Similar Crack Growth Rates Research Articles

The Effect of Crack-Initiating Feature on the Environment-Assisted Cracking Behavior of Sensitized AA5456-H116 in Marine Environments

The influence of crack-initiating feature on the environment-assisted cracking (EAC) behavior of sensitized AA5456-H116 exposed to marine environments is assessed via fracture mechanics-based testing. Specimens that contained either a traditional fatigue precrack or purposefully introduced intergranular corrosion fissures were immersed in 0.6 M NaCl and polarized to select electrochemical potentials while held at a constant force. The measured crack length vs. time relationships from these experiments reveal that the two specimen geometries yield similar crack growth rates at −900 mVSCE and after the onset of accelerated crack propagation at −800 mVSCE. However, precorroded specimens exhibit significantly shorter times to failure than the precracked specimens at −800 mVSCE due to increased crack growth rates at the start of the experiment. The mechanical, environmental, and material factors that could contribute to the initially increased EAC susceptibility of the precorroded specimens are identified using a generalized model for EAC. Analysis of these possible causal factors suggests that the increased susceptibility is due to a residual, initially more deleterious crack chemistry at the occluded corrosion fissure tip from the aggressive galvanostatic polarizations used to accelerate fissure growth. The implications of these results on the efficacy of traditional fracture mechanics-based methods for quantifying EAC susceptibility are discussed.

New Insight Into Crack-Healing Mechanism via Electropulsing Treatment

A combination of experiment and numerical simulation was employed to study the healing mechanism of fatigue crack via a novel route of electropulsing treatment (EPT) named dual-step EPT processing. By applying cyclic loads, similar cracks were generated in 316L stainless steel samples. Then, cracked specimens were subjected to electropulsing treatment under different conditions in order to find the optimum EPT condition to effectively heal the crack. The geometry of the fatigue crack before and after EPT was investigated in 3D by Micro-CT. SEM-EBSD was used to evaluate the microstructure of the healed region. To understand the dominant mechanism of crack healing by EPT, the temperature and stress/strain fields were evaluated by the finite element method (FEM) considering non-uniform Joule heating. The results demonstrate that complete crack healing requires a balanced combination of melting and compressive stress as well as increased peak current density regarding the reduced crack length. The simulations also suggest that Joule heating alone is sufficient to induce the observed melting in the examined samples. The fatigue crack after EPT was found to be effectively healed, as indicated by the similar crack growth rate compared with the samples without electropulsing treatment. In addition, a refined microstructure can be achieved after EPT.

Fatigue crack growth behaviour of wire arc additively manufactured steels

Combining welding technology with robotics, wire arc additive manufacturing (WAAM) is emerging as a viable method of construction. To facilitate its wider application, in particular with structural integrity in mind, an improved understanding of the fatigue properties of WAAM materials is needed. Hence, an experimental investigation into the fatigue crack growth (FCG) behaviour of WAAM plates made of normal- and high-strength steels has been conducted and is reported herein. Following material characterisation, FCG tests on 13 machined compact tension specimens were undertaken, extracted in different directions from WAAM plates built using a parallel deposition strategy. Fractography of the tested specimens was also performed to analyse the mechanisms of crack growth. The material testing demonstrated an approximately proportional relationship between the hardness and ultimate strength. From the FCG tests, Paris’ law constants were derived for the WAAM normal- and high-strength steel specimens, revealing consistently lower material constants m for the latter. Similar FCG behaviour to that of equivalent, conventionally-produced steels, with no significant anisotropy, was found. The observed response was shown to be generally well captured by the FCG laws given in BS 7910, with the recommended curve for unwelded steels providing a close match to the experimental data and the recommended curve for welded steels providing conservative predictions. Additional FCG data on WAAM steels printed using the parallel and oscillatory deposition strategies were collected and analysed. It was shown that the two deposition strategies led to similar crack growth rates for WAAM normal-strength steels, but distinct FCG behavioural trends for WAAM high-strength steels. Finally, the fractographic analysis showed that the WAAM specimens exhibited principally brittle behaviour during FCG but somewhat ductile behaviour before fracture.

Crack growth behaviour in biaxial stress fields: Calculation of K-factors for cruciform specimens

The aim of this study is to propose a simplified method for calculating the Mode I stress intensity factor K for cruciform specimens under planar biaxial loading. It is assumed that two cracks have to grow with a similar crack growth rate. The straight crack paths of the two single cracks with the length a should also be similar. The calculations are carried out on an aluminum and a steel specimen, respectively. For different load cases and materials, the stresses resulting from the forces are first considered. It was found that the elastic constants E and ν have only a small influence of less than 3%. In addition, the coupling between the forces of the load axes, which should be minimized by slotted arms, is considered. Moreover, K-factors are calculated by finite elements (FE) for different crack lengths. These K-values together with the transmission factor allow to find a K-factor formula for cruciform specimens, which is based on the prescribed forces. Finally, the results of the FE-calculation of the exact straight crack paths were compared to experimentally determined crack paths. Furthermore, a comparison of the fatigue crack growth rates versus K-factor range for different loading is shown.

Cruciform specimens for the determination of crack growth behaviour in biaxial stress fields: calculation of K-factors

The aim of this study is to propose a simplified calculation of the Mode I stress intensity factor K for the cruciform specimen design proposed by Brown and Miller. To calculate K, both cracks have to grow with a similar crack growth rate and the crack paths of the two single cracks with the length a should also be similar. The calculations are carried out on an aluminum specimen and a steel specimen. For all load cases and materials, the stresses resulting from the forces are first considered. It was found that the elastic constants E and ν have only a small influence of less than 3 %. In addition, the coupling between the forces of the load axes, which should be minimized by the slotted arms, is considered. Furthermore K-factors are calculated by FE for different crack lengths. These K-values together with the transmission factor allow to find a K-factor formula for cruciform specimens, which is based on the prescribed forces. Finally, the results of the FE calculation of the exact straight crack paths were compared to experimentally determined crack paths.

Environmentally assisted cracking of forged 316LN stainless steel and its weld in high temperature water

The corrosion fatigue (CF) and stress corrosion cracking (SCC) crack growth rates (CGRs) of type 316LN SS and its welds that are used for pressurized water reactor primary water components were evaluated. Effects on CGRs of loading condition, water chemistry and metallurgical microstructure were studied. Experimental results indicated that stress intensity factor K, loading ratio, and frequency control the CGRs of materials along with corrosion potential, chloride, and pH also significantly influencing the cracking behavior. Similar CGRs under various conditions were observed in the base metal and its weld, with low SCC susceptibility in hydrogenated water, but high SCC susceptibility in oxygenated water. The weld metal showed lower SCC CGRs than base metal in oxygenated high purity water but shifted to higher values after adding ppb levels of chloride because of the presence of δ-ferrite in the weld.

Creep crack growth properties of P91 and P22 welded joints

AbstractThe ferritic P22 and bainitic‐martenstic P91 steel welded joints operating at elevated temperatures are highly vulnerable to premature failure, because microstructure gradients can lead to cracking in the heat‐affected zone, commonly referred to as type IV cracking. Creep crack growth tests have been performed using compact tension specimens made of P91 and P22 weld joints at temperatures of 600 and 550 °C, respectively. The creep crack growth characterization through empirical equation of C* was the main fracture parameter for relating behaviour of component's regions. Results have shown that, for a specific value of C* parameter, the weld metal and heat‐affected zone had similar crack growth rates, which were about two times faster than base metal rates, because of lower creep strength that leads to a conclusion that the creep cracks will most likely initiate in the heat‐affected zone region.

Thermomechanical fatigue crack growth in a cast polycrystalline superalloy

Thermomechanical fatigue (TMF) crack growth testing has been performed on the polycrystalline superalloy IN792. All tests were conducted in mechanical strain control in the temperature range between 100 and 750 °C. The influence of in-phase (IP) and out-of-phase (OP) TMF cycles was investigated as well as the influence of applying extended dwell times (up to 6 hours) at the maximum temperature. The crack growth rates were also evaluated based on linear elastic fracture mechanics and described as a function of the stress intensity factor KI. Without dwell time at the maximum temperature, the crack growth rates are generally higher for the OP-TMF cycle compared to the IP-TMF cycle, when equivalent nominal strain ranges are compared. However, due to the fact that the tests were conducted in mechanical strain control, the stress response is very different for the IP and OP cycles. Also the crack closure level differs significantly between the cycle types. By taking the stress response into account and comparing the crack growth rates for equivalent effective stress intensity factor rages ΔKeff defined as Kmax − Kclosure, very similar crack growth rates were actually noticed independent of whether an IP or OP cycle were used. While the introduction of a 6 hour dwell time significantly increased the crack growth rates for the IP-TMF cycle, a decrease in crack growth rates versus ΔKeff were actually seen for the OP-TMF cycle. The fracture behaviour during the different test conditions has been investigated using scanning electron microscopy.

The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy

The effects of thermomechanical processing and subsequent heat treatment on the small fatigue crack growth (FCG) behavior of an AM60 (Mg-6.29Al-0.28Mn wt pct) alloy were evaluated. The effects of mechanical loading parameters, such as maximum stress and load-ratio, on the small FCG behavior were also determined. Maximum stress did not appear to affect the crack propagation rate of small cracks in the stress and crack size ranges considered. Materials with different microstructures and yield stresses, introduced by different processing conditions, showed similar crack growth rates at equivalent stress intensity factor ranges. The effect of load ratio on small crack growth rates was recorded. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior.

SA508 Gr.3 Cl.2 저합금강과 용접부의 290℃ 수화학 환경에서 피로균열거동 분석

The fatigue crack growth behaviors of SA508 Gr.3 Cl.2 low alloy steel in high temperature water environment were investigated. Overall, weld metal showed similar crack growth rate as that of base metal. At 0.01 Hz, fatigue crack growth rate (FCGR) was higher than that in air while the difference was smaller at 0.1 Hz. Also, FCGR showed <TEX>${\Delta}K$</TEX> dependency at 0.1 Hz only, indicating that the environmental effect was much greater at slower loading frequency of 0.01 Hz. FCGR of SA508 Gr.3 Cl.2 low alloy steel was compatible to or smaller than the ASME Sec. XI fatigue reference curves in high temperature water environment.

Near threshold fatigue crack growth in aluminium alloys at low and ultrasonic frequency: Influences of specimen thickness, strain rate, slip behaviour and air humidity

Near threshold fatigue crack growth in ambient air and in vacuum has been studied with ultrasonic fatigue testing equipment (cycling frequency 20 kHz) and with servo-hydraulic equipment (20 Hz). Aluminium alloy 2024-T3 and 7075, heat treated to promote planar slip (7075-UA) and homogeneous slip (7075-OA), respectively, have been tested at load ratio R=-1. In vacuum, no strain rate influences were found and similar crack growth rates were observed at both frequencies. Specimen thickness does not influence the threshold stress intensity amplitude ( K max,th) determined at 3.5×10 –13 m/cycle in vacuum whereas crack growth resistance increases with decreasing specimen thickness at higher growth rates. K max,th determined at 10 -10 m/cycle in ambient air are similar at 20 Hz and 20 kHz and are 45–55% of those measured in vacuum. In ambient air and at higher stress intensities, fatigue cracks may propagate by a factor of 5–50 faster at the lower frequency. In vacuum, 7075-UA shows better crack growth properties than 7075-OA, more pronounced at higher growth rates. In ambient air, the planar slip materials show a plateau like regime in the (Δa/ΔN vs K max)-curves, where growth rates are hardly affected by the stress intensity.

FATIGUE CRACK GROWTH BEHAVIOUR OF FIBRE‐METAL LAMINATE GLARE‐1 AND METAL LAMINATE 7475 WITH DIFFERENT BLUNT NOTCHES

Abstract— The fatigue crack growth behaviour of the fibre metal laminate “GLARE‐1” has been investigated for different blunt notches in Constant Amplitude (CA) tests. In order to investigate the influence of the fibres, the same laminate material but containing no fibres (Laminate 7475) was also tested. The fatigue crack growth properties of GLARE‐1 are superior to those of Laminate 7475. GLARE‐1 shows lower crack growth velocities at the same Knom values and in addition the crack growth rates decrease with increasing crack length. The Laminate 7475 shows typical metal behaviour for single crack propagation and accelerating crack growth with increasing crack length. In GLARE‐1, multiple crack propagation takes place. The cracks propagate independent of each other and have similar crack growth rates, in part due to closure effects caused by the unbroken fibre layers.The crack growth rates of specimens having a small root radius are higher in both materials than in specimens with a large notch radius. In GLARE‐1, the superiority of a larger notch radius is more pronounced than in the Laminate 7475 and is attributed to a stronger crack closure effect owing to fibre bridging. The reason for the higher bridging capability in specimens containing larger notches is that less fibres are broken or damaged in the notch vicinity.