Permanent indentation characterization for low-velocity impact modelling using three-point bending test

This paper deals with the origin of permanent indentation in composite laminates subjected to low-velocity impact. The three-point bending test is used to exhibit a non-closure of matrix crack which is assumed as a cause of permanent indentation. According to the observation at microscopic level, this non-closure of crack is produced by the blocking of debris inside matrix cracking and the formation of cusps where mixed-mode delamination occurs. A simple physicallybased law of permanent indentation, ‘‘pseudo-plasticity’’, is proposed. This law is qualitatively tested by three-point bending finite element model and is lastly applied in low-velocity impact finite element model in order to predict the permanent indentation. A comparison between low-velocity impact experiments and simulations is presented

Mechanical behavior of transparent fiber reinforced polyester composites at extreme temperatures

Selecting materials for harsh or extreme environmental conditions can be a challenge. The combination of a harsh environment, large forces over extended periods and the need for lowest possible cost restricts the choice of materials. One potential material is glass fiber reinforced polymers that are widely used in structural systems as load bearing elements, they are relatively low cost and can be tailored to achieve a range of mechanical properties. This investigation presents the preparation of transparent glass fiber reinforced unsaturated polyester composite and the evaluation of its optical and mechanical properties under extreme conditions of temperature. The polyester resin was reinforced with E-glass fibers to manufacture a composite using the hand layup method. Transparency was achieved by modifying the refractive index of the polyester resin to match that of the glass fibers. This investigation also presents the evaluation of glass fiber reinforced unsaturated polyester under quasi-static tension loading and puncture testing using a drop weight at extreme conditions. The results showed that the reinforced composite had a higher fracture stress and chord modulus at all temperatures ranging from +60 [degree]C to -80 [degree]C as compared to the unreinforced polyester matrix. The unreinforced polyester has a higher stiffness at lower temperatures due to reduced polymer chain mobility and higher clamping pressure of the matrix on the glass fiber reinforcement. The damage created by the impact reduces with decreasing temperatures, while the energy absorb remains constant with temperature.Includes bibliographical reference

A self healing smart syntactic foam based grid stiffened sandwich structure

Syntactic foams are composite materials synthesized by dispersing microballoons in a polymeric, ceramic or metallic matrix. In the past three decades, syntactic foams have gained immense importance as a lightweight and damage-tolerant material when used in foam-cored sandwich structures. Because of the structural-length scale damages by low velocity impact such as tool drops, runway debris etc., sandwich structures usually have a very low residual structural capacity. Unfortunately, macro-length scale damage, in particular internal damage such as impact damage, is very difficult to repair. Therefore, there is a genuine need to develop impact-tolerant and self-healing syntactic foams which can be used as a core in sandwich structures. In this study, a new shape memory polymer (SMP) based syntactic foam was proposed, fabricated, characterized, and tested using DSC, TEM, SEM, and stress-controlled programming and free shape recovery by association with the foam cored sandwich. A micromechanics based model was employed to clearly visualize the microstructure and to quantify the geometrical and mechanical properties of the smart foam composite in the linear elastic region. An orthogrid stiffened SMP based syntactic foam cored sandwich was then fabricated, programmed, impacted, healed (sealed), and compression tested, for the purposes of sealing impact damage. Two impact energy levels (30J and 53J), two prestrain levels (3% and 20%), and two confinement conditions (2-D confined and 3-D confined) were used in the low velocity impact test, strain-controlled programming and constrained shape recovery, respectively. C-scan and visual observation were also conducted to visualize impact damage and evaluate the degree of sealing achieved. It is found that the shape memory functionality of the SMP based syntactic foam can be utilized for the purpose of sealing impact damage with the developed programming and shape recovery. The developed foam and the hybrid sandwich structure are able to heal (or seal) structural-length scale damage (here impact damage) repeatedly (up to 7 rounds of impact-healing cycles), efficiently (with a healing efficiency over 100%); and almost autonomously (the only human intervention is by heating). This study lays a solid foundation for the next generation of smart self-healing composite structures in engineering applications

About the impact behavior of woven-ply carbon fiber-reinforced thermoplastic- and thermosetting-composites: A comparative study

This study is aimed at comparing the response of TS-based (epoxy) and TP-based (PPS or PEEK) laminates subjected to low velocity impacts. C-scan inspections showed that impact led to diamond-shaped damage resulting from different failure mechanisms: fiber breakages in warp and weft directions, more or less inter-laminar and intra-ply damage, and extensive delamination in C/PEEK and C/epoxy laminates. The permanent indentation can be ascribed to specific mechanisms which mainly depend on many factors including the ultimate out-of-plane shear strength, and the interlaminar fracture toughness in modes I–II–III. In TP-based laminates, the matrix plasticization seems to play an important role in matrix-rich areas by locally promoting permanent deformations. Fiber-bridging also prevents the plies from opening in mode I, and slows down the propagation of interlaminar and intralaminar cracks in modes II–III. Both mechanisms seem to reduce the extension of damages, in particular, the subsequent delamination for a given impact energy. In epoxy-based laminates, the debris of broken fibers and matrix get stuck in the cracks and the adjacent layers, and create a sort of blocking system that prevents the cracks and delamination from closing after impact

Effect of corrosive solutions on composites laminates subjected to low velocity impact loading

In recent years, there has been a rapid growth in the use of fibre reinforced composite materials in engineering applications and this phenomenon will be continuing. In this context, composite structures can be exposed to a range of corrosive environments during their in-service life, which causes degradation in terms of material properties. Some works can be found in open literature, but the studies presented are not sufficient to establish a full knowledge about this subject. Therefore, the aim of this work is study the low velocity impact response of Kevlar/epoxy laminates and carbon/epoxy laminates, after immersion into hydrochloric acid (HCl) and sodium hydroxide (NaOH). The aggressive solutions affect significantly the impact strength, but their effects are strongly dependent of the concentration. On the other hand, a significant effect of the temperature can be found, independently of the aggressive solution, on the impact performance and residual bending strength.Nos últimos anos temos assistido a um aumento significativo da utilização de materiais compósitos reforçados com fibras nos mais variados campos da engenharia e este fenómeno tende a continuar. Neste contexto, as estruturas em materiais compósitos podem ser expostas a uma enorme variedade de ambientes corrosivos provocando, deste modo, a degradação das suas propriedades mecânicas. Na verdade podem ser encontrados na literatura alguns trabalhos, mas os estudos apresentados não se revelam suficientes para estabelecer um conhecimento aprofundado nesta temática. Então, este trabalho visa estudar a resposta ao impacto de baixo velocidade de laminados Kevlar/epóxi e laminados carbono/epóxi após imersão em ácido clorídrico (HCl) e hidróxido de sódio (NaOH). As soluções agressivas mostraram afetar significativamente a resistência ao impacto, mas o seu efeito é fortemente dependente da concentração da solução. Por outro lado, a temperatura também apresenta um efeito significativo, independentemente da solução agressiva, no desempenho ao impacto dos referidos laminados e na sua resistência residual à flexão

Resistance of Carbon Fibre Reinforced Composites to Quasi-static and Ballistic Perforation

The failure mechanisms, as well as the indentation and penetration resistance, of carbon fibre reinforced plastic (CFRP) cross-ply laminates were investigated under quasi-static and ballistic loading. In this thesis, the two most prominent failure modes were indirect tension and shear plugging. To characterise the indirect tension mechanism, CFRP cross-ply coupons with various matrix shear strengths were subjected to uniaxial out-of-plane compression between lubricated platens, while CFRP cross-ply beams were subjected to quasi-static indentation between a flat bottom indentor and a lubricated back support. The out-of-plane compressive strength was accurately predicted by finite element simulations and analytical models. To characterise the shear plugging mechanism, quasi-static cropping tests were performed on CFRP cross-ply beams. A beam configuration was selected to allow for ease of identifying the failure mechanisms. The investigation was extended to consider the effect of matrix shear strength on the ballistic performance of simply supported CFRP cross-ply beams impacted by a flat projectile. Laminates with high matrix shear strength failed by shear plugging, and the penetration velocity increased with decreasing matrix shear strength. As the matrix shear strength decreased further, the failure mode switched to indirect tension and subsequently the penetration velocity remained elevated, independent of the matrix shear strength. Having established that shear plugging is associated with low impact resistance, a new type of bilayer CFRP composite (comprising one low and one high matrix shear strength layer) was developed with the intent of suppressing this shear plugging mode. The ballistic penetration resistance of the bilayer beams was compared to that of the above monolithic CFRP beams using the same ballistic set-up. It was observed that the shear plugging mode in the high strength layer was suppressed when the layer was placed at the distal face; failure switched to a back face tensile mode, and the impact resistance was improved. The investigation was extended to a more realistic impact environment: CFRP cross-ply laminates in a plate configuration were perforated by a steel ball. Specimens were tested under quasi-static and ballistic loading with either a back-supported condition (simulating a thick laminate) or an edge-clamped condition. The CFRP plates failed by indirect tension when back-supported but failed by shear plugging when edge-clamped. It was found that the addition of a protective aluminium alloy layer did not alter the failure mechanism of the CFRP, but did produce a load spreading effect that increased the penetration resistance.The tuition of this author was fully sponsored by the Croucher Foundation and the Cambridge Commonwealth, European & International Trust through the Cambridge Croucher International Scholarshi

Testing of Materials and Elements in Civil Engineering

This book was proposed and organized as a means to present recent developments in the field of testing of materials and elements in civil engineering. For this reason, the articles highlighted in this editorial relate to different aspects of testing of different materials and elements in civil engineering, from building materials to building structures. The current trend in the development of testing of materials and elements in civil engineering is mainly concerned with the detection of flaws and defects in concrete elements and structures, and acoustic methods predominate in this field. As in medicine, the trend is towards designing test equipment that allows one to obtain a picture of the inside of the tested element and materials. Interesting results with significance for building practices were obtained

Localised low velocity impact performance of FLAX/PLA biocomposites

Natural fibre composites are fast emerging as a viable alternative to traditional materials and synthetic composites. Their low cost, lightweight, good mechanical performance and their environmentally friendly nature makes them an ideal choice for the automotive sector. The automotive industry has already embraced these composites in production of non-structural components. At present, however, research studies into composites made of natural fibres/bio-sourced thermoplastic resins are at infancy stage and such works are rare in the literature. This study therefore focuses on the mechanical properties of poly(lactic) acid (PLA) flax reinforced composites for structural loaded components. The aim was to investigate the performance of flax/PLA biocomposites subjected to localized low velocity impacts. To start with, a detailed literature study was conducted covering biocomposites and PLA in particular. Next, a series of composite samples were manufactured. Morphological and thermal studies were also conducted in order to develop an in-depth understanding of their thermo-mechanical properties, including crystallinity, thermal response and their related transition temperatures. This was followed by localized impact studies. The influence of temperature, water uptake and strain rates to the material tensile strength and modulus, as well as the damage characteristics and limits that lead to failure were studied. Furthermore, in the present work different methods and existing material models to predict the response of biocomposites were assessed. A case study was then performed using these models to understand, develop and improve the side crash performance of a superlight city car prototype. ...[cont.

Interfacial Mechanical Strength Characterization in Multilayered Materials via Nanoscale Impact and Nano Mechanical Raman Spectroscopy Experiments

A composite materials strength can significantly depend on the constitutive description of interfaces. A computational model of composite deformation should, therefore, incorporate interface constitutive behavior. These interfaces poses several challenges in studying them due their length scales of micrometer to nanometer as well the coupling of other factors such as confinement during the loading. Thus, separating main phase constitutive behavior from interface constitutive behavior in mechanical property measurement experiments is an arduous task. In this work, an epoxy interface between glass plates is analyzed under quasistatic and dynamic loading conditions to obtain a description of interfacial constitutive response at strain rates from 10-2 to 103 s-1. The experiments were conducted with indenters of radius 1, 10 and 100 µm on the interfaces thicknesses of 1, 10 and 100 µms within the spatial error tolerance of less than 3 µms. The interface thickness was verified with the Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis. The approach relies on describing interfaces as a confined material phase between two unconfined phases. Dynamic microscale impact tests are used to obtain stress-strain response as a function of strain rate for the analyzed interfaces. The data was then subjected to statistical analysis to remove experimental errors. An analytical model was developed to find the confinement effect and the solution was verified by capturing stress maps with Nanomechanical Raman Spectroscopy (NRS) experiments pre and post experiments to analyze the change in the stress distribution around interfaces. Based on the analyses of confinement effects, a constitutive model is proposed to predict the interface deformation behavior with a dependence on both strain rate and confinement effect. This model is further used in the finite element simulations to predict and quantify the role of interfaces in multilayered materials