An analysis of non-propagating fatigue cracks

An analysis for the formation of nonpropagating fatigue cracks at (the base of V-shaped) notch roots, based on the considerations of the extent of the critically stressed region ahead of a notch or a crack tip, and the resulting volumetric strength effect, is developed. Assuming that the minimum local cyclic stress required for crack initiation from a notch root is equal to the unnotched fatigue limit, σ_e, and that the minimum local cyclic stress required for the propagation of the crack is equal to the theoretical strength of the material, σ_e, a model of notch fatigue limit is proposed that shows that nonpropagating cracks should form at the notch base if ρ≤ ρ⁰, a critical root radius, provided the notch is sufficiently deep,i.e. d ≥ ρ₀. The radius ρ₀ is a material constant and can be estimated from known material properties. The estimated values of ρ₀ are in fairly good agreement with available experimental values for steels and pure copper. For stresses near the notch fatigue limit it is suggested that p₀ be regarded as a radius above which notch fatigue limit is essentially initiation controlled and below which essentially propagation controlled. The notch fatigue limit based on complete fracture can then be estimated more accurately with mild as well as sharp notches.     

The Influence of Notch Root Radius and Austenitizing Temperature on Fracture Appearance of As-Quenched Charpy-V Type AISI4340 Steel Specimens

Charpy-V type samples either step-quenched from 1200 °C or directly quenched from the usual 870 °C temperature, fractured by a slow bend test procedure, have been fractographically examined. Their notch root radius,ρ, ranged from almost zero (fatigue precrack) up to 2.0 mm. The fracture initiation process at the notch differs according to root radius and heat treatment. Conventionally austenitized samples withρ values larger than 0.07 mm approximately (ρ _eff) always display a continuous shear lip formation along the notch surface, whereas specimens with smaller notches do not exhibit a similar feature. Moreover, shear lip width in specimens withρ >ρ _eff is linearly related to the applied J-integral at fracture. In high temperature austenitized samples similar shear lips are almost nonexistent. The above findings, as well as overall fractographic features, are combined to explain why blunt notch AISI 4340 steel specimens display a better fracture resistance if they are conventionally heat treated, whereas fatigue precracked samples show a superior fracture toughness when they are step-quenched from 1200 °C. Variations of fracture morphologies with the notch root radius and heat treating procedures are associated with a shift toward higher Charpy transition temperatures under the combined influence of decreasing root radii and coarsening of the prior austenitic grain size at high austenitizing temperatures. D. FIRRAO, J. A. BEGLEY were both formerly with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210.     

An analysis of non-propagating fatigue cracks

An analysis for the formation of nonpropagating fatigue cracks at (the base of V-shaped) notch roots, based on the considerations of the extent of the critically stressed region ahead of a notch or a crack tip, and the resulting volumetric strength effect, is developed. Assuming that the minimum local cyclic stress required for crack initiation from a notch root is equal to the unnotched fatigue limit, σ_e, and that the minimum local cyclic stress required for the propagation of the crack is equal to the theoretical strength of the material, σ_e, a model of notch fatigue limit is proposed that shows that nonpropagating cracks should form at the notch base if ρ≤ ρ⁰, a critical root radius, provided the notch is sufficiently deep,i.e. d ≥ ρ₀. The radius ρ₀ is a material constant and can be estimated from known material properties. The estimated values of ρ₀ are in fairly good agreement with available experimental values for steels and pure copper. For stresses near the notch fatigue limit it is suggested that p₀ be regarded as a radius above which notch fatigue limit is essentially initiation controlled and below which essentially propagation controlled. The notch fatigue limit based on complete fracture can then be estimated more accurately with mild as well as sharp notches. D. N. LAL, formerly a Graduate Assistant in Materials Science, Syracuse University     

Local fatigue damage accumulation around notch attending crack initiation

The subsequent recrystallization technique was used to study the process of local damage accumulation around a notch under conditions of low-cycle fatigue. A 0.8-in. compact tension specimen of 304 stainless steel with a notch radius of 1 mm was used. The accumulated plastic zone around notch increases with the number of cyclesN. The accumulated plastic strain within the zone also increases withN, producing the strain gradient (damage gradient). A fatigue crack initiates when the accumulated plastic strain at the notch root reaches a critical value equal to the fracture strain of the material; that is, when the accumulated plastic work at the crack initiation site becomes critical. The fatigue crack emanating from a notch root grows through the pre-existing damaged zone. It is shown that this local damage accumulation approach can explain the fast growth of a short crack from a notch.     

Effect of Environment on Creep Crack Growth in PM/HIP René-95

The creep crack growth rates (CCGR) of PM/HIP René-95 were measured from 10^-9 m per second to 10^-4 m per second in air and in high purity argon at 760°C and 650°C using single edge notched (SEN) specimens. The crack length was monitored by the D.C. potential difference technique. The data were reported asda/dt vs the elastic stress intensity factor,K,, since PM/HIP René-95 is a creep-brittle material. The CCGR were shown to be strongly environment sensitive. The CCGR were up to 1000 times faster in air than in argon for a given value ofK _I . The temperature andK _I dependence of CCGR in air were shown to correlate with a modified formulation of the Larson-Miller parameter. Notched stress rupture (NSR) tests were performed at 650°C in air in order to study the effect of notch root radius on the time to initiate a creep crack. A comparison of the NSR data with SEN data for the same values of initialK, shows that the crack initiation times are a strong function of the notch root radius. It was observed that ninety percent of the rupture time is spent in crack initiation when the notch root radius is finite, while no incubation time was observed for creep crack growth from fatigue precracked specimens. Formerly with Massachusetts Institute of Technology, Cambridge, MA     

Influence of Non‐linear Effects on Fatigue Assessment of a Welded Structure

In this paper root cracks in a welded structure are investigated using the effective notch stress approach and Linear Elastic Fracture Mechanics (LEFM). The problem is complicated by the fact that the welds subjected to endurance stress are located close to a screw joint. The Finite Element (FE) model used for evaluating the fatigue life includes both this screw joint as well as adjacent components and the contact behaviour between the structures. Several important problems in the evaluation of stress intensity factors and notch stresses are investigated. The results can give useful information in the work of performing a fatigue analysis. The areas examined are the significance of the load direction, mesh density, the importance of the right global stiffness in the model, the crack deflection angle and the modelling of a root notch versus the requirements on the drawing.     

Influence of specimen geometry on fracture mechanics values at the onset of stable crack extension

The influence of specimen geometry was studied on 80 mm thick plates of Fe 510 steel. The multi-specimen technique according to ASTM E 813 was compared with SZW and DCPD measurements. Using the ASTM standard lower J_lc values are determined with increasing specimen thickness and overall geometry dimensions, because the calculated blunting line doesn't represent the material's blunting behaviour and the conditions for ‘J-controlled’ crack growth are not applied sufficiently strict in this standard. The initiation values J_i are geometry independent and are lower than J_lc. A proposal to determine a technical J_0.2 value has been suggested for a constant value of Δa = 0.2 mm, which is less complicated concerning the procedure being used, which is resulting in minor geometry independence and is approaching the physical initiation values.     

Critical assessment of the local cleavage stress σ ƒ * in notch specimens of C-Mn steel

An elastic-plastic finite element method (FEM) was used to calculate the stress and strain distributions ahead of notches with various root radii in a bending specimen of C-Mn steel with grain sizes of 10 and 30 μm. By accurately measuring the distance of the cleavage initiation site from the root of the notch, the local cleavage stress σ _ƒ ^* was measured. When the notch radius increased from 0.25 to 1.0 mm, the distribution of high stress had a definite variation but the σ _ƒ ^* remained relatively constant. In notch specimens with different root radii, the critical fracture event is identical,i.e., propagation of a ferrite grain-sized crack into the neighboring matrix. Therefore, the σ _ƒ ^* is mainly determined by the length of the critical microcrack, here, the size of ferrite grain instead of the high stress volume for finding an eligible brittle particle. The critical strain for initiating a crack was about 1 pct. The cleavage site ahead of a notch was related to the relative distributions of stress and strain and the random distribution of the weakest grains. The higher fracture load of the fine-grain material can be attributed to its higher value of σ _ƒ ^* /σ_o as compared with the coarse-grain. The σ _ƒ ^* /σ_o is a potential engineering parameter for toughness assessment in notch specimens.     

High‐Strength Steels in Welded State for Light‐Weight Constructions under High and Variable Stress Peaks

In design codes (Eurocode, British Standard and others) for the dimensioning of welded joints, no distinction is made between low, medium and high strength steels. Because of a lack of general knowledge about the benefits of high‐strength steels and also because of missing information in design codes, in many cases design engineers still use low or medium strength steels (R_p0.2 < 400 MPa) and compensate for high loads under constant or variable amplitude loading or overloads by increasing dimensions. Given this situation, it was deemed necessary to establish criteria for the design of light‐weight welded constructions under high and variable stress peaks using new classes of high strength steels, such as S355N (normalized), S355M (thermomechanically treated), S690Q (water quenched) and S960Q (water quenched), and to perform more reliable evaluations of the fatigue performance of high strength steel structures subjected to complex loading with regard to light‐weight design and economics. For the comparison of the fatigue strengths of the investigated steels the notch factors present were taken into account. Additionally, the real damage sums were determined in order to give recommendations for the fatigue life estimation. Under constant amplitude loading, no significant difference in the bearable local stress amplitudes for the butt welds can be detected for the four investigated steels. Under variable amplitude loading, the butt welded (lower notch factor) high strength steel S960Q has advantages in the case of the normal Gaussian spectrum and in the case of overloads, especially under pulsating loading. For the transverse stiffeners (high notch factor), slight advantages for the high strength steel S960Q exist, only in the case of pulsating overloads. However, the advantages of high strength steels in case of static loading are indisputable. In most of the investigated cases, overloads lead to a benefit in fatigue life.     

Structure and mechanical properties of the welded joints of large-diameter pipes

The structure and mechanical properties of the technological welded joints of large-diameter pipes of strength class K60 produced by two companies are studied. Along with standard mechanical properties (σ_0.2, σ_u, δ, ψ), specific work of deformation a (tensile toughness) and true rupture strength S _f are estimated from an analysis of the stress-strain diagrams constructed in true coordinates. The mechanical behavior is found to be different for samples cut from different zones of a welded joint (central weld, heat-affected zone, and base metal). The mutual correlation between parameters a, S _f, and impact toughness KCV is considered.     

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityK _II of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m^1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔK _II values but of quasi-cleavage at higher ΔK _II values and were different from those in air.     

Microstructure and Mechanical Properties of Fiber-Laser-Welded and Diode-Laser-Welded AZ31 Magnesium Alloy

The microstructures, tensile properties, strain hardening, and fatigue strength of fiber-laser-welded (FLW) and diode-laser-welded (DLW) AZ31B-H24 magnesium alloys were studied. Columnar dendrites near the fusion zone (FZ) boundary and equiaxed dendrites at the center of FZ, with divorced eutectic β-Mg₁₇Al₁₂ particles, were observed. The FLW joints had smaller dendrite cell sizes with a narrower FZ than the DLW joints. The heat-affected zone consisted of recrystallized grains. Although the DLW joints fractured at the center of FZ and exhibited lower yield strength (YS), ultimate tensile strength (UTS), and fatigue strength, the FLW joints failed at the fusion boundary and displayed only moderate reduction in the YS, UTS, and fatigue strength with a joint efficiency of ~91 pct. After welding, the strain rate sensitivity basically vanished, and the DLW joints exhibited higher strain-hardening capacity. Stage III hardening occurred after yielding in both base metal (BM) and welded samples. Dimple-like ductile fracture characteristics appeared in the BM, whereas some cleavage-like flat facets together with dimples and river marking were observed in the welded samples. Fatigue crack initiated from the specimen surface or near-surface defects, and crack propagation was characterized by the formation of fatigue striations along with secondary cracks.     

超声冲击对P355NL1钢焊接接头超高周疲劳性能影响

 通过超声疲劳试验探究超声冲击对P355NL1钢焊接接头超高周疲劳性能的影响。结果表明,由疲劳[S-N]曲线可知,在105～109寿命区间内,冲击态试样的疲劳性能要高于焊态试样,在1.0×108的疲劳寿命下,焊态试样的疲劳强度为139 MPa,冲击态的疲劳强度为217 MPa,冲击态疲劳强度相较于焊态提高了56%,这表明超声冲击可以明显提高焊接接头的疲劳强度。利用扫描电镜（SEM）观察断口形貌可以发现,裂纹源位于焊接接头焊根区表面。P355NL1钢焊接接头疲劳断裂为准解理断裂,超声冲击可以提高焊接接头的疲劳强度,但不会改变其疲劳失效机理。超声冲击可以降低焊根处应力集中,引入有益压应力和表面晶粒细化,从而提高焊接接头的疲劳强度。     

热输入对S43932超纯铁素体不锈钢焊接接头组织性能的影响

对1.0 mm厚度的S43932不锈钢冷轧板分别进行88、101、132、188 J/mm的钨极氩弧焊焊接,研究了焊接区的显微形貌和相组成。详细分析了垂直焊缝、平行焊缝和带缺口焊缝试样的拉伸性能及焊缝杯突试验结果,当热输入为101 J/mm时,焊接接头具有最佳的拉伸及成形性能匹配,之后随热输入＞101 J/mm,焊接接头抗拉强度和埃里克森杯突值急剧下降。焊接接头良好的力学性能与焊接区小尺寸晶粒的形成及大量富Ti(Nb,Ti)(C,N)相的析出有关。     

Evaluation of toughness in AISI 4340 alloy steel austenitized at low and high temperatures

It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,K _Ic, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenK _Ic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onK _Ic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required. formerly with Effects Technology, Inc., Santa Barbara, CA     

焊接参数对超细晶粒钢SS400焊接接头组织和硬度的影响

分别采用J442(/%:≤0.12C、0.3~0.6Mn)、J506(/%:≤0.12C、≤1.6Mn)和J507(/%:≤0.12C、≤1.25Mn)焊条对7 mm SS400钢板(/%:0.18C、0.04Si、0.43Mn、0.022Al、0.002Ca)在热输入6~14 kJ/cm下进行手工电弧焊实验,并通过光学显微镜和显微硬度计观察和测试焊接接头的组织和硬度。结果表明,随焊条合金元素含量的增加,焊缝组织中针状铁素体量增加,当热输入≤10 kJ/cm时,焊接热影响区粗晶尺寸变化不大,当热输入大于10 kJ/cm时,粗晶尺寸明显增大;不同焊接工艺焊接接头的显微硬度均高于母材的显微硬度,没有接头软化现象。     

Low cycle fatigue crack propagation in hastelloy-X at 25 and 760 °C

Fatigue crack propagation tests were conducted on Hastelloy-X in air, at 25 °C and at 760 °C under controlled plastic strain amplitudes in the fully plastic low cycle fatigue regime. The crack growth rate data for different strain levels were correlated with the range of theJ integral ΔJ. The ΔJ values were calculated from finite element numerical solutions. It was found that the assumption thatda/dN =A(Δε _ρ ) ^α a is only an approximation of the more general equationda/dN =B(ΔJ) ^α in a narrow range of crack lengths. It is shown that theoretical models predicting low cycle fatigue lives by integrating the fully plastic crack growth rates will be in error if the (da/dN, ΔJ) relationship is not used.     

Comparisons of experimental measurements and a theoretical model for specimen self-heating during fatigue of type 316 LN stainless steel

The Type 316 stainless steel is being considered as a candidate target-container material for the spallation neutron source (SNS) being built at the Oak Ridge National Laboratory. Satisfactory behavior under fatigue loading is a requirement for the target container. Stress-controlled fatigue experiments were performed on the 316 stainless steel at 0.2 and 10 Hz with an R ratio of −1, where R=σ _min./σ _max.; σ _min. and σ _max. are the minimum and maximum applied stresses, respectively. At R=−1, a large specimen-temperature increase at 10 Hz was observed, which approached approximately 350 °C at a stress amplitude of 263 MPa, and affected fatigue lives. The specimen temperature at 0.2 Hz was about room temperature. The fatigue lives at 10 Hz were found to be shorter than those at 0.2 Hz. Different specimen temperatures were achieved by varying test frequencies. Significant differences in fatigue lives as a function of test frequency were observed with shorter fatigue lives at higher frequencies. The higher specimen temperature at 10 than at 0.2 Hz reduced the fatigue life at 10 Hz. A model based on the dissipation energy of the specimen during fatigue tests was developed to explain the fatigue-life result and predict the specimen-temperature evolution. The present research is sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, United States Department of Energy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC.     

Effect of Notch Root Radius on the Apparent Fracture Toughness in CNT and Carbon Fiber Reinforced, Epoxy-Matrix Hybrid Composite

Significantly improved fracture resistance (in terms of fracture toughness) can be imparted to monolithic materials by adopting composite design methodology based on fiber and nano particulate reinforcement technology. The present work describes the fracture behaviour of one such reinforced material; in this case, carbon fiber (C_f)- and carbon nanotube (CNT)- reinforced epoxy composite. The C_f and CNT reinforced, epoxy-matrix hybrid composite in longitudinal and transverse orientations with varied finite notch root radii (in the range of 120–750 μm) are subjected to mode-I (tensile) fracture. The fracture toughness/resistance (K_Q) of the material is then evaluated and analyzed by investigating the influence of varying notch root radii in longitudinal and transverse orientations. Such an analysis has revealed that the present unidirectional epoxy hybrid composite exhibits a critical notch root radius of 270 μm in longitudinal and 390 μm in transverse orientation.     

The Temperature Dependence of the Fracture Stress of Notched Tensile Specimens

The fracture stresses of unnotched tensile specimens exhibit various temperature dependences. It is shown that such various dependences also occur in tensile tests on notched specimens. For that purpose tests were performed between ‐140 °C and +82 °C on notched specimens with notch root radii of 7.0, 2.0, and 0.075 mm. The observed data indicated seven temperature ranges with different temperature dependences of the fracture stress (and area reduction). These dependences were described by seven different fitting curves. The intersections of these fitting curves lead to the transition temperatures cleavage fracture ‐ mixed fracture and mixed fracture ‐ fibrous fracture. For notch root radii of 7.0 and 2.0 mm these tests yielded the same temperature dependences as were expected from unnotched tensile specimens. Specimens with a notch root radius of 0.075 mm showed some differences in the temperature dependence which are discussed in detail.