Steel Fiber Composites Research Articles

Effect of curing temperature on hydration and microstructure evolution of cement-based composites with extremely low w/b ratio

The challenge of high geo-temperature often occurred during the construction of deep buried tunnels, and to uncover the effect of varied curing temperature on hydration and microstructure evolution of cement-based composites with extremely low w/b ratio, samples were prepared with a w/b ratio of 0.18 and mixed with different fibers, silica fume and fly ash. Hydration kinetics, main hydration product phases and the evolution of C–S–H inside the samples were analyzed. It was found that the time for maximum heat release peak under 40 °C and 60 °C was earlier than that under 20 °C. The hydration heat of steel fiber composites was lower than that of PP fiber composites. 40 °C curing temperature can motivated the chemical activity of samples and promoted the hydration process, multi-stage peaks appeared at 60 °C curing condition. Q0-Q4 of C–S–H were observed in samples, and the mean silicate chain length (MCL) of C–S–H from 29Si NMR analysis was increased mostly by 15.6% with the elevated curing temperature. This study will provide much support to the safe construction of deep buried tunnels.

Suspension behaviour of kenaf and hemp fibre composites leaf spring using finite element analysis

In this study, the suspension behaviour of Kenaf and Hemp reinforced Epoxy composites leaf springs were evaluated and compared to that of steel and synthetic fibre composites leaf spring. The analysis to investigate the bio-composites performance under static and dynamic loads was carried out using the finite element ANSYS software. The results showed that both natural composite leaf springs having approximately 50 times the deformation, strain energy, and similar stresses as structural steel, and a weight reduction of 83.1 %. The dynamic analysis revealed that the first natural frequencies of the Kenaf and Hemp fibre reinforced epoxy composites leaf springs were higher than the typical road irregularity frequency. The natural frequency of bio-composites leaf springs is 18.506 Hz for kenaf fiber/epoxy composites and 18.502 Hz for hemp fiber/epoxy composites, which is approximately 54 % higher than the frequency of road irregularities of 12 Hz.

Effect of Sizing Agent on the Mechanical, Thermal, and Electrical Performance of Basalt Fiber/Epoxy Composites.

Basalt fiber and its resin composites have gradually supplanted traditional steel and glass fiber composites due to their superior strength, heat resistance, and corrosion resistance. However, basalt fiber still has significant flaws that restrict the functionality and use of its composites, such as less active functional groups and poor resin adherence. This study examines the effects of sizing agent on the characteristics of basalt fiber/epoxy resin composites. Epoxy resin emulsion and acrylate emulsion are employed as the primary auxiliary film-forming agents in this study. Polyurethane emulsion with various content levels is also used. The findings indicate that a 1% wt. of polyurethane emulsion concentration produces the greatest results, increasing the composite’s flexural strength, flexural modulus, tensile strength, and interlaminar shear strength by 122%, 34.0%, 102%, and 10.2%, respectively. At the same time, the storage modulus and Tg of the material will decrease. In addition, the breakdown strength can be raised by 112%, and insulation parameters such as leakage current and dielectric loss factor can be decreased by 26.4% and 15.6%, respectively. The effect of sizing agent B is the best.

Effect of moisture and temperature on thermal and mechanical properties of structural polyurethane adhesive joints

The aim of this study was to evaluate the effect of humidity and temperature in the mechanical and thermal properties of two polyurethane adhesives joints. Single and double component adhesives were selected, in order to compare their different properties to join dissimilar materials (steel and carbon fiber composite). By means of two devices specially developed for the study, durability and heat-aging tests were performed. The ability of the adhesives to cope with temperature (72 °C) and humidity (82%), and facing temperature variations (from 26 °C to 72 °C) was studied. Shear tests and Shore D hardness measurements were carried out after the specimens were subjected to different exposure times and adverse environments. Joint reliability was studied by means of a simplified Weibull model. Besides, differential scanning calorimetry and infrared spectroscopy were made to understand the changes produced in the adhesives. Different behaviors were found for each adhesive, but they showed good characteristics to be used in structural applications under adverse environments.

The effect of stacking sequence of basalt and Kevlar fibers on the Charpy impact behavior of hybrid composites and fiber metal laminates

In this study, the effect of the arrangement of Kevlar and basalt fibers as the reinforcements on the Charpy impact behavior of hybrid epoxy composites was investigated. Also, the effect of adding metal plates (aluminum 2024-T3 and stainless steel 316L) into the basalt/ Kevlar fibers reinforced epoxy composites to fabricate fiber metal laminates under Charpy impact loads was studied. The fabricated fiber metal laminates in this research consisted of three metal plates and two groups of composite layers placed between them and were fabricated by the hand lay-up technique. Results indicated that the stacking sequence of fibers due to the hybridization effect caused a considerable improvement in the energy absorption value (99%) of hybrid composites, compared to specimens with one kind of fibers. Moreover, the effect of adding aluminum plates for the fabrication of fiber metal laminate was greater than adding steel plates. Considering the weight of composites, fiber metal laminates with aluminum and steel sheets, it was found that the average specific energy absorption value of aluminum fiber metal laminates was about 2.5 times higher than those of steel fiber metal laminates and composites.

A facile approach to the scalable preparation of thermoplastic/carbon nanotube composites

Disentangling and dispersing multiwalled carbon nanotubes (MWCNTs) in thermoplastics by the existing melt compounding facilities where neither solvent nor liquid chemicals are allowed remains a great challenge. This challenge is addressed herein by combining ball milling with melt compounding. Specifically, ball milling is applied to disentangle MWCNTs and to eventually liberate each tube from agglomeration, and then the tubes are melt compounded with SSFs (PA6). A uniformly distributed MWCNT network is observed; obvious load transfer from the matrix to the tubes is evidenced by the presence of many firmly embedded tubes on the fracture surface. The electrical conductivity percolation threshold of PA6/MWCNT composites is found to be 2.5–3.0 wt%, in comparison with 5.0–8.0 wt% for PA6/stainless steel fiber composites. The PA6/MWCNT composite at 10.0 wt% shows a 76% increase in flexural modulus and 58% in flexural strength.

Stress redistribution around fiber breaks in unidirectional steel fiber composites considering the nonlinear material behavior

The use of steel fibers as reinforcement in polymer composites is recently increasing thanks to their ductility, high stiffness and wide range of diameters. Unlike carbon and glass fibers, steel fibers often end up with a non-circular cross-section due to their manufacturing technology. This may influence the stress redistribution around fiber breaks, which is important in longitudinal tensile failure of unidirectional composites. A parametric study was performed by using 3D finite element models with randomly distributed and oriented hexagonal fibers. Rather than the fiber shape, the distance between fibers was shown to have an influence on stress concentrations in terms of both average and peak stress concentrations. The plastic behavior of steel fibers resulted in smaller stress concentrations and faster stress recovery whereas the opposite was observed for the plastic behavior of epoxy. For different strain levels, results were shown to depend on the relative stiffness of steel and epoxy in the plastic region.

Micro-scale numerical study of fiber/matrix debonding in steel fiber composites

Fiber/matrix debonding behavior of steel fiber composites is analyzed using a parametric finite element modeling procedure and compared with conventional composites with carbon and glass fibers. Cohesive surfaces are applied to fiber–matrix interface to simulate the debonding behavior, while the interface strength properties of steel fiber are obtained with and without surface treatment. The effect of various parameters on the debonding behavior is investigated, including stress concentrations, fiber diameter, fiber shape, and fiber volume fraction, using the parametric model. The influence of stress concentrations is determined to be much lower than the debonding strength. Debonding damage is more evident in larger fibers compared to smaller ones. Earlier and sudden interface separation is observed with the polygonal steel fibers compared to the circular ones. Increase in the fiber volume ratio increases the debonding opening distance but does not affect the opening angle significantly. The results can be useful for assessing possibilities to use steel fibers to increase toughness of the composites in comparison with glass and carbon reinforcement.

Estimation of Compressive Strength and Member Size of Steel Fiber Reinforced Concrete Using Stress Wave-Driven Nondestructive Test Methods

Among fiber-reinforced composites, steel fiber has been widely-used for concrete infrastructure such as silos, tunnels, specifically aiming at reducing the weight of concrete and enhancing its strength by overcoming the brittleness. However, there is still little known about appropriate quality management and applicability assessment for steel fiber composites. This study fills this knowledge gap by testing the possibility of maintenance through steel fiber concrete thickness estimation and assessing the applicability of the quality management instrument. To this end, this study utilizes two different stress wave-driven non-destructive test methods: ultrasonic pulse velocity and impact-echo methods. The ultrasonic pulse velocity method was employed to estimate the compressive strength of steel fiber reinforced concrete, while the impact-echo method was applied to estimate the thickness of various steel fiber reinforced concrete members. As a quality management factor of concrete, correlations between steel fiber mixing ratios and compressive strengths were experimentally explored and validated by error ratios for twenty-four specimens. The reliability was relatively high overall. The average error rate of all the specimens with steel fiber mixing ratios of 0, 0.75 and 1% was 3.36%. Accordingly, the results prove the applicability of the non-destructive test methods for building quality management.

A machine tool motorized spindle with hybrid structure of steel and carbon fiber composite

Carbon fiber reinforced plastic (CFRP) is a promising material for enhancing the spindle performance of machine tools because of its high specific stiffness and low thermal expansion. However, when a conventional steel spindle shaft is replaced by a CFRP shaft, the anisotropic property of CFRP makes it difficult to maintain the static stiffness that is equivalent to that of conventional spindles. This study proposes a hybrid structure of steel and CFRP to avoid reduced stiffness of the shaft. Fundamentals of shaft design are described to enable discussion of a favorable steel-CFRP ratio. An experimental evaluation of a developed CFRP spindle is also reported.

On the mechanical behavior of polypropylene, steel and hybrid fiber reinforced self-consolidating concrete

This article presents the results of an experimental investigation on the mechanical behavior of a self-consolidating concrete reinforced with steel, polypropylene and hybrid fibers. Hooked end steel fibers with different lengths and diameters were used as reinforcement in fiber volume fractions of 0.50%, 1.00% and 2.00%. Polypropylene fibers were used as reinforcement in volume fractions of 0.33%, 0.66% and 1.10%. Hybrid composites (HyFRC) were also developed to study the synergy effects of both fibers. The hybrid fiber self-consolidating concrete was produced with the addition of 0.50% of hooked-end steel fibers and 0.66% of polypropylene fibers.Pre-notched SCC prims where tested under monotonic and cyclic three-point bending. The tests where controlled by the crack mouth opening displacement in order to have a better analysis of the post cracking regime. The addition of polypropylene, hooked-end and hybrid fibers was effective in improving the post-peak strength of the composites, even though the use of steel fibers promoted higher values of flexural residual stress due to its geometrical and material properties. The same behavior was observed for the fracture parameters results with an improvement of the crack growth resistance for the analyzed composites.Finally, the effect of the fiber reinforcement on the flexural behavior was studied through structural tests on fiber reinforced self-consolidating concrete beams. Both hooked-end steel fiber and hybrid composites enhanced the resistance at yielding and promoted a lower rate of stiffness degradation, while the polypropylene fiber reinforcement presented an improvement in the ductile behavior of the structural composite. The present work gives an important contribution on the structural and cyclic behavior of steel, PP and hybrid fiber reinforced concrete. The paper makes a comparison on the mechanics of the different composite systems aiming its application in structural elements.

Detection of Interfacial Debonding in a Rubber-Steel-Layered Structure Using Active Sensing Enabled by Embedded Piezoceramic Transducers.

Rubber–steel-layered structures are used in many engineering applications. Laminated rubber–steel bearing, as a type of seismic isolation device, is one of the most important applications of the rubber–steel-layered structures. Interfacial debonding in rubber–steel-layered structures is a typical failure mode, which can severely reduce their load-bearing capacity. In this paper, the authors developed a simple but effective active sensing approach using embedded piezoceramic transducers to provide an in-situ detection of the interfacial debonding between the rubber layers and steel plates. A sandwiched rubber–steel-layered specimen, consisting of one rubber layer and two steel plates, was fabricated as the test specimen. A novel installation technique, which allows the piezoceramic transducers to be fully embedded into the steel plates without changing the geometry and the surface conditions of the plates, was also developed in this research. The active sensing approach, in which designed stress waves can propagate between a pair of the embedded piezoceramic transducers (one as an actuator and the other one as a sensor), was employed to detect the steel–rubber debonding. When the rubber–steel debonding occurs, the debonded interfaces will attenuate the propagating stress wave, so that the amplitude of the received signal will decrease. The rubber–steel debonding was generated by pulling the two steel plates in opposite directions in a material-testing machine. The changes of the received signal before and after the debonding were characterized in a time domain and further quantified by using a wavelet packet-based energy index. Experiments on the healthy rubber–steel-layered specimen reveal that the piezoceramic-induced stress wave can propagate through the rubber layer. The destructive test on the specimen demonstrates that the piezoceramic-based active sensing approach can effectively detect the rubber–steel debonding failure in real time. The active sensing approach is often used in structures with “hard” materials, such as steel, concrete, and carbon fiber composites. This research lays a foundation for extending the active sensing approach to damage detection of structures involving “soft” materials, such as rubber.

Ductility of steel fibre/epoxy composites in function of their microstructure

Recently we showed that continuous stainless steel fibres deliver structural composites with high stiffness and high failure strain at the same time. In the current research we further investigate these new materials focusing on the effect of the microstructure on their tensile facture behaviour. The steel fibre composites are produced from quasi-unidirectional woven (Q-UD) and purely unidirectional (P-UD) fibre architectures in combination with epoxy resin. The main difference in their microstructures is in the fibre distribution. In the Q-UD composite, steel fibres are tightly packed in bundles with 85% local fibre volume fraction and these bundles are separated by resin rich areas. In P-UD composite, steel fibres are homogeneously distributed through the composite with an average volume fraction of 50%. With the highly packed fibres, Q-UD composites achieve a higher strain to failure. In-situ characterization of damage development using acoustic emission registration and crack observations on the specimen surface were used to propose a hypothesis to explain the difference in fracture behaviour, caused by the different microstructures.

Hybridisation of two ductile materials – Steel fibre and self-reinforced polypropylene composites

Hybridisation of self-reinforced polypropylene (SRPP) with ductile steel fibres is proposed as a way to create composites with a high stiffness, failure strain and toughness. The stiffness and yield stress of SRPP were significantly increased without sacrifice of toughness, even after normalisation to the composite density. The measurements revealed a higher stress level and slope after yielding than the predictions by the linear rule-of-mixtures. This effect was attributed to differences in the Poisson contraction of the two constituents and was supported by modelling predictions. The absolute penetration impact resistance of SRPP was increased, whereas the specific impact resistance remained the same, despite the high density of steel fibres. Placing steel fibre layers on the outside of the laminate maximised the penetration impact resistance, as it allowed the steel fibres to exploit their full plastic potential. These results can guide the optimisation of other hybrid composites with two ductile fibres.

Mechanical and Electrical Characterisation of Steel Fiber and Carbon Black Engineered Cementitious Composites

Engineered cementitious composite (ECC) is one kind of cement based material fabricated with various types of functional fillers, which is of superior tensile resistance and energy absorption. ECC made some conductive fillers such as steel fiber (SF) and carbon black (CB) also presents great potential in self-sensing. In this study, the mechanical property, conductive performance and self-sensing capacity of two types of ECC, i.e. SF-ECC fabricated with 0.58% steel fiber and 1.75% PVA, and CB-ECC casted with 1% weight ratio to cementitious materials and 1.75% PVA, have been investigated by experiments and numerical simulation. Four point bending test synchronising AC resistance measurement has been implemented to compare the stress strain relationship with resistance variation. Discrete element method (DEM) has been applied to simulate the crack density development during the flexural test. The outcomes are also compared between these two types of ECC to evaluate the sensitivity of self-sensing property, which will contribute to the ECC optimisation to balance the loading and self-sending capacity.

High-frequency eddy current measurements using sensor-mounted electronics

Eddy current techniques are used widely for the detection of surface-breaking cracks in metal samples and the detection of such defects in metals with low electrical conductivity is challenging. To achieve good sensitivity to small surface cracks, the electromagnetic skin depth of the eddy current needs to be small, which often means operating at MHz frequencies. One of the major challenges in high-frequency eddy current testing is that the capacitance of the cable between the instrument electronics and the sensor head becomes significant in the MHz range, making the system unstable and introducing noise into the system as the cable moves and interacts electrically with objects close to it. There are significant benefits to locating the electrical circuitry directly behind the eddy current sensor coils, reducing issues with cable-induced electrical noise, enabling the detection of smaller defects at earlier stages of growth. Materials such as nickel-based super-alloys, titanium, austenitic steel and carbon fibre composites are often used in safety-critical applications, where the ability to detect surface cracks at the earliest possible stage is vital. Examples are presented that show the detection of small defects in a range of challenging materials at eddy current frequencies up to more than 15 MHz.

Application of a new methodology based on Eurocodes and finite element simulation to the assessment of a romanesque church

This work proposes a methodology, based on finite element simulation, for analyzing masonry historical structures, according to Eurocodes, that has been applied for the assessment of the Saint Sebastian church, located in Piedratajada (Zaragoza, Spain). Settlement pathologies were detected and the aim of the work is to verify the current safety level and to propose reinforcement solutions if necessary. Results confirm the effect of soil settlement and allow establish the maximum admissible value. If that value is reached, a couple of reinforcement solutions, installing sheets of steel or carbon fiber composite, are proposed and analyzed.

Retrofit Optimization for Resilience Enhancement of Bridges under Multihazard Scenario

AbstractBridge retrofitting is a common approach to enhance resilience of highway transportation systems. The study uses a multiobjective evolutionary algorithm to identify retrofit design configurations that are optimal with respect to resilience and retrofit cost. Application of this algorithm is demonstrated by retrofitting a bridge with column jackets and evaluating resilience of the retrofitted bridge under the multihazard effect of earthquake and flood-induced scour. Three different retrofit materials are used: steel, carbon fiber, and glass fiber composites. The inherent disparity in mechanical properties and associated costs of these different materials gives rise to a trade-off between cost and performance when it comes to retrofit operations. Results from the optimization, the Pareto near-optimal set, include solutions that are distinct from one another in terms of associated cost, contribution to resilience enhancement, and values of design parameters. This optimal set offers the best search re...

Penetration impact resistance of novel tough steel fibre-reinforced polymer composites

Conventional composites are susceptible to impact loading due to their low toughness. Novel annealed steel fibres possess a unique combination of high stiffness and high strain to failure. A range of composite laminates incorporating various steel fibre architectures with thermoplastic and thermoset matrices were produced and the low-velocity penetration impact resistance was characterized by falling weight impact tests, showing the high potential toughness of steel fibre composites. The effect of different parameters was determined. High matrix strain to failure allows a better exploitation of the steel fibre ductility. The higher absorbed energy up to penetration, in case of laminates with lower levels of fibre/matrix adhesion is attributed to fibre debonding, potential for more plastic deformation, and pull-out mechanisms. Thinner fibres are more sensitive to premature failure due to inclusions in the steel fibres. Cross-ply laminates show higher energy absorption, probably due to increased delamination, whereas woven fibre structures lead to more localized damage. A preliminary study shows that layer by layer (‘macro’) hybridization of carbon and steel fabrics leads to penetration impact resistance close to the pure steel fibre composite, in case the steel plies are placed on the outside surfaces, showing a significant positive effect of hybridization.

Effect of fibre architecture on the tensile and impact behaviour of ductile stainless steel fibre polypropylene composites

The high stiffness (∼193GPa) and high strain-to-failure (∼20%) of annealed stainless steel fibres present an opportunity to design ductile structural composites. In this research, the effect of the weave architecture on the tensile and impact behaviour of ductile stainless steel fibre/PP composites is investigated. Composites with three different weave architectures are compared: a quasi-unidirectional weave, a basket weave and a satin weave. The tensile test results show that all weave architectures show the same composite strain-to-failure, despite the difference in crimp. The composite with the basket weave (high crimp) has much lower stiffness and yield stress in comparison with the other two composites. This is attributed to significant out-of-plane deformations observed during the tensile test. The penetration impact results show that the high ductility of stainless steel fibre composites in tensile tests is transferred into excellent impact performance.