Center-cracked Tension Research Articles

Computational evaluation of resistance of fracture capacity for SUS304L of liquefied natural gas insulation system under cryogenic temperatures using ABAQUS user-defined material subroutine

A computational method for evaluating the resistance of fracture capacity of austenitic stainless steel (SUS304L) under ambient and cryogenic temperatures was proposed in the present study. To do this, a unified elasto-viscoplastic-damage model which can take into consideration the material nonlinearity under cryogenic temperature was adopted and implicitly formulated. In addition, the developed model is implemented into ABAQUS as a type of user-defined material subroutine UMAT. In order to verify the proposed method, the simulation results of resistance of fracture capacity are compared to a series of crack tests such as double-edge-cracked and center-cracked tension tests of SUS304L plates under ambient and cryogenic temperatures. Namely, the relationships between force and displacement as well as allowable crack length and allowable stress regarding to room and cryogenic temperatures are evaluate/predicted using the proposed method.

Chromate inhibition of environmentally assisted fatigue crack propagation of aluminium alloy AA 2024-T3 in 3.5% NaCl solution

Aluminium alloy AA 2024-T3 has been used in aircraft construction for over the years due to its high strength to weight ratio, extremely good damage tolerance and high resistance to fatigue crack propagation. However, this material is susceptible to environmetally assisted fatigue crack propagation (EAFCP) when it is exposed to water vapour and chloride solution. In aerospace applications, a control of EAFCP is usually carried out by a protective coating system that contains a corrosion inhibitor. In the present investigation, the behaviour of EAFCP of aluminium alloy AA 2024-T3 in 3.5% NaCl solution containing various levels of chromate (Cr2O42-) inhibitor, typically 0.1%, 0.3% and 0.5% has been studied using centre-cracked tension (CCT) specimens in the longitudinal–transverse (LT) orientation. The fatigue experiment was carried out using a constant amplitude with stress ratio (R) of 0.1 and frequency of 11Hz. The nature of chromate inhibitor in chloride environment was studied using electrochemical polarisation method in combination with optical microscopy and SEM equipped with EDX-spectrometer. Results of this investigation show that the additions of chromate inhibitor to the bulk 3.5% NaCl solution tend to reduce corrosion rate, increase passivity as indicated by the presence of anodic passivation plateaus in potentiodynamic polarisation curves and lower fatigue crack propagation rate (da/dN) of AA 2024-T3 at low ΔK. The mechanism of chromate inhibition on AA 2024-T3 aluminium alloy is complex involving mechanical, chemical and microstructural interactions and is discussed in the present investigation.

A Comparative Evaluation of Fatigue and Fracture Characteristics of Structural Components of Liquefied Natural Gas Carrier Insulation System

This study examined the fatigue strength and fracture toughness of the structural components of membrane type liquefied natural gas carrier (LNGC) insulation systems, such as reinforced poly-urethane foam (R-PUF, insulation material) and 304 L stainless steel (STS 304 L, Primary barrier membrane), at both ambient and cryogenic temperatures. The fatigue strength of the LNGC insulation system was compared with that of low density R-PUF (130 kg/m3) and high density R-PUF (210 kg/m3). The fracture toughness of R-PUF and STS 304 L was investigated in terms of the density effect of R-PUF and the difference in the nickel composition of STS 304 L, STS 304 L (10.2%Ni) versus STS 304 L (9.4%Ni) at both ambient and cryogenic temperatures. In this study, the high density R-PUF (210 kg/m3) and STS 304 L (9.4%Ni) were proposed to improve the structural strength of the LNGC insulation system and reduce the cost. The fracture toughness was characterized in terms of the critical strain energy release rate (GIC) in the context of linear elastic fracture mechanics (LEFM). The geometries of the fracture toughness test used were the center-cracked tension (CCT) and double-edge-cracked tension (DECT) specimens according to ASTM STP381 standard.

Fatigue crack growth rate behaviour of friction-stir aluminium alloy AA2024-T3 welds under transient thermal tensioning

Friction stir welding (FSW) has become a serious candidate technology to join metallic fuselage panels for the next generation of civil aircrafts. However, residual stress introduced during welding which subsequently affects fatigue performance is still a major problem that needs to be paid attention. The present investigation aims to improve fatigue crack growth resistance of friction stir aluminium alloy AA2024-T3 welds using transient thermal tensioning (TTT) treatment. In this investigation, aluminium alloy AA2024-T3 plates were joined using FSW process with and without TTT. The welding parameters used including tool rotation speed (Rt) and the plate travelling speed (v) were 1450rpm and 30mm/min respectively. The TTT treatments were carried out by heating both sides of friction stir weld line using moving electric heaters ahead of, beside and behind the tool at a heating temperature of 200°C. Subsequently, a sequence of tests was carried out including microstructural examination, hardness measurement, tensile test and fatigue crack growth rate (FCGR) test in combination with fractography using scanning electron microscopy (SEM). The FCGR test was carried out using a constant amplitude fatigue experiment with stress ratio (R) of 0.1 and frequency (f) of 11Hz whereas specimens used were centre-crack tension (CCT) type with the initial crack located at the weld nugget. Results of this investigation showed that at low ΔK, typically below 9MPam0.5, the friction stir welds under TTT treatments lowered fatigue crack growth rate (da/dN) and the lowest (da/dN) was achieved as the heaters were located ahead of the tool. This improved weld fatigue performance was associated with stretching effect generated by movingly localised secondary heating which might alter the magnitude and distribution of residual stress in weld region and in a such condition, TTT seemed to act as local preheating.

Validation of a finite element model for fracture mechanics specimens

Single parameter formulations have shown to be insufficient to describe constraint effects infracture mechanics specimens. This has lead researchers to a two parameter approach like the J-Q theory.In order to investigate constraint effects, the authors have developed a generic finite element model. Priorto drawing conclusions this model must first be validated, which is the topic of this paper. This validationhas been done by comparing analytical expressions of the J-integral with those obtained from theperformed simulations. The compared geometries were center cracked tension (CCT) and double edgenotched tension (DENT) fracture mechanics specimens. The results showed good agreement with theanalytical expressions and, as such, the model can now be confidently applied to determine values of the Jintegral. This is a first step towards evaluating two parameter J-Q constraint.

Numerical Simulation of Crack Tip Behavior under Fatigue Loading

Fatigue damage is a localized phenomenon controlled by the near-tip crack behavior. This paper presents an application of a dislocation distribution technique to the simulation of crack tip behavior under fatigue loading. A centre-cracked tension specimen under uni-axial fatigue loading is used in the study. Crack opening and plastic deformation around the crack tip are simulated by distributions of dislocation dipoles in crack plane and four inclined planes ahead of the crack tip. Climb dislocation dipole is used to model the opening and closing of the crack while glide dislocation dipole is used to simulate the backward and forward slip in the inclined planes during loading and unloading of the fatigue cycle. Stress field around the crack tip is obtained by the superposition of the contributions of the applied external load and the distributed dislocation dipoles. Correct boundary conditions of the model are achieved by employing a quadratic programming technique to minimize a properly constructed non-negative object function. It is found that the simulated crack closure variations under the constant amplitude fatigue load agree well with the result of a previously developed modified strip yield model with an appropriate constraint factor.

Application of split Hopkinson tension bar technique to the study of dynamic fracture properties of materials

A novel approach is proposed in determining dynamic fracture toughness (DFT) of high strength steel, using the split Hopkinson tension bar (SHTB) apparatus, combined with a hybrid experimental-numerical method. The center-cracked tension specimen is connected between the bars with a specially designed fixture device. The fracture initiation time is measured by the strain gage method, and dynamic stress intensity factors (DSIF) are obtained with the aid of 3D finite element analysis (FEA). In this approach, the dimensions of the specimen are not restricted by the connection strength or the stress-state equilibrium conditions, and hence plane strain state can be attained conveniently at the crack tip. Through comparison between the obtained results and those in open publication, it is concluded that the experimental data are valid, and the method proposed here is reliable. The validity of the obtained DFT is checked with the ASTM criteria, and fracture surfaces are examined at the end of paper.

Effect of Grain Size on Fatigue Crack Propagation in Extruded AZ31 Magnesium Alloys

This paper describes the fatigue crack propagation behavior of extruded AZ31B magnesium alloys (average grain size: approximately 15 and 119 μm, respectively). Fatigue crack propagation tests were performed on center cracked tension (CCT) specimens at a stress ratio of R=0.1 and a frequency of 10 Hz at room temperature. Loading axis was parallel to the extrusion direction; crack face was perpendicular to basal plane of each grain. The crack growth rate (da/dN) of the coarse-grained specimen was approximately 5 times higher than that of the fine-grained specimen. Fracture surfaces of the fine-grained and coarse-grained specimens showed various directional steps independent of macroscopic crack growth direction.

Characterization of three-dimensional crack border fields in creeping solids

Creep fracture of solids at high temperature is vital to applications of many advanced materials, but most of the previous works are performed within the frame of two-dimensional theory. By using the out-of-plane stress constraint factor T z , here we derive out three-dimensional asymptotic fields near the border of mode-I through-thickness cracks in power law creeping solids. It is found that the asymptotic fields near the crack border are dominated by both T z and C( t) integral. Detailed finite element analyses are carefully performed for single-edge cracked specimens and centre-cracked tension specimens to investigate the dominance of the asymptotic solution for the crack border fields. It is shown that the C( t) − T z description based on the obtained three-dimensional asymptotic solution can provide efficient prediction for the tensile stress ahead of the crack front under small scale creep condition. Under extensive creep conditions, a third parameter Q ∗ should be introduced to take into account of the loss in the in-plane constraint caused by in-plane geometries and loading configuration at extensive creeping, and a three-parameter C( t) − T z − Q ∗ description is proposed and proven to be efficient to predict the tensile stress on the ligament ahead of the crack for both specimens. Therefore, the two-parameter C( t) − T z and three-parameter C( t) − T z − Q ∗ descriptions can provide advanced theoretical basis for small and extensive creeping fracture assessments, respectively.

SIFs of CCT plate repaired with single‐sided composite patch

ABSTRACTConstant amplitude load fatigue tests are performed to obtain crack propagation data for LF2‐aluminium centre crack tension (CCT) plates un‐repaired and repaired with single‐sided composite patches. Then, the James–Anderson method, an experimental method, is used to obtain the stress intensity factor (SIF) formula for the repaired CCT plates with carbon–fibre composite patches. At last, crack propagation life prediction and finite element (FE) calculation are carried out to validate the experimental SIF formula. It is shown that the present SIF formula can exactly predict the fatigue‐crack propagation life of the patched CCT plates and is close to the FE results, which implies the effectivity of the experimental SIF formula in the present paper.

Delamination in Fiber Metal Laminates (GLARE) during fatigue crack growth under variable amplitude loading

This paper presents a study on the influence that load variations have on delamination shapes in Fibre Metal Laminates. Previously fatigue tested centre-crack tension specimens have been chemically etched to obtain the final delamination shapes. In addition, fatigue crack growth tests on similar specimens have been performed to investigate the formation of delamination shapes. For this purpose, digital image correlation has been used as strain measurement technique to record the delamination shapes in situ testing. An explanation is put-forward in order to understand the effects of variable amplitude loading on the formation of delamination shapes. A transition in delamination shapes was observed, but evaluating this observation using an analytical fatigue crack growth prediction model including the observed change in delamination shapes revealed no significant effect on subsequent crack growth.

OS2019 多結晶マグネシウム合金の疲労き裂進展挙動に及ぼすミクロ組織の影響(OS20-4 疲労と強度,OS-20 HCP金属の実験力学と計算力学)

This paper describes the fatigue crack propagation behavior of extruded AZ31B magnesium alloys (average grain size: approximately 15 and 119 μm, respectively). Fatigue crack propagation tests were performed on center cracked tension (CCT) specimens at a stress ratio of R=0.1 and a frequency of 10 Hz at room temperature. Loading axis was parallel to the extrusion direction; crack face was perpendicular to basal plane of each grain. The crack growth rate (da/dN) of the coarse-grained specimen was approximately 5 times higher than that of the fine-grained specimen. Fracture surfaces of the fine-grained and coarse-grained specimens showed various directional steps independent of macroscopic crack growth direction.

Crystallographic Orientation Dependence of Fatigue Crack Propagation Behavior in Rolled AZ31 Magnesium Alloy

This paper deals with fatigue crack propagation behavior of rolled AZ31B magnesium alloy. Two types of specimens with the loading axis parallel to rolling direction were machined; fatigue crack propagation direction was parallel to transverse direction (L-T specimen), and short transverse direction (L-S specimen). Fatigue crack propagation tests were performed with center cracked plate tension specimen with stress ratio R=0.1 and frequency of 10Hz at room temperature. Crack propagation rate of L-T specimen was approximately 10 times higher than that of L-S specimen. SEM-EBSD observations revealed that the c-axis direction is unfavorable for the fatigue crack propagation in textured polycrystalline magnesium alloy.

Crack observation methods, their application and simulation of curved fatigue crack growth

Abstract The observation of cracks with curved crack path has to be done optically. It is shown that a modified commercial flat bed scanner and a combination of a high-resolution scanner camera system with 10,000 × 10,000 pixels with a telecentric lens are appropriate for high-resolution (up to 8 μm/pixel) optical recording of multiple crack ends on sample areas up to 210 mm × 290 mm. The high-resolution photographs are suitable for determination of crack lengths. It is also possible to observe crack paths or geometrical crack tip parameters and strain fields by image correlation. The method is used to determine static crack resistance and cyclic crack growth curves on center crack tension and biaxial cruciform samples. Furthermore, the paper presents an improved finite element technique for the simulation of curved fatigue crack growth in a multiple arbitrarily pre-cracked isotropic sheet under biaxial plane stress loading applying a predictor–corrector procedure in combination with the modified virtual crack closure integral (MVCCI) method including the consideration of the plastic limit loads. For this, the program PCCS-2D was extended to analyse the crack growth and the plastic limit load for each crack propagation step in a fully automatic simulation. The proposed solution algorithm provides a powerful tool for flaw assessment with the failure assessment diagram procedure in combination with a numerical crack path simulation. Finally, the simulation is verified experimentally.

Application of a modified Wheeler model to predict fatigue crack growth in Fibre Metal Laminates under variable amplitude loading

Abstract This paper presents the investigation regarding fatigue crack growth prediction in Fibre Metal Laminates under variable amplitude fatigue loading. A recently developed constant amplitude analytical prediction model for Fibre Metal Laminates has been extended to predict fatigue crack growth under variable amplitude loading using the modified Wheeler model based on the Irwin crack-tip plasticity correction and effective stress intensity factor range (ΔKeff). The fatigue crack growth predictions made with this model have been compared with crack growth tests on GLARE center-cracked tension specimens under selective variable amplitude loading as well as flight simulation loading. The accuracy of the model is discussed in comparison with the experimental fatigue crack growth data.

Fatigue crack growth behaviour of Inconel 718 with high temperature hold times

In this work, fatigue crack growth measurements have been made on center-cracked tension specimens of Inconel 718, where the focus has been to observe the effect of high temperature hold times on the fatigue crack growth behaviour of the material. The material testing has been done at three different temperatures, namely 450 °C, 550 °C and 650 °C. All testing were done in an isothermal LCF context with a standard test method for measuring the fatigue crack growth rates.

Linear Damage Accumulation for Predicting Fatigue in Fiber Metal Laminates

DOI: 10.2514/1.42486 This paper presents the experimental and analytical research on the applicability of the linear damage accumulation approach for fatigue crack growth in fiber metal laminates under variable amplitude loading. A recently developed constant amplitude analytical prediction model for fiber metal laminates has been extended to predict fatigue crack growth under variable amplitude loading using a linear damage accumulation rule. The modified model has been compared with crack growth tests on fiber metal laminates center-cracked tension specimen. In the end, it is discussed to what extent or under which conditions the linear damage accumulation predictions are sufficiently accurate for fiber metal laminates structures. Nomenclature a = total crack length aD = delay distance ai = current crack growth increment a0 = initial/total crack length of previous loading cycle Ccg, ncg = crack growth relation constants Kbridging = bridging stress intensity factor Kfar-field = far-field stress intensity factor Ktip = crack-tip stress intensity factor N = number of loading cycles ND = number of delay cycles NOL = number of overload cycles Smax = maximum stress magnitude SOL = overload stress magnitude R = stress ratio RKtip = crack-tip stress ratio ROL = overload stress ratio � Keff = effective stress intensity factor range

The Assessment of Residual Stress Effects on Ductile Tearing Using Continuum Damage Mechanics

This paper presents the results of a numerical study undertaken to assess the influence of residual stresses on the ductile tearing behavior of a high strength low toughness aluminum alloy. The Gurson–Tvergaard model was calibrated against conventional fracture toughness data using parameters relating to void nucleation, growth, and coalescence. The calibrated model was used to predict the load versus ductile tearing behavior of a series of full-scale and quarter-scale wide-plate tests. These center-cracked tension tests included specimens that contained a self-balancing residual stress field that was tensile in the region of the through-wall crack. Analyses of the full-scale wide-plate tests indicated that the model provides a good prediction of the load versus the ductile tearing behavior up to approximately 3mm of stable tearing. The influence of residual stress on the load versus the crack growth behavior was accurately simulated. Predictions of the load versus the crack growth behavior of full-scale wide-plate tests for crack extensions greater than 3mm and of the quarter-scale tests were low in terms of predicted load at a given amount of tearing. This was considered to result from (i) the “valid” calibration range in terms of specimen thickness and crack extension, (ii) the development of shear lips, and (iii) the differences in the micromechanism of ductile void formation under plane strain and under plane stress conditions.

Reliability Analysis of Corrosion Fatigue Crack Growth for 2024-T3 Aluminum Alloy

Fatigue crack growth rate experiments of center-cracked tension (CCT) specimens of the 2024-T3 aluminum alloy under constant-amplitude load in corrosive environment are carried out with 3 kinds of loading frequency. The fatigue crack growth rates in 3.5% NaCl solution are obtained by using seven-point incremental polynomial method. A probabilistic approach is presented for fatigue crack growth rate in corrosive environment with log-normal random variable model. The reliability analysis of crack propagation is conducted based on the experimental data. The crack exceedance probability at given service time and the distribution of the service time at given crack size are obtained by using the reliability analysis approach. The effect of loading frequency on crack propagation is studied. It is shown that the fatigue crack growth rate is increasing with loading frequency decreasing in corrosive environment. The predicted results by the presented method match the experimental results very well.

Liquid metal embrittlement of an austenitic 316L type and a ferritic–martensitic T91 type steel by mercury

The susceptibility to liquid metal embrittlement (LME) of 316L and T91 steels by mercury has been studied at room temperature. A dedicated experimental device using center crack tension (CCT) specimens was built. We developed a specimen preparation procedure that must be rigorously applied in order to investigate the embrittling effect of Hg. The high strength ferritic–martensitic steel of type T91 is embrittled by Hg at room temperature over a large range of crosshead speeds, between 6.67 × 10 −7 and 6.67 × 10 −3 m s −1. More surprisingly, the austenitic steel of type 316L is also embrittled by Hg between 1.67 × 10 −8 and 2.5 × 10 −4 m s −1. The fracture of the T91 and 316L CCT specimens in contact with Hg occurs by shear band decohesion over the above-mentioned range of crosshead speeds.