Pseudo-ductile Behavior Research Articles

Pseudo-ductile unidirectional high modulus/high strength carbon fibre hybrids using conventional ply thickness prepregs

Pseudo-ductile hybrid composites usually require thin plies. In contrast, in the present paper unidirectional fibre-hybrid composites with conventional ply thicknesses are investigated. A prepreg with DIALEAD fibres and a 0.22 mm thickness was used as the high-modulus/low-elongation (LE) layer, combined with a prepreg with T800 fibres and a 0.11 mm thickness as the intermediate modulus/high-elongation (HE) layer. For hybrids with an LE volume fraction of 33% and below, pseudo-ductile behaviour in tension was obtained. The hybrid with an LE volume fraction of 50% has shown a transitional behaviour, mixing pseudo-ductility with a sharp stress drop after the failure of LE layer. The fracture mechanisms were interpreted using edge optical observation, full-field registration of the surface strain maps and analysis of the amplitude and frequency of acoustic emission signals. The pseudo-ductility was caused by the stable, progressive delamination of the LE/HE interface.

Low velocity impact and pseudo-ductile behaviour of carbon/glass/epoxy and carbon/glass/PMMA hybrid composite laminates for aircraft application at service temperature

Carbon/glass hybrid composite (CGHC) laminates are some of the most promising composites for lightweight applications. Sometimes these laminates are used in warm environment, such as aircraft frame structures, and this may affect their performance. In order to investigate this issue, the present research aims to study the effect of temperatures on the impact behavior and pseudo-ductile behaviour of CGHC in presence of different types of thermosets “epoxy” and thermoplastic “acrylic poly-methyl methacrylate-PMMA”. The experiments were started with making of CGHC laminates from different stacking sequences of unidirectional carbon and woven glass fibre layers, using a vacuum-assisted resin transfer method followed by curing treatment. In addition to CGHC laminates, four other neat batches (Carbon/epoxy, Carbon/PMMA, Glass/epoxy, Glass/PMMA) were prepared for comparison. The low velocity impact behaviour of the fabricated panels was evaluated at high temperatures (60 °C and 80 °C) according to ISO 6603-2 standard, using drop tower, while pseudo-ductile behaviour and ductility index (DI) of the specimens were estimated based on the measured total energy and elastic energy. Also, the low-velocity impact response was modeled mathematically based on a modified energy-balance model to predict the absorbed energies. Finally, the failure mechanisms were examined using optical microscope to determine the influence of these damage growth on DI of the composites under different temperatures. The results showed that the impact energy response of both hybrid composites i.e. epoxy and PMMA was stable even as the temperature rose, however, carbon/glass/PMMA exhibited better performance compared with carbon/glass/epoxy with an increase in impact energy response estimated at 50% (25 °C) and 53% (80 °C). Also, the pseudo-ductile phenomenon was strongly evident, which facilitates the predictablility of failure.

Pseudo-ductile behaviour in fibre reinforced thermoplastic angle-ply composites

Pseudo-ductile behaviour has previously been demonstrated in a [±45n]s carbon fibre composite via fibre rotation. To further develop the pseudo-ductile response, the high strain properties of the matrix should be considered to avoid strain localisation and potentially exploit strain-hardening in an amorphous thermoplastic matrix. For the first time, polycarbonate, a high strain-to-failure matrix is utilised in a pseudo-ductile composite design and compared to a high performance thermoset fibre composite containing an epoxy resin. The use of polycarbonate leads to enhanced pseudo-ductile strain (49%), longitudinal strength (24%) and apparent in-plane shear strength (26%), albeit with a reduction in elastic modulus (26%), shear chord modulus (22%) and yield strength (26%).

High performance quasi-isotropic thin-ply carbon/glass hybrid composites with pseudo-ductile behaviour loaded off-axis

The aim of this work was to investigate the effect of loading angle variation on the pseudo-ductility of quasi-isotropic (QI) hybrid composite laminates. Previously, hybrids of thin-ply carbon fibres and standard glass fibres were found to have an excellent pseudo-ductile behaviour both in unidirectional (UD) and QI configurations when subjected to axial tension in the fibres’ orientations. In this work, the QI laminates, with 60° intervals, have been subjected to a quasi-static tensile load at various off-axis orientations – i.e. 5°, 10° and 20°. The QI hybrid composites were made by sandwiching a QI T300-carbon laminate between the two halves of a QI S-glass laminate. The results showed a pseudo-ductile behaviour with a linear elastic part and a desirable plateau for all the loading directions, however the pseudo-ductile strain decreases when increasing the off-axis angle. Comparing the 20° off-axis with the other cases, there was more active matrix cracking damage before fragmentation in the 20° off-axis plies and it failed earlier than the other samples. Acoustic emission (AE) results confirmed this, with more matrix cracking related AE signals in the 20° off-axis case compared to the other configurations.

Progressive failure in interply hybrid composites of self-reinforced polypropylene and glass fibre

Hybridisation of two composites has the potential of tailoring properties to reach a composite structure with advantages of both constituents where the drawbacks are lessened. The current work reports this practice for self-reinforced polypropylene (SRPP) composites and glass fibre to create an interply hybrid composite with enhanced stiffness and ductility. The effect of glass fibre volume fraction, matrix adhesion, and layup, was studied as the parameters to affect progressive failure through pseudo-ductility. Hybrids with up to 6.5% glass fibre volume fraction showed pseudo-ductile behaviour. However, matrix adhesion and layup interfered with this effect. It was found that composites with low adhesion matrix exhibited higher pseudo-yield strain, and for the laminates with an interleaved layup, the maximum glass fibre volume fraction for progressive failure increased by 100%. The impact behaviour of the composite laminates also demonstrated that brittleness of the glass fabric reduces the energy absorption capability of the laminates compared to SRPP composite of the same thickness.

Fiber Volume Fraction Influence on Randomly Distributed Short Fiber Tungsten Fiber‐Reinforced Tungsten Composites

For future fusion reactors, tungsten (W) is currently the main candidate for the application as plasma‐facing material due to its several advanced properties. To overcome the brittleness of W, randomly distributed short W fiber‐reinforced W (Wf/W) composites have been developed using field‐assisted sintering technology (FAST). Herein, Wf/W materials with different fiber volume fraction (20–60%) are manufactured by FAST process to study the fiber volume fraction influence on the composite properties. Wf/W with ductile fibers and brittle fibers is produced using different tool setups during the production. Three‐point bending tests on prenotched samples, 4‐point bending tests, and tensile tests have been performed to determine the fracture behavior and flexural/tensile strength of the material. Wf/W materials with 30–40% fiber volume fraction exhibit a promising pseudoductile behavior, similar to fiber‐reinforced ceramic composites. However, Wf/W with 20% and >50% fiber volume fraction shows only a limited extrinsic toughening effect. In terms of flexural strength, with increasing fiber volume fraction, the tensile/flexural strength does not show a clear increasing tendency, or even lightly decreases.

Mode I and II interlaminar critical energy release rates in all-carbon interlayer unidirectional fibre-hybrids based on ultrahigh-modulus and high-strength fibres

Delamination resistance governs the damage mode in interlayer hybrid composites. The interlaminar critical energy release rates (CERR) GIc and GIIc are for the first time reported for the different interfaces in hybrids composed of ultrahigh-modulus, low-elongation (LE) carbon fibre layers and high-strength, high-elongation (HE) carbon fibre layers in an epoxy matrix. The GIc and GIIc values at the LE/LE interface were several times lower than for the HE/HE. The GIc of the LE/HE interface was0 close to that of the HE/HE interface, whereas its GIIc was significantly higher than the average of those of the LE/LE and HE/HE interfaces, and 1.5 times higher than LE/LE value. This synergy should be taken into account for the design of LE/HE carbon fibre UD hybrids featuring stable pseudo-ductile behaviour.

Development of tungsten fiber-reinforced tungsten with a porous matrix

Tungsten is the main candidate for the plasma-facing material in future fusion reactors. To overcome the brittleness of tungsten, tungsten fiber-reinforced tungsten (Wf/W) composites have been developed using a powder metallurgy process. In this study, a novel type of Wf/W with a porous matrix has been developed using field-assisted sintering technology. Compared with conventional Wf/W, the avoidance of a fiber–matrix interface simplified the production process. Initial mechanical testing showed that Wf/W with a porous matrix can establish a promising pseudo-ductile behavior with an increased fracture toughness compared with pure tungsten.

Optimization of the microstructure of unidirectional hybrid composites under uniaxial tensile loads

Typical composite structures are characterized by having brittle failure and thus higher safety factors. Alternatively, hybrid composites with pseudo-ductile behaviour can contribute to safer and more efficient designs.The objective of this work is to achieve a “pseudo-ductile” behaviour in the response of unidirectional hybrid composites when subjected to uniaxial traction. To understand under what circumstances such behaviour is obtained, optimization problems are formulated and solved here.A Spring Element Model is used to predict the hybrid composite failure with relatively low computational cost. This cost is an important issue since optimization requires several analyses of the composite response. Two different types of optimization problems are considered. Firstly, one finds out the optimal properties of fibres to hybridize and get the pseudo-ductile behaviour. Once an optimal hybridization is found, another optimization problem is solved in order to understand the influence of the fibre dispersion on the composite response. Both optimization problems are solved using a Genetic Algorithm.The optimal results obtained show hybrid composites having a considerable pseudo-ductile behaviour. It is also shown that the degree of spatial fibre dispersion at the composite microstructure level greatly affects the composite response. High fibre dispersion favours the pseudo-ductile behaviour.

Novel ceramic matrix composites with tungsten and molybdenum fiber reinforcement

Ceramic matrix composites usually utilize carbon or ceramic fibers as reinforcements. However, such fibers often expose a low ductility during failure. In this work, we follow the idea of a reinforcement concept of a ceramic matrix reinforced by refractory metal fibers to reach pseudo ductile behavior during failure. Tungsten and molybdenum fibers were chosen as reinforcement in SiCN ceramic matrix composites manufactured by polymer infiltration and pyrolysis process. The composites were investigated with respect to microstructure, flexural- and tensile strength. The single fiber strengths for both tungsten and molybdenum were investigated and compared to the strength of the composites. Tensile strengths of 206 and 156 MPa as well as bending strengths of 427 and 312 MPa were achieved for W/SiCN and Mo/SiCN composites, respectively. The W fiber became brittle across the entire cross section, while the Mo fiber showed a superficial, brittle reaction zone but kept ductile on the inside.

Engineering tensile behavior of hybrid carbon fiber/self-reinforced polypropylene composites by bio-inspired fiber discontinuities

Carbon fiber layer failure is vital for the tensile behavior of interlayer hybrid carbon fiber (CF)/self-reinforced polypropylene (SRPP) composites. Introducing cuts, termed here as discontinuities, into the carbon layer is a promising way to tailor its failure behavior. Inspired by structural features of biological composites, we designed and produced hybrid composites with hierarchical and polygonal arrangements of discontinuities in the carbon layer. Increasing the number of levels in the hierarchical patterns delayed the onset of carbon layer failure, hence improving the damage resistance of the hybrid composites. A progressive carbon layer failure was achieved with the polygonal patterns by creating a transition from fiber bundle pull-out to fiber bundle fracture. Spreading the polygonal patterns throughout the specimen resulted in a unique diffused delamination distribution that has not been reported in the literature. Pseudo-ductile behavior was achieved by creating dispersed fiber bundle pull-out with the fully dispersed polygonal patterns. The resulting hybrid CF/SRPP composites demonstrated a rare combination of stiffness (10 GPa) and ductility (~16% failure strain) with a pseudo-ductile strain over 14%. This paper delivers and proves the concept of utilizing discontinuities to engineer the tensile behavior of hybrid composites.

Strength and deformation properties of structural lightweight concrete under true tri-axial compression

In this paper, the strength and deformation of structural all lightweight concrete (ALWC), structural semi-lightweight concrete (SLWC) and normal weight concrete (NWC) subjected to uniaxial, equibiaxial and truly tri-axial compression was experimentally studied and compared. The stress-strain curves in three principal directions were recorded, and the failure modes of different specimens under various stress states were observed. Test results showed that the failure modes were mainly dependent on the stress ratio, but independent of concrete types. For all types of concrete, the peak strength under biaxial and triaxial compression is substantially higher than uniaxial compressive strength. At the lower compression stress ratio, i.e. σ1/σ3 = 0.1, all the concrete exhibited relatively brittle behavior, the relative tri-axial compressive strength of ALWC and SLWC was in a range of 1.61–2.05 and 1.93–2.63 respectively, compared to 2.25–3.21 for NWC. The peak deformation in three principal directions for ALWC and SLWC was far greater than that of NWC. At the higher compression stress ratio, i.e. σ1/σ3 = 0.25, all concretes showed pseudo-ductile behavior, the relative tri-axial compressive strength and peak deformation in three principal directions of ALWC and SLWC became close to those of NWC. Considering the role of the intermediate principal stress for concrete under true triaxial compression, a 4-parameter failure criterion model with different material constants was developed. The proposed failure surface was verified against experimental data of NWC, ALWC, and SLWC under multiaxial loads in this paper, as well as low strength LWC under truly tri-axial compression in the literature. It provided the experimental and theoretical foundations for strength analysis of LWC structures subject to complex loads.

Fracture behavior of random distributed short tungsten fiber-reinforced tungsten composites

In future fusion reactors, tungsten is considered as the main candidate material for plasma-facing components. However, the intrinsic brittleness of tungsten is of great concern during operation. To overcome this drawback, tungsten fiber-reinforced tungsten composites (Wf/W) are being developed relying on extrinsic toughening principles. Tungsten (W) fibers with extremely high tensile strength and ductility are used to reinforce a tungsten matrix.In this work, field assisted sintering technology (FAST) is used to produce Wf/W material. Mechanical characterizations including Charpy impact and 3-point bending tests are performed. Based on the 3-point bending test results, the Wf/W materials can facilitate a promising pseudo-ductile behavior even at room temperature, similar to fiber reinforced ceramic composites. Fracture energy density and fracture toughness together with the crack-resistance curves (R-curves) are measured. Compared to conventional pure tungsten, Wf/W shows significant improvement in fracture toughness.

Rapid Preparation of 2D C/C Composite by Moulding Method Based on Recycled Carbon Fiber

Efforts were made on recycling carbon fiber (CF) from carbon fiber reinforced epoxy resin (CF/EP) composite by pyrolysis and reused to rapid fabricate 2D C/C composite in this work, which provides a new direction for high-value reusing recycled carbon fiber (rCF). The results showed that a small amount of pyrolytic carbon was retained in rCF, but the rCF was not markedly damaged and kept comparable properties to virgin carbon fiber (vCF) after pyrolysis treatment. The rCF was subsequent used to rapid fabricate 2D C/C composite though moulding method. No significant negative effect of pyrolytic carbon on densification of rCF C/C composite was observed and there was no obvious structural defect within the rCF C/C composite. The as-obtained rCF C/C composite has a comparable flexural strength of 90.25 MPa with that of the vCF C/C composite of 95.45 MPa. Both the rCF C/C and vCF C/C composite exhibited a pseudo-ductile fracture behavior after reaching the maximum value of the stress-deflection curves. Surprisingly that the pyrolytic carbon has no fatal effect on the mechanical properties of the prepared rCF C/C composite, which is expected to be used as substitute in certain fields.

Post-Impact Behaviour of Pseudo-Ductile Thin-Ply Angle-Ply Hybrid Composites.

This work experimentally explores the post-impact behaviour of thin-ply angle-ply pseudo-ductile carbon fibre laminates subjected to tensile load. Indentation and low-speed impact tests were performed on standard tensile test specimens. Non-destructive tests were used to investigate the damage propagation. Digital Image Correlation (DIC) was adopted to detect the strain distribution during tensile tests. Post-damage pseudo-ductile behaviour was retained in angle-ply hybrid composites subjected to tensile loading conditions.

Fracture surfaces of tungsten wires used in fiber-reinforced plasma facing components: Effect of potassium doping and high temperature annealing

We have studied the microstructure of tungsten fibers, which are considered as reinforcement elements for the advanced tungsten composites (Wf/W) known to exhibit pseudo ductile behaviour at room temperature. The potentially negative impact of the high temperature annealing, expected under operation in fusion environment, remains to be explored and mitigated. Doping by potassium is considered as a main option to delay recrystallization and grain growth inside the drawn tungsten wires to a higher temperature. Here, we have performed a systematic analysis of the fracture surface of pure and K-doped tungsten wires which were annealed prior to a uniaxial tensile test. The results demonstrate that the fracture mechanism depends strongly on the annealing temperature and presence of potassium doping. Four fracture mechanisms were clearly distinguished and classified by converting into deformation maps as a function of annealing and test temperature. By summarizing all previously available results and current ones, one can demonstrate that potassium doping delays massive grain growth (and subsequent loss of strength) by ∼600 °C as compared to the pure W wire and this positive effect holds for the deformation not only at room temperature but at least up to 500 °C.

Development of Pseudo ductile behavior of carbon fiber reinforced composites-An Overview

Abstract Hybridization in polymer matrix composites (PMCs) is one of the novel approaches to improve the mechanical behavior. But in case of structural design most of the failure occurs without warning i.e. brittle failure. Current high-performance composites are stiff, strong and light but fail suddenly and catastrophically. These types of limitations can be overcome by introducing pseudo ductile behavior into the failure process. An overview of significant achievement in pseudo ductile behavior is presented by focusing over rate of fiber dispersion, rule of mixture, orientation of fibers, and introduction of continuous and discontinuous fibers obtained by HiPerDiF (High Performance Discontinuous Fibers) method. HiPerDiF methods offers flexibility in shape of hybrid composites along with type of fiber, fiber length, preform pattern, resin type. The most crucial arrangement of HEF (High Elongation Fiber) and LEF (Low Elongation Fiber) leads to enhance the pseudo ductile performance.

ABAQUS analysis of the SiC cladding fuel rod behavior under PWR normal operation conditions

Silicon Carbide (SiC) cladding is anticipated to be superior to zirconium-based cladding with regard to tolerating light water reactor (LWR) accidents, but it is also crucial that the cladding performance under normal operation conditions be well understood. This paper focuses on the modeling of thermomechanical behavior of a two-layer SiC cladding in Pressurized Water Reactor (PWR) fuel rod under normal operation conditions. For this purpose, a set of ABAQUS subroutines has been written to incorporate material properties and behavior models of Chemical Vapor Deposited (CVD) monolithic SiC, Chemical Vapor Infiltration (CVI) SiCf/SiC Ceramic Matrix Composite (CMC) and UO2. Especially, apart from UO2 irradiation behaviors and fuel rod phenomena models (gas pressure, gap heat transfer, etc.) which have been extensively studied, an alternative swelling model of SiC dedicated for transient simulation and a pseudo-ductile behavior model of CVI SiCf/SiC have been implemented. Both models are important for conducting best estimate analysis of the fuel rod in-pile behavior. Then after benchmarking with the BISON fuel performance analysis code, the ABAQUS model is applied to a downscaled PWR fuel rod with two-layer SiC cladding design, considering different cladding thicknesses and covering the conditions of simple constant power levels, power ramps, power operation mode, reactor hot standby and cold shutdown. The effect of swelling differential between CVD SiC and CVI SiCf/SiC is also discussed. With the help of finite element analysis (FEA), the tightly coupled thermomechanical behavior of SiC cladding fuel rod can be well studied.

The effect of heat treatments on pure and potassium doped drawn tungsten wires: Part II – Fracture properties

Advanced tungsten fibre-reinforced composites (Wf/W), showing pseudo ductile behaviour even at room temperature, are a promising option for future fusion reactors as the intrinsic brittleness of tungsten can be mitigated effectively. The drawn tungsten wires used as reinforcements are the key components of the composites, thus their mechanical properties and thermal stability define the allowed operation/fabrication temperature of the composite material itself. In this work, the room temperature fracture behaviour of the pure and potassium doped tungsten wires was investigated, focusing on the evolution of the fracture micromechanisms in respect to annealing. Single-edge-notched specimens were used, with the crack growth direction perpendicular to the drawing axis of the wire. The occurrence of either a fractographic brittle or a ductile response in the as-received state of both materials is a strong indication that the ductile-to-brittle transition temperature is about room temperature. Pure, annealed tungsten wires experience a tremendous deterioration of the fracture toughness with a very prominent transition of the failure mode. The observed embrittlement by annealing can be related to the loss of the fibrous, elongated microstructure. In contrast to this, the results of the annealed, doped wires demonstrate that the microstructural stability and preservation of the initial, beneficial grain structure is directly reflected in the crack resistance of the material. Predominately ductile behaviour, with characteristic knife-edge necking, is seen even after annealing at 1600 °C.

The effect of heat treatments on pure and potassium doped drawn tungsten wires: Part I - Microstructural characterization

Advanced tungsten fiber-reinforced composites (Wf/W), showing pseudo ductile behaviour even at room temperature, are a promising option for future fusion reactors as the intrinsic brittleness of tungsten can be mitigated effectively. The drawn tungsten wires used as reinforcements are the key components of the composites, thus their mechanical properties and thermal stability define the allowed operation/fabrication temperature of the composite material itself. In this work, a comprehensive characterization of the pure and potassium doped tungsten wires was performed, focusing on the influence of various heat treatments on different microstructural features (nature of grain boundaries, grain shape and size, texture analyses) and mechanical properties. Annealing in the temperature range from 900 to 1600 °C enables the investigation of the microstructural stability of the two materials and arising annealing phenomena – recovery, recrystallization and grain growth. The results demonstrate that the pure tungsten recrystallizes fully in the temperature range 1300–1500 °C accompanied with tremendous coarsening and a complete loss of the initial fibrous, elongated grain structure. In contrast to this, potassium doped wire shows superior high temperature properties, where the performed heat treatments cause only milder microstructural changes, consequently suppressing recrystallization and grain growth to temperatures well above the investigated ones. Furthermore, hardness measurements and observed softening complement the discussion of the grain morphology evolution.