GIGACYCLE FATIGUE BEHAVIOR OF CAST ALUMINUM IN TENSION AND TORSION LOADING

An improved understanding of fatigue behavior of a cast aluminum alloy（2-AS5U3G-Y35）in very high cycle regime is developed through the ultrasonic fatigue test in axial and torsion loading.The new developed torsion fatigue system is presented.The effects of loading condition and frequency on the very high cycle fatigue（VHCF）are investigated.The cyclic loading in axial and torsion at 35 Hz and 20 kHz with stress ratio R=-1 is used respectively to demonstrate the effect of loading condition.S-N curves show that the fatigue failure occurs in the range of 105—1010 cycles in axial or torsion loading and the asymptote of S-N curve is inclined,but no fatigue limit exists under the torsion and axial loading condition.The fatigue fracture surface shows that the fatigue crack initiates from the specimen surface subjected to the cyclic torsion loading.It is different from the fatigue fracture characteristic in axial loading in which fatigue crack initiates from subsurface defect in very high cycle regime.The fatigue initiation is on the maximum shear plane,the overall crack orientation is on a typical spiral 45° to the fracture plane and it is the maximum principle stress plane.The clear shear strip in the torsion fatigue fracture surface shows that the torsion fracture is the shear fracture.

Low cycle fatigue behavior of zirconium−titanium−steel composite plate

The low cycle fatigue behavior of zirconium−titanium−steel composite plate under symmetrical and asymmetric stress control was studied.The effects of mean stress and stress amplitude on cyclic deformation,ratcheting effect and damage mechanism were discussed in detail.The results show that under symmetric stress control,the forward ratcheting deformation is observed.Under asymmetric stress control,the ratcheting strain increases rapidly with mean stress and stress amplitude increasing.Under high stress amplitude,the influence of mean stress is more significant.In addition,by studying the variation of strain energy density,it is found that the stress amplitude mainly promotes the fatigue damage,while the mean stress leads to the ratcheting damage.In addition,fractographic observation shows that the crack initiates in the brittle metal compound at the interface,and the steel has higher resistance to crack propagation.Finally,the accuracy of life prediction model considering ratcheting effect is discussed in detail,and a high-precision life prediction model directly based on mean stress and stress amplitude is proposed.

Effect of Reinforcement Clustering on Crack Initiation Mechanism in a Cast Hybrid Metal Matrix Composite during Low Cycle Fatigue

The reinforcement distribution of metal matrix composites (MMCs) plays an important role in low cycle fatigue. Thus, it is essential to study the effect of reinforcement clustering on the crack initiation mechanism of MMCs. In this study, the effect of reinforcement clustering on the microcrack initiation mechanism in a cast hybrid MMC reinforced with SiC particles and Al2O3 whiskers was investigated experimentally and numerically. Experimental results showed that microcracks always initiated in the particle-matrix interface, located in the cluster of the reinforcements. The interface debonding occurred in the fracture which created additional secondary microcracks due to continued fatigue cycling. The microcrack coalesced with other nearby microcracks caused the final fracture. To validate the experimental results on the microcrack initiation, three dimensional unit cell models using finite element method (FEM) were developed. The stress distribution in both the reinforcement clustering and non-clustering regions was analyzed. The numerical analysis showed that high stresses were developed on the reinforcements located in the clustering region and stress concentration occurred on the particle-matrix interface. The high volume fraction reinforced hybrid clustering region experienced greater stresses than that of the SiC particulate reinforced clustering region and low volume fraction reinforced hybrid clustering region. Besides, the stresses developed on the non-clustering region with particle-whisker series orientation were reasonably higher than that of the non-clustering region with particle-whisker parallel orientation. The high volume fraction reinforced hybrid clustering region is found to be highly vulnerable to initiate crack in cast hybrid MMC during low cycle fatigue.

Effect of low cycle fatigue pre-damage on very high cycle fatigue

Carbon-manganese steel is often applied in components of pipes in nuclear plant. Ultrasonic fatigue tests following low cycle fatigue （LCF） cycles damaged are used to study the strength of very high cycle fatigure （VHCF）. The comparison of test results of simple VHCF and cumulative fatigue （LCF plus VHCF） shows that LCF load influences the following VHCF strength. Continuum damage mechanics model is extended to VHCF region.

Low cycle fatigue behavior of high strength gun steels

The low cycle fatigue (LCF) behavior of two high strength steels, withnominal chemical compositions (mass fraction, %) of 0.40C-1.5Cr-3Ni-0.4Mo-0.2V (PCrNi3MoV) and0.25C-3Cr-3Mo-0.8Ni-0.1Nb (25Cr3Mo3NiNb), was investigated by using the smooth bar specimenssubjected to strained-controlled push-pull loading. It is found that both steels show cyclicsoftening, but 25Cr3Mo3NiNb steel has a lower tendency to cyclic softening. 25Cr3Mo3NiNb steel hashigher fatigue ductility, and its transition fatigue life is almost three times that of PCrNi3MoV.25Cr3Mo3NiNb steel also shows higher LCF life either at a given total strain amplitude above 0.5% orat any given plastic strain amplitude, despite its lower monotonic tensile strength than that ofPCrNi3MoV. It also means that 25Cr3Mo3NiNb steel can endure higher total strain amplitude andplastic strain amplitude at a given number of reversals to failure within 10~4. 25Cr3Mo3NiNb steelis expected to be a good gun steel with high LCF properties because only several thousand firingsare required for gun barrel in most cases.

RELIABILITY ANALYSIS FOR AN AERO ENGINE TURBINE DISK UNDER LOW CYCLE FATIGUE CONDITION

Reliability analysis methods based on the linear damage accumulation law (LDAL) and load-life interference model are studied in this paper. According to the equal probability rule, the equivalent loads are derived, and the reliability analysis method based on load-life interference model and recurrence formula is constructed. In conjunction with finite element analysis (FEA) program, the reliability of an aero engine turbine disk under low cycle fatigue (LCF) condition has been analyzed. The results show the turbine disk is safety and the above reliability analysis methods are feasible.

Nonproportional Low Cycle Fatigue for 316L Stainless Steel

A series of tests are performed for 316L stainless steel under multiaxial nonproportional low cycle fatigue(LCF). The microstructures of the steel in the process of nonproportional LCF are observed with transmission electron microscopy (TEM). Based on macroscopic and microscopic experiments, the micromechanism of additional hardening and the decrease in LCF life under nonproportional cyclic loading are studied. The results of the tests indicate that 316L stainless steel obviously exhibits nonproportional cyclic additional hardening, which is mainly due to rotation of maximum shear stress plane during the LCF under nonproportional cyclic loading.

Low cycle fatigue estimation of low yield point steel

With the development of technology for earthquake resistant,the research of the low yield point(LYP) steel which used for the fabrication of energy dissipation damper were paid more and more attention.The common studies of the low yield point steel is mainly about the performance with constant amplitude and constant frequency.The low cycle fatigue properties of low yield piont steel were studied by series of test with continuous varying amplitude and varying frequency with the materials testing system by us.The test results showed that low yield point steel of Baosteel have excellent low cycle fatigue properties,which meet the requirement for steel used for the fabrication of energy dissipation damper completely.The low cycle fatigue performance of low yield point steel of Baosteel mainly depended on the amplitude in test.And the effect of varying frequency for the low yield point steel was more less than varying amplitude.

In vitro corrosion-fatigue behavior of biodegradable Mg/HA composite in simulated body fluid

Magnesium and its composites as biodegradable materials offer especial capabilities to be used as bio-absorbable implants.However,their poor corrosion and fatigue properties in the physiological environment can restrict their applications.In this study,corrosion-fatigue tests have been performed on the extruded magnesium and magnesium/hydroxyapatite(Mg/HA)composites in a high cycle regime.To produce the composites,pure magnesium was reinforced by 2.5 wt.%and 5 wt.%of hydroxyapatite submicron particles using an electromagneticmechanical stirring method and hot extrusion process.The experimental density measurement exhibits that the porosity increases in the extruded samples with more hydroxyapatite particles.To investigate the corrosion and corrosion-fatigue behavior of the specimens,simulated body fluid(SBF)was used during in vitro tests.The results of the potentiodynamic polarization corrosion test show that the composite with 2.5 wt.%of hydroxyapatite(Mg/2.5 wt.%HA)and the pure magnesium specimen exhibit the highest and the lowest corrosion resistance,respectively.Regarding the elemental mapping analysis of the corroded samples,this behavior could be due to the formation of strong phosphorus-calcium based layers on the composite surface.The results obtained from the mechanical tests indicate that Mg/5 wt.%HA offered the highest tensile and compressive yield strengths,as well as the most promising high cycle fatigue behavior.During the corrosionfatigue test,the simultaneous effects of fatigue and corrosion damages led to a similar corrosion-fatigue behavior in both composites.The fracture surfaces of the corrosion-fatigue samples suggest that the cracks are initiated in the corroded regions of the samples surface,which reduces the crack initiation step and subsequently decreases the fatigue life.In comparison with the pure magnesium,both composites exhibit more promising corrosion and corrosion-fatigue behaviors with a significant fatigue life improvement in the physiological environment.

Fatigue Property of Additively Manufactured Ti-6Al-4V under Nonproportional Multiaxial Loading

The low cycle fatigue strength properties of the additively manufactured Ti-6Al-4V alloy are experimentally investi-gated under proportional and nonproportional multiaxial loading.The fatigue tests were conducted using hollow cylinder specimens with and without heat treatments,at room temperature in air.Two fatigue tests were conducted:one for proportional loading and one for nonproportional loading.The proportional loading was represented by a push-pull strain path(PP)and the nonproportional loading by a circle strain path(Cl).The failure lives of the additively manufactured specimens were clearly reduced drastically by internal voids and defects.However,the sizes of the defects were measured,and the defects were found not to cause a reduction in fatigue strength above a critical size.The fracture surface was observed using scanning electron microscopy to investigate the fracture mechanisms of the additively manufactured specimens under the two types of strain paths.Different fracture patterns were recognized for each strain paths;however,both showed retention of the crack propagation,despite the presence of numerous defects,probably because of the interaction of the defects.The crack propagation properties of the materials with numerous defects under nonproportional multiaxial loading were clarified to increase the reliability of the additively manufactured components.

Discussion on New Evaluation Technology of Non-Metallic Composite Continuous Pipe for Oil and Gas Field

In view of the serious lack and lag of the test and evaluation technology of non-metallic composite continuous pipe, and focusing on the characteristics of the application of non-metallic composite continuous pipe in oil field, this paper discusses a series of new full-scale test and evaluation technologies for accurately evaluating the product quality and practical application performance of non-metallic composite continuous pipe, which effectively solves the major technical problem that the new products of non-metallic pipe cannot be accurately evaluated. Based on the characteristics of the application of non-metallic composite continuous pipe in oil field, a series of new full-scale test evaluation technologies which can accurately evaluate the product quality and practical application performance of non-metallic pipe are designed through a large number of tests. The test and evaluation technology can accurately evaluate the key performance of high and low pressure cycle, high and low temperature cycle, gas permeability resistance, minimum bending radius etc. It provides a scientific evaluation basis for the standardized application of non-metallic continuous pipe and a reliable quality control method for the selection of products in oil field.

Predicting the Fatigue Life in Steel and Glass Fiber Reinforced Plastics Using Damage Models

Three cumulative damage models are examined for the case of cyclic loading of AISI 6150 steel, S2 glass fibre/epoxy and E glass fibre/epoxy composites. The Palmgren-Miner, Broutman-Sahu and Hashin-Rotem models are compared to determine which of the three gives the most accurate estimation of the fatigue life of the materials tested. In addition, comparison of the fatigue life of the materials shows the superiority of AISI 6150 steel and S2 glass fibre/epoxy at lower mean stresses, and that of steel to the composites at higher mean stresses.

Masing Behavior and Microstructural Change of Quenched and Tempered High-Strength Steel Under Low Cycle Fatigue

Low cycle fatigue behavior of a quenched and tempered high-strength steel(Q960 E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the dislocation structural evolution and fracture mechanism were examined and studied by transmission electron microscopy and scanning electron microscopy(SEM). The results showed that this Q960 E steel showed cyclic softening at different strain amplitudes, and the softening tendency was more apparent at strain amplitude of ±(0.6–1.2)% than that at ± 0.5%. The reduction in dislocation density with increasing strain amplitude is responsible for the softening tendency of cyclic stress with the strain amplitude. The material illustrates near-Masing behavior at strain amplitude ranging from ± 0.6% to ± 1.2%. The near-Masing behavior of Q960 E high-strength steel can be the result of stability of martensite lath at different strain amplitudes. Partial transformation from martensite laths to dislocation cells is responsible for the derivation from ideal Masing behavior. In the SEM examination of fracture surfaces, transgranular cracks initiate on the sample surface. Striations can be found during the crack propagation stage.

Mathematical simulation of low cycle fatigue of high-loaded engine parts

The paper discusses main aspects of low cycle fatigue influence on the lifetime ofengine parts.The importance and history of the problem,the main experiments and the effectsimpacting the low cycle fatigue of structural materials are described.A hypothesis about theexistence of a thermomechanical surface of structural material,generalized to the case of acyclical loading was used to approximate the loops of cyclic nonisothermal elastoplasticdeformation curves.The cyclic deformation curve model is based on the following threeparameters:the elastic modulus during unloading,the Bauschinger effect and the conversionparameter of the nonlinear part of its first halfcycle.This model also accounts for theaccumulated plastic strain,as well as the testing temperature.The criterion of durability isformulated,based on the dependence between the number of halfcycles to failure and theaccumulated plastic deformation.Deformation theory of plasticity,generalized to the case ofcyclic deformation,in combination with the durability model and technology of"dying"elements is applied to the finite element analysis of low cycle fatigue of gas turbine engineparts.The results of calculations are demonstrated.

航空发动机高温探针高低周复合疲劳寿命预测方法

在发动机试车过程中高温探针的疲劳断裂十分危险,建立高温探针疲劳寿命预测方法对于试车安全性十分重要。首先,基于气—热—固耦合方法建立高温探针仿真模型,依据实际工况计算探针在热冲击和振动环境下的载荷;然后,结合S-N曲线和三区间法分别计算探针的低周疲劳寿命和高周疲劳寿命;最后,基于累积损伤理论获得探针的高低周复合疲劳寿命。研究结果表明:探针在热冲击载荷和振动载荷下的最大应力分别为96.2 MPa和33.3 MPa;探针低周疲劳寿命和高周疲劳寿命分别为6.63×10^(6)和2.19×10^(8)次循环;在实际工况下,探针的高低周复合疲劳寿命为4826个周循环,即496个小时,该结果与探针的额定使用寿命基本一致。

Effect of laser shock peening on combined low- and high-cycle fatigue life of casting and forging turbine blades

Laser shock peening （LSP） is a novel effective surface treatment method to improve the fatigue performance of turbine blades. To study the effect of LSP on combined low- and high-cycle fatigue （CCF） life of turbine blades, the CCF tests were conducted at elevated temperatures on two types of full-scale turbine blades, which were made of K403 by casting and GH4133B by forging. Probabilistic analysis was conducted to find out the effect of LSP on fatigue life of those two kinds of blades. The results indicated that LSP extended the CCF life of both casting blades and forging blades obviously, and the effect of LSP on casting blades was more evident; besides, a threshold vibration stress existed for both casting blades and forging blades, and the CCF life tended to be extended by LSP only when the vibration stress was below the threshold vibra- tion stress. Further study of fractography was also conducted, indicating that due to the presence of compressive residual stress and refined grains induced by LSP, the crack initiation sources in LSP blades were obviously less, and the life of LSP blades was also longer; since the compressive residual stress was released by plastic deformation, LSP had no effect or adverse effect on CCF life of blade when the vibration stress of blade was above the threshold vibration stress.

基于滞回环数据的42CrMo高强度钢低周扭转疲劳性能分析

滞回环是低周塑性加载过程中连续监测到的应力-应变数据环,包含丰富的材料疲劳损伤累积信息。但由于滞回环数据采集、特征提取及分析等方面存在困难,相关研究报道较少。为深入评估42CrMo高强度钢低周扭转疲劳性能,提出基于滞回环特征数据的疲劳试验分析方法。采用扭转疲劳试验机获取材料的滞回环数据,设计特征提取算法获得循环应力、剪切模量、残余应力和塑性应变等随加载周期的演化规律以及材料的循环软化特性;通过试验设计及分析证实42CrMo材料不满足Masing特性,并且加载应变幅越大,非Masing特性越显著;通过低周扭转疲劳试验数据分析材料的循环应力-应变关系特性和应变-寿命关系特性,计算得到剪切模量G、循环强度系数K′、循环应变硬化指数n′、疲劳强度指数b、疲劳强度系数τ′f、疲劳延性指数c、疲劳延性系数γ′f等。基于滞回环特征数据的疲劳试验分析方法,充分利用扭转疲劳试验中间大量加载滞回环数据提取每一周期的滞回环特征,用丰富的分析数据描述材料疲劳性能。

晶体塑性理论及其在金属高低周疲劳寿命预测中的应用

作为结构材料的主要失效方式,金属的高低周疲劳性能影响设备的服役寿命。晶体塑性理论从微观结构出发用于描述材料微观结构和性能的关系,在金属材料微观结构和力学性能方面的研究还存在很多待解决的问题。针对金属材料高低周疲劳的问题,本文对晶体塑性基本理论与本构模型进行了梳理,列举了基于晶体塑性有限元法的疲劳指示因子,介绍了近15年来晶体塑性有限元法在金属材料高低周疲劳中的应用,并对应用晶体塑性有限元法于金属材料超高周疲劳进行了拓展,最后对未来发展趋势和有待加强的研究方向进行了展望。

Effects of Strain Rate on Low Cycle Fatigue Behaviors of High-Strength Structural Steel

The low cycle fatigue （LCF） behavior of a high-strength structural steel was investigated in the strain rate range of 4×10^-6 -0.12 s^-1 （0. 001-3 Hz） under constant total strain （±1%） control. The cyclic stress response at all strain rates exhibited behavior of rapid softening in the early stage of fatigue life and subsequent saturation up to failure. It was found that the stress amplitude, the plastic strain amplitude, the plastic strain energy density and the fatigue life depend mainly on the strain rate. The strain rate of 0. 012 s-1 was found as a transition point where the LCF of the steel showed different behavior from low strain rate to high strain rate. The relationship between the time to failure and strain rate was expressed well by a power law relation. The fracture surfaces of the fatigue sam-ples were characterized by using a scanning electron microscope （SEM） and the fracture mechanisms were discussed in terms of time-dependent deformation of the steel.

Energy-Based Prediction of Low Cycle Fatigue Life of High-Strength Structural Steel

Energy-based models for predicting the low-cycle fatigue life of high-strength structural steels are presented. The models are based on energy dissipation during average of cycles, cycles to crack propagation and total cycles to failure. Plastic strain energy per cycle was determined and found as an important characteristic for initiation and propagation of fatigue cracks for high-strength structural steels. Fatigue strain-life curves were generated using plastic energy dissipation per cycle (loop area) and compared with the Coffin-Manson relation. Low cycle fatigue life was found similar from both methods. The material showed Masing-type behavior. The cyclic hysterisis energy per cycle was calculated from cyclic stress-strain parameters. The fracture surfaces of the fatigue samples were characterized by scanning electron microscope and the fracture mechanisms were discussed.