Event-based lattice modeling of strain-hardening cementitious composites

Temperature gradient type ESSO test is one of the most popular test methods for evaluating the brittle crack arrest toughness, \(K_{ca}\). However, test conditions which are specimen shape, tab plate shape, applied stress and temperature gradient affect \(K_{ca}\). This document reports effects of specimen geometries that are specimen width, tab plate length, tab plate thickness and tab plate width on \(K_{ca}\) evaluation. In addition, effects of applied stress and temperature gradient have also been investigated. Temperature gradient type ESSO tests are conducted at three different steel mills in Japan. Then, test conditions were varied and test results were compared. In the result, influence range of effect specimen width, tab plate thickness, applied stress and temperature gradient were demonstrated. The applicable range of specimen geometry, applied stress and temperature gradient were clarified and implemented to the brittle crack arrest standard.     

Novel water permeability device for reinforced concrete under load

The presence of cracks in reinforced concrete structures is recognized to increase the penetration of water and aggressive agents into concrete and thus accelerate its deterioration. In order to gain knowledge on the influence of cracking on concrete durability and assess admissible loads to ensure long-term performance of structures, an innovative water permeability device was developed to estimate water flow in plain and cracked reinforced concrete. Permeability measurements were taken simultaneously with the application of a uniaxial tensile load on the testing specimen. The device permitted the estimation of the average stress in the reinforcing bar and the maximum crack opening in the concrete specimen. The experimental program comprised studies on result repeatability and the influence of testing parameters, such as pressure gradient, pressure regulation, loading rate and loading control mode. Test results showed that the modification of the testing parameters had a negligible impact on water permeability. Moreover, correlations were established between the water permeability, the average stress in the steel reinforcement, and the crack opening width in the reinforced concrete. Analysis of the results demonstrated the potential of the research results to improve the design criteria of reinforced concrete at serviceability limit states.     

Evaluation of the crack-growth resistance of ceramics in the stage of subcritical crack growth

We propose a procedure for the evaluation of the parameters of crack growth in ceramic materials based on the application of soft and rigid loading to a standard beam specimen with lateral notch. The kinetic diagrams of fracture of aluminooxide ceramics obtained as a result enable one to predict the durability of products made of this material under specific service conditions.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 6, pp. 81–85, November – December, 1995.     

Prediction of the Durability of Cyclically Loaded Structural Elements

A procedure of determination of the durability of cyclically loaded notched specimens is proposed and experimentally verified. The procedure is based on the concepts of the unified model of fatigue fracture treating the processes of initiation and propagation of a fatigue macrocrack from the common point of view. For specimens with structural stress concentrators of two types, we compute the periods of initiation and growth of a fatigue macrocrack and the number of loading cycles to failure on the basis of the diagrams of fatigue crack growth rates. The numerical results agree with the experimental data with an error of at most 38% depending on the method of calculations and durability. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 4, pp. 39–44, July–August, 2005.     

The effect of loaded volume and stress gradient on the fatigue limit

In this paper an investigation of multiaxial stress based criteria and evaluation methods is presented. The criteria are used with the point, gradient and volume methods. The purpose is to determine the combination of criteria and methods that is best suited for design against the fatigue limit. The evaluation is based on elastic FE-analysis of 15 geometries for which the fatigue limit loads are known. The point method is based on the maximum values of the fatigue stress in each specimen. With the gradient method, the fatigue stress is adjusted with the relative or absolute gradient of the fatigue stress itself. With the volume method, a statistical size effect is considered, by use of a weakest link integral. Thus, the probability of fatigue depends on the fatigue stress distribution. Also, the gradient and volume methods are combined. The results show that the point and gradient methods are not good for prediction of the fatigue limit. It is recommended to use the volume method in fatigue design. It is accurate enough for prediction of the fatigue limit, straightforward to use and easy to interpret. The choice of method is much more important than the choice of criteria.     

Durability‐based design for endurance strength of hydroforming tools

Hydroforming tools must be designed fatigue resistant because in case of tool rupture high costs for repair and production downtime occur. Available guidelines for lifetime estimation of cyclically loaded parts usually are of general nature. In order to examine their applicability to the special concerns of hydroforming tools, stress states occuring during operation were investigated by means of numerical simulation. Basing on the results, specimen geometries were developed, representing a typical stress state. The estimated durability was calculated using the “FKM‐guideline”. The real durability was determined experimentally in form of Wöhler‐graphs. Eventually calculation and experiments were compared. Hence, the qualtity of the lifetime prediction was evaluated and correction factors were proposed.     

Direct determination of cohesive stress transfer during debonding of FRP from concrete

Interface cohesive stress transfer between FRP and concrete during debonding is typically obtained using measured surface strains on the FRP, along the direction of the fibers. The cohesive material law is derived under a set of assumptions which include: (a) the bending stiffness of the FRP laminate is insignificant with respect to that of the concrete test block; (b) the strains in the bulk concrete produced by debonding are negligible, thus concrete substrate can be considered rigid; (c) there is stress transfer between FRP and concrete through the FRP–concrete interface which is of zero thickness; and (d) the axial strain in the FRP composite is uniform across its thickness. In this paper, a test procedure for directly obtaining the through-thickness strains in the FRP and the concrete substrate during cohesive stress transfer associated with debonding is presented. The displacement and strain fields are measured on the side of a direct-shear specimen with the FRP strip attached on the edge. Based on the experimental results, the influence of the assumptions which have been introduced to determine the cohesive law is discussed. Within the stress transfer zone there is a sharp gradient in the shear strain. The location of the interface crack within the stress transfer zone and the cohesive stress transfer during the propagation of the interface crack are determined.     

Correlation between the accuracy of a SHPB test and the stress uniformity based on numerical experiments

Split Hopkinson pressure bar (SHPB) has become a frequently used technique to measure the uniaxial compressive stress–strain relation of various engineering materials at high strain rate. Using the strain records on incident and transmitter bars, the average stress, strain and strain rate histories within the specimen can be calculated by SHPB formulae based on one-dimensional wave propagation theory. The accuracy of a SHPB test is based on the assumption of stress and strain uniformity within the specimen, which, however, is not always satisfied in an actual SHPB test due to the existence of some unavoidable negative factors, e.g., friction and specimen size effects. Two coefficients are introduced in the present paper to measure the stress uniformity in axial and radial directions of the specimen in a numerical SHPB test. It is shown that the accuracy of a SHPB test can be correlated to these two stress uniformity coefficients. An assessment and correction procedure for SHPB test results is illustrated through a numerical example.     

Experimental investigation on low cycle fatigue and fracture behaviour of a notched Ni‐based superalloy at elevated temperature

In order to investigate the effects of stress concentration on low cycle fatigue properties and fracture behaviour of a nickel‐based powder metallurgy superalloy, FGH97, at elevated temperature, the low cycle fatigue tests have been conducted with semi‐circular and semi‐elliptical single‐edge notched plate specimens at 550 and 700 °C. The results show that the fatigue life of the notched specimen decreases with the increase of stress concentration factor and the fatigue crack initiation life evidently decreases because of the defect located in the stress concentration zone. Moreover, the plastic deformation induced by notch stress concentration affects the initial crack occurrence zone. The angle α of the crack occurrence zone is within ±10° of notch bisector for semi‐circular notched specimens and ±20° for semi‐elliptical notched specimens. The crack propagation rate decreases to a minimum at a certain length, D, and then increases with the growth of the crack. The crack propagation rate of the semi‐elliptical notched specimen decelerates at a faster rate than that of the semi‐circular notched specimen because of the increase of the notch plasticity gradient. The crack length, D, is affected by both the applied load and the notch plasticity gradient. In addition, the fracture mechanism is shown to transition from transgranular to intergranular as temperature increases from 550 to 700 °C, which would accelerate crack propagation and reduce the fatigue life.     

Applying modified Weibull failure theory to bimaterial specimen under thermal loading

Failure of a thermally loaded bimaterial specimen was investigated in this research. A high-stress gradient due to thermal loading occurs around the free edge of the interface of the bimaterial specimen. This high-stress gradient plays an important role in the failure of the specimen. The Weibull failure theory has been shown to be unable to account for high-stress gradients in externally loaded bimaterial specimens. The objective of this work is to develop a weight function method to calculate the effective stress intensity factors in the vicinity of a high-stress gradient in a thermally loaded bimaterial specimen, and to develop a modified Weibull failure theory to handle the high-stress gradient. It was found that the modified Weibull failure theory generated monotonous trends for the probability of failure with respect to increasing Weibull moduli, as demonstrated in the literature.     

3D full field study of drying shrinkage of foam concrete

Drying shrinkage (DS) of concrete is important. The graded and heterogeneous DS inside the concrete may lead to cracking and further deteriorate the mechanical and durability properties. To elaborate the drying gradient and deformation heterogeneity, the full field DS distributions of foam concrete have been studied using an expanded Digital Volume Correlation method, which has a high precision of 0.01 voxel (about 0.6 μm) in displacement. The effectiveness of DS in local sub-volume is verified from bulk shrinkage of the whole specimen. The DS gradient due to drying is clearly revealed, and DS heterogeneity in spatial domain and in frequency domain is identified. A full view of foam concrete's drying processes is built. At the middle drying stage, three different states exist simultaneously, especially a drying front arises with high drying shrinkage.     

An analytical relation between the Weibull and Basquin laws for smooth and notched specimens and application to constant amplitude fatigue

Starting from the classical definition of stress‐life Wöhler curve in the form of the Basquin law, an analytical procedure for the calibration of the four parameters' Wöhler curve (the Weibull law) for a plain specimen is proposed. The obtained parameters are then adjusted by means of an additional slope factor preserving the inflection point of the curve while changing its slope in order to model the experimental observations in which an increase of the scatter in life prediction is observed when reducing the stress amplitude. The same approach has then been adopted to calibrate the Weibull law parameters for a notched specimen, and the fitting slope factor has been found to be a value that changes with the material but remains constant with the stress concentration factor. The findings have been validated with existing experimental data on 2024‐T3 aluminum alloy and normalized SAE 4130 steel.     

Fatigue tolerant design of steel components based on the size of large inclusions

Fatigue failures in high strength steel components often originate from large, brittle inclusions. The durability of the components is strongly dependent on the size of the inclusions and the magnitude of the local stresses caused by the applied loads. A successful design must consider both the size and the number of large inclusions as well as the stress distribution arising from the geometry and loading of the component. This paper presents a new approach to the safe fatigue design of steel components based on the size distribution of large inclusions in a component with a given stress distribution. The procedure is illustrated using the example of the stress distribution around a hole in a plate, with the size of large inclusions in a large volume of steel estimated by the Generalized Pareto Distribution (GPD) method. It is found that the single largest inclusion is unlikely to lie in a highly stressed volume, but that the more frequently occurring slightly smaller inclusions contribute more to the probability of a fatigue failure. Knowledge of the shape of the size distribution over a range of large sizes, not solely that of the largest size, is therefore essential. The new approach offers a quantitative measure of the improvement in durability to be expected from reduction of the design stress range of a component and from improvements in steel cleanness.     

On specimen design for size effect evaluation in ultrasonic gigacycle fatigue testing

Literature datasets showed that gigacycle fatigue properties of materials may be affected by the specimen risk‐volume, i.e., the part of the specimen subjected to applied stress amplitudes above a prescribed percentage of the maximum applied stress amplitude. The paper proposes a Gaussian specimen shape able to attain large risk‐volumes for gigacycle fatigue tests, together with a general procedure for its design: wave propagation equations are analytically solved in order to obtain a specimen shape characterised by a uniform stress distribution on an extended length and, as a consequence, by a larger risk‐volume. The uniformity of the stress distribution in the Gaussian specimen is numerically verified through a finite element analysis and experimentally validated by means of strain gauge measurements.     

Comparative estimation of mechanical properties of cemented carbides by the kinetics of penetration of a rigid punch under contact cyclic loading

We suggest a method of contact fatigue testing of cemented carbides. It involves cyclic loading of a specimen (half-space) with a flat circular punch by a pulsating compressive stress. Both the loading punch and the test specimens are made of cemented carbides. It is found that the parameters of contact fatigue of a cemented carbide include the depth of the punch penetration into the specimen, σ, after a given number of cycles, the form of the dependence of the penetration depth on the number of cycles, the number of loading cycles between abrupt changes in the growth of the σ values, and the number of these changes. With these parameters determined by laboratory tests on small specimens, we can predict the durability of large carbide components subjected in operation to contact cyclic loads. V. N. Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 28–37, May–June, 1998.     

Analysis of the crack growth propagation process under mixed-mode loading

In the present paper, a computational model for crack growth analysis under Mode I/II conditions is formulated. The focus is on two issues – crack path simulation and fatigue life estimation. The finite element method is used together with the maximum principal stress criterion and the crack growth rate equation based on the equivalent stress intensity factor. To determine the mixed-mode stress intensity factors, quarter-point (Q-P) singular finite elements are employed. For verification purposes, a plate with crack emanating from the edge of a hole is examined. The crack path of the plate made of 2024 T3 Al Alloy is investigated experimentally and simulated by using the finite element method with the maximum tangential stress criterion. Then, the validation of the procedure is illustrated by applying the numerical evaluation of the curvilinear crack propagation in the polymethyl methacrylate (PMMA) beam and the Arcan specimen made of Al Alloy for which experimental results are available in the literature. In order to estimate fatigue life up to failure of the plate with crack emanating from the edge of a hole, the polynomial expression is evaluated for the equivalent stress intensity factor using values of stress intensity factors obtained from the finite element analysis. Additionally, the fatigue life up to failure of the Arcan specimen is analyzed for different loading angles and compared with experimental data. Excellent correlations between the computed and experimental results are obtained.     

Enhanced stress durability of nano resonators with scandium doped electrodes

To explore mechanical stress durability of thin aluminum–scandium (AlSc) films, 0.86 GHz nano resonators with AlSc electrodes have been manufactured. Four different samples have been prepared altering the Sc content in the alloy between 0.0% and 2.5%. A final lift-off step accomplished manufacture procedure of the devices. The resonators have been operated with heavy load to determine power durability. The resonators with AlSc electrodes show increased power durability compared to conventional Al metallized devices. Texture and grain structure of all films have been investigated by means of electron backscatter diffraction (EBSD) and atomic force microscopy (AFM). Material fatigue of electrodes has been visualized by scanning electron microscopy (SEM). The refined grain structure of these alloys can explain the enhanced mechanical stress durability of AlSc electrodes.     

Optimal design of composite ChamberCore structures

An optimization procedure has been developed to uniquely and efficiently determine the “best” local geometry design of a new composite ChamberCore structure. This procedure is based on minimization of the total mass of a single composite ChamberCore subject to a set of design and stress constraints. The stress constraints are obtained in closed form based on the composite box-beam model for various composite lamination designs and loading conditions. The optimization problem statement is constructed and then solved using the VMCON optimization program, which is an iterative sequential quadratic programming (SQP) technique based on Powell's algorithm. The sensitivity of the solution of the optimal geometry to the values of parameters that characterize the structural durability and the failure mechanism is discussed.     

Optimum Design of a Ceramic Tensile Creep Specimen Using a Finite Element Method

An optimization procedure for designing a ceramic tensile creep specimen to minimize stress concentration is carried out using a finite element method. The effect of pin loading and the specimen geometry are considered in the stress distribution calculations. A growing contact zone between the pin and the specimen has been incorporated into the problem solution scheme as the load is increased to its full value. The optimization procedures are performed for the specimen, and all design variables including pinhole location and pinhole diameter, head width, neck radius, and gauge length are determined based on a set of constraints imposed on the problem. In addition, for the purpose of assessing the possibility of delayed failure outside the gage section, power-law creep in the tensile specimen is considered in the analysis. Using a particular grade of advanced ceramics as an example, it is found that if the specimen is not designed properly, significant creep deformation and stress redistribution may occur in the head of the specimen resulting in undesirable (delayed) head failure of the specimen during the creep test.     

A three-point bend specimen with partly tapered cross-section sides

The stress intensity factor and the J-integral have been derived analytically and numerically for a modified three-point bend specimen with partly tapered sides, for various crack lengths, taper and specimen cross-section proportions, in order to allow full-thickness testing of tapered samples, common in older steel structures, to obtain a fair effective fracture toughness value for a through thickness crack in inhomogeneous materials. The stress intensity factor is obtained with the approximate analytical method of Kienzler and Herrmann, based on the concept of material forces. The J-integral is calculated numerically with a 3D finite element model for a linear elastic material and an elastic ideal-plastic material. A simple single specimen fracture toughness evaluation procedure is proposed. It is found that the effect of taper in the range encountered in practice is small, of the order of a few percent.