Crack Propagation Phase Research Articles

Fatigue behaviour of an additively manufactured ductile gas turbine superalloy

Additive manufacturing (AM) offers new possibilities in gas turbine technology by allowing for more complex geometries. However, the fatigue performance, including crack initiation and crack propagation of AM gas turbine material, is not fully known. In addition, AM materials shows anisotropic properties due to the columnar grain growth in the building direction during the AM process, which needs to be accounted for. Also, an AM component often solidifies with a cellular dendritic structure during the manufacturing process. In the present study, the bulk material of an AM adopted nickel-based superalloy based on Hastelloy X was subjected to low-cycle fatigue (LCF) loading at room temperature. The LCF tests were conducted in strain control on additively manufactured smooth bars,with two different build orientations (with an angle of 0° and 90° relative to the building platform). The LCF results showed that the major part of the fatigue life is spent in the crack initiation phase, namely 78% to 99% of the total fatigue life. Based on the experiments, a model to predict the crack initiation life was developed that takes the anisotropic material behaviour into account. The last part of the fatigue life, the crack propagation phase, was studied on a microstructural level, where initial fractography of the ruptured LCF specimens revealed that the dendritic structure was visible on the fracture surface. It was noted that the dendritic structure could easily be mistaken for regular striations although they represent a different fracture mechanism. The fracture surfaces were therefore cross sectioned and possible correlations between fracture surface characteristics and underlying microstructure were studied using electron backscatter diffraction and electron channelling contrast imaging. The outcome showed that the dendritic structure had some effect on the LCF crack propagation behaviour by interdendritic tearing, which was discussed.

Aspect ratio effect of multi-walled carbon nanotubes and carbon fibers on high-performance cement mortar matrices

Formulation of high-performance cement mortar matrices reinforced with varying aspect ratios of carbon nanotubes and carbon fibers have been devised. Investigations on comparative effect of varying aspect ratios of carbon nanotubes and carbon fibers, with varying loads at two different water to cement ratios (w/c) of 0.45 and 0.50 in cement mortar matrices, were studied for the fracture response of the composites in crack initiation and propagation phases. The experiments concluded that an optimum addition of carbon nanotubes results in substantial improvements in terms of fracture properties. However, in case of carbon fibers, optimum increase in flexural strength was attained for carbon fibers having 12 mm length with 0.45 w/c, with the inclusion percentage of 0.15 by weight of cement indicating the increase in flexural strength up to 62.47%, followed by carbon fibers of 6 mm length at 0.45 w/c with the increase of 41.02%. Research also provides novelty for selection of optimum formulations among different aspect ratios. The field emission scanning electron microscopy evidenced a uniform level of tubes/fibers dispersion, the effective reinforcing effect by the tubes/fibers on the host matrix by providing the fracture mechanics phenomena of crack initiation, crack suppression, crack bridging and fiber–matrix debonding.

Study on flexural behavior of carbon fibers reinforced coral concrete using digital image correlation

In this paper, the flexural behavior, including the flexural damage process, ultimate flexural strength, peak deflection, and flexural toughness, of carbon fibers (CFs) reinforced coral concrete has been investigated by the four-point loading test. The strain fields on the surface of all the specimens were obtained by digital image correlation and used to analyze the flexural damage process. The results show that the flexural damage process can be divided into three phases: Microcrack initiation phase, Macrocrack evolution phase, and Main crack propagation phase. The incorporation of CFs can prolong the deformation process of coral concrete and therefore improve its flexural performance and gradually change its failure mode from brittle to plastic. The optimum dosage of CFs is about 1.5% by weight of the binder, and under the optimum dosage, the ultimate flexural strength, peak deflection, and flexural toughness can be increased by 20.8%~33.3%, 116.2%~142.1%, and 367%~586% for coral concrete with different strength grades, respectively.

Fatigue Properties of Maraging Steel after Laser Peening

Maraging steels are precipitation hardening steels used for highly loaded components in aeronautical and tooling industry. They are subjected to thermomechanical loads and wear, which significantly shorten their service life. Improvements of their surface mechanical properties to overcome such phenomena are of great interest. The purpose of our research was to investigate the influence of pulse density and spot size of a laser shock peening (LSP) process on the surface integrity with the fatigue resistance of X2NiCoMo18-9-5 maraging steel. Surface integrity was analyzed through roughness, residual stress, and microhardness measurements. The tests performed on resonant testing machine confirmed LSP is a promising process for increasing fatigue resistance of a component. Fatigue crack occurs, when the resonance frequency decreases. This moment, when the fatigue crack initiation phase ends and the fatigue crack propagation phase starts, was chosen as the moment of failure. We have proved LSP is a successful method in improving fatigue resistance of maraging steel by appropriate combination of laser spot size and pulse density tested in our research.

Crack evolution behavior of rocks under confining pressures and its propagation model before peak stress

The understanding of crack propagation characteristics and law of rocks during the loading process is of great significance for the exploitation and support of rock engineering. In this study, the crack propagation behavior of rocks in triaxial compression tests was investigated in detail. The main conclusions were as follows: 1) According to the evolution characteristics of crack axial strain, the differential stress-strain curve of rocks under triaxial compressive condition can be divided into three phases which are linear elastic phase, crack propagation phase, post peak phase, respectively; 2) The proposed models are applied to comparison with the test data of rocks under triaxial compressive condition and different temperatures. The theoretical data calculated by the models are in good agreement with the laboratory data, indicating that the proposed model can be applied to describing the crack propagation behavior and the nonlinear properties of rocks under triaxial compressive condition; 3) The inelastic compliance and crack initiation strain in the proposed model have a decrease trend with the increase of confining pressure and temperature. Peak crack axial strain increases nonlinearly with the inelastic compliance and the increase rate increases gradually. Crack initiation strain has a linear relation with peak crack axial strain.

Crack Initiation and Propagation Fatigue Life of Ultra High-Strength Steel Butt Joints

The division of the total fatigue life into different stages such as crack initiation and propagation is an important issue in regard to an improved fatigue assessment especially for high-strength welded joints. The transition between these stages is fluent, whereas the threshold between the two phases is referred to as technical crack initiation. This work presents a procedure to track crack initiation and propagation during fatigue tests of ultra high-strength steel welded joints. The method utilizes digital image correlation to calculate a distortion field of the specimens’ surface enabling the identification and measurement of cracks along the weld toe arising during the fatigue test. Hence, technical crack initiation of each specimen can be derived. An evaluation for ten ultra high-strength steel butt joints reveals, that for this superior strength steel grade more than 50% of fatigue life is spent up to a crack depth of 0.5 mm, which can be defined as initial crack. Furthermore, a notch-stress based fatigue assessment of these specimens considering the actual weld topography and crack initiation and propagation phase is performed. The results point out that two phase models considering both phases enable an increased accuracy of service life assessment.

Experimental analysis of the tensile behaviour of textile reinforced cementitious matrix composites using distributed fibre optic sensing (DFOS) technology

This work presents an experimental study of the mechanical behaviour of textile reinforced cementitious matrix composites (TRCMC) subjected to direct tensile loading. Distributed optical fibres embedded in the core of the composites were used as strain sensors. The measurements were based on the Rayleigh backscattering principle. The sensors were able to measure the strain in the specimen during the test at different locations because of their spatial resolution and reduced dimensions and thus make it possible to analyse and identify the behaviour of the matrix, the textile, and the textile/matrix interface. Six configurations were tested and analysed in this study: two types of matrices with three reinforcement ratios.The local and global behaviours were analysed, and the test results during the three typical stages of the mechanical behaviour of TRCMC composites are explained. The parameter controlling the behaviour of the crack propagation phase is identified, and an explanation of its influence on crack distribution is highlighted. The validity of the law of mixtures and the hypotheses of the textile/matrix interface were examined and compared to experimental results. The influence of the reinforcement ratio was also identified. The obtained results allow a better understanding of the local and global mechanical behaviours of TRCMC composites and the assessment of present theoretical hypotheses in literature related to these behaviours.

A Transition Size of Dividing Crack Initiation and Propagation Phases and the Fatigue Total Life Prediction Approach

How to quantitatively analyze the transition size and establish a total life model has always been a research topic. Crack transition size plays the role of the divider of crack initiation and propagation phases, and the total fatigue life is in the treatment of the summation of fatigue initiation and propagation periods. In this article, a unified model of combination of the corrected local stress (strain) method with the concept of fracture mechanics is applied to estimate the total fatigue life of a notched member and to validate the predictions proposed in this paper, which is compared with the experimental tests, and the results showed good consistency for the notched structure of a certain aluminum alloy.

Improvement of fracture toughness of ultra fine grained Al–Li 8090 alloy processed through multi axial forging

In the present work, mechanical properties, microstructural characteristics and fracture behavior of severely deformed Al–Li 8090 alloy processed through multi axial cryo forging has been investigated and compared with the room temperature forged conditions. Enhancement in the mechanical properties, LEFM fracture toughness and EPFM toughness parameter (J-integral) of severely deformed Al–Li 8090 alloy with a cumulative true strain of 1.2, 2.4 and 3.6 is observed at Cryo forged (CF) condition as compared to room temperature forged (RTF) conditions due to the accumulation of higher dislocation density, dislocation substructure and formation of UFG microstructure. Effective retardation of dynamic recovery and climb at liquid nitrogen temperature (LNT, −196 °C) is responsible for higher dislocation accumulation in CF sample compared to the samples subjected to room temperature forging. The improved fracture toughness of CF samples is attributed to the delay in crack initiation as well as crack propagation phase resulting from Hall-Petch strengthening, grain boundary strengthening, and dislocation strengthening.

Production of Realistic Flaws and Their Meaning for Ultrasonic Testing

This work is designed to artificially create test specimens with flaws that behave the same way as real-function flaws when observed by nondestructive testing (NDT) technologies. Thus, the understanding of the detection limitations of NDT methods is needed. In this study, real, realistic, and artificial flaws were compared by ultrasonic phased array technology. Fatigue flaws, which belong to the most common structural issues (Ruzicka, M., Hanke, M., and Rost, M., 1987, Dynamicka Pevnost a Zivotnost, CVUT, Prague, Czech Republic, p. 75), are investigated. Measurements have revealed significant differences in the amplitude of ultrasonic echo from fatigue cracks in distinct phases of crack propagation. Studied specimens with realistic flaws have demonstrated their quality for calibration, staff training, and NDT system qualification. More realistic test specimens will increase ultrasonic test result reliability.

Fatigue life estimation of cast aluminium alloys considering the effect of porosity on initiation and propagation phases

Porosity is one of the main parameters in the fatigue life of cast alloys as it affects both crack initiation and propagation phases. However, most fatigue assessments present a lack of accuracy as they only consider the effect of the initial pore on the fatigue limit. Therefore, in the present study, first, the effect of the main porosity characteristics on the crack initiation and propagation phases is evaluated. Then, an alternative fatigue assessment model is proposed and implemented for Low Pressure Die Casting (LPDC) A356-T6 aluminium alloy. The model accounts for the effect of the initial pore size and distance to surface on the initiation phase as well as fractographic porosity and average pore size on the fracture surface on the propagation phase. Finally, the model was validated by means of experimental tests. Results have shown that the proposed model is able to qualitatively predict the scatter trend of samples tested at the same load. In addition, the model presents high quantitative accuracy in life estimation with an average logarithmic error below 2% and an absolute average error below 16%.

Influence of ply-angle on fracture in antisymmetric interfaces of CFRP laminates

Laminated composites are prone to fracture at layer interfaces. Such damage impairs their structural response and engenders important constraints in design. In this work, the influence of ply orientation on crack growth resistance was studied for three different antisymmetric interfaces and compared to a unidirectional reference one, using double cantilever beam specimens with equivalent stiffness, loaded under mode I conditions. Fracture toughness at initiation was found interface-independent. In all angle-ply specimens, distinct slow and fast phases of crack propagation were observed. Crack increments due to fast growth, were characterized using experimental energy release rates and verified from fracture surface analysis. The slow propagation phases were accompanied by large scale bridging involving intra-ply growth in the adjacent plies, with toughness increasing inversely with the angle. Mechanistic investigations suggest a consistent fracture pattern in terms of the interface angle. For each interface angle, a single traction-separation relation, obtained from the experimental energy release rate and crack opening displacements, was sufficient to model two consecutive slow propagation phases. These relations were used in 2D cohesive element models to predict very well the loading history.

Influence of fillet end geometry on fatigue behaviour of welded joints

This paper presents a fatigue analysis of a type of fillet welded joint representative of one main joint of the steel box girder of the Alcácer do Sal railway bridge. From previous studies, it was found that the welded joint between the box girder diagonal and the central hanger gusset is one of the most stressed details of the bridge. This welded joint was not fully manufactured according to current construction procedures, as regards the fillet weld end configuration. In order to assess the fatigue behaviour of such welded joint, the present study combines an experimental campaign and numerical analysis. A total of four welded joint series were produced in order to allow the comparison of the fatigue performance of similar type of welded joint of the Alcácer do Sal bridge with welded joints produced according to existing recommendations, such as EC3. Since scale-down specimens were considered, two different thicknesses were included in this study for each joint configuration, to allow the verification of any thickness effect. Concerning the numerical analyses, two main numerical tools were used: the standard Finite Element Method (FEM) with ANSYS and the eXtended Finite Element Method (XFEM) with ABAQUS. Fatigue life predictions were performed including both fatigue crack initiation and fatigue crack propagation phases. The number of cycles to initiate a fatigue crack was computed using local notch strain-life approaches, and the number of cycles for fatigue crack propagation was computed by integrating the Paris fatigue crack growth law with stress intensity factors computed with ANSYS (virtual crack closure technique) and ABAQUS (contour integral method, 3D XFEM model). Experimental tests demonstrated little influence of fillet weld end geometry on fatigue behaviour of welded joints and plate thickness effects were also reduced as also confirmed by the similar fatigue crack propagation rates. Both numerical simulations provided very accurate predictions of the experimental S-N curves, however the XFEM modelling opens new possibilities for mix-mode fatigue crack propagation simulations.

Fatigue crack initiation and subsequent crack growth in fillet welded steel joints

The fatigue damage evolution in fillet welded steel joints where cracks are emanating from the weld toe is investigated. Based on existing experimental data for as-welded joints including crack depth measurements of the early crack growth it is proposed to make a distinction between the crack initiation phase and the subsequent crack growth phase. The welded detail in question is an F class detail with plate thickness 25 mm made of medium strength carbon steel. It is found that the crack initiation phase defined at a crack depth of 0.1 mm is close to 25% of the fatigue life even at a relatively high constant stress range of 150 MPa. At lower stress ranges it is concluded that the initiation phase is the dominating part of the fatigue life. The present work is focusing on the crack propagation phase that is defined from a crack depth of 0.1 mm to final failure of the detail. It is demonstrated that the recommendation given in rules and recommendations (DNVGL and BS 7910) for applying Linear Elastic Fracture Mechanics (LEFM) for the crack propagation phase is valid for the propagation of such small surface breaking cracks. A model based on the rule-based formulas for the Stress Intensity Factor Range (SIFR) and the growth parameters C and m in Paris law agree well the with the measured crack growth curves. For these small semi-elliptical cracks at the weld toe notch important topics like the existence of a threshold limit for the SIFR and the influence of the stress ratio R are discussed. Due to the inherent scatter in the variables characterizing the fatigue damage evolution stochastic approaches are applied for the analyses. Observations and measurements are presented by descriptive statistics and simulations are carried out using Monte Carlo techniques.

Study on small fatigue crack initiation and growth for friction stir welded joints

Small fatigue crack initiation and growth behavior for 7075 aluminum alloy friction stir welded joints are investigated with the replica technique. The effects of microstructure on the initiation and propagation of small cracks are studied under different loading stress levels. The test results show that the initiation and propagation of small cracks are mainly affected by strengthening particles, grain boundaries, rough pits and hardness distribution. Small cracks begin to grow when the cycle numbers are about less than 20% of the fatigue life at the stress ratio of 0.1. With the decrease of stress ratio to − 0.3, the small cracks grow until the cycle numbers are about 30–45% of the fatigue life. The phase of small crack initiation and propagation accounts for about 50–80% of the fatigue life. The growth rate of the major crack is similar to that of other small cracks in the stage of small cracks growth. In the stage of long cracks growth, the growth rate of the major crack is far greater than that of other small cracks. The microstructures of different regions of the joint have an effect on the fracture mode.

A two-phase approach to estimate fatigue crack initiation and propagation lives of notched structural components

Total fatigue lives of notched structural components are considered as a summation of crack initiation and propagation phases and a two-phase approach to fatigue life prediction is presented. The local strain approach and fracture mechanics-based approach are used to predict the lives spent in the two phases. A strain-life curve corresponding to the transition crack size where the two phases are divided is formulated based on the equivalence between the Coffin-Manson law and a simple Elastic-Plastic Fracture Mechanics crack growth law in the low cycle fatigue regime. A crack growth model that includes the short crack behaviour is used to predict the crack propagation life. The transition crack size, related to the average grain size of the material, is chosen in such a way that the proposed approach can be appropriately adopted in both phases, considering the stress gradient and microstructure effects. The proposed approach is applied to predict fatigue lives of various notched specimens. The three-dimensional effect is considered in the assessment. The predictions are in good agreement with the experimental results.

Notched Ti-6Al-4V titanium bars under multiaxial fatigue: Synthesis of crack initiation life based on the averaged strain energy density

The fatigue life of notched components subjected to multiaxial loading conditions may be influenced by extrinsic mechanisms operating during crack propagation phase, such as sliding contact, friction and meshing between crack surfaces. These extrinsic mechanisms may disrupt the applicability of local approaches, such as that based on the strain energy density (SED) averaged over a structural volume having size R0 and surrounding the crack initiation point. In the present contribution, the multiaxial fatigue behaviour of circumferentially notched specimens made of titanium grade 5 alloy, Ti-6Al-4V, has been analysed. Pure bending, pure torsion and in-phase as well as out-of-phase combined bending-torsion fatigue tests have been carried out on notched specimens characterized by two different root radii, namely 0.1 and 4 mm. Fatigue crack initiation and subsequent propagation have been monitored by adopting the direct current potential drop (DCPD) technique. In principle, crack initiation life has been defined when the initiated fatigue crack fractures the structural volume, which corresponds to a given potential drop increase calibrated by means of electrical finite element (FE) analyses. The structural volume size R0 has been determined by fatigue testing plain and sharp V-notched specimens under pure axial or pure torsion loadings. Finally, the averaged SED approach has been adopted to correlate the experimental fatigue results expressed in terms of crack initiation life, in order to reduce the effect of extrinsic mechanisms.

Analysis of a main fatigue crack interaction with multiple micro-cracks/voids in a compact tension specimen repaired by stop-hole technique

Fatigue is a process in engineering materials in which damage accumulates due to the fluctuating loading. One solution for a component under the fatigue process is to arrest the crack propagation before the final failure using different available retardation methods, such as drilling/stop-hole technique. In addition, structural components may also suffer from the existence of micro-cracks or voids due to their forming process or service lives. These micro-cracks/voids are very critical to study, since they can effectively play an important role in the behavior of the existing main crack in a component. This article aims to investigate the effect of the stop-hole retardation technique and multiple micro-cracks/voids with different characteristic lengths and geometries on the fatigue crack propagation in a compact tension specimen. A modified Forman equation, the so-called NASGRO equation is used to define the transition between crack initiation and crack growth period. Also, the extended finite element method is adapted in the crack propagation phase in order to model crack path in the geometry eliminating the need for remeshing procedure. The whole analyses are conducted in a commercial package through a user-written code that handles all fatigue crack growth analysis. The reference solutions from the literature are used to compare and to validate results obtained from current work.

Development of an efficient approach for fatigue crack initiation and propagation analysis of bridge critical details using the modal superposition technique

The fatigue damage assessment of large bridges is highly conditioned by the required computational high demands. Generally, in order to overcome the multi-scale problem, global and local models are needed to properly account for both global structural behaviour and the local nature of the fatigue damage. The analysis of such structural problems using direct time-integration algorithms is impracticable in most of the cases, which leads to the necessity of developing alternative methodologies in order to increase the computational efficiency and the accuracy of fatigue cracking assessments. In this respect, effective computational algorithms based on the modal superposition technique have been proposed and implemented in previous works. Overall, such workflow considers the interaction between the global and local models combined with the application of the modal stress intensity factor concept. Aiming at performing an efficient and accurate assessment of the fatigue damage, firstly, combining the Fracture Mechanics principles and crack propagation laws, the crack propagation phase in a complex bridge detail is analysed. In this regard, the present paper aims at proposing relevant improvements to the above-mentioned methodology, namely: i) the refinement of the implemented submodelling techniques in order to increase the accuracy of stress and strain fields computation and allow to account for smaller initial crack lengths; ii) the analysis and limitation of the considered number of vibration modes to the relevant ones for the local dynamic response; and iii) the implementation of a parallel computing approach for the calculation of the modal stress intensity factors related to the vibration modes defined in ii). The fatigue assessment procedures were applied to an assumed cracked welded detail of a recent railway composite bowstring bridge located in Portugal. Also, since the assumption of a pre-existing crack may lead to very conservative predictions, the modal superposition technique is further extended to evaluate the fatigue crack initiation phase, demonstrating the safety of the analysed case study in the absence of existing defects.

Review of modelling crack initiation and propagation in flexible pavements using the finite element method

Cracking is a major failure mechanism that directly affects the functionality and performance of flexible pavements. The objective of this study is to present the state of knowledge on the use of finite element method (FEM) including cohesive zone model (CZM) to simulate cracking failure mechanisms in flexible pavements under field conditions. Based on this review, a knowledge base is established on how FEM can be effectively applied to predict pavement performance against cracking failure mechanisms. Two main approaches are presented in this study. In the first approach, focused finite element (FE) meshes are used to simulate the crack initiation and propagation phases. Focused meshes allow for estimating the J-integral and the stress intensity factor (SIF) through different contour lines. Since the SIF is a measure of the stress and strain environment around the crack tip (a greater SIF indicates a faster rate of propagation), the rate of crack propagation per cycle can be estimated based on empirical equations such as Paris’ Law. In the second approach, the propagation of damage is simulated using FE and CZM. This approach allows one to study the loading mechanisms responsible for progressing cracking damage in the pavement structure. In summary, both approaches have merits in modelling cracking in flexible pavements. While focused meshes may be used to predict the number of cycles for crack propagation, CZM may be used to study the controlling stresses and mechanisms responsible for crack propagation.