Transverse Fracture Toughness Research Articles

A novel 3D fibre-reinforcement architecture for high performance natural fibre reinforced composite adhesively bonded joints

ABSTRACT In this paper, the effect of a novel fibre reinforcement architecture in the adhesively bonded joint efficiency of natural fibre reinforced composites (NFRC) was investigated. Two different reinforcement techniques were used: intralaminar reinforcement (2D) and orthogonal-through-the-thickness reinforcement (3D). The aim of the novel architecture is to enhance the transverse properties of the adherend (transverse strength and fracture toughness) in order to delay or avoid delamination failures. A jute bidirectional fabric was used as a base primary reinforcement phase and curauá, sisal, ramie, hemp and glass fibres were used as secondary reinforcement phases for the 2D and 3D fibre reinforcement architectures. Single lap joints (SLJs) bonded with an epoxy adhesive used in the automotive industry were fabricated with these adherends and the efficiency of the joints was investigated by comparing them to glass (GFRP) and carbon (CFRP) pure synthetic fibre reinforced composite joints. It was found that the novel architecture was successful in reaching the failure load of the synthetic composite joints for SISAL 3D, CURAUÁ 2D and CURAUÁ 3D SLJs. Therefore, NFRC bonded joints can be a viable replacement for synthetic fibre composite joints at no load-bearing loss.

Effect of Mn addition on the microstructure, interface structure and mechanical properties of MoCoB–Co cermets prepared by in-situ synthesis

MoCoB–Co cermets with different Mn contents were synthesized using an in-situ sintering reaction, and the effect of Mn addition on the microstructure, interfaces, and mechanical properties was investigated. The addition of Mn inhibited the transition of the binder phase from fcc to hcp. Adding an appropriate Mn can increase the density of the material and refine the hard phase grain. With increasing Mn content, the hardness decreased marginally and then gradually increased, and the transverse rupture strength (TRS) and fracture toughness (KIC) gradually decreased. The cermet with 6 wt% Mn addition had the smallest grain size and lower porosity. The hardness of the cermet increased from HRA 89.0 to HRA 91.7. The interface bonding strength of MoCoB/hcp-Co is higher than that of MoCoB/fcc-Co. After isothermal aging treatment, the content of hcp-Co in the binder phase increased markedly; thus, the KIC and TRS increased from 11.3 MPa m1/2 and 1200 MPa–13.3 MPa m1/2 and 1750 MPa, respectively.

Additive manufacturing of cemented carbide using analogous powder injection molding feedstock

In this study, WC-8%Co cemented carbide was fabricated by extrusion-based additive manufacturing process using analogous powder injection molding feedstock. Feedstocks with powder loading content of 51 vol% (PW51) and 57 vol% (PW57) were prepared based on thermoplastic binder system composed of paraffin wax, high density polyethylene and ethylene-vinyl acetate. The results show that the extrusion of feedstock is steady and easily controlled. The binder is efficiently removed by solvent leaching and thermal decomposition. After sintering at 1400 °C, densified samples with a relative density beyond 99% are obtained. Sintered PW57 presents less and more isotropic shrinkage below 17.0%, showing a great significance in print accuracy and deformation control. Sintered PW57 has larger transverse rupture strength and fracture toughness of 1695 MPa and 7.27 Pa·m0.5, respectively. Vickers hardness of sintered PW51 is slightly larger because of finer WC grain size. Mechanical properties, microstructure and surface roughness of sintered samples are all acceptable in actual application. Three-dimensional printing using an extrusion-based additive manufacturing with analogous powder injection molding feedstock is an extremely practicable method to fabricate cemented carbide parts with complicated shape.

On the reaction mechanism of Mo2FeB2-based cermets and wear transition induced by self-lubricating tribo-oxide layer

During the sintering process, the phase evolution of Mo2FeB2-based cermets mainly occurred in the solid stage. When the liquid formed, the phase composition remained constant and the “dissolution-reprecipitation” process predominated the microstructural evolution. At 1250 °C, the hardness, transverse rupture strength and fracture toughness of Mo2FeB2-based cermets reached 88.6 HRA, 1480 MPa and 14.1 MPa⋅m1/2, respectively. The tribology and wear behaviors of Mo2FeB2-based cermets were evaluated from 25 °C to 800 °C. The abrasive wear prevailed at 25 °C, while the wear mechanism transitioned to oxidative mild wear due to the formation of the tribo-oxide layer at 400 °C and 600 °C. Besides the anti-wear Fe2O3, the lubricant MoO3 and B2O3 were also formed. The multi-component self-lubricating tribo-oxide layer simultaneously reduced the wear rate and coefficient of friction of Mo2FeB2-based cermets. However, by further increasing the temperature to 800 °C, the cracked matrix and impaired tribo-oxide layer resulted in the transition of wear mechanism to oxidative severe wear. The appropriate combination of load and temperature was necessary for the formation and stability of the tribo-oxide layer.

The Effect of Carbon Content on the Microstructure and Mechanical Properties of Cemented Carbides with a CoNiFeCr High Entropy Alloy Binder.

CoNiFeCr high entropy alloy (HEA) was used as a binder in cemented carbides for developing a new high-performance binder. The microstructure of WC-HEA cemented carbides with different binder components and carbon contents was subsequently studied. It was observed that the (Cr,W)C phase precipitated at the WC/HEA interface, and a coherent interface with a low degree of misfit was formed between WC and (Cr,W)C, thereby resulting in a significant reduction in the interfacial energy and stress concentration. The (Cr,W)C phase exerted a pinning force (Zener-drag) on the moving grain boundaries, which effectively inhibited the growth of WC grains. As a result, compared with WC-Co, WC-CoNiFeCr had smaller WC grain size, smoother grain shape and larger mean free path (MFP) of the binder, which resulted in slightly lower hardness and higher transverse rupture strength (TRS) and fracture toughness. The lower limit of carbon content in WC-CoNiFeCr was higher than that of WC-Co. With the addition of Ni, the width of the two-phase region became wider, whereas the width of the two-phase region became narrower with the addition of Fe and Cr.

Fracture toughness-based models for web-crippling of pultruded GFRP profiles

This paper presents a numerical study on the web-crippling failure of pultruded glass fibre reinforced polymer (GFRP) profiles, under external-two-flange (ETF) and internal-two-flange (ITF) configurations. The numerical study is validated through experimental tests recently conducted by the authors on four I-section profiles and one U-section profile, obtained from four different suppliers. These web-crippling tests were simulated through shell finite element (FE) models where the transverse compressive fracture toughness of each GFRP material was implemented as a damage evolution parameter. In order to quantify the influence of material damage and instability effects on the web-crippling failure, three different analyses were implemented, accounting for (i) material damage, (ii) instability, and (iii) both effects. The results obtained through the analysis accounting for both material damage and instability effects showed a good agreement with experimental results, in terms of stiffness, failure modes and ultimate loads; moreover, the transverse compressive strain distributions obtained from the numerical models also agreed well with experimental results. Finally, the transverse compressive stresses were found to influence web-crippling the most, regarding both damage initiation and ultimate load stages.

Preparation and wear resistance of dual-scale structure cemented carbide spherical teeth

Based on commercially available WC (particle sizes of 0.9 μm and 2.4 μm) and Co, a dual-scale structure WC-6Co cemented carbide was prepared via conventional roller ball milling and following vacuum sintering. Effect of ball milling process on the microstructure, densities and mechanical properties were investigated together with the toughening mechanism of the dual-scale structure. The results showed that the dual-scale structured cemented carbide has excellent comprehensive mechanical properties, and its transverse rupture strength, hardness and fracture toughness are 3077 MPa, 1570 HV30 and 10.6 MPa m1/2, respectively. The drilling spherical teeth was prepared by the optimized technology and subjected to impact wear test, the failure mechanism of cemented carbide spherical teeth in drilling operations is studied. The CM-2 spherical teeth had a more reasonable distribution structure of coarse and fine of WC, and exhibited more excellent impact wear resistance in the impact wear test of granite.

A Modified Compact Tension Test for Characterization of the Intralaminar Fracture Toughness of Tri-Axial Braided Composites.

The application of braided composite materials in the automotive industry requires an in-depth understanding of the mechanical properties. To date, the intralaminar fracture toughness of braided composite materials, typically used for describing post-failure behavior, has not been well-characterized experimentally. In this paper, a modified compact tension test, utilizing a relatively large specimen and a metallic loading frame, is used to measure the transverse intralaminar fracture toughness of a tri-axial braided composite. During testing, a relatively long fracture process zone ahead of the crack tip was observed. Crack propagation could be correlated to the failure of individual unit cells, which required failure of bias-yarns. The transverse interlaminar fracture toughness was found to be two orders of magnitude higher than the reference interlaminar fracture toughness of the same material. This is due to the fact, that intralaminar crack propagation requires breaking of fibers, which is not the case for interlaminar testing.

Improvement of microstructure, mechanical properties and cutting performance of Ti(C,N)-based cermets by ultrafine La2O3 additions

Ti(C,N)-WC-Mo2C-TaC-Co-Ni cermets with various content of La2O3 were prepared by gas-pressure sintering at 1450 °C. The effects of ultrafine La2O3 additions (0, 0.05, 0.1 and 0.2 wt%) on the microstructure, mechanical properties, wear resistance and cutting performance of cermets were explored. In the microstructure of cermets, the La2O3 particles and dissolved La element in binder phases were observed, which could inhibit the dissolution-precipitation process of ceramics phases during liquid-sintering. Furthermore, the La2O3 could absorb and react with the impurity Al element with low melting point from raw powders, avoiding the appearance of liquid phase at the low temperature and partial overheating during sintering process. These mechanisms could inhibit the abnormal growth of Ti(C,N) core-(Ti,W,Mo,Ta)(C,N) rim structures effectively, leading to the thinning of brittle rim phases and coarsening of wear-proof Ti(C,N) particles. The decrease of proportion of brittle rim phase and ultrafine Ti(C,N) particles promoted the fracture toughness. The increase of proportion and grain size of Ti(C,N) improved the hardness, wear resistance and cutting performance significantly. However, the excessive addition of La2O3 would result in the agglomeration of La2O3, causing the sharp decline of mechanical properties and cutting performance. The cermet with 0.1 wt% La2O3 addition possessed the optimal mechanical properties with Vickers hardness, transverse rupture strength and fracture toughness of 1710 (HV30) Kgf/mm2, 2480 MPa and 11.7 MPa m1/2, respectively.

Effect of ultrafine WC contents on the microstructures, mechanical properties and wear resistances of regenerated coarse grained WC-10Co cemented carbides

The effects of ultrafine WC content on the microstructure, mechanical properties and wear resistance of regenerated coarse grained WC-10Co cemented carbides have been investigated by SEM, mechanical property tests and wear test. The results show that with the increase of ultrafine WC content, the average grain size of the regenerated cemented carbides increases first and then decreases. When the ultrafine WC content is 10%, the average grain size of the regenerated cemented carbides is the maximum (4.06 μm). Meanwhile, the irregular shape grain decreases and the growth of grain is improved. When the ultrafine WC content is 10%, the regenerated cemented carbides have the highest density (99.84%), which could alleviate the negative impact of coarse grains on hardness. The transverse rupture strength and fracture toughness of regenerated cemented carbides increases on account of crack deflection, crack branching, crack bridging, the increase of transgranular fracture of WC and dimple fracture of Co phase (TRS: 2520 MPa, KIC: 20.28 MPa·m1/2). Therefore, when the ultrafine WC content is 10%, the regenerated cemented carbides have more optimal mechanical properties. The addition of ultrafine WC particles balances the hardness and fracture toughness of the regenerated cemented carbides in the sliding wear experiment, which is found that the friction coefficient, wear rate, wear scar width and depth of regenerated cemented carbides are the smallest when the content of ultrafine WC is 15%.

Effect of W/B atomic ratio on the microstructure and mechanical properties of WCoB-TiC ceramic composites: first-principles calculations and experiment

The thermodynamic stability, mechanical properties and electronic properties of the WCoB and W2CoB2 hard phases in WCoB-TiC ceramic composites are analyzed by the first-principles calculations. The microstructure, hardness, transverse rupture strength (TRS) and fracture toughness (KIC) of WCoB-TiC ceramic composites with various W/B atomic ratios are measured by experiments. First-principles calculations indicate that W2CoB2 has better thermodynamic stability and comprehensive mechanical properties than WCoB. The poor mechanical properties of WCoB are due to the heterogeneous nature of Co-W anti-bonds. The better mechanical properties of W2CoB2 are mainly derived from the stronger BB covalent bonds. Experimental results show that the density of WCoB-TiC ceramic composites increases rapidly with increasing of W/B atomic ratio initially and then gradually stabilized. Hardness, TRS and KIC of WCoB-TiC ceramic composites increases firstly and then decreases with the increase of W/B atomic ratio. When W/B atomic ratio is 0.6, hardness is the highest at 92.3 ± 0.3 HRA. When the W/B atomic ratio is 0.5, the WCoB-TiC ceramic composite has the highest TRS and KIC, which are 853.6 ± 30.3 MPa and 11.48 ± 0.33 MPa m1/2, respectively.

A study of the preparation and properties of dense binderless titanium carbonitride-based ceramics

In this study, the surface oxidation phenomenon of raw powders in the preparation of binderless titanium carbonitride-based ceramics was investigated, which might hinder the densification process. In order to reduce the oxidation and strengthen mutual diffusion, the micrometer-sized powder mixtures of Ti(C,N) and carbides additives, pre-alloyed (Ti,W,Mo,Ta)(C,N) solid solution powders were used. The utilization of solid solution raw powders can reduce the oxidation and promote densification effectively. In this way, the binderless ceramics with high relative density and great mechanical properties were fabricated. The hardness, transverse rapture strength and fracture toughness of materials prepared by solid solution powders were 2030 HV, 1280 MPa and 7.8 MPa m1/2, respectively. Moreover, the obtained binderless titanium carbonitride-based ceramics were found with low friction coefficient, excellent wear and oxidation resistance. The great oxidation resistance was attributed to the formation of rutile-type oxidation products facilitated by the existence of other metal elements in the ceramics.

Compressive transverse fracture behaviour of pultruded GFRP materials: Experimental study and numerical calibration

This paper presents a study on the transverse fracture behaviour of pultruded glass fibre reinforced polymer (GFRP) materials in compression, namely the assessment of transverse compressive fracture toughness (G2−) and transverse compressive residual strength (σr). These properties were assessed through experimental Compact Compression (CC) tests of six different GFRP materials, which were coupled with a data reduction method to determine experimental predictions of G2− and numerically calibrated as a function of the experimental load vs. displacement curves. Through this process, the expected experimental overestimation of G2−, attributed to contact stresses behind the notch tip, was evaluated and accurate numerical estimates of G2− were determined. The numerical calibration considered both G2− and σr, by considering a bi-linear cohesive law. Through the numerical calibration, the G2− results ranged between 36 and 67 N/mm and σr values varied from 9% to 16% of transverse compressive strength (7.6 and 16.9 MPa, in absolute values). Finally, the G2− results were analysed as a function of transverse fibre reinforcement percentage and compared to transverse tensile fracture toughness results (G2+) determined in a previous work. This study showed that G2− is generally higher than G2+ and that the compressive-to-tensile ratio is inversely proportional to the transverse reinforcement percentage.

Effect of multilayer graphene as a reinforcement on mechanical properties of WC-6Co cemented carbide

Different contents of graphene was added to the WC-6Co alloy powder to prepare cemented carbide by low pressure sintering. The microstructure, hardness, transverse rupture strength, fracture toughness and thermal conductivity of cemented carbide were investigated. The results show that graphene was distributed as a reinforcement at the Co phase, WC/WC interface and WC/Co interface. The hardness of the cemented carbide decreased slightly with the increase of graphene content. The transverse rupture strength increased first and then decreased with the increase of graphene content, and reached 2321.2 MPa when the content of graphene is 0.05 wt%. The fracture toughness increased to 13.1 MPa m1/2 after adding 0.05 wt% graphene. The thermal conductivity of the sample with 0.05 wt% graphene was greatly improved. After comprehensive consideration, the cemented carbide with 0.05 wt% graphene has excellent performance and meet the performance indexes of high hardness, strength and toughness.

Interfaces and mechanical properties of dual grain structure Ti(C,N)-based cermets prepared by in situ reduction of TiO2

Ti(C,N)-based cermets with dual grain structure were prepared by partially replacing coarse Ti(C,N) particles with TiO2 and carbon via in situ carbothermal reduction. The microstructure and interface structure of the samples were characterized using SEM, EDX, and TEM. With optimized TiO2 content, significant enhancements of the transverse rupture strength (3324 ± 52 MPa) and fracture toughness (18.6 ± 0.5 MPa m1/2) were obtained in the present cermets without losing much of the hardness. The orientation relationship and coherency state at both the rim/binder and rim/rim interfaces in the present cermets changed considerably compared with conventional cermets. Based on detailed analysis of the interface structure, it was demonstrated that interfacial characteristics were the dominant factors for the significantly increased strength and toughness in the present dual grain structured cermets. This study provided an effective method for obtaining an excellent combination of transverse rupture strength and toughness in cermets.

Microstructural evolution and mechanical properties of 300M steel produced by low and high power selective laser melting

300M ultra-high strength steel has been widely used in critical structural components for aviation and aerospace vehicles, owing to its high strength, excellent transverse plasticity, fracture toughness and fatigue resistance. Herein, low and high power selective laser melting (SLM) of 300M steel and their microstructural evolution and mechanical properties have been reported. The results show that the optimal energy density range with the highest relative density for SLMed 300M steel is between 60 and 160 J/mm3. Furthermore, molten pools for deposition exhibit a conduction mode with semi-elliptical shape at a lower laser power of 300–600 W but a keyhole mode with “U” shape at a higher laser power of 800–1900 W. The heterogeneous microstructure of as-built samples is characterized by a skin-core structure which is that tempered troostite with the coarse non-equiaxed grains in the molten pool is wrapped by tempered sorbite with the fine equiaxed grains in the heat-affected zone. The skin-core structure of SLMed 300M steel has the characteristics of hard inside and soft outside. The average microhardness of samples varies from 385 to 341 HV when laser power increases from 300 to 1900 W. Interestingly, ultimate tensile strength (1156–1193 MPa) and yield tensile strength (1085–1145 MPa) of dense samples fabricated at different laser powers vary marginally. But, the elongation (6.8–9.1%) of SLMed 300M steel is greatly affected by the laser power.

Influence of Cr and W addition on microstructure and mechanical properties of multi-step sintered Mo2FeB2-based cermets

The influence of Cr and W addition on microstructure and mechanical properties of Mo2FeB2-based cermets, prepared by multi-step sintering, is systematically evaluated by XRD, SEM, EDS, and TEM. After liquid-phase sintering, the final microstructure of Cr-doped Mo2FeB2-based cermets is similar to that of traditional Mo2FeB2-based cermets. The cermet, with 2.5% Cr content, exhibits superior comprehensive mechanical properties, i.e., Rockwell hardness, transverse rupture strength (TRS) and fracture toughness of 87.6 HRA, 2179 MPa and 21.7 MPa m1/2, respectively. In contrast, for W-doped Mo2FeB2-based cermets, a large amount of W-enriched (Mo,Fe,W)3B2 solid solutions precipitated around undissolved Mo2FeB2 particles, resulting in the formation of black core/gray rim structure. Particularly, a transition layer, with several atomic layer thickness, is observed between (Mo,Fe,W)3B2 rim phase and Fe-based binder phase, which corresponds to B-deficient (Mo,Fe,W)3B2−x orthorhombic boride phase. Moreover, the lattice mismatch between (Mo,Fe,W)3B2 and (Mo,Fe,W)3B2−x is found to be 1.9%. When doped with 5 wt% W, Mo2FeB2-based cermet exhibit superior comprehensive mechanical properties with Rockwell hardness of 84.6 HRA, TRS of 2088 MPa and fracture toughness of 24.4 MPa m1/2. The increase of fracture toughness (KIC) can be ascribed to the presence of W-rich transition layer.

Effect of VC/Cr3C2 additions on the microstructure, interface structure and mechanical properties of Ti(C,N)-based cermets prepared by in situ carbothermal reduction of nano TiO2

Ti(C,N)-based cermets were prepared by in situ carbothermal reduction of nano TiO2. The effect of VC/Cr3C2 additions on the microstructure, interface structure and mechanical properties of the cermets were investigated using SEM, EDX, and TEM. The microstructure of the cermets was primarily characterized by white core/gray rim grains and black core/gray rim grains, in addition to a small amount of white rimless grains and gray grains without core/rim structure. The addition of VC/Cr3C2 refined the grain size, but the microstructure of the cermets showed little difference with varying VC/Cr3C2 weight ratios. Lattice distortion occurred at the rim/binder and ceramic/binder interfaces, and new orientation relationship and coherency state formed at these interfaces. The transverse rupture strength, hardness and fracture toughness all decreased with increasing VC content, but on the contrary increased with increasing Cr3C2 content. Overall, cermet E with 1.5 wt% Cr3C2 showed the superior comprehensive mechanical properties.

Effect of WC content on the microstructure and mechanical properties of Ti(C,N)-based cermets fabricated by in situ carbothermal reduction of TiO2

The effect of WC content on the microstructure and mechanical properties of Ti(C,N)-based cermets fabricated by in situ carbothermal reduction of TiO2 was investigated using XRD, SEM, TEM, and EDX. After sintering, the final microstructure of the cermets prepared via this novel method was mainly characterized by white core/gray rim grains and black core/gray rim grains, and the morphology of the ceramic grains was different from that in traditional cermets. It was found that WC directly participated in the formation of white cores, thus the volume fraction of grains with white cores increased as the content of WC increased, while the volume fraction of the grains with black cores decreased with increasing WC content. Moreover, the interfaces between the binder phase and rims outside the white cores were rough, whereas the interfaces between the binder phase and rims outside the black cores were smooth and an orientation relationship of (200)R//(111‾)B with a perfectly coherent boundary between them. The average grain sizes of the cermets decreased as the content of WC increased, especially when the WC content exceeded 8 wt%. In addition, macropores appeared in the cermets as the WC content exceeded 12 wt%. Overall, the microstructure of the cermets with 8 wt% WC content was the most homogeneous and densest. The transverse rupture strength, hardness and fracture toughness all initially increased and then decreased as the WC content increased.

Effect of Cr3C2 on the microstructure and properties of TiWMoC-based cermets

Effect of Cr3C2 on the microstructure and mechanical properties of TiWMoC-based cermets were studied. The microstructure was investigated by using x-ray diffraction (XRD) and scanning electron microscope (SEM) in combination with energy dispersive spectrometry (EDS). Mechanical properties of transverse rupture strength (TRS), fracture toughness (KIC) and Vickers hardness (HV) were measured. The results show that Cr3C2 mainly dissolved in the binder phase and the dissolution of Cr3C2 into the binder phase reduced the solubility of TiC, WC, and Mo2C. As the Cr3C2 addition increased from 0.4 wt% to 1.2 wt%, the proportion of black core with medium grains increased with a concomitant decrease of coarse grains. Additional dimples appeared in the fracture surface. The porosity decreased with the increase of Cr3C2 addition, and the mechanical properties of TiWMoC-based cermets were improved. However, owing to decreased solubility of TiC in the binder phase, some coarse grains formed with thicker rims. The crack propagation path became a transgranular fracture-led model, the porosity increased and the mechanical properties decreased. The cermet with 1.2 wt% of Cr3C2 demonstrated better mechanical properties.