热疲劳裂纹扩展的数值模拟

本文根据热疲劳损伤力学和有限元技术,研究了复杂温度环境下的热疲劳损伤现象,建立了复杂热力学边界条件下的热应力分析模型,研究了多变量耦合作用下的温度场、热应力场等的分布规律及高温交变热冲击条件热疲劳裂纹的扩展特性,并进行了实验验证,对于类似条件下工作设备的热损伤问题具有重要指导意义.     

Detection of fatigue crack propagation with acoustic emission

Since its inception, acoustic emission has held great promise as a tool for detection and evaluation of damage in structures. Of particular concern is the occureence of fatigue manifested by the processes of crack initiation followed by crack growth to failure. Early research related acoustic emission parameters to physical variables, including load, stress, pressure etc. causing damage. In this paper, relationships relating acoustic emission to fatigue crack growth are developed. Acoustic emission equations, similar to Paris' law, are derived which allow determination of the stress intensity factor and the crack growth rate. These principles are embodied in a second-generation system which automatically and continuously determines these parameters. The approach is applied in the laboratory as well as to structures including bridges, air frames and transmission towers for electrical power.     

Effect of interstitial content on high- temperature fatigue crack propagation and low- cycle fatigue of alloy 720

Alloy 720 is a high-strength cast and wrought turbine disc alloy currently in use for temperatures up to about 650 °C in Allison’s T800, T406, GMA 2100, and GMA 3007 engines. In the original composition in-tended for use as turbine blades, large carbide and boride stringers formed and acted as preferred crack initiators. Stringering was attributed to relatively higher boron and carbon levels. These interstitials are known to affect creep and ductility of superalloys, but the effects on low-cycle fatigue and fatigue crack propagation have not been studied. Recent emphasis on the total life approach in the design of turbine discs necessitates better understanding of the interactive fatigue crack propagation and low-cycle fatigue behavior at high temperatures. The objective of this study was to improve the damage tolerance of Alloy 720 by systematically modifying boron and carbon levels in the master melt, without altering the low-cy-cle fatigue and strength characteristics of the original composition. Improvement in strain-controlled low-cycle fatigue life was achieved by fragmenting the continuous stringers via composition modifica-tion. The fatigue crack propagation rate was reduced by a concurrent reduction of both carbon and bo-ron levels to optimally low levels at which the frequency of brittle second phases was minimal. The changes in composition have been incorporated for production disc forgings.     

Study of crack propagation behavior during low cycle fatigue in spheroidal graphite cast iron based on observation of surface and fracture sections

In this paper, in order to clarify the effect of crack coalescence for crack growth rate, low cycle fatigue tests were carried out using two kinds of spheroidal graphite cast iron (SGI). Crack propagation behavior from crack initiation to fracture was investigated by observing the surface of specimens and their fracture section. The main results obtained are as follows: (1) The variation range in the crack growth curves in SGI is larger than that in its matrix material. This tendency is caused by the occurrence of crack coalescence and the difference in crack initiation length. (2) In most of the cases where the crack growth rate accelerated, there was microshrinkage on the neighboring surface. (3) Crack coalescence is not the main factor in accelerating crack growth rate.     

Discrete dislocation modeling of fatigue crack propagation

Analyses of the growth of a plane strain crack subject to remote mode I cyclic loading under small-scale yielding are carried out using discrete dislocation dynamics. Cracks along a metal–rigid substrate interface and in a single crystal are studied. The formulation is the same as that used to analyze crack growth under monotonic loading conditions, differing only in the remote stress intensity factor being a cyclic function of time. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation being incorporated through a set of constitutive rules. An irreversible relation is specified between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip in order to simulate cyclic loading in an oxidizing environment. The cyclic crack growth rate log(da/dN) versus applied stress intensity factor range log(ΔK_I) curve that emerges naturally from the solution of the boundary value problem shows distinct threshold and Paris law regimes. Paris law exponents in the range 4 to 8 are obtained for the parameters employed here. Furthermore, rather uniformly spaced slip bands corresponding to surface striations develop in the wakes of the propagating cracks.     

WCp/钢基表面复合材料热疲劳裂纹萌生及扩展机理

为了对经受热疲劳的表面复合材料的设计提供理论依据,采用热震试验方法对通过真空实型铸渗(V-EPC)方法制备的WC/钢基表面复合材料的热疲劳性能进行了研究,重点讨论了表面复合材料热疲劳裂纹萌生及扩展的影响因素.结果表明,在本实验条件下Ni6025WC体积分数为15％时的热疲劳性能较5％时得到较大改善.表面复合材料的热疲劳裂纹的萌生和扩展的影响因素有以下几个方面:WC颗粒本身微观质量和热导率、复合层与基材中主要元素以及WC颗粒的平均热膨胀系数、复合层中WC颗粒之间的间距、由热震而产生的界面交变循环应力等.通过改善以上影响因素可以提高复合材料的热疲劳性能.     

喷射沉积Al-Si/SiCp制动盘材料的热疲劳微裂纹扩展行为

采用V形缺口试样,研究喷射沉积Al-Si/SiCp复合材料制动盘在25(450 ℃热循环下的热疲劳行为.通过金相显微镜和扫描电镜观察了复合材料的组织和热疲劳裂纹形貌,研究热疲劳裂纹形成与扩展机制.结果表明:热疲劳主裂纹主要从V形缺口处萌生;在同样的热循环次数下,热处理前的试样要比热处理后的试样先出现裂纹,且裂纹扩展的速率较快;裂纹绕过Si颗粒向前扩展以及裂纹穿过Si颗粒向前扩展是裂纹与Si颗粒相互作用的主要机制;SiC颗粒与热疲劳裂纹有明显的交互作用.因此,改善Si相的形态和分布以及加强Al/SiC颗粒间的界面结合有利于提高热疲劳裂纹扩展的抗力.     

Metallurgical control of fatigue crack propagation in superalloys

Low-cycle fatigue life of turbine engine disk alloys is determined by the initiation and propagation of fatigue cracks. Performance improvements can be achieved through the combination of clean melting technology, to reduce the defect size, and a new generation of high-strength superalloys with fatigue cracking resistance. Metallurgical control of fatigue crack propagation in high-strength superalloys becomes feasible only through a clear understanding of the fatigue cracking mechanism, as well as the micro-structure/property relationships. Many metallurgical parameters have been identified to control the fatigue cracking resistance at high temperatures. One of the most effective methods, applicable to all high γ′ content superalloys, is to modify the grain boundary structure by means of a controlled cooling from a supersolvus solutioning. The precipitation reaction occurring on the grain boundaries during cooling generates a serrated structure that exhibits a good stress oxidation resistance for fatigue cracking.     

Modelling the crack propagation rate for corrosion fatigue at high frequency of applied stress

The application of standard mathematical techniques for the solution of mass transport equations, in the case of advection that is caused by the pulsating movement of crack walls in the case of corrosion fatigue, can be very time consuming. This problem arises, due to the requirement that the time step that must be employed, when solving the non-stationary equations numerically, must be significantly smaller than the period of oscillation. For overcoming these time-consuming limitations, a simple algorithm, which is based on eliminating the convective term from the equations of mass transfer in the pulsating slab by a suitable change of variables, was developed. The estimation of the advection effect on the rate of corrosion fatigue has been performed for the cases of diffusion and mixed kinetic control at high frequencies of applied stress. It is shown that, in many cases, it is possible to use codes that were developed for describing stress corrosion cracking, i.e. for the case of mass transfer without advection at zero loading frequency, to predict corrosion fatigue crack propagation rate, by simply substituting an effective crack length. Numerical calculations that have been performed in this work also show that the method developed here yields results that are applicable not only to the elevated frequencies, but to the any arbitrary frequency from 0 to ∞ for estimating corrosion fatigue crack propagation rate.     

Roles of microstructure and carbides in fatigue crack propagation in high V-Cr-Ni cast irons

This paper describes the roles of microstructure and carbides during fatigue crack propagation (FCP) in high V-Cr-Ni cast irons, with varying C and V contents from 1 to 3% and 3.5 to 10%, respectively. FCP tests have been performed using CT specimens in laboratory air at ambient temperature, and FCP behaviour and fracture mechanisms were discussed on the basis of crack closure, crack path profile and fracture surface analysis. In the materials with non-spheroidal vanadium carbide (VC), the effect of C and V contents, i.e. microstructure, was seen when the data were characterized in terms of nominal stress intensity factor range. Also in the materials with spheroidal VC, the effect of microstructure was recognized. After allowing for both crack closure and elastic modulus, the intrinsic FCP resistance still became lower with increasing C and V contents, particularly remarkable in the materials with 3% C and 10% V regardless of VC morphology. In these materials, there existed the mutually competitive mechanisms: one was that accelerated FCP rates such as preferential growth into VCs and VC fracture, and the other was that decelerated FCP rates such as crack deflection, crack closure, secondary cracks, and uncracked-ligament bridging. The former exerted much larger influence on the overall FCP rates than the latter did, thus resulting in the lower intrinsic FCP resistance.     

Damage mechanism of a nodular cast iron under the very high cycle fatigue regime

Fatigue behaviour in the very high cycle regime (VHCF) of 10¹⁰ cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = −1.

The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An advanced, high-speed, and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels caused by internal damping due to a very high frequency cycling. The temperature field on the surface specimen was determined by using a non-destructive measurement technique called infrared pyrometer. An infrared camera made up of a matrix of 320 × 240 detectors was used.

The S-N curves obtained show that fatigue failure occurred beyond 10⁹ cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Under the current test conditions, the high cycle fatigue (VHCF) behaviour of the cast iron exhibited a typical fatigue crack growth process, that is, fatigue initiation takes place always at the surface graphite or subsurface void; the distinctive stable fatigue crack growth zone can be found around the fatigue crack initiation site, the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition.

Under lower stress amplitude and high cycle condition, surface graphite fatigue initiation is predominantly depended on cyclic stress amplitude; subsurface void fatigue initiation is determined by maximum cyclic stress.

In the process of small crack propagation, the temperature in local plastic zone increase very sharply. The temperature field of ultrasonic fatigue specimen can be changed with the cooling condition; internal heating can accelerate surface fatigue crack initiation and propagation.

Fatigue properties in VHCF regime were studied for cast iron (GS51) at 20 kHz frequency and for the first time, crack initiation and propagation stages were analyzed using a high-sensitivity infrared camera. The new Paris's model for fish eye formation in the gigacycle fatigue were also confirmed by this study.     

Factors influencing fatigue crack propagation behavior of austenitic steels

In the present study, the fatigue crack propagation (FCP) behaviors of austenitic single phase steels, including STS304, Fe18Mn and Fe22Mn with different grain sizes ranging from 12 μm to 98 μm were investigated. The FCP tests were conducted in air at an R ratio of 0.1 using compact tension specimens and the crack paths and fracture surfaces were documented by using an SEM. The highest ΔK_th value of 9.9MPa·m^1/2 was observed for the Fe18Mn specimen, followed by 5.2MPa·m^1/2 for the Fe22Mn specimen and 4.6MPa·m^1/2 for the STS304 specimen, showing a substantial difference in the near-threshold FCP resistance for each microstructure. The crack path and fractographic analyses suggested that the near-threshold FCP behavior of these austenitic steels was largely influenced by the degree of slip planarity, as determined by stacking fault energy and grain size, rather than the tensile properties. In the Paris’ regime, the slip planarity still played an important role while the tensile properties began to affect the FCP. The FCP behavior of austenitic steels with different microstructural features are discussed based on detailed fractographic and micrographic observations.     

A356铸造铝合金的疲劳裂纹萌生及短裂纹扩展研究

研究了A356-T6铸造铝合金的缺口疲劳裂纹萌生与早期扩展行为及机制.结果表明,热等静压试样的疲劳抗力优于非热等静压试样.对于钝缺口试样,疲劳裂纹萌生于缺口根部附近的多个平面,最终哪个裂纹源扩展成主裂纹取决于局部微观组织.对于缺口几何形状不同的热等静压和非热等静压疲劳试样,在疲劳过程中,不管是在高应力状态下,还是在低应力状态下,都出现了铝基体的循环塑性变形和共晶硅粒子断裂导致疲劳裂纹萌生.对于非热等静压试样,铸造缩孔在构件的疲劳过程中起着重要作用,但即使缺口根部存在较大尺度的铸造缩孔,导致了疲劳裂纹萌生,但也同时观察到疲劳裂纹从共晶硅粒子、金属间化合物、铝基体的滑移带和铁基金属间化合物等处萌生.对于脆性的A356铸造铝合金可采用修正的断裂力学参量ΔKn、局部应力范围Δσ或局部应变幅Δε/2作为控制参量来表征疲劳裂纹萌生行为,而缺口有效应力强度因子范围ΔKneff和ΔJs参量可用来表征缺口场中短裂纹扩展行为.     

铝锂合金2198-T8高周疲劳性能及其裂纹萌生机理

选用第3代铝锂合金2198-T8材料,对其力学以及高周疲劳性能进行了研究。结果表明,相对于其它航空铝合金材料,2198-T8在抗拉强度和屈服强度上均有不同程度的提升,并具有较强的抗疲劳性能,但疲劳数据的离散性却随着施加应力的减小而增大。利用扫描电子显微镜(SEM)及其自带的能谱分析(EDS)功能对疲劳裂纹萌生机理进行深入研究。研究发现,位错的增殖、运动和塞积是2198-T8疲劳裂纹萌生的主要原因,疲劳裂纹易于从驻留滑移带(PSB)以及挤出带形成的粗糙区域萌生。不同应力状态下,由材料内部缺陷引起的疲劳断裂现象是致命的,对疲劳寿命有巨大影响,最高可使疲劳寿命降低90%以上。裂纹萌生机制的不同可引起疲劳寿命的较大变化,合理地解释了疲劳数据的离散性问题。     

Interface effects on crack deflection and bridging during fatigue crack growth of titanium matrix composites

The effect of the interface on the crack deflection and crack bridging behavior of continuous fiber-reinforced titanium matrix composites has been investigated using three interfaces with significantly different mechanical characteristics. Each of these composites exhibited stress ranges in which fiber bridging was present and stress ranges in which stable fiber bridging was not present. The fatigue crack growth rate for all composites, even for the ones that did not exhibit fiber bridging was significantly below that of the matrix. This phenomenon, believed to be an effect of elastic crack shielding, was most significant for composites with the strongest interfacial bond. Interface failure ahead of the crack tip and its influence on the local stress intensity factor is believed to be responsible for the decrease in the shielding effect of low strength interfaces. Interface debonding was observed in all three composites, and damage to the interface ahead of the crack tip was seen in two of the three composites. A stress-based criterion for predicting debonding appeared to effectively explain the crack deflection behavior for the three composites. Evidence of crack deflection even for the strongest interface suggests that there is scope to increase the interface bond strength in SiC/Ti-alloy system for improved transverse properties without compromising the fatigue life.