Characteristics of aluminum/CFRP short square hollow section beam under transverse quasi-static loading

Energy absorption capability and bending collapse behavior of an aluminum (Al)/carbon fiber reinforced plastic (CFRP) short square hollow section (SHS) beam were investigated under transverse quasi-static loading. The Al SHS beam was reinforced by CFRP, and the specimen was co-cured via an autoclave curing process. Three-point bending test was performed with five different lay-up sequences and three different laminate thicknesses. Stable bending collapse accompanying plastic hinge was observed in all specimens. Individual bending collapse behaviors were different depending on the lay-up sequences. The specific energy absorbed (SEA) was improved by up to 29.6% in the Al/CFRP SHS beam specimen with a [0/+45°/90°/−45°]_n lay-up sequence and laminate thickness of 1.168 mm (thickness ratio of Al: CFRP = 1: 0.87, 8 plies of prepreg) compared to the Al SHS beam. The SEA was not related with damage area of the Al/CFRP SHS beam. Finite element analysis and theoretical analysis based on Kecman’s model were performed to investigate the effect of reinforcement by CFRP on the Al SHS beam.     

Effect of superheated steam treatment of carbon fiber on interfacial adhesion to epoxy resin

To elucidate the effect of superheated steam (SHS) treatment of carbon fiber on the adhesion to epoxy resin and surface states, virgin unsized carbon fiber was exposed to SHS with or without N₂ in the temperature range of 500–800 °C. The interfacial shear strength (IFSS) between the carbon fiber and epoxy resin was successfully improved by SHS treatment with N₂, and the IFSS of fiber treated above 700 °C was the same as or higher than that of a commercial sized fiber. SHS treatment without N₂ resulted in an increase of total acidic groups on the fiber surface accompanied with the increase of phenolic hydroxyl groups, whereas that with N₂ resulted in a simultaneous increase of total acidic and basic functional groups. The significant improvement in the IFSS after SHS treatment with N₂ is considered to be due to the increase of basicity on the fiber surface.     

The effects of repeated transverse impact load on the burst pressure of composite pressure vessel

The present experimental study deals with the repeated transverse impact effect on the burst pressure of composite pressure vessels. Filament winding method is used to produce the vessels. Glass fiber reinforced (GFR) vessels are manufactured by using E-glass and epoxy resin. Composite pressure vessel was manufactured from fibers oriented [+55°/−55°/+55°/−55]_2s and the impact energies were chosen as 10, 15, 20, 25, 30 J for empty vessel during the impact tests. In addition, 10, 15, 20, 25 J for water filled conditions at 25 and 70 °C. The transverse impact load was applied in single and three times repeated form. The results show that when the impact load and water temperature increases, the burst pressure decreases.     

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/−45₂/0₂]_S, [45/90/−45/0]_2S and [45/90/−45/0]_4S were examined. These were selected on the basis that under quasi-static loading the [45₂/90₂/−45₂/0₂]_S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]_2S and [45/90/−45/0]_4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1 × 10⁶ cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.     

Influence of embedded gap and overlap fiber placement defects on the microstructure and shear and compression properties of carbon–epoxy laminates

This paper presents results from an experimental study of the influence of embedded defects created during automated fiber tape placement, on the mechanical properties of carbon/epoxy composites. Two stacking sequences have been examined, [(−45°/+45°)₃/−45°] and [90°₄/0°₃/90°₄], in which gaps and overlaps have been introduced during fiber placement. These materials have been cured in an autoclave either with or without a caul plate, then analyzed by ultrasonic C-scan. The microstructures were characterized by scanning electron microscopy. In-plane shear tests were performed on the ±45° laminates and showed that the use of a caul plate does not affect mechanical behavior of plies in the embedded defect region. Compression tests were performed on 0°/90° laminates and in this case the presence of a caul plate is critical during polymerization as it prevents thickness variations and allows defects to heal.     

Split Hopkinson pressure bar testing of 3D multi-axial warp knitted carbon/epoxy composites

High-strain-rate compression experiments were performed on 3D MWK carbon/epoxy composites with different fiber architectures at room and elevated temperature using an SHPB apparatus. Macro-fracture and SEM micrographs were examined to understand the failure mechanism. The results show the dynamic properties increase with the strain rate and show a high-strain-rate sensitivity. Meanwhile, composites with [0°/0°/0°/0°] have higher properties. Moreover, composites show temperature sensitivity and the properties decrease significantly, especially for composites with [0°/90°/+45°/−45°]. The results also indicate composites take on more serious damage and failure with the strain rate. The failure of composites with [0°/0°/0°/0°] behaves in multiple delaminating, overall expansion and 0° fibers tearing. While that of composites with [0°/90°/+45°/−45°] is mainly interlaminar delaminating, local fibers tearing and fracture on different fiber layers. In addition, with increasing the temperature, the composite shows less fracture and becomes more plastic. The damage of matrix yielding, interface debonding and twisting of fibers increase significantly.     

Fatigue life prediction of carbon fibre reinforced laminates by using cycle-dependent classical laminate theory

In this work a study about the adaption of the classical laminate theory for fatigue loads is presented. Cycle dependent stiffnesses of single UD 0°, UD 45° and UD 90° plies are implemented in order to calculate the fatigue-induced stiffness decrease of a multidirectional lay-up with the stacking sequence [0°/+45°/−45°/90°/90°/−45°/+45°/0°]. As second input alternative, UD 0°, UD 90° and ±45° plies are used. The calculated cycle-dependent stiffness parameters are compared to experimentally measured fatigue data of the multidirectional lay-up. The experimental test procedure used for the measurement of cycle-dependent stiffness parameters has been published previously. Results show that the experimentally measured stiffness decreases of the multidirectional lay-up can be estimated accurately based on the cyclic unidirectional input parameters.     

A novel carbon fiber reinforced lattice truss sandwich cylinder: Fabrication and experiments

To get a strong, stiff and weight efficient cylindrical shell, a novel carbon fiber reinforced corrugated lattice truss-core sandwich cylinder (LTSC) was designed and fabricated. The core is made up of orthogonal corrugated trusses and manufactured by mould pressing method. The LTSC is fabricated by filament winding and co-curing method. The face sheets have layups of [0°/30°/−30°/−30°/30°/0°] to improve the fundamental frequency as it is controlled by the circumferential stiffness. In end-free vibration the fundamental frequency of the LTSC is 112.18 Hz, higher than the referenced quasi-isotropic Isogrid-core sandwich cylinder. Determined by the skin fracture, the compression strength of the LTSC is 328.03 kN, stronger than the referenced Isogrid-core sandwich cylinder failed at rib buckling and the post-failure deformation is ductile. According to the optimization scheme jointly constrained by the strength and the fundamental frequency, an ultra-light and strong cylinder with high fundamental frequency was successfully fabricated.     

Improvement of interlaminar mechanical properties of CFRP laminates using VGCF

In order to improve the interlaminar mechanical properties of CFRP laminates, hybrid CFRP/VGCF laminates have been fabricated by using a newly-developed method, i.e., powder method, where the powder of vapor grown carbon fiber (VGCF) is added at the mid-plane of [0°/0°]₁₄ CFRP laminates. Experimental results of double cantilever beam (DCB) tests indicate the improvement on the interlaminar mechanical properties of Mode-I fracture behavior with much higher critical load P_C and fracture toughness G_IC with VGCF interlayer. Crack propagation and fracture surface have also been observed to interpret this improvement mechanism. Moreover, based on experimental G_IC, numerical simulations using finite element method (FEM) with cohesive elements have been carried out to analyze the delamination propagation. The interlaminar tensile strength of hybrid CFRP/VGCF laminates, which is obtained by matching the numerical load–COD (crack opening displacement) curves to experimental ones, is higher than that of base CFRP laminates.     

Analysis of multi-layered composite cylinders under hygrothermal loading

In this study, a general stress analysis is developed for thick or thin multi-layered composite cylinders under hygrothermal loadings. The layers are oriented symmetrically and antisymmetrically for [0°/90°]₂, [30°/−30°]₂, [45°/−45°]₂ and [60°/−60°]₂ orientations. The solution is carried out on composite cylinders for plane-strain, open end and closed end conditions. Uniform and parabolic temperature distributions are chosen for the thermal loads. All the integration constants are found from the radial stress and displacement in the normal direction of layers. The hygrothermal and other mechanical properties are measured on a glass-epoxy composite layer. Some analytical solutions are compared with the finite element solutions, in which commercial software ANSYS 7.0 is utilized, and close results are obtained between them.     

High-temperature dielectric and electromagnetic interference shielding properties of SiCf/SiC composites using Ti3SiC2 as inert filler

Ti₃SiC₂ filler has been introduced into SiC_f/SiC composites by precursor infiltration and pyrolysis (PIP) process to optimize the dielectric properties for electromagnetic interference (EMI) shielding applications in the temperatures of 25–600 °C at 8.2–12.4 GHz. Results indicate that the flexural strength of SiC_f/SiC composites is improved from 217 MPa to 295 MPa after incorporating the filler. Both the complex permittivity and tan δ of the composites show obvious temperature-dependent behavior and increase with the increasing temperatures. The absorption, reflection and total shielding effectiveness of the composites with Ti₃SiC₂ filler are enhanced from 13 dB, 7 dB and 20 dB to 24 dB, 21 dB and 45 dB respectively with the temperatures increase from 25 °C to 600 °C. The mechanisms for the corresponding enhancements are also proposed. The superior absorption shielding effectiveness is the dominant EMI shielding mechanism. The optimized EMI shielding properties suggest their potentials for the future shielding applications at temperatures from 25 °C to 600 °C.     

Alumina–tantalum composite for femoral head applications in total hip arthroplasty

Dense composite laminates of alumina (Al₂O₃) and tantalum (Ta) were fabricated by hot pressing and tested in vitro for potential use as a femoral head material in total hip arthroplasty (THA). Al₂O₃–Ta composite laminates hot pressed at 1450 °C and 1650 °C had flexural strengths of 940 ± 180 MPa and 1090 ± 340 MPa, respectively, which were far larger than the values of 420 ± 140 MPa and 400 ± 130 MPa for Al₂O₃ hot pressed at 1450 °C and 1650 °C, respectively. The interfacial shear strength, determined by a double-notched specimen test, was 310 ± 80 MPa for the composite laminate hot pressed at 1650 °C, indicating strong interfacial bonding between Al₂O₃ and Ta. Scanning electron microscopy (SEM), energy dispersive X-ray (EDS) analysis, and X-ray mapping of polished sections of the hot-pressed laminates showed the presence of an interfacial region formed presumably by diffusion of O (at 1450 °C) or O and Al (1650 °C) from Al₂O₃ into Ta. Composite femoral heads of Al₂O₃ and Ta could combine the low wear of an Al₂O₃ articulating surface with the safety of a ductile metal femoral head.     

Experimental study on the tension fatigue behavior and failure mechanism of 3D multi-axial warp knitted composites

An experimental study is described in this paper dealing with the tension–tension fatigue and failure mechanism of 3D MWK composites with different fiber architectures and material sizes. Macroscopic fracture morphology and SEM micrographs are examined to understand the fatigue damage and failure mechanism. The results show the fatigue properties and failure mechanism of composites can be affected significantly by the fiber architecture and material size. The fatigue life of material A(0°/0°/0°/0°) with small fiber orientation angle is significantly longer than that of material B(+45°/−45°/+45°/−45°). For material A, the fatigue properties of the long composite are better than that of the short one. It is 0° fiber bundles fracture under fatigue stress which cause the material failure and the long composite provides more space for the formation and propagation of local fatigue micro-cracks. However, for material B, the short composites have better fatigue properties. Moreover, the materials show typical ±45° zigzag fatigue fracture and obvious shear behavior. The fatigue cracks for the long composite can be spread more quickly along the fiber/matrix interface due to the fiber bundles realignment.     

Defect detection in CFRP by infrared thermography with CO2 Laser excitation compared to conventional lock-in infrared thermography

This paper presents a NDT by a CO₂ Laser infrared thermography applied to defect detection in CFRP. The CO₂ Laser is an infrared laser with the wavelength of 10.6 μm. This excitation has a controllable heating beam by a geometric relation D = 0.01575·d, which allows to heat the samples at a specific position (placed at the distance “d”) and area (of a diameter “D”). The PPT interpretation principle was used to reduce the non-uniformity’s effect of the excitation causing inhomogeneous heat. The test with this excitation is much faster than the tests with conventional lock-in thermography method.     

Repair of shear-deficient normal weight concrete beams damaged by thermal shock using advanced composite materials

The use of advanced composite materials such as Fiber Reinforced Polymers (FRPs) in repairing and strengthening reinforced concrete structural elements has been increased in the last two decades. Repairing and strengthening damage structures is a relatively new technique. The aims of this study was to investigate the efficiency and effectiveness of using Carbon Fiber Reinforced Polymer (CFRP) to regain shear capacity of shear-deficient normal weight high strength RC beams after being damaged by thermal shock. Sixteen high strength normal weight RC beams (100 × 150 × 1400 mm) were cast, heated at 500 °C for 2 h and then cooled rapidly by immersion in water, repaired, and then tested under four-point loading until failure. The composite materials used are carbon fiber reinforced polymer plates and sheets. The experimental results indicated that upon heating then cooling rapidly, the reinforced concrete (RC) beams exhibited extensive map cracking without spalling. Load carrying capacity and stiffness of RC beams decreased about 68% and 64%, respectively, as compared with reference beams. Repairing the thermal damaged RC beams allowed recovering the original load carrying without achieving the original stiffness. Repaired beams with CFRP plates with 90° and 45° regained from 90% to 99% of the original load capacity with a corresponding stiffness from 79% to 95%, whereas those repaired with CFRP sheet on the web sides and a combination of CFRP plates and sheet regained from 102% to 107% of the original load capacity with a corresponding stiffness from 81% to 93%, respectively. Finally, finite element analysis model is developed and validated with the experimental results. The finite element analysis showed good agreement as compared with the experimental results in terms of load–deflection and load–CFRP strain curves.     

Scaling effects in the tensile behavior of fiber-metal laminates

A study of scaling effects in a new lightweight fiber-metal laminate (FML) system has been undertaken. The FMLs were based on an aluminum alloy, a self-reinforced polypropylene composite and a polypropylene film acting as an interlayer adhesive. The investigation focuses on the feasibility of using scale models to predict the full-scale behavior of laminates based on [Al_n, 0°/90°_n]_s and [Al_n, ±45°_n]_s stacking configurations. Tensile tests were undertaken on laminates that were prepared using three different scaling approaches, these being 1D (scaling the thickness dimension), 2D (scaling the in-plane dimensions) and 3D scaling where all of the dimensions were scaled appropriately. Geometrical effects were investigated by monitoring the stiffness, strength, strain to failure and the stress/strain response of the FMLs. In the 1D and 3D-scaled samples, a small decrease in tensile strength was observed with increasing scale size, an effect that was associated with a change in failure mode from tensile fracture to interply delamination. In contrast, the strength of the 2D-scaled samples increased with increasing scale size, an effect that was attributed to the decreasing influence of edge delaminations in the larger test samples. The elastic modulus of the fiber-metal laminates did not exhibit any scale effect, remaining roughly constant over the four sizes of specimen considered here.     

Ultrasound-assisted brazing of Cu/Al dissimilar metals using a Zn–3Al filler metal

Ultrasound-assisted brazing of Cu/Al dissimilar metals was performed using a Zn–3Al filler metal. The effects of brazing temperature on the microstructure and mechanical properties of Cu/Al joints were investigated. Results showed that excellent metallurgic bonding could be obtained in the fluxless brazed Cu/Al joints with the assistance of ultrasonic vibration. In the joint brazed at 400 °C, the filler metal layer showed a non-uniform microstructure and a thick CuZn₅ IMC layer was found on the Cu interface. Increasing the brazing temperature to 440 °C, however, leaded to a refined and dispersed microstructure of the filler metal layer and to a thin Al_4.2Cu_3.2Zn_0.7 serrate structure in the Cu interfacial IMC layer. Further increasing the brazing temperature to 480 °C resulted in the coarsening of the filler metal and the significantly growth of the Al_4.2Cu_3.2Zn_0.7 IMC layer into a dendrite structure. Nanoindentation tests showed that the hardness of the Al_4.2Cu_3.2Zn_0.7 and CuZn₅ phase was 11.4 and 4.65 GPa, respectively. Tensile strength tests showed that all the Cu/Al joints were failed in the Cu interfacial regions. The joint brazed at 440 °C exhibited the highest tensile strength of 78.93 MPa.     

Tailoring of the interfacial properties of polymeric single fibre-reinforced epoxy composites by non-oxidative plasma treatments

The interfacial properties of epoxy composites reinforced with a single, plasma-treated fibre of either poly(p-phenyleneterephthalamide) (PPTA) or poly(p-phenylenebenzobisoxazole) (PBO) have been investigated with a focus on evaluating the effect of two non-oxidative (He and N₂) microwave plasma treatments on interfacial adhesion properties. Tensile testing of single filaments revealed that their tensile strength does not diminish with the plasma treatments, despite the fact that their surface properties have been both physically and chemically modified. Interfacial characterisation by Raman spectroscopy indicated that the quality of adhesion was substantially enhanced following exposure of the fibres to microwave plasma treatment in either pure helium or pure nitrogen flows for just one minute. Such improvement was higher than that attained when O₂ was used for blowing the plasma, under the same operational conditions. Moreover, no swelling effect was observed by AFM after exposure of the He or N₂ plasma-treated fibres to ambient conditions for as much as 24 h.     

Tunable BT@SiO2 core@shell filler reinforced polymer composite with high breakdown strength and release energy density

Barium titanate@silicon dioxide (BT@SiO₂) core@shell fillers with an average diameter of 100 nm were prepared by a facile sol–gel synthesis. The thickness of SiO₂ shell can be easily tuned by varying different mass ratio of BT to tetraethyl orthosilicate (TEOS). Polyvinylidene fluoride (PVDF) based composite films reinforced by BT and BT@SiO₂ were fabricated via a solution casting method. The effects of SiO₂ shell on morphology structure, wettability, interfacial adhesion, dielectric, electrical and energy performances of composites were investigated. Compared with BT/PVDF, BT@SiO₂/PVDF composites show significantly increased breakdown strength due to enhanced interfacial adhesion and suppressed charge carrier conduction. Benefiting from enhanced breakdown strength and reduced remnant polarization induced by SiO₂ shell, BT@SiO₂/PVDF shows increased release energy density (energy density which can be fully discharged and applicable). Especially, BT@SiO₂/PVDF with SiO₂ thickness of 4 nm exhibits the highest release energy density of 1.08 J/cm³ under applied electric field of 145 kV/mm.     

Effect of temperature on low velocity impact damage and post-impact flexural strength of CFRP assessed using ultrasonic C-scan and micro-focus computed tomography

The effect of temperature on the low velocity impact resistance properties and on the post-impact flexural performance of CFRP laminates were studied. With this aim, 150 × 75 mm cross-ply carbon fibre/epoxy laminates with a [0/90/90/0]_2s layup, therefore with a total of sixteen layers, were impacted at ambient temperature (30 °C) and at elevated temperatures (55, 75 and 90 °C) at a velocity of 2 m/s using a drop weight impact tower. This was followed by flexural tests carried out at ambient temperature using a three-point bending rig. Damage assessment of impact and post-impact behaviour were carried out using ultrasonic C-scan and microfocus X-ray computed tomography (μCT). Interrupted flexural tests using μCT allowed delamination propagation to be observed. In general, lower projected damage was observed at elevated temperatures, which resulted also in a possible hindrance to delamination and shear cracks propagation during impact and in a greater amount of retained flexural strength after impact.