Modelling impact response of agglomerated cork

Cellular materials have been intensively used in engineering applications where a good energy absorption capability is a desired feature. Cork is a natural cellular material capable of absorbing considerable amounts of energy. When compared to synthetic cellular materials, cork also appears as a sustainable alternative, once it is fully recyclable. The purpose of this work is to simulate cork’s compressive behaviour when subjected to impact, including the material’s relaxation after dynamic compression. This study comprises experimental and numerical tests at quasi-static and dynamic strain rates under axial compressive loading. Numerical simulations are performed using Finite Element Analysis, and the material model developed is validated against experimental results. After validation, a dynamic test resorting to a drop tower is carried out successfully validating the model and representing adequately cork’s mechanical behaviour under dynamic compressions.     

Sandwich composites impact and indentation behaviour study

In order to better exploit the natural cork available in Algeria, an experimental characterisation of a jute/epoxy–cork sandwich material to impact and indentation was undertaken. The aim of this work is to evaluate the impact energy and cork density influence over the sandwich plate damage behaviours by instrumented static and dynamic tests. The results show that the onset damage force, the maximum force and the damage size are influenced by the cork density and the impact energy. The sandwich material, with the heavy agglomerated cork having a density of 310 kg/m³ is characterised by a weaker energy dissipation capacity, by about 3.72% for impact test and 3.29% for indentation one, than the sandwich with lighter cork (160 kg/m³). This difference is an infusion process consequence. The infiltrated resin into the agglomerated cork pores changes the material local rigidity. Also, under impact loading the sandwich laminates dissipate 11% more energy than with the quasi-static indentation test.     

Experimental study of agglomerated-cork-cored structures subjected to ballistic impacts

The present paper examines the high-velocity impact behaviour of agglomerated cork-cored structures. The ballistic performance was studied by impact-perforation tests. Three different types of specimens were tested: an agglomerated cork, two spaced thin aluminium plates, and a pair of thin aluminium plates separated by an agglomerated-cork core. The behaviour of the agglomerated cork and the effects of the cork core were analysed in terms of the ballistic limit, residual velocity, and energy absorption. The ballistic limit of cork-cored structures increased slightly, whereas the absorbed energy was strongly augmented by the presence of the cork core.     

How does hydration affect the mechanical properties of wine stoppers?

Data related to the comparison of the mechanical properties of the different stoppers used in the wine industry are scarce. This study aims at comparing the effect of hydration (from 0 to 100 % relative humidity at 25 °C) on the mechanical properties of four widely used types of stoppers: natural corks, agglomerated corks, technical stoppers and synthetic (co-extruded) stoppers. For both natural and agglomerated corks, the Young’s modulus was significantly and similarly affected by hydration, with a constant plateau value up to 50 % relative humidity (RH) and a mean value around 22 and 14 MPa, respectively. For higher RH, the increase in water content leads to a decrease in the material rigidity (Young’s modulus <10 MPa), which is attributed to water clusters formation between polymer chains. Technical stoppers revealed a similar profile, but with a much smaller impact of the water content and with overall lower Young’s moduli values, around 5 MPa, throughout the RH range. The stiffness of synthetic closures was not affected by hydration, in agreement with the hydrophobic behavior of polyethylene. Differential scanning calorimetry and dynamic mechanical thermal analysis allowed us to identify a glass transition temperature (T _g) in cork (around 0 °C), and another one in agglomerated cork and technical stoppers (close to ?45 °C, corresponding to additives). All together, for the first time the data highlight the comparative mechanical properties of such materials of the wine industry, and the progressive loss of the “cork-like” behavior of cork composites when other components are mixed with cork.     

发泡聚丙烯材料的动态力学行为研究

目的研究发泡聚丙烯材料的厚度对其冲击性能的影响。方法对4种不同厚度的发泡聚丙烯材料进行动态压缩试验,分析其接触力、最大位移、最大应变以及吸收能,研究动态条件下发泡聚丙烯材料的力学性能。结果当冲击能一定时,增加发泡聚丙烯材料的厚度,其接触力会逐渐减小,接触时间会逐渐增加;冲击能和厚度一定时,厚度与最大位移、吸收能成正比例相关,但对最大接触力和最大应变无明显影响;任意厚度的发泡聚丙烯材料,其冲击能和厚度的增加会导致其最大接触力、最大位移、最大应变、吸收能的增加。结论在研究的冲击能量和厚度范围内,吸收能不受发泡聚丙烯材料厚度的影响,由冲击能决定。     

New composite liners for energy absorption purposes

This work studies the capacity of cork to act as material for the absorption of impact energy. Focus is given on the viability of hybrid paddings consisting of micro-agglomerate cork (MAC) and expanded polystyrene (EPS) through simulations of multi-impacts. EPS is a widely used material for energy absorption applications. However, once deformed, it shows no springback, which means that its capacity for energy absorption is greatly reduced after the first impact. On the other hand, cork is a viscoelastic material that has a good level of energy absorption capacity with almost total springback. An example in which EPS is commonly used is motorcycle helmet liners. In the first part of this work, a compression test is used to assess the effectiveness of the material laws chosen to model the cellular materials under study. Results show that the constitutive laws employed for EPS and micro-agglomerate cork adequately model the actual behaviour in springback absence. In the second part of this work, it was developed a simplified model of a road helmet energy absorption liner. This representative padding was subjected to double impacts as specified in an international helmet standard. The work also studies the use of MAC and EPS arrangements on the energy absorption linear through various configurations. Results were obtained with regard to the head centre of gravity acceleration, final padding thickness and the final weight of a helmet. Conclusions are drawn about the best configuration for the application under study.     

Cork agglomerates as an ideal core material in lightweight structures

The experiments carried out in this investigation were oriented in order to optimize the properties of cork-based agglomerates as an ideal core material for sandwich components of lightweight structures, such as those used in aerospace applications. Static bending tests were performed in order to characterize the mechanical strength of different types of cork agglomerates which were obtained considering distinct production variables. The ability to withstand dynamic loads was also evaluated from a set of impact tests using carbon-cork sandwich specimens. The results got from experimental tests revealed that cork agglomerates performance essentially depends on the cork granule size, its density and the bonding procedure used for the cohesion of granulates, and these parameters can be adjusted in function of the final application intended for the sandwich component. These results also allow inferring that optimized cork agglomerates have some specific properties that confirm their superior ability as a core material of sandwich components when compared with other conventional materials.     

Compressive response of glass–fiber reinforced polymeric composites to increasing compressive strain rates

The utilization of composite materials instead of traditional materials in structural high-speed applications has induced the need for a proper knowledge of dynamic behavior as well as static behavior of them. The material and structural response vary significantly under dynamic loading as compared to static loading conditions. In order to investigate the dynamic responses of composite materials under dynamic loading at various strain rates, special testing machines are needed. Most of the researches in this field are focused on applying real loading and gripping boundary conditions on the testing specimens.The present study is carried out in order to characterize the compressive properties of unidirectional glass–fiber reinforced polymeric composites using a servo-hydraulic testing apparatus at varying strain rates, ranging from 0.001 to 100 s⁻¹. For performing practical tests, a jig and a fixture are designed and manufactured, which could insure the alignment of axial loads on the specimens. During of tests, the performance of the test jig is evaluated. It is found that the designed jig and the fixture perform very well during the test process. The results of the dynamic tests are compared with the results of the static tests carried out on specimens with identical geometry. Based on the experimental results obtained from the tests, empirical functions for the mechanical properties are proposed in terms of strain rates. The results of the study indicate that strain rate has a significant effect on the material response. It is found that the compressive strength and modulus both increased with increasing the strain rate. Also, the results show that the compressive strain to failure is generally insensitive to strain rate.     

Impact and compression after impact experimental study of a composite laminate with a cork thermal shield

The aim of this paper is to present an experimental study of impact and compression after impact (CAI) tests performed on composite laminate covered with a cork thermal shield (TS) intended for launchers fairing. Drop weight impact tests have been performed on composite laminate sheets with and without TS in order to study its effect on the impact damage. The results show the TS is a good mechanical protection towards impact as well as a good impact revealing material. Nevertheless, totally different damage morphology is obtained during the impact test with or without TS, and in particular at high impact energy, the delaminated area is larger with TS. Afterwards, CAI tests have been performed in order to evaluate the TS effect on the residual strength. The TS appears to increase the residual strength for a same impact energy, but at the same time, it presents a decrease in residual strength before observing delamination. In fact, during the impact tests with TS, invisible fibres’ breakages appear before delamination damage contrary to the impacts on the unshielded sheets.     

高柔性隔热软木复合材料的制备与性能

以单组分湿固化的聚氨酯(U030)作为胶黏剂制备的软木复合材料(RC16),密度为0.40g/cm~3,具有优异的柔韧性和力学性能。考察了胶黏剂的含量、软木粒子的含水率、固化条件等因素对RC16软木隔热性能、柔韧性及力学性能的影响。结果表明:合成的胶黏剂U030与软木粒子之间具有良好的相容性和粘接性,可完全包裹软木粒子且分散均匀;材料的力学性能随着软木粒子含水率的增加呈现先提高后降低的趋势,在7.0%时达到最大值;随着U030含量的增加,RC16软木的各项性能均呈现先上升后趋于稳定的趋势,但其热导率也相应提高,当U030含量为30%时材料的综合性能最佳;确定RC16软木的最佳固化工艺条件为:固化温度110℃,固化时间1.5h;RC16软木具有良好的耐热性,可在20s内承受800℃高温烧蚀。     

Bioinspired Materials with Self-Adaptable Mechanical Properties

Natural structural materials, such as bone, can autonomously modulate their mechanical properties in response to external loading to prevent failure. These material systems smartly control the addition/removal of material in locations of high/low mechanical stress by utilizing local resources guided by biological signals. On the contrary, synthetic structural materials have unchanging mechanical properties limiting their mechanical performance and service life. Inspired by the mineralization process of bone, a material system that adapts its mechanical properties in response to external mechanical loading is reported. It is found that charges from piezoelectric scaffolds can induce mineralization from surrounding media. It is shown that the material system can adapt to external mechanical loading by inducing mineral deposition in proportion to the magnitude of the stress and the resulting piezoelectric charges. Moreover, the mineralization mechanism allows a simple one-step route for fabricating functionally graded materials by controlling the stress distribution along the scaffold. The findings can pave the way for a new class of self-regenerating materials that reinforce regions of high stress or induce deposition of minerals on the damaged areas from the increase in mechanical stress to prevent/mitigate failure. It is envisioned that the findings can contribute to addressing the current challenges of synthetic materials for load-bearing applications from self-adaptive capabilities.     

Dynamic tensile behaviour of fibre reinforced concrete with spiral fibres

This paper performs drop-weight splitting tests to study the dynamic tensile properties of fibre reinforced concrete (FRC) materials with different steel fibres. A renovated splitting tensile testing method was developed to ensure a more qualified experimental process. The splitting tensile impact tests were conducted with an instrumented drop-weight impact system consisting of a hard steel drop weight, a fast-response load cell, a high-speed video camera and a high-frequency data acquisition system. The quasi-static compressive and splitting tests were also conducted to obtain the static properties of the FRC materials. The commonly used hooked-end steel fibre and a new spiral shaped steel fibre were tested in this study. The high-speed video camera was used to capture the detailed failure process, deformation and cracking process of the tested specimens. Average strain rates and the cracking extension displacement and velocity under impact loading were estimated by analysing the recorded high-speed images. The strains were also measured by the strain gages on the specimen surface. The dynamic stress–strain and stress–COD (cracking opening displacement) relations, the rate sensitivity of tensile strength and the corresponding energy absorption capacity of plain concrete and FRC with different fibres were obtained, compared and discussed. The advantage and effectiveness of the new spiral fibre in increasing the performance of FRC under dynamic tensile loading were examined. The results show that FRC with spiral fibres outperforms that with hooked-end fibres, and is a promising construction material in resisting dynamic loadings.     

Evaluation of the performance of recycled textile fibres in the mechanical behaviour of a gypsum and cork composite material

Given the need for using more sustainable constructive solutions, an innovative composite material based on a combination of distinct industrial by-products is proposed aiming to reduce waste and energy consumption in the production of construction materials. The raw materials are thermal activated flue-gas desulphurization (FGD) gypsum, which acts as a binder, granulated cork as the aggregate and recycled textile fibres from used tyres intended to reinforce the material.This paper presents the results of the design of the composite mortar mixes, the characterization of the key physical properties (density, porosity and ultrasonic pulse velocity) and the mechanical validation based on uniaxial compressive tests and fracture energy tests. In the experimental campaign, the influence of the percentage of the raw materials in terms of gypsum mass, on the mechanical properties of the composite material was assessed.It was observed that the percentage of granulated cork decreases the compressive strength of the composite material but contributes to the increase in the compressive fracture energy. Besides, the recycled textile fibres play an important role in the mode I fracture process and in the fracture energy of the composite material, resulting in a considerable increase in the mode I fracture energy.     

近海大气环境下锈蚀平面钢框架抗震性能试验研究及有限元分析

为研究近海大气环境下锈蚀钢框架结构的抗震性能,采用人工气候加速腐蚀技术对42件钢材材性试件和4榀平面钢框架进行加速腐蚀试验,然后对不同锈蚀程度的钢材材性试件进行拉伸破坏试验,获得锈蚀Q235B钢材力学性能指标（屈服强度、极限强度、伸长率和弹性模量）与其失重率的函数关系。并对4榀平面钢框架进行低周往复加载试验,研究了锈蚀对平面钢框架的破坏机制、滞回曲线、骨架曲线、刚度退化、延性及耗能能力等的影响。结果表明:4榀平面钢框架试件均发生了延性较好的破坏,呈现出混合屈服机制;但随着锈蚀程度的增加,试件承载力、耗能能力显著降低,强度和刚度退化明显,延性变差。在试验研究的基础上,利用通用软件ABAQUS对试验平面钢框架进行了非线性有限元分析,研究了轴压比对其力学性能的影响,结果表明:随着轴压比的增大,试件承载力、延性不断降低。     

Impact tolerant and healable aluminum millitube reinforced shape memory polymer composite sandwich core

In order to improve impact tolerance and energy absorption of sandwich panel under impact loading, a new aluminum hollow tube reinforced shape memory polymer (AHTR-SMP) composite sandwich core is designed and fabricated. Physical/mechanical properties were examined through a variety of tests, including axial compression, three-point bending, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and shape recovery tests. In order to characterize its dynamic performances, low velocity impact test was conducted. According to the tests results, this new AHTR-SMP core demonstrated considerable impact tolerance and damage healing functionality, and may be considered as a promising option for critical structural applications featured by tolerating repeated impacts.     

Deformation and impact energy absorption of cellular sandwich panels

The response and energy absorption capacity of cellular sandwich panels that comprises of silk-cotton wood skins and aluminum honeycomb core are studied under quasi-static and low velocity impact loading. Two types of sandwich panels were constructed. The Type-I sandwich panel contains the silk-cotton wood plates (face plates) with their grains oriented to the direction of loading axis and in the case of Type-II sandwich panel, the wood grains were oriented transverse to the loading axis. In both of the above cases, aluminum honeycomb core had its cell axis parallel to the loading direction. The macro-deformation behavior of these panels is studied under quasi-static loading and their energy absorption capacity quantified. A series of low velocity impact tests were conducted and the dynamic data are discussed. The results are then compared with those of quasi-static experiments. It is observed that the energy absorption capacity of cellular sandwich panels increases under dynamic loading when compared with the quasi-static loading conditions. The Type-I sandwich panels tested in this study are found to be the better impact energy absorbers for low velocity impact applications.     

Low velocity impact tests of laminate glass-fiber-epoxy matrix composite material plates

This work deals with the characterization of composite material suitable for constructing structural parts devoted to dissipate kinetic energy during impacts. In particular glass-fiber-epoxy matrix laminates are considered, both with unidirectional layers and with woven layers stacking, with three different layers orientations.Experimental tests are performed according to ASTM standards using a free-fall drop dart testing machine. The specimens are plates completely constrained on a circular edge by the clamping fixture. Two energy absorption parameters (namely saturation impact energy and damage degree), two relevant characteristic values of the impact force history (namely the first damage force and the maximum force) and the sensitivity of the material mechanical characteristic to the strain rate effect are considered in order to describe the impact behavior of the material. Diagrams are presented to show the history of relevant kinematical, dynamic and energetic quantities, both to synthesize the dependency of the energy parameters and force threshold values on the impact velocity. The considered materials, under the considered loading conditions, show no sensitivity to the strain rate effect.     

Mechanochemical Adhesion and Plasticity in Multifiber Hydrogel Networks

The extracellular matrix (ECM) has force-responsive (i.e., mechanochemical) properties that enable adaptation to mechanical loading through changes in fibrous network structure and interfiber bonding. Imparting such properties into synthetic fibrous materials will allow reinforcement under mechanical load, the potential for material self-adhesion, and the general mimicking of ECM. Multifiber hydrogel networks are developed through the electrospinning of multiple fibrous hydrogel populations, where fibers contain complementary chemical moieties (e.g., aldehyde and hydrazide groups) that form covalent bonds within minutes when brought into contact under mechanical load. These fiber interactions lead to microscale anisotropy, as well as increased material stiffness and plastic deformation. Macroscale structures (e.g., tubes and layered scaffolds) are fabricated from these materials through interfiber bonding and adhesion when placed into contact while maintaining a microscale fibrous architecture. The design principles for engineering plasticity described can be applied to numerous material systems to introduce unique properties, from textiles to biomedical applications.     

CorkSTFμfluidics – A novel concept for the development of eco-friendly light-weight energy absorbing composites

CorkSTFμfluidics are environmentally friendly composites consisting of a laminar sheet of compacted micro-agglomerated cork engraved with a network of microchannels by laser and filled with a concentrated aqueous solution of cornstarch (shear thickening fluid). Thus the mechanical properties of these composites will result from the combination of the mechanical properties of the micro-agglomerated of cork and the enhanced shear thickening response of the STF flowing through the network of microchannels. In this work we have performed low velocity impact tests using a drop weight testing machine in order to asses the improvement of these composites with regards to the micro-agglomerated of cork alone. A numerical analysis was also performed and allowed to understand how shear thickening behavior is triggered by the appropriate combination microchannels’ size and the inlet velocity. Thus, for the 60P/40S/M sample, the peak force was larger than the one given by the cork sheets without engraved microchannel nor STF.