壓力艙結構塑性變形對疲勞強度影響之評估

STUDY OF PLASTIC DEFORMATION OF PRESSURE CHAMBER AND EFFECT ON FATIGUE STRENGTH

本研究主要目標為利用計算與實驗，建立高降伏強度金屬在歷史塑性變形作用過後，塑性應變累積對疲勞強度的影響，並提出可供參考的S-N曲線修正建議。這項研究的緣由主要是因為壓力艙結構中，調節船重的水櫃分為經常性與非經常性使用櫃，在載具操作過程中必須承受所謂的工作壓力與潛深壓力，經常性水櫃用在一般運作狀態需要經常性承受任何操作所需要的水壓，所以強度上有很高的要求。而非經常性水櫃僅需在調節重量與姿態時使用，也不需要直接承受外部的水壓，所以此種結構一般強度設計上可以較低，也是結構優化時可以利用的部分。但是非經常性水櫃在完成建造後，仍然需要經過最極端狀況下之水壓測試，這種較高的測試載重在後續操作中比較不常遇到，僅在緊急狀態下會達到，但是由於次數很少，比較類似極端強度估算。因此，如果能在第一次測試時使用部分降伏後的應變硬化，可以在符合極端強度與疲勞強度估算的情況下，大幅度降低結構重量。為了瞭解結構行為，本研究任務包含：1.模擬水櫃在工作與潛深壓力情況下塑性變形與應變硬化累積程度；2.設計並進行高強度鋼材在經歷塑性變形後的疲勞實驗；3.找出可能的疲勞強度與歷史塑性變形關係。結果顯示歷史塑性應變對疲勞壽命有顯著的影響，本研究也提出根據回歸分析所求得的相關修正曲線。

The objective of this research is to use numerical modeling and experiment to establish the effect of historical plastic strain on high strength metal under cyclic loading conditions. The results will be used as a reference to adjust the S-N curve based on the historical plastic strain due to sea trial, blast loading and extreme operating conditions. The original ideal of this research comes from the need of designing the optimization of water attached to the pressure vessel. These water tanks are classified into frequently-used tanks and non-frequently-used tanks. Frequently-used tanks usually need to sustain any water pressure required for general operation, which include working pressure and deep-diving pressure. On the other hand, non-frequently-used tanks usually do not experience the deep-diving water pressure during operation except for the emergency scenario. And its structure can be optimized to reduce overall weight. How-ever, when the construction of the pressure vessel is completed, all water tanks still need to go through testing conditions, which represent the deep diving condition. If plastic deformation occurs, those strength-reduced water tanks may have less fatigue life due to the plastic strain accumulation. To understand the effect of the initial plastic deformation, this research performs the following tasks: 1. Numerical modeling of water tanks under operation and deep diving loading conditions to obtain the strain hardening behavior; 2. Design and perform fatigue experiments of high strength metal with pre-plastic deformation history; 3. Possible relation between fatigue strength and historical plastic deformation. The result shows historical plastic strain shows obvious influence on the fatigue life. And a S-N adjustment process is proposed.