Fracture Properties Of Materials Research Articles

Numerical Simulations of Fatigue Crack Growth in a Steam Turbine Rotor Blade Groove

With increasing share of renewable energy sources in the electricity production strict demands are placed on thermal power plants that have to cover the power shortages more frequently. Increasing number of steam turbine (ST) start-ups and shutdowns, as well as requirements on higher ramping of operating conditions, has detrimental effect on the overall lifetime of ST components. In the ST design process, this situation has to be dealt by applying advanced prediction methodologies handling the thermo-mechanical fatigue mechanism, for instance. On the other hand, in the case of currently operating STs, regular inspection and maintenance schedule as well as technologies for turbine operation control have to be reconsidered or newly developed. To cope with these challenges, the international consortium of energetic turbine producers and research institutes initiated the TURBO-REFLEX project funded by EU’s H2020 program. One of the principal aims of the project is development of a damage tolerance approach that may be suitable for scheduling the ST rotor maintenance, for instance. Decisive factors in this effort are ST rotor operating conditions, material fracture properties and geometry that constitute the crack initiation site and crack growth rate and direction. This forms a complex task that has to be handled numerically by using a Finite Element (FE)-based code accompanied by in-house scripts for detecting the most probable way of crack propagation. In this contribution, the adopted fracture–mechanics approach applied to low-pressure section of ST rotor and results that have been achieved are presented.

Residual strain effects in needle-induced cavitation.

Needle-induced cavitation (NIC) locally probes the elastic and fracture properties of soft materials, such as gels and biological tissues. Current NIC protocols tend to overestimate properties when compared to traditional techniques. New NIC methods are needed in order to address this issue. NIC measurements consist of two distinct processes, namely (1) the needle insertion process and (2) the cavitation process. The cavitation process is hypothesized to be highly dependent on the initial needle insertion process due to the influence of residual strain below the needle. Retracting the needle before pressurization to a state in which a cylindrical, tube-like fracture is left below the needle tip is experimentally demonstrated to reduce the impact of residual strain on NIC. Verification of the critical cavitation pressure equation in this new geometry is necessary before implementing this retraction NIC protocol. Complementary modeling shows that the change in initial geometry has little effect on the critical cavitation pressure. Together, these measurements demonstrate that needle retraction is a viable experimental protocol for reducing the influence of residual strain, thus enabling the confident measurement of local elastic and fracture properties in soft gels and tissues.

Micro Indentation Fracture Toughness Assessed by Energy-Based Method: The Method Improvement and Affecting Factors

This paper reported an improved energy-based method to assess the fracture properties of a quasi-brittle material (cement paste) by using micro indentation technique. The model was initiated by the additionally generated fracture energy that is assumed to be averagely distributed in indentation, and only involves the load-displacement curves that follow power functions. An explicit equation for estimating the plastic energy considering the effect of holding procedure that has not been considered before was established, which, together with the principle of energy balance, allows us to assess the fracture energy, energy release rate and fracture toughness of quasi-brittle materials. The accuracy, reliability and robustness of the model were validated by the experimental data with changing the maximum load, loading speed and holding time. Overall, the micro fracture testing method developed here provides an efficient way to assess the fracture properties of quasi-brittle materials in micro scale.

The determination of the stress intensity factor solutions for the new pipe-ring specimen using FEA

For safe transport and reliable supply of petrochemical substances, it is crucial to ensure the structural integrity of the equipment, pipelines in particular. Regarding the large diameters and high mass per distance, pipelines for natural gas are designed as thin-walled cylindrical structures. To ensure the structural integrity of the unknown material means to measure and therefore empirically test the material fracture properties of the laboratory specimen for giving an assessment of the accepted defect size. Compared to standards and regulations, such as the ASTM E-1820, BS 7448 standards and the GKSS procedure, the production of standard specimens for measuring the fracture toughness is commonly very difficult or even impossible for applications. On the basis of the Slovenian–Russian bilateral project, we investigate and propose a solution for this issue with a new kind of specimen, called the pipe-ring specimen. The specimens were made from a segment of the observed thin-walled pipeline from the construction filed or stored in a warehouse. The measurement procedure is similar as for the standard SENB specimens and extensometer because of the geometry of the specimen, which is cut from the pipe and contains only a machine-made notch. The next step is to test specimens axially on the three-point bending load test on the hydraulic machine. Because the ring as the specimen is not standardized, it is necessary to show and prove how, and if it is possible to use ring specimens as an alternative option to the standard specimens for testing and determining fracture properties of testing material for thin-walled pipelines. In the frame of the three main experimental, analytical and numerical approaches, this publication shows the numerical approach of defining the stress intensity factor (SIF) for crack opening mode I with and without prior fatigue pre-cracking. Besides the limit load, the SIF presents one of two main parameters for developing the failure assessment diagram and estimating the possible accepted defects in a material relating to its’ structural integrity.

Pre-notched dog bone small punch specimens for the estimation of fracture properties

In recent years, the pre-notched or pre-cracked small punch test (P-SPT) has been successfully used to estimate the fracture properties of metallic materials for cases in which there is not sufficient material to identify these properties from standard tests, such as CT or SENB specimens. The P-SPT basically consists of deforming a pre-notched miniature specimen, whose edges are firmly gripped by a die, using a high strength punch. The novelty of this paper lies in the estimation of fracture properties using dog-bone-shaped specimens with different confinement levels. With these specimens, three confinement variations have been studied. The results obtained enable the establishment of a variation of fracture properties depending on the level of confinement of each miniature specimen and selection of the most appropriate confinement for this goal.

Assessment of fracture resistance data using p-SPT specimens

Determination of fracture properties of aged materials is important to assess degradation of structural materials subjected to in-service loads and also to assess residual life of the component. The pre-cracked small punch test is an alternative method for the determination of fracture properties in case of availability of limited quantity of materials insufficient for conducting standard ASTM tests. In this work, a simplified methodology is developed to assess J-R data of structural steels 20MnMoNi55 and T91 using pre-cracked small punch test (p-SPT) specimens. The size of the p-SPT specimen is taken as 10 × 10 × 0.5 mm. Wire EDM technique is used to fabricate specimens having through thickness crack with a/W equals to 0.4, 0.45 and 0.5. The tests are conducted to get load v/s displacement data. FE elastic-plastic analyses of the p-SPT specimen are also carried out in parallel to obtain load v/s displacement data computationally. A comparison of computed and experimental data showed that the peak loads calculated numerically are significantly higher than the experimental values. To resolve this issue, it is presumed that there is some extent of crack growth in the p-SPT specimens during experiment before the onset of peak loads. To make an assessment of this crack growth at peak load, a series of FE elastic-plastic analyses of p-SPT specimens are carried out with different crack lengths. Such FE results are then used to obtain two empirical correlations representing (i) variation of peak loads and crack length (ii) variation of CTOD with peak loads. The first empirical correlation is then used to assess crack lengths (and hence the crack growth) corresponding to experimental peak loads for all the tested specimens. Similarly, the second empirical correlation is used to calculate CTOD corresponding to the experimental peak loads. These two sets of data finally generated CTOD as a function of crack growth. Using the conventional relations between CTOD and J-integral, the data is then converted to J-R curve. This procedure is used to calculate J-R curves for 20MnMoNi55 and T91 materials. The J-R curves generated for both the materials are found to be in reasonable agreement with the experimental data reported in the literature. The methodology described in this paper has the potential to determine J-R data of aged structural materials using pre-cracked small punch test specimens.

Mechanical Response and Failure Evolution of 304L Stainless Steel under the Combined Action of Mechanical Loading and Laser Heating

Deformation and fracture properties of structural materials are greatly influenced by the factors like applied load, state of stress, and temperature. A precise prediction of the material properties of stainless steel at elevated temperature is necessary for determining the load-carrying capacity of structures under severe conditions. The present work reports the deformation and failure characteristics of 304L stainless steel subjected to combined laser heating and mechanical loading. The effect of main parameters on stress-strain, fracture characteristics, failure time, and temperature profile of specimens have been explored. Specimens were subjected to prescribed loading states, and then irradiated by a continuous wave fiber (1.08 µm) laser. The stress-strain curves indicated that the specimens experienced slight strain hardening in a specific temperature range prior to fracture. The specimen’s ultimate failure time is found to be reduced by increasing either laser power density or preload level. Fracture on a microscopic scale was predominantly ductile, comprising dimples as well as micro-void nucleation, growth, and coalescence. With the increase of laser power density, dimples rupture is the primary fracture mode, while with the increase of preload value, relatively more in-depth and severe deformation effects were observed. The description and characterization of 304L stainless steel failure under the simultaneous action of laser heating and tensile stress have been explored in detail.

A critical study of the parameters governing molecular dynamics simulations of nanostructured materials

Molecular dynamics (MD) simulations have been used extensively over the past two decades to determine the mechanical and physical properties of nanomaterials. However, the discrepancy between the reported results from these atomistic studies shadows the reliability of this computationally efficient technique. This inconsistency is attributed to the misuse and incorrect application of MD as evidenced by the arbitrary use of interatomic potentials, cut-off function parameters, strain rate, time increment, and domain size in the conducted simulations. In this paper, we highlight erroneous simulations by investigating the influence of these parameters on the elastic and fracture properties of nanostructured materials; including carbon nanotubes, graphene, and boron nitride (BN) sheets subject to direct and contact loads. The effect of interatomic potential type was investigated by comparing the predicted properties from AIREBO, Tersofff, CVFF, and ReaxFF potentials with those obtained with experimental and DFT techniques. The cut-off function parameters were also investigated to determine the optimum inner and outer cut-off radii selected to capture the actual physical behavior and avoid the reported strain hardening phenomena. Furthermore, MD simulations with strain rates spanning several orders of magnitudes and time increments ranging from 0.1 to 20 fs were performed to define the maximum allowable parameters for each material and loading scheme. Additionally, graphene and BN sheets with side length up to 500 Å were modeled to determine the size and edge effects on the mechanical properties. Finally, a set of parameters is recommended in each investigation to help guiding future atomistic studies obtaining reliable results using the available computational resources.

Hopkinson bar experiments as a method to determine impact properties of brittle and ductile materials

Split Hopkinson bar experiments are fundamental test arrangements to determine elastic, plastic, and fracture properties of materials under high strain rates. A specimen is considered to be in a well‐defined state of uniaxial stress, and depending on the specific device, limit loads or high strains can be achieved through purely elastic loading in the bars where there are no shock waves present. Variations of the conventional Hopkinson bar procedure allow determining of the dynamic fracture toughness of brittle materials. Here, the results of dynamic Brazilian tests on ultra‐high performance concrete are compared with the results of spallation experiments that simultaneously allow for the determination of the specific fracture energy. Similar experiments are performed for water ice. In addition, the strain rate dependence of the plastic hardening of a medium carbon steel is investigated. The yield stress is determined at strain rates up to 1300s−1.

Development of new ηpl, β and limit load equations to evaluate fracture parameters of pre-cracked small punch test specimens

The small punch test (SPT) is an alternative method for determination of the mechanical properties when enough material is not available for conventional testing. Recently SPT specimen configuration is changed to have a crack (known as pre-cracked SPT) for more accurate determination of material fracture properties. Use of p-SPT specimens needs limit and maximum load values and geometric parameters, β and ηpl for evaluation of Stress Intensity Factor (SIF) and plastic J-integral respectively. However, general equations for these parameters are not available in the open literature. The present paper is an attempt to address this aspect of p-SPT specimens. Finite element analysis has been employed to evaluate the above parameters. Square pre-cracked specimens with crack parallel and perpendicular to the thickness direction are considered in this paper. The range of crack size a/W = 0.40–0.60 is considered for the present analysis. The load–displacement curves are extracted for both specimens using 3-D elastic–plastic FE analysis assuming elastic perfectly plastic material behaviour. Maximum load is directly obtained from the FEA load–deflection curves. The maximum load helps to assess the machine capacity to perform the SPT. Using the elastic–plastic J-integral and load–displacement data from FEA, ηpl values are calculated. Limit analysis using Plastic Work Curvature (PWC) criterion is carried out to obtain the limit loads. The geometric factor (β) is found out using elastic FE analysis for SIF calculation. Based on these FEA results, new equations have been proposed for limit load, maximum load, geometry factor (β) and ηpl for both types of p-SPT specimen configurations.

Ductile Fracture Behaviour of Hot Isostatically Pressed Inconel 690 Superalloy

Herein we assess the differences in Charpy impact behavior between Hot Isostatically Pressed and forged Inconel 690 alloy over the temperature range of 300 °C to − 196 °C. The impact toughness of forged 690 exhibited a relatively small temperature dependence, with a maximum difference of ca. 40 J measured between 300 °C and − 196 °C, whereas the HIP’d alloy exhibited a difference of approximately double that of the forged alloy over the same temperature range. We have conducted Charpy impact testing, tensile testing, and metallographic analyses on the as-received materials as well as fractography of the failed Charpy specimens in order to understand the mechanisms that cause the observed differences in material fracture properties. The work supports a recent series of studies which assess differences in fundamental fracture behavior between Hot Isostatically Pressed and forged austenitic stainless steel materials of equivalent grades, and the results obtained in this study are compared to those of the previous stainless steel investigations to paint a more general picture of the comparisons between HIP vs forged material fracture behavior. Inconel 690 was selected in this study since previous studies were unable to completely omit the effects of strain-induced martensitic transformation at the tip of the Chary V-notch from the fracture mechanism; Inconel 690 is unable to undergo strain-induced martensitic transformation due to the alloy’s high nickel content, thereby providing a sister study with the omission of any martensitic transformation effects on ductile fracture behavior.

Influence of inertia and material properties on discrete simulation of dynamic fracture of concrete

It is well known that the material resistance to fracture increases with increasing strain rate. For relatively slow strain rates, the response is strongly dependent on the material fracture properties such as material strength or fracture energy, whereas with further increase of loading rate, the loading force is getting influenced predominantly by the material inertia. The response of the material changes with strain rate in the sense of loading force as well as character of the crack pattern. The contribution is focused on simulations of fracture experiments at various strain rates. Dynamical concrete fracture is simulated using meso-scale discrete model, i.e. a system of interconnected discrete particles. Material properties in the model are randomly distributed within the volume domain. Similar to quasi-static loading rates, the material randomness influences the dynamic fracture as well. Deterministic and probabilistic simulations are compared and the influence of material parameters and their strain rate dependence are discussed.

Nonlinear deformation and failure analysis of laminated composites

An approach to the characterization of different types of nonlinearity in the behavior of composite materials is proposed. One of them is concerned usually observed in experimental studies the dependence of deformation properties of composite materials on the type of external forces. Another type of nonlinearity lies in the fact that the shear stress-strain curves are nonlinear, though they are linear ones when the load is applied along the reinforcement. To describe these effects, the additional matrix is introduced into the proposed constitutive relations. There are different mechanisms of deformation of these materials, which are dependent on the type of reinforcement, matrix properties, loading conditions, directions of loads with respect to reinforcement and some others. These mechanisms and their interactions determine the stress-strain behavior of materials that influence the damage evolution and fracture properties of composite materials. To describe the damage accumulation process, the system of general failure model assumptions is formulated that include the choice of first ply failure criterion, constitutive relations for damaged materials with the use of corresponding damage parameters, the dependence of first ply failure criterion and elastic properties on the damage rate parameters and others. Some particular models are considered taking into account different types of nonlinearities, Lomakin et al. (1981, 2007, 2014, 2015 and 2017). This approach is verified using complex loading experiments. The theoretical dependencies obtained on the base of proposed models are compared with the results of experimental studies and good correspondence of them is shown. The proposed approach to the formulations of constitutive models from simple to more complex ones allows to describe adequately the behavior of composite materials under different loading conditions with the necessary precision.

Influence of some Test Parameters on Automated Ball Indentation Test Results

Automated Ball Indentation (ABI) technique has the potential to evaluate tensile and fracture properties of materials by testing a small volume of material. ABI testing involves various parameters such as surface finish of test specimen, maximum depth of indentation, number of loading-unloading cycles and material of indenter. As ABI test is yet to be standardized, it is necessary to assess and understand the consequences of the various test parameters. In this investigation, the influence of the above test parameters on the tensile properties of 316LN SS has been investigated at 298 K. It was found that by increasing the maximum depth of indentation, number of loading-unloading cycles and surface finish of test specimen at the cost of time and efforts, ABI results were not altered significantly. The selection of indenter material has to be done judiciously, according to test temperature.

Effect of thermomechanical loading on fracture properties of brittle materials: A fully-coupled transient thermoelastic analysis using a lattice approach

The influence of simultaneous thermomechanical loadings on the fracture behavior of linear thermoelastic brittle materials with a propagating crack and transient temperature diffusion is investigated. A planar two-way coupled thermoelastic lattice with a brittle erosion algorithm, implemented in MATLAB, is used in the context of LEFM to study two well-known classic fracture problems in Mode I, i.e., center and double-edge crack configurations, under thermal, mechanical, and thermomechanical loadings for a brittle crystalline Silicon, as an example material. The approach can also be applied to other brittle materials like brick or concrete. Validated by the well-developed analytical solutions for these configurations under separate thermal and mechanical loads, the numerical lattice analyzes the crack tip’s energy release rates and stress intensity factors based on the change of the global stiffness matrix of mesh before and after crack growth with no need for strain/stress calculations. Under the thermomechanical loading scenario, the response of the configurations in terms of the load-displacement and fracture toughness-crack length curves are then analyzed with respect to different loading rates and temperature gradients at crack surface and ambient environment. Having more pronounced effects on the toughness curves than the loading rate does, higher temperature difference results in lower values for the toughness curves. These curves also exhibit a rising jump for very short crack lengths and then approach to constant uniform values as a typical behavior for brittle materials. Considering the average trend of the toughness curves, it is observed that the thermomechanical fracture growth is highly unstable for very short cracks and it approaches a more stable condition as the crack length grows. Having a simple constitutive formulation and failure criterion, the proposed lattice approach is a convenient tool to obtain fracture properties of brittle materials under thermomechanical loading.

Experimental and Numerical Investigation of Rock Dynamic Fracture

Rapid development of engineering activities expands through a variety of rock engineering processes such as drilling, blasting, mining and mineral processing. These activities require rock dynamic fracture mechanics method to characterize the rock behavior. Dynamic fracture toughness is an important parameter for the analysis of engineering structures under dynamic loading. Several experimental methods are used for determination of dynamic fracture properties of materials. Among them, the Hopkinson pressure bar and the drop weight have been frequently used for rocks. On the other hand, numerical simulations are very useful in dynamic fracture studies. Among vast variety of numerical techniques, the powerful extended finite element method (XFEM) enriches the finite element approximation with appropriate functions extracted from the fracture mechanics solution around a crack-tip. The main advantage of XFEM is its capability in modeling different on a fixed mesh, which can be generated without considering the existence of discontinuities. In this paper, first, the design of a drop weight test setup is presented. Afterwards, the experimental tests on igneous (basalt) and calcareous (limestone) rocks with single-edge-cracked bend specimen are discussed. Then, each experimental test is modeled with the XFEM code. Finally, the obtained experimental and numerical results are compared. The results indicate that the experimentally predicted dynamic fracture toughness has less than 8 percent difference with calculated dynamic fracture toughness from extended finite element method

Understanding indentation-induced elastic modulus degradation of ductile metallic materials

Noticeable degradation of the measured elastic moduli of ductile metals has been found in meso-/macro- indentation tests. Although this phenomenon has been exploited to estimate the fracture properties of ductile metallic materials, the mechanism of the indentation-induced damage is rarely studied. Finite element simulations show that the finite stiffness of an indentation system can cause a false degradation of the effective indentation modulus. However, the decay pattern is different from that due to the indentation damage. Through theoretical analysis and experimental measurements, it is shown that the elasticity degradation can be mainly attributed to the anisotropic damage caused by the shear deformation in the indentation test.

Mode II fracture toughness of CFRP as a function of temperature and strain rate

Abstract Knowledge of the fracture properties of composite materials is fundamental to predict the impact behaviour of the lightweight structures currently used in the automotive industry. Although there is substantial research on mode I fracture behaviour of composites, limited information exists on mode II behaviour. This work aims to fulfil this gap, presenting experimental data regarding the influence of temperature and strain rate on the fracture energy in mode II, G IIC , of composite plates. Significant influence of the strain rate and temperature on G IIC of the composite materials was found. Higher strain rates led to a decrease of G IIC up to 17%. An increase of temperature corresponded to an increase of G IIC up to 15% and a decrease of temperature originated a decrease of G IIC up to 7%.

Implementation of nondestructive testing and mechanical performance approaches to assess low temperature fracture properties of asphalt binders

In the present work, three different asphalt binders were studied to assess their fracture behavior at low temperatures. Fracture properties of asphalt materials were obtained through conducting the compact tension [C(T)] and indirect tensile [ID(T)] strength tests. Mechanical fracture tests were followed by performing acoustic emissions test to determine the “embrittlement temperature” of binders which was used in evaluation of thermally induced microdamages in binders. Results showed that both nondestructive and mechanical testing approaches could successfully capture low-temperature cracking behavior of asphalt materials. It was also observed that using GTR as the binder modifier significantly improved thermal cracking resistance of PG64-22 binder. The overall trends of AE test results were consistent with those of mechanical tests.

Effect of structural dynamic characteristics on fatigue and damage tolerance of aerospace grade composite materials

In aircraft structural integrity analysis, the damage tolerance and fatigue life is investigated against a cyclic loading spectrum. The particular spectrum includes the stress/loading levels counted during a flight of certain duration. The occurrences of load factors may include higher gravitational acceleration ‘g’ levels. While maintaining a certain g level occurrence at higher angle of attack, wing structure vibrates with the amplitudes of its natural frequencies. The cyclic stress amplitudes of vibration depend upon the natural frequencies of vibrating structure, i.e. lower frequency gives higher amplitudes and vice versa.To improve the dynamic stability, modal parameters of simple carbon fibre sandwich panels have been adjusted by tailoring the fibre orientation angles and stacking sequence. In this way, the effect of change in structural dynamic characteristics on fatigue life of this simplified structure has been demonstrated. The research methodology followed in this work consists of two phases. In the first phase, aero-elastically tailored design was finalized using FEM based modal analysis and unsteady aerodynamic analysis simulations followed by experimental modal analysis. In the second phase, fatigue and damage tolerance behaviour of material was investigated using different fracture mechanics based techniques. ASTM's standard practices were adopted to determine material allowable and fracture properties. Simulation work was performed after proper calibration and correlation of finite element model with experimentally determined static and dynamic behaviour of panels.It has been observed that the applicable cyclic loading spectra, as major input parameter of fatigue analysis, largely depend upon the natural frequencies, damping and the stiffness of the structure. The results and discussions of the whole exercise may be beneficial while carrying out aero-elastic tailoring of composite aircraft wing. This research work has also a positive contribution towards multidisciplinary structural design optimization of aerospace vehicles.