Pre-cracked Compact Tension Research Articles

Role of precipitate-induced [formula omitted] twins on anisotropic fracture behavior in Mg-3Al-0.5Ce alloys

Anisotropic fracture toughness (FT) in a hot-rolled Mg-3Al-0.5Ce magnesium alloy has been investigated using fatigue pre-cracked compact tension (CT) specimens with the notch (an)‖ to the rolling direction (RD) and transverse direction (TD). During testing, notable non-linear load–displacement behavior before peak load indicated significant crack tip plasticity. Thus, elastic–plastic FT (JIC) was evaluated and found to be 20.7N/mm and 15.8N/mm for an‖ to RD and TD, respectively. Long-aligned Al11Ce3 precipitates were present in this alloy within the elongated grain boundaries along RD. During FT testing, profuse 101¯2〈101¯1〉 extension twins (ET) with c-axis almost ‖ and lamellae morphology almost ⊥ to RD formed between these elongated precipitates, irrespective of the notch, an‖ to RD or TD. The ET lamellae ⊥ and ∥ to crack path lead to trans and inter lamellar crack for an‖ to RD and TD, respectively. Translamellar crack in an‖ to RD leads to comparatively high strain hardening, dislocation density, geometrically necessary boundaries, and grain refinement, indicating larger plastic energy dissipation and thereby has higher JIC than in an‖ to TD with interlamellar crack.

Investigation of the Combined Influence of Temperature and Humidity on Fatigue Crack Growth Rate in Al6082 Alloy in a Coastal Environment

The fatigue crack growth rate (FCGR) of aluminium alloys under the combined influence of temperature and humidity remains a relatively unexplored area, receiving limited attention due to its intricate nature and challenges in predicting the combined impact of these factors. The challenge was to investigate and address the specific mechanisms and interactions between temperature and humidity, as in coastal environment conditions, on the FCGR of aluminium alloy. The present study conducts a comprehensive investigation into the combined influence of temperature and humidity on the FCGR of the Al6082 alloy. The fatigue pre-cracked compact tension specimens were corroded for 7 days and then subjected to various temperature and humidity conditions in a thermal chamber for 3 days to simulate coastal environments. The obtained data were analysed to determine the influence of temperature and humidity on the FCGR of the Al6082 alloy. An empirical model was also established to precisely predict fatigue life cycle values under these environmental conditions. The correlation between FCGR and fracture toughness models was also examined. The Al6082 alloy exhibits a 34% increase in the Paris constant C, indicating reduced FCGR resistance due to elevated temperature and humidity levels. At the same time, fatigue, corrosion, moisture-assisted crack propagation, and hydrogen embrittlement lead to a 27% decrease in threshold fracture toughness. The developed model exhibited accurate predictions for fatigue life cycles, and the correlation between fracture toughness and FCGR showed an error of less than 10%, indicating a strong relationship between these parameters.

Numerical prediction of warm pre-stressing effects for a steam turbine steel

In warm pre-stressing (WPS), the fracture resistance of cracked steel components is raised when subjected to certain temperature-load histories. WPS’s beneficial effects enhance safety margins and potentially prolong fatigue life. However, understanding and predicting the WPS effects is crucial for employing such benefits. This study utilised pre-cracked compact tension specimens made from steam turbine steel for WPS and baseline fracture toughness testing. Two typical WPS cycles were investigated (L-C-F and L-U-C-F), and an increase in fracture resistance was observed for both cycles. The WPS tests were simulated using finite element analysis to understand its effects and predict the increase in fracture resistance. A local approach was followed based on accumulative plastic strain magnitude ahead of the crack tip. Since cleavage fracture is triggered by active plasticity, the WPS fracture is assumed when accumulated plasticity exceeds the residual plastic zone formed at the crack tip due to the initial pre-load.

Crack Growth Sensitivity to the Magnitude and Frequency of Load Fluctuation in Stage 1b of High-pH Stress Corrosion Cracking

Pipelines undergo sequential stages before failure caused by high-pH stress corrosion cracking. These sequential stages are the incubation stage, intergranular crack initiation (Stage 1a), crack evolution to provide the condition for mechanically driven crack growth (Stage 1b), sustainable mechanically driven crack propagation (Stage 2), and rapid crack propagation to failure (Stage 3). The crack propagation mechanisms in Stage 1b are composed of the nucleation and growth of secondary cracks on the free surface and crack coalescence of secondary cracks with one another and the primary crack. These mechanisms continue until the stress intensity factor (K) at the crack tip reaches a critical value, known as KISCC. This investigation took a novel approach to study Stage 1b in using precracked compact tension (CT) specimens. Using precracked specimens and maintaining K at less than KISCC provided an opportunity to study crack initiation on the surface of the specimen under plane stress conditions in the presence of a pre-existing crack. In the present work, the effects of cyclic loading characteristics on crack growth behavior during Stage 1b were studied. It was observed that the pre-existing cracks during Stage 1b led to the initiation of secondary cracks. The initiation of the secondary cracks at the crack tip depended on loading characteristics, i.e., the amplitude and frequency of load fluctuations. The secondary cracks at the crack tip can be classified into four categories based on their positions with respect to the primary crack. Low R-ratio cycles generated an evident cyclic plastic zone, where high density of intergranular cracks were formed. The higher the frequency of the low R-ratio cycles, the higher the density of the intergranular cracks forming in the cyclic plastic zone. The crack growth rate increased with an increase in either the amplitude or the frequency of the load fluctuations. The minimum and maximum crack growth rates were 8 × 10−9 mm/s and 4.2 × 10−7 mm/s, respectively, with the R-ratio varying between 0.2 and 0.9, frequency varying between 10−4 Hz and 5 × 10−2 Hz, and at a fixed stress intensity factor of 15 MPa√m. It was found that avoiding rapid and large load fluctuations slowed down crack geometry evolution and delayed the onset of Stage 2. The implication of these results for pipeline operators is that reducing internal pressure fluctuations by reducing the frequency and/or amplitude of the fluctuations can expand Stage 1 and increase the reliable lifetime of operating pipelines.

Fatigue crack growth behaviour of a nickel base super alloy GTM720 under cold-TURBISTAN spectrum load sequence

Damage tolerance concept is widely used in aerospace industry for designing structural components against fatigue. In this study, an attempt is made to predict fatigue crack growth behavior in an aero-engine material under a standard spectrum load sequence using a closure independent crack driving force parameter, K* and compare with experimental results. Initially, constant amplitude (CA) fatigue crack growth rate (FCGR) behavior of GTM720, a nickel based super alloy was determined at stress ratios, R = σmin/σmax ranging from R = 0.1 to 0.7. Then, empirical fatigue crack growth law was derived from this CA-FCGR data in terms of two parameter crack driving force, K*. Further, the fatigue crack growth behavior was predicted through cycle-by cycle approach in a pre-cracked compact tension (CT) specimen of GTM720 alloy under standard cold-TURBISTAN spectrum load sequence using K* as the crack driving force. Also, experimental fatigue crack growth behavior under the same load sequence was determined in a CT specimen and compared with predicted results. A fairly good correlation was observed with predicted and experimental results. Predicted fatigue crack growth life was conservative and the fatigue crack growth life ratio, Npred/Nexpt was about 0.92.

Numerical assessment on fatigue damage evolution of materials at crack tip of CT specimen based on CPFEM

Fatigue failure of metal parts is common in engineering, and the failure mechanisms remain to be studied. The crystal plastic finite element method (CPFEM) was used to study the damage evolution of materials at the crack tip of precracked compact tension (CT) specimen based on the established crystal plasticity constitutive relation which considered the grain size effect and cyclic hardening. The parameters used in polycrystal model were obtained through reverse optimization method combined with genetic algorithm, which are performed through the combination of commercial software ABAQUS and MATLAB. The stress–strain curve obtained by 2D polycrystal model finite element simulation is closed to the experimental curve of 304 stainless steel. Then, the 3D Voronoi-like polycrystalline model was established to investigate the effects of stress state, grain size and inclusion on the fatigue properties of materials at crack tip of precracked 304SS CT specimen by evaluating the evolution of plastic strain energy (PSE) and Von Mises stress under cyclic load. These factors will affect the fatigue damage evolution of materials from different aspects which were analyzed in detail. The proposed method and model are helpful to study the fatigue failure of metal materials at mesoscopic scale.

Assessment of hydrogen embrittlement susceptibility of an Al-Cu-Mg alloy in humid air

In the present study, we investigated the hydrogen embrittlement susceptibility of Al-4%Cu-1.5%Mg alloys subjected to several heat treatments by means of SSRT tensile test and humid gas stress corrosion cracking(HG-SCC) test. For SSRT tensile test, the tensile test pieces were cut from cold-rolled sheets of 1mm thickness. The test pieces were solution-treated at 500 °C for 1h, water-quenched and aged at 140oC for 72h or 360h. SSRT tensile test was performed in two environments, humid air (HA) and dry nitrogen gas (DNG) at a strain rate of 1.39×10-6s-1. Fracture surfaces were observed with a scanning electron microscopy(SEM). For HG-SCC test, compact tension(CT) test pieces were cut from hot-rolled plate of 6mm thickness. The CT test pieces were solution-treated, water-quenched and aged at 190 °C for 9h, 50°C for 96h or 140°C for 72h. HG-SCC test was carried out based on High Pressure Institute of Japan standards; HPIS E103:2018. The pre-cracked CT specimens with stress loading were kept for 90 days in two environments, HA and DNG. After 90 days, in order to observe whether cracks propagated due to HG-SCC, the specimens were loaded up to fracture rapidly, followed by SEM observation. Tensile properties obtained by SSRT tensile tests were almost the same in two environments. Also, fracture surfaces were not affected by test environments. Moreover, in HG-SCC tests, crack propagation was not observed at each test conditions. Therefore, Al-4%Cu-1.5%Mg alloy had high-resistance to hydrogen embrittlement.

J-integral Fracture Toughness of High-Mn Steels at Room and Cryogenic Temperatures

The J-integral fracture toughness values of high-Mn steel (Fe-25 wt pct Mn) and 304L stainless steel were evaluated at 25 °C, 0 °C, − 50 °C, − 100 °C, − 163 °C, and − 196 °C using precracked compact tension (CT) specimens and compared to those determined using Charpy impact tests. The high-Mn steel exhibited excellent J-integral fracture toughness at both room and cryogenic temperatures, with values comparable to those of 304L stainless steel. However, the trend of the J-integral fracture toughness of high-Mn steel with the decreasing temperature differed from that of the Charpy impact test results. Electron backscattered diffraction and micrographic analyses suggest that the varying stacking fault energies of high-Mn steels at different temperatures affected the deformation behavior in the stretch zone at the crack tip of the CT specimen. The effect of this temperature-dependent deformation behavior of high-Mn steels on the fracture process in the J-integral test could differ from that in the Charpy impact test, resulting in the different trends in the fracture resistance with the decreasing temperature.

Fatigue crack growth behavior of laser-shock processed aluminum alloy 2024-T3

Laser shock processing (LSP) is a surface modification technique aimed at enhancing the resistance to wear, corrosion and fatigue of structural alloys. Recently, LSP without coating (LSPwC) has been gaining ground, using lasers with lower energies, shorter pulse duration, smaller laser spots and higher surface coverage per shot. In the present work, LSPwC treatment was performed in both sides of pre-cracked compact tension specimens of aluminum alloy 2024-T3. A pulsed (9 ns) Nd:YAG laser system operating in the second harmonic (532 nm) at 10 Hz repetition rate and with pulse energy of about 270 mJ was positioned with a 500 mm focal distance lens in order to conduct LSPwC with an estimated power density of 5.2 GW/cm2 and two distinct overlapping rates: 50% and 75%. The objective of the work was to investigate the effect of the LSPwC and cyclic load condition on the crack closure and fatigue crack growth (FCG) behavior shown by the samples. Constant amplitude FCG tests were performed with two distinct load ratios: R = 0.2 and R = 0.5. A small increase in the crack closure loads (Pcl ≈ 1.1-1.2 Pmin) and in the number of cycles to crack propagation was evinced for the specimens tested at R = 0.2 compared to the untreated ones, whereas negligible effect was observed in the R = 0.5 tests. Besides, the obtained results indicate that the increase in overlapping rate is not effective for the adopted LSPwC conditions.

A nonlinear 3D model for iron-based shape memory alloys considering different thermomechanical properties for austenite and martensite and coupling between transformation and plasticity

The paper presents a 3D macroscopic constitutive model for Iron-based shape memory alloys (Fe-SMAs) that uses different thermomechanical properties for austenite and martensite, and considers nonlinear coupling effects between phase transformation and plasticity. The constitutive equations are derived from a potential comprising the Voigt mixture of the free energies of the two phases adapted from the ZM model, and a new interaction energy term. The loading conditions for phase transformation and plastic deformation are obtained by requiring the governing thermodynamic forces to derive from an appropriate dissipation potential, in which a quadratic plasticity-dependent term has been introduced to account for its suppressive effect on forward transformation. The model is implemented in ABAQUS through a user defined material subroutine (UMAT), validated against experimental data taken from the literature, and used to simulate partial unloading and investigate the influences of interaction parameters. Finite element analysis of a precracked compact tension sample is then carried out under both plane stress and plane strain (nonproportional stress fields with strong gradients). The results show highly heterogeneous stress distribution in the specimen. The inelastic strain singularity at the crack front is a consequence of pure phase transformation at low temperature, pure plasticity at high temperature, and a mix of both at intermediate temperatures. During unloading, the crack front accommodates the compression of the surrounding material by undergoing cyclic phase transformation and/or reversed plasticity, which, in turn, induces partial crack closure. If the mechanical loading cycle is operated at low temperature then heating leads to complete crack closure due to martensite→ austenite transformation, while if it is operated at elevated temperature, heating leads to further but not complete crack closure as a result of the thermal-induced plasticity.

Dynamic stress intensity factor of 2A12T4 Al compact tension specimen with loading point displacement and spring mass model

Loading point displacement and spring mass model were introduced into the calculation of the dynamic stress intensity factor for a fatigue pre-cracked 2A12T4 aluminium alloy compact tension specimen. A simple formula of dynamic stress intensity factor for a fatigue pre-cracked compact tension specimen is derived using the vibration analysis method. By determining equivalent mass and equivalent stiffness of 2A12T4 aluminium alloy compact tension specimen, the dynamic stress intensity factor expression has been given. Three examples of simple dynamic loading were calculated, the results were in good agreement with their finite element model values.

Prediction of Crack Growth Propagation of a Super Alloy through ANN Modeling

Linear-elastic fracture mechanics based technique was used to measure the fracture toughness in terms of K1C of a solid solution super alloy. Due to thermal fatigue and high temperature exposure for various application of Alloy 617, it was demanded to measure crack growth behaviour of this alloy. Pre-cracked compact tension (CT) specimens ware used to determine the crack growth rate (CGR) of alloy 617 by direct current potential drop (DCPD) in-situ crack monitoring technique. Artificial Neural network (ANN) statistical model computed different fracture parameters from experimental inputs by feeding information to the network. This feed-forward network calculated the threshold fracture toughness, number of cycle to failure, slope of the Paris curve for the alloy at different temperatures and load ratios. The computational model correlates and converges with the experimental results with a maximum deviation of 6%. Thus, the model is recommended for complex and stochastic application of the nickel base super alloy 617.

Effect of Sour Environment pH on Crack Morphology in Ultra-High Strength Drilling Steel Under Cyclic Stress

Catastrophic failure of a pre-cracked compact tension UD-165 sample after a cyclic stress versus frequency experiment in a 5% NaCl solution buffered with CO2/NaHCO3 and in contact with 0.83 MPa H2S at 20°C, pH=7, occurred in air. Only fatigue crack propagation was observed in a similar sample after a similar test in the pH=12 solution. The log fatigue crack rate measured as a function of log frequency for UD-165 in the pH=7 solution was linear with a gradient of -0.492 while in the pH=12 solution exhibited a plateau indicating different mechanisms based on fracture mechanics theory. The microscopic investigations of the sample after testing at pH=7 revealed the presence of fatigue cracks, initiated by sample pre-cracking. Also, on the surface of the catastrophically fractured base metal, pits and other secondary cracks were observed. The latter propagated along the beach marks. The surface analysis of UD-165 after testing in pH=12 solutions showed corrosion products covering the crack but not crack tip. Intergranular cracks were found on the surface of the fractured base metal in liquid nitrogen. The microscopic surface investigations corroborate the fracture mechanics results.

Corrosion fatigue crack growth of AISI 4340 steel

Corrosion fatigue crack growth (CFCG) experiments of AISI 4340 high strength steel were conducted using pre-cracked compact tension specimens in dry air, distilled water, and 3.5% NaCl aqueous solution at different R-ratios (where R is the ratio of the minimum load over the maximum load in a loading cycle) and different loading frequencies. The material displays typical Type B CFCG behavior: significant environmental effect when the maximum stress intensity factor is above KISCC (the threshold stress intensity for stress corrosion cracking) and minimal environmental effect when the applied load is below the threshold value or above a large stress intensity factor. A new engineering model was developed to consider the interaction and competition between fatigue and environmental contributions. The R-ratio effect was considered using the Walker’s equation. The baseline data required for the model is the data from stress corrosion in a given environment and the data from crack growth of the material in dry air or inert environment. The new model was found to correlate very well the CFCG experiments of AISI 4340 steel.

Effect of laser shock processing on fatigue crack growth of duplex stainless steel

Duplex stainless steels have wide application in different fields like the ship, petrochemical and chemical industries that is due to their high strength and excellent toughness properties as well as their high corrosion resistance. In this work an investigation is performed to evaluate the effect of laser shock processing on some mechanical properties of 2205 duplex stainless steel. Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. A convergent lens is used to deliver 2.5J, 8ns laser pulses by a Q-switched Nd:YAG laser, operating at 10Hz with infrared (1064nm) radiation. The pulses are focused to a diameter of 1.5mm. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is determined by the contour method. It is observed that the higher the pulse density the greater the compressive residual stress. Pulse densities of 900, 1600 and 2500pul/cm2 are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R=0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness if this steel.

Statistical Analysis of Creep Crack Growth Behavior in Modified 9Cr-1Mo Steel

In this paper, a series of statistical studies were conducted on creep crack growth behavior of Grade 9Cr-1Mo steel for next generation reactor. Creep crack growth tests were performed on pre-cracked compact tension (CT) specimens under the applied load ranges from 3800 to 5000N at the identical temperature condition of 600oC. The creep crack growth behavior has been analyzed statistically using the empirical equation between crack growth rate da/dt and C* parameter, namely da/dt=B(C*)q. First, the determination methods of B and q obtained from experiments were investigated by the least square fitting method and the mean value method. The probability distribution functions of B and q have been investigated using the normal, log-normal and Weibull distribution. The constant B and q are followed well 2-parameter Weibull. Second, the creep crack growth rate data were generated by Monte-Carlo simulation method assuming the 2-parameter Weibull in B and q parameters. The probability distribution of creep crack growth rate for arbitrary C* parameter values seems to follow well Weibull distribution.

Preliminary Evaluation of Digital Image Correlation for In-situ Observation of Low Temperature Atmospheric-Induced Chloride Stress Corrosion Cracking in Austenitic Stainless Steels

Digital image correlation has been used to observe the growth of atmospheric-induced chloride stress corrosion cracking in type 304L stainless steel under controlled conditions of temperature, relative humidity and chloride-deposition density in a non-destructive manner. The technique is capable of detecting changes in crack dimensions that are difficult to discern via conventional optical microscopy, i.e. crack growth beneath salt layers and adherent corrosion product deposits, and measurement of crack opening displacements. Our results also demonstrate that suitable specimen design, combined with digital image correlation, will provide the means of comparing the growth behaviour of short atmospheric-induced chloride stress corrosion cracks with data obtained from conventional pre-cracked compact tension specimens as a function of mechanical "driving force".

In situ studies and modeling the fracture of Zircaloy-4

In situ fracture studies were performed on non-irradiated Zircaloy-4 using tensile specimens and pre-cracked Compact Tension (CT) specimens to clarify the mechanism for fracture initiation in the constrained and non-constrained state. Similar approaches have been taken in the literature to understand the role of hydrides on the fracture of Zircaloy-4, but hydride-free Zircaloy-4 has received little study. Both annealed and beta-treated Zircaloy-4 were tested in the longitudinal, transverse, and short-transverse orientations to study the role of microstructure and orientation. Unstable crack extension is shown to occur under plastic constraint by a process of void nucleation, growth, and coalescence initiating from the Laves phase particles in the microstructure. A micromechanical model is developed for ductile tearing by void growth and coalescence. Excellent agreement between the model and experiments are observed. Aspects of the fracture mechanism and model are discussed.

R-curve behavior and toughening mechanisms of resin-based dental composites: Effects of hydration and post-cure heat treatment

Objectives To test the hypothesis that the fracture resistance of two different particulate resin composites degrade after water hydration and improve after post-cure heat treatment, and to correlate those changes with salient failure micromechanisms. Methods Two composites with different filler morphology were selected, denoted microhybrid (Filtek™ Z250) and nanofill (Filtek™ Supreme plus). Following initial light curing, hydrated samples were aged in water for 60 days at room temperature while post-cured samples were heat treated at 120 °C for 90 min. Fracture resistance was assessed using fracture resistance curves ( R-curves) utilizing pre-cracked compact tension, C(T), specimens. The flexural strength of the hydrated composites also was evaluated in four-point bending using unnotched beams. Scanning electron microscopy (SEM) of crack paths and fracture surfaces was performed to determine the micromechanisms of fracture and toughening. The results were compared by two-way ANOVA and Tukey's multiple comparison test ( p ≤ 0.05). Results SEM observations revealed a predominantly interparticle matrix crack path for all cases except the hydrated nanofill composite, which showed evidence of particle matrix debonding. Hydration lowered the strength for both composites and the peak toughness for the nanofill composite. The strength decrease was attributed to resin matrix plasticization and hydrolytic degradation in both cases, with additional interfacial degradation causing a larger strength decline and concomitant peak toughness decrease in the nanofill composite. The post-cure heat treatment noticeably changed the R-curve shape causing the peak toughness to be reached after shorter amounts of crack extension. Such changes help explain the increases in strength reported in other studies and is attributed to improved resin matrix properties. Significance Results from this study provide new insight into the micromechanisms of fracture in resin-based dental composites which should aid the future development and improvement of these materials.

Fracture toughness master-curve analysis of the tempered martensitic steel Eurofer97

We report fracture toughness data for the reduced activation tempered martensitic steel Eurofer97 in the lower to middle transition region. The fracture toughness was measured from tests carried out on 0.35 T and 0.87 T pre-cracked compact tension specimens. The data were first analyzed using the ASTM E1921 standard. The toughness–temperature behavior and scatter were shown to deviate from the ASTM E1921 standard predictions near the lower shelf. Using the method of maximum likelihood, the athermal component of the master-curve was calculated to better fit the data from the lower to the middle transition region. We showed that these master-curve adjustments are necessary to make the T o values obtained near the lower shelf with 0.35 T C( T) specimens consistent with those obtained in the middle transition region with 0.87 T C( T) specimens.