This is a proposal of research topics and approaches to promote them for comprehensive understanding of the fatigue and fracture mechanics that relates to numerous accidents caused by mechanical damage. Initial phenomena as an origin of fatigue fracture are often unexplained. This proposal is aimed at establishing a new axis of scientific and technological basis with regard to the fatigue fracture by deepening the understanding of its initial phenomena of which the mechanical mechanism is largely unknown and the understanding of the whole fatigue fracture based on it.

In the efforts to achieve a carbon-neutral society, there is a rising demand for weight saving and higher efficiency of energy equipment (prime movers) and transport equipment (automobiles, trains, aircraft and ships). However, from the perspective of ensuring reliability and safety of the equipment, these devices are designed by multiplying a safety factor. Therefore, a new approach based on the understanding of physical phenomena of the fatigue fracture is necessary for further pursuit of weight saving and higher efficiency. Deeper understanding of the fatigue fracture phenomena enables shortened development time-to-market and reduction in the development costs. It also allows marketing of a new product with adequate reliability earlier than any other countries, which can lead to improvement of Japan's industrial competitiveness. Hence, it is necessary to understand the fatigue fracture phenomena more substantively as well as establish and strengthen the scientific and technological basis for the fracture phenomenology and fracture engineering.

It is difficult to predict fatigue fracture because the dynamic mechanism of the initial phenomena from microcrack initiation to microcrack growth has hardly been elucidated or the phenomenon is not understood or modeled. Under the current circumstances, strength required for equipment to withstand the fatigue fracture is calculated from statistical and experimental methods by using massively accumulated fatigue test data. Therefore, most of the recent researches on the fatigue fracture are undertaken as part of the development of new equipment and materials and remain as studies on properties of individual materials. In order to break away from such circumstances, it is necessary to conduct research to advance the understanding of the fatigue and fracture mechanics that have not yet been elucidated.

We propose the following two specific research topics to elucidate the microscopic mechanism and mechanics of the initial phenomena of the fatigue fracture and thereby establish the scientific and technological basis.

• ①Discovering the law of the initial phenomena of the fatigue fracture
• ② Establishment of the fatigue phenomenology and the fatigue engineering based on the understanding of the initial phenomena of the fatigue fracture

Significant progress has recently been made in experiments and in the non-destructive/in-situ measurement techniques for the mechanical characteristics and the fatigue characteristics in microscopic size that relate to elucidation of the initial fracture phenomena. With exponentially developed performance of computing machineries, efforts have been progressing towards unified understanding of the fracture phenomena by multi-scale multi-physics simulation. Data science such as machine learning and deep learning are expected as a tool for prediction. By combining these experimental methods, measurement methods and mathematical methods, it is proposed that the law of initial phenomena of the fatigue fracture should be figured out and that a new axis of the fatigue phenomenology and the fatigue engineering should be established based on the understanding of the initial phenomena of the fatigue fracture.

As a first approach for promotion of the above, it is important to form a cross- disciplinary network beyond boundaries of academia and industry, where core human resources should be developed who have understanding of both the basic research focused on the discovery of the law of initial phenomena of the fatigue fracture and the fracture phenomena of actual equipment. As a second approach, creation of a center of excellence should be promoted as a sustainable research environment. The aim of the center is to elucidate the laws of the initial phenomena of the fatigue fracture, which are diverse and complex, and to establish a new axis of the fatigue phenomenology and the fatigue engineering based on it. This field is one of the fundamental scientific and technological fields to protect and maintain national security. Therefore, it is important for the government to promote research and development in this field from a medium-to long-term and continuous perspective.