Middle-crack Tension Research Articles

Effects of shot peening with different coverage on surface integrity and fatigue crack growth properties of 7B50-T7751 aluminum alloy

Engineering design methods based on the assumption of crack growth and the application of fracture mechanics principles are commonly used in aerospace engineering for the design of aluminum structures. In aerospace manufacturing, surface shot peening is one of the most widely used techniques for improving the fatigue performance of structural components. This work reviews the effect of different shot peening coverage on surface integrity and fatigue crack growth (FCG) properties of 7B50-T7751 aluminum alloy. The shot peening was applied to middle-crack tension (MT) specimens with four kinds of peening coverage: 100%, 300%, 600% and 1000%, respectively. The results showed that shot peening treatment reduced specimen surface roughness, moreover, surface shot peening promoted a maximum microhardness increased by 28.1% and induced compressive residual stresses in order of −412 MPa compared to untreated specimens. FCG experiment results showed that the effect of shot peening coverage on da/dN-ΔK curves was quite limited at constant amplitude loading. With the increase of shot peening coverage from 100% to 1000%, the FCG rate decreases first and then increase. At 300% of surface shot peening coverage, the FCG rate is the lowest, which results in the longest fatigue crack growth life.

Suitability Of Standard Fracture Test Specimens For Low Constraint Conditions

Test specimen selection plays a vital role in successful transformation of laboratory fracture parameter findings to the real fracture behavior of any component. The ASTM and BSI Standard test methods for measurement of fracture toughness are dominated by high constraint specimens like Compact Tension (C(T)) and Single Edge Notch Bend (SEN(B)). However, most cracks in real time structure are shallow and surrounded by low constraint. Hence, the standard specimen like Middle Crack Tension (M(T)) in ASTM and Clamped Single Edge Notch Tension (Clamped SEN(T)) specimen in BSI, yield fracture toughness value to be test specimen dependent. The Clamped SEN(T) specimen suitable for various real pipeline (low constraint conditions) application has been widely reported in literature, that however in not the case for M(T) specimen which has scarcely reported. The 3D numerical analysis of SEN(T) and M(T) specimens with an identical flaw size for fracture toughness and constraint parameters (T-stress) under linear-elastic conditions are dealt in this article. The standard specimen that emulates low constraint fracture behaviour forms the basis for recommending suitable specimen for defined real time conditions. The conservatism associated with usage of standard specimen to find fracture parameters for an application has been quantified in this work for field implementation by practitioners in fracture mechanics.

Numerical Study on the Variability of Plastic CTOD.

This paper presents a numerical study on the influence of material parameters and loading variability in the plastic crack tip opening displacement (CTOD) results. For this purpose, AA7050-T6 was selected as reference material and a middle-cracked tension specimen geometry was considered. The studied input parameters were the Young’s modulus, Poisson’s ratio, isotropic and kinematic hardening parameters and the maximum and minimum applied loads. The variability of the input parameters follows a Gaussian distribution. First, screening design-of-experiments were performed to identify the most influential parameters. Two types of screening designs were considered: one-factor-at-a-time and fractional factorial designs. Three analysis criteria were adopted, based on: main effect, index of influence and analysis of variance. Afterwards, metamodels were constructed to establish relationships between the most influential parameters and the plastic crack tip opening displacement (CTOD) range, based on two types of designs: Face-Centered Central Composite Design and Box-Behnken design. Finally, the metamodels were validated, enabling the expeditious evaluation of the variability in the plastic CTOD range; in addition, the variability in the fatigue crack growth rate was also evaluated.

FE analysis of strain constraint around the crack tip under cyclic loading

PurposeThe purpose of this paper is to deal with the FE analysis of strain constraint around the crack tip under cyclic loading and its utilization using crack growth prediction strip yield model (SYM). During cycling, the constraint develops based on the load history. The monotonic loading is analyzed mostly, but during cyclic loading the conditions are different. The constraint is analyzed after several loading cycles applied in upwards part of the cycle and the formula for its development is proposed.Design/methodology/approachThe study is based on the 3D FE analysis of middle-cracked tension specimen M(T). The strain constraint is described by Newman’s factor α. The variability of constraint factor α was analyzed for several load levels and specimen thicknesses. The crack is considered as non-propagating with straight crack front. The material is modelled as elastic-perfectly plastic. The SYM is modified by implementing variable constraint and the experimental results are compared with the simulation.FindingsIn major part of the loading cycle, it was found by FE analysis, that the constraint factor αg is lower after overloads than when creating monotonic plastic deformation on the same load level. The value of αg is governed by the ratio of thickness B over the plastic zone size rp. By implementing the variable constraint factor into the SYM, the improvement of the predicted specimens lives under variable amplitude loading was shown.Originality/valueThe new phenomenon on the variability of strain constraint during cyclic loading is presented. The development of constraint factor αg during cyclic loading is different from the monotonic loading and should be accordingly implemented into prediction models.

A modified model of residual strength prediction for metal plates with through-thickness cracks

A new model, accurate and easy-to-apply, was proposed to predict the residual strength for the metal plates intact or with different damage degrees. In this model, we introduced the damage degree factor (DDF) to quantify the initial damage condition of a plate. The middle crack tension tests and multiple site damage (MSD) tension tests were performed on plate specimens in aluminum alloy LY12-CZ and 2524-T3 respectively. For various damage degrees, the predicted results of this new model show an improved correlation with test results compared to net section yield criterion, K-apparent criterion and Duong’s method (Duong et al., 2001).

Fatigue crack growth in a laser shock peened residual stress field

Laser Shock Peening is a surface treatment technique used in the aerospace sector to increase fatigue life, as well as resistance to fretting fatigue and stress corrosion cracking. In this study, laser shock peening was applied to a 6-mm-thick middle-crack tension specimen made of aluminium 2524-T351. Residual stress was measured with neutron diffraction and the contour method, along the predicted crack path prior to fatigue testing. Fatigue crack growth test results showed that fatigue life improved by a factor of 4 compared to an untreated component, owing to a significant crack growth rate reduction inside the laser peened area. A linear-elastic finite-element crack growth prediction model was also developed, obtaining predicted results in excellent agreement with the experimental data.

Fatigue-crack growth in two aluminum alloys and crack-closure analyses under constant-amplitude and spectrum loading

Fatigue-crack-growth tests were conducted on compact, C(T), and middle-crack tension, M(T), specimens made of 7075-T6 and 7249-T76511 aluminum alloys. The 7075 tests were conducted on thin-sheet (2-mm thick) material; whereas, the 7249 tests were conducted on three thicknesses (2, 3.2 and 6.35-mm). The thin-sheet 7075 alloy did not have residual stresses, but the 7249 alloy (B = 2 mm) had low-level (<10 MPa) residual stresses from measurements. However, the other two 7249 alloy thicknesses were residual-stress free based on measurements. These tests were conducted to generate fatigue-crack-growth-rate data from threshold to near fracture at two load ratios (R = 0.1 and 0.7). Two methods were used to generate near threshold data on C(T) specimens: (1) compression pre-cracking constant-amplitude (CPCA) and (2) compression pre-cracking load-reduction (CPLR). Constant-amplitude tests were conducted on the M(T) specimens at the same two load ratios. Crack-closure measurements using a compliance method were made on some of the low R tests and a crack-closure model were used to develop an effective stress-intensity factor range against rate relation using a constant constraint factor (α = 1.85). The constraint factor accounts for three-dimensional stress-state effects on plastic yielding at crack tips and crack-closure behavior.Simulated aircraft wing spectrum tests were conducted on the M(T) specimens using two modified full-scale-fatigue-test (FSFT) spectra used on the P-3C aircraft. Tensile pre-cracking was used on all of the M(T) specimens, except one 3.2-mm thick 7249 specimen, before the spectra were applied. The same spectrum sequence was applied, but one had tension-compression loads, whereas, the other only tension-tension loads. The spectrum tests were used to calibrate the constraint-loss regime (plane-strain to plane-stress; α = 1.85–1.0) behavior for the FASTRAN strip-yield model. Comparisons were made between the spectrum tests and calculations made with the FASTRAN life-prediction code; and the calculated crack-growth lives were generally within ±20% of the test results.

Crack Size and Shape Considerations Related to Near-Threshold Fatigue Crack Growth Rate Behavior

Abstract It is well recognized that crack size or load history or both have an important influence on near-threshold fatigue crack rate behavior at a low stress ratio. This investigation will compare the behavior of a radial crack, such as a surface flaw or a corner crack, to that of a linear crack to see if crack shape plays a role in physically small crack to long crack behavior. Recent research has unveiled a methodology to experimentally partition plasticity from roughness- and oxide-induced crack closure, showing that nonplasticity-induced closure mechanisms have a greater influence at low Kmax by orders of magnitude. Knowledge of this is key to understanding differences between long crack and physically small crack behavior in the near-threshold regime. In this study, the well behaved 2024-T351 aluminum alloy was used to generate threshold data using numerous specimen geometries, flaw geometries, crack sizes, and test procedures to highlight not only the significance of crack size but also the role of crack shape. It will be shown both experimentally and analytically that a radial crack (surface flaw, corner crack) has significantly less remote closure and, therefore, a lower threshold than a through crack (compact tension, middle crack tension), even if the crack size is the same.

Comparison of J–R curves and JC values of C(T) and M(T) specimens of bidirectional GLARE 3 5/4 0.3 fiber-metal laminates

The experimental evaluation and comparison of J–R curves from compact tension C(T) and middle-crack tension M(T) specimens of commercial GLARE 3 5/4 0.3 fiber-metal laminates was done. Fracture tests were performed on 50mm wide C(T) specimens with crack to width ratio a/W=0.5, and 230mm wide M(T) specimens having two different crack to width ratios, 2a/W=0.25 and 0.35. For C(T) specimens J–R curves were evaluated based on the ASTM E1820 standard methodology, whereas for M(T) specimens the ASTM E561 standard methodology was applied. In both cases crack length was measured through unloading compliance. The results indicate that unloading compliance is applicable to bidirectional GLARE laminates in both geometries, estimating the stable crack growth during the tests with reasonable accuracy. The results also showed good agreement between J–R curves from both geometries, as well as similar J values at the beginning of stable crack growth. The experimental J–R curves were slightly higher for M(T) specimens, which were many times larger than C(T) ones.

Comparison of the Fatigue Crack Propagation Behavior of Two Different Forms of PMMA Using Two-Stage Zone Model

The fatigue crack propagation behavior of two different forms of PMMA was studied using two-stage zone model. First, the fatigue crack length and fatigue crack propagation velocities of different specimens were obtained experimentally. Then the effect of material forms and specimen types on the fatigue crack propagation velocities was analyzed. Finally, the data scatter of da/dN-ΔK curves in different forms and different types of specimens was analyzed. The results show that the expressions of fatigue crack propagation velocities of middle crack tension (MT) specimens and compact tension (CT) specimens in the same form PMMA are similar. And the scatter of MT specimens is larger than CT specimens in two forms of PMMA.

Numerical Studies of Estimation of Fracture Parameters for Mismatched Welded Joints with HAZ Cracks

An idealized tri-material assumption is established to describe the constitutive relationship of mismatched welded joints by considering the influence of heat affected zone (HAZ). The fracture parameters and C* are estimated for mismatched welded joints with HAZ cracks by finite element analysis with ABAQUS. A middle crack tension (M(T)) specimen is utilized in the analysis for different material properties and geometries of the weldment. The influence of mechanical property and geometry on the fracture parameters J and C* of the specimen is discussed for the welded joints with HAZ crack. The results suggest that the HAZ property is a significant factor in the estimation of J and C* for the mismatched welded joint with HAZ crack.

Fatigue crack growth in 7249‐T76511 aluminium alloy under constant‐amplitude and spectrum loading

AbstractFatigue crack growth tests were conducted on compact, C(T), specimens made of 7249‐T76511 aluminium alloy. These tests were conducted to generate crack growth rate data from threshold to near fracture over a wide range of load ratios (R). Four methods were used to generate near threshold data: (1) ASTM E‐647 load reduction (LR), (2) compression pre‐cracking constant‐amplitude (CPCA), (3) compression pre‐cracking LR, and (4) constant crack mouth opening displacement LR method. A crack closure analysis was used to develop an effective stress‐intensity factor range against rate relation using a constraint factor (α = 1.85).Simulated aircraft wing spectrum tests were conducted on middle crack tension, M(T), specimens using a modified full‐scale fatigue test spectrum. The tests were used to develop the constraint‐loss regime (plane strain to plane stress; α = 1.85 to 1.15) behaviour. Comparisons were made between the spectrum tests and calculations made with the FASTRAN life prediction code; and the calculated crack growth lives were generally with ±10% of the test results.

Finite element modeling of plasticity-induced crack closure due to occasional mode II loading on mode I fatigue crack growth

Plasticity-induced fatigue crack closure associated with occasional mode II loading was simulated for two-dimensional middle-crack tension geometry by using the finite element method. When a single mode II cycle was superposed onto the steady-state mode I crack growth, the range of crack tip opening displacement, ΔCTOD, exhibited the initial drop and subsequent recovery under plane strain condition. On the other hand, under plane stress condition, ΔCTOD exhibited the initial jump and drop right after the single mode II loading, and then it showed the recovery. The present results indicate that occasional mode II loading can cause a small retardation for mode I crack growth due to the enhanced plasticity-induced closure.

Effet de l’équilibre glycémique sur le statut antioxydant d’une population de diabétiques de la ville de Constantine

Plasticity-induced fatigue crack closure associated with occasional mode II loading was simulated for two-dimensional middle-crack tension geometry by using the finite element method. When a single mode II cycle was superposed onto the steady-state mode I crack growth, the range of crack tip opening displacement, ΔCTOD, exhibited the initial drop and subsequent recovery under plane strain condition. On the other hand, under plane stress condition, ΔCTOD exhibited the initial jump and drop right after the single mode II loading, and then it showed the recovery. The present results indicate that occasional mode II loading can cause a small retardation for mode I crack growth due to the enhanced plasticity-induced closure.

Syndrome d’interruption de la tige pituitaire de révélation tardive

Plasticity-induced fatigue crack closure associated with occasional mode II loading was simulated for two-dimensional middle-crack tension geometry by using the finite element method. When a single mode II cycle was superposed onto the steady-state mode I crack growth, the range of crack tip opening displacement, ΔCTOD, exhibited the initial drop and subsequent recovery under plane strain condition. On the other hand, under plane stress condition, ΔCTOD exhibited the initial jump and drop right after the single mode II loading, and then it showed the recovery. The present results indicate that occasional mode II loading can cause a small retardation for mode I crack growth due to the enhanced plasticity-induced closure.

Extent of the Surface Region in Notched Middle Cracked Tension Specimens

This article aims at evaluating the extent of the surface region in notched Middle Cracked Tension specimens. Firstly, a fully automatic fatigue crack growth technique is developed to obtain stable crack shapes. After that, the stress triaxiality along the crack front is evaluated for different notch shapes. Then, objective criteria are defined to quantify the extent of the surface region from the stress triaxiality data collected. Next, the extent of the surface region is related to the elastic stress concentration factor of the uncracked geometry by a linear relationship. Finally, empirical two-constant equations able to evaluate the extent of the surface region from the thickness, notch radius, notch depth and elastic stress concentration factor are formulated.

Implementation of a plastically dissipated energy criterion for three dimensional modeling of fatigue crack growth

Fatigue crack growth is simulated using three dimensional elastic–plastic finite element analysis. The crack extension per load cycle, da/dN, as well as crack front profile changes (crack tunneling) under cyclic loading is not specified as an input but evaluated based on a condition that relates plastically dissipated energy to a critical value. Simulation of cyclic crack propagation in a middle-crack tension M(T) specimen using this implementation captures the well established, experimentally obtained crack growth rate reduction accompanying a single overload event. The analysis predicts that the single overload also affects the crack front profile, where a tunneling crack propagates with a flatter crack front in the overload affected zone.

Numerical investigation on the creep crack-tip constraint induced by loading configuration of specimens

In this paper, the creep crack-tip constraint induced by loading configuration of specimens is quantitatively investigated by the finite element method. The results show the creep constraint induced by the loading configuration could be characterized and analyzed by the constraint parameter R. The specimen order in terms of the creep crack-tip constraint from high to low is compact tension, single edge-notched bend, single edge-notched tensile and middle-cracked tension specimen. With increasing C ∗ the constraint decreases in the specimens with different loading configurations. The effect of distances from the crack tips on the creep constraint parameter R is also analyzed.

Fatigue crack propagation into the residual stress field along and perpendicular to laser beam butt‐weld in aluminium alloy AA6056

ABSTRACTLaser beam butt welds in Al‐alloys are very narrow and are accompanied by steep residual stress gradients. In such a case, how the initial crack orientation and the distance of the notch tip relative to the weld affect fatigue crack propagation has not been investigated. Therefore, this investigation was undertaken with two different crack orientations: along the mid‐weld and perpendicular to the weld. Fatigue crack propagation ‘along the mid‐weld’ was found to be faster in middle crack tension specimens than in compact tension specimens. For the crack orientation ‘perpendicular to the weld’, the relative distance between the notch tip and the weld was varied using compact tension specimens to generate either tensile or compressive residual stresses near the notch tip. When tensile residual stresses were generated near the notch tip, fatigue crack propagation was found to be faster than that in the base material, irrespective of the difference in the initial residual stress level and whether the crack propagated along the mid‐weld or perpendicular to the weld. In contrast, when compressive weld residual stresses were generated near the notch tip, fatigue crack arrest, slow crack propagation, multiple crack branching and out of plane deviation occurred. The results are discussed by considering the superposition principle and possible practical implications are mentioned.

One Numerical Simulation Method and its Applications for Fatigue Crack Growth

Abstract: Based on the Miner′s linear accumulative damage theory and the low cycle fatigue damage parameter that is relative to the total strain amplitude and the mean stress of the plastic zone near crack tip, a new numerical simulation method to predict the fatigue crack growth (FCG) of materials is presented. The numerical FCG results for middle-cracked tension (MT) specimens of Cr2Ni2MoV steel, TA12 and TC4 alloys by ANSYS are closed to those by tests. Therefore, the new method has capability to give good FCG prediction accuracy for different materials. Furthermore, the Paris FCG models of TA17 and TC11 alloys were predicted by this method and can be referred to the key engineerings.