Abstract of Special Research Report (RR-25)

National Institute of Occupational Safety and Health, Japan

A Presumption Method of Grinding Wheel Bursting Speed with Consideration of Stress Gradient

RR-25-1

Soichi KUMEKAWA

: Assuming that the rotating strength of grinding wheels is influenced by the maximum stress and the stress gradient at the inner periphery of the wheel, the grinding wheel bursting speed can be calculated from the strength which is got in the bending test under the same stress gradient.
    The grinding wheels used in this experiment are vitrified bonded wheels and resinoid bonded wheels which are shown in Table 1.
    The bending apparatuses shown in Fig.3 are used in the bending test and the bending strength σ_B is calculated by Eqs.(10). The results of the bending test are as Fig.6 (vitrified) and Fig.9 (resinoid).
    The bursting speed of the grinding wheels which are measured in the centrifugal test are shown in Fig.7 (vitrified) and Fig.10 (resinoid).
    Substituting strength σ_B into Eqs.(9), the presumed values V _B of the bursting surface speed are got. And then the presumed values V _B and the experimental values v _B of the bursting surface speed of wheels are plotted in Fig.8 (vitrified) and Fig.11 (resinoid).
    This presumption method gives fairly good agreement with the experimental results as shown in Table 2.

An Electron Fractographic Approach to Quantitative Failure Analysis --Correlations Between Fracture Surface Appearance and Fracture Mchanics Parameters for Stage II fatigue Crack Propagation in an Alminum Alloy and a Mild Steel--

RR-25-2

Yoshio KITSUNAI and Masazumi TANAKA

: In order to analyze the fatigue failure causes in service using electron fractographic methods, fatigue crack growth rate for an aluminum alloy (2017-T3) and for a mild steel (SB42) were determined from measurements of specimen's surfaces, termed macroscopic rate, and from striation spacing fracture, termed microscopic rate. Effect of stress ratios on fatigue crack growth rate also have been, studied as a function of various fracture mechanics parameters, including the stress intensity range (ΔK) and both positive and negative ratios of the minimum to the maximum stress (R).
    A centrally slotted 4 mm thick sheet specimens with 100 mm wide by 200 mm length were made. One side of the specimen's surface was polished and lines scribed 0.5 mm apart so that the length of the fatigue cracks could be estimated to 0.05 mm. The fatigue tests were conducted on a Vibrophor test machine at frequency of 120 - 145 Hz with various stress ratios (R). The values for the stress ratios that were investigated in this study were R = -1 - 0.73 for an aluminum alloy and R = - 1 - 0.6 for a mild steel. Fatigue crack growth rate ware obtained graphically by taking the slopes of the fatigue crack growth curves at various crack lengths.
    Two stage chromium shadowed carbon replicas were obtained from the fatigue fracture surfaces and examined using a transmission electron microscope. Precisely matched regions of matching fracture-surfaces were examined to distinguish between striation and quasi-striation pattern. More than 100 striation spacing have been measured for a given stress intensity level on each of the specimen and were examined statistically. These statistical results of striation spacing were compared with macro-scopic crack growth rate.
    The results may be summarized as follows.
  (1) Fatigue crack growth rate for various stress ratios (R) are correlated with the stress intensity factor (Ke) based on Elber's crack closure phenomenon.
  (2) Fatigue crack growth rate and striation spacing approximately agree with each other for crack growth range from 0.05 μ/c to 0.4 μ/c.
  (3) The scatter of striation spacing in one patch is small as compared with total patches for a given the effective stress intensity level.
  (4) For determining unknown component cyclic loading during failure analysis, item (3) suggests that one should be measured striation spacing obtained from many patches.
  (5) Coefficient of variation of striation spacing for an aluminum alloy is smaller than that of a mild steel.
  (6) The distribution of striation spacing mainly follows to logarithmic normal distribution.
  (7) Striation spacing is related to the effective strain intensity factor (Ke / E) for an aluminium alloy and for a mild steel.
  (8) In the case of the planes of the patches are away from the perpendicular to the maximum principal stress, the features on both fracture halves are considerable difference.

On the Required Conditions for the Temporary Railing --Aabout necessary conditions in the height and the strength--

RR-25-3

Katsunori OGAWA, Yoshimasa KAWAJIRI and Noriyuki HORII

: The temporary railing has been used for the prevention of fall accidents in construction sites and dockyards, while it is under the necessity of standardizing the specification of it. This report is the experimentally investigation about conditions the temporary railing must have, above all next two contents.
  (1) Necessary height of the railing for a man not to get over the railing while at work.
  (2) Necessary strength of the railing for working actions.
    In the experiment about (1), actions against the railing were limited to 2 sorts (action 1 and action 2. Refer to Photo 1) and men and a dummy were employed as subjects. From the experiment the following empirical equations could be obtained among some variables concerning posture of subject.
    Equation in the boundary whether feet of subject floats from floor surface or not.
      H/L = tan(-1.679·H₀/√(H² + L²) + 4.320) ................ about action 1
      H/L = (H₀/√(H² + L²) - 1.300)·tan(0.00341V + 0.824) + 1.345 ....... about action 2
    Equation in the boundary whether body of the subject gets over the railing or not.
      H/L = tan(-1.628·H₀/√(H² + L²) + 4.638) ................ about action 1
  where
     H : Distance between the floor surface and the center of the toprail
     L : Horizontal distance between the toe or heel of subject and the center of the toprail
     H₀ : Body height of subject
     V : Truck speed or walking speed
    Then we carried out the simulation using above equations and calculated the probabilities of getting-over and floating. An digital computer was used for the simulation.

    About (2), first of all, we had to know the forces which act on the railing, so we performed the experiment to make clear the relation between working actions and the forces act on the railing. In the experiment, actions against the railing were limited and assumed to 8 sorts (4 static actions, 4 dynamic actions. Refer to Photo 2) and men or a dummy were employed as subjects.
    As the results of the experiment, the following empirical equations could be obtained concerning the forces act on the railing for typical two actions (action S4, action D1)

  where
      F_s2 : Force acts on the railing in action S4
      F_max : Impact force acts on the railing in action D1
      H, L, H₀, V : As above mentioned
      W₀ : Body weight of subject
      W₁ : Weight of the load in action S4
      K : Spring constant of the railing
      g : Acceleration of gravity
      α₁, α₂, A, B : Experimental coefficients (Refer to Table 9, Table 10)
    Then we carried out the computer simulation using above equations and calculated the forces act on the railing.
    From the results of this experimental investigation we shall propose as follows.
  1. The temporary railing shall be more than 95 cm in height.
  2. The temporary railing shall be classified into two ranks (for light-work use and for heavy-work use) in the strength, according as the places where the railing is installed and the matter of work. The dimensions and members of each railing shall be such that the completed structure shall be capable of withstanding a load of at least follows applied in any direction at any point of the toprail.
  (a) 36 kg / person for light-work use.
  (b) 122 kg / person for heavy-work use.

On the Method of Temperature Compensation to the Output Voltage of a Hot Wire Anemometer and the Calculation Method of the Direction of Air Stream in Low Velocity

RR-25-4

Kinichi KINOSHITA

: This paper deals with the method of compensating the out put voltage of a hot wire anemometer for fluctuating temperature, and the method of calculating the direction of air stream by using a digital computer.
    Firstly, in measuring accurately low air velocity below 2 m/sec, it is necessary to compensate the voltage of the anemometer. Therefore for this purpose, fluctuating air temperature must be measured accurately and continuously. The most simplest method to measure air temperature is to use the tungsten wire of about 5μm in diameter which is generally used in a hot wire anemometer, because this fine wire can be got easily and its temperature coefficient on electrical resistance is comparatively greater than other common metals. The prong of this sensor is made of manganine wire of 60μm, and the step response to temperature of this sensor is estimated to be about 16 Hz.
    The compensations equations written in eq.(3,20), (3,23) are derived from the results that are obtained, by solving eq.(3,8) under the following conditions ; 1) Diameter of hot wire of tungsten is 5.25 μm and 10 μm, 2) Aspect ratio (L/d) of hot wire is in range of 190 - 570, 3) Fluctuating air temperature is in the range of normal temperature and 60 °C, 4) The average heat temperature of hot wire is the cases of 170, 200 and 230 °C. And also the same eq.(3,24), (3,25) as above equations were derived from many experimental data. Temperature compensation by the former equation gives good agreement with that by the latter equations. The error of temperature compensation by using eq.(3,26) is estimated at ± 0.161 % to the out put voltage, and this accuracy is approximately sufficient in measuring low air velocity.
    Secondly, it is an interesting problem to calculate the direction of air stream. In this paper, the method of calculation of two dimensional direction was ascertained empirically by using a x-type probe, and the approximate equations to calculate the direction are obtained. But high accuracy in calculation can not expected with the reason which heat transfer from fine wire is inclined to be insensible to the direction of air stream in the very low air velocity.