The influence of toughening-particles in CFRPs on low velocity impact damage resistance performance

The role of particle-toughening for increasing impact damage resistance in carbon fibre reinforced polymer (CFRP) composites was investigated. Five carbon fibre reinforced systems consisting of four particle-toughened matrices and one system containing no toughening particles were subjected to low velocity impacts ranging from 25 J to 50 J to establish the impact damage resistance of each material system. Synchrotron radiation computed tomography (SRCT) enabled a novel approach for damage assessment and quantification. Toughening mechanisms were detected in the particle-toughened systems consisting of particle–resin debonding, crack-deflection and crack-bridging. Quantification of the bridging behaviour, increase in crack path length and roughness was undertaken. Out of the three toughening mechanisms measured, particle systems exhibited a larger extent of bridging suggesting a significant contribution of this toughening mechanism compared to the system with no particle

Partial volume correction for approximating crack opening displacements in CFRP material obtained from micro-focus X-ray CT scans

This paper presents a partial volume correction technique that applies a measurement weighting based on grey scale intensity values, allowing crack opening displacements (CODs) to be better estimated in micro-focus computed tomography (?CT) scans. These were tested on 3D data obtained from two separate ?CT scanners on particle toughened and non-particle toughened carbon fibre material subjected to low velocity impact. Direct comparisons of COD estimations were made with higher resolution measurements obtained using synchrotron radiation computed tomography (SRCT) scans taken at the European Synchrotron Radiation Facility (ESRF). In this study, partial volume correction is reported to improve the accuracy of these measurements to within 20% of SRCT measurements, whereas measurements based on counting interconnected voxels representing a detectable crack are reported to consistently overestimate crack openings by up to 500%. Scatter in estimations was dependent on material type, noise, and artefacts associated with ?CT volumes

Quasi-static indentation and compression after impact damage growth monitoring using microfocus X-ray computed tomography

In this study interrupted quasi-static indentation and post-impacted compression tests were performed at incremental load steps with X-ray computed tomography performed at each step. This enabled non-destructive, three-dimensional damage assessments to be carried out allowing initiation and propagation of different damage modes to be monitored. Preliminary results from these experiments are reported in this paper

Rupture de fibre : comparaison entre un modèle écrit a l'échelle microstructural et une détection du phénomène par tomographie très haute résolution

International audienceOn présente dans cette étude la discrimination des phénomènes de ruptures de fibre, de microfissuration intralaminaire, de macrofissuration intralaminaire ainsi que du microdélaminage au sein des composites stratifiés d'unidirectionnels orientés, dans le cadre d'une méthode de détection des endommagements basée sur la technique de l'Emission Acoustique. Ces résultats expérimentaux sont confrontés aux prévisions issues d'un modèle de comportement de matériaux composites unidirectionnels, basés sur la physique des phénomènes à l'échelle des constituants

Synchrotron radiation computed tomography for experimental validation of a tensile strength model for unidirectional fibre-reinforced composites

Synchrotron radiation computed tomography has been used to analyse fibre break accumulation in unidirectional composites loaded in tension. The data are compared to model predictions. The model only slightly overestimated the composite failure strain, but predictions of fibre break density were too high, which can be mainly attributed to errors in the Weibull distribution. Both the number and percentage of interacting fibre break clusters were under-predicted by the model. This was attributed to an underestimation of stress concentrations in the model. While the experimental observations revealed mainly co-planar clusters, the model predicted mainly diffuse clusters. The experiments showed that the clusters did grow any further after their formation, while the model predicted a gradual development. Both local and dynamic stress concentrations were hypothesised to be key features for further exploration. The discrepancies identified, inform suggestions for directions advancing the state-of-the-art strength models of UD composites

Asynchronous food-web pathways could buffer the response of Serengeti predators to El Niño southern oscillation

Understanding how entire ecosystems maintain stability in the face of climatic and human disturbance is one of the most fundamental challenges in ecology. Theory suggests that a crucial factor determining the degree of ecosystem stability is simply the degree of synchrony with which different species in ecological food webs respond to environmental stochasticity. Ecosystems in which all food-web pathways are affected similarly by external disturbance should amplify variability in top carnivore abundance over time due to population interactions, whereas ecosystems in which a large fraction of pathways are nonresponsive or even inversely responsive to external disturbance will have more constant levels of abundance at upper trophic levels. To test the mechanism underlying this hypothesis, we used over half a century of demographic data for multiple species in the Serengeti (Tanzania) ecosystem to measure the degree of synchrony to variation imposed by an external environmental driver, the El Niño Southern Oscillation (ENSO). ENSO effects were mediated largely via changes in dry-season vs. wet-season rainfall and consequent changes in vegetation availability, propagating via bottom-up effects to higher levels of the Serengeti food web to influence herbivores, predators and parasites. Some species in the Serengeti food web responded to the influence of ENSO in opposite ways, whereas other species were insensitive to variation in ENSO. Although far from conclusive, our results suggest that a diffuse mixture of herbivore responses could help buffer top carnivores, such as Serengeti lions, from variability in climate. Future global climate changes that favor some pathways over others, however, could alter the effectiveness of such processes in the future

Performance, milk fatty acid composition and behaviour of high-yielding Holstein dairy cows given a limited grazing period

The effects of a limited grazing period on the performance, behaviour and milk composition of high‐yielding dairy cows were examined. A total of 56 Holstein cows yielding 44.7 ± 0.42 kg/day were allocated to one of four treatments in one of two, 4‐week periods. Treatments were as follows: control (C)—cows housed and offered TMR ad libitum; early grazing (EG)—cows grazed for 6 hr after morning milking then housed; delayed grazing (DG)—cows returned to housing for 1 hr after morning milking followed by grazing for 6 hr, then housed; restricted TMR (RT)—cows grazed for 6 hr after morning milking, then housed and fed TMR at 75% of ad libitum. Intake of TMR was highest in cows receiving C, intermediate in EG and DG, and lowest in RT at 26.9, 23.6, 24.7 and 20.3 kg DM/day respectively. Pasture intake was similar in cows receiving EG or DG, but was higher in RT at 2.4, 2.0 and 3.5 kg DM/day respectively. Milk yield was similar between cows receiving C, EG or DG, but lowest in RT at 45.7, 44.2, 44.9 and 41.7 kg/cow, respectively, while milk fat content of C18:3 n‐3 was increased by grazing. Cows in C spent more than 55 min/day longer lying and had three additional lying bouts/day, while lying bouts were shorter than for cows receiving EG, RT or DG. It is concluded that high‐yielding cows can be grazed for 6 hr/day with little impact on performance, provided TMR is available ad libitum when housed

See-Saw Type Mixing and νμ→ντ \nu_{\mu} \to \nu_{\tau} Oscillations

We consider $\nu_{\mu} \to \nu_{\tau}$ oscillations under the assumption that there is a see-saw type mixing of the light neutrinos with heavy Majorana particles. It is shown that the existing data, including the recent LEP data, do not exclude the possibility that the additional terms in the transition probability due to this mixing could be of the same order of magnitude as the usual oscillating term. Detail investigations of $\nu_{\mu} \to \nu_{\tau}$ transitions in future CERN and Fermilab experiments could allow to get informations not only about the neutrino masses and mixing but also about the mixing of neutrinos with heavy Majorana particles.Comment: DFTT 66/92. LaTeX file, 7 pages, no figure