Safety Factor Example

The type and number of tests to be conducted, natural or simulated, as usual are dependent on such factors as end item performance requirements, time and cost limitations, past history, performance safety 

The example of a building roof structure represents the simplest type of problem in static loading in that the loads are clearly long term and well defined. Creep effects can be easily predicted and the structure can be designed with a sufficiently large SF to avoid the probability of failure. 

A seating application is a more complicated static load problem than the building example just reviewed because of the loading situation. The 

Throughout this book as the viscoelastic behavior of plastics has been described it has been shown that deformations are dependent on such factors as the time under load and the temperature. Therefore, when structural components are to be designed using plastics it must be remembered that the standard equations that are available for designing springs, beams, plates, and cylinders, and so on have all been derived under the assumptions that (1) the strains are small, (2) the modulus is constant, (3) the strains are independent of tlie loading rate or history and are immediately reversible, (4) the material is isotropic, and (5) the material behaves in the same way in tension and compression. 

Since these assumptions are not always justifiable when applied to plastics, the classic equations cannot be used indiscriminately. Each case must be considered on its merits, with account being taken of such factors as the time under load, the mode of deformation, the service temperature, the fabrication method, the environment, and others. In particular, it should be noted that the traditional equations are derived using the relationship that stress equals modulus times strain, where the modulus is a constant. From the review in Chapters 2 and 3 it should 

---

Health and Safety Factors. Fluorocarbons containing bromine or iodine are more toxic than the corresponding chloro compounds. When the ratio of the fluorine to other halogens is high, the toxicity can be quite low, especially for bromofluorocarbons. Perfluoro-l-bromooctane [423-55-2] has an LD q of greater than 64 mL/kg when adininistered into the gastrointestinal tract, and has Htde effect when instilled into the lungs (49). Other examples are included in Table 7. 

Health and Safety Factors. Animal-feeding studies of DMPPO itself have shown it to be nontoxic on ingestion. The solvents, catalyst, and monomers that are used to prepare the polymers, however, should be handled with caution. Eor example, for the preparation of DMPPO, the amines used as part of the catalyst are flammable toxic on ingestion, absorption, and inhalation and are also severe skin and respiratory irritants (see Amines). Toluene, a solvent for DMPPO, is not a highly toxic material in inhalation testing the TLV (71) is set at 375 mg/m, and the lowest toxic concentration is reported to be 100—200 ppm (72). Toxicity of 2,6-dimethylphenol is typical of alkylphenols (qv), eg, for mice, the acute dermal toxicity is LD q, 4000 mg/kg, whereas the acute oral toxicity is LD q, 980 mg/kg (73). The Noryl blends of DMPPO and polystyrene have PDA approval for reuse food apphcations. 

Let s take the example of benzene, which at 12,000 ppm, is 100% LEL. The National Fire Protection Association (NFPA) states that equipment can operate, without LEL monitors or controls, if the LEL is less than 25% LEL. For benzene then, 25% LEL is equal to 3,000 ppm. This upper boundary becomes a dictating factor in the selection and design of the oxidation equipment. If the concentration is higher than 25% LEL, the NFPA requirements state that an LEL monitor is required. Using an LEL monitor, NFPA guidelines allow operation up to 50% LEL (a 2 1 safety factor). Thus, 100% LEL is explosive if the stream is at 25%, a factor safety of four exists. 

Example 2.10 The polypropylene snap fit shown in Fig. 2.22 is to have a length of 10-30 mm. If the insertion force is not to exceed 4 N and the yield stress of the plastic is 30 MN/m, calculate suitable cross-sectional dimensions for the snap fit. The short-term modulus of the polypropylene is 900 MN/m and the coefficient of friction is 0.3. The safety factor on stress is to be 2. 

Although equations (2.112), (2.113) and (2.115) can be useful they must not be used indiscriminately. For example, they are seldom accurate at short times but this is not a major worry since such short-time failures are usually not of practical interest. At long times there can also be inaccurate due to the embrittlement problem referred to earlier. In practice therefore it is generally advisable to use the equations in combination with safety factors as recommended by the appropriate National Standard. 

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and superimposed on this is an alternating axial load of 100 N. If the fatigue limit for the material is 13 MN/m and the creep rupture strength at the equivalent time is 40 MN/m, estimate a suitable diameter for the rod. Thermal effects may be ignored and a fatigue strength reduction factor of 1.5 with a safety factor of 2.5 should be used. 

In risk characterization, step four, the human exposure situation is compared to the toxicity data from animal studies, and often a safety -margin approach is utilized. The safety margin is based on a knowledge of uncertainties and individual variation in sensitivity of animals and humans to the effects of chemical compounds. Usually one assumes that humans are more sensitive than experimental animals to the effects of chemicals. For this reason, a safety margin is often used. This margin contains two factors, differences in biotransformation within a species (human), usually 10, and differences in the sensitivity between species (e.g., rat vs. human), usually also 10. The safety factor which takes into consideration interindividual differences within the human population predominately indicates differences in biotransformation, but sensitivity to effects of chemicals is also taken into consideration (e.g., safety faaor of 4 for biotransformation and 2.5 for sensitivity 4 x 2.5 = 10). For example, if the lowest dose that does not cause any toxicity to rodents, rats, or mice, i.e., the no-ob-servable-adverse-effect level (NOAEL) is 100 mg/kg, this dose is divided by the safety factor of 100. The safe dose level for humans would be then 1 mg/kg. Occasionally, a NOAEL is not found, and one has to use the lowest-observable-adverse-effect level (LOAEL) in safety assessment. In this situation, often an additional un-... 

Caution No safety factor is included in this example calculation. Additional checkup must be done if the obtained value of the torque is not greater than the recommended makeup torque for tool joints. 

There are no hard-and-fast rules to follow in setting safety factors for any given material unless experience exists. The most important consideration is of course the probable consequences of failure. For example, a little extra deflection in an outside wall or a hairline crack in one of six internal screw bosses might not cause concern, but the failure of a pressure vessel or aircraft wing might have serious safety or product-liability implications. 

A seating application is a more complicated static load problem than the building example just reviewed because of the loading situation. The self load on a chair seat is a small fraction of the normal load and can be neglected in the design. The loads are applied for relatively short periods of time of the order of 1 to 5 hours, and the economics of the application requires that the product be carefully designed with a small safety factor. 

The easiest means for assessing occupational exposure hazards associated with materials used in a process is through the use of Permissible or Occupational Exposure Limits (OEL or PEL) which go by a variety of names for example, TLV (U.S. - American Conference of Government Industrial Hygienists), MAK (Germany), or individual company established values. Occupational exposure limits are usually set based on a combination of the inherent toxicological hazard of a chemical and a series of safety factors such as intraspecies variability in test results, nature and severity of the effect, adequacy and quality of... 

Following are some examples of safety factors selected by choosing from the comparative table of LC50, LVE, MVE and IDLH substances, which are hardly, moderately and highly toxic. The vapour pressures of the substances come from the tables in Part Three, the estimation techniques in paragraph 1.1.2 should be applied, if need be. 

An example of the effects of waste settlement can be illustrated by a recent incident at a hazardous waste landfill facility in California.5 At this facility, waste settlement led to sliding of the waste, causing the standpipes (used to monitor secondary leachate collection sumps) to move 60-90 ft downslope in 1 day. Because there was a very low coefficient of friction between the primary liner and the geonet, the waste (which was deposited in a canyon) slid down the canyon. There was also a failure zone between the secondary liner and the clay. A two-dimensional slope stability analysis at the site indicated a factor of safety (FS) greater than 1. A three-dimensional slope stability analysis, however, showed that the safety factor had dropped below one. Three-dimensional slope stability analyses should therefore be considered with canyon and trench landfills. 

This gives an example of fate modeling in which the risks of an insect growth inhibitor, CGA-72662, in aquatic environments were assessed using a combination of the SWRRB and EXAMS mathematical models.. Runoff of CGA-72662 from agricultural watersheds was estimated using the SWRRB model. The runoff data were then used to estimate the loading of CGA-72662 into the EXAMS model for aquatic environments. EXAMS was used to estimate the maximum concentrations of CGA-72662 that would occur in various compartments of the defined ponds and lakes. The maximum expected environmental concentrations of CGA-72662 in water were then compared with acute and chronic toxicity data for CGA-72662 in fish and aquatic invertebrates in order to establish a safety factor for CGA-72662 in aquatic environments. 

There are two approaches that may be taken in integrating all of these factors. The first is a full calculation of the hazard from fires. This calculation requires the use of a computer and a sizable amount of expert judgment. The second approach involves a derived index. This is generally an algebraic combination of a few pieces of data leading to a value indicative of relative fire safety. An example of each approach is provided below. 

The need for special facilities for work involving neutron activation analysis and radiochemical measurements has been referred to above in Section 4.3.6. Other safety factors may also influence your choice of method. For example, you may wish to avoid the use of methods which require toxic solvents, such as benzene and certain chlorinated hydrocarbons, or toxic reagents, such as potassium cyanide, if alternative procedures are available. Where Statutory Methods have to be used, there may be no alternative. In such cases, it is essential that staff are fully aware of the hazards involved and are properly supervised. Whatever method is used, the appropriate safety assessment must be carried out before the work is started. Procedures should be in place to ensure that the required safety protocols are followed and that everyone is aware of legislative requirements. 

Strictly, natural exposure can be carried out for any of the environmental agents. For example, if the product is to spend its life in water at 70 °C then exposure to water at 70 °C can be considered natural ageing. Exposure to water at 80 °C could be called natural ageing at worst possible conditions or with a safety factor. Natural exposure defined in this way is carried out by adapting the standard laboratory methods for air ageing and exposure to liquids as there are no specific natural exposure standards. 

Safety factors are used in engineering design to reduce the design load (or equivalent parameter) to allow for the predicted degradation, to allow for a confidence limit based on statistical evaluation, or to allow simply for uncertainty. It is clear that any predicted degradation should be allowed for in full. Confidence limits can be calculated. Uncertainties must be estimated. For example, to allow for the extrapolation necessary... 

The Lehman-Fitzhugh approach has been very widely used for setting limits on exposures to chemicals, not only in food, but in all other environmental media, but it has undergone significant refinement in recent years. The EPA, and others, for example, now uses the term uncertainty factor for those factors that reflect a true scientific uncertainty, and distinguishes these from safety factors, which reflect the injection of policy judgments that go beyond scientific uncertainties in the establishment of acceptable intakes. The EPA has dropped the... 

The 100-fold safety factor has traditionally been interpreted as the product of two factors with default values of 10. For example, according to WHO/IPCS (1987), the safety factor is intended to provide an adequate Margin of Safety (MOS) by assuming that the human being is 10 times more sensitive than the test animal and that the difference of sensitivity within the human population is in a 10-fold range. 

---