Prediction of very high cycle fatigue failure for high strength steels, based on the inclusion geometrical properties

This article deals with the experimental and predicted fatigue endurance of the high strength steels, European 100C6 (martensitic and bainitic) and the Japanese SUJ2 in the gigacycle regime. Tests were carried out with stress ratio R = −1 in tension–compression condition at room temperature. To attain the high number of cycles required in a reasonable period of time, an ultrasonic test machine working at 20 KHz was used to obtaining 1.7 × 10⁹ cycles in approximately 24 h. The relationship between the geometrical properties of inclusions associated with fatigue failure and the fatigue life of these steels was studied. Thereafter, with basis on a simplified evaluation of the highest stress in the elliptical inclusion for fatigue Mode I, three models to predict the fatigue life for these high strength steels were proposed adjusting non-linear regression curves to the corresponding experimental results.     

On effect of hydrogen on very high cycle fatigue behaviours of high strength steels

Abstract

In order to analyse the effect of hydrogen on very high cycle fatigue properties, hydrogen was precharged into two high strength steels. The applied stress intensity factor range at the periphery of inclusions before and after being precharged is approximately proportional to the cubic root of inclusion size. In addition, the applied stress intensity factor range at the periphery of inclusions after being precharged was lower compared with uncharged specimens. The additional stress intensity factor range generated by hydrogen ΔK_H is raised after the hydrogen was precharged. A simple prediction equation of S–N curve was proposed by introducing the hydrogen influence factor. The proposed prediction equation can reasonably describe the S–N curves for precharged specimens.     

Effects of SiCp size and volume fraction on the high cycle fatigue behavior of AZ91D magnesium alloy composites

High cycle fatigue tests (i.e., stress-controlled, axial) were conducted on monolithic AZ91D and AZ91D magnesinm alloy composites processed via squeeze casting and extrusion to contain either 15 gm or 52 gm size SiC particles, at both the 20% and 25% volume fraction reinforcement level. The effects of changes in SiC particle size and volume fraction on the high cycle fatigue behavior have been determined. In addition, the number of cracked particles on the fatigue fracture surfaces, as well as the level of damage beneath the fatigue fracture surfaces were quantified in order to determine the effects of particle size on the evolution of damage during fatigue and during overload failure. Commercial purity Mg specimens containing a large grain size were also tested in fatigue for comparison with the alloy and composite data.     

Effect of toughness on low cycle fatigue behavior of pipeline steels

The effect of toughness on the fatigue behavior of pipeline steels was investigated, including the fatigue crack propagation rate and low cycle fatigue test under the loading condition simulating the actual operation of pipelines. The results indicate that the toughness can strongly influence the fatigue behavior of pipeline steels (i.e., the steels with high toughness possess high resistance to fatigue crack propagation and high tolerance of damage, which are much beneficial to obtaining a long life for line pipe structures).     

Multifunctional properties of high volume fraction aligned carbon nanotube polymer composites with controlled morphology

Advanced composites, such as those used in aerospace applications, employ a high volume fraction of aligned stiff fibers embedded in high-performance polymers. Unlike advanced composites, polymer nanocomposites (PNCs) employ low volume fraction filler-like concepts with randomly-oriented and poorly controlled morphologies due to difficult issues such as dispersion and alignment of the nanostructures. Here, novel fabrication techniques yield controlled-morphology aligned carbon nanotube (CNT) composites with measured non-isotropic properties and trends consistent with standard composites theories. Modulus and electrical conductivity are maximal along the CNT axis, and are the highest reported in the literature due to the continuous aligned-CNTs and use of an unmodified aerospace-grade structural epoxy. Rule-of-mixtures predictions are brought into agreement with the measured moduli when CNT waviness is incorporated. Waviness yields a large (10×) reduction in modulus, and therefore control of CNT collimation is seen as the primary limiting factor in CNT reinforcement of composites for stiffness. Anisotropic electron transport (conductivity and current-carrying capacity) follows expected trends, with enhanced conductivity and Joule heating observed at high current densities.     

Factors influencing the GBF size of high strength steels in the very high cycle fatigue regime

The influences of major factors including applied stress amplitude, inclusion size and hydrogen content on granular-bright-facet (GBF) size of high strength steels in the very high cycle fatigue regime were studied in this article. It was found that the GBF size is determined by the applied stress amplitude and material hardness. If the applied stress amplitude is lower, the GBF size is larger. When a specimen containing bigger inclusions, the applied stress amplitude to form GBF can be reduced which results in the increase of GBF size. Hydrogen has different effects on the GBF size. The related reasons were discussed.     

Effect of surface properties on high cycle fatigue behaviour of shot peened ductile steel

Abstract

Experimental investigations into shot peened ductile steel have been carried out, applying three surface finishing conditions: as machined, standard shot peening using 100% coverage and severe shot peening with 1000% coverage (high exposure time). The properties of the shot peened surfaces were examined and characterised, and specimens were then submitted to three point bending tests. The fatigue limit was determined for each case. In this way, the dependence of fatigue behaviour on initial surface finishing properties was determined, and a relationship is suggested to describe and correlate fatigue limits with initial surface properties. A phenomenological approach is proposed to characterise and to correlate qualitatively and quantitatively the influence of local shot peened surface properties on fatigue limit of treated specimens. The Crossland multiaxial failure high cycle fatigue criterion is used in this approach to model the influence of each surface property.     

Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

再结晶对DD6单晶高温合金轴向高周疲劳性能的影响

为了研究再结晶对二代单晶高温合金DD6高周疲劳性能的影响,对标准热处理的DD6合金进行表面吹砂处理,然后分别在1120℃和1315℃保温4h,以获得不同类型的再结晶组织。在疲劳试验机上分别测试了光滑和含再结晶的DD6合金试样在1070℃的轴向高周疲劳寿命。采用SEM观察DD6合金再结晶组织及疲劳断口。结果表明:胞状再结晶和等轴再结晶降低了DD6合金的轴向高周疲劳性能,胞状再结晶作用小于等轴再结晶;含再结晶的DD6合金试样的轴向高周疲劳断裂机制为类解理断裂和枝晶间的局部韧窝断裂共存的混合断裂;再结晶使DD6合金试样变为多源疲劳断裂。高温条件下,再结晶晶界的存在加快合金试样的氧化损伤,显著缩短早期疲劳裂纹的萌生和扩展时间,降低合金的轴向高周疲劳性能。     

Determination of volume fraction of bainite in low carbon steels using artificial neural networks

Artificial neural networks have been used to estimate the volume fraction of bainite in low carbon steels containing various alloying elements. The network predicts the volume fraction for a given composition, isothermal transformation temperature and isothermal transformation time. Additionally, the maximum transformation temperature at which bainite formation takes place is also provided as an input to the neural network. The network was trained using the experimental data from three low carbon steels and it was found to perform quite well in predicting the volume fraction of bainite. The impact of the composition of alloying elements on the volume fraction of bainite was also studied and the results were in agreement with the known metallurgical theory.     

Effect of fibre volume fraction on fatigue behaviour of glass fibre reinforced composite

The aim of this paper is to study the fatigue behavior of GFRP composites manufactured by vacuum bagging process by varying the volume fraction. Constant‐amplitude flexural fatigue tests were performed at zero mean stress, i.e. a cyclic stress ratio R=?1 by varying the frequency of the testing machine. The relationship between stiffness degradation rate and fibre volume fraction, was observed, and the influence of volume fraction on the tensile strength was also investigated. The results show that, as the volume fraction increases the stiffness degradation rate initially decreases and then increases after reaching a certain limit for the volume fraction. Graph between volume fraction and Young's modulus shows that as the volume fraction increases Young's modulus also increases and reaches a limit and then it decreases with further increase in volume fraction, due to the increase in fibre content which changes the material properties of the composite material. The obtained results are in agreement with the available results.     

Characteristics of metal magnetic memory signals of different steels in high cycle fatigue tests

The detection of crack initiation in steel Q235, 16MnR and 20 g was carried out by metal magnetic memory (MMM) technique. Fifty specimens of welded and base metal were tested. Both MMM testing and metallographic examination were done to them when unloaded at the different phase of high‐cycle fatigue testing, and hence MMM signals before and after crack initiation could be recorded. The values of magnetic intensity gradient were calculated, and whose critical value was determined and proposed for early defects detection. The results show that the magnetic intensity (Hp) curve became concave responding to the occurrence of stress concentration, and its gradient (dHp/dx) increased greatly; at the critical dHp/dx, no change occurred to the microstructure, but beyond the critical value, dHp/dx increased suddenly and a large number of intragranular slips were found in the microstructure. Three different kinds of materials have different critical values of magnetic intensity gradient.     

Effect of predamage from low cycle fatigue on high cycle fatigue strength of Ti-6Al-4V

Effects of prior low cycle fatigue (LCF) cycling on the subsequent high cycle fatigue (HCF) limit stress corresponding to a life of 10⁷ cycles are investigated for Ti-6Al-4V at room temperature. Tests are conducted at 420 Hz on an electrodynamic shaker-based system at several different LCF maximum loads and under subsequent HCF at R=0.1, 0.5 and 0.8 using a step loading procedure. Under these load combinations, which include the possibility of overload or underload effects if cracks form, there is no statistically significant effect of the prior LCF on the subsequent HCF limit stress. While LCF loading at a high stress of 900 MPa is seen to result in strain ratcheting, no distinct features on the fracture surface and different mechanisms of crack propagation from those obtained at lower maximum loads were observed. LCF loading up to 50% of expected life did not produce any indications of crack formation from either the stress limit data or the fracture surfaces.     

Effect of microstructure of simulated heat‐affected zone on low‐ to high‐cycle fatigue properties of low‐carbon steels

To clarify the effect of microstructural changes on the fatigue property of the weld heat‐affected zone (HAZ), low‐ to high‐cycle fatigue tests were conducted on 16 types of simulated HAZ specimens that had been prepared using thermal processes. The results showed the fatigue S‐N curves of the HAZ to be widely scattered as a function of strength level. These fatigue data were divided into two groups: coarse grain (CG) and fine grain (FG) HAZ, when strain amplitude was used to represent S‐N curves. The fatigue data for the CGHAZ group showed a relatively short fatigue life. Based on surface observations, the initiated fatigue crack size of CGHAZ was larger than that of FGHAZ as a function of microstructural unit size. Hence, fatigue crack growth life, which is almost the same as total fatigue life of CGHAZ, decreased.     

Notable size effects on very high cycle fatigue properties of high-strength steel

Very high cycle fatigue (VHCF) properties were compared between two types of specimens: enlarged specimens and our standard specimens. Fatigue tests were conducted by ultrasonic fatigue testing; the material used was commercial spring steel. All tests ended in internal fracture, with large-size effects observed, i.e., the enlarged specimens showed lower VHCF strength than the standard specimens. Most of the internal fracture origins were oxide-type inclusions that were larger in the enlarged specimens than in the standard specimens, indicating the size effect to be caused by the difference in oxide-type inclusion sizes at the origins of internal fractures. The large-size effect strongly urges the use of large specimens when conducting VHCF tests on high-strength steel. Moreover, the large-size effect implies that fatigue strength cannot in this case be determined using the conventional S-N curve approach, since the S-N curve depends on the specimen size. The evaluation of the VHCF strength thus needs two steps: an estimation of the maximal inclusion size, followed by an estimation of the VHCF strength based on the maximal inclusion size.     

Hydrogen permeability of annealed and deformed carbon steels with a granular cementite structure

Hydrogen permeability of St.40, U8, and U12 steels with granular cementite structures was studied. It was established that polytherms of the rate of hydrogen permeation obtained for annealed specimens of these steels have deflection points at t_cr=400°C. The permeability parameters (E, p₀) of these steels for t < t_cr increase to a lesser extent than in the case of specimens consisting of lamellar pearlite. After plastic deformation followed by annealing at various temperatures, substantial changes in the permeability are also observed but only for t < t_cr; the effect is smaller, however, than that observed for the same steels with a lamellar pearlite structure. The results obtained were attributed to the influence of ferritecarbide phase boundaries whose adsorption power is changed as a result of plastic deformation and subsequent annealing.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 5, No. 5, pp. 588–591, September–October, 1969.     

Effect of microstructure on non-linear behavior of ultrasound during low cycle fatigue of pearlitic steels

Influence of microstructural changes on the second harmonics of sinusoidal ultrasonic wave during low cycle fatigue (LCF) deformation in pearlitic steel was studied. Fatigue tests were interrupted and at every interruption, non-linear ultrasonic (NLU) parameter (β) was determined. Microstructures of cyclically deformed specimens at various cycles were examined by transmission electron microscopy (TEM). The variation of β with fatigue cycles was correlated with the microstructural changes and the results were explained through the variation in dislocation morphology and carbon content of the steel.     

Metallurgical aspects on the fatigue of solution-annealed austenitic high interstitial steels

Austenitic stainless steels have been used for over 100 years for their combination of strength and ductility. In order to further improve the mechanical and chemical properties of austenitic high nitrogen steels (AHNS) were developed. Ni reduces the solubility of N and, therefore, was substituted by Mn in order to allow for up to 1 weight-% N to be alloyed. AHNS show an even higher strength for the solution annealed state, which can be increased further by cold working. Unfortunately the endurance limit did not follow this trend as it is known to for cold-worked Ni-containing steels. The solution annealed Ni-containing austenites allow for wavy slip and the generation of dislocation cells while the Mn-alloyed AHNS only show planar slip with twins and stacking faults. While the stacking fault energy was thought to be the main reason for planar slip, early results showed that there must be other near-field effects. The density of free electrons, which is mainly influenced by the sum and the ratio of C and N, might be responsible. Strain-controlled fatigue tests were carried out in CrMn-alloyed austenitic steels with different sums (C + N: 0.65–1.2) and ratios (C/N: 0.13–∞) of C and N. Manson–Coffin analyses revealed distinct differences in the fatigue behaviour to CrNi-alloyed C + N steels investigated earlier. This contribution presents these differences and discusses them in relation to microstructural characteristics as well their alterations under cyclic loading.     

The fatigue properties of some cast steels

Fatigue properties of some steels are presented with the aim of highlighting both the need, and areas, for future work on these materials. Possible explanations for the lack of acceptance, by design engineers, of cast steels are given. Specific areas in which further research is required are indicated - the biggest problem is predicted as being the characterization and mathematical modelling of real defects. Discontinuities worthy of investigation are listed.