Abstract of Special Research Report (RR-20)

National Institute of Occupational Safety and Health, Japan

Quenching Distance of Solids

RR-20-1

K. KOMAMIYA

: For the purpose of fire prevention in the oxygen-enriched atmospheres, flame quenching distances of various combustible solids have been measured.
    Experiments were carried out with a metallic holder displayed in Fig.2, putting the sample, all of which are cited in Table 1, between the two plates of the holder, in oxygen, oxygen-nitrogen mixed gas and nitrous oxide under pressures of 0-4 kg/cm² G.
    The results are showed in Table 6 - 8 and in Fig.8 - 13. Good reproducibilities were obtained in these measurements. In oxygen the quenching distances of polyethylene and Japanese paper (a sort of paper which is made peculiarly and traditionally in Japan and is composed of almost pure cellulose) have nearly the same values as that of hydrogen. Further, in this experiment an interesting phenomenon like one showed in Fig.17 was observed. Therefore, it has been recognized that the quenching distances of solids are measurable in practically the same way as in the case of combustible gases. Accordingly, design data for fire prevention in the oxygen-enriched atmospheres were also able to be obtained.
    And then, application of this method was investigated.
  1. A portable oxygen analyzer based on quenching distance.
    Very reproducible quenching distance being obtained in each condition, the values in various oxygen nitrogen mixed gases were measured, using thin Japanese paper. As the result, it was revealed that such a scale as the one showed in Fig.20, compared with the corresponding combustion distance of Japanese paper, could be put to use as a handy type oxygen analyzer.
  2. A combustion test based on quenching distance.
    In Fig.10 quenching distance shows a good correlation with oxygen index and this measurement appears in principle to be utilized as a combustion test. Moreover, it also seems to be used as a simple method for discrimination of plastics according to the appearances of their flames in Table 6.

Effciency Improvement of the Safety Net --About Special Character of the Safety Net--

RR-20-2

K. KINOSHITA and K. OGAWA

: The safety net has been used for the prevention of fall accidents in construction sites and others, while it is under the necessity of standardizing the specification of it. This report investigates experimentally into the dynamical characteristics of the safety net made of synthetics fiber for the above necessity.
    In the experiment, fall impacts were made on the following conditions ;
  1) The dummy in a hollow cylinder is made of iron, 90 kg in weight, and 33 cm in diameter.
  2) The position to fall it is the center of a net.
  3) The number of suspending points of a net is eight points.
  4) The height of free fall is measured upward from the level where the weight of the dummy balances with the reactionary force of the net.
    Then, the deceleration acting on the dummy, tensile forces in suspending points, and the distance of displacement of the dummy were measured.
    From the experiment we get the following empirical equations among some variables and other results ;
  1) In the first impact, relations between the maximum deceleration, D ₁ (9.8 m/sec² ) and the ratio the length of one side of a net, L (m) to the height of free fall, H (m) are found, without regard to the material of nets, as follows ;
      D ₁ = 6.50 L / H + 3.85      10 cm meshes
      D ₁ = 7.79 L / H + 3.46       5 cm meshes
  2) When the fall-impact is repeated on the same conditions, the following equation is found,
      D _N = a (1 - e^-b(N-1)) L / H + D ₁
      a, b : Constant value due to the characteristics of nets,
      D _N : The maximum deceleration in the nth impact (9.8 m/sec² ).
  3) In the penetrated cases by fall impacts, the relations between the average breaking strength of twines, T (kg) and the maximum deceleration, D _b (9.8 m/sec² ) is regarded as follows
      D _b = 0.044 T + 6.4       10 cm meshes
      D _b = 0.112 T + 7.1        5 cm meshes
  4) The ratios of tensile forces in suspending points to the maximum impact load are estimated about 30 - 40%
  5) With the distance of maximum displacement, S (m) from the level of initial sag, following empirical equations are found ;
      S / L = 0.045 H / L + 0.44        10 cm meshes
      S / L = 0.070 H / L + 0.39         5 cm meshes
  6) It is confirmed that, in comparison with the value of maximum deceleration of each net, barely Nylon nets and Polypropylene nets are superior to vinylon nets in buffer effect but practically they have the same effect.

Some Studies on Combustion of Irradiated Polyethylene

RR-20-3

S. MORISAKI

: Vicat softening point determination, differential thermal analyses and thermogravimetries were carried out for the purpose of investigating the thermal stability of polyethylenes irradiated with electron beams up to 100 Mrad. The softening temperatures for the irradiated low density polyethylenes much more increased with increase of radiation dose than those for the irradiated high density polyethylenes. From the results of thermogravimetry, the irradiated polyethylenes decompose in three stages in air but in one stage in a nitrogen atmosphere. Those irradiated polyethylenes decompose easily in nitrogen as it has also been shown by the calculation for activation energies, and begin to decompose at relatively low temperature in air with increasing radiation dose. The irradiated polyethylenes, however, are less combustible than the unirradiated polyethylenes, since the initial temperature for the second stage degradation, which probably shows an ignition point, and the residue after the degradation increase with increase of radiation dose. In addition, the differential thermal analysis was also carried out to investigate the mechanism of thermal decomposition of the irradiated polyethylenes.

Electron Fractographical Study on th Causes of Metals Failure (2nd Report) --Analysis of the fatigue striations in the high strength aluminum alloy--

RR-20-4

Eiji AKIYAMA Taiji KONDO and Yoshio KITSUNAI

: Fractographical features of the high strength aluminum alloy, and especially the nature of the fatigue striation formed during Stage II crack growth, were examined quantitatively, to apply the fractographical technique for the analysis of the failure causes and to clear the fatigue fracture mechanism.
    Mechanical properties and chemical composition of the test material are shown in Tables 1 and 2. The shape of the fatigue test specimen is illustrated in Fig.1. Fatigue tests were carried out under tension and compression by means of Vibrophor. Macroscopic crack growth rate on both surfaces of the specimen was measured by two optical microscopes with low magnification (x 20).
    Specimens for the electron microscope observation were made by the two stage carbon replica method. Those replicas were shadowed with chromium at 45° to the macroscopic crack growth direction.
    Measurements on transmissivity for analysis of striations were carried out by the micro photometer. Transmissivity curves of the film were automatically recorded by the pen-deflection recorder. As illustrated in Fig.6, one cycle in the transmissivity curves coincided with a striation spacing.
    Striation spacing was measured for each location and stress amplitude applied during the test. Distribution of striation spacing was shown with histograms in Fig.7. Moreover, mean value, standard deviation and coefficient of variation of the striation spacing were recorded in Fig.7 and Table 3.
    The relation between macroscopic growth rate and striation spacing is exhibited in Fig.9. Striation spacing was plotted against the stress intensity factor range ΔK computed from Eq.(2). The correlation of striation spacing to ΔK is shown in Fig.10.
    Assuming that transmissivity curves are similar to shapes of the striation, shapes of the striation and fluctuation of the striation spacing were examined. Fourier analysis of the transmissivity curves was attempted to show the shape of the striation. Typical examples of the result are shown in Eqs.(6) and (7). Auto correlation function of the transmissivity curves were also calculated to exhibit the fluctuation of the striation spacing (Figs.11, 12 and 13).
    The main results may be summarized as follows.
  (1) Stress amplitude, crack length, crack growth rate, tear line, inclusions and other particles have influence on the formation of the striation.
  (2) Striation spacing tends to increase in proportion to crack length and stress amplitude, therefor, striation spacing depends on the stress intensity factor range ΔK .
  (3) Compared with each coefficient in the results of the Fourier analysis on the transmissivity curves, the shapes of the striation may be estimated in micro-range.
  (4) Fluctuations of the striation spacing are indicated by the auto correlation function of the transmissivity curves.
    For (3) and (4), however, correlations between the absolute value of the shapes on the fracture surfaces and the transmissivity curves obtained from the electron micrograph are necessary to make study still more.

Decaying of Gaseous Detonation by Expansion (First Report) --Experiments in Flat Channels for Oxyacetylene Detonations--

RR-20-5

Hidenori MATSUI

: Detonation expansion processes were investigated in this paper in order to know the properties of gaseous detonation decaying from the view point on the problem of safety precaution in handling explosive mixtures.
    When a steady detonation wave suddenly expands into a larger space, an induction period increases due to the decrease of the pressure and temperature, so that the separation of the combustion zone from the shock front occurs.
    Oxy-acetylene gaseous detonation waves were initiated in the rectangular cross section tubes which were made out of transparent acrylic resin plates. Each of the tubes was enlarged in width at the halfway, the tubes before-enlargement were 4, 8, 20 and 40 mm in width and 50 cm in length, and the all tubes after-enlargement were 100 mm in width and 50 cm in length. The depth was 3 mm through out the whole tubes.
    Streak and still photographs were taken to obtain the detonation decayed distance(induction distance) , the limiting initial pressure of decay or P _decay and variation of the flame velocity after expansion, for various expansion ratios of the tubes and gas compositions.
    Still photographs gave qualitative explanation of the wave expansion (Fig.5). At a constant expansion ratio and gas composition, as the initial pressure was low, so the detonation decaying easily occurred, and the more the initial pressure increased, the shorter the decayed distance became. At a certain initial pressure, the detonation suddenly stopped to decay (Fig.6). This limiting pressure of decay varied with gas compositions (Fig.7).
    In the case of detonation decaying after expansion, the flame velocity suddenly decreased and gradually increased until the detonation re-established. Just after the re-establishment, the detonation overdrove and showed extremely high velocity for an instant, then proceeded in steady one.
    The limiting pressure of decay was proportional to expansion ratio to the power of 0.6 - 0.7 in common with each gas composition (Fig.8). It is shown as follows : P _decay = K ·R ⁿ
      n : 0.6 - 0.7
      K : Constant for each gas composition
      R : Expansion ratio/

JNIOSH