Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K31/00—Processes relevant to this subclass, specially adapted for particular articles or purposes, but not covered by only one of the preceding main groups
  • B23K31/12—Processes relevant to this subclass, specially adapted for particular articles or purposes, but not covered by only one of the preceding main groups relating to investigating the properties, e.g. the weldability, of materials
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
  • F02B77/00—Component parts, details or accessories, not otherwise provided for
  • F02B77/08—Safety, indicating, or supervising devices
  • F02B77/081—Safety, indicating, or supervising devices relating to endless members
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L5/00—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes
  • G01L5/0047—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes measuring forces due to residual stresses
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
  • G01N23/20—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by using diffraction of the radiation by the materials, e.g. for investigating crystal structure; by using scattering of the radiation by the materials, e.g. for investigating non-crystalline materials; by using reflection of the radiation by the materials

Definitions

• This invention generally relates to a method for managing the service life of fatigue-limited metal components.
• This invention also generally relates to a method for managing and extending the service life of fa igue-limited " metal components. More specifically, this invention is related to a management method using a non-destructive technique for measuring the remaining useful service life of fatigue-limited metal components by determining the residual compressive stress in the critical surfaces of the individual components. Using the method of this invention, a metal component is either removed from service or reworked to increase its residual compressive stress once the residual compressive stress is reduced or falls below a predetermined value.
• This invention allows an increase in both safety and economy in the management and operation of turbine engines and other machines containing fatigue-limited metal components by providing a reliable means for non- de ⁇ tructively measuring the remaining service life of the fatigue-limited metal components.
• This invention is especially adapted for managing populations of fatigue- limited rotating parts in gas turbine engines, including aircraft engines, and the like. Using the present methods of this invention to manage and measure the residual compressive stress in such parts or components, it is now possible to determine the appropriate time (i.e. , prior to permanent deterioration from residual tensile stress crack initiation) for reworking the part to increase or restore its residual compressive stress so that the service life of the part or component can be extended.
• the overall service life of a population of components used in, for example, jet engines or turbine engines can be maximized without a significant decease in safety.
• the present invention can provide both increased safety and economy for the aviation and other industries.
• Fatigue-limited metal components of gas turbines or jet engines, or other machine components subject to metal failure or fatigue, must be carefully managed in order to avoid failure during operation.
• the failure for example, of a critical component of a jet engine during operation may result in loss of life or other catastrophic consequences.
• Currently in the aviation industry (commercial and military) there are three general types of management techniques or approaches used for the management of fatigue-limited machine components in order to prevent possible catastrophic failure due to metal fatigue. Each of these approaches attempts to balance safety and economic concerns based on available data. See, for example, S. Sure ⁇ h, Fatic e of Metals. 499-502 (1991), which generally discusses the three commonly used management approaches.
• the most conservative of these approaches is based on the estimated fatigue life established through analysis and comparable experience by the engine manufacturer. This approach attempts to estimate the point at which the shortest-lived part or component in the total population would be expected to fail. After allowing for a suitable safety margin, an arbitrary retirement point is adopted for that component. This retirement point is normally measured in total take-off cycles or hours. Once a part reaches the retirement point, it is removed from service and mutilated to prevent further, unauthorized use. Although generally allowing for the greatest margin of safety, significant economically useful service life of such parts is lost. In effect, this "safe life” approach is based on, and controlled by, an estimation of the lifetime of the weakest part or component in the total population.
• a somewhat less conservative management technique is the so-called "fail safe” approach.
• a maximum service life is determined by the total accumulated service hours or cycles (whichever is shorter) at which the first crack is detected in an actual part (i.e. , disk or drum rotor used in a compressor, turbine, or engine) in the population of like parts.
• the United States Air Force has successfully adopted an even less conservative management technique, the so-called "retirement for cause” approach, for its management of some critical engine components.
• the parts are periodically examined non-destructively for cracks using, for example, fluorescent dye penetration or magnaflux techniques.
• the present invention relates to methods for the management of populations of fatigue-limited metal components. This invention also relates to methods for the detection of the remaining service life of individual fatigue-limited metal components.
• the metal components to be managed by the present invention include metal components having relatively high levels of residual compressive stress as manufactured and which are subject to fatigue-related failure.
• the relatively high residual compressive stress of such a metal component as manufactured may be the result of the actual manufacture process used and/or subsequent working of the metal component by shot peening or other cold working processes after actual production to increase the residual compressive stress.
• the residual compressive stress as manufactured is in the range of about 50,000 to 200,000 pounds per square inch and, more preferably, in the range of about 150,000 to 180,000 pounds per square inch.
• Components having residual compressive stresses higher or lower than these ranges can, of course, be managed by the methods of the present invention.
• the components as manufactured should have sufficient residual compressive stress for their intended use. Using the methods of this invention, increases in both safety and economy in the management of such metal components is expected.
• Fatigue failures in metal components almost always develop from cracks generated in the surface layer of the metal components exposed to high stress environments.
• great care is normally taken in the manufacture of such metal components to ensure that the initial residual stress in the critical surface layers of the crystalline structure of the metal are in relatively high compression (often up to 170,000 pounds per square inch or higher) .
• the residual compressive stress of the component gradually diminishes over time. Once the residual compressive stress reaches zero, the trend continues and builds up residual tensile stress in these areas. Over time, the residual tensile stress can increase to levels in excess of the ultimate tensile strength of the surface of the material and cracks develop.
• the present invention provides methods for managing metal components whereby conditions involving significant residual tensile stress and, therefore, crack initiation are avoided.
• the present invention provides a management program which does not rely on either expected or actual crack formation as the management criteria.
• a non ⁇ destructive technique i.e. , x-ray diffraction
• x-ray diffraction a non ⁇ destructive technique
• the metal components is simply removed permanently from service.
• the metal component is reworked (using, for example, shot peening) to increase its residual compressive stress and then returned to service.
• the remaining service life of that component can be determined.
• Such information should be useful (especially as considerable historical data for the population becomes available over time) in matching components for use in particular engines or applications (i.e. , matching components which have comparable remaining service life) or for scheduling routine teardown ⁇ and maintenance.
• One object of the present invention is to provide a method for managing a population of metal parts in order to determine when to remove an individual metal part from service, wherein said metal parts are manufactured having relatively high levels of residual compressive stress and said metal parts are subject to fatigue-related failure, said method comprising:
• Another object of the present invention is to provide a method for managing a population of metals parts in order to extend the service life of individual metal parts in the population, wherein said metal parts are manufactured having relatively high levels of residual compressive stress and said metal parts are subject to fatigue-related failure during service, said method comprising, for each individual metal part in the population:
• the present invention is preferably directed towards methods for the management of large populations of similar type metal parts, it can also be used to test individual metal parts.
• the present invention can also be used for spot checking metal parts throughout their expected service life as part of routine or scheduled preventive maintenance or during repairs or teardown procedures necessitated by breakdowns.
• still another object of the present invention is to provide a method for determining when to remove a metal part from service, wherein said metal part is manufactured having relatively high levels of residual compressive stress and said metal part is subject to fatigue-related failure, said method comprising: (1) measuring the remaining residual compressive stress of the surface of the metal part in one or more areas of stress concentration using x-ray diffraction techniques;
• Still another object of the present invention- is to provide a method for extending the service life of a metal part, wherein said metal part is manufactured having relatively high levels of residual compressive stress and said metal part is subject to fatigue-related failure during service, said method comprising:
• Figure 1 illustrates a typical disk from a jet engine showing areas of stress concentration in which residual compressive stress should be determined.
• Figure 2 shows a typical plot of residual compressive stress as a function of service hours for turbine disks operating under different load and temperature conditions.
• the invention generally relates to methods for managing the service life of fatigue-limited metal components and to methods for managing and extending the service life of fatigue-limited metal components.
• the methods of this invention employ a non-destructive technique for measuring the remaining useful service life of fatigue-limited metal components by determining the residual compressive stress in the critical surfaces of the individual component.
• the residual compressive stress can be correlated with the remaining service life of the individual component. If the residual compressive stress has not fallen below a predetermined value, the component can be returned to service. If the residual compressive stress reaches or falls below a predetermined value, the component can be permanently removed from service. Or, if desired and appropriate, the component can be reworked to increase the residual compressive stress to a level above the predetermined value, preferably a value approaching the compressive stress of the component as originally manufactured, and then returned to service.
• FIG. 1 illustrates such a component, specifically a disk 10 for use in a gas turbine engine. Failure of such a disk is often caused by cracks forming in the surface layers of areas of high stress concentration such as the inside radii or bottom 12 of dovetail or firtree slots 14. These dovetail or firtree slots are used for attachment of the compressor and turbine blades (not shown) . To reduce the likelihood of crack formation, great care is normally taken in the manufacture of such components to ensure that the initial residual stress in the critical surface layers of the crystalline structure of the metal are in high compression.
• turbine disks such as the one illustrated in Figure 1
• the curves in Figure 2 (labeled 20, 22, 24, 26, and 28) are for different turbine disks 10 (i.e. , different stages) used in a gas turbine engine.
• each disk is subjected to different load and temperature conditions during operation.
• the rate of decrease of the residual compressive stress is different for each disk or stage.
• residual tensile stress can build up in these areas. Over time, the residual tensile stress can increase to levels in excess of the ultimate strength of the surface of the material and cracks will initiate. Such cracks in a component left in service propagate until they reach a critical length, at which time catastrophic failure will occur.
• the methods of this invention monitor the residual compressive stress in. areas of stress concentration in order to prevent the initial formation of cracks in the surface.
• the present methods allow individual components to be used in a manner in which surface cracks are not formed or are, at least, formed at a significantly lower rate as compared to current management methods.
• the present methods allow for achievement of the maximum service life of components without increased failure or safety risks.
• the present methods allow for significantly extending the service life of individual components without increased failure or safety risks.
• the residual compressive stress in the surface layer of the individual component is measured in one or more areas of stress concentration using x-ray diffraction techniques.
• the actual area measured is normally in the range of about M by M inches to about 1 by 1 inch, although smaller or larger areas can be used if desired.
• the measured value (or values or averaged value) is compared to a predetermined value. If the measured value is above the predetermined value, the part can be returned to service. If however, the measured value equals or falls below this predetermined value, the component can be treated in several ways. In a first option, the component can be permanently removed from service. In such cases, it is generally preferred that the component be mutilated, or otherwise marked, to prevent further, unauthorized use. In a second option, the component can be reworked to increase its residual compressive stress and then returned to service.
• such a component can be reworked and returned to service a fixed number of times or cycles (i.e. , until other failure mechanisms predominate or the component no longer meets design criteria or specifications) .
• the acceptable number of cycles for reworking such components will generally be determined on a case-by-case basis. For a given component, such as the disk shown in Figure 1, the cyc ⁇ e of service and reworking can generally be repeated so long as the component retains its dimensional and icro ⁇ tructural stability.
• the residual compressive stress when the residual compressive stress is above, but close to or approaching, the predetermined value, it may be preferred to rework that part at that time rather than wait for the residual compression stress to fall below the predetermined value.
• the measured residual compressive stress suggests that the part has only a relatively short service life remaining before reworking will be required (see, for example, curve 22 in Figure 2 at 15,000 hours with a predetermined value of zero pounds per square inch)
• the predetermined value of the residual compressive stres ⁇ for any given population of components will depend, at least in part, on the residual compressive stress of the components as originally manufactured, the specific physical and metallurgical characteristics of the components, the environment in which the components are used, and any appropriate safety factors. For populations of different parts, this predetermined value will likely be different because of different designs of the parts and exposure to different stresses during use. Populations of the same parts, but operated under different conditions and environments, may also have different predetermined values. Moreover, for a given component, different areas of stress concentration may have different predetermined values. For example, different areas of metal component will normally be exposed to, or will experience, different levels of stress and may, therefore, experience changes in the compressive stress at different rates.
• the area that reaches its predetermined value first will normally control the disposition of that component.
• the initial compre ⁇ ive re ⁇ idual ⁇ tre ⁇ of the metal component ⁇ be mea ⁇ ured or otherwise known before they are placed in service, or shortly thereafter. Measurement of the residual co pres ⁇ ive stres ⁇ of a component as originally manufactured can help insure that only components meeting specifications are used and can provide benchmarks for later measurements of remaining re ⁇ idual compre ⁇ ive stres ⁇ .
• such initial residual compres ⁇ ive ⁇ tre ⁇ s data, along with the data generated by the present methods, can be used to define or redefine component specifications and design criteria as appropriate.
• the predetermined value can be expressed in terms of absolute numbers (e.g. , a specific value in suitable units for the residual compressive stres ⁇ ) or in relative number ⁇ (e.g. , a percentage of the remaining compre ⁇ sive ⁇ tres ⁇ of the component as compared to the re ⁇ idual compres ⁇ ive ⁇ tre ⁇ a ⁇ originally manufactured) .
• the predetermined value for a given population may change over time as more historical data becomes available. For example, for newly designed components, it may be desirable to use a relatively high predetermined value to guard against unexpected failures for increased safety. As service life data becomes available, however, it may be appropriate to decrease the predetermined value if significant safety or failure related problems are not found in the population.
• a predetermined value of about zero pounds per square inch (or other appropriate units) or 100 percent decrease in residual compressive stress may be appropriate.
• the use of zero pounds per square inch or 100 percent decrease as the management criteria might, for example, be appropriate to maximize the service life of a component where reworking the component is not practical or is otherwise not anticipated.
• a predetermined value of a value greater than zero pounds per square inch or less than 100 percent decrease will generally be preferred and appropriate based on safety considerations. Such higher predetermined values will be especially preferred where reworking of the component to restore all or part of the residual compressive stress is anticipated.
• a predetermined value of less than zero pounds per square inch or greater than 100 percent decrease may be appropriate.
• the residual compressive stress is measured non-destructively using conventional x-ray diffraction techniques.
• the residual compressive stress is measured using portable x-ray diffraction equipment. Examples of such x-ray equipment and techniques can be found in U.S. Patent 5,125,016 (June 12, 1992); Taira & Tanaka, "Residual Stress Near Fatigue Crack Tips," 19 Transactions of the Iron & Steel Institute of Japan, 411-18 (1979) ; Harting & Fritsch, "A Non-destructive Method to Determine the Depth-dependence of Three-dimensional Residual Stress States by X-ray Diffraction," 26 J. Phvs. D: APPI. Phvs..
• Portable x-ray equipment which i ⁇ generally preferred in the present invention, can be obtained commercially from, for example, Technology for Energy Corporation (P.O. Box 22996, Lexington Drive, Knoxville, Tennes ⁇ ee 37933) or American Stress Technologies, Inc. (61 McMurray Road, Pittsburgh, Pennsylvania 15241) .
• x-ray diffraction equipment or techniques can also be used in the present invention.
• Normally such measurement ⁇ should be made, at a minimum, during scheduled teardowns and other maintenance events. In some case ⁇ , however, it may be de ⁇ irable to make such measurements more often than regularly scheduled maintenance events, especially during the early service life of a population of newly designed components lacking a exten ⁇ ive service life history. Normally, such ea ⁇ urements of the residual compressive stress will be made on the individual parts during teardowns. For some components, however, it may be possible to make the necessary measurements without having to perform complete teardowns. As noted above, x- ray diffraction measurement ⁇ of residual compres ⁇ ive stress should be made in areas of high stress concentration (e.g.
• the bottom 12 of the firtree slots 14 on the disk 10 shown in Figure 1) the bottom 12 of the firtree slots 14 on the disk 10 shown in Figure 1) .
• areas of high stres ⁇ concentration are those areas in which crack formation ha ⁇ been ob ⁇ erved or is more likely to occur. It is not necessary, however, to make such measurements in each and every area of high stress concentration in a given component, especially where such areas are operated under similar load and temperature conditions.
• measurements might be taken on the bottom 12 of the firtree slots 14 located at 0, 90, 180, and 270 degrees, rather than at the bottom of every slot 14. The individual measurements at these representative locations, or an average of the individual measurements, are compared to the predetermined value.
• the number and location of mea ⁇ urement ⁇ for a given di ⁇ k (or other population of component ⁇ ) can be modified as appropriate.
• a component can either be removed permanently from service or reworked to increase its residual compre ⁇ ive ⁇ tre ⁇ to a level above the predetermined value and then returned to ⁇ ervice.
• the disk represented by curve 28 in Figure 2 should be removed from service or reworked after about 10,000 hours of service,* the disks represented by curves 24 and 26 should be removed from ⁇ ervice or reworked after about 15,000 hours; the disk ⁇ repre ⁇ ented by curve ⁇ 20 (especially) and 22 (to a lesser extent) have service lives greater than 15,000 hours.
• the residual compressive ⁇ tre ⁇ in such reworked components is returned to a level approaching the original residual compres ⁇ ive stress as manufactured.
• Reworking such components can be carried out using conventional procedures for increasing or achieving residual compres ⁇ ive ⁇ tres ⁇ .
• Such methods include, for example, shot peening and other methods of cold working the surface (e.g. , hammer peening, rolling, or burnishing methods) .
• the predetermined value used as the management criteria preferably is higher ⁇ i.e. , a higher predetermined value of residual compressive stres ⁇ ) than in ca ⁇ e ⁇ where reworking i ⁇ not anticipated.
• reworking increases the residual compressive stress to at least 50 percent of the residual compres ⁇ ive stres ⁇ in the part as originally D manufactured. More preferably, the rework level for the residual compre ⁇ ive ⁇ tress is at least 80 percent of the value a ⁇ originally manufactured. Even more preferably, the reworked level i ⁇ comparable to the value a ⁇ originally manufactured.
• the method ⁇ of thi ⁇ invention generally allow one to maximize the service life of a metal component by providing a procedure to determine when to optimally rework the metal component to restore all or part of the residual compres ⁇ ive stres ⁇ .
• Excessive reworking can D actually decrease the service life since reworking techniques such as shot peening increase the dimension of the metal part in the reworked area.
• the present invention provides procedures whereby the maximum service life can be obtained with the minimum number of reworkings.
• the effect of reworking on dimensional stability should be taken into account when establishing the 0 appropriate predetermined value for a given component.
• the methods of the present invention provide improved management procedures by providing management criteria (i.e. , re ⁇ idual compressive stress compared to predetermined values) which are observable well before irreversible crack initiation or formation occurs.
• management criteria i.e. , re ⁇ idual compressive stress compared to predetermined values
• the maximum service life of such component ⁇ can be obtained in a safe and effective manner.
• at least some components can be reworked to increase the residual compressive stress and placed back in service, thereby providing increased efficiency without compromising safety.
• the methods of the present invention are expected to achieve improved efficiency and improved safety as compared to the management methods currently in use since the management criteria is based on residual compressive stresses rather than on actual or expected crack formation.
• the margin of safety i ⁇ expected to be increased, perhaps significantly.
• the methods of the present invention are ideally suited for augmenting computer tracking of metal components during their service life.
• the data generated from the present methods i.e. , residual compressive stres ⁇
• data regarding the service life and history of individual components can be used to develop a management database for the metal components.
• the database might show that only components having a service live in excess of 10,000 hours ever have re ⁇ idual compre ⁇ ive stresses approaching or below the predetermined value. In such a case, only components with service lives greater than this value will need to have the residual compressive ⁇ tress measured during a routine teardown.
• components with abnormalities or components subjected to extreme service events should be evaluated regardless of the cumulated service life.
• considerable ⁇ aving ⁇ can be obtained.
• the method u ⁇ ed to predict this likelihood must be proven over time to be both effective and safe.
• the present methods are not, however, intended to replace all non-destructive testing of such metal components.
• the present methods are de ⁇ igned to manage failures and defects related to crack initiation mechanism ⁇ commonly a ⁇ ociated with the re ⁇ idual compressive stres ⁇ and re ⁇ idual ten ⁇ ile ⁇ tre ⁇ level ⁇ in the ⁇ urface layers.
• Other non-destructive testing methods for identifying other types of failures and/or failure mechanisms, especially early in the ⁇ ervice life of such components ⁇ i.e. , during the so-called "infant mortality" period) should be continued a ⁇ appropriate. Incorporation of such data from other non-de ⁇ tructive testing procedures in the management database di ⁇ cussed above may allow even more complete tracking and management of populations of such metal components.
• components of approximately equal remaining service life i.e. , approximately equal re ⁇ idual compres ⁇ ive ⁇ tre ⁇ s
• the frequency of such teardowns in individual engines will be controlled by the component having the shortest remaining service life.
• early teardown of engines necessitated by a single component with only a short remaining service life, can be avoided.
• the frequency of teardowns over the entire population can be reduced.

Landscapes

• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Physics & Mathematics (AREA)
• Mechanical Engineering (AREA)
• General Physics & Mathematics (AREA)
• Analytical Chemistry (AREA)
• General Health & Medical Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Combustion & Propulsion (AREA)
• Biochemistry (AREA)
• General Engineering & Computer Science (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Analysing Materials By The Use Of Radiation (AREA)
• Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
• Investigating And Analyzing Materials By Characteristic Methods (AREA)
• Testing Of Devices, Machine Parts, Or Other Structures Thereof (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)

Priority Applications (10)

Applications Claiming Priority (2)

Publications (1)

Family

ID=22924973

Family Applications (1)

Country Status (14)

Cited By (3)

Families Citing this family (21)

Citations (1)

Family Cites Families (10)

• 1994
  • 1994-05-18 US US08/245,011 patent/US5490195A/en not_active Expired - Lifetime
• 1995
  • 1995-04-19 IL IL11342195A patent/IL113421A/en not_active IP Right Cessation
  • 1995-05-03 JP JP52968595A patent/JP4117902B2/ja not_active Expired - Lifetime
  • 1995-05-03 RU RU96123898/28A patent/RU2150691C1/ru not_active IP Right Cessation
  • 1995-05-03 BR BR9507733A patent/BR9507733A/pt not_active IP Right Cessation
  • 1995-05-03 DE DE69524218T patent/DE69524218T2/de not_active Expired - Lifetime
  • 1995-05-03 WO PCT/US1995/005488 patent/WO1995031715A1/en not_active Ceased
  • 1995-05-03 AU AU23733/95A patent/AU686378B2/en not_active Ceased
  • 1995-05-03 EP EP95917804A patent/EP0764267B1/en not_active Expired - Lifetime
  • 1995-05-03 AT AT95917804T patent/ATE209784T1/de not_active IP Right Cessation
  • 1995-05-03 CA CA002190354A patent/CA2190354C/en not_active Expired - Lifetime
  • 1995-05-08 ZA ZA953706A patent/ZA953706B/xx unknown
• 1996
  • 1996-11-15 FI FI964586A patent/FI964586A7/fi not_active Application Discontinuation
  • 1996-11-15 NO NO964848A patent/NO964848L/no not_active Application Discontinuation

Patent Citations (1)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events