Physical System Research Group

 


Ships operate in the harsh corrosive environment of the sea and are constantly subjected to fluctuating loads of waves, so various damages are generated to the hull. To contribute to the realization of a safe and secure society and to enhance the competitiveness of the maritime industry, we are engaged in a variety of research projects that contribute to the advancement of construction technology, focusing on technologies related to damage assessment and control.


 

Members 

(◎: Head of the Group)

 




Overview of our research

 
1.  Fatigue Life Estimation by Fracture Mechanics
 There are many cases of accident due to fatigue damage in large welded structures such as ships. To develop an evaluation method based on fracture mechanics into practical use in the fatigue strength design and maintenance planning of hulls, we have been working on the following topics.

  1. Quantitative Estimation of Fatigue Surface Crack Propagation Behavior Initiated from Surface Defect in Three-Dimensional Objects
  2. Quantitative Evaluation of Multiaxial Stress Effects on Fatigue Crack Propagation Behavior

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2.  Evaluation of Methods for Improving Fatigue Strength
 As hull structures have become larger, the plate thicknesses have also increased. On the other hand, it is generally known that the apparent fatigue strength (, which is based on nominal stress) decreases due to the plate thickness increases. It is called the "thickness effect". In recent years, improving techniques for fatigue strength by smoothing weld bead and imparting compressive residual stress has been attracting attention. Our Group is working on the following topics to reflect the effect of improving techniques for fatigue strength such as High-Frequency Impact treatment in the fatigue design guidelines.

  1. Identification of the effect of compressive stress
  2. Identification of the fatigue strength by secondary treatment
  3. Identification of the index of thickness effect
  4. Clarification of cost effectiveness

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3.  New material utilization technologies that utilize new joining technologies in shipbuilding
 As one of the core technologies to enhance international competitiveness and promote the creation and development of related industries, research and development of optimal design and joining adhesion technologies for multimaterials has been drawing attention. In recent years, the use of new materials has been expanding in other industries, as well as improving the strength of joints between dissimilar materials, therefore it is also necessary to promote the use of new materials for general merchant ships in shipbuilding industry. In order to utilize new materials that have not been commonly used for ships, due to the improvement of flexibility in design with high-functional materials and the efficiency of the construction process, our group is conducting research on improving the long-term reliability of adhesive joining technology for dissimilar materials in consideration of aging deterioration by combining environmental deterioration acceleration test and various strength tests.

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4.   Improvement of Technology for Evaluation of Hull Structural Integrity under Corrosion Condition
 Ships and offshore structures have been in service for more than 25 years under harsh environments. Among them, seawater tanks suffer the most severe corrosion damage and require advanced structural integrity management. Therefore, in order to develop a corrosion simulation system that has a function of post-update by monitoring, our group is researching the measurement method to detect of the initial under-film corrosion on coated steel panel and development of coating deterioration/metal corrosion coupled simulation method.

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5.  Brittle Fracture (International Standardization of Materials Strength Evaluation Methods)
 Brittle fracture suddenly causes enormous damage and is difficult to predict. Therefore, it is one of the biggest concerns for people involved in ships, and it is necessary to accurately grasp the material strength (fracture toughness value) of the weld area, which is particularly prone to fracture. The shape of the test piece used in the experiment to determine the fracture toughness value is defined by the standard, and it is necessary to grow the fatigue crack straight from the machine notch, but the fatigue crack does not grow straight with the welds. However, it is known that by compressing the tip of the notch in advance (local compression), even if it is a welded material, the fatigue crack will grow straight. Therefore local compression have often been done before precracking. In the typical local compression, the material is compressed directly at the point of fracture, i.e. crack tip. That degrades the material and lowers the strength of the material. In addition, a large testing machine is required to compress ultra-thick pieces because of the increased load required. Therefore, our group is conducting research on the international standardization of a new local compression method that allows fatigue cracks to grow straight without directly pressing the fracture initiation point and can be carried out with the load capacity of a small testing machine.

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