Effects of Si addition on microstructure, mechanical and thermal fatigue properties of Zn-38Al-2.5Cu alloys

The influence of Si addition on microstructure, mechanical properties and thermal fatigue behavior of Zn-38Al-2.5Cu alloys was investigated. The results show that constitutional supercooling of ZA38 alloys is formed because of the Si addition. Zn-38Al-2.5Cu-0.55Si alloy shows the dramatically refined microstructure and the best mechanical properties. When the Si addition exceeds 0.55%,αdendrites develop and Si phases become larger and aggregate along the dendrites boundaries, decreasing the mechanical properties. Oxides and pits formed by the plastic deformation are the main factors of cracks initiation. During the early stage of crack propagation, the cracks grow at a high speed well described by Paris law because of the porous and loose oxide, and mainly propagate along the dendrites boundaries. During the slow-growth stage, secondary cracks share the energy of crack growth, delaying the propagation of cracks, and the cracks propagate and fracture by the mixture of intergranular and transgranular modes.

Fatigue behaviour of coke drum materials under thermal-mechanical cyclic loading

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in cracking. There were some studies on coke drums in the form of bulging and the fatigue life estimation for the coke drums, but most of them were based on strain-fatigue life curves at constant temperatures, which do not consider simultaneous cyclic temperature and mechanical loading conditions. In this study, a fatigue testing system is successfully devel- oped to allow performing thermal-mechanical fatigue （TMF） test similar to the coke drum loading condition. Two commonly used base and one clad materials of coke drums are then experimentally investigated. In addition, a comparative study between isothermal and TMF lives of these materials is conducted. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums and more accurate prediction of fatigue life of coke drum materials.

Thermal-Mechanical Fatigue Behavior and Life Analysis of Cast Ni-base Superalloy K417

In-phase (IP) and out-of-phase (OP) thermal-mechanical fatigue (TMF) behavior of cast Ni-base superalloy K417 was studied. All experiments were carried out under total strain control with temperature cycling between 400-850℃. Both in-phase and out-of-phase TMF specimens exhibited cyclic hardening followed by cyclic softening at the minimum temperature. Besides, they cyclically hardened in the early stage of life followed by cyclic softening at the maximum temperature. OP TMF life was longer than that of IP TMF. Various damage mechanisms operating in different controlled strain ranges and phasing were discussed. A few life prediction methods for isothermal fatigue were used to handle TMF fatigue and their applicability to superalloy K417 was evaluated. The SEM analysis of the fracture surface showed that transgranular fracture was the principal cracking mode for both IP and OP TMF. Oxidation was the main damage mechanism in causing shorter fatigue life for IP TMF compared with OP TMF.

EFFECTS OF STRAIN RATIO ON THERMAL-MECHANICAL CYCLIC STRESS-STRAIN RESPONSE AND FATIGUE LIFE IN DS SUPERALLOY DZ125

The effects of strain ratio on thermal-mechanical cyclic stress-strain response and fatigue life in DS superalloy DZ125 have been studied by performing tests at various strain ratio experiments, under strain-controlled and temperature cycling from 550 to 1000℃. It is shown that thermal-mechanical cyclic stress-strain response behavior not only depend on magnitude of strain, and temperature-loading phase angle, but also strain ratio. Fatigue life at strain ratio Rε=-0.3 is longer than that of strain ratio Rε=-1.0, under in-phase thermal-mechanical loading. However, Fatigue life at strain ratio Rε=-0.3 is shorter than that of strain ratio Rε=-1.0, under out-of-phase thermal-mechanical loading. The thermal-mechanical fatigue (TMF) damage model was discussed. Results of fractography show that fatigue, creep and oxidation damage always occur during TMF. The main damage mode depends on loading wave, strain ratio and magnitude of strain.

FRACTURE MECHANICS ANALYSIS OF THERMAL STRAINFATIGUE LIFE FOR THE SINTERING MACHINE PALLET

In recent years elastic-plastic fracture mechanics has developed rapidly and is widely used to solve various engineering problems. The application of elastic-plastic fracture mechanics on the pallet of sintering machine is approached in detail for the first time in the present study. The theoretical results were compared with the actual data determined from sintering machine pallet. Results show that good agreement was achieved between the method suggested by the author and the actual data. The basis of determining design of the sintering machine pallet in iron and steel engineering has been provided and it will result in great economic benefits.

Morphological evolution ofγ'phase in K465 superalloy during thermal fatigue

The alternative heating/cooling cycles (thermal fatigue) of K465 superalloy were carried out. The specimens were held at 1 050℃for 300 s, then quenched into 20℃recycling water for 10 s as a cycle. During thermal fatigue,γ' precipitates changed typically from cubical to irregular shape after 10 cycles, to complex configuration after 20 cycles and raft-like shape after 30 cycles. The very fineγ' particles precipitated inter the originalγ' particles. The elastic energy dominated morphological evolution of largeγ' precipitates, and the thermal stress induced the directional growth of precipitates that minimized the total energy of the system, and the nucleation theory controlled the formation of fineγ' precipitate. The results show that the volume fraction ofγ' precipitates is increased with the increase of heating/cooling cycles, which improves the mechanical property of this alloy.

Effects of quenching temperature on microstructure and mechanical properties of H13 mandrel steel

Optical microscope, stereomicroscope, scanning electronic microscope （SEM） and mechanical property testing were used to research the effects of different quenching temperatures on the microstructure and mechanical properties of the H13 mandrel steel. The results indicate that following an increase in the quenching ,the degree of alloying is enhanced due to the carbides dissolving gradually in austenite, which improves the hardenability of the specimens, as well as their room and high-temperature strength. At the same time, the fracture toughness increases due to the increment of the martensite number and the interparticle distance of impact toughness. Optimal performance Consequently,the service life of the H13 the carbides. However, extremely coarse grain and martensite can decrease the can be obtained after quenching at 1 060℃ and double tempering at 620℃mandrel steel is extended significantly.

Influence of the cylinder head structure of a diesel engine on fatigue strength

Using finite element method,influence of diesel cylinder head structure on fatigue strength is investigated.A simplified head model with function characteristics is built for thermal-mechanical simulation.From the simulation results,the influence of valve bridge structure and roof transition fillet dimension on fatigue strength are obtained.And a new valve bridge structure which can effectively improve the fatigue life is proposed.

Constitutive modeling of a directionally solidified nickel-based superalloy DZ125 subjected to thermal mechanical creep fatigue loadings

A transversely isotropic continuum elasto-viscoplasticity model,which was developed from Chaboche’s unified constitutive model,was formulated to capture the thermal mechanical creep fatigue deformation behavior of a directionally solidified nickel-based superalloy.A fourthorder tensor was introduced to model material anisotropy.In order to model the tertiary creep behavior,the Kachanov damage evolution equation was coupled into the stress tensor.Based on the test results of uniaxial tensile,fatigue,and creep loadings at isothermal temperature conditions,the material parameters are obtained.Thermal mechanical fatigue(TMF) and creep-fatigue interaction test results were used to verify the robustness of the model.Additionally,strain-temperature-dependent stress-strain responses under TMF loadings were analyzed using the present model.Under strain-controlled conditions,both of the stress ranges and mean stresses are strongly influenced by the strain-temperature phases,a key parameter for TMF tests.

In-phase thermal–mechanical fatigue investigation on hollow single crystal turbine blades

Thermal-mechanical fatigue（TMF）is the primary cause of failure of nickel-based single crystal turbine blades.TMF experiments have been performed on the critical section which is subjected to the most serious damage and determined by numerical calculation combined with service failure experience.An experimental system including the loading,heating,air cooling,water cooling,and control subsystems,is constructed to satisfy the TMF experimental requirements.This experimental system can simulate the stress feld,temperature feld,air cooling process,and TMF spectrum on the critical section under service conditions in a laboratory environment.A metal loading device and a new induction coil are developed to achieve the required stress and temperature distributions on the critical section,respectively.TMF experimental results have indicated that cracks initiated at the trailing edge of the suction surface on the critical section.Based on these experiments,life prediction and failure analysis of hollow single crystal turbine blades can be investigated.

Biomimetic coupling effect of non-smooth mechanical property and microstructural features on thermal fatigue behavior of medium carbon steel

Some kinds of particular functions possessed by natural organisms are often formed by coupling up the multiple typical features on their body surfaces. Inspired by the coupling phenomenon in biological system, the medium carbon steel specimens with the coupling effect of non-smooth mechanical property and microstructural features were fabricated by laser processing. Thermal fatigue behavior of specimens with biomimetic coupling surface was investigated and compared. The results confirmed that such a biomimetic method has the beneficial effect on improving the thermal fatigue property of medium carbon steel specimens. The related mechanisms behind the biomimetic coupling effect for explaining the enhanced thermal fatigue resistance were discussed preliminarily.

A new model for life prediction of GH4133 under TMF conditions

Thermal mechanical cyclic strain tests were carried out under in-phase andout-of-phase conditions on a Nickel-base Superalloy GH4133 in the temperature range of 571-823 degC. Based on analyzing the present models of TMF (thermal mechanical fatigue) life prediction, a newmodel for predicting nickel-base superalloy TMF lifetime was proposed. TMF life of superalloy GH4133was calculated accurately based on the new model. Experimental TMF life has been compared with thecalculated results and all results fall in the scatter band of 1.5. The calculating results showthat the new model is not only simple, but also precise. This model will play great roles as lifeprediction of the metal materials and the engineering components subjected to non-isothermal serviceconditions.

BREAKAGE MECHANISM OF TUNGSTEN BASED ALLOY BLOCK FOR ELECTRO-HEAT UPSETTING

The breakage mechanism of W-Ni-Fe alloy in the process of electro-heat upsetting studied both theoretically and experimetnally, and also the behaviors of crack formation and propagation were analysed. Alloy suffers from corrosion and thermal-mechanical fatigue mutual function. Simultaneously, the practical ways to improve the anvil life was discussed.

Study on Higher Efficiency Thermal Cycling Profile for HALT

HALT(highly accelerated life test) is a new reliability test technique.This paper uses nonlinear finite element method to analyze the stress strain characteristic of solder joints of PQFP(plastic quad flat packaging) and BGA(ball grid array) under thermal cycle test,and studies influences of profile parameters of the thermal cycle,such as hot and cold soak temperature,hot and cold soak time and temperature change rate,on elastic strain range,accumulated plastic strain,fatigue life and test efficiency of two types of solder joints.Based on the above research and experimental verification,this paper presents the method to build an optimal thermal cycling profile for HALT of electronic components.

镍基高温合金的热机械疲劳寿命预测模型研究

针对发动机热端部件常用材料镍基高温合金GH4169进行了200~450℃及400~650℃条件下的同相位热机械疲劳(TMF)试验,考虑TMF条件下多晶材料在弹性阶段产生的微观损伤应变能,提出一种适用于多晶材料的TMF寿命预测模型,并结合试验数据确定模型参数;采用GH4169、IN718、DD8三种高温合金对该模型的TMF寿命预测能力进行评估,结果表明,提出的寿命模型预测精度高于TMF寿命预测常用的Manson-Coffin模型和Ostergren模型。

陶瓷封装球栅阵列焊点失效机理研究

研究了陶瓷封装球栅阵列(CBGA)的焊点在长期使用后开裂的失效机理,提出了改善此类元器件焊点可靠性的有效措施。通过X-ray分析、显微剖切、扫描电镜和能谱分析(SEM&EDS)等手段,研究分析了CBGA封装器件焊点失效的机理与成因;基于失效机理,提出了针对性的改善措施,并验证了相关措施的有效性。研究结果表明,由于CBGA器件本体与印制板之间存在较大的热膨胀系数差异,导致焊点在长期使用后出现了热疲劳开裂失效,通过更换焊球类型以增加焊点高度可以显著地改善焊点的可靠性。

热冲击高强钢焊接接头的低周疲劳及断裂机理研究

为了研究热冲击下高强钢焊接接头的低周疲劳性能及断裂机理,用电弧焊焊接Q345结构钢,分别在-20~400℃、-40~650℃、-60~900℃下对焊接接头进行循环冲击,用疲劳试验机加载低周疲劳载荷,分析热冲击温度对低周疲劳性能及疲劳强度的影响,并用扫描电子显微镜等分析焊接接头的断裂机理。结果表明:相同疲劳加载次数下,焊接接头的总应变幅值随热冲击循环次数的增加匀速下降;相同热冲击循环次数下,疲劳加载次数越多,焊接接头的应变幅值越小;相同疲劳加载次数和相同热冲击循环次数下,随冲击温度范围的扩大,焊接接头的总应变幅值逐渐减小,但总应变幅值整体变化与冲击温度范围正相关。

干热岩柔性压裂裂缝起裂与扩展规律

考虑岩石热孔隙弹性效应、循环载荷下岩石强度疲劳劣化本构关系、弹脆性破坏准则和井筒应力叠加效应,建立并验证了热流固-疲劳损伤耦合的干热岩柔性水力压裂裂缝扩展数值模型,在此基础上开展数值模拟研究,探究不同温度和循环载荷共同作用下干热岩裂缝起裂及扩展特征。研究表明:①周期注入、流体渗透、孔隙压力累积与岩石强度劣化共同引发柔性压裂岩石疲劳破坏;②干热岩柔性压裂裂缝扩展模式由温差和循环载荷共同控制,温差越大,热应力越强,利于形成复杂缝网;循环载荷降低,热应力波及范围增大,循环载荷为90%pb和80%pb(pb为常规水力压裂时岩石破裂压力)时,改造面积相较于常规水力压裂分别提升88.33%和120%(注入温度为25℃);③随着循环载荷的进一步降低,储层改造效果减弱,循环载荷降低至70%pb时,距离井筒较远处流体压力无法达到岩石最低破裂压力,不会产生宏观水力裂缝。

重复使用火箭发动机推力室疲劳寿命研究进展

再生冷却推力室内壁的热-机械疲劳失效严重影响重复使用液体火箭发动机的可靠性和使用寿命,疲劳分析在内壁损伤机理研究、寿命预测和结构优化设计中具有重要作用。简要回顾了推力室再生冷却结构热-机械疲劳分析方法的发展历程,重点围绕材料本构关系、热-力响应计算和疲劳寿命模型,对比梳理已有方法,讨论其特点及应用。基于研究进展与工程需求,从全服役周期瞬态载荷环境、材料本构关系、热-机械损伤模型及验证、基体与涂/镀层耦合失效分析和基于有限数据的工程方法等方面给出了进一步研究的方向和建议。