1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-02-18a71 Center Point of Contact: GRCa71 Submitted by: Wil HarkinsSubject: NASA Preferred Reliability Practices; Design and Test Practices for Aerospace Systems; Environmental Factors Practice: At the onset of the design proc
2、ess, identify the operating conditions that will be encountered during the life of the equipment.Programs that Certify Usage: Programs That Certified Usage: Space Electronic Rocket Test (SERT) I and II, Communication Technology Satellite (CTS), ACTS, Space Experiments, Launch Vehicles, Space Power S
3、ystems, and Space Station Freedom.Center to Contact for Information: GRCImplementation Method: This Lessons Learned is based on Reliability Practice NO. PD-EC-1101 from NASA Technical Memorandum 4322A, NASA Preferred Reliability Practices for Design and Test.Benefits:Each of the identified environme
4、ntal factors requires consideration in the design process. This assures that adequate environmental strength is incorporated into the design to ensure reliability.Implementation Method:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-To ensure a relia
5、bility-oriented design, determine the needed environmental resistance of the equipment. The initial requirement is to define the operating environment for the equipment. A Life-Cycle Environment Profile, containing this information, should be developed.A Life-Cycle Environment Profile is a forecast
6、of events and associated environmental conditions that an item experiences from manufacturing to retirement. The life cycle includes the phases that an item will encounter such as: handling, shipping, or storage prior to use; disposition between missions (storage, standby, or transfer to/from repair
7、 sites); geographical locations of expected deployment; and platform environments. The environment or combination of environments the equipment will encounter at each phase should be determined. All deployment scenarios should be described as a baseline to identify the environments most likely to be
8、 associated with each life cycle phase. The following factors should also be taken into account:a. Hardware configuration.b. Environment(s) that will be encountered.c. Platform/hardware interfaces.d. Interfaces with other equipment.e. Absolute and relative duration of exposure phase.f. Probability t
9、hat environmental condition(s) will occur.g. Geographical locations.h. Any other information that will help identify environmental conditions that may impact the item.The steps in developing a Life-Cycle Environment Profile are as follows:1. Describe anticipated events for an item of equipment, from
10、 final factory acceptance through terminal expenditure or removal from inventory.2. Identify significant natural and induced environments or combination of environments for each anticipated shipping, storage, and logistic event (such as transportation, dormant storage, stand-by, bench handling, and
11、ready modes, etc.).3. Describe environmental and stress conditions (in narrative and statistical form) to which equipment will be subjected during the life cycle. Data may be derived by calculation, laboratory tests, or operational measurements. Estimated data should be replaced with actual values a
12、s determined. The profile should show the number of measurements used to obtain the average value of these stresses and design achievements as well as their variability (expressed as standard deviation).This analysis can be used to: develop environmental design criteria consistent with anticipated P
13、rovided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-operating conditions, evaluate possible effects of change in environmental conditions, and provide traceability for the rationale applied in criteria selection for future use on the same program or other
14、 programs.A listing of typical environmental factors is included in Table 1.Table 1: Environmental Coverage Checklist (Typical) Natural InducedAlbedo, Planetary IRCloudsElectromagnetic RadiationElectrostatic DischargeFogFreezing RainFrostFungusGravity, LowHailHumidity, HighHumidity, HightIceIonized
15、GasesLightningMagnetics, GeoMeteoroidsPollution, AirPressure, HighPressure, Low, VacuumRadiation, Cosmic, SolarRainSalt SpraySand and DustSleetSnowTemperature, HighTemperature, LowWindAccelerationChemicalsCoronaElectromagnetic, LaserElectromagnetic RadiationElectrostatic DischargeExplosionIcingMagne
16、ticsMoistureNuclear RadiationShock, Pyro, ThermalSpace DebrisTemperature, High, Aero. Heating, FireTemperature, Low, Aero. CoolingTurbulenceVapor TrailsVibration, Mechanical, MicrophonicsVibration, AcousticTechnical Rationale:Provided by IHSNot for ResaleNo reproduction or networking permitted witho
17、ut license from IHS-,-,-Given the dependence of equipment reliability on the operating conditions encountered during the life cycle, it is important that such conditions be identified accurately at the beginning of the design process. Environmental factors that strongly influence equipment reliabili
18、ty are included in Table 1, which provides a checklist for environmental coverage (typical).Concurrent (combined) environments may be more detrimental to reliability than the effects of a single environment. In characterizing the design process, design/test criteria must consider both single and/or
19、combined environments in anticipation of providing the hardware capability to withstand the hazards identified in the system profile. The effects of typical combined environments are illustrated in a matrix relationship in Figure 1, which shows combinations where the total effect is more damaging th
20、an the cumulative effect of each environment acting independently. For example, an item may be exposed to a combination such as temperature, humidity, altitude, shock, and vibration while it is being transported. The acceptance to end-of-life history of an item must be examined for these effects. Ta
21、ble 2 provides reliability considerations for pairs of environmental factors.Each environmental factor that is present requires a determination of its impact on the operational and reliability characteristics of the materials and parts comprising the equipment being designed. Packaging techniques sh
22、ould be identified that afford the necessary protection against the degrading factors.In the environmental stress identification process that precedes selection of environmental strength techniques, it is essential to consider stresses associated with all life intervals of the equipment. This includ
23、es operational and maintenance environments as well as the pre-operational environments, when stresses imposed on the parts during manufacturing assembly, inspection, testing, shipping, and installation may have significant impact on equipment reliability. Stresses imposed during the pre-operational
24、 phase often are overlooked; however, they may represent a particularly harsh environment that the equipment must withstand. Often, the environments to which systems are exposed during shipping and installation are more severe than those encountered during normal operating conditions. It is probable
25、 that some of the environmental strength features that are contained in a system design pertain to conditions that will be encountered in the pre-operational phase rather than during actual operation.Figure 1: Effects of Combined EnvironmentsProvided by IHSNot for ResaleNo reproduction or networking
26、 permitted without license from IHS-,-,-refer to D descriptionD (Click image for a larger view) Table 2: Various Environmental Pairs High Temperature and HumidityHigh Temperature and Low PressureHigh Temperature and Salt SprayHigh temperature tends to increase the rate of moisture penetration. The g
27、eneral deterioration effects of humidity are increased by high temperatures.Each of these environments depends on the other. For example, as pressure decreases, outgassing of constituents of materials increases; as temperature increases, outgassing increases. Hence, each tends to intensify the effec
28、ts of the other.High temperature tends to increase the rate of corrosion caused by salt spray.High Temperature and Solar RadiationHigh Temperature and FungusHigh Temperature and Sand and DustProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-This is a m
29、an-independent combination that causes increasing effects on organic materials.A certain degree of high temperature is necessary to permit fungus and microorganisms to grow. However, fungus and microorganisms cannot develop above 160F (71C).The erosion rate of sand may be accelerated by high tempera
30、ture. However, high temperature reduces sand and dust penetration.High Temperature and Shock and VibrationHigh Temperature and AccelerationHigh Temperature and Explosive AtmosphereSince both environments affect common material properties, they will intensify each others effects. The degree to which
31、the effect are intensified depends on the magnitude of each environment in combination. Plastics and polymers are more susceptible to this combination than metals, unless extremely high temperatures are involved.This combination produces the same effect as high temperature and shock and vibration.Te
32、mperature has minimal effect on the ignition of an explosive atmosphere but does affect the air-vapor ratio, which is an imporant consideration.Low Temperature and HumidityHigh Temperature and OzoneRelative humidity increases as temperature decreases, and lower temperature may induce moisture conden
33、sation. If the temperature is low enough, frost or ice may result.Starting at about 300F (150C) temperature starts to reduce ozone. Above about 520F (270C), ozone cannot exist at pressures normally encountered.Low Temperature and Solar RadiationLow Temperature and Low PressureLow Temperature and Sal
34、t SprayProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Low temperature tends to reduce the effects of solar radiation and vice versa.This combination can accelerate leakage through seals, etc.Low temperature reduces the corrosion rate of salt spray.L
35、ow Temperature and Sand and DustLow Temperature and FungusLow temperature increases dust penetration.Low temperature reduces fungus growth. At sub-zero temperatures, fungi remain in suspended animation.Low Temperature and Shock and VibrationLow Temperature and AccelerationLow Temperature and Explosi
36、ve AtmosphereLow temperature tends to intensify the effects of shock and vibration. However, it is a consideration only at very low temperatures.This combination produces the same effect as low temperature and shock and vibration.Temperature has minimal effect on the ignition of an explosive atmosph
37、ere but does affect the air-vapor ratio, which is an important consideration.Low Temperature and OzoneHumidity and Low PressureHumidity and Salt SprayOzone effects are reduced at lower temperatures but ozone concentration increases with lower temperatures.Humidity increases the effects of low pressu
38、re, particularly in relation to electronic or electrical equipment. However, the actual effectiveness of this combination is determined primarily by the temperature.High humidity may dilute the salt concentration and could affect the corrosive action of the salt by increasing the coverage, thereby i
39、ncreasing the conductivity.Humidity and Fungus Humidity and Sand and DustHumidity and Solar RadiationHumidity helps the growth of fungus and microorganisms but adds nothing to their effects.Sand and dust have a natural affinity for water and this combination increases deterioration.Humidity intensif
40、ies the deteriorating effects of solar radiation on organic materials.Humidity and Vibration Humidity and Shock and AccelerationHumidity and Explosive AtmosphereProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-This combination tends to increase the ra
41、te of breakdown of electrical material.The periods of shock and acceleration are considered too short for these environments to be affected by humidity.Humidity has no effect on the ignition of an explosive atmosphere but a high humidity will reduce the pressure of an explosion.Humidity and Ozone Lo
42、w Pressure and Salt SprayLow Pressure and Solar RadiationOzone meets with moisture to form hydrogen peroxide, which has a greater deteriorating effect on plastics and elastomers than the additive effects of moisture and ozone.This combination is not expected to occur.This combination does not add to
43、 the overall effects.Low Pressure and Fungus This combination does not add to the overall effects.Low Pressure and Sand and DustLow Pressure and VibrationLow Pressure and Shock or AccelerationThis combination only occurs in extreme storms during which small dust particles are carried to high altitud
44、es.This combination intensifies effects in all equipment categories but mostly with electronic and electrical equipment.These combinations only become important at the hyperenvironment levels, in combination with high temperature.Low Pressure and Explosive AtmosphereSalt Spray and Fungus Salt Spray
45、and DustAt low pressures, an electrical discharge is easier to develop but the explosive atmosphere is harder to ignite.This is considered an incompatible combination.This will have the same effect as humidity and sand and dust.Salt Spray and Vibration Salt Spray and Shock or AccelerationSalt Spray
46、and Explosive AtmosphereThis will have the same combined effect as humidity and vibration.This combinations produce no added effects.This is considered an incompatible combination.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Salt Spray and Ozone S
47、olar Radiation and FungusSolar Radiation and Sand and DustThis combination is similar to but more corrosive than humidity and ozone.Because of the resulting heat from solar radiation, this combination probably produces the same combined effect as high temperature and fungus. Further, the ultraviolet
48、 in unfiltered radiation is an effective fungicide.It is suspected that this combination will produce high temperatures.Solar Radiation and OzoneFungus and Ozone Solar Radiation and Shock or AccelerationThis combination increases the rate of oxidation of materials.Fungus is destroyed by ozone.These
49、combinations produce no added effects.Solar Radiation and VibrationSand and Dust and VibrationUnder vibration conditions, solar radiation deteriorates plastics, elastomers, oils, etc., at a higher rate.Vibration might possibly increase the wearing effects of sand and dust.Shock and Vibration Vibration and AccelerationThis combination produces no add
