Stopping Crack Research Articles

Self-Healing Poly(urea formaldehyde) Microcapsules: Synthesis and Characterization.

Smart coatings and smart polymers have been garnering great interest in recent times due to their novel characteristics, such as being self-restoring, self-cleaning, and self-healing. However, most self-healing materials have a low glass transition temperature (Tg) and are inadequate for the repair of advanced composites. Because of their low Tg, the conventional self-healing materials plasticize and weaken the composites. In this study, moderate to high temperature self-healing microcapsules, capable of healing and thus stopping crack propagation, are prepared. The microcapsules were prepared using a two-step process involving the synthesis of poly(urea formaldehyde) (PUF) prepolymer, followed by the encapsulation of hexamethylene diisocyanate (HDI) in an oil-in-water emulsion to form a crosslinked PUF shell. Diisocyanates are of particular interest as self-healing encapsulants because of their diversity of structure and fast rate of hydrolysis. Successful encapsulation was verified by Fourier transform infrared spectroscopy (FTIR) and optical microscopy. Thermogravimetric analysis (TGA) was used to characterize the thermal properties of microcapsules. The onset temperature for microcapsule degradation varied from 155 °C to 195 °C. Dynamic mechanical analysis (DMA) was used to determine the thermomechanical response of microcapsule/epoxy films. DMA showed that the glass transition temperature (Tg) of the epoxy/microcapsule composite was greater than the Tg for neat epoxy and varied between 34 and 65 °C. The TGA analysis of the epoxy/microcapsule composite shows that the thermal stability and char retention of the epoxy/microcapsule composite increased and the low temperature decomposition peak at 150 °C, associated with the microcapsule, disappeared after the DMA test, indicating the occurrence of a reaction between HDI and the epoxy to form a crosslinked polyurea network structure.

Short fiber-reinforced resin-based composites (SFRCs); Current status and future perspectives.

One technique for placing of resin-based composite for large posterior cavities is the use of short fiber-reinforced resin-based composite (SFRC) to replace dentin in a biomimetic approach. As endurance under mastication cycles is a significant consideration in the clinical success of resin-based composite posterior restorations, the use of SFRC as a base material may prevent restorative fracture due to the fibers' effectiveness in stopping cracks. This review article specifies the characteristics of SFRC and describes the major underlying mechanisms of short fiber reinforcement for resin-based composite. Insights are further taken from laboratory studies used to define the short fiber-related properties of resin-based composite and the performance of currently available materials, focusing on aspects that are relevant to the reinforcement of resin-based composite. Finally, future standpoints on the development of SFRCs with nano fibers and different resin monomers, and their role in digital dentistry, are discussed.

Structural behaviour of deep beam using hybrid fiber reinforced concrete with M−sand

The modern construction world comes out with the strong growth of high-rise building, in many developing countries. As a primary need in Construction of high rise building, deep beams have gained popularity in present construction practices and Conventional flexural beam theory cannot be used to understand the structural behaviour of deep beam. Inclusion of fibre has improved strength and durability of the properties of concrete structures and moreover hybridization of different types of fibre can play a significant role in stopping cracks and thus achieving high concrete efficiency. Hybrid fiber reinforced concrete has many economic and technological advantages. In this project, five different mixes of concrete with 0.3 percent of glass fibre, 0.75 percent and 1 percent of steel fibre, and 0.3 percent of polypropylene fibres were included to the amount of concrete and were casted as Deep beams. For advancing towards sustainability, the mix was designed using the IS code approach for M20 grade concrete, with 100 percent substitution of natural sand by M−sand. The mechanical properties and Load versus Deflection behavior of Hybrid Fiber Reinforced Concrete (HFRC) are examined in this study. The results indicate that hybrid fibre reinforced deep beams are excelled than standard RC deep beams in terms of strength and structural behaviour when loaded.

Оценка напряженно-деформированного состояния полимерного композиционного материала с гибридной матрицей

Properties of polymer composite material consisting of carbon fabric and two matrix types (epoxy and organic-silicon) have been studied. The main purpose of the organic-silicon matrix is to relax stresses arising in carbon-filled plastic components under loads and to decelerate (or stop) crack growth. The structure of the composite material was determined by using a tomography. On the basis of the structure, a finite-element model was developed and a calculation of the stressed-deformed state depending on elastic characteristics of the matrix was performed. A safety factor calculation was made. It was found out that the addition of organic-silicon material in the composite composition made possible to decrease stress values in the crack tip that considerably increased the service life of such materials.

Mechanism of stopping crack propagation in continuous fiber reinforced self-healing ceramic

Mechanism of stopping crack propagation in continuous fiber reinforced self-healing ceramic

Fretting wear mechanism of plasma-sprayed CuNiIn coating on Ti-6Al-4V substrate under plane/plane contact

The fretting wear mechanism of plasma-sprayed CuNiIn coating on Ti-6Al-4V substrate under plane/plane contact conditions was investigated systematically using a bench level test. The obtained experimental results showed that the fretting wear mechanism of the CuNiIn coating can be divided into three stages: (I) In the initial stage, the asperities on the rough surface were flattened to form debris and fill the nearby pits. The contact surface gradually flattened, which was accompanied by slight abrasive wear. (II) With increasing number of fretting cycles, the fretting wear process of the CuNiIn coating entered the delamination wear stage. Under the action of alternating shear loads, the lamellar structure and pores in the CuNiIn coating rapidly cracked and propagated to the surface. In addition, some fatigue cracks propagated into the matrix along the direction of maximum shear stress. (III) Eventually, the CuNiIn coating entered a stable wear state under the lubrication of a third body, which was accompanied by local delamination. The lamellar structure greatly enhanced the fretting wear resistance of the CuNiIn coating. In the process of fretting wear, the lamellar structure of the CuNiIn coating effectively hindered fatigue crack growth, guided crack deflection along the lamellar interface, and induced crack bifurcation. In addition, the pores can blunt the crack tip. • A new self-balancing device was developed to improve geometrical adaptation of friction pairs with plane/plane contact. • The fretting wear resistance of CuNiIn coating was better than that of Ti-6Al-4V. • The wear rate of the CuNiIn coating was non-monotonic with increasing number of cycles. • The lamellar structure of CuNiIn coating was conducive to delamination wear. • The lamellar structure of CuNiIn coating could induce crack deflection, stop crack propagation, and guide crack bifurcation.

Epoxidized soybean oil grafted with CTBN as a novel toughener for improving the fracture toughness and mechanical properties of epoxy resin

We prepared epoxidized soybean oil (ESO) grafted with carboxyl-terminated poly(acrylonitrile-co-butadiene) (CTBN) (ESO-g-CTBN) by a ring-opening reaction between the epoxide group and the carboxyl group. The structural features of the resulting product were determined using modern analytical techniques, such as Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, and gel permeation chromatography (GPC). The ESO-g-CTBN was applied as a toughener for an epoxy resin-based composite that was fabricated by blending the epoxy resin with diethylenetriamine (DETA) as the curing agent. The main aim of this procedure is to simultaneously improve the mechanical properties and fracture toughness of a bisphenol A-based epoxy resin. When 15 phr of ESO-g-CTBN was added to the EP/DETA mixture, the resin fracture toughness (KIC) and tensile strength increased from 0.65 to 1.09 MPa m1/2 and from 34.42 to 42.55 MPa, respectively. The ESO-g-CTBN existed in the EP matrix as a separate phase and induced an increase in the KIC via stopping crack growth or changing the crack direction. The epoxydized soybean oil grafted with CTBN (ESO-g-CTBN) was synthesized from ring opening reaction between epoxide group and carboxyl group. The ESO-g-CTBN help to improve the fracture toughness of epoxy resin by the mechanism as shown in the figure.

Stress concentrations and bone microdamage: John Currey's contributions to understanding the initiation and arrest of cracks in bone.

The microarchitecture of bone tissue presents many features that could act as stress concentrators for the initiation of bone microdamage. This was first identified by John Currey in a seminal paper in 1962 in which he presented the mechanical and biological evidence for stress concentrations at the bone surface, within the bone through the action of stiffness differentials between architectural features including between lamellae, and at the level of the lacunar and canalicular walls. Those early observations set the stage to consider how microscopic damage to bone tissue might affect the properties of bone at a time when most in the scientific community dismissed microcracks in bone as artifact. Evidence collected in the nearly 60 years since those important initial observations suggest that some of these architectural features in bone tissue are more effective as crack arrestors than as crack initiators. Sites of higher mineralization in the bone matrix, particularly interstitial sites in both cortical and trabecular bone, may serve preferentially as locations for crack initiation, whereas those boundaries identified by Currey as both stress concentrators and stress arrestors are more effective at stopping cracks than at initiating them.

Self-Healing Nanotextured Vascular-like Materials: Mode I Crack Propagation.

Here, we investigate crack propagation initiated from an initial notch in a self-healing material. The crack propagation in the core-shell nanofiber mats formed by coelectrospinning and the composites reinforced by them is in focus. All samples are observed from the crack initiation until complete failure. Due to the short-time experiments done on purpose, the resin and cure released from the cores of the core-shell nanofibers could not achieve a complete curing and stop crack growth, especially given the fact that no heating was used. The aim is to elucidate their effect on the rate of crack propagation. The crack propagation speed in polyacrylonitrile (PAN)-resin-cure nanofiber mats (with PAN being the polymer in the shell) was remarkably lower than that in the corresponding monolithic PAN nanofiber mat, down to 10%. The nanofiber mats were also encased in polydimethylsiloxane (PDMS) matrix to form composites. The crack shape and propagation in the composite samples were studied experimentally and analyzed theoretically, and the theoretical results revealed agreement with the experimental data.

Wing cross veins: an efficient biomechanical strategy to mitigate fatigue failure of insect cuticle.

Locust wings are able to sustain millions of cycles of mechanical loading during the lifetime of the insect. Previous studies have shown that cross veins play an important role in delaying crack propagation in the wings. Do cross veins thus also influence the fatigue behaviour of the wings? Since many important fatigue parameters are not experimentally accessible in a small biological sample, here we use the finite element (FE) method to address this question numerically. Our FE model combines a linear elastic material model, a direct cyclic approach and the Paris law and shows results which are in very good agreement with previously reported experimental data. The obtained results of our study show that cross veins indeed enhance the durability of the wings by temporarily stopping cracks. The cross veins further distribute the stress over a larger area and therefore minimize stress concentrations. In addition, our work indicates that locust hind wings have an endurance limit of about 40% of the ultimate tensile strength of the wing material, which is comparable to many engineering materials. The comparison of the results of the computational study with predictions of two most commonly used fatigue failure criteria further indicates that the Goodman criterion can be used to roughly predict the failure of the insect wing. The methodological framework presented in our study could provide a basis for future research on fatigue of insect cuticle and other biological composite structures.

Enhanced fatigue behavior under internal pressure of CNT reinforced filament wound cracked pipes

Abstract In this study, the fatigue behavior of filament winding pipes was determined experimentally under internal pressure. Samples were produced by ultrasonication and the filament wound methods. In order to produce the CNTs/E Glass/Epoxy nanocomposite, CNTs and E-glass reinforcements and Bisphenol-A epoxy polymer matrix were used. Test specimens had antisymmetric six layers. All specimens had ±55° winding angle. During all the experiments, the surface crack, fatigue crack growth rate and reinforcement condition of carbon nanotube were investigated. Two different rates of CNT (0.5% and 1%) were compared with the pure epoxy material. Effect of CNT on fatigue life was investigated experimentally. Cracks on the pipes for the fatigue tests had the aspect ratio of a/c = 0.2 and crack depth range between a/t = 0,25 and 0,50. Each test was repeated at least 3 times. Three different maximum stress levels were determined for the tests, which were 40%, 50% and 60% of the ultimate hoop stress. Fatigue life of nanocomposite pipes was found to increase with enhanced inter-laminar adhesion via CNTs. Stopping crack progression, bridging mechanism and mechanical interlocking between fibers of CNTs had been justified by SEM images. As a conclusion, material properties of GRP pipes were discussed by determining the effect of the reinforcement of carbon nanotubes.

Возможность использования ребер для защиты трубопроводов от распространения продольных трещин

This research aims to build a finite element model of oil or gas steel pipelines transmission. The model has been developed in that crack propagation in pipes can be reduced as possible to avoid the failure. The suggestion that is taken into consideration for this purposeis to stopping crack propagation as possibly, is by using ribs as arresters. Different thicknesses have been tested for the ribs used in this study. The first one was of 0.02 m, while as the second thickness was of 0.05 m. The dimensions of pipe are of 0.01 m thickness, and diameter of 0.5 m. The model of pipe consists of three parts. The first part has two different lengths 2 m and 3 m. The second, which is the rib, is of length of 0.15 m. The third part of pipe has the length of 1 m. So all the three parts are made up the case study that was studied in this work. The value of stress intensity factor K1 for different lengths of cracks has been calculated by the finite element model deve-loped by using ANSYS package. These calculations were applied with and without ribs. The results showed that when using the ribs, the stress intensity factor would be reduced significantly. This leads to reduce the possibility of failure.

Example of Arresting Crack Growth in Welded Parts

The unstable growth of fatigue cracks brings a material or structure to catastrophic failure. It is better than any fatigue crack does not initiate in engineering applications. However, fatigue crack initiation cannot be prevented in some applications because maintaining their efficiency or performance is important factor for usage. When a crack is initiated in machine equipment, it should be repaired or the equipment replaced. Sometimes, the cracked material was repaired and many repair methods were proposed. In this report, a simple method of stopping crack growth is proposed to apply to a cracked plate with varying thickness. The stop-hole and crack arrester were applied in the present method.

18CrNiMo7-6 with TRIP-Effect for Increasing the Damage Tolerance of Gear Components — Part I: Alloy Design

High performance components such as gear wheels shall be resistant to rolling-contactfatigue. This type of failure is usually caused by effects occurring on a microscopic scale, such ascrack initiation at non-metallic inclusions. Much effort has been invested so far in improving thesteel cleanliness. However, these high performance components often do not reach the desiredservice life. Preliminary failure within the guarantee terms still occurs which leads to high warrantycosts. Alternative to improving steel cleanliness, the damage tolerance of high performancecomponents could be increased by inducing the TRIP-effect around the crack tip. Due to high localstrain hardening, martensite transformation occurs. The high compressive stresses related to it coulddelay or stop crack propagation by reducing stress concentrations via plastic deformation. As aresult, rolling-contact fatigue resistance of carburized steels may be increased and preliminaryfailure may be avoided. Part I of this study focuses on modifying the chemical composition ofconventional 18CrNiMo7-6 steel with Al to develop a high-strength, yet ductile matrix with a highwork hardening potential. Dilatometric tests on laboratory melts analyze the possibility of adjustinga microstructure able to produce a TRIP-effect. Both isothermal annealing and Quenching andPartitioning (Q&P) are used to stabilize residual austenite and optimum process routes areidentified.

Repairing and Preserving Bridge and Steel Structure Using an Innovative Crack Arrest Repair System

Cracking in steel road or rail bridges is a major concern for continued safe operation of the bridge. Traditional crack arrest hole (CAH) or drill stops placed at the end of the detectable crack have been shown to be ineffective in stopping cracks. A method adapted from the aerospace industry, which utilizes a high interference fit bushing expanded into the CAH, has been tested in large A36 steel coupons under typical cyclic bridge loading. Expansion of the bushing into the hole yields the surrounding steel and induces a residual compressive stress around the hole to shield it from the cyclic load and prevent further crack growth. Not only did this method completely arrest the crack with a life improvement factor exceeding 60:1 over the conventional CAH but it also increased the load factor capability of the coupon by over 20%. This paper will describe the methodology, present the results of the test program and show how this innovative method dramatically increases the effectiveness of a typical CAH that could forestall major structural failure.

Method of arresting crack growth for application at a narrow working space

The many fracture accidents in engineering applications are related to fatigue crack initiation and growth. Therefore, the inspection of fatigue crack initiation and the arrest of crack growth are very important for the safe operation of a machine and the maintenance of a structure. Non destructive inspection has been developed in order to prevent a fracture accident resulting from the cracks or defects. When a crack initiates in machine equipment, continuous observation of the crack is carried out and the crack growth is arrested by some method. To arrest the crack growth, stop-holes are drilled at the crack tips in a case of initiation of a small number of cracks. Drilling the stop-hole is an effective method to prevent crack growth because it reduces stress concentration at the crack tip. Application of residual stress around the stop-holes and making a plastic hardening zone around the crack tips are effective to arrest crack growth. Applying a patch for repairing cracked parts is more effective than drilling stop-holes. In practical cases, patches are applied to repair cracked parts in airplane bodies, bridges, etc. However those methods cannot be used in some cases, because of the conditions of working space, for example. In this paper, an example of the application of a crack arrester at a car facility is introduced. Then, the effectiveness of the crack arrester is discussed. When many cracks are initiated and working space is very small, the crack arrester is a good tool for repairing the cracked parts. It is necessary to develop a method for stopping crack growth in order to replace cracked equipment, because new equipment should be designed carefully for preventing crack initiation again.

Influence of anodization on the fatigue strength of 7050‐T7451 aluminium alloy

ABSTRACTIn recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050‐T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material.A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.

An Analysis on Enhancement of Fatigue Durability of Polyurethane by Incorporating Organoclay Nanofillers

AbstractSummary: One of the important applications of PUEs is as damping materials such as the spring aids used in cars, which need to have excellent cyclic fatigue durability. Our results show that the fatigue durability of PUE can be significantly improved by incorporating organoclay. The best result was obtained for 3 wt.‐% organoclay, and the fatigue durability of the PUE‐organoclay nanocomposites was increased on an average to about ten times that of pure PUE. The addition of organoclay resulted in a reduction in the size of hard‐segment domains. The organoclay nanoplatelets show reversible deformation behaviour during a fatigue test confirmed by WAXD and computer simulation. These could benefit in stopping crack propagation, releasing concentrated stress and giving rise to fatigue resistance.Schematic diagram of d‐spacing on nanoscale change during stretching.magnified imageSchematic diagram of d‐spacing on nanoscale change during stretching.

Atomistic and continuum studies of a suddenly stopping supersonic crack

Large-scale atomistic simulations are used to study a suddenly stopping crack under mode I and mode II loading conditions. For both cases, linear system solutions are established with harmonic interatomic potentials and it is shown that these reproduce the continuum mechanics solution. We find that the Griffith criterion gives predictions for the onset of fracture in good agreement with the atomistic simulation. The simulation results of the suddenly stopping crack agree well with continuum mechanics and experiments. At the atomic scale, we observe a thermalization effect where mechanical energy is dissipated as heat soon after crack stopping. This result does not affect the basic nature of crack tip fields as predicted by continuum mechanics. The harmonic models are further used as reference systems to probe crack dynamics in the nonlinear materials. We discuss cracks stopping from super-Rayleigh and supersonic velocities in hyperelastic solids. We demonstrate via ab initio crack dynamics that it is difficult to define a unique wave front in the nonlinear solid. This observation supports the idea that there is no unique wave speeds near the crack tip. There exists a train of “longitudinal” and “shear” waves associated with the rapidly changing stress state near the crack tip. For supersonic mode II cracks, the mother crack seems to transform from a singularity into a less localized stress concentration after the daughter crack is nucleated. In addition to this result, we observe an anisotropy of wave velocities close to the crack tip leading to elliptical instead of circular wave fronts. This study exemplifies joint atomistic and continuum modelling of nanoscale dynamic systems.

An approach for prediction of fatigue strength of shot peened components

In this paper the problem of predicting and optimising the fatigue strength of shot peened specimens is dealt with. A series of notched cylindrical specimens, peened by using different parameters, was fatigue tested, being the aim to determine the improvement of the fatigue limit of the treated specimens. After the tests, the specimens were observed at the SE microscope and non-propagating microcracks were found. On the basis of this experimental evidence it was considered that the fatigue alleviation due to shot peening is mainly due to the ability of stopping crack propagation. A finite element procedure of the cracked notched specimens was established, with the aim of calculating the stress intensity factor including the applied load, the residual stresses and the contact between the crack faces. The results enable the prediction of the improvement of fatigue strength due to shot peening.