High velocity impact characteristics of MWNT added CFRP at LEO space environment

In this paper, multi-wall carbon nanotube (MWNT) added carbon fiber reinforced plastics (CFRP) composites are suggested as solutions to improve the impact energy absorbing capability of CFRP for spacecraft application because it was proven that the resistance against LEO environment and the quasi-static material properties of CFRP can be improved by adding MWNT in previous papers. To verify the effect of MWNT on the impact energy absorbing capability of composite materials, normal CFRP and MWNT-reinforced CFRP were prepared and tested by using a two-stage light gas gun that can accelerate an aluminum ball of a diameter of 5.56 mm to 1 km/s. And the applicability of MWNT against hypervelocity impact of space debris was studied. In addition, accelerated ground simulation experiments were performed for each material model to simulate the aging of composite materials to verify the effect of LEO environmental aging on impact absorbing capability of composites. For the aging experiment, the impact specimens were simultaneously exposed to high vacuum, atomic oxygen, ultra violet light, and thermal cycling. After being exposed to simulated LEO environment, high velocity impact tests were performed for each material. As a result, MWNT did not have a significant improvement on the impact energy absorbing capability of CFRP under high velocity impact, even though the quasi static material properties are improved by adding MWNT. This is caused by the early generation of fiber breakages on the impact surface before enough generation of progressive failure which is one of the impact energy absorbing mechanism. Similarly, MWNT has less effect on the impact energy absorbing capability of CFRP under LEO environment.     

Exploring the recovery of fatigue damage in bituminous mixtures: the role of rest periods

Recovery capability of bituminous materials plays a significant role in the development of new technologies for extending the service life of asphalt pavements. This capability originates from various phenomena such as thixotropy, cooling, relaxation of hardening, or healing. However, their real effect on mechanical response is not clear. This article aims to investigate how rest periods (RPs) available between traffic loads can contribute to the damage recovery of bituminous materials. For this purpose, different types and durations of RPs were applied during the laboratory evaluation of fatigue resistance of these materials using the University of Granada Fatigue Asphalt Cracking Test method. The results indicate that the addition of RPs to the loading regime could lead to an extension in the fatigue life of bituminous materials. Additionally, an increase in the RP duration showed a positive impact on the resistance of the materials against cyclic loading. Nonetheless, these benefits are not only related to the recovery of lost properties during RPs, but also a growth in the amount of plastic deformations as a result of the applying RPs could delay the appearance of damages (i.e. cracking). Consequently, the bituminous material can tolerate a higher number of load cycles during fatigue test.     

Numerical simulations of low-velocity impact on an aircraft sandwich panel

The potential hazards resulting from a low-velocity impact (bird-strike, tool drop, runway debris, etc.) on aircraft structures, such as engine nacelle or a leading edge, has been a long-term concern to the aircraft industry. Certification authorities require that exposed aircraft components must be tested to prove their capability to withstand low-velocity impact without suffering critical damage.

This paper describes the results from experimental and numerical simulation studies on the impact and penetration damage of a sandwich panel by a solid, round-shaped impactor. The main aim was to prove that a correct mathematical model can yield significant information for the designer to understand the mechanism involved in the low-velocity impact event, prior to conducting tests, and therefore to design an impact-resistant aircraft structure.

Part of this work presented is focused on the recent progress on the materials modelling and numerical simulation of low-velocity impact response onto a composite aircraft sandwich panel. It is based on the application of explicit finite element (FE) analysis codes to study aircraft sandwich structures behaviour under low-velocity impact conditions. Good agreement was obtained between numerical and experimental results, in particular, the numerical simulation was able to predict impact damage and impact energy absorbed by the structure.     

Finite element simulation of low velocity impact on shape memory alloy composite plates

Delamination of composite materials due to low velocity impacts is one of the major failure types of aerospace composite structures. The low velocity impact may not immediately induce any visible damage on the surface of structures whilst the stiffness and compressive strength of the structures can decrease dramatically.

Shape memory alloy (SMA) materials possess unique mechanical and thermal properties compared with conventional materials. Many studies have shown that shape memory alloy wires can absorb a lot of the energy during the impact due to their superelastic and hysteretic behaviour. The superelastic effect is due to reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and the SMA undergoes a large hysteresis loop and a large recoverable strain is obtained. This large strain energy absorption capability can be used to improve the impact tolerance of composites. By embedding superelastic shape memory alloys into a composite structure, impact damage can be reduced quite significantly.

This article investigates the impact damage behaviour of carbon fiber/epoxy composite plates embedded with superelastic shape memory alloys wires. The results show that for low velocity impact, embedding SMA wires into composites increase the damage resistance of the composites when compared to conventional composites structures.     

Impact Response Characterization in Composite Plates—Experimental Validation

This paper presents an experimental study for the normalized low-velocity impact response of composite plates. It is demonstrated that a characterization diagram that shows the relationship of three non-dimensional parameters with the normalized maximum impact force can be used to fully characterize the response. With the governing non-dimensional parameters obtained experimentally, it is shown that impact tests having the same non-dimensional parameters, have dynamic similarity and the same non-dimensional response. Furthermore, the experiments can be placed in appropriate dynamic regions in which simplified dynamic models can be used to predict the response.     

碳基材料超高速粒子侵蚀的数值模拟

应用动力学理论建立了碳基材料超高速粒子侵蚀的数学模型。在模型中引入了表征材料抵抗侵蚀破坏能力的参数: 冲击破坏侵蚀能和剪切破坏侵蚀能。通过定义多重碰撞修正因子β, 给出了多粒子侵蚀材料体积损失的理论计算公式。计算了石墨和C/ C 靶材在Al₂O₃粒子撞击下, 体积损失比随粒子入射角及入射速度的变化规律。结果表明, 脆性碳基材料在超高速粒子撞击作用下, 冲击破坏与剪切破坏同时发生, 冲击破坏在材料体积损失中起主导作用。计算结果与已有的实验值吻合较好。      

Progress of ecomaterials toward a sustainable society

This review paper provides an overview of research activities in Japan in the field of ecomaterials. Ecomaterials are the materials which are used in the life-cycle design of products aimed at reducing environmental impact. Ecomaterials research includes the fields of ‘materials containing less hazardous substances’, ‘materials with green environmental profiles’, ‘materials with a higher potential for recycling’, and ‘materials with higher resource productivity’. They are evolving from being an idealised concept to produce real solutions for materials selection. As part of this trend, multi-performance capability, as required for usage including processability, is becoming an important problem to be solved by materials scientists and engineers.     

Plate impact testing method for GRC materials

The paper reports on the impact performance evaluation of glass fibre reinforced cement (GRC) materials. The paper includes a review of the principles of impact testing methodology and a proposal for a new test method consisting of a drop weight instrumented impact on plate specimens. The significance of impact test data is discussed with regard to the parasitic effects on impact load versus time records. An application of digital filtering treatment provided the load-displacement characteristics reflecting the specimen response alone. The following impact features of the GRC elements were determined: the maximum impact load, the energy absorbed up to the maximum load and the energy absorbed up to total failure. The energy absorption capability of GRC plates was studied as a function of plate thickness and impact velocity. A comparison of the impact data and the reference quasi-static tests yielded a ratio of impact-to-static energy absorption of 1·7–1·8 for the GRC elements considered.     

A review of recent research on mechanics of multifunctional composite materials and structures

In response to the marked increase in research activity and publications in multifunctional materials and structures in the last few years, this article is an attempt to identify the topics that are most relevant to multifunctional composite materials and structures and review representative journal publications that are related to those topics. Articles covering developments in both multiple structural functions and integrated structural and non-structural functions since 2000 are emphasized. Structural functions include mechanical properties like strength, stiffness, fracture toughness, and damping, while non-structural functions include electrical and/or thermal conductivity, sensing and actuation, energy harvesting/storage, self-healing capability, electromagnetic interference (EMI) shielding, recyclability and biodegradability. Many of these recent developments are associated with polymeric composite materials and corresponding advances in nanomaterials and nanostructures, as are many of the articles reviewed. The article concludes with a discussion of recent applications of multifunctional materials and structures, such as morphing aircraft wings, structurally integrated electronic components, biomedical nanoparticles for dispensing drugs and diagnostics, and optically transparent impact absorbing structures. Several suggestions regarding future research needs are also presented.     

Illuminating Origins of Impact Energy Dissipation in Mechanical Metamaterials

Elastomeric mechanical metamaterials have revealed striking ability to attenuate shock loads at the macroscopic level. Reports suggest that this capability is associated with the reversible elastic buckling of internal beam constituents observed in quasistatic characterizations. Yet, the presence of buckling members induces non‐affine response at the microscale, so that clear understanding of the exact energy dissipation mechanisms remains clouded. In this report, the authors examine a mechanical metamaterial that exhibits both micro‐ and macroscopic deformations under impact loads and devise an experimental method to visualize the resulting energy dissipation mechanisms. By illuminating the dynamic distribution of strain in the metamaterial, the authors uncover a rational way to program the macroscopic deformation and enhance impact mitigation properties. The results emphasize that mechanical metamaterials clearly integrate materials science and structural engineering, encouraging future interdisciplinary studies to capitalize on the opportunities.

     

X-ray microtomography analysis of the damage micromechanisms in 3D woven composites under low-velocity impact

3D woven composites reinforced with either S2 glass, carbon or a hybrid combination of both and containing either polyethylene or carbon z-yarns were tested under low-velocity impact. Different impact energies (in the range of 21–316 J) were used and the mechanical response (in terms of the impact strength and energy dissipated) was compared with that measured in high-performance, albeit standard, 2D laminates. It was found that the impact strength in both 2D and 3D materials was mainly dependent on the in-plane fiber fracture. Conversely, the energy absorption capability was primarily influenced by the presence of z-yarns, having the 3D composites dissipated over twice the energy than the 2D laminates, irrespective of their individual characteristics (fiber type, compaction degree, porosity, etc.). X-ray microtomography revealed that this improvement was due to the z-yarns, which delayed delamination and maintained the structural integrity of the laminate, promoting energy dissipation by tow splitting, intensive fiber breakage under the tup and formation of a plug by out-of-plane shear.     

Experimental Designs for Constrained Regions

The familiar factorial, fractional factorial, and response surface designs are designs for regularly-shaped regions of interest, typically cuboidal regions and spherical regions. An irregularly shaped region of experimentation arises in situations where there are constraints on the factor level combinations that can be run or restrictions on portions of the region of exploration. Computer-generated designs based on some optimality criterion are a logical alternative for these problems. We give a brief tutorial on design optimality criteria and show how one of these, the D-optimality criteria, can lead to very reasonable designs for constrained regions of interest. We show through a simulation study that D-optimal designs perform very well with respect to the capability of selecting the correct model and accurately estimating the design factor levels that result in the optimal response.     

Mechanical properties of textile-inserted PP/PP knitted composites using injection–compression molding

This paper concentrates on the experimental investigation of the self-reinforced all-polypropylene composites. There exists an optimum processing condition to produce high quality specimens by injection–compression molding. Tensile and 3-point bending properties of the virgin PP materials were nearly unaffected by the introduction of reinforcing knit layer(s) due to very low fibre content of the knitted fabrics used. 3-point bending properties were also unaffected by the surface of indentation-flexure. The applied impact energy was maintained at 5 J for the homo-PP and 27 J for the block-PP materials, respectively, to cause penetration during drop-weight impact tests. It is interestingly noteworthy that the self-reinforced homo-PP composites exhibited superior energy absorption capability when compared with the virgin matrix materials. The corresponding plate bending performances of the self-reinforced homo-PP composites also revealed consistent improvement as compared to their virgin counterparts. On the other hand, although virgin block-PP material exhibited better impact performances than its composite reinforced by the homo-PP knitted fabric, a notably small increase in the reinforcement fibre content revealed considerable improvement in the impact properties comparable to those of the virgin block-PP matrix materials. These self-reinforced homo-PP/block-PP materials have clearly indicated that they have the potential to out-perform the block-PP materials via modification and/or manipulation of the reinforcement knit structural/geometrical parameters and the content of reinforcement fibres. Both static and dynamic impact properties are likely to be affected by the local area properties of the tested face under indentation, and thereby contributing to the improved performances of the composite specimens with the knit face under the impact.     

Integral transform procedures for forced response of composite domain micropolar elasticity problems

A piecewise weighted orthogonality principle is developed for the free vibration modes of 3-D composite configurations consisting of finitely many distinct subdomains modelled by micropolar elasticity theory. Based on this orthogonality procedure a new piecewise weighted integral transform technique is developed which can handle the forced response of composite domain configurations which are subject to time dependent external and interdomain inhomogeneities caused by either thermal or prestress fields. The techniques generality is such that the response of composite configurations with time dependent materials can also be handled. To illustrate its capability, the piecewise weighted integral transform technique is specialized to treat the forced response of a technically important problem.     

Nanotechnology and the environment: A European perspective

The potential positive and negative effects of nanotechnology on the environment are discussed. Advances in nanotechnology may be able to provide more sensitive detection systems for air and water quality monitoring, allowing the simultaneous measurement of multiple parameters and real time response capability. Metal oxide nanocatalysts are being developed for the prevention of pollution due to industrial emissions and the photocatalytic properties of titanium dioxide nanoparticles can be exploited to create self-cleaning surfaces that reduce existing pollution. However, while nanotechnology might provide solutions for certain environmental problems, relatively little is known at present about the environmental impact of nanoparticles, though in some cases chemical composition, size and shape have been shown to contribute to toxicological effects. Nanotechnology can assist resource saving through the use of lightweight, high strength materials based on carbon nanotubes and metal oxide frameworks as hydrogen storage materials. Other energy related applications include nanostructured electrode materials for improving the performance of lithium ion batteries and nanoporous silicon and titanium dioxide in advanced photovoltaic cells. It is important to develop an efficient strategy for the recycling and recovery of nanomaterials and methods are needed to assess whether the potential benefits of nanotechnology outweigh the risks. Life cycle analysis will be a useful tool for assessing the true environmental impacts.     

Impact of Thin Film Thermophysical Properties on Thermal Management of Wide Bandgap Solid-State Transistors

Wide bandgap semiconductor solid-state transistors continue to have a wide array of applications that include power supplies, communications, electronic warfare, and multifunctional RF systems. Two viable wide bandgap semiconductor materials currently under investigation are silicon carbide (SiC) and gallium nitride (GaN). One interesting aspect of these devices is their ability to operate at elevated temperatures on the order of 500°C. At higher temperatures the heat capacity of semiconductors is constant, while the phonon mean free path is inversely proportional to the lattice temperature. This causes a significant reduction in the thermal conductivity over the operating temperature range. The submicron-scale conducting channels and junctions of wide bandgap devices can create highly localized heat fluxes on the order of several hundred kilowatts per square centimeter. Since these heat fluxes lead to localized hot spots within the electrically critical regions of the transistors, they can have a strong impact on device gain, power capability, and reliability. Quantifying the thermophysical properties of the underlying thin film materials is of critical importance for the accurate prediction of these localized temperature extremes.     

Two-dimensional imaging biosensor for the monitoring of lactate released from brain slices

Real-time monitoring of lactate release from brain slices has been studied with an optical two-dimensional (2D) imaging biosensor. The 2D biosensor is prepared by direct immobilization of lactate dehydrogenase (LDH) molecules onto a flat silica glass surface through a covalent binding mechanism. The biosensor is able to spatially differentiate lactate concentration variations with conventional optical microscopic spatial resolution. This biosensor has the capability to effectively detect lactate down to a concentration of 100 nM. The 2D biosensor responds uniformly with 2.5% RSD from pixel to pixel. With a 100 ms response time, this 2D biosensor has the capability of monitoring simultaneously many cells in one image. We have studied the impact of KCI on lactate release from brain slices. Clear differences have been observed in lactate release for different regions of the tissue. The real-time determination of the newly released lactate from the mouse brain slices clearly demonstrates the feasibility of monitoring lactate release from living specimens. The 2D biosensor will enable us to study cellular communications and possibly other biological processes that require simultaneous temporal and spatial resolution.     

A computationally efficient metamodeling approach for expensive multiobjective optimization

This paper explores a new metamodeling framework that may collapse the computational explosion that characterizes the modeling of complex systems under a multiobjective and/or multidisciplinary setting. Under the new framework, a pseudo response surface is constructed for each design objective for each discipline. This pseudo response surface has the unique property of being highly accurate in Pareto optimal regions, while it is intentionally allowed to be inaccurate in other regions. In short, the response surface for each design objective is accurate only where it matters. Because the pseudo response surface is allowed to be inaccurate in other regions of the design space, the computational cost of constructing it is dramatically reduced. An important distinguishing feature of the new framework is that the response surfaces for all the design objectives are constructed simultaneously in a mutually dependent fashion, in a way that identifies Pareto regions for the multiobjective problem. The new framework supports the puzzling notion that it is possible to obtain more accuracy and radically more design space exploration capability, while actually reducing the computation effort. This counterintuitive metamodeling paradigm shift holds the potential for identifying highly competitive products and systems that are well beyond today’s state of the art.     

Comparison of the low and high velocity impact response of cfrp

A series of low and high velocity impact tests has been conducted on a wide range of cfrp laminates to examine the initiation and development of damage under these two widely differing loading conditions. For conditions of low velocity impact loading the size and shape of the target determines its energy-absorbing capability and therefore its impact response. High velocity impact loading by a fast moving projectile induces a localized form of target response and the level of damage incurred does not, therefore, appear to be governed by the areal size of the component. The effect of such loading on the residual tensile strength has also been assessed. High velocity impact loading by a small projectile is generally more detrimental to the integrity of a composite structure than low velocity drop-weight impact loading.     

Effect of microscopic damage events on static and ballistic impact strength of triaxial braid composites

The reliability of impact simulations for aircraft components made with triaxial braided carbon fiber composites is currently limited by inadequate material property data and lack of validated material models for analysis. Methods to characterize the material properties used in the analytical models from a systematically obtained set of test data are also lacking. A macroscopic finite element based analytical model to analyze the impact response of these materials has been developed. The stiffness and strength properties utilized in the material model are obtained from a set of quasi-static in-plane tension, compression and shear coupon level tests. Full-field optical strain measurement techniques are applied in the testing, and the results are used to help in characterizing the model. The unit cell of the braided composite is modeled as a series of shell elements, where each element is modeled as a laminated composite. The braided architecture can thus be approximated within the analytical model. The transient dynamic finite element code LS-DYNA is utilized to conduct the finite element simulations, and an internal LS-DYNA constitutive model is utilized in the analysis. Methods to obtain the stiffness and strength properties required by the constitutive model from the available test data are developed. Simulations of quasi-static coupon tests and impact tests of a represented braided composite are conducted. Overall, the developed method shows promise, but improvements that are needed in test and analysis methods for better predictive capability are examined.