Initial Stage Of Crack Propagation Research Articles

Investigation of fatigue crack growth behavior and crack tip plastic zone characteristics in welded structures based on local displacement fields

The fatigue crack growth behavior in welds is difficult to evaluate due to complex internal stresses and microstructural differences. This paper uses the CJP model based on displacement field variations to describe the fatigue crack growth behavior and the size of the crack tip plastic zone in welds. First, fatigue crack growth rate tests were conducted on base metal and as-welded specimens. The digital image correlation technique was used to capture the speckle pattern variations on the specimen surface during crack growth, obtaining the crack tip displacement field required by the CJP model. Based on this, by calculating the stress intensity factor ranges ΔK and ΔKCJP, two da/dN–ΔK(ΔKCJP) curves defined by the traditional and CJP models were obtained, proving the model’s applicability for describing fatigue crack growth behavior in welded joints. Finally, the CJP model was used to calculate and compare the crack tip plastic zones of base metal and as-welded specimens. It was found that the maximum error for the as-welded specimen was 37.84%, occurring at the initial stage of crack propagation and gradually decreasing with the fatigue process.

Energy transfer and release during the fracture of UD fiber-reinforced thermoplastic composites

This study delves into the complex dynamics of energy transfer and release during the fracture of unidirectional (UD) fiber-reinforced thermoplastic composites. Conducted through meticulous experimental observations and rigorous energy analysis, the research presents groundbreaking conclusions on the fracture toughness and energy conversion efficiency of UD carbon fiber composites under various loading conditions. A prominent focus is on how these mechanical properties are intricately linked to the sample’s fiber orientation angles and the external conditions applied during testing.The work employed tapered double cantilever beam (TDCB) as the far-field boundary to investigate the energy changes, transfer, and conversion processes during the crack propagation of unidirectional fiber reinforced thermoplastics (UDFRTPs) structures. It was found that throughout the entire propagation process, blunt extension occurs first followed by inertial extension. This is because in the initial stage of crack propagation, the stored elastic energy in the structure is relatively small, making the crack tip prone to blunting. This continuous blunting impedes the release of internal strain energy in the structure, with blunt extension accounting for a very small proportion of the entire fracture process. When the internal strain energy in the structure continues to increase to a certain extent, the blunting of the crack tip cannot prevent the release of a large amount of elastic energy inside the structure. At the same time, the excess released strain energy converts into kinetic energy and continually impacts the blunted region at the crack tip. At this point, the propagation transitions into the inertial extension stage, which occupies a significant portion of the entire fracture process.

Hole-edge crack monitoring in attachment lug with large bolt hole based on guided wave and circular piezoelectric sensor array

The crack damage monitoring of aircraft structures is very significant for ensuring aircraft safety, reducing maintenance costs and extending service life. Due to the extreme service environment, the attachment lug is prone to initiate crack damage at the hole edge, which leads to crack propagation and fracture failure. Structural health monitoring technology based on piezoelectric guided wave has been widely studied, promoting the development of crack monitoring. However, at present, research on hole-edge crack damage monitoring of attachment lugs still needs to be further carried out. It is difficult to monitor small cracks at the initial stage of crack propagation, and the accuracy of crack monitoring needs to be improved. By focusing on the accuracy of the crack monitoring in the attachment lug, a crack damage monitoring method based on the circular piezoelectric sensor array is proposed in this paper. Combined with damage alarming and localization imaging, this method comprehensively evaluates the hole-edge crack damage monitoring situation and improves the monitoring effect. The method is verified by an experiment in attachment lug, and this verification includes small crack monitoring and crack propagation monitoring. The experimental results demonstrate that this method can achieve correct damage alarming results, and the maximum localization error of crack damage is only 3.02 mm, which provides a research idea for the accurate monitoring of crack damage at the hole edge.

Effect of ultrasonic rolling on crack propagation behavior of Ti6Al4V titanium alloy laser welded joints

This study investigated the effect of ultrasonic rolling on the fatigue resistance of Ti6Al4V titanium alloy laser welded joints, with an emphasis on crack growth rate. Multiple analyses were conducted on samples subjected to different numbers of rolling passes, including analysis of surface roughness, morphology, cross-section structure, hardness, fatigue fracture, and residual stress. Ultrasonic rolling was found to considerably enhance the life cycle of the sample. One, three, and five rolling passes resulted in fatigue life increases of 17.14%, 46.38%, and 66.61%, respectively, when compared with untreated samples. Treated samples exhibited rough and intricate crack propagation paths on fracture surfaces, along with numerous secondary cracks. Fatigue striation width decreased by 48.6%, 62.5%, and 69.4% for samples receiving one, three, and five rolling passes, respectively. These fatigue striations, which were narrower and more densely distributed, indicate that ultrasonic rolling effectively increases fatigue life. The ultrasonic rolling treatment enhanced fatigue performance and reduced the rate of crack propagation due to changes in surface compressive residual stress, microstructure, microhardness, and improved surface quality. Crack retardation primarily occurred in initial stage of crack propagation and stable propagation stages, whereas inhibitory effects weakened during the final fracture stage due to superposition and offset of the higher driving force with compressive residual stress. In summary, the study demonstrates that treatment via ultrasonic rolling is an effective method for improving fatigue life and fracture toughness in Ti6Al4V titanium alloy laser-welded joints.

Stress and Strain State Analysis of Crack Front in Dissimilar Metal Welded Joints with Dual Field of Mechanical Heterogeneity and Residual Stress

The micro-mechanical state at the crack front is one of the key factors affecting the stress corrosion cracking (SCC) growth behavior. The mechanical heterogeneity and residual stress in the dissimilar metal welded joint (DMWJ) induce the micro-mechanical state at the crack front to become more complex. The sandwich model and dual-field model of the DMWJ with inner surface axial crack were established in this study. The stress and strain states at the crack front with different crack locations and lengths under the interaction of the mechanical property and the residual stress were investigated. The results show that a more accurate evaluation of stress and strain states can be obtained when using the dual-field model to describe the material mechanical property and residual stress of the DMWJ. The sandwich model overestimates the crack driving force including the stress and strain at the crack front. The tensile stress in the middle of shallow cracks is smaller than that at both ends, while the tensile stress in the middle of deep crack is larger than that at both ends. The variation trend of the tensile stress and normal strain at the crack apex is basically the same as that of the residual stress with the crack depth. However, there is almost no normal plastic strain in the initial stage of crack propagation due to the small residual stress in the initial stage.

Multiscale Pore Structure Characteristics and Crack Propagation Behavior of Coal Samples from High Gas Seam.

Investigation on the pore-fracture features and crack propagation behavior of coal is necessary to prevent coal mine disasters. The pore structure features of coal samples taken from high gas seam were obtained by mercury injection porosimetry (MIP) and gas adsorption methods. The process of deformation and failure for coal samples under three-point bending conditions were obtained. The results demonstrate that the adsorption pores with diameter less than 100 nm are the most developed and their surfaces are the roughest (the average surface fractal dimension Ds is 2.933). The surface of micro-cracks is smoother (Ds is 2.481), which is conducive to gas seepage. It may be the explanation for that 14-3# coal seam is a high gas seam, while there was almost no gas outburst accident so far. At the initial stage of crack propagation, the main crack on the coal sample expanded along the direction of the natural cracks. In the process of crack propagation, the surface fractal dimension of the main crack increased, suggesting that the bending degree of the main crack enhanced. The brittle characteristics of coal samples can be reflected by the ratio of the dissipated energy to the accumulated energy.

Analytical Determination of the Shape of a Growing Crack in a Specimen Subjected to Bending

The paper presents an analytical approach to the prediction of the crack front at the initial stage of crack propagation. The effect of stress triaxiality on the shape of the crack front is accounted for when the cracked specimen is subjected to various types of loading modes including static and dynamic loading under room and low temperatures.

Influence of the Undercut Anchor Head Angle on the Propagation of the Failure Zone of the Rock Medium.

The paper presents the results of a numerical analysis (FEM) describing the effect of the undercutting head angle on the formation of the rock mass failure zone during the initial stages of failure propagation. The research was carried out in the context of developing a technology for rock extraction by controlled pull-out of undercut anchors installed in the rock mass. The focus was on the initial stage of crack propagation and its trajectory for anchors embedded at an assumed constant depth and a value of the friction coefficient of the rock against the anchor head. It is shown that smaller angles favor smaller stripping angles and an increased radial impact of the head on the rock material (in the plane perpendicular to the head axis), while the impact of heads with larger angles is found to favor larger fracture penetration angles and faster penetration towards the free rock surface.

Initial Crack Propagation and the Influence Factors of Aircraft Pipe Pressure.

In aircraft engineering, an increase of internal pressure in a hydraulic pipe increases the probability of pipe damage, leading to crack propagation becoming a serious issue. In this study, the extended finite element method (XFEM) is applied to simulate initial crack propagation in hydraulic pipes and to investigate the influence factors. Stress intensity factors are extracted to verify the mesh independence of XFEM, which is based on the level set method and unit decomposition method. A total of 30 finite element models of hydraulic pipes with cracks are established. The distribution of von Mises stress under different initial crack lengths and internal pressures is obtained to analyze the change of load-carrying capacity in different conditions. Then, a total of 300 finite element models of hydraulic pipes with different initial crack sizes and locations are simulated under different working conditions. The relationship between the maximum opening displacement and crack length is analyzed by extracting the opening displacement under different initial crack lengths. The length and depth of the initial crack are changed to analyze the factors affecting crack propagation. The opening size and crack propagation length are obtained in different directions. The results show that radial propagation is more destructive than longitudinal propagation for hydraulic pipes in the initial stage of crack propagation.

Path-dependent J-integral evaluations around an elliptical hole for large deformation theory

AbstractAn exact expression is obtained for a path-dependent J-integral for finite strains of an elliptical hole subject to remote tensile tractions under the Tresca deformation theory for a thin plate composed of non-work hardening material. Possible applications include an analytical resistance curve for the initial stage of crack propagation due to crack tip blunting.

On computational homogenization of microscale crack propagation

SummaryThe effective response of microstructures undergoing crack propagation is studied by homogenizing the response of statistical volume elements (SVEs). Because conventional boundary conditions (Dirichlet, Neumann and strong periodic) all are inaccurate when cracks intersect the SVE boundary, we herein use first order homogenization to compare the performance of these boundary conditions during the initial stage of crack propagation in the microstructure, prior to macroscopic localization. Using weakly periodic boundary conditions that lead to a mixed formulation with displacements and boundary tractions as unknowns, we can adapt the traction approximation to the problem at hand to obtain better convergence with increasing SVE size. In particular, we show that a piecewise constant traction approximation, which has previously been shown to be efficient for stationary cracks, is more efficient than the conventional boundary conditions in terms of convergence also when crack propagation occurs on the microscale. The performance of the method is demonstrated by examples involving grain boundary crack propagation modelled by conventional cohesive interface elements as well as crack propagation modelled by means of the extended finite element method in combination with the concept of material forces. © 2016 The Authors. International Journal for Numerical Methods in Engineering Published by John Wiley & Sons Ltd.

High Speed Computation of Muscle Stress Problem Using Crack Propagation Modelling on DPCS Platform

Muscle stress is a disease resulting from performing too much effort, sport fitness or high blood pressure problem. This paper focuses on the mathematical modeling of crack propagation that applies on the human muscle stress. The modeling describes the relationship between the temperature and crack propagation among targeted region, node and element. The contribution of this paper is to predict the stress level based on the mathematical modeling for nodal displacements of crack propagation with respect to time and space. The initial stage of crack propagation that leads to an initiation of crack propagation is being modeled satisfactorily the discretization of finite element method (FEM) of the specific region. The sequential algorithm of the crack propagation simulation for muscle stress will be solve using Gauss Seidel method (GS). The parallel algorithm will be solve using Red Black Gauss Seidel method (RBGS). Some comparisons based on numerical analysis and parallel performance evaluation will be analyzed in terms of convergent rate of FE solution results to the exact solution of clinical dataset, run time prediction, time execution, speedup, efficiency, effectiveness and temporal performance. C and Parallel Virtual Machine (PVM) software with open source operating system on distributed parallel computer system (DPCS) will support the huge computation cost of the parallel algorithms.

Life prediction of titanium and aluminum alloys under fretting fatigue conditions using various crack propagation criteria. Part 1. Experimental and calculation techniques

Using the analysis of state-of-the-art techniques of fretting fatigue studies and results of description of multi-stage fatigue crack propagation in fretting zone within fracture mechanics framework, we have refined the calculation-and-experimental technique earlier proposed by the author. This technique allows one to predict current values of the angle and rate of inclined crack propagation in the subsurface layers of material under fretting conditions using calculated stress intensity factors KI and KII for contact and bulk loads, as well as experimental crack resistance diagrams by KI and/or KII types. We have performed comparative analysis of various techniques for construction of crack resistance diagrams by KI and KII types and obtained the results for AMg6N and Al 7075-T6 aluminum alloys and VT9 titanium alloy. It is shown that the initial stage of crack propagation in the above alloys can occur in the maximal shear stress plane by shear mechanism or in the maximal tensile stress plane by cleavage mechanism according to the Otsuka two-parameter criterion or to the Richard empirical criterion.

R-curve characteristics of adhesives modified with high rubber content under mode I loading

For rubber-modified epoxy adhesives with high rubber content, stable crack propagation is observed under mode I loading. In such a situation, a further increase in rubber content not only extends the plastic zone in front of the crack tip, but also promotes the formation and growth of voids in the adhesive layer. To clarify the effect of a further increase in rubber content on crack initiation and propagation behavior, fracture toughness tests were conducted under mode I loading using adhesively bonded double cantilever beam (DCB) and compact tension (CT) specimens: these specimens were used for evaluating the crack growth resistance and fracture toughness in the initial stage of crack propagation, respectively. Stable behavior in crack growth was observed for adhesives with rubber contents of both 10 and 14 wt%. The crack growth resistance of the adhesive with 14 wt% rubber was higher than that with 10 wt%, whereas in the initial stage of crack propagation the energy release rate for 10 wt% rubber was higher than that for 14 wt%. The crack growth behavior of the DCB specimen was discussed on the basis of Gurson’s model. The estimated R-curve agreed well with the experimental result in the initial stage of crack propagation, although the difference between estimated and experimental plots increased with crack growth.

The dynamic stress intensity factor analysis of adhesively bonded material interface crack with damage under shear loading

This paper studies the dynamic stress intensity factor (DSIF) at the interface in an adhesive joint under shear loading. Material damage is considered. By introducing the dislocation density function and using the integral transform, the problem is reduced to algebraic equations and can be solved with the collocation dots method in the Laplace domain. Time response of DSIF is calculated with the inverse Laplace integral transform. The results show that the mode II DSIF increases with the shear relaxation parameter, shear module and Poisson ratio, while decreases with the swell relaxation parameter. Damage shielding only occurs at the initial stage of crack propagation. The singular index of crack tip is −0.5 and independent on the material parameters, damage conditions of materials, and time. The oscillatory index is controlled by viscoelastic material parameters.

Comparison of failure mechanisms between rubber-modified and unmodified epoxy adhesives under mode II loading condition

The effect of rubber modification on fracture toughness of adhesive joints under mode II loading condition was investigated in comparison with that under mode I loading, wherein the two adhesives rubber-modified and unmodified were used. To evaluate the fracture toughness on the basis of R-curve characteristics under mode II loading condition, four-point bend tests had been conducted for the adhesively bonded end-notched flexure (ENF) specimens. Thus obtained R-curves revealed the following trend: its behavior did not appear for the unmodified adhesive, whereas the rubber-modified adhesive exhibited a typical behavior. In the initial stage of crack propagation, GIIC of the rubber-modified adhesive is lower than that of the unmodified adhesive, but becomes greater in the range of Δa > 25 mm. Nevertheless, the significant improvement of the fracture toughness with the rubber modification under mode I loading condition was not observed under mode II loading. Moreover, FEM analysis was made to elucidate the relation between the above fracture behavior and stress distributions near the crack tip. The results gave the reasonable relationship between evolution of plastic zone and the area with high void-fraction as well as the R-curves behavior. In addition, macroscopic and SEM observations for the fracture surfaces were also conducted.

高強度鋼の疲労破面に及ぼす大気／高真空／材料内部環境の影響

Uniaxial tension fatigue tests of high strength steel, SNCM439, were carried out in air and high vacuum environments. In order to clarify how different environments affect the fatigue fracture surfaces and interior fatigue crack propagations, fracture surfaces were compared through fractographic analysis under air, high vacuum, and inner environment of materials. The following results were obtained; (1) Compared with surface fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) High vacuum environment strongly influenced the fatigue fracture surface in initial stage of crack propagation. (3) When the transition of crack propagation from Optically Dark Area (ODA) to Stage II a occurred, there was the possibility that some mechanical factors were involved in the transition. The formation of ODA was closely related to the mean stress. (4) Inner environment of materials gave more similar effects on crack propagation with high vacuum rather than air. (5) When fracture origins were non-metallic inclusions, interior fatigue cracks of materials in Fish eye were propagated in the following order; ODA → Stage II a → Stage II b → Stage II c.

307 高強度鋼の疲労破面に及ぼす大気/高真空/材料内部環境の影響(OS1-1 高強度材の超高サイクル疲労(1))(OS1 高強度材・表面改質材の疲労と破壊)

Uniaxial tension fatigue tests of high strength steel, SNCM439, were carried out in air and high vacuum environment. As a result, the following were obtained : (1) Compared with surface fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) High vacuum environment strongly influenced the fatigue fracture surface in the initial stage of crack propagation. (3) High vacuum environment was similar to inner environment of materials. (4) The formation of Optically Dark Area (ODA) closely related to mean stress.

Three-dimensional variational analysis of composite structures using bernstein polynomial approximations. Report 2

The application of the previously developed 3D varionational analysis approach to the investigation of crack propagation in composite bonded joints is presented. In this application, the propagation of three different types of a 2D planar crack (adhesive, cohesive, and interfacial) is modeled by relaxing the respective continuity conditions for displacements between adjacent bricks in the mosaic structure. The crack propagation process is then characterized by the release rate of the total potential energy between two consecutive states of the mosaic body with different crack lengths. Numerical examples illustrate the 3D analysis of double-lap adhesively bonded joints with unidirectional and cross-ply laminated composite adherends. The numerical results provide an illustration of various characteristics of the crack propagation process. The values of the ultimate failure load predicted by analyzing the initial stage of crack propagation are found to be in a good agreement with experimental data.

Fatigue, Cyclic Deformation and Microstructure. The Effects of Straining Frequency and Stress Ratio on Polarization Current Responded to Cyclic Strain in a Commercial Iron.

Corrosion fatigue tests of a commercial iron were carried out using a cylindrical specimen with a hole in a borate buffer solution containing 5 mol/m 3 NaCI at a constant passive potential. Polarization current behavior induced by strain cycles has been investigated in order to elucidate the effects of straining frequency and stress ratio. The amplitude of polarization current synchronized with the strain cycle and the phase shift between the polarization current and the strain were calculated. The polarization current obtained prior to the crack initiation was found to be the charging and discharging current of the electric double layer capacitance. The corrosion damage increased as decreasing the straining frequency in the initial stage of crack propagation. The increased corrosion damage was observed at the reduced stress ratio.