Fracture Mechanics Studies Research Articles

THE EFFECT OF FIBER REINFORCEMENT ON FRACTURE TOUGHNESS ASSESSMENT OF NANOCLAY FILLED POLYMER COMPOSITES

This paper represents the nanomechanical properties of various loading levels of montmorillonite clay filled polyester composites and randomly distributed jute fiber reinforced hybrid composites through Vickers micro-hardness test. The study of indentation fracture mechanics in polymer materials is a simple and cost-effective technique for the determination of fracture toughness. Ultrasonication technique was used to disperse the clay in the polyester matrix. The hand layup method was adapted to prepare the test specimens. Incorporation of 5[Formula: see text]wt.% montmorillonite clay into the polymer matrix results in an enhancement in hardness of 26.52% and the modulus of elasticity increased from 4205.21[Formula: see text]MPa for neat polyester to 5051.46[Formula: see text]MPa with the addition of clay. Fracture toughness was observed to depend on the montmorillonite clay content, and the maximum value was observed at 5[Formula: see text]wt.% nanoclay and 25[Formula: see text]wt.% jute fibers. The results show that the increase in the fiber content reduces the crack propagation in hybrid composites and increases the fracture toughness. To predict the crack size, the scanning electron microscope images are used.

CONCENTRATION OF STRESSES NEAR V-SHAPED ELEMENTS

The study of stress concentrators, especially those that cause the formation of singularities of σ→∞ form, is of great scientific and applied importance. The most famous representative of such stress concentrators is the crack, which is the subject of study of fracture mechanics. Cracks can be normal, located in a homogeneous material, interfacial, intersecting the phase plane between two different materials. Depending on this, stress fields are formed with different features. Another class of similar stress concentrators is V-shaped elements that form a dihedral angle with a disclosure angle greater than zero. V-shaped elements can be formed in both homogeneous and dissimilar materials. The singularity features depend on the mechanical properties of the materials and the angles occupied by these materials. If the opening angle is zero, then the corresponding formulas are transferred to the well-known formulas for cracks. The next class of stress concentrators forming the singularity is the internal elements with V-shaped material boundaries. Such elements are formed around non-metallic inclusions with sharp corners, in fibrous composite materials at the ends of the fibers. Unlike the previous class, in this case there are no empty sectors. However, the resulting formulas transfer into the well-known expressions, if the mechanical properties of one of the materials tend to zero. Thus, all classes of tasks related to the formation of a singularity of a σ→∞ form can be considered as one common task with an increased number of parameters. Such work is done in this article. This made it possible to obtain new patterns of stress state and to determine the directions for the further development of mechanics destruction. As an example, using the complex potentials method with the application of the Kolosov-Muskhelishvili equations, a theoretical analysis of the stress state at the top of the V-shaped boundary separating two homogeneous materials with different normal elastic moduli E1 and E2 was carried out. The calculation showed that in the local zone near the top of the angle, the straightened state acquires a singular character, the stress distribution is described by terms of the form σ ≈ К/rλ, and the parameters of the singularity λ1, λ2, λ3 depend on the ratio of the elastic characteristics of the materials. Relevant patterns are studied.

Numerical Comparison of Cruciform weld and Butt weld simulation and a Study of Fracture Mechanics on Two Types of Welds

The modeling of the welding is desirable to guess the deformation of the components during manufacture, the position and the magnitude of maximum residual stresses and to envisage metallurgical effects in specific zones. The welding are problems of complex modeling requiring the thermal and structural solutions. This has to lead to the development of several software packages and codes for simulation by finite elements. The welding condition, the properties of the structure and their interactions have significant influences on the thermal and structural responses (temperature history, distortion, and residual stress) in welded structures. This paper presents a finite element procedure for the prediction of welding-induced residual stresses and distortions. Comparison is made with tow numerical example. The first example is a butt welded joint of two plates, while the second is a Cruciform welded joint of two plates with four passes. Moreover, a comparison between the Cruciform weld and the butt-weld which we can obtain the parameters of the linear fracture mechanics; stress intensity factor K and energy release rate G.

A fracture model for exfoliation of thin silicon films

The direct exfoliation of thin films from silicon wafers has the potential to significantly lower the cost of flexible electronics while leveraging the performance benefits and established infrastructure of traditional wafer-based fabrication processes. However, controlling the thickness and uniformity of exfoliated silicon thin films has proven difficult due to a lack of understanding and control over the exfoliation process. This paper presents a new silicon exfoliation process and model which enables accurate prediction of the thickness and quality of the exfoliated thin-film based on the exfoliation process parameters. This model uses a parametric, finite element, linear elastic fracture mechanics study with nonlinear loading to determine how each process parameter affects the crack propagation depth. A metamodel is then constructed from the results of numerous simulations to inform the design and operation of a novel exfoliation tool and predict thickness of produced films. In order to manufacture uniform, high-quality films, the tool creates a controlled peeling load that is able to propagate a crack through the silicon in a controlled manner. Finally, exfoliated silicon samples produced with the prototype tool are evaluated and compared to metamodel projections, confirming the ability of the tool to steer crack trajectory within ± 3 microns of the crack depth predictions.

Study of localized shear fracture mechanisms in alloys under dynamic loading

Study of localized shear fracture mechanisms in alloys under dynamic loading

Cold resistance of materials as an integrity factor of railway transport in the Extreme environment

The integrity of structures and responsible units of railway transport in Extreme environment depends on the efficiency materials such as locomotive and rail steels at low temperature. Understanding of materials and structures behavior under the impact loading it is necessary to improve the characteristics and reliability of all railway transport. So the study of locomotive steel fracture mechanism at low temperature has been carried out. The optical and electron microscopy used to study the fracture surface of impact toughness test samples. The analysis reveals the sources of the radial fracture pattern. In case of locomotive wheel bandage it is the globular particle located in the steel grains. It is obvious that the spheroidization of non-metal imperfection happens under the thermal influence during the variable cyclic loading over the operation time. The stochastic modeling shows the high susceptibility of the crack growth velocity to structural size. So the problem of low reliability and lifetime could be solved by improving the impact toughness ant low temperature through the thermal and mechanical processing of steel and microalloying with refractory and rare-earth element compositions.

A basic study of fracture mechanics due to hydraulic fracturing in unconsolidated sands

A basic study of fracture mechanics due to hydraulic fracturing in unconsolidated sands

Consistent finite-dimensional approximation of phase-field models of fracture

In this paper we focus on the finite-dimensional approximation of quasi-static evolutions of critical points of the phase-field model of brittle fracture. In a space discretized setting, we first discuss an alternating minimization scheme which, together with the usual time-discretization procedure, allows us to construct such finite-dimensional evolutions. Then, passing to the limit as the space discretization becomes finer and finer, we prove that any limit of a sequence of finite-dimensional evolutions is itself a quasi-static evolution of the phase-field model of fracture. In particular, our proof shows for the first time the consistency of numerical schemes related to the study of fracture mechanics and image processing.

Numerical Simulation of a Steel Weld Joint and Fracture Mechanics Study of a Compact Tension Specimen for Zones of Weld Joint

To evaluate the integrity of a structure consists to prove his ability to perform his mechanical functions for all modes of loading, normal or accidental, and throughout its lifespan. In the context of nuclear safety, for the most important welded structures such as the tank or the primary circuit, we consider that the presence of a degradation grouping several aspects, such as cracks which created during welding. We seek then, from the sizing, to show the mechanical strength for this degraded mode. We also seek; to show the mechanical strength of a structure in the presence of a crack when defects have been detected during an inspection, in this context, the fracture mechanics provides the necessary tools to analyze cracked components. Its purpose is to establish a fracture criterion to foreordain the loading margins in normal or accidental operating conditions. Each type of rupture must be the subject of a specific characterization.

Evaluation of uncertainty in the measurement of the crack-tip stress field using photoelasticity

Photoelasticity has always played an important role in the experimental study of fracture mechanics. It can, with a degree of simplicity, be used to measure the parameters that characterize the crack-tip stress field. The so-called over-deterministic method is the most widely used in that regard. Although that particular method functions correctly, it has some metrological limitations: no measurement uncertainties are provided, observations are assumed to be independent and of equal accuracy, and its results may be skewed due to the misalignment of the coordinate axes in the data collection. In this article, it is shown that these limitations no longer apply when using the generalized least squares by Lagrange multipliers method. It provides not only an estimate of the quantities to be measured, as the over-deterministic method, but also a fitted estimate of the value of each input quantity; the covariance matrix of all these estimates from which both the standard uncertainties and correlation coefficients can be calculated; a chi-square value that can be used to test the consistency of the measurement model; and the normalized deviations between the input estimates and their fitted values, which are tools to identify potential outliers.

Uncertainty evaluation of crack-tip parameters from displacement field data

In the study of fracture mechanics, various techniques are used to determine the parameters that characterize the crack-tip stress field. What these techniques obtain is the full-field displacement distribution and then, the parameters are extracted through the so-called over-deterministic method. This method has some metrological limitations: it does not provide the uncertainties of the measurements, it assumes that the observations are independent and have the same accuracy, and its results are very sensitive to the misalignment of the coordinate axes. In this paper it is shown that the Generalized Least Squares by Lagrange Multipliers method overcomes all that limitations.

소성가공에 따른 STS 304L재의 초음파피로시험시 작은 표면균열의 거동에 관한 파괴역학적 연구

소성가공에 따른 STS 304L재의 초음파피로시험시 작은 표면균열의 거동에 관한 파괴역학적 연구

Enhancing the Environmentally Assisted Cracking Resistance of Aircraft Quality Al Alloy of Type AA7075 Stiffened with Polymer Matrix Composite Using Cerium Chloride Inhibitor

This study explores a methodology to raise environmentally assisted cracking resistance of alclad Al–Zn–Mg–Cu (AA7075) alloy, used in aircraft structures, stiffened with polymer matrix composite asymmetric patch in order to suppress growth of fatigue cracks. Fracture mechanics studies of adhesively bonded center-pre-cracked Al alloy specimens having stiffeners were carried out in 3.5 wt% sodium chloride environment to understand the effect of cerium chloride inhibitor on the threshold stress intensity for stress corrosion cracking as well as the second-stage (steady-state) crack growth rate. It was observed that in the peak- and the two-step-aged tempers, the crack growth rate of the alloy reduced by two and a half, and four times, respectively, while the threshold stress intensity increased by 14–15% due to the addition of 1000 ppm of this inhibitor. The study offers a method to enhance significantly life of aging aircrafts stiffened with polymer matrix composite.

Finite Element Study on Normal Stress Distribution to the Kerf Plane in a Finite Plate Under Mode I Loading

The normal stress distribution to the crack plane in fracture mechanics studies is well recognized. However, the detailed normal stress distribution to the kerf plane is still unknown. A recent study has proven that the kerf can be used as alternative in crack interactions under tension loading and as a crack arrestor in fracture mechanics. By definition, kerf is the amount of material removed by machining tool on a single pass plus tools cutting tolerance. The use of kerf in fracture mechanics studies as an alternative to the crack has a great advantage as it is much easier to fabricate. Therefore, it is essential to study normal stress distribution to the kerf plane to further understand the behaviour of the kerf in fracture mechanics. The finite element analysis has been used to model a finite plate which contains various length of kerf at the edge. A simple experiment was conducted to measure actual dimensions of the kerf for finite element model. The normal stress value to the kerf plane along the horizontal distance from the kerf tip has been studied and compared with the results obtained using crack plane model. It can be concluded that stress distribution normal to the kerf plane is comparable to the crack plane. The only difference is that the use of a kerf would not produce high value of stress near its tip as demonstrated by a crack. Results have been compared with theoretical and they show an approximated maximum error less than 5 %.

An analytical model for the fracture behavior of the flexible lithium-ion batteries under bending deformation

To understand the influence of the bending deformation on the stress evolution and crack propagation in nano flexible electrode during electrochemical cycling, an analytical model is developed based on core-shell structure in a cylindrical electrode. In the model, the analytical solution of stress specialized for the cylindrical electrode in the process of bending deformation and phase transformation is clarified. Further, the weight function is utilized to calculate the time-dependent stress intensity factor by combining the stress profiles as found in the analytical work. Thus, the behavior of the preexisting center and edge cracks in the electrode is discussed to investigate the effect of the initiation position on crack propagation. It is found that cracks tend to spread more easily in the early stages of discharging process due to the larger slope of the SIF curve within small edge cracks. Further, an analytical fracture mechanics study is presented and the formula of critical size for the flexible electrode is derived based on Griffith criterion, below which the crack does not spread under the superposition of the diffusion stress and the bending stress. In the light of the fracture mechanics study of the flexible electrode, the present work sheds some light on stress engineering and structural design of durable flexible lithium-ion batteries.

Multi-scale strategy for modeling macrocracks propagation in reinforced concrete structures

This paper introduces a new approach to model cracking processes in large reinforced concrete structures, like dams or nuclear power plants. For these types of structures it is unreasonable, due to calculation time, to explicitly model rebars and steel-concrete bonds. To solve this problem, we developed, in the framework of the finite element method, a probabilistic macroscopic cracking model based on a ulti-scale simulation strategy: the Probabilistic Model for (finite) Elements of Reinforced Concrete (PMERC).The PMERC's identification strategy is case-specific because it holds information about the local behaviour, obtained in advance via numerical experimentations.The Numerical experimentations are performed using a validated cracking model allowing a fine description of the cracking processes.The method used in the inverse analysis is inspired from regression algorithms: data on the local scale would shape the macroscopic model.Although the identification phase can be relatively time-consuming, the structural simulation is as a result, very fast, leading to a sensitive reduction of the overall computational time.A validation of this multi-scale modelling strategy is proposed. This validation concerns the analysis of the propagation of a macrocrack in a very large Double Cantilever Beam specimen (DCB specimen usually used in the framework of Fracture Mechanics studies) containing rebars. Promising results in terms of global behaviour, macrocracking information and important reduction in simulation time are obtained.

Evaluation of mode-I SIF, T-stress and J-integral using displacement data from digital image correlation – Revisited

Digital image correlation (DIC) is a unique technique for determining displacements because in a single experiment one can get ux and uy displacements if it is 2D DIC and all the three components if one uses 3D DIC. Hence, for data processing one has many options. While processing displacements, rigid body translation and rotation need to be accommodated properly. There has been no comprehensive study in the literature. In this paper, a systematic study is done to determine the choice of suitable individual displacements or a combination to evaluate not only mode-I SIF but also T-stress. Study on influence of data collection zone, a revisit to the multi-parameter displacement field equations on the nature of additional terms to account for rigid body motion, role of out-of-plane displacement in influencing error in estimating SIF and T-stress has been discussed. A simple procedure to reduce the error due to improper identification of crack-tip has been implemented. This study has recommended the use of 2D DIC if one wants to evaluate only SIF. However, if one wants to evaluate T-stress also, then 3D DIC gives better accuracy.J-integral evaluation is essential for elasto-plastic fracture mechanics (EPFM) studies. Curve fitting the local displacement data is necessary for J-integral evaluation and suitable polynomial function and grid size need to be selected. Theoretical study carried out in this work showed that a third order polynomial fitted to displacement data in a local grid of size 0.5 mm (approximately ten times the plastic zone size) gave accurate J values even for paths of small radius (closer to crack-tip) where strain gradients are very high. Using these parameters, J-integral is obtained for epoxy (brittle polymer) and polycarbonate (ductile polymer). For polycarbonate, a method has been suggested to determine the Dugdale plastic zone size from the measured J-integral and out-of-plane displacement results.

Monitoring System of Rail Surface Crack Propagation

Abstract Industrially produced rails can contain some inherent failures without evident damage. If a fracture propagates beyond a critical size, it can lead to breakage. The study of fracture mechanics suggests many different theories for detecting the fracture. Continuous monitoring of the rail surface state is necessary in order to assure uninterrupted and safe transportation.

Fracture mechanics of shear crack propagation and dissection in the healthy bovine descending aortic media

An intimal tear of an apparently healthy aortic wall near the aortic arch is life-threatening because it may lead to full rupture or to wall dissection in which delamination of the medial layer extends around most of the aortic circumference. The mechanical events underlying dissection are not definitively established. This experimental fracture mechanics study provides evidence that shear rupture is the main mechanical process underlying aortic dissection. The commonly performed tensile strength tests of aortic tissue are not clinically useful to predict or describe aortic dissection. One implication of the study is that shear tests might produce more fruitful simple assessments of the aortic wall strength. A clinical implication is that when presented with an intimal tear, those who guide care might recommend steps to reduce the shear load on the aorta.

Generalised fracture mechanics approach to the interfacial failure analysis of a bonded steel-concrete joint

Steel-concrete joints are often made by welded shear studs. However, this connection reduces the fatigue strength, especially in situations where locally concentrated loads occur with a large number of load cycles e.g. in bridge decks. In this paper the shear bond strength between steel and ultrahigh performance concrete (UHPC) without welded mechanical shear connectors is evaluated through push-out tests and a generalized fracture mechanics approach based on analytical and finite element analyses. The connection is achieved by an epoxy adhesive layer gritted with granules. In the tests, specimens made with various manners of preparation of the epoxy interlayer are tested experimentally. Numerical-analytical 2D and 3D modelling of a steel-concrete connection is performed without and with the epoxy interlayer. The model of a bi-material notch with various geometrical and material properties is used to simulate various singular stress concentrators that can be responsible for failure initiation. Thus conditions of crack initiation can be predicted from knowledge of the standard mechanical and fracture-mechanics properties of particular materials. Results of the fracture-mechanics studies are compared with each other and with experimental results. On the basis of the comparison, the 2D simulation of the steel-concrete connection without the epoxy interlayer is shown to be suitable for the estimation of failure conditions.