Standard Compact Tension Specimens Research Articles

Effect of plasma nitriding of a stop hole along the fatigue crack path on crack propagation

The effect of nitriding of the fastener hole along the Fatigue Crack Growth (FCG) path on FCG was investigated. A standard Compact Tension (CT) specimen made of AISI 4140 steel with a 5 mm diameter hole drilled on the FCG path was used. It was subjected to a plasma nitriding process. Crack growth was monitored using the digital imaging method. The results were compared with the paired sample t-test. As compared to the un-nitrided sample without a hole (control group), FCG lifetime was increased 1.5 times un-nitrided without a hole and nitrided with a hole samples. And 2.5 times the total FCG lifetime was observed in drilled un-nitrided samples. It was determined that nitriding treatment of the drilled samples eliminated the positive effect of the hole on FCG life.

An investigation of the validity of LEFM at the nanoscale in amorphous materials using the atomistic J-integral including entropic effect

The objective of this paper is threefold: (a) to determine the validity of applying the continuum-based linear elastic fracture mechanics (LEFM) to investigate fracture processes at the discrete atomic level in amorphous materials, such as polymers, (b) to quantify the contribution from entropic effects to the J-integral at the nanoscale due to the thermal motion of atoms, and (c) to demonstrate that the atomistic J-integral can be applied to extract the cohesive traction-separation law at a nanoscale notch-tip as a material property in a polymer when specific modeling criteria are obeyed. Verification of the atomistic J-integral algorithm was performed using toughness data for a monocrystalline graphene sheet, before application to a polymer. A generic epoxy polymer (EPON 862-DETDA) was selected for this study, and the simulations were carried out using molecular dynamics (MD). Furthermore, atomistic J-integral was employed as a nanoscale fracture metric to explore flaw tolerance at the nanoscale and to build a methodology to evaluate and predict the material’s initiation fracture toughness. For this purpose, three MD models of edge-notched specimens that are geometrically similar to the ASTM standard compact tension (CT) specimens were developed, each with a different nano-size notch length to study the effect of notch size on toughness and strength. Significant deviation of peak atomistic J-integral values from LEFM-based macro-scale fracture toughness values for epoxy was observed at notch lengths below a critical threshold crack length, indicating that LEFM is not applicable below this length scale. Entropic effects arise in materials at the micro/nano scale due to the random thermal motion of atoms and molecules, especially near crack faces. Results from our MD simulations demonstrated a significant entropic contribution to the atomistic J-integral, especially at elevated temperatures. Notch-size independence of the cohesive traction-separation law near a nanoscale subcritical notch and the effect of temperature were also demonstrated using the atomistic J-integral.

Measurement of Fatigue Crack Growth in Compact Tension Specimen Using an Optical Extensometer

Abstract In this investigation, fatigue crack growth (FCG) tests are performed on transformation induced plasticity steels to characterize the relevance of stress intensity and crack growth rates with standard compact tension (CT) specimens. An optical extensometer that can achieve real-time displacement measurement is developed based on digital image correlation. Instead of a displacement gauge, the optical extensometer is adopted in FCG tests to measure CT specimen crack opening displacement (COD). Compared with stress intensity, it is difficult to calculate crack growth rates with COD. Therefore, the compliance equation and incremental polynomial method are programmed with C++ languages to obtain the steady crack growth curve directly. The proposed method is verified with experimental results obtained by the displacement extensometer. In addition, the precision of the displacement measurement is assessed, and the error induced by displacement output rates is discussed.

Stochastic modeling of fatigue crack growth for bolt holes in turbine disc

The stochastic fatigue crack growth (FCG) modeling method is established for bolt holes in the turbine disc. Through analyzing the FCG experimental of standard compact tension (CT) specimens, a Wiener process is firstly established based on the failure mechanism to describe the stochastic FCG process, and then extended to the component-like and component level. The accuracy of the established model is verified by the experiments of bolt hole plate (BHP) specimens. Finally, the reliability analysis framework is constructed and verified for the turbine disc with multiple cracks based on the Copula function.

A study on the fatigue crack growth behaviour of GTM718 nickel based super alloy under cold-TURBISTAN spectrum loads

In this study, the fatigue crack growth (FCG) behavior of GTM718 nickel base super alloy subjected to cold-TURBISTAN spectrum load sequence was determined by three different methods i.e., (i) Experimental, (ii) Analytical prediction using cycle-by-cycle method and, (iii) Prediction by finite element analysis (FEA). In the experimental method, a standard compact tension (CT) specimen was prepared and pre-cracked to produce a sharp fatigue crack at the notch root. Then, the specimen was subjected to spectrum load blocks repeatedly and crack growth was measured as a function of applied number of flights. Two such tests were carried out in a servo-hydraulic universal test machine with triangular waveform and at an average frequency of 2 Hz. In the second analytical method, the individual load cycles in cold-TURBISTAN spectrum block were separated through rain flow cycle counting method. For each of this counted cycle, the crack extension was determined using crack growth law which was based on two-parameter crack driving force, ΔK*. An in-house MATLAB code was written and used to determine the FCG behavior by cycle-by-cycle method. In the third FEA method, a global model of CT specimen of GTM718 was created in HYPERMESH and a local model with a pre-crack was created in FRANC3D. The FCG behavior under spectrum load was determined using FRANC3D in conjunction with MSC NASTRAN. Two different types of ‘FCG law - stress ratio effect’ combinations were employed i e., Paris’-Walker (P-W) and Bi-linear-Walker (B-W). The required constants for these laws were derived from the available experimental data of the material. It was observed that the predicted FCG behavior by analytical and FEA methods, although conservative, were fairly good when compared to experimental results.

Analysis of Dynamic Response Behavior of Crack under Impact Stress Wave

The structural parts of construction machinery mostly fail due to impact load, but current research on the failure behavior of the impact load has not established a complete theoretical system. Based on wave theory and fracture mechanics, this paper analyzed the wave behavior of shock stress waves and established a model of shock stress wave propagation. Given the dynamic response behavior of the stress and strain field at the crack tip, dynamic fracture mechanics theory was used to solve the dynamic fracture strength stress factor and evaluate the dynamic fracture performance of the structure with crack damage under shock waves. Through dynamic response analysis and numerical calculation of the typical SHPB (split Hopkinson pressure bar) test standard compact tension (CT) specimens under the short-term strong shock stress wave, the stress and strain evolution law of the material under the shock wave was analyzed, and the correlation of the shock stress wave was verified. This research work can meet the requirements of engineering design and has practical engineering significance, playing an important role in material safety design.

Incorporation of Obstacle Hardening into Local Approach to Cleavage Fracture to Predict Temperature Effects in the Ductile to Brittle Transition Regime.

Ductile-to-brittle-transition refers to observable change in fracture mode with decreasing temperature—from slow ductile crack growth to rapid cleavage. It is exhibited by body-centred cubic metals and presents a challenge for integrity assessment of structural components made of such metals. Local approaches to cleavage fracture, based on Weibull stress as a cleavage crack-driving force, have been shown to predict fracture toughness at very low temperatures. However, they are ineffective in the transition regime without the recalibration of Weibull stress parameters, which requires further testing and thus diminishes their predictive capability. We propose new Weibull stress formulation with thinning function based on obstacle hardening model, which modifies the number of cleavage-initiating features with temperature. Our model is implemented as a post-processor of finite element analysis results. It is applied to analyses of standard compact tension specimens of typical reactor pressure vessel steel, for which deformation and fracture toughness properties in the transition regime are available. It is shown that the new Weibull stress is independent of temperature, and of Weibull shape parameter, within the experimental error. It accurately predicts the fracture toughness at any temperature in the transition regime without relying upon empirical fits for the first time.

Probabilistic life prediction for FCG degradation process of turbine disc with small sample data

A probabilistic fatigue crack growth (FCG) life prediction is conducted on a turbine disc considering small sample data involving material properties and geometric dimensions in this study. Wiener process with two transformed time scales is used to quantify the time-varying dispersion of FCG degradation process. The accuracy of Weiner process model is verified by using the FCG experimental data for standard compact tension (CT) specimens. Then, FCG tests on the component-like specimens are conducted to establish the time-varying FCG model considering geometrical features. During this process, Bootstrap method is introduced to expand the small test samples into larger Bootstrap samples. Meanwhile, the dispersion for geometric dimensions of the small sample is described by using probability box method. Finally, probabilistic FCG analysis is conducted on a turbine disc through using quasi-Monte Carlo sampling method. Results show that the average relative error between the lower bound of Ns and Na is within 20%, and the average relative error between the upper bound of Ns and Na is less than 15%, which further demonstrates the accuracy of probabilistic FCG method.

Revisiting the first Sandia Fracture Challenge with transient deformation heating and strain localization considerations

Here, we present a systematic experimental study and accompanying theoretical analysis of the dependence of ductile fracture on strain localization, strain hardening rates, deformation-induced thermal softening, and transient heat conduction under spatially uniform as well as spatially heterogeneous deformation. Spatially uniform cases are studied via standard dogbone shaped specimens subject to uniaxial tension. Spatially heterogeneous deformation is studied via the so-called Sandia Fracture Challenge (SFC) specimen, which is a standard compact tension specimen modified with three machined holes in front of a blunt notch (Boyce et al. in Int J Fract 186(1–2):5–68, 2014). We utilize the same precipitation hardened martensitic stainless steel used in the first SFC experiments, i.e. 15-5 PH with an H1075 heat treatment. We also study 15-5 PH in Condition A and with the H900 heat treatment, each of which has a different hardening behavior and ductility. We find that the ductility does not correlate with the deformation to first crack initiation in the SFC specimen. Instead, hardening rates are better correlated. Moreover, a re-examination of (Boyce et al. in Int J Fract 186(1–2):5–68, 2014) finds that an accurate calibration of the hardening rates is strongly correlated with accurate blind predictions of ductile fracture in the SFC specimens. Given that thermal softening can greatly affect hardening rates, we provided a thermographic analysis of deformation-induced heating and transient heat transfer in both the dogbone shaped samples and the SFC specimens. Transient heating in stainless steels is found to have first-order effects on the strain at the onset of necking, strain to fracture, as well as strain localization and ductile fracture in complex geometries even under so-called quasi-static loading rates.

Uncertainty Modeling of Fatigue Crack Growth and Probabilistic Life Prediction for Welded Joints of Nuclear Stainless Steel.

Welded joints are widely used in the pipeline connection of nuclear power plants. Defects in these joints are an important factor leading to the failure of welded joints. It is critical to study the fatigue crack growth and life prediction methods for the welded joints with defects, to reduce their likelihood. In this paper, we present our study of the uncertainty of fatigue crack propagation and probabilistic life prediction for welded joints of nuclear stainless steel. The standard compact tension (CT) specimens were fabricated according to the American Society for Testing and Materials (ASTM) standard. Fatigue crack propagation tests with different stress ratios were performed on CT specimens, using the Mei Te Si (MTS) fatigue test system. A fatigue crack propagation rate model considering the uncertainty of material parameters, and based on the Paris formula and crack propagation experimental data, was established. A probabilistic life prediction method based on Monte Carlo simulation was developed. The fatigue crack propagation prediction result of a CT specimen was compared with the actual tested result, to verify the effectiveness of the proposed method. Finally, the method was applied to an embedded elliptical crack in welded joints of nuclear stainless steel, to predict the fatigue crack growth life and evaluate the reliability.

Effect of microstructural constituents on mechanical properties and fracture toughness of Inconel 718 with anomalous deformation behavior at 650 °C

In the current work, Inconel 718 superalloy was subjected to five different heat treatment schedules to develop five different microstructures for studying the effect of γ’, γ” and δ phases on mechanical properties of IN-718 at room temperature and 650 °C as the working temperature of IN-718 is near 650 °C. Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Vickers hardness tests, tensile tests and fracture toughness (JIC) tests were performed for different heat-treated samples at room temperature. Further tensile tests were carried out at 650 °C for all heat-treated specimens and one high-temperature JIC test at 650 °C was performed for Standard heat-treated compact tension specimen. It had been demonstrated that γ′ and γ″ phases caused a substantial increase in hardness of the material while δ phase was attributed to low hardness of this superalloy. Also, it was found that γ″ phase provided maximum strength to IN-718 superalloy followed by γ′ at room temperature but on contrary, γ′ was found to be primarily responsible for higher strength than γ″ at 650 °C. The δ phase furnished poor strength at all temperatures. However, the formation of Nb + Ti carbides and oxides at 650 °C caused premature failure of the tensile specimens with lower elongation. In case of fracture toughness test, the maximum value of critical J-integral (JIC) and tearing modulus (T) were demonstrated by γ′ phase. A decrease in the JIC value of the material was observed at higher temperature (650 °C). Both critical J-integral (JIC) and tearing modulus (T) were found to be critical parameters for crack growth characterization of this superalloy.

Fatigue Crack Growth Properties of AA 5754 Aluminum Alloy Gas Tungsten Arc Welding and Friction Stir Welding Joints

Friction stir welding (FSW) and gas tungsten arc welding (GTAW) were employed to manufacture the spiral pipes by a double-sided synchronization approach. The fatigue crack growth properties for base metal (BM) and welded metal (WM) in FSW and GTAW were studied using standard compact tension specimens manufactured from the welded pipes of the AA 5754 aluminum alloys. The highest crack growth rate occurred in WM of GTAW specimen, while WM of FSW specimen showed the lowest crack growth rate. Moreover, the crack growth rate of BM specimen fell in between them. This was because of the combination of the coarse grains and a high residual stress in WM of GTAW in comparison with WM of FSW and BM. The increase in fatigue crack rate matched well with the variation in the plastic region at the crack tip.

Suitability of constraint and closure models for predicting crack growth in generic configurations

The effective stress intensity factor range ΔKeff is commonly considered to govern the fatigue crack growth and may be estimated through the crack opening stress Sop and the constraint factor α. In the literature closed-form expressions have been developed for estimating both variables in a center-cracked plate with elastic-perfectly plastic behavior subjected to remote uniform stresses. However, such expressions are frequently adopted regardless of geometry, material and loading and, moreover, α is often given a fixed value. The final objective is to assess the suitability of existing constraint and closure models for performing accurate crack-growth predictions in configurations that do not meet the original hypotheses. The region-II growth of a hole-edge crack is predicted in the 42CrMo4 steel by integrating crack growth laws calibrated with data from standard compact tension (CT) specimens. The results show overall better predictions by using the same constraint model for estimating α in both geometries, rather than by treating it as a fitting parameter in CT specimens. Unexpectedly, the predictions obtained at different stress ratios were, in general, more accurate by integrating a non-closure law based on ΔK and independent of Sop and α.

Load separability and ηpl factor determination for CT specimen of Zr-2.5Nb pressure tube material

For fracture testing of pressure tubes, specimens are machined directly from the pressure tube without disturbing its curvature. The compact tension (CT) specimens thus obtained are called curved compact tension (CCT) specimens. Apart from being curved, the CCT specimens do not satisfy width/thickness criteria as mentioned in ASTM E1820-15. In spite of these differences, CCT specimens use the same ηpl and χpl factors as that of standard CT specimens for fracture toughness evaluation. In this work, 17 mm wide blunt notched CT specimens of different crack length obtained from a Zr-2.5Nb pressure tube, which were not meeting the thickness criteria, were used to investigate load separation and to evaluate the ηpl factor experimentally. Load separation existed between a/W of 0.45 and 0.70. The ηpl factor had a constant value of 2.078 in this range when calculated using the load separation method by power law fit. The ηpl factor calculated using a third order polynomial fit passing through the origin showed linear increase with b/W similar to but marginally lower than standard CT specimen.

Experimental Fatigue Life Investigation on a Standard PMMA Compact Tension Specimen Equipped by the Lock Technique

The lock technique is a method to decrease crack growth by putting a lock on the crack which the focus of this study is to experimentally characterize the influence of the lock technique on crack growth behavior and life assessment. In this study, polymethyl-methacrylate rectangular standard compact tension plates were used as the test samples. Three different edge crack and lock configurations were tested for total six samples in order to assess fatigue life and fracture resistance, and experiments were conducted using a high-frequency fatigue test machine that applies cyclic tension–compression loads to the samples. In this study, effects of varying lock and hole geometry on crack initiation and propagations are investigated. Detecting of the crack initiation life and assessing crack propagation life from crack initiation until fracture moment are investigated. The experimental results show that the crack growth life is reduced in the lockless standard cracked specimen compared to the locked samples, and similarly, fracture strength in specimens equipped with the lock is more in comparison with the lockless specimens.

Microscopic analysis of non-propagating fatigue crack tips

Herein, the phenomena around a non-propagating fatigue crack tip in polycrystalline interstitial-free steel were investigated on a microscopic scale. A non-propagating fatigue crack was introduced in a standard compact tension specimen using a load-shedding procedure. The near-tip area was subsequently investigated via scanning electron microscopy and transmission electron microscopy. The wake in the tip area was covered with an oxide layer. Discrete dislocations, whose densities were relatively low, were only found in front of the crack. These results may suggest that an increase of the opening point was attributable to the oxide-induced crack closure rather than the plasticity-induced one.

Assessing the cracking behavior of auxetic cellular structures by using both a numerical and an experimental approach

Auxetic cellular structures are advanced materials with negative Poisson’s ratios, which exhibit some unique features which are useful for various applications. The objective of this paper is the numerical simulation and experimental analysis of evolution propagation under quasi-static loading conditions in selected auxetic cellular structures with different geometries and orientations of unit cells that is: (i) honeycomb structure, (ii) re-entrant structure, and (iii) rotated re-entrant structure. The failure modeling capability of Simulia Abaqus code for ductile materials is used for the numerical simulation. The paper presents in detail the calculation methodology for determination of ductile damage material parameters that, are used for further numerical simulations of the damage initiation and evolution. Standard Compact Tension (CT) specimens are selected for the numerical simulation and experimental testing. Here, the specimens are made from 7075-T651 aluminum plate, cut using water jet cutting technology. Using a numerical approach, the fracture behavior of the selected auxetic structures is estimated first. Experimental testing is also carried out to validate the numerical results. The comparison between computational and experimental results regarding crack propagation path and force-displacement diagrams showed a reasonable agreement.

Effect of plate thickness on the fatigue crack growth rate of a low carbon bainitic steel

An investigation has been conducted to figure out the effect of plate thickness on the fatigue crack growth rate (FCGR) of a low carbon bainitic steel. Based on the standard ASTM E647, standard compact tension specimens with three different plate thicknesses were used in fatigue crack growth test and the crack closure was taken into consideration when calculating crack lengths. When analyzing the data obtained in fatigue tests, two different mathematic models were used to fit the relationship curve between crack length a and cycle number N. Results show that the low carbon bainitic steel with a bigger plate thickness has a lower crack growth rate in a short crack growing period. More information for crack growth behavior was obtained via the scanning electron microscope technique.

Influence of retrogression and re-ageing heat treatment on the fatigue crack growth behavior of 7010 aluminum alloy

Aluminum alloys are widely used in aircraft structural components where light weight, high strength and good corrosion resistance are the primary requirements. These alloys are generally used in peak-aged (T6) condition in which they are susceptible for stress corrosion cracking. In the recent years, retrogression and re-ageing (RRA) treatment on aluminum alloy is carried out to enhance their corrosion resistance maintaining the ultimate tensile strength. The aim of this work was to study the influence of RRA treatment on the fatigue crack growth rate (FCGR) behavior. The 7010 aluminum alloy was heat treated to two different conditions i.e., T6 and RRA. The microstructures of these alloys were characterized by using TEM. Standard compact tension (CT) specimens were prepared and FCGR tests were carried out by using a 100 kN servo-hydraulic test machine as per ASTM E647-15e1. The constant amplitude FCGR tests were carried out at a stress ratio, R = 0.5 using sine wave loading pattern at 10 Hz. Crack length was monitored by following compliance technique. Microstructural studies show that RRA treated alloy contain fine and densely populated precipitates in the matrix along with coarse and discontinuous precipitates in the grain boundary. The fatigue crack growth rate was observed to reduce along with an increase in the threshold stress intensity factor range (ΔKth) for RRA treated alloy compared to the T6 alloy. The mechanisms for reduction in fatigue crack growth rate of RRA treated alloy is attributed to the microstructural modifications. The increased resistance is expected to enhance the damage tolerance capability of the alloy.

Improved fatigue crack growth resistance by retrogression and re‐aging heat treatment in 7010 aluminum alloy

AbstractAircraft grade 7010 aluminum alloy was heat treated to two different conditions: (1) standard peak aging (T6) and (2) retrogression and re‐aging (RRA). The microstructures of these alloys were characterized by using transmission electron microscope. Fatigue crack growth rate (FCGR) tests were conducted using standard compact tension specimens, following ASTM standards. Tests were conducted at various stress ratios, R ranging from 0.1 to 0.7. The RRA‐treated alloy was observed to contain coarsened η′ (MgZn2) precipitates with higher inter‐particle spacing when compared with T6‐treated alloy. The grain boundary precipitates (GBPs) were also coarsened and discontinuous in RRA‐treated alloy as compared with continuous GBPs in T6 condition. The FCGR was lower and ΔKth was higher in RRA‐treated alloy compared with T6‐treated alloy at all the stress ratios investigated. Improved fatigue crack growth resistance in RRA‐treated alloy was correlated to the modified microstructure and enhanced crack closure levels.