were bound together so that two sets of data could be imaged in one operation, thus time and facilities access costs were reduced. Reviewing the data and images, more defects could be found in the ramie fibre surfaced composite than the carbon fibre, which is consistent with the low failure load achieved in a mechanical pull-off test reported elsewhere. These features are related to (i) surface finishing, (ii) voids in the matrix and (iii) an uneven application of adhesive. The scans provide useful datasets that can be used to both inform improvements in the manufacturing process and give insight into the performance of the material during mechanical testing

Global warming and recurrent mass bleaching of corals

During 2015–2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs

Harnessing 3D microarchitecture of pterosaur bone using multi-scale X-ray CT for aerospace material design

Pterosaurs were the largest animals to have achieved powered flight in the history of life on Earth, possessing wingspans akin to some modern light aircraft. Vertebrate fossils have shown their potential to retain information on the chemical, physical, and mechanical properties of precursor bone. However, the fossil record is not a traditional source of inspiration for engineers to create palaeo-bioinspired designs. To explore its potential, this study has imaged the three-dimensional porosity of pterosaur bone intending to inspire and improve the mechanical properties of aerospace materials. Historically, two-dimensional histological analysis has resolved fine-scale structures in fossilised bone, which damages the sample. By applying advanced X-ray imaging techniques in this study (using Image Quality Indicators) we show it is possible to non-destructively resolve/verify the microarchitecture of pterosaur bone not previously seen in three dimensions. Pterosaur bone porosity has helped map the macroscopic stresses of this biomaterial but ultimately presents an opportunity to inspire advanced manufactured materials. This microarchitecture of bone offers a unique geometry where self-healing materials with internal monitoring systems can be developed. The iterative process of Darwinian natural selection has evolved multiple engineering solutions that can be reverse engineered to solve challenges facing industry in the 21st Century