Faster Crack Growth Rates Research Articles

Towards understanding stress corrosion cracking of austenitic stainless steels exposed to realistic sea salt brines

Stress corrosion cracking behavior of stainless steel 304 L was investigated in full immersion, evaporated artificial sea salt brines (ASW) at 55 °C. It was observed that brines representative of thermodynamically stable brines at lower relative humidity (40% RH, MgCl2-dominant) had a faster crack growth rate than high relative humidity brines (76% RH, NaCl-dominant). Observed crack growth rates (da/dt) under constant stress intensity (K) conditions were determined to be independent of transitioning procedure (rising K or decreasing frequency) regardless of solutions investigated for the orientation presented. Further, positive strain rates had little to no impact on the observed da/dt. The observed behavior suggests an anodic dissolution enhanced hydrogen embrittlement mechanism for SS304L in concentrated ASW environments at 55 °C. Additional explorations further examined environmental influences on da/dt. Nitrate additions to 40% ASW at 55 °C solutions were shown to decrease measured da/dt and further additions stopped measurable crack growth. After sufficient nitrate had been added to fully stifle crack growth, a temperature increase to 75 °C induced cracking again, and a subsequent decrease to 55 °C once again stopped da/dt. These tests demonstrate the importance of ascertaining both brine-specific chemical and dynamic environmental influences on da/dt.

A study of high-velocity penetration on icy lunar regolith simulants

Understanding the mechanical response of a high-speed penetrator penetrating icy lunar regolith (ILR) is essential for designing penetrators in lunar permanently shadowed regions and interpreting the detection data from the device. Experimental research on the penetrators is limited in engineering due to the difficulties in preparing large-scale icy lunar regolith simulants (ILRS). Such limitation urges the need to construct a theoretical model and verify a numerical simulation model based on the scaled-down penetration experimental results, which provide insights into the mechanical response of penetrator penetrating ILR. Projectile penetration experiments were conducted on ILRS targets with four typical water content levels in a cryogenic chamber at 110 K. The experimental results show that the ILRS with higher water content exhibits greater brittleness and a faster crack growth rate. Consequently, the diameters of cratering and scabbing areas are augmented on the target surface upon projectile penetration. Moreover, increased mechanical strength decreases the plugging height on the ILRS targets. Based on the projectile residual velocities, the equivalent target strength parameter R were calculated and fitted to a functional relationship with the uniaxial compressive strength. RHT model parameters were calibrated using the test results of the dynamic and static mechanical properties of the simulants. Numerical simulation of projectile penetration into semi-infinite and thick targets were conducted using the calibrated model. The simulation results demonstrate high consistency with the experimental and theoretical calculations, indicating the effectiveness of the constitutive model in describing the mechanical response of the ILRS under projectile penetration.

Notch fatigue fracture and crack growth behaviors on a steel sheet under out-of-plane bending

For industrial products made of sheets, it is crucial to prevent the formation of fatigue cracks and the rapid destruction by stress risers. To carry out tests under out-of-plane bending load, this study used not only smooth sheet specimens made of a common structural steel, but also specimens with a through-wall circular hole in the center and specimens with a V-shape notch on one edge of the plate. The fracture mechanics parameters were investigated while fatigue crack initiation, growth, and the process leading up to a break were observed. In the high cycle region, the notch factor of the Hole specimen was 1.07, and that of the V-notch specimen was 2.22. With the same initial set stress amplitude, the V-notch specimen had fewer cycles at the time of crack initiation and faster crack growth rate than the Hole specimen. The state and the extent of crack initiation and growth were affected by the strain concentration and cyclic loading levels with different notch geometries. The threshold and unstable crack growth stress intensity amplitudes were identified, while the fracture mechanism and affecting factors were examined. By extending the Paris-Erdogan equation, a crack growth rate prediction model was proposed and validated with good approximation accuracy. It becomes possible to quantitatively predict the occurrence of fatigue, crack propagation, fracture, and fatigue life in industrial machinery and structures made of similar materials in out-of-plane bending.

Study on fracture parameters and fracture process zone of manufactured-sand recycled aggregate concrete

This paper studied the fracture parameters and fracture process zone (FPZ) of manufactured-sand recycled aggregate concrete (MSRAC) with different replacement rates of recycled coarse aggregate (RCA) by three-point bending test on single-edge notched beams. A threshold crack identification method (TCIM) was proposed to locate the crack tip and identify FPZ. Combined with TCIM and digital image correlation (DIC) technology, the fracture mechanism of MSRAC was investigated and the FPZ evolutions of MSRAC and manufactured-sand normal aggregate concrete (MSNAC) were comparatively analyzed. The results indicated that the crack tip and FPZ could be well identified by TCIM and it had good stability. The initial fracture toughness of MSRAC was not obviously different from that of MSNAC, while the unstable fracture toughness and fracture energy of MSRAC were significantly reduced. The brittleness of MSRAC increased with the RCA replacement rates. According to the difference of fracture behaviors between MSRAC and MSNAC, the FPZ evolutions of MSRAC were divided into three stages, including FPZ stable development, FPZ unstable development, and FPZ retraction development. The FPZ length increased with the RCA replacement rates while the FPZ width and FPZ area were opposite. The RCA reduced the aggregation interlocking effect of MSRAC, resulting in a faster crack growth rate and fewer secondary cracks and damage behaviors of MSRAC than MSNAC.

Fatigue crack growth behavior and fracture model of the hydrogenated Ti-0.3Mo-0.8Ni alloy gas tungsten arc welded joints

Fatigue crack growth behavior of the hydrogenated Ti-0.3Mo-0.8Ni alloy gas tungsten arc welded (GTAW) joints was investigated. Fracture model of hydrogen-induced GTAW joint was revealed in details. The results show that as hydrogen level increased, crack growth rate in Paris and instability growth zones increased, the more the hydride, the more crack growth channels, the faster crack growth rate, and brittle fracture was intensified. There was the mutation of dislocation density on both sides of the hydride, and dislocation tangles and dislocation lines were concentrated in the vicinity of the hydride. Dislocation weakening caused by solute hydrogen and internal stress generated by hydrogen-rich clusters respectively promoted crack initiation. Evolution of hydride morphology and distribution caused by the interaction between the hydride and persistent slip bands (PSB) significantly reduced crack initiation resistance. Hydride inside the PSB as a hardening phase accelerated to separate from the matrix or itself fracture, which further promoted crack initiation. Cracks in Paris zone rapidly propagated along grain boundary of coarse size “cluster”, alpha / beta phase boundary and the hydride, or cut through alternating lamellar alpha and fine needle beta grains and the hydride.

A closure model for predicting crack growth under creep-fatigue loading

A strip-yield model was formulated to simulate creep-fatigue crack growth and to quantify the influence of hold time on plasticity-induced crack closure during creep-fatigue crack growth. Creep-fatigue experiments have shown that longer creep hold times result in faster crack growth rates in subsequent fatigue cycles. This model advances the idea that a decrease of plasticity-induced crack closure is experienced by the crack during fatigue loading when a longer hold time is applied each creep-fatigue cycle. Consequently, the crack tip experiences an increase in the effective stress intensity factor range causing faster growth rate during the fatigue loading. The weight function method was used to compute stress intensity factors and surface displacements for cracks embedded in a material experiencing elastic, plastic and creep deformations at elevated temperatures. It is shown that the longer the hold time, the larger the creep deformation and crack opening displacements in the near crack-tip region. In turn, this leads to a decrease in the crack-tip opening stress/load and faster crack growth rates during the subsequent fatigue cycle. The model was used to perform simulations of creep-fatigue crack growth at elevated temperatures in a nickel-base superalloy and AISI 316 austenitic steel.

The influence of combined gradient structure with residual stress on crack-growth behavior in medium carbon steel

Gradient materials produced by various external surface treatments are widely used in various mechanical structures. In this study, we illustrated how the combined gradient structure and residual stress influences crack-tip plasticity and crack-growth behavior in medium carbon steel S38C. The material used as train axles has a gradient layer of thickness about 8 mm, with fine nanoscale structures in the surface to microstructures of tens of micron in the core. From the surface, the strength in the gradient layer decreases from about 1600 MPa to about 400 MPa, and the corresponding tensile ductility increases from 2% to 13%. Through in-situ fatigue tests, we further revealed slower crack-initiation and faster crack-growth rate in the gradient samples in contrast to their gradient-free coarse-grain counterparts. Using the key material parameters obtained from experiments, we employed finite-element simulations to examine the crack-growth behavior in the gradient structure with compressive residual stress. We showed that both the gradient structure and compressive residual stress reduce the plastic deformation size and hence slow down crack initiation. However, the lower ductility in the high strength gradient layer gives rise to even faster crack growth after the formation of a sharp crack. Based on the definition of the cyclic ΔJ-integral for small scale yielding, we saw lower ΔJ in the presence of the residual stress distribution, hence smaller driving force for the crack-growth rate as the latter monotonically increases with ΔJ.

Effects of microstructure on fatigue crack propagation behavior in a bi-modal TC11 titanium alloy fabricated via laser additive manufacturing

In this study, the crack propagation behaviors in the equiaxed and equiaxed-columnar grain regions of a heat-treated laser additive manufacturing (LAM) TC11 alloy with a special bi-modal microstructure are investigated. The results indicate that the alloy presents a special bi-modal microstructure that comprises a fork-like primary α (αp) phase surrounded by a secondary α colony (αs) in the β phase matrix after the heat treatment is completed. The samples demonstrate a fast crack growth rate with larger da/dN values through the equiaxed grain sample versus across the equiaxed-columnar grain sample at low ΔK values (<13.8). The differences that are observed between the crack propagation behaviors (in the crack initiation stage) of the samples can be mostly attributed to the different size and morphology of the αp lamellae and αs colony within the grains in the equiaxed and columnar grain regions rather than the grain boundaries. The cracks prefer to grow along the α/β boundary with a smooth propagation route and a fast propagation rate in the equiaxed grain region, where the αp and α clusters have a large size. However, in the columnar grain region, small and randomly distributed αp lamellae generate a zigzag-shaped propagation path with a reduction in the da/dN value. Additionally, the change in the size of the αp lamellae in the equiaxed grains (heat affected bands, HAB) is also observed to influence the propagation behavior of the crack during the crack initiation stage.

Modeling of creep-fatigue interaction effects on crack growth at elevated temperatures

The well-known load frequency effect on creep-fatigue crack growth is explained by the interactions between fatigue and creep loading and is quantified using the concept of plasticity-induced crack closure. It is shown that the hold time during creep loading affects crack growth rates during subsequent fatigue cycles. Longer hold times lead to lower crack-tip opening stresses and faster crack growth rates during fatigue loading. To model the impact of hold time on crack opening stresses during fatigue loading, a strip-yield model was developed for creep-fatigue crack growth. The strip-yield model computes crack-tip opening stresses, which determine the effective stress intensity factor range and crack growth rate during the fatigue portion of each loading cycle. Maximum stress intensity factor is used to compute the crack growth rate during the creep portion of each cycle. The proposed strip-yield model is used to compute creep-fatigue crack growth rates for several structural materials, i.e., an Astroloy, aluminium alloy 2650 and 316 stainless steel. The model predictions of crack growth rates compare well with published experimental data for these alloys. This model achieves reliable predictions of crack growth rates and life prediction on components subjected to creep-fatigue loading at elevated temperatures by considering loading interaction effects.

Creep-fatigue crack growth behavior in GH4169 superalloy

This study aims to examine the crack growth behavior of turbine disc GH4169 superalloy under creep-fatigue loading. Crack growth experiments were performed on compact tension specimens using trapezoidal waveform with dwell time at the maximum load at 650 °C. The crack growth rate of GH4169 superalloy significantly increased with dwell time. The grain boundaries oxidize during the dwell process, thereby inducing an intergranular creep-fatigue fracture mode. In addition, testing data under the same dwell time showed scattering at the crack growth rate. Consequently, a modified model based on the Saxena equation was proposed by introducing a distribution factor for the crack growth rate. Microstructural observation confirmed that the small grain size and high volume fraction of the δ phase led to a fast creep-fatigue crack growth rate at 650 °C, thus indicating that two factors, namely, fine grain and presence of the δ phase at the grain boundary, increased the amount of weakened interface at high temperature, in which intergranular cracks may form and propagate.

Relationship between unusual high-temperature fatigue crack growth threshold behavior in superalloys and sudden failure mode transitions

An investigation of high temperature cyclic fatigue crack growth (FCG) threshold behavior of two advanced nickel disk alloys was conducted. The focus of the study was the unusual crossover effect in the near-threshold region of these type of alloys where conditions which produce higher crack growth rates in the Paris regime, produce higher resistance to crack growth in the near threshold regime. It was shown that this crossover effect is associated with a sudden change in the fatigue failure mode from a predominant transgranular mode in the Paris regime to fully intergranular mode in the threshold fatigue crack growth region. This type of a sudden change in the fracture mechanisms has not been previously reported and is surprising considering that intergranular failure is typically associated with faster crack growth rates and not the slow FCG rates of the near-threshold regime. By characterizing this behavior as a function of test temperature, environment and cyclic frequency, it was determined that both the crossover effect and the onset of intergranular failure are caused by environmentally driven mechanisms which have not as yet been fully identified. A plausible explanation for the observed behavior is proposed.

Effect of Particle Size on the Micro-cracking of Plasma-Sprayed YSZ Coatings During Thermal Cycle Testing

The failure of plasma-sprayed thermal barrier coatings (TBCs) during service or thermal cycle testing usually results from internal cracking in the top coat, erosion and CMAS (calcium-magnesium-alumina-silicate)-induced damage, etc. The microstructure of ceramic coatings affects their durability and other properties of TBCs. In the present study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. In addition, the effect of particle size on damage evolution in the top coat was investigated. It is found that the coatings deposited using coarse particles show the higher thermal cycle life. Crack length grew with increasing numbers of thermal cycles. The faster crack growth rate can be found for the coatings deposited from fine particles. The porosity of the coating made from the coarse powder is larger than the porosity of the coating made from fine powder both in the as-sprayed condition and after thermal cycling. The changes in crack growth rate and the porosity are related to the effect of sintering and stress evolution in coatings during the thermal cyclic tests.

Failure analysis for power car wheels based on contact positions and tread slope

Abstract During service operation, the thermal cracks and shelling at power car wheel are initiated before a regular profiling of railway wheel. The damaged railway wheel is reprofiled prior to regular inspection and the maintenance cost for the railway wheel is sharply increased. Therefore, it is necessary to analyze and understand the damage mechanism of power car wheel in order to reduce maintenance cost. In the present paper, to analyze the damage cause and reduction for the power car wheel with tread brakes, the observations of by replication test was performed and fracture characteristics depend on contact zones are carried out. Contact stress is analyzed with respect to the wheel tread slope and contact positions. The result shows the zone between tread center and wheel rim demonstrated white layer by higher hardness and lower fracture toughness and faster crack growth rate in view point of fracture mechanic characteristics. The 1/20 of wheel tread slope can effectively mitigates surface damage of power car wheel.

Fatigue‐Crack‐Growth Behavior of Two Pipeline Steels

The fatigue‐crack‐growth behavior of two types of pipeline steels, Alloy B [Fe–0.05C–1.52Mn–0.12Si–0.092Nb, weight percent (wt%)] and Alloy C (Fe–0.04C–1.61Mn–0.14Si–0.096Nb, wt%), have been investigated. Compact‐tension (CT) specimens have been tested at various frequencies (10, 1, and 0.1 Hz) and different R ratios (0.1 and 0.5, R = Pmin/Pmax where Pmin is the minimum applied load, and Pmax is the maximum applied load) in air. It is concluded that higher R ratios lead to faster crack‐growth rates (FCGRs), while frequency does not have much influence on FCGRs. Moreover, Alloy B tends to have better fatigue resistance than Alloy C under various test conditions in air.

Pressure, Temperature and Dwell Time Effects on Fatigue Life in 304 Stainless Steel Using a R5-based Mechanistic Fatigue Model

This paper aims to evaluate the effects of pressure, temperature, pipe wall thickness and dwell time on fatigue life in 304 stainless steel. For a given dwell time and temperature-internal pressure combination, fatigue life is calculated using the mechanistic fatigue model (MFM) presented. In addition, the influence of pipe wall thickness is also examined. The MFM uses Tresca strain range for initiation and Rankine strain range to account for crack growth rate up to a crack limiting length. The results showed that fatigue life was generally lower for a given load combination in the thinner pipe considered given its smaller surface area compared to the thicker pipe. This led to higher plastic strains and consequently, faster crack growth rates. Also, dwell time influences fatigue life with longer dwell times found to be more damaging. However, the influence of dwell time is tightly coupled with pipe wall thickness as it determines the nature of the thermal gradients developed.

Effect of fiber positioning on mixed-mode fracture of interfacial debonding in composites

Under transverse tensile loading, fibers oriented perpendicular to the tensile direction can undergo fiber/matrix debonding. Experiments show that the first stage of fiber/matrix interface debonding is mode-I dominated fracture with very fast crack growth rate. Subsequent stable crack propagation along the interface is due to mixed mode I/II fracture. The aim of this study is to explore ways to stabilize the early stage of debonding so that it becomes possible to determine the mixed mode interfacial fracture properties for the entire mode-mixity range by in-situ observations. Therefore, the objective of this study is to stabilize crack initiation in the dominant mode-I fracture by changing the position of one fiber with its neighboring fiber or hole using the finite element analysis. The progressive fiber/matrix debonding is studied by focusing on the interaction of one fiber with its neighboring fiber or hole. The results show that decrease of the position angle stabilize the crack growth at the interface in the ligaments. This effect is more significant in the cases with small ligament thickness. In the two-fiber model and at very small ligaments the results show that the crack growth stops when the crack tips meet each other in the ligament and further crack growth is under dominant mode-II fracture. In the fiber-hole model, both the crack initiation and propagation are stabilized by decrease of the position angles at very thin ligaments. This paper suggests to use two fibers instead of a single fiber in order to ease the characterization of interfacial properties.

Subcritical Crack Growth in Unpoled and Poled PZT Ceramics under Applied Static and Oscillating Electric Fields. I. Experimental Observation

The effects of both electric field and mechanical stress on the subcritical crack growth (SCG) behavior of lead zirconate titanate (PZT) ceramics are investigated. In Part I, the growth of indented crack in unpoled and poled soft PZT ceramics subjected to different static and oscillating electric fields is studied. The experimental results indicated a faster crack growth rate initially followed by a slower growth rate with time. The SCG behaviors along directions perpendicular and parallel to the applied electric fields are different and depend on the field intensity, field polarity, frequency, and wave-form of the applied oscillating fields. It is suggested that the crack driving forces for the SCG behavior in PZT ceramics are due to the residual stress from indentation and the application of the electric field.

Experimental investigation of specimen size effect on creep crack growth behavior in P92 steel welded joint

In order to clarify the effect of constraint induced by specimen size on creep crack growth behavior of P92 steel welded joint, creep crack tests were carried out on the compact tension specimens with thick thickness and thin thickness, the crack tip of which were located at different distinct zones of welded joint. Tested results revealed that even in thin thickness specimens, fine grained heat affected zone specimens exhibited a fast creep crack growth rate compared with other micro-zones specimens due to a low creep crack resistance and a high multistress state. The fractographies of these specimens exhibited an accelerated number of spherical particles that were caused by the coalescence of creep voids. Furthermore, the correlation of C* with creep crack growth rate was dependent on specimen thickness. As the specimen thickness increased from 10 to 30mm, the creep crack growth rate increased. This was due to the increase in constraint level ahead of crack tip during creep crack propagation.

Thresholds of time dependent intergranular crack growth in a nickel disc alloy Alloy 720Li

At high temperatures in air, introducing a dwell period at the peak stress of fatigue cycles promotes time dependent intergranular crack growth which can increase crack growth rates by upto a few orders of magnitude from the rates of transgranular fatigue crack growth in superalloys. It is expected that time dependent intergranular crack growth in nickel-based superalloys may not occur below a critical mechanical driving force, Δ K th−IG , analogous to a fatigue threshold (Δ K th ) and a critical temperature, T th . In this study, dwell fatigue crack growth tests have been carefully designed and conducted on Alloy 720Li to examine such thresholds. Unlike a fatigue threshold, the threshold stress intensity factor range for intergranular crack growth is observed to be highly sensitive to microstructure, dwell time and test procedure. The near threshold crack growth behaviour is made complex by the interactions between grain boundary oxidation embrittlement and crack tip stress relaxation. In general, lower Δ K th−IG values are associated with finer grain size and/or shorter dwell times. Often a load increasing procedure promotes stress relaxation and tends to lead to higher Δ K th−IG . When there is limited stress relaxation at the crack tip, similar Δ K th−IG values are measured with load increasing and load shedding procedures. They are generally higher than the fatigue threshold (Δ K th ) despite faster crack growth rates (d a /d N ) in the stable crack growth regime. Time dependent intergranular crack growth cannot be activated below a temperature of 500 ∘ C.

Effect of microstructural stability on fatigue crack growth behaviour of nanostructured Cu

Constant amplitude fatigue crack growth tests were carried out on commercial and high purity nanostructured copper processed by High Pressure Torsion (HPT). Due to strong grain refinement the HPT processed materials show higher tensile strength but also faster crack growth rates when compared to coarse grained material. Crack growth curves of nanostructured copper determined at different stress levels, however, showed that the occurrence of grain coarsening at low stress amplitudes leads to a retardation of crack growth in commercial and high purity HPT Cu. This effect was not observed for high purity HPT Cu with a bimodal microstructure. Crack propagation rates depend significantly on the coarsening phenomenon which on the other hand depends on the applied stress amplitude. A comparison of these results with cyclic deformation tests in the high cycle fatigue regime suggests that grain coarsening during crack growth depends more on the stored energy of the materials while a similar coarsening during cyclic deformation depends more on the activation enthalpy for annealing of defects.