Mode Crack Growth Research Articles

Corrosion Fatigue Studies on a Bulk Glassy Zr-Based Alloy under Three-Point Bending

Corrosion fatigue (CF) tests were carried out on bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 (Vitreloy 105) samples under load-controlled three-point bending conditions with a load ratio of R = 0.1 in 0.01 M Na2SO4 + 0.01 M NaCl electrolyte. During cyclic testing, the bar-shaped specimens were polarized in situ at constant potentials and the current was monitored. Three different anodic potentials within the interval between the pitting potential EP and the repassivation potential ER, and three different load amplitudes were applied. In some cases, in situ microscopic observations revealed the formation of black corrosion products in the vicinity of the crack tip during anodic polarization. Fractographic analysis revealed a clear distinction between two modes of crack growth characterized by smooth dissolution induced regions on the one hand and slim fast fracture areas on the other hand. Both alternating features contributed to a broad striated corrosion fatigue fracture surface. Moreover, further fatigue tests were carried out under free corrosion conditions yielding additional information on crack initiation and crack propagation period by means of the open circuit potential (OCP) changes. Thereby, a slight increase in OCP was detected after rupture of the passive layer due to bare metal exposed to the electrolyte. The electrochemical response increased continuously according to stable crack propagation until fracture occurred. Finally, the fracture surfaces of the corrosion fatigue samples were investigated by energy dispersive X-ray with the objective of analyzing the elemental distribution after anodic dissolution. Interestingly, anodic polarization at a near repassivation potential of -50 mV vs. SCE (Saturated Calomel Electrode, E = 0.241 V vs. SHE, Standard Hydrogen Electrode) led to favorable effects on the fatigue lifetime. In conclusion, all results are conflated to a corrosion fatigue model for bulk glassy Vitreloy 105 under anodic polarization in chloride-containing electrolyte and compared to the previously proposed stress corrosion mechanisms under similar conditions.

時効硬化Al合金のき裂伝ぱ形態に及ぼす大気環境の影響（回転曲げ疲労と超音波疲労）

時効硬化Al合金のき裂伝ぱ形態に及ぼす大気環境の影響（回転曲げ疲労と超音波疲労）

Structural Performance of Inconel 625 Superalloy Brazed Joints

The purpose of this work was to investigate tensile and fatigue behaviors of Inconel 625 superalloy brazed joints after transient liquid-phase bonding process. Brazing was performed in a vacuum furnace using a nickel-based filler metal in a form of paste to join wrought Inconel 625 plates. Mechanical tests were carried out on single-lap joints under various lap distance-to-thickness ratios. The fatigue crack initiation and crack growth modes were examined via metallographic analysis, and the effect of local stress on fatigue life was assessed by finite element simulations. The fatigue results show that fatigue strength and endurance limit increase with overlap distance, leading to a relatively large scatter of results. Fatigue cracks nucleated in the high-stressed region of the weld fillets from brittle eutectic phases or from internal brazing cavities. The present work proposes to rationalize the results by using the local stress at the brazing fillet. When using this local stress, all fatigue-obtained results find themselves on a single S-N curve, providing a design curve for any joint configuration in fatigue solicitation.

An investigation of the small fatigue crack growth behavior in Ti-6.5Al-2Zr-1Mo-1V alloy

In this paper, we describe the experimental generation of a small fatigue crack in an SENT (single-edge notched tension) specimen of Ti-6.5Al-2Zr-1Mo-1V alloy. The growth behavior of the crack was analyzed using optical microscopy, scanning electron microscopy, transmission electron microscopy and other methodologies. We determined that when the crack length was approximately equal to the size of the plastic deformation zone at the crack tip, a knee point appeared in the da/dN-ΔK curve. At the knee point, the crack fracture mode transitioned from a crystallographic-faceted fracture to a striation-type fracture. Before the knee point, the change in the crack growth mode caused a zigzag crack growth path. The phase boundary and crack closure resulted in an irregular FCGR (fatigue crack growth rate). After the knee point, the small crack propagated forward by sliding while retaining traces of slip bands at the crack tip. The directions of the slips in the plastic deformation zone could be affected by the crystal orientations. Therefore, the crack growth path transitioned from a zigzag to a linear path due to the similar crystal orientation of the parallel α lamellae. In addition, the crack driving force far exceeded the resistance of the microstructures, which resulted in a significant acceleration of the FCGR.

Prediction of effective mode II fatigue crack growth threshold for metallic materials

The fatigue crack growth threshold consists of the intrinsic component (effective threshold) and the extrinsic component (crack closure). Theoretical formulae for prediction of the mode II effective threshold in metallic materials are presented. The model is based on physical interpretation of experimental findings. For a given material the dominant local crack growth mode at the kinked crack front of a remote mode II loaded crack is identified and the local stress intensity factor is expressed, which represents a new concept for estimation of the mode II effective threshold. The local crack growth mode depends on microstructure and crystallographic structure of the material. Moreover, new experimental data for mode II effective thresholds of the Ti6Al4V alloy (ΔKIIeff,th=1.8MPam1/2) and pure zirconium (ΔKIIeff,th=1.3MPam1/2) are presented. These data extended the available database for metallic materials and confirmed a broad validity of the proposed theoretical relationships.

Analysis on Low Cycle Fatigue Life and Fatigue Fracture Surface Morphology of TC25 Titanium Alloy

At room temperature, the low cycle fatigue tests for smooth specimens of TC25 titanium alloy under various stress ranges are operated at a CSS280I-20w Electro Hydraulic Servo Universal Testing Machine with a microscopic observation system, and the low cycle fatigue lifetimes are measured. Based upon the analysis of stress-strain hysteresis loop of low cycle fatigue of TC25 titanium alloy, a simplified Manson-Coffin formula is derived according to both the experimental characteristics and the stress-strain constitutive model, the fatigue lifetimes are plotted against stress ranges, and a stress-fatigue life curve for TC25 titanium alloy is obtained by the linear regression analysis method. Finally, the fracture surface morphologies of TC25 specimens are investigated using a JSM-6360 Scanning Electron Microscopy, and the fatigue fracture mechanisms of low cycle fatigue are studied. It shows that the plastic deformations are mainly formed at the accelerated fracture stage, and various shear lips can be observed on the fracture surfaces, which demonstrates that the shear stress results in the final rupture of TC25 titanium alloy. During the fracture of low cycle fatigue, the cleavage nucleation leads to the formation of fatigue crack initiation region, the fatigue crack growth exhibits a mixed transgranular and intergranular crack growth mode, and in the final rupture region, the fracture surface of low cycle fatigue of TC25 titanium alloy appears as a typical semi-brittle fracture mode.

Friction and roughness induced closure effects on shear-mode crack growth and branching mechanisms

The understanding of shear-mode crack growth mechanisms and crack branching phenomena is of great interest for a variety of practical engineering situations. Despite this fact, relatively little research is available regarding these topics. Of the studies that have been performed, few provide a means of quantifying such effects and most consider crack growth starting from a precrack. The current study is aimed at trying to fill some of the research voids in these areas by investigating the effects of microcrack coalescence, loading level, and superimposed normal stresses on the shear-mode crack behavior of naturally initiated fatigue cracks. Based on the experimental results and subsequent analyses, it was determined that microcrack networks and coalescence have little to no effect on the experimentally observed crack paths, regardless of the applied loading level. Instead, the preferred crack growth mode was shown to have a dependence on the applied shear stress magnitude and stress normal to the crack plane, indicating a significant role of fiction and roughness induced crack closure in the crack growth process. A simple model was then proposed to quantify these effects based on the idea that crack face interaction reduces the effective shear-mode driving force by allowing a portion of the nominally applied loading to be transferred through a crack. Crack growth predictions based on the model were compared to experimental results with respect to both crack branching and crack growth rate. The model was found to agree well, both qualitatively and quantitatively, with experimentally observed trends for a variety of multiaxial loading conditions.

Memory effects in variable amplitude and multiaxial fatigue crack growth: an incremental approach

This paper presents an incremental approach for modelling fatigue crack growth with memory effects due to the non-linear behavior of the material. This approach is developed at LMT since 2003, in collaboration with several industrial partners, mainly with Snecma, SAFRAN Group, EDF and AREVA and SNCF. The first part of this paper presents the context and the objectives, and the key assumptions on which the model is based. The second part presents some examples of applications of the model, fatigue crack growth in mode I conditions, under variable amplitude loading; non-isothermal situations; crack growth in coupled environmental and fatigue loading conditions; extension of the model to non-proportional mixed mode loading conditions, and to short cracks. The last part presents some ongoing work, possible developments and scientific challenges that remain to be overcome.

A simplified life estimation method for the spherical hull of deep manned submersibles

Creep and fatigue are involved in the loading of deep manned submersible, which is a rather complex variable amplitude pattern. Dwell effects resulting in lower life than pure fatigue are observed in the prior experimental research while no proper prediction methods are available to explain the phenomenon. Recently, the authors proposed a modified crack growth rate model to explain the creep effect under the cyclic loading which is validated for the crack growth of some titanium alloys under cyclic creep loading, however, its application is restricted to the condition where lots of parameters have to be determined based on many experiments and it is not very convenient in the material selection and primary design stage of a component. In this paper, a simplified prediction model for the load pattern of constant amplitude cyclic loading is proposed aiming at one of the most applicable materials for the pressure hull of submersibles, Ti–6Al–4V ELI. The method can be easily used to estimate the life of the pressure hull based on two series of basic mechanical properties of the material and validated by the modified crack growth rate model with parameters determined by large amounts of experimental data.

Crack paths in smooth and precracked specimens subjected to multiaxial cyclic stressing

The understanding of shear-mode crack growth mechanisms and crack branching phenomena is of great interest for a variety of practical engineering situations. Despite this fact, relatively little research is available regarding these topics. Of the studies that have been performed, few provide a means of quantifying such effects and most consider crack growth starting from a precrack. The current study is aimed at trying to fill some of the research voids in these areas by investigating the effects of microcrack coalescence, loading level, and superimposed normal stresses on the mode II crack behavior of naturally initiated fatigue cracks. Based on the experimental results and subsequent analyses, it was determined that microcrack networks and coalescence have little to no effect on the experimentally observed crack paths regardless of the applied loading level. Instead, the preferred crack growth mode is shown to have a dependence on the applied shear stress magnitude and stress normal to the crack plane, indicating a significant role of fiction and roughness induced crack closure effects in the crack growth process. A simple model is then proposed to quantify these effects based on the idea that crack face interaction reduces the effective mode II SIF by allowing a portion of the nominally applied loading to be transferred through a crack. The model agrees qualitatively with the experimentally observed trends for pure torsion loading and predicts crack branching lengths within a factor of 2 for all loadings considered.

Experimental Study of the Fatigue Crack Propagation and Overloading Retardation in TA2 Plate

In this paper, I+II mixed mode notch-crack fatigue propagation in titanium alloy steel TA2 was tested with the emphasis on the crack mode transition and retardation under constant amplitude loading or overloading. Finite element method was employed to calculate the stress distribution at the crack tip in order to explain the crack growth behavior. It is found that after initiation from the crack tip, the new formed crack propagates in a mode I form, regardless of the magnitudes of the inclined angle of the crack and the overloading applied. The relationship between the fatigue crack growth rate and mode I stress intensity factor range was calculated. After overloading, crack initiation and propagation will be significantly slowed, or in other words, there exists overloading retardation. Calculation shows that after overloading, a plastic zone with residual compressive stress is formed which is responsible for the retardation of the crack growth rate. Micro-morphologies of the crack growth path and the crack fracture surface were also observed and analyzed.

Failure mode and fatigue behavior of weld-bonded lap-shear specimens of magnesium and steel sheets

Failure modes and fatigue behaviors of ultrasonic spot welds in lap-shear specimens of magnesium AZ31B-H24 and hot-dipped-galvanized mild steel sheets with and without adhesive were investigated. The spot welded specimens failed from the kinked crack growth mode. The adhesive-bonded specimens failed from the cohesive failure through the adhesive and the kinked crack growth through the magnesium sheet. The weld-bonded specimens failed from the cohesive failure through the adhesive, the interfacial failure through the spot weld, and the kinked crack growth through the magnesium sheet. The estimated fatigue lives for the adhesive-bonded and weld-bonded specimens failed from the kinked crack growth mode are lower than the experimental results.

The effect of NbC on mechanical properties and fracture behavior of WC–10Co cemented carbides

In an attempt to improve the understanding of fracture behavior of brittle cemented carbides with NbC addition, a detailed study was conducted with the main goal of characterizing the fracture character and crack growth mode. The study showed that NbC had a significant influence on the mechanical properties of WC–10Co alloys. The maximum hardness and transverse rupture strength were observed when the NbC content was 1wt.%. However, with the further addition of NbC, the TRS and fracture toughness degraded. The fracture origin was cleavage fracture from coarse WC when the addition of NbC was 1wt.% and 2wt.%. But in the materials with 3wt.% and 4wt.% NbC addition, the brittle (W,Nb)C solid solution increased considerably, and this leaded to a low stress brittle fracture. In the WC–10Co alloy, intergranular growth was the dominant mode of crack propagation. The ratio of transgranular crack increased because of the grain refinement of WC and solid strengthen effect of binder by NbC, and the transgranular crack in brittle (W,Nb)C solid solution increased when NbC content was increased to 3wt.% and 4wt.%.

High cycle fatigue crack growth in Mode I of adhesive layers: modelling, simulation and experiments

The capability to predict high cycle fatigue properties of adhesive joints is important for cost-efficient and rapid product development in the modern automotive industry. Here, the adaptability of adhesives facilitates green technology through the widening of options of choosing and joining optimal materials. In the present paper a continuum damage mechanics model is developed based on the adhesive layer theory. In this theory, through-thickness averaged variables for the adhesive layer are used to characterise the deformation, damage and local loading on the adhesive layer. In FE-simulations, cohesive elements can thereby be used to model the adhesive layer. This simplifies simulations of large scale complex built-up structures. The model is adapted to experimental results for two very different adhesive systems; one relatively stiff rubber based adhesive and one soft polyurethane based adhesive. The model is able to reproduce the experimental results with good accuracy except for the early stage of crack propagation when the loads are relatively large. The model also predicts a threshold value for fatigue crack growth below which no crack growth occurs. The properties of the model are also compared with the properties of Paris’ law. The relations between the parameters of the continuum damage mechanics law and the parameters of Paris’ law are used to adapt the new law. It also shows that the properties of a joined structure influence the Paris’ law properties of the adhesive layer. Thus, the Paris’ law properties of an adhesive layer are not expected to be transferable to joints with adherends having different mechanical properties.

Investigation of mode III fatigue crack growth in rubber based on the pioneering work of Alan N. Gent and coworkers

From 1998 to 2003, Alan N. Gent and some co-workers published a series of papers devoted to the first measurements of fatigue crack growth properties in rubber in mode III loading conditions (Rubber Chemistry and Technology, vol. 71, pp. 76–83; vol. 71, pp. 84–94; vol. 76, pp. 1276–1289). The present paper proposes to revisit this seminal work benefiting from the capabilities of new multiaxial fatigue machines. Then, a new mode III fatigue test for elastomers is developed. A thin bonded disk of rubber with a circumferential cut is subjected to cyclic torsion to investigate the fatigue crack growth in mode III. The corresponding measurement methods for the crack growth rate and the fracture energy are proposed. As a constant small axial extension can be applied and controlled to prevent the cracking surfaces from abrasion and to ensure almost mode III loading conditions, the quality and the relevance of the experimental data are highly enhanced. As a results, the method is applied to an unfilled natural rubber and the crack growth rate is related to the fracture energy for several loading cases. The effects of axial extension, alternate cycles and positive minimum angle cycles are examined. In particular, it is shown that for fully unloaded cycles the crack growth rate laws in modes I and III coincide in the regime of high fracture energies.

EBSD Analysis of Fatigue Crack Growth of 2124 Aluminum Alloy for Aviation

Abstract The fatigue crack growth of 2124-T851 aluminum alloy was investigated by Electron Backscatter Diffraction (EBSD) analysis. The result shows that the mode of fatigue crack growth is mainly transcrystalline, and the preferential orientation changes with the difference of grain orientation. When the crack extends to the grain boundary, the deflection of the crack against the normal extensive path will take place as a result of the misorientation of adjoining grains, but the deflection of the crack in the grain inner is mainly caused by the coarse second phase particles which are poor in coordination of deformability under cyclic stress. The phenomenon of bifurcation during the process of the crack propagation is relative with the specific crystallographic direction, which is caused by the simultaneous movement of numerous equivalent {111} slip system on crack-tip.

Mechanical properties of composites made from locally manufactured carbon fabrics and the composites produced from air-textured glass yarns

Composite materials have a wide range of applications in structural components because of their high strength-to-weight and stiffness-to-weight ratios. However, the most crucial and common life-restricting crack growth mode in laminated composites i.e. delamination is of great concern. Air jet texturing was selected to provide a small amount of bulk to the glass yarn. The purpose was to provide more surface contact between the fibres and resin and also to increase the adhesion between the neighbouring layers. These were expected to enhance the resistance to delamination in the woven glass composites. The development and characterisation of core-and-effect textured glass yarns was presented in the previous paper. This paper describes the comparison of the mechanical properties of composites produced from air-textured glass yarns and the composites made from locally manufactured carbon fabrics. The tensile, flexure and inter-laminar shear strength (ILSS) were compared and it was observed that although glass fibres are inferior to carbon fibres in terms of mechanical properties however, the flexure strength and ILSS of glass based composites increases after texturing and were found closer to the properties of carbon based composites.

Near‐threshold behaviour of shear‐mode fatigue cracks in metallic materials

ABSTRACTThis review paper presents a brief state‐of‐the art of the research on long fatigue shear‐mode cracks and describes some recent results on effective crack growth thresholds and mode I branching conditions achieved by the authors for ARMCO iron, titanium with two different microstructures, nickel and stainless steel. A special technique for preparation of fatigue precracks enabled us to substantially suppress the crack closure (friction) effects at the beginning of the experiment, and the measured threshold values could be considered to be very close to the effective ones. In all investigated materials, the effective thresholds under the remote mode II loading were found to be about 1.7 times lower than those under the remote mode III loading. Effective thresholds under mode II loading of investigated materials were found to follow a simple formula assembled by the shear modulus G, the magnitude of Burgers vector b and a goniometrical function nα of the mean deflection angle that depends on the number of available crystallographic slip systems. These quantities determine the intrinsic material resistance to mode II crack propagation at the threshold. A simple criterion for mode I branching in terms of effective threshold values well reflects a transition from the shear‐mode to the opening‐mode controlled crack propagation at the threshold. The associated transition deflection angle of 40° is a material independent constant.

Ultrasonic Spot Welding of Aluminum to High-Strength Low-Alloy Steel: Microstructure, Tensile and Fatigue Properties

The structural applications of lightweight aluminum alloys inevitably involve dissimilar welding with steels and the related durability issues. This study was aimed at evaluating the microstructural change, lap shear tensile load, and fatigue resistance of dissimilar ultrasonic spot-welded joints of aluminum-to-galvanized high-strength low-alloy (HSLA) steel. Two non-uniform layers were identified in between Al and HSLA steel via SEM/EDS and XRD. One was an Al-Zn eutectic layer and the other was a thin (<2 μm) layer of intermetallic compound (IMC) of Al and Fe in the nugget zone. The lap shear tensile testing gave a maximum load of 3.7 kN and the sample failed initially in between the Al-Zn eutectic film and Al-Fe IMC, and afterward from the region containing Al on both matching fracture surfaces. The fatigue test results showed a fatigue limit of about 0.5 kN (at 1 × 107 cycles). The maximum cyclic stress at which transition of the fatigue fracture from transverse through-thickness crack growth mode to the interfacial failure mode occurs increases with increasing energy input.

Fracture mechanics analysis of asperity cracking due to adhesive normal contact

A two-dimensional linear-elastic fracture mechanics analysis of asperity cracking induced by adhesive normal contact was performed with the finite element method. Normal contact between two elastic asperities was analyzed with the equivalent contact system of an elastic asperity with equivalent radius of curvature and effective elastic modulus compressed by a rigid plane. Surface adhesion was modeled by nonlinear springs obeying a constitutive force-distance law derived from the Lennard–Jones potential. The maximum ranges of the tensile and shear stress intensity factors were used to determine the crack-growth direction and the dominant mode of asperity fracture in terms of the Maugis parameter (a function of the equivalent radius of curvature, work of adhesion, effective elastic modulus, and intermolecular equilibrium distance), friction coefficient at the crack interface, maximum surface interference, and crack position. Finite element simulation results indicate that the direction and the rate of crack growth are mostly affected by the Maugis parameter and the maximum surface interference. A transition from shear to tensile dominant mode of crack growth is encountered with the increase of the Maugis parameter and/or the decrease of the maximum surface interference. Opening, slip, and stick between the crack faces during loading and unloading are discussed in the context of crack mechanism maps.