1,721,199 research outputs found
Assessment of the ultimate actual strength of rock-climbing protection devices: Extraction tests in the field and the human capability to predict the ultimate strength
No full textBackground. Rock climbing protection devices are crucial for climbing practice safety and for mountaineering in general. The use of these devices, together with appropriate tech-niques, reduces injuries in the critical event of a climber’s fall. Although European stan-dards and rules support the manufacturer in the design, production and laboratory test-ing, a thorough investigation of their behaviour in a real environment and during an actual placement has not yet been performed. Methods. The aim of this work is to present an insight into the strength of such devices through the application of a monitored, quasi-static, increasing force in a field environ-ment. Results from several types of devices (pitons, nuts and cams) are presented and crit-ically evaluated with respect to the values of the loads acting on the anchors due to the fall of the climber. Results. As far as the piton actual strength is concerned, the present activities show that the characteristics requested by EN specifications and rules are functional for product qualification purposes, but of very little use when defining the load holding capabilities once the devices are in place. However, even if the actual strength does not match the requirement of the standard, the comparison with the actual load applied is fairly encour-aging. With regards to nuts and cams, it is worth underlining the importance of a correct placement: when placed correctly, the actual strength achieved by the device in the field complies and is higher than the classification of the EN standard. Moreover, an investigation of human capability to predict the ultimate strength of rock-climbing protection devices placed in the field has been carried out, with the aim of verifying the reliability of the climber’s judgement, and, possibly, improve the safety of the in-field decision-making process. Conclusions. The lesson learned from the experiments is that modern equipment shows one step better behaviour and, similarly to pitons, the device-rock coupling dictates the pairs actual strength, assuming of course a sound placement. To the author’s best knowl-edge, the present work represents the first attempt to investigate the human capabilities to assess the reliability of a protection placement in-field
An investigation into mechanical properties of the nanocomposite with aligned CNT by means of electrical conductivity
Full text linkIn the present study, a novel modelling approach based on electrical properties was proposed to replicate the mechanical behaviour of aligned carbon nanotube/polymer nanocomposites. Firstly, an electrical analytical model with Monte-Carlo method involved was established and validated by accurately predicted electric conductivity. The microstructure of the nanocomposite was then determined according to the electric property. Subsequently, a large-scale representative volume element model based on the predicted distribution of the carbon nanotubes was built to replicate the mechanical response of the nanocomposite under tension, which can be validated by existing experiments. To consider the crystalline structure of the matrix, two cases on the nanocomposites with crystalline and amorphous polymer were investigated, locating their difference on the bonding condition of the interface between CNT and matrix. Results evidenced that the electrical properties of nanocomposites can be used to identify the internal microstructure of nanocomposite. Moreover, the effects of the loading direction, the interfacial strength and the weight fraction were studied by numerical models. The reinforcement effect of the carbon nanotubes was significant when loaded along the aligned direction, but the effect was limited in the other directions. The modulus and the strength of nanocomposite were improved by the increase of the weight fraction of CNTs, while the increase of interfacial strength improves the strength of nanocomposite along CNT-aligned direction significantly, but had negligible effect on its modulus
Modeling approaches for ballistic simulations of composite materials: Analytical model vs. finite element method
Full text linkDevelopment of predictive models for woven composite materials under ballistic impact is of great importance for their further applications as protective structures in aerospace and related fields. There are mainly two numerical methodologies widely used in the community: analytical models and finite element methods. As a popular method, finite element modeling has been widely investigated and applied in ballistic simulations, which can provide accurate results. However, high time consumption and complex calculation process cannot be avoided due to the complicated fiber architecture of woven composites. Alternatively analytical modelling approaches can provide a reliable prediction for ballistic simulation through a relatively portable modeling process with a high computational efficiency. However, limited attention has been paid to replicating the ballistic behavior of deformed projectiles versus woven composites, especially with a full metal jacket projectile. Therefore, in the current work the capability of different numerical modeling methods to simulate ballistic behaviors of woven composites impacted by a full metal jacket projectile is investigated. For analytical models, an innovative approach named ghost projectile method has been proposed with the focus on the effect of the deformable jacket of the projectile during impact loading. Regarding the finite element method, damage assessment by MAT_162 in Ls-dyna was used with optimized parameters. Experimental data on a Kevlar tile impacted by a full metal jacket projectile (0.357 Magnum) was used as a reference for comparison with numerical models. The capability of the two different numerical modeling methodologies in the current work was compared with respects to the ballistic curves, load history and projectile deformation
FE coupled to SPH numerical model for the simulation of high-velocity impact on ceramic based ballistic shields
No full textPredictive models are an important tool in the design and optimization of ballistic shields. Indeed, several authors in the literature have developed numerical models for simulating high-velocity impact on ceramic-based ballistic shields which are based on the finite element method. Element erosion is usually implemented in finite element models simulating impact to remove excessively distorted elements but, it leads to energy loss, which in turns may lead to the production of incorrect results. Due to the absence of a fixed mesh, the smoothed particle hydrodynamics method is well suited for large deformation problems, overcoming the limitations of the finite element method. On the other hand, the smoothed particle hydrodynamics method is computationally more expensive than the finite element method. Thus, a numerical model combining the lower computational cost of finite elements and the capability of smoothed particle hydrodynamics of dealing with crack formation and fracturing would be an interesting solution for the simulation of high-velocity impact on ceramics. The aim of this work is therefore to develop a finite element coupled to smoothed particle hydrodynamics numerical model for the simulation of high-velocity impact on ceramic-based ballistic shields. High-velocity impact tests were performed on Al2O3 tiles and the experimental results were used for the calibration of the numerical models; furthermore, high-velocity impact test were performed on multilayer targets with Al2O3 front layer and AA6061-T6 backing layer for the validation of the numerical models. This study proved that this approach is more appropriate for the simulation of the response of ceramic materials rather the common finite element model
Numerical investigation on the uniaxial compressive behaviour of an epoxy resin and a nanocomposite
Full text linkThe current work aims at exploring the relationship between complex failure behaviour and the presence of defects for RTM6 epoxy resin as well as hyperbranched polyester (HBP)/RTM6 nanocomposite under compressive loading. Numerical simulations were performed in LS-DYNA by means of a statistical approach that exploits different failure strains among elements. It allows a phenomenological description of the effect of defects and different stress triaxialities on the failure modes of polymer/nanocomposite materials. Additionally, a parameter describing defects, named defect severity, was added to the model in order to quantify and explore the effect of defects on the mechanical behaviour during the damage process. Both the generalized incremental stress-state dependent damage model (GISSMO) and Monte Carlo method were employed to simulate the effect of stress triaxiality and the spatial distribution of defects on the mechanical performances. The relationship between the defect severity and the failure modes (tensile-domain and shear-domain) was also discussed. Numerical results of neat RTM6 showed that the presence of a large number of defects can lead to more brittle (tensile-domain) failure, while numerical results of HBP/RTM6 nanocomposite presented that the addition of nanoparticles can compensate the negative effect of the existing defects in polymer materials under uniaxial compression, which provides a novel insight for potential applications of nanomaterials
Analytical and empirical methods for the characterisation of the permanent transverse displacement of quadrangular metal plates subjected to blast load: Comparison of existing methods and development of a novel methodological approach
Full text linkThe behaviour of blast loaded structures has been extensively investigated over the past fifty years through experimental tests. These tests are quite challenging and require dedicated infrastructures to be efficiently and safely performed, however, the obtained data are useful to develop predictive approaches. Among them, analytical approaches are capable of efficient and satisfactory characterisation of blast events and the related effects on structures. In particular, among the analytical methods two categories can be identified: those methods exploiting fully analytical relationships, e.g., the Jones’ theory, and those based on model fitting to experimental results, e.g., the Nurick and Martin's method. More recently, numerical models have been proposed to define the response of structures to blast loads: the main numerical methods considered to assess the structural response to blast loading are the coupled Eulerian-Lagrangian, uncoupled Eulerian-Lagrangian and Analytical-Lagrangian analyses. In this context, this paper aims at establishing a detailed comparison of the main fully analytical and empirical methods available in the literature, exploiting consolidated experimental evidence and results from numerical simulations. The focus of this work is on the estimation of the permanent transverse deflection of a quadrangular, initially flat plate subjected to blast loading, considering both close-range and far-field explosions. Moreover, a modelling framework is herein presented, which serves as a fast and reliable predictive tool for estimating blast load effects on plates
Calibration of the material parameters of a CFRP laminate for numerical simulations
No full textThe aim of present paper is to show a procedure to calibrate mechanical properties to be used in a finite element model for a carbon fibre-reinforced plastic laminate that use solid elements. A reduced experimental programme including tensile test, tensile test on specimen with a central hole, three-point bending test and three-point bending test on short beam test were carried out. Every test was numerically reproduced by means of an explicit solver. Properties are determined from the tensile test and unmodified for the other load scenarios which are used as validation benchmarks. Finally, it is demonstrated that the properties determined with the simple tensile tests can guarantee accurate results when adopted to simulate much more complicated stress patterns
Numerical Investigation of the Effect of Open Holes on the Impact Response of CFRP Laminates
Full text linkThe presence of open holes changes the behaviour of composite laminates when subjected to mechanical loads creating critical zones with a high probability of interlaminar and intralaminar damage initiation. While open holes in composite laminates are a requirement in many situations such as assembly needs, wiring, and maintenance access, their influence on the impact response of composite laminates is still poorly understood. In this paper, a numerical study was performed on Carbon Fibre Reinforced Polymer (CFRP) composite laminates with open holes subjected to low velocity impacts. The influence of the distance between open holes to impact origin, hole diameter, and the number of open holes on mechanical response and failure was studied using a FE model based on the inter-fibre failure criterion of Cuntze to account for the progressive intralaminar failure. The interlaminar failure was considered by using zero thickness cohesive elements based on the cohesive zone model. The results showed that i) open holes change the shape and size of the damage caused by low velocity impact and ii) that the presence of an open hole close to the impact origin in-plane spread of damage is stopped resulting in more severe damage and a smaller projected damage area compared to the control specimen. In addition, the presence of open holes in most cases did not change the locality of the low velocity impact but rather changed the severity of the damage in the local impact zone
- …
