LOW CYCLE FATIGUE BEHAVIOUR OF STRUCTURAL STEEL MEMBERS AND CONNECTIONS

Abstract— An experimental study was carried out to investigate the low-cycle fatigue behaviour of steel beam and beam-column members, as well as of steel connections, with particular emphasis on beam-to-column joints. Several full-scale specimens were tested, in a multi-specimen testing program, using constant-amplitude displacement histories, to develop a cumulative-damage model. Variable amplitude tests were carried out in order to verify the validity of the proposed model.
It is shown that the S-N curves given by codes for high-cycle fatigue can be used to interpret the low-cycle fatigue behaviour of steel members and connections. Miner's rule can be used, together with the previously defined S-N curves and with a cycle counting method to define a unified cumulative damage model valid for both high and low cycle fatigue. A general failure criterion is proposed which considers the actual energy dissipation through plastic deformation.     

ELASTIC-PLASTIC BEHAVIOUR AND UNIAXIAL LOW CYCLE FATIGUE LIFE OF NOTCHED SPECIMENS

Experimental and numerical analyses were carried out to examine the elastic-plastic behaviour and low cycle fatigue (LCF) life of EN15R steel notched specimens. Notch root strains were measured and compared with estimates obtained from three methods: Neuber, Glinka and hite element (FE) analyses. All methods provided fairly accurate estimates of cyclic strain up to net section yield, from which point the Neuber and Glinka predictions were greater than measured. The finite element results compared well with measured results. The estimated notch root strains were used to predict the life of the notched specimens based on LCF results from unnotched specimens. Uniaxial Coffom-Manson and multiaxial Lohr-Ellison approaches were used. Improved fatigue life predictions were obtained when the FE predictions of the multiaxial strains were combined with a multiaxial strain parameter. The possible influence of strain gradient is inferred by comparing LCF lives for hollow thin-walled and solid bar specimens.     

THE INFLUENCE OF NITROGEN ON HIGH TEMPERATURE LOW CYCLE FATIGUE BEHAVIOUR OF AUSTENITIC STAINLESS STEELS

Abstract— High strain fatigue properties of AISI 316 , AISI 316N and Sandvik 253MA have been investigated at 600°C. The two latter alloys, which contain significant amounts of N, exhibit a higher resistance to fatigue than 316. This effect is accompanied by a planar dislocation slip mode in 316N and 253MA as opposed to a wavy slip mode in 316. The results provide strong evidence that N improves fatigue strength in austenitic stainless steels, by inhibiting cross-slip of screw dislocations.     

SLOW FATIGUE CRACK GROWTH AND THRESHOLD BEHAVIOUR IN IMI 685

Abstract— Fatigue thresholds and crack growth rates up to 10⁻⁴ mm cycle ⁻¹ have been measured in β processed IMI 685. The results obtained in laboratory air for material having an aligned α microstructure and a random basketweave microstructure displayed a pronounced load ratio dependence which increased with decreasing ΔK. This sensitivity to mean load was also apparent from the threshold results determined in a vacuum of 5 ± 10⁻⁶ torr.
Fractographic observations, compliance measurements, pd output and crack path replication have indicated that contacts can be made between the fracture faces at a number of points behind the crack tip during the load cycle. These contacts wedge the crack open, thus preventing the stress intensity from falling to the value associated with the minimum applied load. A critical stress intensity, K _op, has been determined which relates to the crack being fully "open" and the results are reanalysed and discussed in terms of an effective stress intensity range, Δ K _eff.     

A STUDY OF FATIGUE AND CREEP BEHAVIOUR OF FOUR HIGH TEMPERATURE SOLDERS

Creep and cyclic deformation behavior of two lead-free high temperature solder alloys, 95Sn-5Ag and 99Sn-1.0Cu, a high lead alloy 97.SPb-1.SAg-1.0Sn, and an Ag-modified eutectic alloy 62.SSn-36.1Pb-1.4Ag, were studied. Room temperature and high (100°C and 150°C) temperature fatigue tests (with cyclic strain amplitude up to 6.0%) for the four solders were conducted, with the fatigue lives ranging from a few cycles to more than 100,000 cycles. It is shown that among the alloys studied, 62.SSn-36.1Pb-1.4Ag (the modified Sn-Pb eutectic alloy) has the lowest fatigue resistance in term of low cycle fatigue life (strain controlled). The high lead alloy, 97.SPb-1.5Ag-1.0Sn, has the highest strain fatigue resistance in the large strain region (Δ > 2.0%). Temperature has a significant effect on alloys 95Sn-5Ag and 99Sn-1.0Cu, but has a negligible effect on the Ag modified Sn-Pb eutectic alloy 62.5Sn-36.1Pb-1.4Ag and 97.5Pb-1.5Ag-1.0Sn. Creep studies show that these alloys generally have a very significant primary creep regime (up to 20%); thus, any realistic constitutive relation has to take such a primary creep phase into consideration. Cyclic deformation of alloy 95Sn-SAg was simulated by using a constitutive relation built upon a 2-cell model, which covers both primary and secondary creep. This model provides a good estimate of the peak stresses (the minimum stress and the maximum stress in each cycle); it agrees with experimental results when the applied cyclic strain is small and/or the applied strain rate is very low.     

SLOW FATIGUE CRACK GROWTH AND THRESHOLD BEHAVIOUR IN AIR AND VACUUM OF COMMERCIAL ALUMINIUM ALLOYS

Abstract— Fatigue crack growth and threshold behaviour have been examined in three commercial aluminium alloys in both air and vacuum environments. It was observed that, in air, the threshold stress intensity range Δ K _t, varied linearly with the Δ K _t ratio. In contrast Δ K , in vacuum was found to be independent of R. Over the whole growth rate range examined fatigue crack growth in vacuum was Δ K controlled and failure occurred by a dimple and ductile striation mechanism. This also applied to failure in the intermediate growth rate ranges in air. However, at slow growth rates in air, fatigue crack growth was structure sensitive and crystallographic facets were formed during the crack propagation process.     

NEAR THRESHOLD FATIGUE CRACK PROPAGATION BEHAVIOUR OF A HETEROGENEOUS MICROSTRUCTURE

The threshold regime of a heterogeneous, pearlitic-ferritic microstructure was investigated at R= 0.1. Not only did the ΔK_th values vary depending on the test procedure in that da/dN vs ΔK curves before and after ΔK_th determination were not identical but transition ΔK values also differed. The observations are correlated with inherent micro- structural features.     

HIGH CYCLE FATIGUE AND A FINITE ELEMENT ANALYSIS

Abstract— We aim to develop a systematic method of designing structures, by finite element methods, for high cycle fatigue under periodic constant load systems. After having defined a precise terminology, we quickly list those multiaxial fatigue criteria which can be found in the literature. Some criteria, derived from a microscopic approach (Dang Van's, Papadopoulos' and Deperrois' criteria) are extensively presented. The criteria which can be reasonably retained for numerical analyses of structures are underlined and compared to one another. As a conclusion, we describe a high cycle fatigue CAD system which can be derived from this analysis.     

FATIGUE BEHAVIOUR OF A RAIL STEEL UNDER LOW AND HIGH LOADING RATES

Abstract— In this study, both sinusoidal wave loading and rectangular pulse (impact) loading are applied to a sample of rail steel in order to compare the growth rates for a surface fatigue crack. The results show that the rectangular pulse (impact) loading gives a crack growth rate on average 5 times that of the sinusoidal wave loading but can be almost 100 times in some instances. There are distinct differences between the fractograph patterns under the two modes of loading. The fractograph under sinusoidal wave loading clearly exhibits the pearlite structure of the material but under impact loading this microstructure is not revealed; only a quasicleavage pattern is observed. Analysis of the experimental fatigue crack growth data indicated that the surface fatigue crack growth rate behaviour could be described by the Paris-type relationship.     

一种单晶高温合金的高周疲劳行为(英文)

本文对一种单晶高温合金在800℃经高周疲劳实验的断裂特征和微观变形机制进行了详细研究,发现该单晶合金存在两种断裂形貌,一种是定向解理断裂,一种是蜂窝状撕裂,这两种断裂方式分别与按确定取向的滑移变形机制和位错胞变形机制相联系。还观察到了小角晶界的变形机制,合金中存在的极微量 MC 碳化物仍然是重要的疲劳源,该单晶高温合金的高周疲劳强度极限较之相应的普通铸造 IN738合金和低偏析定向凝固 DE38G 合金大幅度提高。     

HIGH TEMPERATURE LOW CYCLE BIAXIAL FATIGUE OF TWO STEELS

Abstract— Biaxial low cycle fatigue tests at various temperatures and strain rates were performed on 1% Cr-Mo-V steel and AISI 316 stainless steel under combined torsional and axial loads. A correlation for fatigue strength has been derived, and it is also shown that if the Gough ellipse quadrant criterion is rephrased in terms of strain amplitudes, it may be used as a safe design rule for ductile metals in both the low and high cycle fatigue regimes.     

EFFECT OF HOLD-TIME DAMAGE ON HIGH TEMPERATURE FATIGUE BEHAVIOUR

Abstract— —A series of tests has been carried out to investigate the effect of sequential high frequency fatigue cycles interspersed with hold times on elevated temperature crack growth rates in AISI type 304 stainless steel. Each test sequence included different combinations of fatigue cycles and hold-times. Those sequences which included the smallest number of fatigue cycles resulted in the fastest crack propagation rates. The sequence with a block of 1000 cycles and a 120min hold time had the slowest crack propagation rate, similar to that for a reference 5 Hz fatigue test. Examination of the fracture surfaces revealed alternating regions of transgranular and intergranular fracture corresponding to the respective fatigue block and hold-time period. Intergranular fracture was observed to be characteristic of maximum time-dependent and time-independent damage interaction, which was associated with those specimens subjected to sequences including a block of 50 fatigue cycles, possessing the fastest crack propagation rates. The results may be explained using a model based on fatigue-creep-environment interaction. During the hold-time a damaged zone due to oxidation induced cavitation formed at the crack tip. Depending on the stress intensity factor range, the subsequent fatigue cycles continued to extend the crack through part or whole of the remaining portion of this region. For the remainder of the fatigue block the crack propagated in its normal transgranular mode. Oxide induced closure at low Δ K levels brought about the lowest crack propagation rate for the sequence consisting of a block of 1000 cycles and a 120 min hold period.     

HIGH TEMPERATURE FATIGUE BEHAVIOUR OF A CAST COBALT-BASE SUPERALLOY

Abstract The initiation and the propagation of fatigue cracks and the low cycle fatigue life of a cast cobalt base superalloy was studied at 293K, 973K and 1173K by optical and scanning electron microscopy. A substantial decrease in fatigue life occurred at 973 and 1173K when compared to room temperature life. A time-dependent bulk damage was evidenced at 1173K which was determined by quantitative microscopy for two plastic strain levels. High strain fatigue crack propagation experiments were carried out at room temperature and at 1173K. From these experiments the decrease of the overall fatigue life at high temperature was shown to result from a considerable reduction of the initiation period due to oxidation and also from a significant acceleration of the crack propagation rate in the presence of oxidation and bulk damage.     

EFFECT OF PROCESSING ON THE HIGH TEMPERATURE LOW CYCLE FATIGUE PROPERTIES OF MODIFIED 9Cr-1Mo FERRITIC STEEL

The high temperature low cycle fatigue properties of modified 9Cr-1Mo ferritic steel in a hot forged and a hot rolled condition have been evaluated. The hot forged material exhibited inferior fatigue properties as compared to the finer grained hot rolled material. Analysis of the data indicates that a larger grain size adversely affects the initiation stage but has little effect on the propagation stage. A steeper slope on the Coffin-Manson plot results when the number of cycles to initiation is reduced.     

HIGH TEMPERATURE LOW-CYCLE FATIGUE BEHAVIOUR OF TWO TURBINE-DISC ALLOYS*

Abstract— The high temperature low-cycle fatigue behaviour of two turbine disc alloys was studied at 600 and 650°C, at total strain ranges, between 0·6 and 2·1%. These results showed that the life of both alloys may be predicted by a Coffin-Manson type equation. The fracture surfaces of the specimens were examined by scanning electron microscopy.     

STUDY ON FATIGUE THRESHOLD BEHAVIOUR AND FATIGUE CRACK PROPAGATION IN A CAST Co-Cr-Mo ALLOY USED FOR SURGICAL IMPLANTS

Abstract— —Fatigue crack propagation rates (d a /d N ) and fatigue crack thresholds (Δ K _th) have been studied in a cast Co-Cr-Mo alloy used for surgical implants with various grain sizes. Results for materials with average grain sizes of about 400 and 60μm respectively are presented. Threshold values close to 10–15 MPam have been measured with decreasing values observed on increasing the grain size. Similar effects of grain size are found on the crack propagation behaviour at higher growth rates, where a coarse grain size material show a higher crack growth rate than a fine grain size material at the same Δ K levels. The effects of microstructure on fatigue properties of the cast Co-Cr-Mo alloy are caused not only by grain size variation but are also attributed to the microstructural differences: a coarse-grained material with a directionally grown dendritic structure vs a fine-grained material with an equiaxed grain structure.     

各种平均应力下高周疲劳极限间的定量关系

疲劳极限是材料工程应用中的一个重要特性参数,但它并不是材料常数。在不同循环载荷作用下,材料会有不同的疲劳极限值。疲劳极限值很大程度上依赖于循环载荷中的平均应力或最大应力幅值,两者之间的关系是材料自身固有的特性关系。因为在接近疲劳极限的低循环应力幅下,S-N曲线受疲劳极限控制,所以在工程应用中对其定量关系有迫切的需求,但现有的经典经验关系只能对其进行非常粗略的估算,不能满足工程应用中有关寿命设计的需要。现状是只能通过大量的实验,得出其实验关系。该文提出了一种函数形式的关系式,该关系式可以足够精确地描述各种材料的疲劳极限和平均应力之间的关系,且只包含三个材料常数。只要这三个材料疲劳特性常数被事先确定,则任意疲劳载荷下的疲劳极限值,都可由该关系式估计得到,因此可以作为材料本身固有的疲劳强度的状态关系式使用。