Showing papers on "Fracture toughness published in 2018"

A Critical evaluation of indentation techniques for measuring fracture toughness

Abstract: The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.

Extreme Toughening of Soft Materials with Liquid Metal.

Abstract: Soft and tough materials are critical for engineering applications in medical devices, stretchable and wearable electronics, and soft robotics. Toughness in synthetic materials is mostly accomplished by increasing energy dissipation near the crack tip with various energy dissipation techniques. However, bio-materials exhibit extreme toughness by combining multi-scale energy dissipation with the ability to deflect and blunt an advancing crack tip. Here, we demonstrate a synthetic materials architecture that also exhibits multi-modal toughening, whereby embedding a suspension of micron sized and highly deformable liquid metal (LM) droplets inside a soft elastomer, the fracture energy dramatically increases by up to 50x (from 250 ± 50 J m-2 to 11,900 ± 2600 J m-2 ) over an unfilled polymer. For some LM-embedded elastomer (LMEE) compositions, the toughness is measured to be 33,500 ± 4300 J m-2 , which far exceeds the highest value previously reported for a soft elastic material. This extreme toughening is achieved by (i) increasing energy dissipation, (ii) adaptive crack movement, and (iii) effective elimination of the crack tip. Such properties arise from the deformability of the LM inclusions during loading, providing a new mechanism to not only prevent crack initiation, but also resist the propagation of existing tears for ultra tough, soft materials.

Effects of nano-graphite content on the characteristics of spark plasma sintered ZrB2–SiC composites

Abstract: In this study, ZrB2–25 vol% SiC composite containing 0, 2.5, 5, 7.5 and 10 wt% graphite nano-flakes were prepared by spark plasma sintering (SPS) process at 1900 °C for 7 min under 40 MPa. The fabricated composite samples were compared to examine the influences of nano-graphite content on the densification, microstructure and mechanical properties of ZrB2–SiC-based ultrahigh temperature ceramics. Fully dense composites were obtained by adding 0–5 wt% nano-graphite, but higher amounts of additive led to a small drop in the sintered density. The growth of ZrB2 grains was moderately hindered by adding nano-graphite but independent of its content. The hardness linearly decreased from 19.5 for the graphite-free ceramic to 12.1 GPa for the sample doped with 10 wt% nano-graphite. Addition of graphite nano-flakes increased the fracture toughness of composites as a value of 8.2 MPa m½ was achieved by adding 7.5 wt% nano-graphite, twice higher than that measured for the graphite-free sample (4.3 MPa m½). The in-situ formation of ZrC and B4C nano-particles as well as the presence of unreacted graphite nano-flakes led to a remarkable enhancement in fracture toughness through activating several toughening mechanisms such as crack deflection, crack bridging, crack branching and graphite pullout.

Mechanical glass transition revealed by the fracture toughness of metallic glasses

Abstract: The fracture toughness of glassy materials remains poorly understood. In large part, this is due to the disordered, intrinsically non-equilibrium nature of the glass structure, which challenges its theoretical description and experimental determination. We show that the notch fracture toughness of metallic glasses exhibits an abrupt toughening transition as a function of a well-controlled fictive temperature (Tf), which characterizes the average glass structure. The ordinary temperature, which has been previously associated with a ductile-to-brittle transition, is shown to play a secondary role. The observed transition is interpreted to result from a competition between the Tf-dependent plastic relaxation rate and an applied strain rate. Consequently, a similar toughening transition as a function of strain rate is predicted and demonstrated experimentally. The observed mechanical toughening transition bears strong similarities to the ordinary glass transition and explains the previously reported large scatter in fracture toughness data and ductile-to-brittle transitions. Understanding the fracture toughness of metallic glasses remains challenging. Here, the authors show that a fictive temperature controls an abrupt mechanical toughening transition in metallic glasses, and can explain the scatter in previously reported fracture toughness data.

Simple and accurate model of fracture toughness of solids

Abstract: Fracture toughness $K_{IC}$ plays an important role in materials design. Along with numerous experimental methods to measure fracture toughness of materials, its understanding and theoretical prediction is very important. However, theoretical prediction of fracture toughness is challenging. By investigating the correlation between fracture toughness and elastic properties of materials, we have constructed a fracture toughness model for covalent and ionic crystals. Furthermore, by introducing an enhancement factor, which is determined by the density of states at the Fermi level and atomic electronegativities, we have constructed a universal model of fracture toughness for covalent and ionic crystals, metals and intermetallics. The predicted fracture toughnesses are in good agreement with experimental values for a series of materials. All the parameters in the proposed model of fracture toughness can be obtained from first-principles calculations, which makes it suitable for practical applications.

Mode I fracture toughness and fractographic investigation of carbon fibre composites with liquid Methylmethacrylate thermoplastic matrix

Abstract: Laminated polymer composites are extensively used in various applications ranging from aerospace to automotive, building to marine and offshore, and much more. These composites possess higher mechanical properties in their in-plane directions, but lower interlaminar properties. Especially, low interlaminar fracture toughness (ILFT) makes them susceptible to delamination. In the current research, a novel thermoplastic-based thin-ply composite system is conceptualized and manufactured with an aim to improve the through-the-thickness properties and which can be a competitive solution to traditional epoxy-based composites as well as other class of thermoplastic composites. The detailed experimental investigation on determining the Mode I ILFT properties of these thin ply carbon fibre thermoplastic composites, along with thin ply thermoset composites for benchmarking, is carried out. Quasi-isotropic composite laminates were manufactured using a room temperature cure epoxy, and the novel reactive Methylmethacrylate (MMA) liquid thermoplastic resin. The thin ply/liquid MMA composites have shown 30% and 72% higher Mode I ILFT properties compared to the thick ply/liquid MMA and thin ply/Epoxy composites respectively. Surface morphological studies were conducted to understand and differentiate damage mechanisms in these composites. From the comprehended damage mechanisms, it was deduced that strong fibre-matrix interface, plastic deformation as well as features like ductile drawings in liquid MMA composites make them more resistant to crack propagation.

Sintering of WC-12%Co processed by binder jet 3D printing (BJ3DP) technology

Abstract: A study was carried out to evaluate the sintering densification, shrinkage and mechanical properties of WC-12%Co powders processed by binder jet 3D printing (BJ3DP). After debinding, vacuum sintering at 1435–1485 °C for 45 min yielded low sintered densities in the range of 13.1–13.5 g/cm3. Near theoretical densities of 14.1–14.2 g/cm3 were achieved by sintering the parts under a pressure of 1.83 MPa at 1485 °C for 5–30 min. Shrinkage in the range of 22.2–24.4% and 20.9–26.2% was observed in the parts sintered in vacuum and under pressure, respectively. The samples densified to near theoretical density showed hardness of 1256 HV30 and fracture toughness of 17 ± 1 MPa m1/2. The hardness and fracture toughness are in line with the properties of conventionally produced WC-12%Co with medium grain size. The results from the present study confirm the suitability of the binder jet 3D printing (BJ3DP) process to manufacture WC-Co parts with good mechanical properties.

Microstructural tailoring and mechanical properties of a multi-alloyed near β titanium alloy Ti-5321 with various heat treatment

Abstract: A new near β-Ti alloy Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe (Ti-5321) with a unique combination of high strength and good fracture toughness was designed. The microstructure was tailored by changing the solution and ageing conditions, and the influences of microstructural evolution on tensile properties and fracture toughness of the alloy were investigated. The results showed that the volume fraction and size of primary α phase were decreased with increasing the solution temperature, while the morphology of secondary α precipitates was related to ageing temperature. The ultimate tensile strength (UTS), total elongation (EL) and fracture toughness can be achieved in a range of 1147–1439 MPa, 3–26% and 57–76 MPa m1/2, respectively, depending on the heat treatment parameters. An excellent balance of high strength and good ductility was realized after the solution treatment at 830 °C and ageing at 620 °C for 480 min, in which the UTS, EL and fracture toughness were 1238 MPa, 20% and 73 MPa m1/2, respectively. Morphological features of the fractography were discussed against the different microstructural morphologies, and this provided further information on the fracture behavior of the alloy.

Nonlinear fracture toughness measurement and crack propagation resistance of functionalized graphene multilayers

Abstract: Despite promising applications of two-dimensional (2D) materials, one major concern is their propensity to fail in a brittle manner, which results in a low fracture toughness causing reliability issues in practical applications. We show that this limitation can be overcome by using functionalized graphene multilayers with fracture toughness (J integral) as high as ~39 J/m2, measured via a microelectromechanical systems–based in situ transmission electron microscopy technique coupled with nonlinear finite element fracture analysis. The measured fracture toughness of functionalized graphene multilayers is more than two times higher than graphene (~16 J/m2). A linear fracture analysis, similar to that previously applied to other 2D materials, was also conducted and found to be inaccurate due to the nonlinear nature of the stress-strain response of functionalized graphene multilayers. A crack arresting mechanism of functionalized graphene multilayers was experimentally observed and identified as the main contributing factor for the higher fracture toughness as compared to graphene. Molecular dynamics simulations revealed that the interactions among functionalized atoms in constituent layers and distinct fracture pathways in individual layers, due to a random distribution of functionalized carbon atoms in multilayers, restrict the growth of a preexisting crack. The results inspire potential strategies for overcoming the relatively low fracture toughness of 2D materials through chemical functionalization.