JNIOSH

Abstract of Special Research Report (RR-97)

National Institute of Occupational Safety and Health, Japan

Application of Thermoelastic Effect to Determination of Stress Concentration Factors and Stress Intensity Factors

RR-97-1
Yoshio KITSUNAI, Takashi HONDA and Tetsuya SASAKI

: The local stresses and strains near notches and cracks are very important for fracture analysis of cyclically loaded structural components. A number of experimental techniques such as photo elasticity and numerical methods such as finite element analysis have been developed to evaluate stresses or strains near notches. A thermoelastic method is a relatively new stress analysis technique based on the measurement of infrared radiation emitted from the surface of a body as a result of change in temperature arising from a change of stress. In this study, to confirm the validity of the thermoelastic method, stress concentration for several notches in sheet specimens and stress intensity factors of CT and CCT specimens were analyzed by the thermoelastic stress analysis technique. The results were compared with theoretical and analytical results.
    The materials used in this study are SUS 316NG steel with 12 mm thick, HT60 and JIS SM490A steels with 16 mm thick. The sheet specimens with several kinds of basic notches were made of these materials. Moreover, cross-shaped sheet specimens made of SM490A steel were also used to examine the stress concentrations near weld toe in a gusset plate. The shapes of fillet of the crossshaped specimens were machined on referring to the weld toe shape in the gusset welded plate. CT specimens with 100 mm wide, 14 mm thick and CCT specimens with 100 mm wide, 6 mm thick and 300 mm long were machined of SM490A steel plate to measure stress intensity factors.
    Prior to stress measurement, all specimens were cleaned with acetone and then coated with matt black spray paint to maximize their emissivity. To achieve adiabatic thermal condition, each specimen was cyclically loaded at a frequency of 5 Hz, using a servo-hydraulic fatigue machine of 196 kN capacity. The stress measurement by thermoelastic effect was carried out using an infrared stress analysis system developed by JEOL. Finite element study was carried out under plane stress conditions with two-dimensional mesh.
    The main results obtained in this study are as follows:
  (1) The stress concentration factors measured by the infrared technique were slightly lower than finite element or theoretical result, and the error tended to increase with increasing stress concentration. However, the stress gradient agreed with both the results.
  (2) The stress intensity factors for mode I, ΔKI derived from the infrared technique agreed with other comparable independent values in the range of error within 9%.

Evaluation of Human Pain Tolerance for Human-Oriented Robot Design

RR-97-2
Hiroyasu IKEDA and Noboru SUGIMOTO

: Domestic and international standards or regulations for industrial robots strictly recommend that these robots should be isolated from operators and other workers. However, there will be many fields in the future where humans will have to allow robots to work besides them. One typical application will be a robot usage in labor assistance and rehabilitation for the aged people or with disabilities. Other applications may include a non-autonomous robotic system designed for cooperative tasks with a human operator.
    This paper describes human pain tolerance for the design standard of a human-robot coexistence system. Among human sensations, somatic pain is like an emergency call. It is regarded as a stable sensation having neither individual variations nor adaptations. Therefore, somatic pain which can tolerate repeated mechanical stimuli is adopted.
    This study carries out following experiments to measure the threshold of human pain tolerance using a biomechanical method and statistical analysis.
  1) 16 subjects for the experiment are arranged, and 12 measurement points on their bodies which are considered to have a high possibility of contacts in human-robot coexistence environment are selected.
  2) A pneumatic cylinder with a contact disc prove of 10 mm diameter is used as an actuator to give mechanical stimuli to the subjects.
  3) The mechanical stimuli applied to the subjects are divided into two categories: static and dynamic forces.
    The values of the human pain tolerance are given as tolerable limit of these mechanical stimuli. The mean value of the pain tolerance in terms of a static force ranges from 65N to 146N, while the minimum value ranges from 13N to 46N, depending on the measurement point. As a result of applying dynamic force, the dynamic human pain tolerance relates to a mean impact force and an impact time. Human pain tolerance is characterized uniformly in terms of the pain intensity curve which is fitted to the minimum values of both the static and dynamic pain tolerance. Moreover, this paper investigates the relationship between the deformation and velocity of some human sections, when the human subjects feel somatic pain.
    Human pain tolerance needs to be analyzed further, but it will be a trigger to build up an advanced safety design standard.

Statistical Analysis on Human Factors concerning Falling-accident in Construction Works

RR-97-3
Yoshimi SUZUKI, Shin-nosuke USUI, Yoshiyuki EGAWA and Takuro SHOJI

: Although industrial injuries in Japan have decreased as a whole, the number of cases in construction industry accounts for about 40% of the total casualties(deaths) and trend in the cases by falling-accident is increasing recently.
    In order to establish the ergonomic countermeasures against the falling-accident in construction works, statistical analysis on human factors concerning construction work sites are carried out.
    Procedures in this study arc summarized as follows;
  1) By means of the utility program in the "Occupational Accident Report Database System" developed in The National Institute of Industrial Safety (NIIS), pattern analysis on human behavior in 154 falling-accident cases have been made.
  2) In order to investigate the potential factors concerning human factors in falling-accidents, these 154 accident cases are checked from 198 view-points which are listed as latent factor of accidents considering actual construction work sites.
  3) Then, to estimate relationship between 198 view-points and 154 accident cases, statistical quantification analysis are applied on data which are obtained by above step 2).
    Main results of these investigations are as follows;
  1) From the pattern analysis on human behavior in falling-accidents, actual falling-accidents in construction work sites are classified into 5 types as follows;
    Type I : Falling at walking on scaffold, Type II: Falling in reaction of fallen worker's task on scaffold, Type III: Falling by shaking/swaying of scaffold/equipment, Type IV: Falling caused by mal-handling of scaffold/equipment by cooperated worker, Type V: Falling caused by malhandling of scaffold/equipment by fallen worker-himself.
    Almost 90% of 154 falling-accident cases are occurred by causes of Type I and II. From the point of difference of the kind of works, Type I accident are frequent in Wooden building works, and Type II accident are much more in Steel and reinforced concrete building works.
  2) As the results of investigation on accident cases from 198 view-points of latent factor of accident, some latent factors concerning human factors are extracted as follows: Psycho-physiological factors of workers, Behavioral factors of workers, Safety-equipment factors and Management factors in construction processing.
  3) As the results of statistical quantification analysis, these factors mentioned above are clarified in scatter diagrams of category weights of 198 view-points, and these view-points can be regrouped as background factors of falling-accident as follows: Psychological & behavioral factor, Management factor on working-process, Education or experienced factor and so on.

Stability of Composite Ground Improved by Deep Mixing Method during Construction of Embankments

RR-97-4
Noriyuki HORII, Yasuo TOYOSAWA, Satoshi TAMATE, Hideo HASHIZUME and Yasuhiko OKOCHI

: Deep Mixing Method(DMM) is often utilized to control the large deformation or failure during construction of embankments and other soil structures. However, the evaluation method for the strength of improved composite ground has not been established in a practical aspect. In this study, centrifuge model tests were carried out to investigate the effect of improvement by the deep mixing method. Four kind of model tests were carried out. These were (1) non improved ground, (2) deep improved ground under the toe of the embankment, (3) deep improved ground under the shoulder of the embankment, and (4) shallow improved ground under the shoulder of the embankment. In addition to centrifuge model tests, numerical analyses by means of Finite Element Method and Circular Sliding Method were performed to appreciate the experimental results.
Following conclusions were obtained from the test results.
  1) Both ground and soil improvement column were moved together in spite of using a relatively low replacement ratio of 10%.
  2) In the case that the ground was improved under the shoulder of the embankment, the effect of soil improvement was the biggest of other cases.
  3) Also, the case of using shallow improvement, the effect of the improvement was observed.
  4) Circular sliding method could be adopted for stability analysis in case of non improved ground and shallow improved ground.
  5) FEM could simulate the behavior of the tendencies of the deformation of the test results.

Deformation and Failure Behavior of Anchored Retaining Wall Induced by Excessive Exacavation in Centrifuge Model Tests

RR-97-5
Yasuo TOYOSAWA, Noriyuki HORII and Satoshi TAMATE

: Accidents due to collapse of anchored retaining wall occasionally occur in excavation site. To study the deformation and failure behavior of anchored retaining wall, centrifuge model tests were carried out by using an in-flight excavator. Three different models which consisted of different layer such as (1) Kanto loam, (2) sand and (3) a stratum of Kanto loam layer and sand layer with anchors were tested to observe the anchored sheet pile wall's collapse. Based on the results of these centrifuge model tests, the development of the earth pressures and the strains on a sheet pile wall during excavation process and failure mechanisms are discussed.
  1) In-flight excavator which we newly developed works accurately under centrifugal field of up to 60G.
  2) The earth pressures with the depth on the wall and the tensile force acting on an anchor's head during excavation showed deference depending on the models. It is estimated these values resulted from the inter-relationship between stress and strain when the soil behind the wall activates.
  3) Failure occurred suddenly along a failure line with settlement of wall. Settlement of wall induced by the downward load of wall in the case when an anchor is installed at an acute angle.
  4) There is a close relationship among the tensile force acting on an anchor, the sheet pile wall's movement and the ground displacements. For example, downward movement of a sheet pile wall without horizontal displacements would cause the anchor to reduce its tensile force.
  5) Earth pressure on the wall and the tensile force on an anchor would be influenced by the direction of wall inclination when the failure occurred. The failure by overturning about toe would cause further increasing both earth pressure and tensile force acting on an anchor.
  6) During excavation process, the tensile force and the earth pressure of an active side remained almost constant. Just before the failure, it was observed the earth pressure of active side at the lower sheet pile wall increased 1.5 times bigger than the former value.
  7) It is difficult to find out the causes of accident, because of the disturbance of site during the rescue and the collapse itself. Centrifuge model tests gave useful information to approach the causes of accidents. To have better understanding of the accidents, a further improvement of both quality and quantity in the centrifuge model test are expected.

Dust Explosibility of Magnesium and Its Alloys

RR-97-6
Toei MATSUDA

: A large number of violent dust explosions have been caused by light metals, such as aluminum and magnesium, which show the most severe dust explosion hazard because of their high heats of combustion, particularly in finely divided states. This fact would lead to the requirement that all aspects of explosion hazards of these dusts should be disclosed and understood for their safer industrial use and manufacture. The ignition and combustion of light metals have been fairly well studied by many researchers and useful information on metal dust explosibilities has been provided by the U.S. Bureau of Mines and German organizations. However, their dust explosibility data in recent testing apparatuses have been limited, and the available data on the explosibility characteristics of metal alloys are contradictory in part.
    Dust explosibility characteristics of Mg powders (volume median diameters of 37 and 61μm) were measured in the Siwek 20 L spherical explosion chamber with a 10 kJ ignitor to compare their reactivities in air, nitrogen and carbon dioxide. The reactivity or the volume-normalized rate of pressure rise (Kst) for Mg in CO2 is nearly half of that in air, and the main explosion product of Mg dusts is MgO in CO2, although a small quantity of carbon is formed in the higher dust concentrations. The Mg dust used did not ignite in nitrogen at room temperature. The Al-Mg alloy dusts (median particle sizes: 61-76μm) with a Mg content of more than about 40 wt.% showed an explosion hazard in CO2, in which only the Mg component reacted.
    The dust explosibility data for Al-Mg and Mg-Si alloys were also obtained in the 20-l chamber to study the relation between the explosibility and those of the components of the binary alloys. It seems that an increase in the Mg content of the alloy causes an increased explosion hazard of the dusts over those of the other component of the alloy, likely due to higher reactiveties of Mg, when the particle size is enough small( median diameters less than about 26μm). In particular, the Kst values for the alloy dusts of Al-Mg and Mg-Si systems with nearly the same particle size distributions show the overall trends of increasing with increasing Mg content over that for pure Al or Si and show a strong dependency on particle sizes as metal dusts generally do.
    Measurements of ignition temperatures of dust-air mixtures were made for Mg and its alloys, and the results may support the contribution of heterogeneous reaction to their ignitions.
    These data may be useful for evaluating the explosion hazards of newly emerging complex dusty materials such as ceramics and intcrmctallics.

Study on Chemical Reaction Hazards of Organic Solvents (2nd Report) --Reaction Mechanism of Epichlorohydrin-Dimethylsulfoxide Mixture--

RR-97-7
Takayuki ANDO

: Although the potential hazards of the solvents are usually discussed on fire or explosion hazard, some solvents show thermal hazards such as exothermic decomposition and/or exothermic reaction with the solute, which are found, for example, in distillation processes.
    In our previous report, we measured the exothermic properties of the mixture of epichlorohydrin(ECH) with dimethyl sulfoxide (DMSO), and analyzed the reaction products so as to investigate the reaction mechanism of the mixture, and have concluded that the two distinct exothermic peaks in a DSC curve for the mixture corresponds to the intermediate formation(the first exotherm) and decomposition(the second exotherm) of mono- and di-chlorohydrins.
    In this paper, the exothermic properties of the mixtures of mono- and di-chlorohydrins with DMSO were measured by use of DSC, and the effect of addition of water or hydrochloric acid to the mixture of ECH with DMSO was investigated to certify the validity of the reaction mechanism proposed in the previous report.
    The results are summarized as follows:
  (1) The second exothermic peak in a DSC curve for a mixture of ECH with DMSO is in the similar temperature range for the exothermic peaks in DSC curves for mixtures of chlorohydrins with DMSO. It is consistent with our conclusion in the previous report that the second exotherm is due to decomposition of the chlorohydrins formed in the first exotherm.
  (2) The first exothermic peak in a DSC curve for a mixture of ECH with DMSO moves toward lower temperature by addition of water or hydrochloric acid. It shows that the first exotherm is not due to some reaction of ECH with DMSO, but is likely to correspond to the decomposition of ECH accelerated with the polar effect of DMSO.

Estimation of Activation Energy for Decomposition of Unstable Substances

RR-97-8
Teruhito OHTSUKA

: When a chemical storaged, it's very important to know how fast they decompose and how much heat flow comes from them. But in some cases, the temperature to start decomposition and its melting point are very close, the decomposition temperature measurement will be impossible. In this paper, the method to measure some properties about chemicals like above by measuring with pressure, is proposed. Lauroyl peroxide (LPO) was chosen as an example.
    In LPO case, only the decomposition from solid state was occurred at the temperature lower than 50 °C. So two experiments are performed. One is upper temperature for solid and liquid mixture decomposition. The other one is lower temperature for solid state decomposition.
    It is easy to know how much quantity decomposed from pressure. The heat of decomposition was obtained as the ratio of heat value and decomposed rate.
    The results are summarized as follows;
  1) The heat of decomposition is 708 J/g in liquid state and 551 J/g in solid state. So the difference between 551 J/g and 708 J/g, 157 J/g, is the heat of melting. And also melting point was obtained as 53.72 °C from the extrapolation of decomposed rate and Temperature.
  2) An appropriate model was made and analyzed for the decomposition speed with that model from a point of view of heat. According to that model, activation energy of solid state LPO is 281 kJ/mol and 101 kJ/mol in liquid state.
  3) Computer simulation was also performed. From that model, regarding the elapse time until finishing the melting as the holding time to ignite, the simulation shows proper time dependency on temperature very close to the experimental result on the literatures. According to that simulation, the temperature equivalent to SADT of LPO is 46.19 °C.

Insulation Degradation of Printed Circuit Board Caused by Electrochemical Migration

RR-97-9
Tatsuo MOTOYAMA and Kenji ICHIKAWA

: Recently, high density wiring boards have been used in electronic devices in order to achieve its small sizing, therefore the intensity of electric field between lines has become high. On the other hands, electronic devices such as a control apparatus, measuring systems and so on have been used in atmosphere of not only indoor but also outdoor especially in chemical plants where corrosive gases exist. Humidity and corrosive gases accelerate insulation degradation caused by electrochemical migration and failure of electronic devices occurs.
    Electrochemical migration is a kind of electrochemical phenomena effected by electric field strength and water with electrolyte. Metal of line is dissolved in water and ionizes. And they are moved by electric field strength and deposits on the board between lines. After the migration occurred, there are some cases that the resistance of deposits is recovered according to decreasing of humidity.
    In this paper, the dependence on testing condition to progression of its migration and the reason why the resistance of deposits is recovered are studied to prevent the accidents caused by electrochemical migration. Three experiments in different condition are conducted, mainly. Experiment 1 is constant temperature and humidity test under condition of 40 °C, 95% RH, 100V. Experiment 2 is cyclical test under condition of (20°C, 60% RH(20 min.) -- 40°C, 90% RH(20 min,))/10V. Experiment 3 is the same test as Experiment 2 except for atmosphere containing 35.5 ppm of SO2 . Main experimental results are summarized as follows :
  (1) The exposed time that resistance between lines was decreased until 107 Ω was much different among Experiment 1, Experiment 2 and Experiment 3. In case the distance between lines was 0.4 mm on phenolic resin with paper (Ph), it took 800 hours on Experiment 1,200 min. (equivalent to 5 cycles) on Experiment 2, and 40 min. (equivalent to 1 cycle) on Experiment 3. In case of the same distance on epoxy resin with glass fiber (GE), it took over 800 hours on Experiment 1, 88 hours (equivalent to 132 cycles) on Experiment 2, and 40 min. (equivalent to 1 cycle) on Experiment 3.
  (2) Tip of dendrite or dendrite like deposits was composed of many hemisphere-particles with the diameter of some micro-m.
  (3) Resistance between lines with deposit generated by electrochemical migration increased at nor mal experimental room condition. It mainly depended on formation of micro space caused by water evaporation in deposits or increase of resistance caused by oxidation or chemical combination of deposits.

Self-Consistent Simulation of Charged Powders
and its Application to Evaluating Electrostatic Hazards

RR-97-10
Atsushi OHSAWA

: During the handling and processing of powders, it is well known that charge accumulate on the powder particles. When these charged powders enter and accumulate in a vessel, a large amount of charge can be stored and can lead to electrostatic hazards. To prevent such hazards, it is important to understand the mechanism of the formation of the hazards. In this paper, we present a self-consistent simulation of charged powder entering a vessel for modeling electrostatic phenomena and evaluating electrostatic hazards.
   The particle-in-cell technique has been used to simulate charged powders entering a vessel. The motion of charged particles and the electric field inside the vessel have been solved self-consistently. The particle size distribution of the powder has also been taken into account. The probability of incendiary discharges is considered based on calculated local electric fields and electrostatic energies.
   It was found that the motion of particles strongly depends on particle size. Lighter particles in the powder were trapped in the upper region by a self-generated and heap electric fields. Since there was no large charged cloud of dispersed powder in air space, no lightning-like discharge is likely to occur under the simulated conditions. The position of the strongest electric field was initially in vicinity of the inlet pipe until the heap is formed or on the heap from the heap is completely formed. The possibility of incendiary discharges on the top of the heap is demonstrated.

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