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.
    For development of maintenance method suitable to mobile cranes, following subjects have been carried out in this specific research.
(1) Load monitoring for mobile cranes
  * Active load measurements for lattice jib type mobile cranes
  * Stress measurements for wheel crane booms under operating conditions
(2) Improvement of damage detecting technique for mobile crane components
  * The improvement of Nondestructive inspections using thermal stress analysis system
  * Ultrasonic testing for welded joints of thin plate modeled upon the welded joint on corner of mobile crane boom
(3) Development of fatigue damage monitoring technique for joints
  * Development of fatigue damage monitoring technique for bolted joints.

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.
    Active load measurements for lattice jib type mobile cranes were conducted. Objectives of them were to reveal actual state of stresses loading on members of cranes. They were split into two phases. One was the in yard measurement of a crane simulating some actual operations such as slewing, hoisting, derricking, combined operation and so on, aiming at grasping typical features of stresses and associating them with operations of the crane. The other was the measurement for a crane operated for actual building construction works, aiming at clarifying actual state of stresses.
    The results and significance of this study are mainly as follows:
(1) Main factors of changes of stresses acting on the tower and jib of cranes are the posture of the crane, the mass of the load and speed of slewing.
(2) Amplitudes of stress changes by dynamic lift off and touch down are much smaller than those by posture shifting and slewing with loading.
(3) Acting stresses at the bottom of crane towers by fast slewing can be comparable to those by posture shifting, and they cannot fully be evaluated with overloading alarming instruments equipped on lattice jib type mobile cranes.
    We suggest that new instruments for recording load history of members of cranes should be ones that are capable of evaluating actual state of loading, and that they must include a measuring device for transversal loads.

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.
    Two types of stress measurements for the booms were conducted on rough terrain cranes. In the first type, the measurements were made under three typical movements observed in crane operations, in a testing yard. Three typical movements are; a continuous hoisting and slewing, luffing with simultaneous slewing and hoisting with simultaneous slewing. The characteristics in histogram of stress amplitude estimated by Rain Flow Method were investigated and the relationships between speed of movement, or mass of loading and the estimated stress amplitude were analyzed for each type of movement. In the second type, stress measurements were done in a construction material center. The aim of this measurements was to clarify the stress changes on the boom during actual crane operations.
    The main results obtained in this study are as follows.
(1) The stresses on the boom change according to the posture of the crane, the mass of loading and the bending vibrations in the boom. The vibration in the horizontal plane is caused by slewing and that in the vertical plane is caused by lift off and touch down of the load.
(2) In the case of slow operating speed, the posture of the crane and the mass of loading are the major factors of stress changes on the boom. The amplitudes of these stress changes are much larger than those of stress fluctuations caused by the bending vibrations in the boom.
(3) At higher operating speed, bending vibration in the horizontal plane has a great influence upon the stress fluctuations. Some of the stress fluctuations in the section near the boom root by slewing are comparable to stress changes by mass of loading and posture shifting. The stress fluctuations by lift off and touch down of the load are smaller than those by slewing.

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.
    A number of non-destructive test such as penetration, magnetic, radiation and ultrasonic test have been developed to detect defects in structural components. Among those methods, the thermographic non-destructive technique is relatively new method based on the measurement of infrared radiation emitted from the surface of a body. Since this method can not only detect a flaw but also measure it's size in two-dimensional, the authors have been investigating NDT using thermal stress analysis (TSA) system. As the results, it was found that 4mm crack was detectable and crack lengths estimated from measurement images also agreed well with those measured by the optical microscope. However, stress intensity factor ranges evaluated by measurement results, which is the most important parameter of estimating the remaining life, were somewhat lower than comparative numerical values.
    In this study, in order to improve the accuracy of stress intensity factor range evaluated by TSA, the source of the error between measurement values and numerical values was clarified and the improve method aided by numerical analyses was devised. Also, TSA was applied to several types of tubular specimens in order to confirm the accuracy of measurement results for curved surfaces. The main results obtained in this study are as follows:
  (1) The accuracy of stress intensity factors calculated by the hybrid expansion method can not be improved due to inaccuracy of stress values in the vicinity of a crack tip.
  (2) Stress intensity factor ranges evaluated by the hybrid method aided by finite element analyses were often larger than the values obtained by the extrapolation method.
  (3) TSA is applicable to stress measurements of tubular specimens because the stress distributions measured were constant from the center to the edge of specimens.
  (4) The stress concentrations at the weld toe in tubular T joint specimen can be observed by TSA. Also, the artificial crack, of which length is 4 mm, in the tubular specimen can be detected by TSA.

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
    The main results obtained in this study are as follows.
  (1) In the butt-welded specimen, each prepared flaw was detected by TOFD, angle beam and immersion techniques. The length of fatigue cracks could be measured by each technique and the measured length was considered to be close to the actual crack length.
  (2) The height of fatigue cracks could be measured by TOFD and immersion techniques, but could not be measured by angle beam technique. TOFD and immersion techniques gave similar values for the crack height
  (3) In the L-shaped specimen, some flaws were not detected by TOFD technique, though all flaws were detected by the other techniques. The height of fatigue cracks could be, however, measured by TOFD technique.
  (4) In case of flaw which was an aggregation of several separated voids, the length measured by TOFD and angle beam techniques was different from that of immersion technique. These techniques were considered to lose accuracy in measuring the length of flaws of this sort.

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.
    In this study, built-in sensor bolts are used to overcome inherent difficulties of monitoring fatigue damage of is a strain metals. The damage sensor embedded in a bolt is a strain gauge which is not only low cost but also highly sensitive and reliable. Fatigue tests are conducted using two types of specimens; shear type bolted joint specimens and flange type bolted joint specimens, to demonstrate the ability of fatigue damage monitoring with built-in sensor bolts. The shear type bolted joint specimens are used to monitor fatigue damage of base metal. On the other hand, the flange type bolted joint specimens are used to monitor fatigue damage of bolts. Following conclusions are reached from this study:
  (1) Fatigue cracks in body metal of shear type bolted joint specimens can be detected before fracture of a joint using built-in sensor bolts although the change of sensor signal is relatively small.
  (2) Fracture of bolts due to fatigue in the flange type bolted joint specimens can be easily detected using built-in sensor bolts because fracture of one bolt results in the change of axial forces of other unbroken bolts.

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