Abstract of Special Research Report (SRR-No.30)
National Institute of Occupational Safety and Health, Japan
Development of Damage Estimation Technique for Maintenance of Construction Machines
|Etsuji YOSHIHISA and Yutaka MAEDA|
: Since various machines used over long terms increased recently, measures to extend the lives of machines were adopted to save resources and to cut costs. The same tendency is also observed in construction machines such as mobile cranes. Fracture accidents sometimes occur due to damages which are generated in stress concentrated parts such as welded joints in mobile cranes, as they are mainly used under severe conditions. To prevent fracture of mobile cranes used over long terms, it is important to conduct appropriate maintenance, in which reliable flaw detection, risk evaluation for the detected flaws are conducted and interval between inspections is based on active load. Cranes operated under fixed working are classified by loading conditions, but it is difficult to classify mobile cranes in similar manner, owing to various kinds of their working. Though a rule to determine the method of inspection and overhaul on the basis of loading conditions, are presented in ISO 12482-1 Crane - Condition Monitoring - Part 1: General, concrete method is not given. Active load measurements for mobile cranes, therefore, are considered to be useful, and the improved inspection methods, in which reliable detection and sizing for flaws are possible, are desired to develop.
Active Load Measurements for Lattice Jib Type Mobile Cranes
|Masayuki YOSHIMI, Etsuji YOSHIHISA and Yutaka MAEDA|
: Recently, maintenance of machines is of great concern not only as economic issue but also as safety issue. Suitable inspection of flaws and adequate estimation of the risk of failure are essential in applicable maintenance. From a point of view of fatigue, evaluation of the load history is necessary in order to assess the remaining life of a structure or a member.
Stress Measurements for Wheel Crane Boom under Operating Conditions
|Etsuji YOSHIHISA, Masayuki YOSHIMI, Yutaka MAEDA, Tetsuya SASAKI and Takashi HONDA|
: Life extension of various machines is of great concern in recent years. Material fatigue is a major factor that influences the lives of machines. The importance of fatigue strength in structural components therefore increases for construction machines such as mobile cranes. To estimate the fatigue strength of the components of actual mobile cranes, it is necessary to grasp the load history of the cranes under operating conditions.
The Improvement of Non-destructive Inspections by Using Thermal Stress Analysis System
|Takashi HONDA and Etsuji YOSHIHISA|
: Defects in construction machines are inspected at the regular interval of a time period without any consideration in the amount and the frequency of a load in a construction field. In order to ensure the safety of construction machines till next inspection, the periodical inspection based on a remaining life estimated by nondestructive test (NDT) and working condition in service is necessary. The NDT mainly applied to construction machines is the visual inspection. However, detection of a flaw using this technique is difficult since machinery is usually painted in order to prevent it from rust. Therefore, the effective non-destructive technique which replace the visual inspection is needed for the life prediction of construction machines.
Ultrasonic Testing on Welded Joints of Thin Plate Modeled upon the Welded Joint on the Corner of a Mobile Crane Boom
|Etsuji YOSHIHISA and Takashi HONDA|
: The importance of maintenance increases for construction machines such as mobile cranes to prevent the fracture accidents and to extend the lives of the machines. Welded joints are of major concern as they are frequently the sites of localized damage. Non-destructive inspection techniques for detection and sizing of flaws in welded joints of the machines have been developed. Ultrasonic TOFD(Time of Flight Diffraction) technique is one of those techniques. In this study, TOFD technique was applied to measure the flaw height(through-wall height of flaws) and flaw length(length in the direction parallel to the surface) for two types of specimen. One was a thin plate specimen with butt-welded joint and the other was a L-shaped specimen which is modeled upon the welded joint on the corner of box-type boom of a mobile crane. In these specimens, some artificial flaws, which were surface fatigue cracks, blow holes, lack of fusions and incomplete penetrations, were prepared in the joints. Angle beam and immersion technique were also applied to the specimens for comparison with TOFD technique in the ability of detecting and sizing of flaws
Development of Fatigue Damage Monitoring Technique for Bolted Joints
|Tetsuya SASAKI and Takashi HONDA|
: To prevent fracture of construction machines such as cranes and concrete boom pump trucks, which are widely used at construction site, is essentially important to assure safety of workers and the neighborhood. Although there are some fracture types, fatigue is a major factor that must be considered to prevent fracture accidents of construction machines. Basically, prevention of fracture due to fatigue used to be attained by employing safe life design or damage tolerance design. In the safe life design, it is set so that the design life may not exceed predicted fatigue life. In the damage tolerance design, nondestructive inspection (NDI) is periodically applied to materials to assure structural integrity during service life. However, these two design concepts are not perfect because predicted fatigue life is inaccurate and all cracks are not always detected by NDI. These fundamental problems of the safe life design and the damage tolerance design are rejected if all fatigue damages which may result in a fatal accident can be monitored at any time using some sort of damage sensors. This idea has being applied to composite materials in which damage sensors can be easily embedded during their fabrication process. In spite of it, quite a few attempts of fatigue damage monitoring have been made for metals, the most widely used materials for any industrial machines and installations, since it is difficult to embed damage sensors in them.