Thickness-dependence of the translaminar fracture toughness: Experimental study using thin-ply composites

The concept of translaminar fracture toughness of 0° plies has enabled the development of a considerable number of ply-level numerical models for structural failure of laminated composites. Using thin-ply pre-pregs, this paper demonstrates that this translaminar toughness is not an absolute, but rather in-situ, property and depends strongly on the 0° ply-block thickness, even in situations where delamination and diffuse damage are inhibited. We used two different grades of a thin-ply carbon-epoxy system to produce four different 0° ply-block thicknesses ranging from 0.03 mm to 0.12 mm, and measured the respective translaminar fracture toughness using compact tension tests. SEM and X-ray analysis showed no delamination nor diffuse damage. Yet, the translaminar fracture toughness increased from 46 to 104 kJ/m² (initiation), and from 49 to 160 kJ/m² (propagation), for the thickness range above. This finding has significant implications for the development and use of ply-level numerical failure models, for structural design with thin-ply composites, and for the development of thin-ply material systems.     

Microstructure and mechanical properties of titanium carbonitride whisker reinforced β-sialon matrix composites

The microstructure, hardness, fracture toughness and thermal shock resistance were investigated for 15 vol.% TiC_0.3N_0.7 whisker reinforced β-sialon (Si_6−zAl_zO₂N_8−z with z=0.6) composites with additions of three different volume fractions 2, 5 and 20 vol.%, of an yttrium-containing glass oxynitride phase. The composites were prepared by hot pressing at 1750°C for 90 min under a uniaxial pressure of 30 MPa in nitrogen atmosphere. The TiC_0.3N_0.7 whiskers were found to survive without deteriorating in morphology or reacting with the β-sialon matrix and/or the glass phase. The TiC_0.3N_0.7 whiskers had no obvious influence on the matrix microstructure, but their presence improved both the hardness and the fracture toughness of the composites. The highest hardness was obtained for the whisker composite with 2 vol.% glass phase (H_v=18.6 GPa). The fracture toughness and thermal shock resistance improved with increasing glass content. The whisker reinforced composite containing 20 vol.% glass showed the highest fracture toughness (K_1C=6.8 MPa m^1/2). No unstable crack extension occurred during the thermal shock test of the obtained composites in the temperature interval 90-700°C, but above 700°C severe oxidation of the whiskers precludes further evaluation of thermal shock properties by the indentation-quench method applied.     

Fabrication and mechanical properties of monolithic MoSi2 by spark plasma sintering

Fine MoSi₂ powders containing a small amount of Mo₅Si₃ have been prepared by self-propagating high-temperature synthesis (SHS), followed by spark plasma sintering (SPS) for 10 min at 1200-1500°C and 30 MPa. Dense MoSi₂ materials, in which the grain size is ∼7.5 μm, have been fabricated at 1300°C. They exhibit excellent mechanical properties: Vicker’s hardness H_v (10.6 GPa), fracture toughness K_IC (4.5 MPa m^1/2), and bending strength σ_b (560 MPa). The strength of 325 MPa can be retained up to 1000°C.     

Microstructure and mechanical properties of silicon nitride-titanium nitride composites prepared by spark plasma sintering

Si₃N₄-TiN composites were prepared by spark plasma sintering (conventional sintering (SPS1) and in situ reaction sintering (SPS2)). Homogeneous distribution of equiaxed TiN grains in Si₃N₄ matrix results in the highest microhardness (21.7 GPa) and bending strength (621 MPa) of sample SPS1 sintered at 1550 °C. Dispersion of elongated TiN grains in Si₃N₄ matrix results in the highest fracture toughness (8.39 MPa m^1/2) of sample SPS2 sintered at 1300 °C.     

Preparation and properties of negative thermal expansion Zr2WP2O12 ceramics

Using solid-state reaction method, Zr₂WP₂O₁₂ powder was synthesized for this study. The optimum heating condition was 1200 °C for 4 h. The obtained powder was compacted and sintered. The relative density of the Zr₂WP₂O₁₂ ceramics with no sintering additive was 60%. That of samples sintered with more than 0.5 mass% MgO was about 97%. The average grain size (D₅₀), as estimated from the polished surface of a sample sintered at 1200 °C for 4 h was about 1 μm. The obtained ceramics showed a negative thermal expansion coefficient of about −3.4 × 10⁻⁶ °C⁻¹. Young's modulus, Poisson's ratio, three-point bending strength, Vickers microhardness, and fracture toughness of the obtained ceramics were, respectively, 74 GPa, 0.25, 113 ± 13 MPa, 4.4 GPa and 2.3 MPa m^1/2.     

Mechanical and thermal properties of LaMgAl11O19

Lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉) powders were synthesized successfully at 1300 °C for 4 h by solid-state reaction, and LaMgAl₁₁O₁₉ ceramic was prepared at 1700 °C for 6 h by pressureless sintering. Phase composition, microstructure, mechanical and thermophysical properties of LaMgAl₁₁O₁₉ ceramic were investigated. Results show that the flexural strength and fracture toughness of LaMgAl₁₁O₁₉ ceramic are 353.3 ± 12.5 MPa and 4.60 ± 0.46 MPa m^1/2. Young's Modulus and Poisson ratio is 295 GPa and 0.23, respectively. The linear thermal expansion coefficient of LaMgAl₁₁O₁₉ ceramic from 473 K to 1473 K is 9.17 × 10⁻⁶/K, and thermal conductivity at 1273 K is 2.55 W/m K.     

Transparent polycrystalline ruby ceramic by spark plasma sintering

Fine-grained and transparent polycrystalline ruby ceramics (Cr₂O₃-doped Al₂O₃) were successfully prepared by spark plasma sintering (SPS). The effect of Cr₂O₃ concentration on the grain size, hardness, fracture toughness and thermal conductivity of ruby ceramics was investigated systematically. For 0.05 wt.% Cr₂O₃, high in-line transmittance of 85% at 2000 nm can be reached, further increase of Cr₂O₃ concentration leads to the decrease in transmittance. High hardness of 23.95-25.05 GPa can be achieved due to the fine grain size in all ruby ceramics. The fracture toughness of 1.9-2.29 MPa m^1/2 indicates that no improvement in fracture toughness over pure Al₂O₃ can be obtained by Cr₂O₃ doping in these submicron grained ruby ceramics. High thermal conductivity of 28-29.8 W/(m K) at room temperature, close to that of single crystal sapphire, can be achieved. The change in grain size for different Cr₂O₃ concentrations is the major reason for the change in mechanical and thermal properties, but not for the change in optical properties.     

Preparation of AlON-TiC composites via reaction-bonding

AlON-TiC composites were fabricated via a reaction-bonding technique, using Al, Al₂O₃ and TiC powders as the starting materials. A composite sample sintered at 1850 °C after nitriding is highly densified and the Vickers hardness and fracture toughness of the sample are about 1751.1 kg/mm² and 5.3 MPa m^1/2, respectively. The composition and microstructure of the sample are characterized by means of XRD and SEM/EDX.     

Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.     

Size effect on fracture toughness of freestanding copper nano-films

We conducted fracture toughness experiments on freestanding copper films with thicknesses ranging from about 800 to 100 nm deposited by electron beam evaporation to elucidate the size effect on fracture toughness in the nano- or submicron-scale. It was found that initially, the crack propagated stably under loading, and then the crack propagation rate rapidly increased, resulting in unstable fracture. The fracture toughness K_C was estimated on the basis of the R-curve concept to be 7.81 ± 1.22 MPa m^1/2 for the 800-nm-thick film, 6.63 ± 1.05 MPa m^1/2 for the 500-nm-thick film and 2.34 ± 0.54 MPa m^1/2 for the 100-nm-thick film. Thus, a clear size effect was observed. The fracture surface suggested that the crack underwent large plastic deformation in the thicker 800-nm and 500-nm films, whereas it propagated with highly localized plastic deformation in the thinner 100-nm film. This size effect in fracture toughness might be related to a transition in deformation and fracture morphology near the crack tip.     

Pretreatment and sintering of Si3N4 powder synthesized by the high-temperature self-propagation method

Self-propagation high-temperature synthesis (SHS) was applied for the synthesis of low-cost Si₃N₄ powder. The powder was purified and ground until its particle size reached submicron levels and its purity reached 98%. Using this pretreated powder, with α/β = 60/40 content, fully dense Si₃N₄ ceramics, having improved mechanical properties, were obtained by liquid-phase sintering in the presence of (Y, La)₂O₃-AlN. The mechanical properties achieved finally were as follows: strength, 784 MPa; hardness, 15.1 GPa; and fracture toughness, 5.2 MPa m^0.5. The behaviors of the SHS-Si₃N₄ powders before and after the pretreatment were compared. The relation between microstructure and mechanical properties of the sintered specimens and the effect of different β content in the powder on the sintering process of Si₃N₄ were also studied.     

The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite

Alumina reinforced with 1 wt% single-wall carbon nanotubes (SWNTs) was fabricated by hot-pressing. The fracture toughness of SWNTs/Al₂O₃ composite reaches 6.40 ± 0.3 MPa m^1/2, which is twice as high as that of unreinforced alumina. Nanoindentation introduced controlled cracks and the damage were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SWNTs reinforcing mechanisms including CNT pullout, CNT fracture, CNT bridging and crack deflection were directly observed, and the relationship between carbon nanotubes microstructures in the matrix and mechanical properties was also discussed in detailed.     

Interaction of laminate damage and adhesive disbonding in composite scarf joints subjected to combined in-plane loading and impact

Impact tests were carried out on composite laminates and composite scarf repairs, while both were subjected to in-plane loading with tensile pre-strain levels up to 5000 microstrain. The results show that pre-straining of the composite laminates has no noticeable influence on the size of the delamination area for the given impact energy of 8 J, which represents a typical barely-visible impact on thin-skin composite structures. For composite scarf joints, however, resulting damage has been found to be a combination of adhesive disbonding and matrix cracking (delamination and intraply cracking) in the composite laminate. The size of this mixed type of damage increases significantly with increasing pre-strain levels. A finite element model was developed to investigate the interaction between adhesive disbonding and composite delamination. The computational results reveal that both delamination and adhesive disbonding are dominated by the mode II fracture. Since the critical mode II fracture energy release rate for composite laminates (G_IIC = 1.08 kJ/m²) is much less than that pertinent to the adhesive (G_IIC = 3.73 kJ/m²), delamination tends to occur first in the composite laminates, which then shield the growth of disbonding in the adhesive.     

Synthesis of TiC-TiB2-Ni cermets by thermal explosion under pressure

TiC/TiB₂-based cermets were fabricated in situ by means of the thermal explosion under pressure technique starting from Ti-B₄C powders with the addition of varying contents of Ni metal binder to achieve near-net-shape bulks. The combustion reaction was ignited in a graphite die heated by current. Full conversion of the reactants was obtained by thermal explosion and the process yielded TiC-TiB₂-Ni materials characterised by a fine microstructure. Appreciable differences in terms of microstructure, hardness and fracture toughness by indentation were observed between core and external surface of the products due to fast cooling caused by heat transfer to the die walls. Cermets with a high content of Ni showing high hardness and fracture toughness were obtained, with values of HV₅ = 2182 and K_Ic = 8.8 MPa m^1/2 for 30 wt.% Ni and of HV₅ = 1684 and K_Ic = 12.7 MPa m^1/2 for 47 wt.% Ni.     

Study on the structure and properties of fine-grained alumina fast sintered with high heating rate

Fine-grained alumina was obtained in 2 min by a new densification method based on Self-propagating High-temperature Synthesis plus Quick Pressing (SHS/QP). The sample was densified to more than 99% of theoretical density under a large mechanical pressure (100 MP) and a fast heating rate (1600 °C/min). Compared with the alumina sample obtained by spark plasma sintering (SPS) at lower heating rates (100 or 500 °C/min), almost no grain growth was found in the sample obtained in this work. The microstructure and mechanical properties were studied. Hardness value of 18.6 ± 0.4 GPa and fracture resistance value of 3.4 ± 0.3 MPa m^1/2 were measured for fine-grained alumina of this work. The densification mechanism was discussed.     

Jute and glass fibre hybrid laminates

Hybrid laminates have been fabricated from randomly oriented jute fibre mats and woven glass fabrics with a common polyster resin matrix. Hand lay up techniques were used to simulate practical production methods in the field. A variety of laminate constructions were mechanically tested and some laminates were in addition assessed for environmental stability. Modified rule of mixtures expressions successfully predicted the tensile properties of the laminates and the jute plies were seen to control the failure of hybrid laminates at about 0.8% strain. Fracture toughness measurements of G_IC andK _IC indicate that hybrid laminates have maximum toughness (G _IC 12 kJ m^–2 when jute plies are sandwiched between glass fabric facings. All the hybrid laminates were found to be tough in impact, although here fabric plies used as the laminate core maximize the work of fracture at a value of approximately 45 kJ m^–2. Hybrid laminates with jute facings are, as expected, least able to withstand hot moist environments. However, significant moisture uptake by the polyester resin matrix was measured for all laminates. Optical and scanning electron microscopy have been used to explain the mechanical performance and environmental resistance of the hybrid laminates.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

The production of grain oriented lanthanum titanate (La2Ti2O7) ceramics by uniaxial hot-forging process for improved fracture toughness

The layered-structural ceramics, such as lanthanum titanate (La₂Ti₂O₇), have been known for their good electrical and optical properties at high frequencies and temperatures. However, few studies have been conducted on the mechanical properties of these ceramics. The interest in ceramic hot-forging (HF) has been greatly increased recently due to the enhancement in fracture toughness via bridging effect of oriented grains. In this study, grain oriented lanthanum titanate was produced by the hot-forging process. The characterizations of the samples were achieved by density measurement, scanning electron microscopy (SEM), optical microscopy, X-ray diffraction (XRD), Vickers indentation and three-point bending test. According to X-ray diffraction patterns, the orientation factor (f) was found to be 0.73 for certain hot-forging conditions resulting an improved fracture toughness. The improved fracture toughness of La₂Ti₂O₇ (3.2 MPa m^1/2) reached to the value of monolithic alumina (Al₂O₃) between 3 and 4 MPa m^1/2.     

Sensitivity of structurally loaded sandwich panels to localized ballistic penetration

This paper describes a series of tests focused on the combination of structural loading (bending, shear) and simultaneous penetrating impact on sandwich panels with thin GFRP face-sheets, with emphasis on the specific damage morphologies and developments depending on the type and magnitude of structural loading. The test specimens were sandwich panels, length 250 mm and width 150 mm, with carbon fibre prepreg face-sheets ([0°/90°], thickness t_f ≅ 0.5 mm) bonded to the faces of a foam core (density 80 kg/m³, thickness H = 10 mm). The impact velocity was approximately 420 m/s, using a spherical steel impactor, diameter 10 mm, with a mass of 4.1 g. A high-speed camera was used for registration of panel response. It was demonstrated, that, at preload levels above a specific limit, the impact would cause catastrophic failure, i.e., complete or near-complete loss of structural load carrying capacity. Developments of failure morphology, consistent with the observed evidence, were derived and outlined.     

ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing

ZrB₂–Nb (ZN) composites were prepared through hot-pressing at a temperature of 1800 °C. A contribution of Nb was believed a significant influence on the sinterability, microstructure and mechanical properties of ZN composites. The values of flexural strength of ZN composites rang from 395 to 773 MPa, who are dependent on Nb contents. The highest strength obtained for the ZN composite containing 25 vol.% Nb (773 MPa). A fracture toughness of 7.1 MPa m^1/2 of ZN was revealed, which was much higher than that of monolithic ZrB₂. The improvement in fracture toughness strongly depended on an introduction of Nb–ZrB₂ matrix. Crack deflection and branching were believed to be the toughening mechanism of ZN.