随着发动机往低排放、高功率密度方向发展，由复杂循环的高热载荷引起的表面热疲劳失效已成为影响其使用及安全性能的关键工程问题，因此，热疲劳实验是测试和评价燃烧室部件及其材料热疲劳性能的重要手段。以脉冲激光为加热热源，可以充分利用脉冲激光束在时间和空间上的高可控性，模拟受热部件及材料在不同温度场作用下的热疲劳损伤过程，突破传统加热方式难以实现的空间上非对称、非均匀温度分布和时间间隔为毫秒级的温度波动的局限性。针对现有脉冲激光热疲劳试验技术中存在的问题，本文开发了脉冲激光热疲劳实验平台及变幅热疲劳实验方法，采用数值模拟和疲劳实验相结合的方式，研究了蠕墨铸铁材料变幅热疲劳裂纹演变过程，分析了脉冲激光变幅热循环加载下的温度场/应力应变场演化，提出了变幅热疲劳裂纹演变机理。本文主要内容及研究成果如下：

1. 建立了基于先进温度测量技术和闭环控制系统的脉冲激光变幅热疲劳实验平台，根据材料在不同激光参数下的温度响应特性，利用不同闭环控制手段和不同冷却方式的组合，实现不同幅值的热循环加载，形成不同循环载荷类型下的变幅热疲劳实验方法。采用低单脉冲能量、高重复率的激光参数组合进行高幅热循环加载，在此基础上叠加高单脉冲能量、低重复率的脉冲激光能量输入，实现变幅热循环加载。

2. 系统研究了蠕墨铸铁材料在脉冲激光变幅热循环加载下的裂纹演变过程。实验结果表明，铸铁材料的裂纹萌生阶段较短，随后在试样表面及深度方向上不断发生裂纹的粗化和扩展。变幅热循环加载下产生的热疲劳裂纹中，以次生裂纹和短裂纹为主，长裂纹的数量较少。随着深度的增加，网状裂纹总长度呈现指数减小的趋势，在变幅热疲劳实验中观察到的裂纹深度在200~300mm之间。通过数值模拟研究了脉冲激光变幅热循环加载下的温度场/应力应变场的分布及演变规律。建立了二维瞬态热-结构分析数值计算模型，提出了两个反映变幅热疲劳过程的重要热参量，即加热阶段的持续时间t_h、高周加载阶段的温度变化范围∆T_HCF，通过调整数值模型中的裂纹参数，可以准确反映出试样在不同循环次数后的温度历程变化。计算结果表明，脉冲激光在试样厚度方向上产生较大的温度梯度，瞬态温度波动只发生在表面及以下薄层区域内。随着循环次数的增加，裂纹深度和裂纹数量增加，t_h逐渐减小，∆T_HCF逐渐增加。加热-冷却阶段主要产生拉伸应力，产生不可逆的残余塑性变形，并随着热疲劳循环的进行而不断累积。高周循环载荷主要产生峰值压应力，以热弹性变形为主，几乎不产生塑性应变。每循环产生的温度波动∆T_HCF越大，热应力应变波动越大，造成的高周热疲劳损伤也越大。

3. 研究了在脉冲激光变幅热循环加载下的影响因素和裂纹演变机理。加热阶段的上限温度主要影响表面长裂纹的数量，长裂纹的比例随着上限温度的增加呈现线性增加的趋势；当上限温度超过450℃时，长裂纹比例的增速减缓。试样表面热疲劳裂纹的分形维数与塑性应变幅呈指数增加的趋势，随着塑性应变幅的增加，分形维数会逐渐趋向于临界分形维数D₀为1.395。高周加载阶段叠加的温度波动主要影响表面次生裂纹的数量，高周载荷次数的增加导致次生裂纹的比例增加，当高周载荷次数大于10³次时影响开始变得显著。分形维数与高周载荷次数N_HCF之间呈指数增加的趋势。当T_max为450℃时，若N_HCF大于临界值2000后，随着N_HCF的增加，分形维数会逐渐趋向于临界分形维数D₀为1.404。在脉冲激光变幅热循环加载下，除了形成一定数量的主裂纹外，试样表面还有较多次生裂纹，网状裂纹更加密集，裂纹数量较多，表现出与恒幅热循环加载下不同的热疲劳裂纹特征。基于分形维数和循环应变能密度的量化关系提出一种类似Paris公式的损伤演变模型，用于描述蠕墨铸铁材料的变幅热循环损伤演变机理。

本文研究结果提供了一种在多层次、多过程、多尺度范围内深入研究各类复杂变幅受热表面热疲劳损伤演变过程及机理的研究方法，为受热部件寿命预测和设计提供工程参考依据，同时对脉冲激光热疲劳实验技术的工程应用有重要指导意义。

Thermal fatigue failure caused by complicated thermal loading has become a critical problem which influences the serviceability and safety performance, with development of engine to lower emission and higher power density. Thermal fatigue experiment is an effective method to test and evaluate the thermal fatigue performance of combustion chamber components and materials. Pulsed laser has been adopted as heating source due to the high spatial and temporal controllability. Thermal loading process of heated components and materials under different temperature fields has been simulated. This approach breaks through the limitations of traditional heating methods in realizing the spatial asymmetric and inhomogeneous temperature field and the temperature fluctuation within millisecond-interval. In view of the problems in the current thermal fatigue test, an intelligent system of thermal fatigue test induced by pulsed laser and experimental method for variable amplitude test are developed in this paper. By means of numerical simulation and experiment, the evolution of temperature field and stress/strain field under thermal cyclic loading of variable amplitude induced by pulsed laser, and the evolution process and influence factors of thermal fatigue crack in compacted cast iron are studied. The main contents and conclusions are as follows:

1. A test system of thermal fatigue under variable amplitude by pulse laser is developed, based on advanced temperature-measurement technology and intelligent closed-loop control system. According to the temperature response characteristics of materials under different laser parameters, the thermal cycle loading of different amplitudes is realized by the combination of different closed-loop control modes and cooling methods, therefore, a thermal fatigue method of variable amplitude is developed, which is suitable for different types of cyclic loads. Low pulse energy and high repetition rate are used to fulfill thermal cycle loading of high amplitude. On this basis, pulsed energy input of high pulse energy and low repetition rate is added to achieve thermal cycle loading of variable amplitude.

2. The process of crack evolution for compacted cast iron under thermal cycle with variable amplitude is systematically studied. The experimental results show that, the stage of crack initiation is short, and then the cracks coarsen and propagate on the surface and in the depth direction. Among all the surface cracks produced by the thermal cyclic loading of variable amplitude, the secondary cracks and short cracks are the main crack patterns, and the number of long cracks is the least. With the increasing depth, the total length decreased exponentially, and the observed crack depth is between 200~300mm. The distribution and evolution of the temperature field and stress/strain field under the thermal cycle of variable amplitude are studied by numerical simulation. A two-dimensional numerical model for transient thermal-structural analysis is proposed. Two important parameters reflecting thermal fatigue process of variable amplitude are pointed out, namely the duration of heating stage t_h, and the temperature range of high cycle loading stage ∆T_HCF. The temperature change after different cycles can be reflected accurately by adjusting the crack parameters in the numerical model. The results show that, large temperature gradient in the thickness direction is caused by pulsed laser, and the transient temperature fluctuation occurs only on the surface and a thin layer below the surface. As the cycles going, both the crack number and crack depth increase, causing the decreased t_h and the increased ∆T_HCF. The heating/cooling stage mainly produces tensile stress, causing irreversible plastic deformation and damage accumulation during the tests. High cycle loading mainly produces peak compressive stress, causing thermal elastic deformation, and almost no plastic strain is induced. The larger the temperature fluctuation ∆T_HCF is, the larger the thermal stress/strain fluctuation is, and the greater the thermal fatigue damage is.

3. The influence factors and mechanism of crack evolution for compacted cast iron under thermal cycle with variable amplitude are studied. The maximum temperature T_max in the heating stage mainly affects the number of long cracks, and the proportion of long cracks increases linearly with the increasing temperature. When T_max is over 450℃, the growth rate of long cracks slows down. With the increase of plastic strain amplitude, the fractal dimension increases exponentially, and gradually tend to the critical fractal dimension D₀ of 1.395. The temperature fluctuation in the high-cycle stage mainly affects the number of secondary cracks. The increased number of high cycle loading causes larger number of secondary cracks, and such effect becomes obvious when the superimposed cycle number N_HCF is larger than 10³. The fractal dimension increased exponentially with the increase of N_HCF. When T_max is 450℃, if N_HCF is larger than the critical value 2000, the fractal dimension will develop to the critical fractal dimension D₀ of 1.404 with the increase of N_HCF. Under the thermal cycle loading of variable amplitude induced by pulsed laser, there are more secondary cracks on the surface, besides a certain number of main cracks. The crack network is denser, and the crack number is larger, which shows different characteristics with the crack caused by the thermal cycling of constant amplitude. A model of damage evolution, which is similar to the Paris equation and based on the relationship between fractal dimension and cyclic energy density, is proposed to describe the damage evolution of thermal cycle with variable amplitude for compacted cast iron.

The results provide a method which can be used to study the influence factors and rules of thermal fatigue damage to various kinds of heating surface in multi-level, multi process and multi scale range. This method can provide reference for the lifetime prediction and design of heated components. At the same time, it has important guiding significance to the engineering application of thermal fatigue test induced by pulsed laser.