Showing papers on "Fracture toughness published in 2013"

Fracture Mechanics of Ceramics

Abstract: The failure of ceramic materials is caused by the extension of small flaws. Therefore, linear-elastic fracture mechanics can be applied to describe the failure behaviour. The main problem in the application of the simple fracture mechanics relation is the existence of a rising crack growth resistance curve, which is caused by crack bridging forces behind the advancing crack tip or by transformations in front of the crack tip. The increasing crack growth resistance leads to problems in the transformation of results from specimens with macrocracks to components with natural cracks.

Electron Beam Additive Manufacturing of Titanium Components: Properties and Performance

Abstract: This research evaluates the fatigue properties of Ti-6Al-4V specimens and componentsproduced by Electron Beam additive manufacturing. It was found that the fatigue per-formance of specimens produced by additive manufacturing is significantly lower thanthat of wrought material due to defects such as porosity and surface roughness. However,evaluation of an actual component subjected to design fatigue loads did not result in pre-mature failure as anticipated by specimen testing. Metallography, residual stress, staticstrength and elongation, fracture toughness, crack growth, and the effect of post process-ing operations such as machining and peening on fatigue performance were alsoevaluated. [DOI: 10.1115/1.4025773]Keywords: additive manufacturing, electron beam, titanium, fatigue, fracture

Effects of carbon nanotube alignment on electrical and mechanical properties of epoxy nanocomposites

Abstract: This paper reports the alignment of multi-walled carbon nanotubes (MWCNTs) in an epoxy matrix as a result of DC electric fields applied during composite curing. Optical microscopy and polarized Raman spectroscopy are used to confirm the CNT alignment. The alignment of CNTs gives rise to much improved electrical conductivity, elastic modulus and quasi-static fracture toughness compared to those with CNTs of random orientation. An extraordinarily low electrical percolation threshold of about 0.0031 vol% is achieved when measured along the alignment, which is more than one order of magnitude lower than 0.034 vol% with random orientation or that measured perpendicular to the aligned CNTs. The examination of the fracture surfaces identifies pertinent toughening mechanisms in aligned CNT composites, namely crack tip deflection and CNT pullout. The significance of this paper is that the technique employed here can tailor the physical, mechanical and fracture properties of bulk nanocomposites even at a very low CNT concentration.

Modeling of shear ductile fracture considering a changeable cut-off value for stress triaxiality

Abstract: A macroscopic ductile fracture criterion is proposed based on micro-mechanism analysis of nucleation, growth and shear coalescence of voids from experimental observation of fracture surfaces. The proposed ductile fracture model endows a changeable cut-off value for the stress triaxiality to represent effect of micro-structures, the Lode parameter, temperature, and strain rate on ductility of metals. The proposed model is used to construct fracture loci of AA 2024-T351. The constructed fracture loci are compared with experimental data covering wide stress triaxiality ranging between −0.5 and 1.0. The comparison suggests that the proposed model can provide a satisfactory prediction of ductile fracture for metals from compressive upsetting tests to plane strain tension with slanted fracture surfaces. Moreover, it is expected that the proposed model reasonably describes ductile fracture behavior in high velocity perforation simulation since a reasonable cut-off value for the stress triaxiality is coupled with the proposed ductile fracture criterion.

Characterisation of cotton fibre-reinforced geopolymer composites

Abstract: This paper describes the physical, mechanical and fracture behaviour of fly-ash based geopolymer reinforced with cotton fibres (0.3–1.0 wt%). Results show that the appropriate addition of cotton fibres can improve the mechanical properties of geopolymer composites. In particular, the flexural strength and the fracture toughness increase at an optimum fibre content of 0.5 wt%. However, as the fibre content increases, the density of geopolymer composites decreases due to an increase in porosity and tendency of fibre agglomeration.

Fabrication characteristics and mechanical behaviour of rice husk ash – Alumina reinforced Al-Mg-Si alloy matrix hybrid composites

Abstract: The fabrication characteristics and mechanical behaviour of Al-Mg-Si alloy matrix composites reinforced with alumina (Al2O3) and rice husk ash (RHA, an agro-waste) was investigated. This was aimed at assessing the viability of developing high performance Al matrix composites at reduced cost. Al2O3 particulates added with 0, 2, 3, and 4 wt% RHA were utilized to prepare 10 wt% of the reinforcing phase with Al-Mg-Si alloy as matrix using two-step stir casting method. Density measurement, estimated percent porosity, tensile testing, micro-hardness measurement, optical microscopy, and SEM examination were used to characterize the composites produced. The results show that the less dense Al-Mg-Si/RHA/Al2O3 hybrid composites have estimated percent porosity levels as low as the single Al2O3 reinforced grade (< 2.3% porosity). The hardness of the hybrid composites decreases slightly with increase in RHA content with a maximum reduction of less than 11% observed for the Al-4 wt% RHA-6wt% Al2O3 composition (in comparison with the Al-10 wt% Al2O3 single reinforced composition). Tensile strength reductions of 8% and 13%, and specific strengths which were 3.56% and 7.7% lower were respectively observed for the 3 wt% and 4 wt% RHA containing hybrid composites. The specific strength, percent elongation and fracture toughness of the 2 wt% RHA containing hybrid composite was however, higher than that of the single Al2O3 reinforced and other hybrid composite compositions worked on. RHA thus has great promise to serve as a complementing reinforcement for the development of low cost-high performance aluminum hybrid composites.

Measurements of the fracture energy of lithiated silicon electrodes of Li-ion batteries.

Abstract: We have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration. To this end, we have constructed an electrochemical cell capable of testing multiple thin-film electrodes in parallel. The stress in the electrodes is measured during electrochemical cycling by the substrate curvature technique. The electrodes are disconnected one by one after delithiating to various states of charge, that is, to various concentrations of lithium. The electrodes are then examined by optical microscopy to determine when cracks first form. All of the observed cracks appear brittle in nature. By determining the condition for crack initiation, the fracture energy is calculated using an analysis from fracture mechanics. In the same set of experiments, the fracture energy at a second state of charge (at small concentrations of lithium) is measured by determining the maximum value of the stress during delithiation. The fracture energy was determined to be Γ = 8.5 ± 4.3 J/m2 at s...

Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering

Abstract: Flexural strengths at room temperature, at 1400 °C in air and at room temperature after 1 h oxidation at 1400 °C were determined for ZrB 2 - and HfB 2 -based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB 2 with 20 vol.% SiC had room temperature strength of 700 ± 90 MPa, fracture toughness of 6.4 ± 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 ± 0.6 GPa, 1400 °C strength of 400 ± 30 MPa and room temperature strength after 1 h oxidation at 1400 °C of 678 ± 15 MPa with an oxide layer thickness of 45 ± 5 μm. HfB 2 with 20 vol.% SiC showed room temperature strength of 620 ± 50 MPa, fracture toughness of 5.0 ± 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 ± 0.6 GPa, 1400 °C strength of 590 ± 150 MPa and room temperature strength after 1 h oxidation at 1400 °C of 660 ± 25 MPa with an oxide layer thickness of 12 ± 1 μm. 2 wt.% La 2 O 3 addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation.

Application of Cracked Triangular Specimen Subjected to Three-Point Bending for Investigating Fracture Behavior of Rock Materials

Abstract: Numerical and experimental studies were performed on a new fracture test configuration called the edge cracked triangular (ECT) specimen. Using several finite-element analyses, the fracture parameters (i.e., K I, K II, and T-stress) were obtained for different combinations of modes I and II. The finite-element results show that the ECT specimen is able to provide pure mode I, pure mode II, and any mixed-mode loading conditions in between. Also, a series of mixed-mode fracture experiments were conducted on Neiriz marble rock using the proposed specimen. Furthermore, the generalized maximum tangential stress (GMTS) criterion was used to predict the experimental results. The GMTS criterion makes use of a three-parameter model (based on K I, K II, and T) for describing the crack tip stresses. Due to the significant positive T-stresses that exist in the ECT specimen, typical minimum fracture toughness values were expected to be obtained when the ECT specimen is used. The direction of fracture initiation and the path of fracture growth were also obtained theoretically using the GMTS criterion, and good agreement was observed between the experimental fracture path and theoretical simulations. The fracture study of this specimen reveals that the ECT specimen can be also used in mixed-mode fracture studies of rock materials in addition to the conventional circular or rectangular beam test samples.

Fracture toughness and electrical conductivity of epoxy composites filled with carbon nanotubes and spherical particles

Abstract: The attainment of both high toughness and superior electrical conductivity of epoxy composites is a crucial requirement in some engineering applications. Herein, we developed a strategy to improve these performances of epoxy by combining the multi-wall carbon nanotubes (MWCNTs) and spherical particles. Two different types of spherical particles i.e. soft submicron-rubber and rigid nano-silica particles were chosen to modify the epoxy/MWCNT composites. Compared with the binary composites with single-phase particles, the ternary composites with MWCNTs and spherical particles offer a good balance in glass transition temperature, electrical conductivity, stiffness and strength, as well as fracture toughness, exhibiting capacities in tailoring the electrical and mechanical properties of epoxy composites. Based on the fracture surface analysis, the complicated interactions between multiscale particles and the relative toughening mechanisms were evaluated to explain the enhancement in fracture toughness of the ternary composites.

Graphene NanoPlatelets reinforced tantalum carbide consolidated by spark plasma sintering

Abstract: Graphene NanoPlatelets (GNP) reinforced tantalum carbide composites are synthesized by spark plasma sintering (SPS) at processing conditions of 1850 °C and 80–100 MPa. The GNP addition enhances the densification of TaC–GNP composites to 99% theoretical density, while reducing the grain size by over 60% through grain wrapping mechanism. Survival and structure retention of GNP is confirmed through scanning electron microscopy and micro-Raman spectroscopy. Nanoindentation and high load (20–30 N) microindentation are utilized to evaluate elastic modulus and hardness. GNP improves fracture toughness of TaC by up to 99% through toughening mechanisms such as GNP bending, sheet sliding, cracking bridging, and crack deflection.

Crumb rubber aggregate coatings/pre-treatments and their effects on interfacial bonding, air entrapment and fracture toughness in self-compacting rubberised concrete (SCRC)

Abstract: The interfacial-bonding, interfacial transition zone (ITZ), and porosity are regarded as the key factors affecting hardened concrete properties. The aim of this study was to experimentally improve the bonding between the rubber aggregate and cement paste by different methodologies including water washing, Na(OH) pre-treatment, and both cement paste and mortar pre-coating. All methods were assessed by determining mechanical and dynamic properties, then correlating this with ITZ porosity and interfacial gap void geometry, along with quantification of the fracture energy during micro crack propagation using fractal analysis. The results indicated that pre-coating the rubber by mortar gave the best results in terms of fracture toughness and energy absorption showing good agreement between observations made at both micro and macro scales.

Numerical analysis of size effects on open-hole tensile composite laminates

Abstract: The tensile strength of open-hole fibre reinforced composite laminates depends on in-plane, thickness and ply lay-up scaling. Translaminar (fibre direction) mode I fracture toughness has recently been experimentally determined to be thickness dependent. This paper presents a computational study of the tensile strength prediction of open-hole laminates using a cohesive zone model. To the authors’ knowledge, it is for the first time in the literature that the thickness-dependence of translaminar fracture toughness is accounted for in the numerical modelling of composites. The thickness size effect in the strength of open-hole composite laminates failed by pull-out is accurately predicted for the first time by a deterministic model. It is found that neglecting delamination in the numerical models will lead to mesh-dependency and over-estimation on the predicted strength. Smeared crack model with cohesive elements to model delamination is able to predict the correct failure mode; but it is found not suitable for accurate strength predictions for laminates failed by delamination.

Influence of various metal oxides on mechanical and physical properties of heat-cured polymethyl methacrylate denture base resins

Abstract: PURPOSE. To evaluate the effect of various metal oxides on impact strength (IS), fracture toughness (FT), water sorption (WSP) and solubility (WSL) of heat-cured acrylic resin. MATERIALS AND METHODS. Fifty acrylic resin specimens were fabricated for each test and divided into five groups. Group 1 was the control group and Group 2, 3, 4 and 5 (test groups) included a mixture of 1% TiO 2 and 1% ZrO 2 , 2% Al 2 O 3 , 2% TiO 2 , and 2% ZrO 2 by volume, respectively. Rectangular unnotched specimens (50 mm × 6.0 mm × 4.0 mm) were fabricated and droptower impact testing machine was used to determine IS. For FT, compact test specimens were fabricated and tests were done with a universal testing machine with a cross-head speed of 5 mm/min. For WSP and WSL, discshaped specimens were fabricated and tests were performed in accordance to ISO 1567. ANOVA and KruskalWallis tests were used for statistical analyses. RESULTS. IS and FT values were significantly higher and WSP and WSL values were significantly lower in test groups than in control group (P<.05). Group 5 had significantly higher IS and FT values and significantly lower WSP values than other groups (P<.05) and provided 40% and 30% increase in IS and FT, respectively, compared to control group. Significantly lower WSL values were detected for Group 2 and 5 (P<.05). CONCLUSION. Modification of heat-cured acrylic resin with metal oxides, especially with ZrO 2 , may be useful in preventing denture fractures and undesirable physical changes resulting

The effects of manufacturing-induced and in-service related bonding quality reduction on the mode-I fracture toughness of composite bonded joints for aeronautical use

Abstract: The scope of the present work is to investigate the possible effects of manufacturing and in-service induced reduction of bonding quality on the mechanical performance of composite bonded joints appearing in aeronautical applications, due to defects that cannot be detected sufficiently by means of conventional NDT. To this end, an experimental program has been conducted to measure mode-I interlaminar fracture toughness of adhesively bonded CFRP laminates. Five different representative scenarios have been considered; namely, poor curing of the adhesive, release agent contamination and moisture uptake by the composite substrates, which may occur during manufacturing of the joints, as well as Skydrol contamination and thermal degradation of the composite substrates, which are related to in-service life and could affect an adhesively bonded repair. Eight specimens for each scenario were tested. To assess the effects of each scenario, the experimental results have been compared with relative measurements taken from a set of reference specimens. Prior to mechanical testing, and the conventional NDT techniques, ultrasound C-scan and X-ray tests have been conducted to assess the quality of the bondline. All scenarios considered in the present study have been found to have an effect on the mode-I fracture toughness of the composite bonded joints. It was also shown that conventional NDT, such as ultrasonic and X-ray inspection, are not capable to sufficiently detect the defects resulting from the specific scenarios considered and being responsible for the reduction of joint’s performance.

In-vitro strength degradation of dental ceramics and novel PICN material by sharp indentation.

Abstract: Objectives To determine the flexural strength and subsequent strength degradation of a range of dental CAD/CAM ceramic materials and novel PICN (Polymer-Infiltrated-Ceramic-Network) materials by means of pre-damaging with Vickers indentations at various loads. Methods The materials tested included (Mark II, PICN test material 1 and 2, In-Ceram Alumina, VM 9, In-Ceram YZ; Vita Zahnfabrik, Bad Saeckingen, Germany) and (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein). Bending bars were cut and lapped with 15 µm diamond suspension. Initial flexural strength (n=10) was determined in three-point-bending. To evaluate the damage tolerance, Vickers indentations were placed on the bending bars (n=35) with varying loads (1.96–98.07 N). The indented bending bars were subsequently loaded to fracture in three-point-bending. In addition, the fracture toughness was determined by the indentation strength (IS) and the SEVNB technique (n=5). Results With increasing indentation loads the fracture strength of all materials tested decreased. The material with the highest fracture resistance to indentation induced damage, was the PICN test material 1 with an indentation load-flexural strength curve slope of 0.21. In-Ceram YZ exhibited the highest damage susceptibility with a slope of 0.4. The fracture toughness varied with the measurement technique and material in the range of 0.82 (VM 9) to 4.94 (In-Ceram YZ) MPa√m for the SEVNB method and 0.96 (VM 9) to 4.97 (In-Ceram YZ) MPa√m for the IS method respectively. Significance This study aims to indicate the likely clinical behavior by evaluating the damage tolerance and R-curve behavior of dental ceramics by in-vitro strength degradation and fracture toughness measurements.