Failure Of Structural Materials Research Articles

New insight into the role of inclusions in hydrogen-induced degradation of fracture toughness: three-dimensional imaging and modeling

ABSTRACT An in-depth understanding of the role of non-metallic inclusions (NMIs) in the hydrogen-induced failure of structural materials is important in the manufacture and application of steels. In this paper we investigated the type, the amount and the distribution of NMIs in pipeline steels and their relation to the susceptibility to hydrogen-induced cracking (HIC). Scanning Electron Microscope (SEM) observation confirmed the interlinking of the microcracks formed at the inclusion-matrix interfaces. By combining with the nondestructive three-dimensional (3D) mapping of crack propagation paths, based on the synchrotron tomography technique, we obtained direct evidence that the cracking initiation at the NMIs and then interlinking on the (nearly) same plane are the main mechanism of HIC failure in X70 steels. We built an elastic-energy-based model, which can be applied to quantitatively predict the HIC-induced dependence of the fracture toughness on NMIs, based on statistical information of the NMIs. The theoretical results show a good agreement with the experimental observation. We proposed a criterion to determine the susceptibility to HIC by the observation of NMIs. We also developed a thermodynamics-based model to analyze the hydrogen trapping at the inclusion-matrix interface and its dependence on the strength of the steel matrix. For the first time, the direct relationship between NMIs and HIC-induced degradation of fracture toughness is obtained.

Damage model for calculating the failure of structural materials

Damage model for calculating the failure of structural materials

Predicting the kinetics of hysteretic self-heating of GFRPs under high-frequency cyclic loading

Hysteretic self-heating effect emerges in aerospace, automotive and marine structures subject to vibrations with high amplitudes and frequencies, causing the degradation of mechanical properties and, in some cases, failure of structural materials. In this work, nonlinear deformation of GFRP at single and cyclic tension along with warp yarns at quasi-static conditions was investigated. The self-heating effect at the frequency of 25 Hz using a thermal imaging camera was analyzed. As a result, the analytical model of hysteretic heat generation at cyclic loading was developed. For this, we have approximated single loading stress-strain curves by Ramberg–Osgood functions and calculated the width of hysteresis loop at cyclic loading with the given stress amplitude. It helped to develop the methodology for the prediction of samples self-heating under conditions of natural convective heat transfer and compare the calculated and experimental data of thermal progress. The life of GFRP at cyclic loading is estimated using the criterion of reaching glass-transition temperature obtained from the tests on the dynamic mechanical analyzer (DMA).

ВЛИЯНИЕ ПУЗЫРЬКОВ ГАЗА НА ЭРОЗИЮ СТАЛИ В СВИНЦЕ

The erosion of the steel surface in lead-based coolants is a powerful factor that initiates corrosion of the structural material. In the conditions of cavitation effect of gas bubbles on the oxide coating, breaches of its integrity can be created, as a result of which an intensive influence of lead on steel occurs on the "bare" areas. This leads to a multiple increase in the rate of corrosion. The regularities of erosive action of lead or its alloys on structural materials have not been sufficiently studied. In the experiments of different authors, differing data of flow rates of lead or lead-bismuth, to cause the erosion of steel, were obtained. In general, this phenomenon is a combination of a number of simultane-ously occurring thermohydraulic and physicochemical processes. Erosion failure of structural materials in a heavy coolant is rapidly developing and occurs in places with increased coolant velocities, in the field of hydrodynamic disturbances, and so on. Protective coatings formed on steels based on oxide technology are vulnerable from the point of view of erosion wear. The erosion of protective coatings on steels can in turn cause peeling of fuel rod cladding with end-to-end sheath breaking, which can proceed more intensively than the front dissolution of unprotected shells.The most significant factor leading to the erosion of steel in lead is the hydrodynamic effect of the coolant flow. In the presence of gas bubbles in the coolant, erosion occurs at the lowest flow rates, and the erosive effect itself has a cavitation character. The present report presents the results of studies of erosion of steel in lead under gas cavitation. A methodology for appropriate erosion testing was developed. Experiments were carried out on the installation with a rotating cylinder at different cylinder speeds and at various temperatures. A numerical solution of the differential equation of motion of the bubble wall is performed for the different hydrodynamic parameters of the lead flow. The experimental and calculated data obtained make it possible to estimate the process parameters under which a cavitation damage of the oxide coating occurs on the steel surface in the flowing lead.

Failure of Structural Material as a Nonequilibrium Phase Transition

The initial stage of instability in the failure of a structural material treated as a nonequilibrium phase transition is reconstructed. Its mechanism is based on spinodal decomposition (diffusion separation).

THE EVALUATION OF DURABILITY OF STRUCTURAL ELEMENTS USING NEURAL NETWORKS

<p class="Default">While in operation structural elements may have cracks, which usually grow up to critical size due to further cyclic loading, so that the component is likely to be destroyed. Therefore, it is important to study the process of fatigue failure of structural materials. The aim of this work was to evaluate the durability of structural elements during their design phase, taking into account the achieved level of material damage, and to predict the fatigue crack growth (FCG) rate in an aluminium D16chT alloy under regular loading using neural network (NN). Based on experimental data of FCG at stress ratio <em>R</em> = 0, FCG rate was predicted in this alloy. The results obtained by the authors are in good agreement with the experimental data.</p>

The imaging of failure in structural materials by synchrotron radiation X-ray microtomography

The initiation and propagation of short cracks are critical to the detailed understanding of the failure mechanisms of engineering materials and structures. Therefore, for a safety critical component, it is beneficial to be able to follow the progress of cracks in loaded structures and real environments ideally at sub-micron spatial resolution and/or sub-second time resolution. Here we review the great progress that has been made in the application of synchrotron radiation X-ray computed microtomography (SR-μCT) to the study of internal damage accumulation and evolution in structural materials including cast irons and steel, nickel superalloys, lightweight titanium and aluminum alloys as well as metallic, ceramic and polymer composite materials together with additatively manufactured or three-dimensional (3D) printed metallic materials. SR-μCT is shedding light on the complex interactions of cracks with intrinsic porosity, precipitates and intermetallic inclusions and grain structure, as well as macroscopic features such as holes and joints. Furthermore it is enabling qualitative and quantitative relations between the microstructural features and engineering performance to be established and validated under realistic service loading and environmental conditions. Here we introduce the basic 3D and time-lapse (4D) tomographic methods and discuss their strengths and limitations across a range of the applications, as well as identify opportunities for the future.

Development of a stochastic model of failure of structural materials in creep at hardening stage

The results of research of patterns of long-term failure of metals and alloys in isothermal creep considering all its stages are presented. The approach to prediction of long-term strength characteristics of structural materials in isothermal creep under uniaxial steady load is proposed. This approach is original in the probabilistic interpretation of kinetic equations describing the creep, as well as a variety of failure criteria and their relative characteristics. Using the experimental creep curves, the adequate stochastic model was constructed and the method of identification of unknown constants of the model was developed. The calculations of basic probability characteristics of the time to failure on the example of experimental data for PA6 aluminum alloy specimens at various stress and temperatures levels were carried out. According to the research, it is enough to use the quadratic dependencies as dependencies of distribution parameters of random variables a and b, as well as the parameters c and g on the stress and temperature. The choice of the type of dependencies of higher degrees has little impact on calculation results. The proposed model can be used for prediction of the time to failure of various structural materials, as well as the development of methods, standards and guidelines in the field of reliability and durability assessment of structural materials.

Monitoring of Acoustic Emission During the Disintegration of Rock

Many factors and complexities are involved in breaking rock with conventional methods. Water jet technique is an effective solution to the problems faced in the conventional method. Rock deterioration is the most important step in the mining, civil, mineral engineering, the technology of water jet cutting and disintegration of various material attained continuous growth during the last decades. Water jet is widely used in industrial application such as disintegration of materials and rock, cleaning and removal of surface layers. The literature is reviewed the particular attention to the detection, recording and monitoring of fracture evolution, monitoring of collision healing and creep in rock and other material using acoustic emission. Acoustic emission has been widely used in industries and other various field such as steam pipes and pressure vessels, composites materials, monitor the welding and corrosion progress,metals and also applied numerous structural components. It is a technique to direct monitor defects formation and failure in structural materials. However, it can also provide information on the stress field, fatigue and the fracture pattern behaviour.

Preface to the Special Issue on the Sandia Fracture Challenge

Despite many advances in fracture mechanics and computational analysis methods over the past several decades, there are still certain classes of problems in failure of structural materials that are difficult to predict. One such class of problems can be generally categorized as ductile tearing of metals and alloys. Ductile tearing has been an enduring challenge in the metal forming industry, where certain rubrics like forming limit diagrams provide utilitarian solutions, but are limited in applicability. The 2012 Sandia Fracture Challenge was pursued as a means to evaluate the ‘state-ofthe-art’ in prediction of ductile tearing. As a precursor to the Sandia Fracture Challenge, a series of internal blindprediction assessmentswere performed at Sandia National Laboratories. These efforts, some of which were documented in a 2011 report [Report number SAND2011-6801], demonstrated the difficulty in accurate computational prediction of ductile tearing failures. Several deficiencies were highlighted: (a) the need for an accurate assessment of finite deformation as a prerequisite for accurate failure modeling, (b) the limitations of a tensile test as the sole

Stress, Strain, and Energy at Fracture of Degraded Surfaces: Study of Replicates of Rough Surfaces

Due to the aging of structures, the issues of plant life management and license extension are receiving increasing emphasis in many countries. Understanding failure of structures due to random roughness on surfaces at early stages of degradation is therefore crucial. It has been shown that even slightly sinusoidal roughness can increase stress concentration by a factor of two or three, which can be critical for a brittle component due to the significant reduction of its load-carrying capacity, even with slight roughness. A more in-depth fracture analysis of surfaces possessing random roughness is needed in order to more profoundly understand, and, hence, develop models that will predict more accurately, failure of structural materials exposed to degrading, in-service conditions. Using a technique previously developed and successfully applied, replicates of random rough surfaces, imprinted with various levels of degradation, and at three distinct auto correlation lengths, were realized and mechanical testing was performed on them. The stress, strain, and energy at fracture are reported. Finite element analysis was carried out to elucidate experimental results. Besides the expected reduction of energy at fracture with degradation, a relaxation region was observed where the energy slightly increases. This phenomenon implies that even after degradation has progressed there is a local maximum of energy at fracture due to the competing effect of tendons and growth of pits. The results find applications on the early stage of maintenance of surfaces of structures in service.

An application for the fracture characterisation of quasi-brittle materials taking into account fracture process zone influence

The paper introduces a Java application programmed for the advanced determination of the fracture characteristics of silicate-based materials failing in a quasi-brittle manner. The tool reconstructs the progress of a quasi-brittle fracture from the measured load–displacement curve and the knowledge of basic mechanical properties of the material. The main contribution of the proposed approach is that it takes the characteristics of the Fracture Process Zone (FPZ, particularly its extent, i.e. its size and shape) evolving at the tip of the propagating crack during the failure process into account and incorporates them into the fracture-mechanical parameter evaluation procedure(s). This approach is expected to substantially diminish the influence of the test specimen’s size/shape and the test geometry on the values of the parameters of nonlinear fracture models determined from the records of fracture tests on laboratory specimens. The application implements a developed technique for estimation of the size and shape of the FPZ. The technique is based on an amalgamation of several modelling concepts dealing with the failure of structural materials, i.e. multi-parameter linear elastic fracture mechanics, classical nonlinear fracture models for concrete (equivalent elastic crack and cohesive crack models), and the plasticity approach. The knowledge of the FPZ’s extent is employed for the relation of a part of the entire work of fracture to its characteristics within the presented approach. The verification and validation of the developed technique is performed via numerical simulations using the authors’ own computational code based on physical discretization of continuum and selected sets of experimental evidence published in the literature. Reasonable agreement is observed between the outputs of the presented semi-analytical technique and both the simulation results and the experimental data.

Accidental Fires and Radiation Heat Transfer: Investigating the Effects of Flame Impingement on Structures

Accidental fires and their effects on the structural integrity of buildings are responsible for significant loss of human lives, economic assets, and environmental resources in the United States and across the globe. One cause of the collapse of the World Trade Center and the collapse of the Deepwater Horizon drilling platform in the Gulf of Mexico was the failure of structural materials under large heat loads from fires. Flame impingement or flame structure interaction studies are essential for understanding the effects of fire on the structural integrity of buildings. Radiation is an important mode of heat transfer to the structural element, and measurements of radiation intensities from the flame are useful for improving the understanding of heat transfer from the flame to the structure. This study investigates the radiation intensities and radiation heat flux from flames with and without impingement on a flat steel plate based on plate location relative to burner, fuel mass flow rate, and fuel type. The quantitative comparison between measured radiation intensities of impinging and non-impinging flames shows significant increases in radiation intensities near the surface of the plate. The radiation heat flux to the environment is lower for impinging flames, indicating high heat transfer to the plate. Quantitative knowledge gained from this study can be used to estimate structural integrity under large heat loads and to manufacture structural elements that are more resistant to fire.

Fracture and Fatigue in Graphene Nanocomposites

Graphene, a single-atom-thick sheet of sp-bonded carbon atoms, has generatedmuch interest due to its high specific area and novel mechanical, electrical, and thermal properties. Recent advances in the production of bulk quantities of exfoliated graphene sheets from graphite have enabled the fabrication of graphene–polymer composites. Such composites show tremendous potential for mechanical-property enhancement due to their combination of high specific surface area, strong nanofiller–matrix adhesion and the outstanding mechanical properties of the sp carbon bonding network in graphene. Graphene fillers have been successfully dispersed in poly(styrene), poly(acrylonitrile) and poly(methyl methacrylate) matrices and the responses of their Young’s modulus, ultimate tensile strength, andglass-transition temperaturehave been characterized. However, to the best of our knowledge there is no report on the fracture toughness and fatigue properties of graphene–polymer composites. Fracture toughness describes the ability of a material containing a crack to resist fracture and it is a critically important material property for design applications. Fatigue involves dynamic propagation of cracks under cyclic loading and it is one of the primary causes of catastrophic failure in structural materials. Consequently, the material’s resistance to fracture and fatigue crack propagation are of paramount importance to prevent failure. Herein we report the fracture toughness, fracture energy, and fatigue properties of an epoxy polymer reinforced with various weight fractions of functionalized graphene sheets. Remarkably, only 0.125% weight of functionalized graphene sheets was observed to increase the fracture toughness of the pristine (unfilled) epoxy by 65% and the fracture energy by 115%.Toachievecomparableenhancement,carbonnanotube (CNT) and nanoparticle epoxy composites require one to two orders of magnitude larger weight fraction of nanofillers. Under fatigue conditions, incorporation of 0.125% weight of functionalized graphene sheets drastically reduced the rate of crack propagation in the epoxy 25-fold. Fractography analysis

The effect of carbon nanotube dimensions and dispersion on the fatigue behavior of epoxynanocomposites

Fatigue is one of the primary reasons for failure in structural materials. It has beendemonstrated that carbon nanotubes can suppress fatigue in polymer composites viacrack-bridging and a frictional pull-out mechanism. However, a detailed study of the effectsof nanotube dimensions and dispersion on the fatigue behavior of nanocomposites has notbeen performed. In this work, we show the strong effect of carbon nanotube dimensions(i.e. length, diameter) and dispersion quality on fatigue crack growth suppression in epoxynanocomposites. We observe that the fatigue crack growth rates can be significantlyreduced by (1) reducing the nanotube diameter, (2) increasing the nanotubelength and (3) improving the nanotube dispersion. We qualitatively explain theseobservations by using a fracture mechanics model based on crack-bridging and pull-outof the nanotubes. By optimizing the above parameters (tube length, diameterand dispersion) we demonstrate an over 20-fold reduction in the fatigue crackpropagation rate for the nanocomposite epoxy compared to the baseline (unfilled) epoxy.

Suppression of fatigue crack growth in carbon nanotube composites

Fatigue is one of the primary causes for catastrophic failure in structural materials. Here, we report an order-of-magnitude reduction in the fatigue crack propagation rate for an epoxy system with the addition of ∼0.5wt.% of carbon nanotube additives. Using fractography analysis and fracture mechanics modeling, we show that the crack suppression is caused by crack bridging, which results in an effective crack-closing stress due to the pull out of nanotube fibers in the wake of the crack tip. Carbon nanotubes therefore show potential to significantly enhance the reliability and operating life of structural polymers that are susceptible to fatigue failure.

Effect of Composition and Combustion of Metallized Solid Fuels on Intensity of Failure of Structural Materials Due to a Jet of Combustion Products

The relationship between the initial composition of typical metallized solid fuels, physicochemical transformations during their combustion, and intensity of failure of structural materials under the action of jets of combustion products is experimentally studied. Condensed components of combustion products (aluminum and iron oxides) are found to play the determining role in the process of failure of structural materials. Key words: solid fuel, combustion, composition, failure, condensed components.

Strength, longevity, and failure of structural materials under multicycle loading (review)

This review provides a brief historical outline of the development of research on fatigue strength. Basic attention is focused on studies of the strength, longevity, and failure of metallic materials and structural components under cyclic loads, which have been conducted at the S. P. Timoshenko Mechanics Institute, National Academy of Sciences of Ukraine during the years of its existence. Developments of related organizations are touched upon briefly only to that degree in which they are associated with the problems under consideration.

An approximating model of failure of structural materials and a problem of a failure wave

A method of numerical calculation of a failure wave in a brittle material in pulsed loading is proposed. The method is based on the variation of the elasticity modulus in transition through the wave front. The results of calculating the movement of a spherical failure wave are presented.

Maximum likelihood estimation techniques for concurrent flaw subpopulations

Failure of structural materials is often caused by the presence of two or more types of defect subpopulations. The maximum likelihood estimation technique for evaluating the Weibull parameters of these underlying subpopulations in the case of known fracture origin is presented. The maximum likelihood estimation equations are derived, and solved by means of nonlinear programming. The estimators obtained therefrom are tested for both accuracy and consistency against a series of simulation runs. For data sets containing a relatively small sample size, the advantage of the method of maximum likelihood over two established nonparametric techniques is demonstrated.