Static Crack Growth Research Articles

Fatigue crack growth of deformed pearlitic rail steels under multiaxial loading

A new non-standard experiment is developed to measure fatigue crack propagation in the shear crack growth mechanism occurring in rail and crossing parts. Pre-cracked tubular specimens are loaded under cyclic Mode-II and static compressive Mode-I stresses to reproduce those shear crack growth conditions. To investigate the influence of severe plastic deformation as found in the top of rails in track, specimens are pre-deformed using high pressure torsion. For the pearlitic rail steel, the crack path and crack growth behavior changes distinctively with material pre-deformation: In the undeformed material cracks bifurcate under the Mode-II loading and turn into a Mode-I dominated mixed-mode. After severe plastic deformation, the crack propagates in Mode-II at significantly lower load levels without bifurcating. A detailed analysis of the fracture surfaces as well as the correlation of the crack path and microstructural alignment confirm these findings. Therefore, the proposed experimental procedure could provide Mode-II fatigue crack growth data required to predict crack propagation within rolling contact fatigue in rails and crossings.

Steady state crack growth in viscoelastic solids: A comparative study

We compare the predictions of different theories on the steady state growth of a Mode I crack in linear viscoelastic solids. The theories studied in this work include those by Knauss, Schapery, Persson and Brener. The comparisons focus on the fractional dissipation rate and the relationship between crack growth velocity and fracture energy. Analytical solutions are carried out using realistic constitutive models such as the Generalized Maxwell Solid (GMS) and the Power Law Solid (PLS). These theories are tested against two different sets of experimental data reported in the literature. We also present new results such as the strain field directly ahead of the crack tip, the residual strain on the crack surfaces and the crack opening displacement (COD). We use the expressions for COD to study the shape and size of the “viscoelastic trumpet” proposed by de Gennes. Using a new approach, we study the shape and size of the viscoelastic dissipation zone around the crack tip and discuss its dependence on the crack growth velocity.

A study of pure hydrolysis of carbon fibre reinforced polyamide 6 composites tested under mode I loading

This paper focuses on the durability of carbon/polyamide 6 thermoplastic composites subjected to hydrolytic aging and tested under mode I static crack growth loading. In this study, aging was performed at temperatures ranging from 100 to 140 °C for durations up to 3 months in oxygen-free water. Results show that following wet aging the fracture toughness (energy release rate) decreases significantly, from an unaged value of 3.4 kJ/m2 down to values below 1 kJ/m2 after extensive degradation. This decrease was associated with a change from ductile to brittle behaviour, which allowed a critical molar mass M’c, to be determined for the first time on long carbon fibre reinforced composites.

Fatigue crack propagation behaviors in Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with STA and BASCA heat treatments

Fatigue crack propagation behaviors of a novel high strength Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with equiaxed and lamellar microstructures were systematically investigated. Fatigue crack propagation tests at both increasing and decreasing stress intensity factor range values were carried out with a frequency of 15 HZ and stress radio R of 0.1 by using compact tension samples. The sample with lamellar microstructure displayed the faster fatigue crack propagation rates in near-threshold regime compared to the sample with equiaxed microstructure. The fatigue threshold (ΔKth) was 2.7 MPa·m1/2 for lamellar microstructure, and 3.2 MPa·m1/2 for equiaxed microstructure. The steady state crack growth rate in lamellar microstructure was slightly slower than that in equiaxed microstructure in the Paris regime, while much faster in the rapid growth regime. The interactions between crack path and microstructures as well as the microscopic fracture surface morphology in both microstructures were detected and analyzed by scanning electron microscope (SEM), revealing the different crack propagation behaviors in three regimes. The results showed that the crack propagation behaviors were a result of two contributing factors: interface obstacles and crack front profile. The increased fatigue crack propagation resistance of equiaxed microstructure in near-threshold regime can be attributed to the interface obstacles which dominated over the rougher crack front profile. While the higher fatigue crack growth resistance for lamellar microstructure in Paris regime was attributed to the more positive effect of rougher crack front profile. In the rapid growth regime, the crack in lamellar microstructure was found to be extended along the grain boundary α, which was regarded as a low energy crack path and led to the faster crack propagation rate.

Fatigue and Static Crack Growth Rate of Alloy 718 Under Cathodic Polarization

Fatigue crack growth rate was developed on three heats of Alloy 718 (UNS N07718) under cathodic polarization, over a wide range of loading conditions. Fatigue crack growth rate increased with decreasing frequency over a range of Kmax and ΔK conditions. In most cases, there was no evidence of a plateau in fatigue crack growth rate at low frequencies. The fatigue crack growth rate over the range of conditions evaluated was influenced by static crack growth rate at Kmax. The principle of superposition of fatigue crack growth and static crack growth was used to rationalize the observed crack growth rate response. Static crack growth rate of Alloy 718 measured under constant K conditions was lower than that measured under rising displacement conditions. A crack tip strain rate-based model was used to rationalize the fatigue crack growth rate behavior and the static crack growth rate behavior under constant K. However, the formulation of the model for the rising K was not able to rationalize the crack growth rate under rising displacement conditions.

Features of heat treatment of a new steel for the manufacture of hot deformation dies

The influence of heat treatment parameters on the structure and properties of the promising 70Cr3Mn2WTiB die steel, which is offered for the manufacture of mold parts for aluminum alloy injection molding machines, is revealed. A rational mode of thermal hardening is recommended for the developed steel, including spheroidizing annealing at a temperature of 780 °C with combined cooling; quenching at 1000 °C with oil cooling; high-temperature tempering at 550–600 °C with cooling in calm air. The structure formation and behavior of carbide phases in microalloyed steels at various stages of heat treatment are evaluated. The features of phase transformations and the mechanism of dispersion hardening during high-temperature tempering that provide the required set of mechanical and technological properties are determined. The influence of heat treatment modes on mechanical and operational properties under cyclic loading and heat exposure at normal and elevated temperatures is evaluated. Fractographic studies of zones of cyclic and static crack growth in fractures of samples after testing for crack resistance under cyclic and static loading were performed. The main regularities of changes in the thermal structural stability of chromium steels under cyclic temperature and force influences, depending on the nature of alloying and heat treatment modes, are substantiated, which made it possible to reasonably recommend the developed 70Cr3Mn2WTiB steel for implementation.

Design of lamina orientation for biaxially loaded off-axis tunnelling cracks

The primary objective of this work is to demonstrate how the condition for biaxially loaded off-axis tunnelling crack growth can be reproduced in a modified laminate under uniaxial loading. More specifically, laminates with the stacking sequences [0/θ/0/-θ] s and [0/α/0/-α] s have been considered for the biaxial and uniaxial loadings, respectively. A steady-state crack growth condition is analysed for an isolated off-axis tunnelling crack. The crack tip stress state is expressed in terms of a through thickness averaged mode mixity and energy release rate. The presented framework uses an energy accounting method with the crack opening displacements extracted from a finite element analysis. Results shows that the tunnelling crack tip stress state of the modified laminate under uniaxial loading matches with the biaxially loaded laminate. With a modified off-axis lamina orientation and the additional load amplitude from the biaxially loaded laminate, a simplified uniaxial testing laminate can be designed that yields the same biaxial tunnelling crack tip conditions.

Study of quasi-static crack growth in aluminum/alumina functionally graded material using crack gauge

Quasi static crack growth of Al/Al2O3 functionally graded materials prepared through powder metallurgy up to 30% weight fraction of Al2O3 is studied with crack on stiff (FGM I) and compliant (FGM II) sides using crack gauge in three point bend test. The variation of weight fraction of Al2O3 with distance from 100% aluminum face is measured through image analyzers to identify the composition corresponding to crack tip location. The measurement of crack tip location, load point displacement and load with time are carried out simultaneously. It is observed that crack grows stably when the crack traverses from stiff to compliant side and unstably from Compliant to stiff side. For stable quasi static crack growth (crack on stiff side), variation of the fracture toughness and crack tip opening displacement (CTOD) are evaluated and increase with distance from stiff side. Further, the fracture toughness is also evaluated using Raveendran’s micromechanics prediction techniques and found that the experimental values are closer to Raveendran’s lower bound for greater weight fraction of alumina and closer to Raveendran’s upper bound for lower ceramic weight fraction. From fractography, mixed mode failure with greater deviation of crack path for FGM I is observed as compared to mode-I failure for FGM II.

Environmental cracking performance of new generation thick plate 7000-T7x series alloys in humid air

The hydrogen environmentally-assisted cracking (HEAC) behaviour of new generation (new-gen) Al-Zn-Mg-Cu thick-plate alloys (AA7449, AA7085, and AA7037), under accelerated humid warm-air exposure, has been compared to the aerospace industry benchmark alloy AA7050 in over-aged T7x type tempers. Constant load time-to-failure and DCB crack growth tests have been performed at a relative humidity of 85 %, at 70 °C. It has been found that in constant load tests the failure time of the new generation alloys can be reduced by at least an order of magnitude and in some cases cracking was observed in only a few days at stress levels of 85 % of yield. Analysis of the failed fractured specimens has confirmed that the fracture behaviour is predominantly intergranular, and highly localised to high angle grain boundaries. The progression from surface initiation to self-propagating cracks occurred much more rapidly in the new higher Zn content alloys, but with less prior surface chemical attack. In DCB v-K tests stage II crack growth rates were also found to be significantly higher (by a factor of 6–20 times) in the new-gen alloys and the K1HEAC threshold stress intensity was less than half that for AA7050-T7651. Under long crack growth conditions, propagation thus required a higher mechanical driving force and crack growth stagnated more readily in the AA7050-T7651 benchmark material, compared to in the new generation alloys. The results have been discussed in the context of the potential influences of differences in the alloy’s chemistries and microstructures.

Stress intensity factors for mixed-mode crack growth in imitation models under biaxial loading

In this study a procedure to stress intensity factors calculation for imitation models made of titanium alloy is described. In service fatigue cracks are detected in a disk and blade “dovetail type” attachment. On the base of this attachment dimensions and with taking into account the biaxial loading conditions of rotating compressor disk the two geometries of imitation model of gas turbine engine compressor disk are developed. To accurate verification of biaxial loading conditions, the first imitation model of constant thickness is used. In order to fully reproduce the geometry of the compressor disk and conditions of mixed mode crack growth, the second imitation model with reduced cross section is proposed. The fatigue crack growth experiments of imitation models were carried out at room temperature on a biaxial testing machine. Two different stress ratio values are applied several times to each imitation model in order to fix the experimental crack front positions. The elastic and plastic stress intensity factors used for the representation of the experimental results are computed by using full-size 3D Finite Element analysis of the imitation models with surface quarter elliptical and through-thickness cracks.

Crack growth behavior of a biomedical polymer-ceramic interpenetrating scaffolds composite in the subcritical regimen

We subjected a commercial dental composite formed by interconnecting polymer-ceramic scaffolds to extensive quasi-static, static and cyclic experiments under biaxial flexure. By this we meant to obtain static and cyclic subcritical crack growth exponents that, based on established relationships describing the degradation of frictional bridging mechanisms, challenging the notion of a suggested R-curve behavior. By exploring the fracture statistics of specimens with increasing effective volumes and effective areas, we demonstrated the presence of a bi-modal defect size distribution in disaccord with the Weibull behavior across length scales. Lifetime distributions seemed to follow the strength distributions, and were used to derive crack growth velocity diagrams for combined levels of applied stress. Ultimately, the claim of an R-curve behavior could not be supported based on the absence of any significant cyclic fatigue effect, i.e. bridging degradation.

Environmentally Assisted Cracking of Subsea Pipelines in Oil & Gas Production Environments—Effect of Static Loading

Fatigue crack growth rate of line pipe steels in sour environments typically exhibits a steady-state value at low frequencies. However, in highly inhibited sour environments, there is no evidence of a steady-state fatigue crack growth at low frequencies. This is likely a result of static crack growth rate at Kmax. Stable static crack growth measured under constant stress intensity factor (K) conditions in inhibited sour environments was in the range of 10−7 mm/s to 10−8 mm/s. The crack growth rate in inhibited sour environments is likely associated with crack tip processes associated with metal dissolution/film formation and associated hydrogen evolution. The results obtained were modeled based on a crack tip strain rate based approach, where the rate limiting step was the metal dissolution/FeS formation and the corresponding hydrogen generation reaction.

Poroelastic effects on steady state crack growth in polymer gels under plane stress

Fracture experiments on polymer gels are often conducted with thin specimens, which are close to plane stress in two-dimensional models. However, many of the previous theoretical and numerical studies on fracture of polymer gels have assumed plane strain conditions. The subtle differences between the plane stress and plane strain conditions are elucidated in this paper based on a linear poroelastic formulation for polymer gels, including the asymptotic crack-tip fields and finite element simulations of steady-state crack growth in long strip specimens. Moreover, a poroelastic cohesive zone model is adopted to study the rate-dependent fracture process of polymer gels. It is found that, without the cohesive zone model, the normalized crack-tip energy release rate at the fast crack limit is greater than the slow crack limit, suggesting reduced poroelastic toughening for fast crack growth under plane stress conditions, while the two limits are identical under plane strain conditions. With a solvent-permeable cohesive zone for the case of immersed specimens, solvent diffusion within the cohesive zone enhances the poroelastic toughening significantly as the crack speed increases, leading to a rate-dependent traction-separation relation. On the other hand, with no solvent diffusion in the cohesive zone for the not-immersed case, the poroelastic toughening effect diminishes as the crack speed increases. Based on the present study, the intrinsic steady-state fracture toughness of a poroelastic gel can be determined using long-strip pure-shear specimens, which in general is smaller than the applied energy release rate.

Development of a Novel Test Method to Characterize Material Properties in Corrosive Environments for Subsea HPHT Design

Abstract High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. Fatigue endurance (i.e., S–N) and fracture mechanics design are both permitted, depending on the criticality of the component. Both design approaches require material properties generated in corrosive environments, such as seawater with cathodic protection and/or sour production fluids. In particular, it is necessary to understand sensitivity to cyclic loading frequency (for both design approaches), crack growth rates (CGR) (for fracture mechanics approach) as well as fracture toughness performance. For many subsea components, the primary source of fatigue loading is associated with the start-up and subsequent shutdown operation of the well, with long hold periods in-between, during which static crack growth (CG) could occur. These are the two damage modes of most interest when performing a fracture mechanics based analysis. This paper presents the preliminary results of a novel single specimen test method that was developed to provide fatigue crack growth rate (FCGR) and fracture toughness data in corrosive environments, in a timeframe that is compatible with subsea HPHT development projects. Test data generated on alloy 625+ in seawater with cathodic protection are presented along with a description of how the test method was developed. A crack tip strain rate based formulation was applied to the data to rationalize the effect of frequency, stress intensity factor range (ΔK), and maximum stress intensity factor (Kmax).

A Mathematical Model for Root Canal Preparation Using Endodontic File: Part B

The aim of this article is to develop the mathematical model to describe response of endodontic file in curved root canal during the preparation of root canal using fracture mechanics approach. Any obturation process involves the filling of prepared root canal using bio-compatible materials like gutta-percha. During preparation of infected root canal, substrate, dead tissue and pulp is removed and tapered shape is formed so that any practitioner can fill it effectively. During preparation process of root canal, the canal wall applies locking action and causes resistance to motion of endodontic file, finally resulting into fatigue failure. This article described plastic behavior of endodontic file in curved root canal, mathematical model describing the necessary condition for crack growth in endodontic file, a mathematical model describing plastic zone size for crack on surface of endodontic file, a mathematical model describing crack tip opening displacement for crack on surface of endodontic file and mathematical model for pure-combined torsion and bending consideration for design of endodontic file. The mathematical model described is helpful for the Endodontic experts, researchers, design engineers. However the applicability of the described mathematical model limited to assumption of study. The gap between root canal to be prepared and endodontic file is zero while preparation. The endodontic file weight, speed of rotation and substrate removal rate is assumed to be constant. The mathematical model for endodontic file discussed above proved to be efficient tool for studying the root canal preparation.

Fatigue and Static Crack Growth Rate Study of Carbon Steel for Corrosion Prevention of Natural Gas Transmission Pipelines

Internal metallic and non-metallic coatings are being considered as parts of answers to allow current natural gas transmission pipelines to safely batch transport a variety of products, including natural gas, carbon dioxide and hydrogen. Sacrificial anode coatings, that contain Zn, are being considered for internal corrosion protection of standard carbon steel pipeline as they are known to galvanically protect steel. However, this can lead to the creation and ingress of hydrogen into the line pipe material, resulting in increased susceptibility to cracking under high static and cyclic loads associated with both steady state as well as startup and shut down conditions. The current study simulates the conditions expected to be encountered in field by performing crack growth rate measurements under static and fatigue loading conditions on fracture mechanics-based specimen extracted from API 5L X-65 steel. The effect of cyclic loading frequency on fatigue crack growth rate (FCGR) and the effect of applied cathodic polarization on static crack growth rates (SCGR) was investigated in a 3.5% NaCl solution in contact with CO2 at 25°C. The partial pressure of CO2 (pCO2) was 101.325 kPa. The applied cathodic potentials were those associated with mixed potential of the Zn- based coatings.

Crack path and threshold condition for small fatigue crack growth in annealed carbon steels under fully-reversed torsional loading

Torsional fatigue tests at an R of −1 were carried out near the fatigue limit, using smooth, round-bar specimens of annealed carbon steels with banded ferrite/pearlite and isotropic ferrite/pearlite structures. Precise polishing of the specimens made it possible to observe the interior geometry of fatigue cracks. While the crack-plane was macroscopically of shear-mode, it was irregular microscopically, bearing many kinked and branched cracks. Based on calculation of the stress intensity factor for propagating and non-propagating cracks, it was concluded that the torsional fatigue limit was determined by the threshold condition for continuous Mode I propagation into the ferrite/pearlite structure.

Investigation of static and fatigue crack growth mechanism of borated stainless steel using in situ observation method

Investigation of static and fatigue crack growth mechanism of borated stainless steel using in situ observation method

A simplified procedure to prediction of ductile failure modes of a SA333 Gr.6 carbon steel pipe: Fracture instability and Plastic collapse condition

Abstract Ductile failure assessments of circumferentially through walled cracked piping system are an important phenomenon for safety analysis of nuclear power plant. The present work is focused in this direction. The common failure modes of failure of components made of ductile materials are fracture instability (uncontrolled crack growth) and plastic collapse. In the current study, both the failure modes are considered in failure assessment of primary heat transport pipe of PHWR. The material of the piping system is SA333 Gr.6 carbon steel which has a large extent of ductility and thereby, the chances of plastic collapse are observed in certain conditions. The material J-R curve (J - Δa curve) for the material SA 333 carbon steel is determined using a CT specimen. A simplified method for determination of J – R curve is proposed. It is found that the J-R curve obtained by the proposed method is suitable for steady crack growth conditions and thereby, suitable for pipe fracture analysis where the extent of crack growth is large. Straight pipe with different crack angles are modeled in FE analysis under pure bending load. Applied moment versus J (Japp) curve is obtained from FE analysis for different crack angles. J is calculated using 3D version of Rice’s J-integral using domain integration method. Finally instability diagram for pipe is constructed using applied “Japp” vs semi crack angle with applied moments as parameters. Material J-R curve for the material SA333 Gr.6 carbon steel, determined by using CT specimen, is superimposed in this diagram to find the instability condition. This diagram also shows the condition of plastic collapse. Finally a failure chart is tabulated for various initial crack angles and applied moments.

On the theoretical modeling of fatigue crack growth

Although fatigue is by far the most common mode of failure of structural materials, mechanistic understanding is still lacking. For example, the fundamental Paris law which relates the crack growth rate to stress-intensity factor range is still phenomenological and no reliable mechanistic model has been established for a given material or class of materials despite numerous investigations over a half a century. This work is an attempt to theoretically model fatigue crack propagation induced by alternating crack-tip plastic blunting and re-sharpening in the mid-range of growth rates on the basis of inputs from experiments that measure macroscopic material behavior, e.g., response to uniaxial cycling loading. In particular, we attempt to predict Paris law behavior by accounting for the material constitutive behavior in response to cyclic loading by modeling crack advance solely in terms of the underlying plastic dissipation. We obtain the steady-state condition for crack growth based on plastic dissipation, numerically using finite element analysis, which involves a methodology to address plastic closure upon unloading. For a given stress-intensity range, we calculate the crack propagation rate from the steady-state condition through each cycle of loading and unloading of a cracked compact-tension specimen, without resorting to any specific criterion for crack advance.