High R-ratio Research Articles

Relationship between fatigue crack threshold K max,th and K 1SCC

Abstract This article describes the relationship that relates the static threshold parameter of K 1SCC (at ∼10−10 m/s) to ΔK th and K max,th (at ∼10−10 m/cycle) under fatigue load at high R-ratio like R = 0.85. At these high R-ratios (>0.75), applied K max > K 1SCC and the fracture path has a larger component of static fracture. 5083 Al alloy is adapted as an example to demonstrate this relationship at room temperature their relationship with and without GB-β anodic precipitates in inert and a 3.5 % NaCl solution environments. We are interested in the mechanical cyclic ΔK th and K max,th and quasi-static K ISCC driving forces affecting the damage in the presence of a chemical environment due to anodic dissolution of the GB-β precipitates. Detailed discussions involve how these parameters are interrelated.

Mechanical Stress Effects on 550\xa0\xb0C Hot Corrosion Propagation Rates in Precipitation Hardened Ni-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS

Combinations of temperature, stress and hot corrosion may cause environmentally-assisted cracking in precipitation-hardened Ni-base superalloys, which is little understood. This research aims to increase current understanding by investigating the effects of mechanical stress on the hot corrosion propagation rate during corrosion-fatigue testing of CMSX-4, CM247LC DS and IN6203DS. The parameters used during the tests included a high R-ratio, high frequency, and a temperature of 550 °C. The results showed CMSX-4 experienced a predictable increase in the hot corrosion rate, CM247LC DS also experienced increased rates, but no obvious trend was apparent; whilst IN6203DS showed no evidence of an increased rate. These different behaviours appear to be a result of an interaction between the mechanical stress and microstructural features, which include gamma-prime volume fractions in both the matrix and eutectic regions, along with the distribution of the eutectic structure. The different behaviours in the hot corrosion propagation rate subsequently affected the respective corrosion fatigue results, with both CMSX-4 and CM247LC DS experiencing fracture but with significantly more scatter involved in the CM247LC DS results. All IN6203DS corrosion-fatigue specimens completed the respective tests without fracture and showed no evidence of cracking. It, therefore, appears that precipitation hardened Ni-base superalloys, which are susceptible to environmentally-assisted cracking, also experience increased hot corrosion propagation rates.

Mechanism of HCF-creep interaction in a type 316LN stainless steel

HCF-creep interaction in type 316LN stainless steel is investigated at 923 K by conducting HCF tests well below the endurance limit with high mean stress (high R-ratio). High mean stress inflicts creep damage at 923 K, which when imposed on a small alternating stress, leads to significant HCF-creep interaction. Depending on the magnitude of mean stress (R-ratio), three distinct regimes are identified, viz. (a) HCF regime (b) HCF-creep interaction regime and (c) creep regime. Detailed fractographic investigation revealed distinct fracture characteristics pertaining to each of the three regimes. HCF-creep interaction regime is marked by significant intergranularity at low values of mean stress which changes to predominantly dimpled fracture with increasing mean stress. The above observations are corroborated through detailed EBSD investigations which bring out the underlying deformation mechanisms responsible for failure under HCF-creep interaction.

Fatigue crack growth behavior of Ni-Cr-Mo-V steel welded joints considering strength mismatch effect

High strength steel welded joints are usually fabricated by heterogeneous weld metal for reducing the negative impacts of microstructural characteristics and defects on mechanical properties, such as hydrogen embrittlement cracking, joint toughness decreasing, etc. To reflect material heterogeneity in the mechanical characteristics, two kinds of weld filler material are selected to obtain Evenmatched Welded Material (E-WM) and Undermatched Welded Material (U-WM) marine Ni-Cr-Mo-V steel welded joints. The Fatigue Crack Growth (FCG) behaviors of Base Metal (BM) and related welded joints are investigated considering load ratio (0.1, 0.4, 0.7) and specimen state (as-welded and heat-treated) effects. The experimental FCG trends for BM and weldments have been compared with the trends available in standards. The FCG rate (da/dN) results show the U-WM demonstrates higher FCGR curves than BM and E-WM. Additionally, it demonstrates no significant difference about FCGR for E-WM and U-WM under high R-ratios (0.4 and 0.7). Moreover, both E-WM and U-WM in as-welded state presents higher fatigue crack propagation resistance than in the Post Welded Heat Treatment (PWHT) state. The fracture mechanism of FCG was analyzed according to the fatigue crack trajectory in microstructures and fractography. Transgranular fracture behavior were observed with some secondary particles in E-WM, while the intergranular fracture was exhibited with few tiny secondary microcracks in U-WM.

Scrutinizing interlaminar fatigue loading cycle in composites using acoustic emission technique: Stress ratio influence on damage formation

Current models for delamination propagation prediction in fiber-reinforced polymer (FRP) composites exhibit limitations to explain the physics underlying the mechanisms of damage formation in fatigue. In order to contribute in this field, this research focuses on the study of damage development within a single loading cycle of FRP double cantilever beam specimens under different stress ratios (R). The acoustic emission technique was used to investigate damage propagation. Results showed that under high R-ratios, the load cycle spends an increased time above the threshold energy (Uth). This time difference affects the damage distribution within a single loading cycle. Furthermore, the steady-state delamination propagation was influenced by the R-ratio variation due to the modification of the external work applied to the specimen.

Effect of load stress ratio on nominal and effective notch fatigue strength assessment of HFMI-treated high-strength steel cover plates

The International Institute of Welding published IIW-recommendations for the HFMI-treatment, which covers fatigue assessment procedures and quality assurances guidelines. This paper contributes to the validation of the detrimental effect of high R-ratios on HFMI-treated cover plate specimens made of high-strength steel S700. Applying the nominal stress approach, the recommended S/N-curves agree well to the experiments in both cases, R = 0.1 and R = 0.5. In case of the effective notch stress approach, only for R = 0.1 a sound agreement is observed, whereas for R = 0.5 a more pronounced reduction of the fatigue strength may be considered within the recommendations to ensure an overall conservative assessment.

Two-dimensional fatigue debonding in GFRP/balsa sandwich panels

An experimental investigation of the two-dimensional (2D) debonding behavior of GFRP/balsa sandwich panels with embedded circular disbonds at the face sheet/core interface subjected to out-of-plane fatigue loads was conducted. The constant amplitude fatigue experiments were performed under load control and different R-ratios. Two face sheet configurations were investigated: a pure woven ply layup (SPA) and a combination of continuous filament mat (CFM) and woven plies (SPB). For the latter, the CFM layers (which are prone to develop fiber-bridging) were placed above and below the woven plies. In contrast to one-dimensional (1D) beam-like fatigue fracture experiments, decreasing crack growth rates and stable crack propagation were achieved under load control as a result of the 2D growth of the disbonds. The fatigue fracture performance obtained under R = 0.1 for the SPB configuration was less efficient than for the SPA configuration owing to the fiber-bridging crushing associated with high fatigue amplitudes. Under higher R-ratios, a reduced amount of fiber-bridging crushing was observed, leading to a corresponding crack arrest and much longer life. The load-displacement hysteresis loops exhibited an increase in the cyclic stiffness within each fatigue cycle throughout the experiments. This stiffening was mainly caused by in-plane stretching stresses that developed in the debonded part of the face sheets whose magnitude and evolution were evaluated based on the in-plane strains monitored during the experiments.

Redistribution of Welding Residual Stresses of Crack Tip Opening Displacement Specimen by Local Compression.

The demand of crack tip opening displacement (CTOD) test which evaluates fracture toughness of a cracked material is very important to ensure the stability of structure under severe service environment. The validity of the CTOD test result is judged using several criterions of the specification standards. One of them is the artificially generated fatigue pre-crack length inside the specimen. For acceptable CTOD test results, fatigue pre-crack must have a reasonable sharp crack front. The propagation of fatigue crack started from the tip of the machined notch, which might have propagated irregularly due to residual stress field. To overcome this problem, test codes suggest local compression method, reversed bending method and stepwise high-R ratio method to reduce the disparity of residual stress distribution inside the specimen. In this paper, the relation between the degree of local compression and distribution of welding residual stress has been analyzed by finite element analyses in order to determine the amount of effective local compression of the test piece. Analysis results show that initial welding residual stress is dramatically varied three-dimensionally while cutting, notch machining and local compressing due to the change of internal restraint force. From the simulation result, the authors find that there is an optimum amount of local compression to modify regularly for generating fatigue pre-crack propagation. In the case of 0.5% compressions of the model width is the most effective for uniforming residual stress distribution.

Ultra-slow Fatigue Crack Propagation in Metallic Alloys

The influence of frequency (20kHz ultrasonic tests and conventional 20 to 35Hz tests) and environment (air and vacuum) on near-threshold fatigue crack propagation of three metallic alloys, Ti-6Al-4V, 2024-T351 and 12% Cr stainless steel is compared experimentally. The effective stress-intensity factor which is considered as the propagation driving force is determined from closure measurements or tests run at high R-ratio. Based on microfractographic observations, the results are discussed in terms of a preexisting model for intrinsic and environmentally assisted fatigue crack propagation.

A modified cyclic crack propagation description

A new form of the crack propagation law is presented for cyclic crack propagation calculations. It is characterized by a multiplicative juxtaposition of the Paris range, the threshold and critical limit and the R-dependence. The latter is covered by a modified Walker approach, in which the influence of small cycles at high R-ratios is appropriately adjusted. The concrete form of the R-dependence was validated by application to a wide range of materials.

Fatigue crack growth in SiC particle reinforced Al alloy matrix composites at high and low R-ratios by in situ X-ray synchrotron tomography

Metal matrix composites (MMCs) offer high strength, high stiffness, low density, and good fatigue resistance, while maintaining cost an acceptable level. Fatigue resistance of MMCs depends on many aspects of composite microstructure. Fatigue crack growth behavior is particularly dependent on the reinforcement characteristics and matrix microstructure. The goal of this work is to obtain a fundamental understanding of fatigue crack growth behavior in SiC particle-reinforced 2080 Al alloy composites. In situ X-ray synchrotron tomography was performed on two samples at low (R=0.1) and at high (R=0.6) R-ratios. The resulting reconstructed images were used to obtain three-dimensional (3D) rendering of the particles and fatigue crack. Behaviors of the particles and crack, as well as their interaction, were analyzed and quantified. Four-dimensional (4D) visual representations were constructed to aid in the overall understanding of damage evolution.

On the onset of the asymptotic stable fracture region in the mode II fatigue delamination growth behaviour of composites

An experimental investigation aimed at identifying the presence and onset of an asymptotic stable fracture region in the mode II fatigue delamination growth behaviour of composites is presented. The study is motivated by the possibility that experimental data sets, particularly those obtained at high R-ratios, may unknowingly contain data points within the asymptotic stable fracture region that influence its perceived log-linear behaviour and the fit of various log-linear delamination growth models. Results from the experimental investigation indicate that the asymptotic stable fracture region can extend to GIImax values as low as 0.7 GIIc. The implications of this result on characterizing and fitting of various delamination growth models (including models assuming log-linear behaviour) to delamination growth behaviour are discussed.

Effect of Oxygen on Ethanol Stress Corrosion Cracking Susceptibility: Part 2—Dissolution-Based Cracking Mechanism

The mechanism of stress corrosion cracking (SCC) of carbon steel in fuel-grade ethanol was investigated. Repeated potentiodynamic polarization measurements were made during slow strain rate (SSR) tests. Current difference curves were determined from measurements made in the plastic and elastic regions. Peaks in the current difference curves were observed in the same potential region where SCC was found previously with potentiostatic SSR tests, which reveals the relation of cracking susceptibility and slip-enhanced anodic dissolution. Notched bar SSR tests exhibited consistent evolution of measured current vs. strain at constant applied potentials. A high R-ratio and low cyclic frequency crack growth rate (CGR) test was intended to simulate the real service condition of pipelines. The CGR at different applied anodic potentials was higher in the same SCC-susceptible potential range determined by the other approaches. The results support a slip-enhanced dissolution-based mechanism of ethanol SCC.

컴플라이언스법에 의한 다층 맞대기 이음의 잔류응력 추정

Abstract It depends on the joint configuration, dimensions and constraints of the joint whether the residual stress at the root of single-sided butt weld is tensile or not. Therefore, recommendation is generally made that highR ratio should be used in the fatigue test of welded joint in order to prevent excessively long life caused by compressive residual stress. In this research, the residual stress profile in butt weld joint was obtained through compliance method, using successive extension of a slot and measurement of the variation of strainduring the slot extension. The residual stress profile was firstly assumed to be the linear summation of Legendrepolynomials up to 9th order excluding 0th and 1st order. Strain variation on the surface was measured while the slot was being extended by cutting to find out the 8 unknown coefficients of each polynomial term. The cut was made by the electric discharge machine. It was concluded that the residual stress near the surface is positive valued, however, it turned into the negative value as soon as it passed through 2 or 3 mm of the depth. Key Words : Crack compliance method, Residual stress, Electric discharge machine, Finite element analysis,Butt weld

Effect of load ratio on fatigue crack propagation behavior of solid-solution-strengthened Ni-based superalloys at elevated temperature

The fatigue crack propagation (FCP) behavior of two solid-solution-strengthened Ni-based superalloys, INCONEL 617 and HAYNES 230, were studied simultaneously in laboratory air using a constant stress intensity factor (K)-controlled mode with different load ratios (R-ratio) at 700°C. The FCP tests were performed in both cycle and time-dependent FCP domains to examine the effect of R-ratio on the FCP rate, da/dn. For cycle-dependent FCP test, a 1-s sinusoidal fatigue was applied for a compact tension (CT) specimen of INCONEL 617 and HAYNES 230 to measure their FCP rates. For time-dependent FCP test, a 3-s sinusoidal fatigue with a hold time of 300s at maximum load was applied. Both cycle/time-dependent FCP behaviors were characterized and analyzed. The results showed that increasing R-ratio would introduce the fatigue incubation and decrease the FCP rates at cycle-dependent FCP tests. On the contrary, fatigue incubation was not observed at time-dependent FCP tests for both INCONEL 617 and HAYNES 230 at each tested R-ratio, suggesting that association of maximum load (Kmax) with crack tip open displacement (CTOD) and environmental factor governed the FCP process. Also, for time-dependent FCP, HAYNES 230 showed lower FCP rates than INCONEL 617 regardless of R-ratio. However, for cycle-dependent FCP, HAYNES 230 showed the lower FCP rates only at high R-ratios. Fracture surface of specimens were examined using SEM to investigate the cracking mechanism under cycle/time-dependent FCP condition with various R-ratios.

Fatigue strength improvement factors for high strength steel welded joints treated by high frequency mechanical impact

Numerous studies have observed that the fatigue strength of improved welds increases with material yield strength. This paper provides a comprehensive evaluation of published data for high frequency mechanical impact treated welds. In total, 228 experimental results for three weld geometries subject to R=0.1 axial loading have been reviewed. A design recommendation including one fatigue class increase in strength (about 12.5%) for every 200MPa increase in static yield strength is proposed and are shown to be conservative with respect to all available data. Special cautions are given for high R-ratio or variable amplitude fatigue and potential alternate failure locations.

Tension–tension fatigue of hybrid composite rods

The tension–tension fatigue behavior was investigated for a hybrid composite rod comprised of a unidirectional carbon fiber core and a glass fiber shell. Fatigue tests were performed at three R-ratios and four maximum applied stress levels (MAS) while recording the secant modulus at each cycle, and acoustic emission (AE) sensors were employed to monitor the activation of fatigue mechanisms. Fatigue failure occurred when the composite rod was no longer able to support the applied cyclic load. For a MAS level of 70% of the ultimate tensile stress (UTS), composite rods tested at higher R-ratios showed AE activity through a larger percentage of fatigue life, but exhibited a greater resistance to fatigue failure, whereas samples cycled at lower R-ratios displayed AE activity only near the end of fatigue life, and showed a lower resistance to fatigue failure. The hybrid composite showed modes of progressive fatigue damage at high R-ratios and low strain amplitudes in the form of longitudinal splitting of the GF shell. In contrast, failure of the CF core was catastrophic and non-progressive. The fatigue resistance and damage mechanisms of the composite rod were dependent on the MAS level and R-ratio. Fatigue cracks initiated because of fretting between the GF shell and grip surface, which led to the observed longitudinal splitting of the GF shell. Fatigue damage occurred along the GF/CF interface where non-uniform strains developed because of the clamping force of the grip on the GF surface. At an R-ratio of 0.85, a fatigue stress of 70% UTS caused catastrophic fatigue failure, while at lower stresses, composite rods did not fail and withstood cyclic loads up to 1 million cycles. The research conducted is the first to investigate the degradation in fatigue performance arising from grip/composite rod interactions and suggests that the results from the study provide new information for composite materials in industries that utilize unidirectional composites in cylindrical form.

Fatigue Crack Growth Behavior in Fusion Zone of 16MnR Steel Weldment

Fatigue test is carried out for 16MnR steel weldment. The compact tension (CT) samples with 3.8mm thickness are used, and comparisons of fatigue crack growth between base metal and fusion zone in the weldment are considered in Paris region. It is shown that fatigue crack growth rate in the base metal is not sensitive to R-ratio, but the crack growth rate in the fusion zone increases with R-ratio increasing. To the low R-ratio, the crack growth rate in the fusion zone is smaller than that in the base metal, but they approximate the same under the high R-ratio. The mechanism of fatigue crack growth is analyzed through crack growth path in the microstructure, the different of fatigue crack growth behavior is studied for the base metal and the fusion zone.

Local compression에 의한 CTOD 시편내의 용접잔류응력 재분포

When conducting CTOD test, especially in thick welded steel plate, fatigue pre-cracking occasionally failed to satisfy the requirements of standards thus making the test result invalid. Internally accumulated residual stress of test piece has been thought as one of the main reasons. The propagation of fatigue crack, started from the tip of machined notch, which might have propagated irregularly due to residual stress field. To overcome this kind of difficulty three methods to modify the residual stress are suggested in standard i.e. local compression, reverse bending and stepwise high-R ratio method. In this paper not only multi pass welding but also local pre-compressing process of thick steel plate has been simulated using finite element method for clarifying variation of internal welding residual stress. The simulated results show that welding residual stress is compressive in the middle section of the model and it is predominantly increased after machining the specimen. Comparing as-welded state all component of the welding residual stress changing to compressive in the tip of machine notch whereas residual stress of the outer area remain as tensile condition relatively. Analysis results also show that this irregular residual stress distribution is improved to be more uniformly by applying local compression.

Fatigue crack growth – Mode I cycles with periodic Mode II loading

The influence of periodic Mode II loading on Mode I fatigue crack growth is studied using a new type of loading device. The average rate of crack growth is found to be governed mainly by four parameters; Δ K I and the Mode I R-ratio, the magnitude of the Mode II load and the Mode II period, M ( M = number of Mode I load cycles per Mode II load). The Mode II load has to be large enough to create residual tangential displacements of the crack faces otherwise no effect occurs at all. Two mechanisms are found, Mode II-induced crack closure that reduces the crack propagation rate (long range) and a mechanism that increases the growth rate temporarily at every Mode II load (short-range). For a specific Mode I load and Mode II magnitude, it is possible to find a minimum crack growth rate for a certain Mode II period, M opt. At high R-ratios the Mode II-induced closure mechanism disappears, but the temporary increase in growth rate at every Mode II load is still active. A semi-analytical model for the average crack growth rate during sequential Mode I and Mode II loading is presented.