Ermüdung von Stahl × 20 Cr 13. Rißwachstum bei Lastwechselfrequenzen von 10−3 Hz bis zu 21 KHz,Bestimmung des Grenzwertes ΔK0 mit Ultraschall

Fatigue Crack Growth in a 13% Chromium Steel at Frequencies from 10^?3 to 10⁴ Hertz. Determination of the Threshold Stress Intensity ΔK₀ by Ultrasonic Fatigue Fracture mechanics fatigue crack growth tests were run with loading frequencies from 10^?3 to 10⁴ Hertz. The results show that the environmental effects on fatigue crack growth decrease strongly at higher frequencies. Ultrasonic fatigue tests in water yield the same crack growth rates as low frequency fatigue tests in vacuum. Ultrasonic fatigue tests allow the measurement of the fatigue threshold stress intensity ΔK₀ within reasonable testing times.     

Neue Beobachtungen über das Phänomen der Wechseldehnungs-Versprödung an einer Rißfront anhand von Sprödbruchversuchen

New Observations about the Phenomenon of Cyclic Strain Embrittlement at the Fatigue Crack Tip by Means of Brittle Fracture Tests . Fracture tests were performed on flat samples notched on both sides (net section 120 × 30 mm²) of material 19 Mn 5 to determine the influence of a local embrittlement on the nominal fracture stresses. The local embrittlement in a very small area could be produced by pulsating tensile tests due to cyclic strain could be produced by pulsating tensile tests due to cyclic strain embrittlement at 350°C gradually different in the base of the notch respectively at the tip of the growing fatigue crack. Fractures due to “low” nominal stress tend to occur in those cases where the rate of crack growth in the pulsating tensile test is approaching zero or is equal to zero.     

Fatigue and corrosion of a Pd-based bulk metallic glass in various environments

Bulk metallic glasses (BMGs) possess attractive properties for biomedical applications, including high strength, hardness and corrosion resistance, and low elastic modulus. In this study, we conduct rotating beam fatigue tests on Pd₄₃Ni₁₀Cu₂₇P₂₀ bulk metallic glass in air and Eagle's medium (EM) and measure the corrosive resistance of the alloy by submersion in acidic and basic electrolytes. Fatigue results are compared to those of commonly used biometals in EM. Rotating beam fatigue tests conducted in air and in Eagle's medium show no deterioration in fatigue properties in this potentially corrosive environment out to 10⁷ cycles. A specimen size effect is revealed when comparing fatigue results to those of a similar alloy of larger minimum dimensions. Corrosion tests show that the alloy is not affected by highly basic (NaOH) or saline (NaCl) solutions, nor in EM, and is affected by chlorinated acidic solutions (HCl) to a lesser extent than other commonly used biometals. Corrosion in HCl initiates with selective leaching of late transition metals, followed by dissolution of Pd.     

Effect of environment on the low cycle fatigue behaviour of cast nickel-base superalloy IN738LC

Low cycle fatigue tests were conducted on cast nickel-base superalloy IN738LC in vacuum and air and on specimens coated with a layer of NaCl and Na₂SO₄ at 900°C and two different strain rates. The fatigue life, determined in terms of the cyclic plastic strain, decreased markedly as the severity of the environment increased, ie in the order vacuum, air, NaCl + Na₂SO₄. The marked strain-rate effect found in air and saline environments disappeared when the experiments were conducted in a vacuum of 1.3 × 10^?2 Pa. Cyclic plastic deformation had the effect of producing coarsening which was associated with cyclic softening. Crack nucleation occurred in favourably oriented grains in vacuum while for air and for NaCl + Na₂SO₄ coated specimens the cracks always initiated at hot-corroded grain boundaries near the surface or generally at oxide spikes in surface-connected grain boundaries. The crack propagation from these nucleation sites was essentially transgranular for all environments studied.     

Ermüdungsrißwachstum in Kerben

Fatigue Crack Growth in Notches Nowadays it is wellknown that an important part of the fatigue life time, usually differenciated in crack initiation and crack growth, is often controlled by fatigue crack growth of cracks in notches. An elastic-plastic on the J-integral based crack growth model considering the crack opening and closure phenomenon will be described to determine crack growth of cracks in notches between crack initiation and failure. Experimental results and finite element analysis were used to verify the developed model.     

Einige Gesichtspunkte für die Auswahl von Stählen in der chemischen Höchstdrucktechnik

Some Aspects of Selecting Materials in the Chemical High Pressure Technology . Steels for chemical industry in the pressure range between 2000 and 3000 bar must have high yield strengths in order to be able to bear the static internal pressure. Even with high yield strengths the wall thickness is so high that steels must be selected which can be thoroughly and evenly quenched and tempered. They should be insensitive to tempering embrittlement. For components stressed by static internal pressure the safety against bursting and the bursting behaviour play a major rǒle. In addition to that, under pulsating pressures, the fatigue characteristics of a component are of importance. Steels with the high fatigue strengths necessary in high pressure application are notch sensitive. It is shown which measures are to be taken from the metallurgical side and in fabrication to increase their fatigue strength. Results of fatigue tests on high pressure tubes with different surface treatment are given. The fatigue strength can be improved by improving tube manufacturing, mechanically or electrochemically smoothening the inner tube surface, nitriding or autofrettage. For special purposes it is sometimes necessary to use steels whose strength is too low for high pressure applications. One must then apply a strength calculation conceding partial plastic deformation. This is illustrated for an age hardening austentitic steel.     

Threshold and growth mechanism of fatigue cracks under mode II and III loadings

ABSTRACT The fatigue crack growth behaviour of 0.47% carbon steel was studied under mode II and III loadings. Mode II fatigue crack growth tests were carried out using specially designed double cantilever (DC) type specimens in order to measure the mode II threshold stress intensity factor range, ΔK_IIth. The relationship ΔK_IIth > ΔK_Ith caused crack branching from mode II to I after a crack reached the mode II threshold. Torsion fatigue tests on circumferentially cracked specimens were carried out to study the mechanisms of both mode III crack growth and of the formation of the factory‐roof crack surface morphology. A change in microstructure occurred at a crack tip during crack growth in both mode II and mode III shear cracks. It is presumed that the crack growth mechanisms in mode II and in mode III are essentially the same. Detailed fractographic investigation showed that factory‐roofs were formed by crack branching into mode I. Crack branching started from small semi‐elliptical cracks nucleated by shear at the tip of the original circumferential crack.     

Zur Rißausbreitung in Stahl unter schwingender Belastung. Der Einfluß einiger Versuchsparameter und des Gefüges auf die Rißwachstumsgeschwindigkeit

Crack Growth in Steel under Fatigue Conditions. The Influence of Research Parameters and Microstructure on Crack Growth Rates Crack growth rates in a plain carbon eutectoid steel were measured. The microstructure and the stress ratio were variied. The experiments were conducted in air and in a 5% NaCl-H₂O solution. The effect of the research parameters, atmosphere and microstructure on the constants m and C in the PARIS equation were determined. Possible mechanisms for the increase of crack growth rates due to a corrosive medium, an increase of stress ratio R, and/or the mechanical strength of the steel by specific heat treatment are presented.     

Beanspruchung von Hohlzylindern mit exzentrisch eingeschweißten Stutzenrohren unter Innendruck

Internal Pressure Loading of Hollow Cylinders with excentric welded-in Nozzles Investigations were carried out on nozzles welded on to the basic pipe with a V-seam without counter welded root; the material was of St 35 and WB 36. Flat specimens St 37 and BH 51 with a cross seam were delivered for comparative fatigue tests. The highest stressed part was established on the hole edge in the base material for nearly all types during static measurements (with and without longitudinal force compensation). The stress concentration factor α_K, plastic support factor n_0,2 and weakening factor v_0,2 were determined for the highest stressed part on samples of St 35 (exception WB 36). Stresses with longitudinal force compensation were somewhat larger than without, that means the results of internal pressure pulsating tests can be regarded as the lower limit of loading types. With internal pressure pulsating, cracks were predominantly on the hole edge in the base material, i. e. the negative influence of the (ground) seam was mostly smaller than the differences in maximum strains between hole edge and weldment. Compared with the test results of polished test bars, a surface factor f₀ would be defined. As far as the crack was within the range of the weld a “weld factor” f_S could be added so that the fatigue can be calculated with the total factor f_ges - f₀ × f_S from the strain amplitude of the polished bar. When utilizing the static design basis it turns out that the higher strength of WB 36 as against St 35 allows only a factor 2 ÷ 10 lower fatigue life (pulsating loading).     

FATIGUE LIFE OF AN AUTOFRETTAGED THICK-WALLED PRESSURE VESSEL WITH AN EXTERNAL GROOVE

Abstract— Fatigue tests that simulate an autofrettaged thick-walled pressure vessel with an external groove under pulsating internal pressure loading conditions were performed using specimens taken from an autofrettaged thick-walled pressure vessel. Load-controlled simulation fatigue tests using rectangular, elliptic, and shot peened elliptic grooved specimens were performed for three different autofrettage levels of 50, 75, and 100% overstrains. In order to estimate the fatigue life of the thick-walled pressure vessel subjected to pulsating internal pressure, the local strain approach was considered to assess the crack formation life. A cyclic stress-strain relation and fatigue damage models determined from strain-controlled low cycle fatigue tests were employed to estimate the fatigue life of the autofrettaged thick-walled pressure vessel. Larger local stresses and strains were obtained from the Neuber's rule compared to the linear rule and these led to conservative fatigue life estimations. Estimated fatigue lives were obtained within factors of 2 to 4, compared to the experimental fatigue lives determined from the simulation fatigue tests.     

Corrosion fatigue behaviour of brass in aqueous solutions

The corrosion fatigue behaviour of α-brass was investigated in NaCl, NaNO₂, (NH₄)₂SO₄ and (NH₄)₂SO₄ + CuSO₄ solutions by applying the reverse bending technique at 60 cycles/min. With the exception of NaCl, all electrolytes reduced the fatigue life compared with that in air or water. Anodic polarization caused further reduction in the fatigue life in all solutions. In NaCl the failure was mainly due to electrochemical dissolution of the brass, while in ammoniacal solution both thinning and corrosion fatigue occurred. In NaNO₂ solution corrosion was inhibited but severe corrosion fatigue occurred. The fracture mode changed from transgranular in air to mixed mode, with different percentages of the intergranular type depending on the electrolyte.     

Ermüdungsschwellwert: Materialeigenschaft und Auslegungsparameter

Fatigue Crack Propagation Threshold: Material Property and Design Criteria The fatigue crack propagation threshold has been determined by two experimental methods covering the tension-tension fatigue regime. The two experimental methods are boundary conditions for this fatigue regime. A third experimental method positions the threshold somewhere between K_max and K_min such that because of the ?Closure Parameter K_op”? the threshold is not positioned below 0.5 K_max. The results obtained show that a threshold exists under all fatigue loading conditions and – depending on the subject, material – is either a material property or a material parameter which depends on K_max     

LOW-TEMPERATURE AUTOFRETTAGE: AN IMPROVED TECHNIQUE TO ENHANCE THE FATIGUE RESISTANCE OF THICK-WALLED TUBES AGAINST PULSATING INTERNAL PRESSURE

Abstract— It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick-walled tubes against pulsating internal pressure. The physical reason is based on the well-known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick-walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between-110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low-temperature autofrettage in enhancing both the fatigue life in the finite-life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low-temperature autofrettage performance are discussed.     

A unified model of fatigue kinetics based on crack driving force and material resistance

Fatigue in Al-alloys is largely a process of crack growth from pre-existing defects occurring by several different mechanisms, each of which dominates a particular rate-driven segment of fatigue kinetics. These include fatigue void formation through interfacial cracking of secondary particulates, crack extension by brittle micro-fracture (BMF) in near-threshold fatigue, slip driven crack growth in the Paris regime and quasi-static crack extension by the well-known micro-void coalescence (MVC) and the less known fatigue void coalescence (FVC). BMF is mean stress and sequence-sensitive.Mechanism selection for fatigue crack extension in each load cycle occurs on the principle of least resistance to crack driving force represented by ΔK and K_max. Crack extension will switch to a different failure mechanism given reduced resistance to that mechanism by comparison to the current one. Increasing driving force will thus force a switch from BMF to shear and then onto MVC or FVC in that order, over each rising load half-cycle. Higher growth rates will therefore always be associated with a mix of all these mechanisms.     

Mixed-mode crack opening displacement measurements for small fatigue cracks growing in Astroloy at 20 °C

Crack opening displacements were measured for small fatigue cracks in Astroloy being grown with uniaxial stress application under high-cycle fatigue conditions. Four cracks were investigated including one that grew from 27 to 74 μm in three increments. Most of the cracks grew at an angle to the loading axis and all opened bimodally. Crack opening scaled with distance from the crack tip similar to an elastic crack, which allowed the calculation of a local stress intensity factor for both mode I and mode II. The proportion of mode II stress intensity factor was relatively large, varying as 0.06 < Δ K _II /Δ K _I < 0.42, with an average of ~0.3. Thus, uniaxial loading remote to the cracks resulted in a bimodal opening response on the scale of the cracks.     

Fatigue crack growth of a railway wheel

Typically, fatigue crack propagation in railway wheels is initiated at some subsurface defect and occurs under mixed mode (I–II) conditions. For a Spanish AVE train wheel, fatigue crack growth characterization of the steel in mode I, mixed mode I–II, and evaluation of crack path starting from an assumed flaw are presented and discussed.Mode I fatigue crack growth rate measurement were performed in compact tension C(T) specimens according to the ASTM E647 standard. Three different load ratios were used, and fatigue crack growth thresholds were determined according to two different procedures. Load shedding and constant maximum stress intensity factor with increasing load ratio R were used for evaluation of fatigue crack growth threshold.To model a crack growth scenario in a railway wheel, mixed mode I–II fatigue crack growth tests were performed using CTS specimens. Fatigue crack growth rates and propagation direction of a crack subjected to mixed mode loading were measured. A finite element analysis was performed in order to obtain the K_I and K_II values for the tested loading angles. The crack propagation direction for the tested mixed mode loading conditions was experimentally measured and numerically calculated, and the obtained results were then compared in order to validate the used numerical techniques.The modelled crack growth, up to final fracture in the wheel, is consistent with the expectation for the type of initial damage considered.     

Effects of various environments on fatigue crack growth in Laser formed and IM Ti–6Al–4V alloys

The fatigue crack growth behaviors of Laser formed and ingot metallurgy (IM) Ti–6Al–4V alloys were studied in three environments: vacuum, air and 3.5% NaCl solution. Taking the Unified Fatigue Damage Approach, the fatigue crack growth data were analyzed with two intrinsic parameters, stress intensity amplitude ΔK and maximum stress intensity K_max, and their limiting values ΔK^* and . Fatigue crack growth rates da/dN were found increase with stress ratio R, highest in 3.5% NaCl solution, somewhat less in air and lowest in vacuum, and higher in IM alloy than in Laser formed one. In 3.5% NaCl solution, stress corrosion cracking (SCC) was superimposed on fatigue at R=0.9 for where K_max>K_ISCC, the threshold stress intensity for SCC. This and environment-assisted fatigue crack growth were evidenced by the deviation in fatigue crack growth trajectory (ΔK^* vs. curve) from the pure fatigue line where . Furthermore, the fractographic features, identified along the trajectory path, reflected the fatigue crack growth behaviors of both alloys in a given environment.     

Beanspruchungssynchrone elektrochemische Messungen an SwRK-beanspruchten Proben aus rost- und säurebeständigem Stahl

Stress Synchron Electrochemical Measurement with Stainless Steel in Corrosion Fatigue The stress transient corrosion current behaviour (tensile stress of varying magnitude) of stainless steel (Type 306 and AF 22) in corrosion fatigue was studied in aqueous sulfidic acid solutions at temperatures up to 70 °C. It was found that the transient current is set by the alternating strain rate and is no function of the total applied stress. In the state of unfractured specimen surface the transient corrosion current describes the passive layer like an ?mechanical impedance”? and is therefore no function of experimental time. Crack initiation is marked by a typical deforming of the transient current signal (phase shift and increasing amplitude). This indicates the state of starting crack much more earlier than any other signal and is therefore an excellent help in studying corrosion fatigue crack initiation.     

Ermüdung von Keramik unter Schwingbeanspruchung

Fatigue of Ceramics under Cyclic Loading Fatigue of ceramics attracts more attention due to the expected availability of high-performance ceramic components e.g. for engines, However, the knowledge in fatigue of brittle materials is still inadequate, the actual standpoint as taken from literature is shortly reviewed. In this study two experimental-analytical ways are presented which manifest the chances and difficulties in this part of the fatigue research. First, the probabilistic approach to identify a true cyclic fatigue effect in brittle ceramics is envisaged. The second way applies direct observation of crack extension in static and cyclic loading mode in order to define the conditions and characteristic differences of both types of crack propagation. Results are presented from both test methods for several Si₃N₄, SiC and ZrO₂ materials and possible mechanisms are discussed. It is concluded, that toughened ceramics are more prone to cyclic fatigue effects than conventional and pure brittle ceramics (e.g. glass).     

Very high cycle fatigue of a high strength steel under sea water corrosion: A strong corrosion and mechanical damage coupling

This paper is focused on the effect of sea water corrosion on the gigacycle fatigue strength of a martensitic–bainitic hot rolled steel R5 used for manufacturing off-shore mooring chains for petroleum platforms in the North Sea. Crack initiation fatigue tests in the regime of 10⁶ to 10¹⁰ cycles were carried out on smooth specimens under three different environment conditions: (i) without any corrosion (virgin state) in air, (ii) in air after pre-corrosion, and (iii) in-situ corrosion-fatigue under artificial sea water flow. A drastic effect of sea water corrosion was found: the median fatigue strength beyond 10⁸ cycles is divided by 5 compared to virgin state specimens. The crack initiation sites were corrosion pits caused by pre-corrosion or created during corrosion-fatigue under sea water flow. Furthermore some sub-surface and internal crack initiations were observed on specimens without any corrosion (virgin state). Crack propagation curves were obtained in mode I in air and under sea water flow. Calculation of the stress intensity factor at the tip of cracks emanating from hemispherical surface pits combined with the Paris–Hertzberg–Mc Clintock crack growth rate model showed that fatigue crack initiation period represents most of the fatigue life in the VHCF regime. Additional original experiments have shown physical evidences that the fatigue strength in the gigacycle regime under sea water flow is mainly governed by the corrosion process with a strong coupling between cyclic loading and corrosion.