深潜器耐压壳体结构蠕变与疲劳耦合损伤机理及寿命评估方法研究

The pressure hull of deep submergence vehicles endures high hydrostatic pressure during working in ultra-deep water. At present the titanium alloy materials which have higher strength-to-weight ratio and specific stiffness are widely used in the pressure hull manufacturing. However, different from the mechanical responses of ship hull in normal sea state, the creep behavior of the titanium alloy materials under ultra-high hydrostatic pressure can’t be ignored any more. Therefore, when evaluating structural safety of the pressure hull in service-life, it is necessary to propose a method for evaluating the creep and fatigue coupled damage initiation and evolution in the pressure hull under ultra-high hydrostatic pressure condition, as well as the whole service-life of the pressure hull. First of all, due to the imperfection of classical creep damage constitutive relationship, which is mainly applicable to the tensile material in high temperature, we investigate the creep damage mechanism for the titanium alloy materials under ultra-high hydrostatic pressure at room temperature considering the fact that the titanium alloys display different behavior between tensile statement and pressure statement. Secondly, practical models for the evaluation of creep damage in ultra-deep water, fatigue damage with extremely low frequency and the two coupled damage for the titanium alloy materials in the ultra-high hydrostatic pressure environment are proposed based on the continuum damage mechanics. Furthermore, considering the actual working state of the deep manned submersibles, creep recovery effect on pressure hull service-life is studied as well. Finally, we investigate a new damage initiation and evolution rule, which is suitable to finite element simulation, based on the developed creep and fatigue coupled damage accumulation model. And an effective method to evaluate the service-life of the pressure hull considering creep and fatigue coupled damage is also proposed. The innovations of our research are deducing the damage constitutive relation of the titanium alloy materials under ultra-high hydrostatic pressure condition, and proposing a creep and fatigue coupled damage accumulation model considering the creep recovery as well as the method to evaluate the service-life.

深潜器耐压壳体在超深海作业过程中承受极大的深水压力，工程上通常采用具有高比强度和高比刚度的钛合金材料。然而，钛合金材料在超高压下的蠕变问题不容忽视。因此，在对耐压壳体进行全寿命期安全可靠性评估时，需要确立耐压壳体蠕变与疲劳耦合累积损伤及其寿命评估方法。首先，针对目前蠕变损伤本构关系主要适用于材料高温受拉破坏的不足，开展钛合金材料常温、超高静水压力下的蠕变损伤机理研究；其次，基于连续损伤力学理论开展耐压壳体超深水环境下的蠕变损伤、极低频疲劳损伤以及蠕变与疲劳耦合损伤累积模型研究，并进一步考虑潜水器实际工作状态，研究蠕变恢复对耐压壳体蠕变寿命的影响；最后，研究蠕变与疲劳耦合作用下损伤起始和损伤演化准则，提出能够考虑蠕变与疲劳耦合累积损伤的耐压壳体寿命评估方法。该方法的创新性体现在建立了钛合金材料含受压状态下的损伤本构关系，建立了能够考虑蠕变恢复效应的蠕变与疲劳耦合累积损伤模型及寿命评估方法。

深潜器在结构形式突变或不连续加强的局部会产生应力集中，这会导致结构在承受每一作业周期的超大幅值交变载荷时发生疲劳损伤。耐压壳的特殊作业环境，钛合金长时间处于深海高压环境中会发生蠕变损伤，疲劳和蠕变之间存在相互促进的关系，会加速裂纹的萌生与扩展，显著降低耐压球壳的寿命。本研究首先对疲劳损伤进行了研究，建立钛合金材料的低周疲劳损伤累积模型；开展Ti80合金低周性能试验和CT试件裂纹萌生与扩展试验；进一步，对商业有限元软件进行二次开发，建立深潜器耐压壳结构低周疲劳裂纹萌生与扩展的全寿命预测方法。之后对蠕变损伤进行了研究，首先在Lemaitre伤本构方程基础上，发展了一种方便应用于深潜器耐压壳体损伤演化数值模拟的损伤演化方程，可以考虑拉压不同的影响。之后，确立钛合金材料在深海中长期处于高压环境下的蠕变损伤本构关系。并开展单轴实验标定参数。最后，结合室温环境下开展了单轴保载疲劳试验，根据试验前后测得的杨氏模量计算单轴拉伸试件的损伤，结合有限元模拟相互比较分析的方法，得到与疲劳-蠕变以及蠕变-疲劳有关的损伤参数，确立了一种适用于钛合金材料室温环境下疲劳-蠕变非线性耦合损伤模型。进行二次开发，利用降低有效杨氏模量的方法模拟无初始缺陷的CT试件在保载疲劳载荷下从裂纹萌生-扩展-失效的全寿命损伤演化过程，并与室温环境下开展的三组保载疲劳试验结果进行比较分析，证明了本模型的准确性。为证明本模型具有实际工程应用价值，开展了钛合金深潜器复杂结构的非线性有限元计算，模拟其在全寿命期间损伤演化过程并以此进行寿命预测。模拟结果说明在耐压壳焊趾位置处会存在明显的应力集中现象，在实际结构当中，该处不可避免会存在初始缺陷，在经受超高压和超大幅低周疲劳载荷的循环作业下，初始缺陷会因受到疲劳-蠕变耦合损伤的作用不断扩大，显著降低结构的寿命。