Axial capacity and crushing behavior of metal–fiber square tubes – Steel,stainless steel and aluminum with CFRP

Composite metal-carbon fiber reinforced polymer (CFRP) tubes combine the benefits of the high strength to weight ratio of the fiber/resin composite and the stable, ductile plastic collapse mechanism of the metal, to form a composite tube with high strength and energy absorption capability. This paper investigates the axial capacity and crushing behavior of square hollow section (SHS) tubes composed of composite steel-CFRP, stainless steel-CFRP and aluminum-CFRP. Experiments of tubes with different metal SHS geometries and two different matrix layouts of carbon fibers are described, and a general theory to predict the compression buckling, axial capacity, axial collapse and mean crush load of metal–fiber square tubes is developed and validated against the experimental results. It is shown that carbon fiber may be successfully externally bonded to metal SHS, and such application may be provided to improve the performance of existing structures, or to design new structures with enhanced strength-weight and energy absorption-weight ratios. Comparisons are made between the performance of the different types of metals, SHS geometries and carbon fiber matrix layouts.     

Experimental quasi-static axial crushing of cone–tube–cone composite system

An experimental investigation was carried out to study the crashworthiness parameters of cone–tube–cone composite system. The quasi-static crushing behaviour of axially compressed cone–tube–cone composite system has been investigated experimentally. The conical parts of composed system were symmetric. The cone vertex angles used were 10 and 15°. Two arbitrary cone ratios of height-to-base-diameter (C/D₁) have been explored. The tubular part heights were varied between 0 and 50 mm and between 0 and 20 mm for 1.41 and 0.69 of C/D₁ ratios, respectively. Load–displacements curves and deformation histories of typical specimens are presented and discussed. The results showed that structures with ratios (tube-height-to-total-height) vary between 0.06 and 0.11 exhibit a higher crashworthiness performance.     

Dynamic buckling of elastic–plastic square tubes under axial impact—II: structural response

Dynamic elastic–plastic buckling of thin-walled square tubes is studied from the viewpoint of elastic–plastic stress wave propagation, which originates from an axial impact loading. The influence of the impact velocity and the striking mass on the development of the buckling shape is discussed when considering the transient deformation process. It is shown that the maximum load, which results from a high velocity impact load and occurs at t=0, is a function of the impact velocity and is related to the speed of the elastic–plastic stress waves propagating along the tube. The predictions for the initiation of buckling based on a numerical simulation of the axial impact of strain rate insensitive square tubes using the FE code ABAQUS show good agreement with the results from experiments on aluminium alloy tubes impacted at various initial velocities. A comparison between the buckling initiation in square tubes and geometrically equivalent circular tubes reveals differences in the response, which are attributed to the stress wave propagation phenomena and to the structural differences between the two structures.     

Simulation of delamination–migration and core crushing in a CFRP sandwich structure

Following the onset of damage caused by an impact load on a composite laminate structure, delaminations often form propagating outwards from the point of impact and in some cases can migrate via matrix cracks between plies as they grow. The goal of the present study is to develop an accurate finite element modeling technique for simulation of the delamination–migration phenomena in laminate impact damage processes. An experiment was devised where, under a quasi-static indentation load, an embedded delamination in the facesheet of a laminate sandwich specimen migrates via a transverse matrix crack and then continues to grow on a new ply interface. Using data from this test for validation purposes, several finite element damage simulation methods were investigated. Comparing the experimental results with those of the different models reveals certain modeling features that are important to include in a numerical simulation of delamination–migration and some that may be neglected.     

Strength and damage tolerance of composite–composite joints with steel and titanium through the thickness reinforcements

Today’s aeronautic, automotive and marine industry is in demand of structurally efficient, low weight alternatives for composite–composite joints which combine the advantages of low weight input of adhesively bonded joints and high damage tolerance of through the thickness bolted joints. In the present work, composite–composite joints are reinforced through the thickness by thin metal inserts carrying cold metal transfer welded pins (CMT pins). The influence of pin alignment and type of pin on the damage tolerance of single lap shear (SLS) composite–composite joints is investigated. The use of titanium reinforcements is evaluated and compared to stainless steel reinforced, adhesively bonded and co-cured specimens. A detailed analysis of the stress–strain behavior is given and the stiffness and energy absorption of the SLS joints during tensile loading is assessed. The results show that joints reinforced with CMT pins absorb significantly higher amounts of energy, when compared to adhesively bonded and co-cured joints.     

Testing and modeling of a novel FRP-encased steel–concrete composite column

A composite column consisting of steel, concrete and fiber reinforced polymer (FRP) is presented and assessed through experimental testing and analytical modeling. The composite column utilizes a glass FRP (GFRP) composite tube that surrounds a steel I-section, which is subsequently filled with concrete. The GFRP tube acts as a stay-in-place form in addition to providing confinement to the concrete. This study investigates the behavior of the proposed composite columns under axial loading. A total of seven specimens were tested. The influence of concrete shrinkage on the compressive behavior of the composite columns was also investigated. Significant confinement and composite action resulted in enhanced compressive behavior. The addition of a shrinkage reducing agent was found to further improve the compressive behavior of the composite columns. An analytical model was developed to predict the behavior of the composite columns under axial loading.     

Microstructure and mechanical properties of low alloy steel T11–austenitic stainless steel 347H bimetallic tubes

Abstract

A low alloy steel (T11) has been bonded to an austenitic stainless steel 347H by hot coextrusion under industrial conditions. The final product was a seamless bimetallic tube with 347H cladding the exterior for corrosion resistance in severely corrosive environments at high temperatures. The microstructures of the coextruded bonding have been compared to those obtained in the laboratory, after diffusion bonding experiments, using hot isostatic pressing (hipping). In all cases both the interdiffusion of the different elements across the interface and the microstructure have been analysed by optical microscopy, SEM, and TEM. On the 347H side a profuse precipitation, mainly of NbC, was found in a region near the interface. Only in the hipped specimens, as result of nickel and chromium diffusion from the stainless steel to the T11 steel, a martensite band was observed parallel to the interface. The heat treatment performed on the bimetallic tubes, to obtain the optimum combination of mechanical properties and corrosion resistance, consisted of austenitisation between 1050–1100°C, water quenching, and a stabilisation treat ment at 850–900°C, followed by slow cooling.     

Investigation into laminate design of open carbon–fibre/epoxy sections by quasi–static and dynamic crushing

The effect laminate design on the crush performance of carbon–fibre/epoxy “DLR” crush elements has been experimentally investigated. A quasi-isotropic lay-up was found to result in the highest Specific Energy Absorption (SEA) for Four-Harness (4HS) reinforced laminates; however a hybrid of unidirectional weave and 4HS fabric produced the highest SEA of 114 kJ/kg. Interleaving with thin thermoplastic films increased the steady state crushing force, however the increase in laminate density associated with the addition of the film caused no improvement and in some cases a reduction in SEA depending on material and lay-up. Dynamic crush testing of selected laminate designs resulted in a reduction in SEA of between 6% and 15% compared to the quasi–static case.     

Energy absorption and load carrying capability of woven natural silk epoxy–triggered composite tubes

This study investigated the energy absorption response and load carrying capability of woven natural silk/epoxy–triggered composite rectangular tubes subjected to an axial quasi-static crushing test. The rectangular composite tubes were prepared by hand lay-up technique. The tubes consisted of 12, 24, and 30 layers of natural woven silk/epoxy laminate and were 50, 80, and 120 mm long. The crashworthiness of the tubes was evaluated by measuring the specific energy absorption in quasi-static axial compression. Specific energy absorption was obtained from the load–displacement curve during testing. The failure mode of the tubes was analyzed from high resolution photographs obtained. Overall, the tube with 50 mm length and 30 layers showed the best crashworthiness among the tubes. The failure morphology showed that the specimens failed in two distinct modes: local and mid-length buckling. The triggered composite tubes exhibited progressive failure.     

Effect of processing parameters on the microstructure and mechanical properties of Al–steel composite foam

Al–steel composite foams comprise of steel hollow spheres embedded in an aluminum matrix and are processed using a gravity casting technique. The effect of processing parameters such as casting temperature and cooling rate on the microstructure and mechanical behavior was studied to establish structure–property relationships. Results show that the amount and composition of intermetallic phases present in the foam microstructure is directly related to casting temperature and cooling rate. Highest strength and energy absorption were obtained from Al–steel foams with fast solidification rates that minimize the growth of intermetallic phases.     

Analysis of load–strain models for RC square columns confined with CFRP

This article presents the comparison between 6 theoretical models of axially confined concrete columns with the experimental results of 7 tested columns of different authors. This study analysed the accuracy of 6 different confinement models for square columns taking into account the results of experimental tests on 7 RC columns confined with CFRP sheets with different dimensions and carried out by different authors. The profile of curves, the peak/failure values, the stress–strain and axial–to–lateral relations were studied to conclude which models show the best correlation with the experimental test results. Quantification of this deviation was carried out for key parameters. Some models predicted peak values with reasonable accuracy – Manfredi & Realfonzo, Campione & Miraglia, Lam & Teng, Pellegrino & Modena – although for the whole load–strain behaviour only the model of Faustino, Chastre & Paula seemed to be reasonably accurate in most cases.     

Flexural and shear strengthening of RC beams with composite materials – The influence of cutting steel stirrups to install CFRP strips

Experimental, numerical and analytical investigations have revealed that Carbon Fibre Reinforced Polymer (CFRP) strips with larger cross section height improve the effectiveness of the Near Surface Mounted (NSM) technique for the flexural strengthening of existing reinforced concrete (RC) beams. However, this height is limited to the concrete cover thickness of the longitudinal steel bars, since the application of strips of cross section height larger than the cover thickness requires that the bottom arm of the steel stirrups be cut. This work aims to assess the influence, in terms of shear resistance, of cutting the bottom arm of steel stirrups to install NSM strips for the flexural strengthening of RC beams. The obtained results showed that, for monotonic loading, cutting the bottom arm of steel stirrups led to a decrease of the beam’s load carrying capacity of less than 10%. Due to the high effectiveness of the adopted NSM flexural strengthening systems, shear can be a predominant failure mode for these beams. To avoid this type of failure mode, strips of wet lay-up CFRP sheets with U configuration were used, resulting in effective strengthening solutions for RC beams. In the present paper the experimental program is described, and the obtained results are presented and discussed.     

Wet thermo-mechanical behavior of steel–CFRP joints – An experimental study

The long term durability of CFRP strengthened steel structures is a key parameter for their safe use and effective design. Strengthened members can be subjected to different environmental conditions and loading scenarios during their service life, the effect of which on the failure mechanism of the strengthened member requires fundamental investigations. This paper presents an experimental investigation into the effects of wet thermo-mechanical loading on the bond strength and the failure mode of steel–CFRP single lap joints. A total of thirty four steel–CFRP single lap shear specimens were prepared and exposed to different combinations of wet thermal cycle ranges and sustained loads. The results show that these conditions (wet thermal cycles and sustained loads) have little impact on the bond strength of steel–CFRP lap joint when applied separately. However, when applied simultaneously, the bond strength of the joint is significantly reduced with failure observed at less than 30% of the static strength under temperatures that are well below the glass transition temperature of the adhesive.     

Numerical modeling on concrete structures and steel–concrete composite frame structures

A steel–concrete composite fiber beam-column model is developed in this study. The composite fiber beam-column model consists of a preprocessor program that is used to divide a composite section into fibers and a group of uniaxial hysteretic material constitutive models coded in the user defined subprogram UMAT in ABAQUS. The steel–concrete composite fiber beam-column model is suitable for global elasto-plastic analysis on composite frames with rigid connections subjected to the combined action of gravity and cyclic lateral loads. The model is verified by a large number of experiments and the results show that the developed composite fiber model possesses better accuracy and broader applicability compared with a traditional finite element model. Although the fiber beam-column model neglects the slip between the steel beam and concrete slab, there are essentially no effects on the global calculation results of steel–concrete composite frames. The proposed model has a simple modeling procedure, high calculation efficiency and great advantage when it is used to analyze composite frames subjected to cyclic loading due to earthquake.     

Numerical simulation and validation of impact response of axially-restrained steel–concrete–steel sandwich panels

Steel–concrete–steel (SCS) sandwich panels are an effective means for protecting personnel and infrastructure facilities from the effects of external blast and high-speed vehicle impact. In conventional SCS construction, the external steel plates are connected to the concrete infill by welded shear stud connectors. This paper describes a programme of research in which the non-composite SCS panels with axially restrained connections were studied experimentally and numerically. High fidelity finite element models for axially restrained steel–concrete–steel panels subjected to impact loading conditions were developed using LS-DYNA. The simulation results were validated against the dynamic testing experimental results. The numerical models were able to predict the initial flexural response of the panels followed by the tensile membrane resistance at large deformation. It was found that the strain rate effects of the materials and the concrete material model could have significant effect on the numerically predicted flexural strength and tensile membrane resistance of the panels.     

Investigation on diffusion bonding of functionally graded WC–Co/Ni composite and stainless steel

A functionally graded WC–Co/Ni composite (FGWC) and 410 stainless steel (410ss) were successfully bonded by diffusion bonding. With the bonding temperature or holding time increasing, the tensile strength of the joints increased firstly and then decreased. The maximum tensile strength of the FGWC/410ss joints was 195 MPa bonded at 950 °C for 80 min. A diffusion layer was formed between the Ni layer and the 410ss as a result of the interdiffusion of Ni and Fe. The Ni layer could release the residual stresses of the FGWC/410ss joints. The fracture of the FGWC/410ss joints occurred in the Ni layer by the way of ductile fracture.     

Effect of moisture on the mechanical properties of CFRP–wood composite: An experimental and atomistic investigation

Adhesive bonding of fiber-reinforced polymers (FRP) to wood has been proven as a general way to achieve reinforcement and rehabilitation for wood structures. Although a significant mechanical enhancement can be acquired by using such approach, there exists a big concern about the long-term performance of the FRP–wood composite, especially under the effect of moisture. In this paper, both experimental and atomistic approaches are adopted for investigating the moisture effect on the entire FRP–wood composite system. Macroscopic mechanical tests show that its mechanical properties and its fracture behaviors notably change at different levels of ambient humidity. From an atomistic perspective, molecular dynamics (MD) simulations reveal that water molecules significantly reduce the adhesion energy between wood and epoxy. Results from experimental and numerical studies imply that the strength of the FRP–wood interface critically determines the mechanical performance of the entire system. The water molecules absorbed at the interface are crucial to the durability of multi-layer systems and a general mechanism governing the failure modes of such systems is found.