JNIOSH

Abstract of Special Research Report (RR-2002)

National Institute of Occupational Safety and Health, Japan

Time-Frequency Analysis of Ultrasonic Echoes and its Application to Nondestructive Evaluation of Thermal Damage of Steel

RR-2002-1
Shiwei MA, Tetsuya SASAKI, Etsuji YOSHIHISA and Takashi HONDA

: Recent trend to extend the original design life of pressure vessels and piping may increase the possibility of failure accident of these components caused by material degradation due to creep or creep-fatigue. Since sudden rupture of these components usually results in a serious industrial accident, such material damages must be detected using nondestructive evaluation technique to ensure the safety of workers at the industrial sites. Although there are a lot of different methods in nondestructive evaluation technique, ultrasonic pulse-echo technique is supposed to be one of the best methods to detect creep or creep-fatigue damage. In this method, reflected ultrasonic echoes by material internal defects and specimen interface often exhibit critical time and frequency information. However, it is difficult to determine creep or fatigue-creep damage using conventional signal processing technique. To conquer such a difficulty, the time-frequency analysis method is applied in this study. Because the time-frequency analysis method can provide an effective tool to represent a signal in a two-dimensional time and frequency plane, it is possible to obtain frequency dependent ultrasonic characteristics related to certain material properties in a time-frequency plane rather than only in a time domain or frequency, and these characteristics can be applied to identify internal defects or to assess creep damage of crucial structure material.
    In this study, ultrasonic tests with high frequency contact transducers were carried out using thermally degraded steel in order to confirm the effectiveness of the time-frequency analysis method. Measured echo signals were analyzed by the Morlet function based wavelet transform, one of the methods for the time-frequency analysis. It is shown that with the increase of thermal degradation, both the ultrasonic attenuation and its change with frequency gradually increased within a significant frequency band but ultrasonic velocity was relatively unchanged. The effectiveness of the wavelet analysis to ultrasonic signals for the quantitative evaluation of material thermal damage is clarified.

Requirements of Prevention Equipment against Falling from Roof

RR-2002-2
Yasumichi HINO

: Falling Accidents at the construction site of low rise houses have been recognized as the major cause of fatal accidents since the occupational safety and health law was instituted in 1972. There is a decreasing trend of the accidents in the last two or three years. It is thought that the reasons are due to "popular use of precedent scaffold installation method" and "decrease in the number of construction works".
    However, the number of accidents is still large. Especially, about 20 to 30 of falling accidents from roof periodically happened over the last decade and the decreasing trend of the accidents has not been observed. And in recent years, the construction work has changed from the construction of new house to extension or repair work. In some cases of extension or repair work, construction period and the budget for installation of the countermeasure against falling may not enough.
    The purpose of this study is to get the fundamental data for the countermeasure against falling from roof by using many accident data. And also, the requirements of prevention equipment against falling from roof were discussed by using the obtained data.
    The major findings obtained in this study can be summarized as follows:
  1) Archetypal falling patterns of the victims on the roof surface from work position to eaves or verge are clarified by using the analytical results on the basis cause of falling and the motion. From the results, the falling patterns can be categorized into 3 types and the necessity of prevention equipment, which can prevent all these 3 types, is realized.
  2) Falling velocity of the victim passing from the eaves to ground surface is estimated by using the data of falling point based on the many accident data. From the results, it is cleared that the maximum falling velocity is about 15km/h at the eaves, and 50 km/h at the ground surface. This maximum velocity at the ground surface is very fast as it is approximately equal to the setting velocity of the automobile in the experiment for car crash safety.
  3) Based on the estimated results of the falling velocity, the impact force of collision between the prevention equipment and the victim is discussed. From the results, it is cleared that the impact force is very large, and the victim may be injured. Especially, for the accident type of victim slides down the roof surface, the maximum impulse force is larger than the threshold value of head tolerance which was proposed by Dr. GADD. Therefore, it is necessary to apply the prevention equipment which can enlarge the collision area and can extend the collision time.

Fundamental Study on Wind Pressure Acting on Temporary Scaffolds

RR-2002-3
Yasumichi HINO, Katsutoshi OHDO, Songpol PHONGKUMSING, Eizo MARUTA and Makoto KANDA

: The number of construction accidents has gradually decreased since the Occupational Safety and Health Law was instituted in 1972, and the number of fatal accidents has decreased 30 percent since 1972. However, approximately seven hundred fatal accidents still happen every year. And also, 10 percent of collapse accidents in serious accidents in which 3 or more workers were killed or injured are caused by wind. Especially, not only construction workers but also pedestrians were killed or injured by the collapses of temporary scaffolds due to wind. Therefore, the prevention methods for collapses of temporary scaffolds due to wind should be established.
    It is known that not only the wind speed but also the plan shape of structure influences the wind load acting on the structure. At the construction site, the plan shapes of temporary scaffolds and of buildings always vary according to the progress of construction. Therefore, the wind load acting on the temporary scaffolds also varies.
    However, the general design method of the scaffolds does not consider the variation of their shapes due to the progress. Especially, the building under construction usually has many open area on the exterior wall, but there are a few studies about the influence of the open area on wind pressure acting on the temporary scaffolds erected alongside of the building under construction. In fact, some collapse accidents happened due to the strong wind blows into the open area.
    The purpose of this study is to investigate the fundamental characteristics of wind pressure acting on temporary scaffolds erected alongside of the building under construction which has open area on the exterior walls. The wind tunnel experiments using the scaffolds and building models were conducted investigating the influence of the open area and the wind direction.
    From the results of this study, wind pressure acting on the scaffolds which were erected alongside of the building was greatly influenced by existence of the building. Moreover, the open area largely affects the wind pressure acting on the rear side of the scaffolds, while only small influence can be observed on the front side of the scaffolds.

Stability of Counter forted-type Sheet Pile Retaining Wall --For Sandy Ground--

RR-2002-4
Yasuo TOYOSAWA, Noriyuki HORII, Satoshi TAMATE, Makoto ETOH, Mitsuo SATO, Mitsuru EGUCHI and Norio FUJITA

: About 100 workers die in accidents during excavation at construction sites every year in Japan. There are many construction processes involved in the assembly of retaining walls such as installing wales, struts, braces etc, as well as dismantling such materials, and the complexity of these processes is one of the main causes of such accidents.
    The cantilever sheet pile retaining method is the simplest method of earth support, as there is no need to install and dismantle earth supports inside the excavation site. Although this method is an effective way to avoid accidents, it can only be used for shallow excavation. Especially in soft ground, the penetration depth tends to be deeper and the costs higher when designed by some standard designs.
    In order to overcome these problems, the counter forted-type sheet pile method (SCB method) was developed. The SCB method uses counterfort retaining walls with some intervals along the conventional cantilever retaining wall. These counterfort-type walls stabilize the retaining wall and allow deeper excavation. However, there have been few researches on the mechanisms of deformation and collapse of retaining walls made using this method, even though it is likely to be increasingly used in the near future.
    The safety and risks of this method should be understood and a reasonable design method developed. In this study, centrifuge model tests were carried out for clarifying the mechanisms of deformation and collapse, and the following conclusions were drawn.
  1) The counter forted-type retaining wall effectively reduces the inclination of the wall and bending moment generated due to excavation.
  2) The installation interval between counterfort walls greatly affects the stability of the retaining wall. The inclination of the retaining wall was reduced to 60% compared to that of a conventional cantilever retaining wall when the interval between counter fort walls was less than 70 % of the retaining wall length.
  3) The results of centrifuge tests, in which failure occurred due to excess excavation, showed that compared with the conventional cantilever retaining wall, the counter fort retaining wall has greater stability and resistance to toppling failure of the retaining wall. It is suggested that the penetration depth can be decreased by using the counterforted-type retaining wall.

Survey on Commuting Stress and Accidental Falls of Elderly Workers on the Way to and from Work --Building-Maintenance Industry--

RR-2002-5
Hisao NAGATA and Sunyoung LEE

: In the building-maintenance industry where many elderly workers are employed, the ratio of accidents on the way to and from work is higher than other sectors, and accidents due to falling on the way to and from work often occur. Accordingly, we conducted a survey on workers in the building-maintenance industry who are 60 years old and over. In the building-maintenance industry, the labour force in particularly is ageing, and falling accidents on the way to and from work makes up a sizable proportion of the overall figure.
    We chose 645 building-maintenance companies within the Tokyo area, and asked them to fill out the questionnaire by at most 5 persons per company. Questionnaire items were as follows: attributes of respondents; commuting means (trains, buses, bicycles, walking etc.) ; start or finish times; places where the risk incurred and falls experienced; details of falling accidents on the way to and from work etc.
    We obtained 702 responses from 224 companies out of 645 total (response rate: 34.7%). Of the 702 respondents, the number of men was 514 (average age: 64.6) and that of women was 188 (average age: 64.4). This means that men constituted 73.2% of all respondents, while women constitut-ed 26.8%. For the ages of respondents, "60-64 years old" constituted 58.1%, "65-69 years old" 33.9% and "70 years old or over" 8.0%.
    In the question, the "place where you have anticipated the risk of falls when someone rushed or walked past you during congested times," 154 respondents of the 676 (22.8%) answered "No" while 522 respondents answered "Yes" (multiple selection was allowed). "Stairways" constituted 65.6% of all responses, followed by "narrow aisles" at 16.3%, "ramp ways" at 7.6% and "escalators" at 7.5%.
In the case of accidents, "risky places where you have fallen on the way to and from work," 346 respondents answered "No." Of all 247 respondents who answered "Yes" (41.7% of the effective respondents), "stairways on station premises" constituted 35.9% of all responses, followed by "sidewalks" at 20.4% and "roads (with no distinction between sidewalks and streets)" at 8.4%.
    For "whether you have fallen and have been injured on the way to and from work," 85 respondents (12.1%) answered "Yes," and 616 respondents (87.9%) answered "No." For "which you were on the way to or from work when falling accidents occurred," 60% of the 80 respondents answered "on the way to work," For commuting routes when an accident occurred, "on the station premises or at stops" constituted 38.4% of 78 respondents, "from home to station (bus stop)" 29.5%, "from the station (bus stop) to the place of work" 21.8%, "in a vehicle" 3.8% and "other" 6.4%. For ratio of accidents by place, "stairways on station premises" constituted 31.6% of all responses from the 79 effective respondents, followed by "sidewalks" at 21.5% and "roads (with no clear distinction between sidewalks and streets)" at 8.9%. For degrees of congestion, "not congested" constituted 55.1% of the 79 effective respondents and "congested" 44.9%. The total ratio of "quite congested" and "extremely congested" constituted 23.0%. For walking pace, "slightly hurried" constituted 32.9% of 79 effective respondents, "somewhat hurried" 6.3% and "extremely hurried" 3.8%. The total ratios were the above and the ratio of "in a hurry" resulted in 43.0%. The calculated values for the "cause of a fall" were: "stumbled" constituted 40.8% of the 76 effective respondents; "slipped" 22.4%; "missed footing" 18.4%; "pushed and nudged" 15.8% and "unknown" 2.6%.

The Dependence of Initial Pressure for the Autoignition Temperature of Flammable Liquid

RR-2002-6
Haruhiko ITAGAKI

: Autoignition temperatures (AIT) of flammable liquids have been measured by many researchers. Especially, many data by the ASTM method have been reported. However, a fire sometimes happened at the chemical plant due to the autoignition of some flammable liquids though those were dealt with under the lower temperature than the data measured by the ASTM method.
Because, the AIT, by its very nature, is dependent on the chemical and physical properties of the material, the method and apparatus employed for its determination. Actually, the data reported by one literature is often different from the data reported by another literature.
    Therefore, the author paid attention to the size of the test vessel for the measurement of AIT at first. The dependence of AIT on the size of vessel was measured for pentane and decane. If the one liter round bottom flask was used, AITs of pentane and decane were 16 °C and 18 °C, respectively lower than the measurement data by the ASTM method using the 200 ml erlenmeyer flask. The heat conductive model was introduced to estimate the AIT in the larger vessel. As the result, it was reported before that the AIT which depended on the size of the test vessel could be estimated by using the heat conductive parameter which mainly expressed the ratio of the volume and the surface area of the test vessel.
    Next, to study the dependence on initial pressure in the test vessel, AIT for decane was measured by using a 500 ml round bottom flask in a closed container. The AIT decreased with the rise in the initial pressure. AIT in 0.40 MPa was 192 °C that was 10 °C lower than AIT in 0.10 MPa, and the ignition delay time was over ten minutes. The thermal explosion theory was applied to the results. And, the simplified relationship between AIT and the initial pressure was obtained. It was shown that the ratio of AIT difference increased in proportion to the logarithm of the ratio of initial pressure. The relationship between the ratio of initial pressure and the ratio of measured AIT difference for decane fitted straight line well. AIT under another pressure in the same vessel could be estimated when AITs were obtained at some initial pressures.

Study of Terms Connection in the Database for Explosion and Fire Accidents

RR-2002-7
Teruhito OTSUKA and Haruhiko ITAGAKI

: According to the development of Internet, various search methods have been invented recently. Because of large number of web pages, page ranking system has been succeeded on Internet. But such method is not applicable for the database for explosion and fire accidents, because of lack of linkage system and paucity of same accident.
    For the database, there is another requirement, the outputs should be not only the direct reference by the words and phrases which have completely in agreement but also the guessed data which was able to set in order with a certain aspect such as similarity. From the guessed data, the point to notice of the process can be found previously.
    In this study, a simple but quite powerful technique, quantification method of third type, is used for deriving quantitative information from qualitative data, which made from abstracts of accidents. The several way for getting keywords from abstracts in Japanese are compared. As a result, the quantitative distance relation among terms was obtained reasonably.
    The merits of introduction of the quantitative distance relation among terms are summarized as below;
  1) Since it is quantitative information, placing in the order is possible to outputs.
  2) The correlation of the noun obtained can be used not only in the database for explosion and fire accidents used for distance space creation, but in another database with a perspective of explosion and fire accidents.
  3) It is possible to adjust by selection of the axis, the scale of a specific axis, and the applicable number of cases etc. into a parameter for the reduction of the information noise.
  4) According to the simplified nouns, it is possible to take partial match to a reference word. Therefore, reference by the compound noun which does not appear in the database can also return some information.

Characteristics of a New Apparatus for Measuring Ignition Energy of Dust Clouds having a Dispersion Mechanism by Ultrasonic Vibration

RR-2002-8
Mizuki YAMAGUMA, Kwang-SEOK CHOI and Tsutomu KODAMA

: This paper describes a novel apparatus for measuring the minimum electric spark ignition energies of combustible dust clouds. Instead of a transient suspended dust cloud by a blast of compressed air as in the conventional Hartmann tube apparatus, this one forms a curtain-like, constant downstream of dust in an explosion chamber by combining an ultrasonic vibrator, a sieve, and a specially designed dust hopper. While passing through the gap of two electrodes placed in the middle of an explosion chamber, sparks are given to the dust stream to determine its minimum ignition energy (MIE). The concentration of a dust cloud is widely controllable through the current to the vibrator. The apparatus is equipped with two pieces of coarse metal mesh functioning as flame arresters and are placed just below the sieve and over the base of the explosion chamber to protect the chamber and to enable the recycling of unburned dust. By comparing the MIE values of several samples of dust with those obtained by the conventional method, the new method was confirmed to give reliable data.

Electromagnetic Disturbance of Large-Scale Crane due to Medium Broadcasting and Countermeasures

RR-2002-9
Hajime TOMITA

: IWhen a large-scale crane, such as a tower crane or a rough-terrain crane, is used under the influence of a high electromagnetic field caused by a broadcast wave at medium frequency, the voltage induced in the hook can reach 1kV and causes the electric shock to workers. The current induced in the crane can cause the overload-preventive device to malfunction. However, the intensity of induced current in a large-scale crane is not well understood.
    On the other hand, mobile cranes are commonly used on shop floors and work sites where the crane is move frequently. In these cases, quick and easy methods for mitigating the high voltage induced in the hook are necessary. One countermeasure suppressing the voltage in the hook has already been put to practical use.
    In this report, for a countermeasure against the malfunction of the overload-preventive device, the intensity of the induced current in a large-scale crane is estimated numerically using a crane model. A production version rough terrain crane of 50t at maximum rated load is taken as an example of a large-scale crane. The induced current in the boom of the rough terrain crane is measured by means of a grounding wire. The current intensity in the boom of the rough terrain crane is also calculated by means of the magnetic field strength measured around the boom. The results of the calculation and measurement show that the intensity of the current in a boom of the crane is estimated in the range of several amperes under the electric field of 0.79V/m.
    To mitigate the high voltage induced in the hook of a large-scale crane, two methods which can be implemented in a short period are proposed. One method is to decrease the capacitance of the crane by inserting acrylic plates under outrigger floats. The voltage in the hook of the rough-terrain crane described above is attenuated to less than one-half of that before the countermeasure is implemented. The other method is to add a plain knit grounding wire in the hook. In this case, the voltage in the hook of the rough-terrain crane is attenuated to about one-half of that before the countermeasure is implemented.

Questionnaire Survey regarding Manual AC Arc Welding Operation

RR-2002-10
Tatsuo MOTOYAMA, Hajime TOMITA and Eiki YAMANO

: Arc welding is a high-risk operation because it is very difficult to remove exposed live parts, such as the welding rod and the welding rod holder, from a manual AC arc-welding machine. The Voltage-Reducing Device for an AC arc-welding machine (VRD) is designed to prevent electric shock caused by the arc-welding operation.
    Although VRD is very useful for preventing electric shock, the time has come to improve the device by internationalizing the specifications for the standards. A questionnaire survey on the arc-welding operation was carried out among arc-welding workers. Statistical analysis was performed on 453 responses. The results are summarized as follows:
  1) About 58% of the arc-welding workers had suffered electric shock during arc welding.
  2) The live parts touched most often in electric shock incidents were the welding rod (35%), the welding rod holder (30%), and the welding work piece (26%).
  3) Over 50% of the arc-welding workers had suffered electric shock during welding-rod-exchange work. One reason is that the welding rod is exchanged before the VRD has reduced the output voltage of the arc-welding machine to under 30V.
  4) About 95% of the electric shock incidents occurred when clothing, such as coveralls, shoes, or gloves got wet.
  5) When a welding worker wearing wet clothes touches live parts of an arc-welding machine that has a VRD, the device may not operate safely, possibly causing the workers to suffer electric shock. Therefore, it is necessary to consider the starting sensitivity of VRD.
  6) About 30% of the workers thought that the use of VRD was troublesome. Therefore, manufacturers should improve the device.

Study on Usage of Safety Belt and Scope of Airbag for Fall Protection

RR-2002-11
Kiyoshi FUKAYA

: Two fall protection measures were studied. One is a fall arrest system by a safety belt, and another is an airbag system for fall protection which absorbs shock to a worker.
    The study on the fall arrest system concerned the recommendations for use of safety belts. They are necessary because the recommendations for construction of safety belts require specific usages and the other usages may result in accidents. There are two important problems which must be considered in the recommendations for use, because the wrong uses resulted in accidents.
  1) Ropes deteriorate more rapidly than the other parts and need daily checks. Some general construction companies checked safety belts in service and found that 2-4% of the safety belts deteriorated. Strength tests of some of these deteriorated ropes were carried out. The breaking strength of one third of them were under 7kN, which is the expectation value of impact load during fall arrest. In other words one third of deteriorated rope would be cut in case of fall accidents. There was no clear relation between the duration of service for safety belts and the strength. There was relation between the degree of deterioration and the strength.
  2) Hooks are premised on the use in extension load only and the use in the other loads such as bending load or the load to locking keeper might result in an accident. The cross arm use of lanyard involves the possibility of improper usages. And the cross arm use without pad may cause contact to the sharp edge of steel beam which reduces the strength of lanyard.
    The airbag is a shock absorption device in case of fall accidents and it is proved to have a shock absorbing function. In order to clarify its injury reduction ability, relation between degree of impact and degree of injury was investigated in literature search. The relation is probabilistic and the effect of airbag is probabilistic.
    Fatal fall accidents data were analyzed. 60% of death resulted from head injuries. Therefore, the protection of heads is most important for shock absorption devices. There were fatal accidents in the accidents of fall height under 2m. Therefore fall protections are necessary even for work on height under 2m, and the airbag is more effective than safety belt in this range.
    Fall tests of a dummy with/without airbag were carried out. The relation between the fall height and degree of shock was found. In a test without airbag first contact of shoulder to the ground reduced the shock compared to the first contact of head. Fall tests of standing posture were carried out and it was confirmed that the shock to the head was absorbed by the leg. These results support concentration of the protection by the airbag to the back and head.
    In the results of tests in inclined posture airbag had shock absorption function in the range where the contact to the ground of airbag is earlier than that of head.
    These two measures were positioned in fall protection measurements. These two measures belong to measures of reducing injury, and effect of safety belt is deterministic and effect of airbag is probabilistic. Therefore safety belt is more important than airbag.

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