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ESA SME Initiative Course D:Materials

ESA SME Initiative Course D:Materials

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ESA SME Initiative Course D:Materials

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  1. Materials properties requirements and associated test methods for metallic materials ESA SME InitiativeCourse D:Materials Ton de Rooij Principal Metallurgist Materials and Processes Division Product Assurance and Safety Department Materials and Processes Division ESA/ESTEC/TOS-QM

  2. Content of presentation • Materials properties • Constraints on materials • Surface finishes • coatings • Joining • Corrosion testing • Mechanical testing • Macroscopic examination • Microscopic examination • Non-destructive examination • Failure analysis Materials and Processes Division ESA/ESTEC/TOS-QM

  3. Strength Elastic modulus Fatigue Fracture toughness Creep Micro-yielding Coefficient of thermal expansion and coefficient of moisture expansion Stress corrosion Corrosion fatigue Hydrogen embrittlement Wear and fretting Surface finishes Anodising Conversion pickling/etching mechanical Coatings hard barriers Materials Properties Materials and Processes Division ESA/ESTEC/TOS-QM

  4. Metallic Materials used in space • Light metals, such as beryllium, magnesium, aluminium and titanium and their alloys • Steels, such as low-alloy, tool, corrosion resistant, precipitation hardable, and maraging • Nickel and nickel base alloys, including pure nickel, Monel alloys, Inconel alloys, and other nickel- and cobald-base superalloys • Refractory metals, principally niobium and molybdenum • Copper-base alloys, including pure coppers, beryllium coppers, bronzes and brasses • Precious metals • Welding, brazing and soldering alloys • Various plating alloys Materials and Processes Division ESA/ESTEC/TOS-QM

  5. Materials Properties cont.. • Fracture Toughness • The fracture toughness is a measure of the damage tolerance of a material containing initial flaws or cracks. The fracture toughness in metallic materials is described by the plain strain value of the critical stress intensity factor. The fracture toughness depends on the environment. Kic is the plain strain critical stress intensity factor. Kiscc plain strain critical stress intensity factor for a specified environment in which environmentally induced crack growth occurs. • For homogeneous materials the Kic or Kiscc shall be measured according to ECSS- Q- 70- 45. • Metallic materials for use in corrosive surface environments shall be tested for fracture toughness under representative conditions Typical subcritical crack propagation rate versus stress intensity relationship. Stress intensity K, is defined as K=A(C/B), where  is the total tensile stress, C is the crack length, and A and B are geometrical constants Materials and Processes Division ESA/ESTEC/TOS-QM

  6. Materials Properties cont.. • Coefficient of thermal expansion and coefficient of moisture expansion • The difference in thermal or moisture expansion between members of a construction or between the constituents of a composite or a coated material can induce large stresses or strains and can eventually lead to failures. • The thermal mismatch between members shall be minimised to such a degree that stresses generated in the experienced temperature domain are acceptable. • The coefficient of thermal expansion (CTE) of composite materials intended for high stability structural applications shall systematically be determined by means of dry test coupons and dry test conditions. • For hygroscopic materials intended for high stability structural applications,the coefficient of moisture expansion (CME) shall systematically be determined. • For composite materials a sensitivity analysis shall be performed in relation with the inaccuracies due to the manufacturing process. Materials and Processes Division ESA/ESTEC/TOS-QM

  7. Materials Properties cont.. • Mechanical contact surface effects • When clean surfaces are placed in contact they do not touch over the whole of their apparent area. The load is supported by surface irregularities and the following interactions can occur: • elastic or plastic deformation • adhesion • material transfer and removal • heat transfer • chemical reaction • transformation of kinetic energy into heat energy • diffusion/localised melting • For very clean surfaces strong adhesion occurs at regions of real contact, a part of which may be due to cold-welding • The friction behaviour of polymers differs from that of metals. The surfaces left in contact under load may creep and high local temperatures can be generated by frictional heating at regions of real contact. • Wear is the progressive loss of material from the operating surface of a body occurring as a result of relative motion at the surface. Wear is usually detrimental, but in mild form may be beneficial, e.g., during the running-in of engineering surfaces. • The major types of wear are abrasive wear, adhesive wear, erosive wear, rolling wear and fretting. Materials and Processes Division ESA/ESTEC/TOS-QM

  8. Materials Properties cont.. • Stress Corrosion Cracking • Stress corrosion maybe defined as the combined action of sustained tensile stress and corrosion that may lead to the premature failure of materials. Certain materials are more susceptible than others. If a susceptible material is placed in a corrosive environment under tension of sufficient magnitude, and the duration of service is sufficient to permit the initiation and growth of cracks, failure will occur at a stress lower than that which the material would normally be expected to withstand. The corrosive environment need not be severe in terms of general corrosive attack. • Service failures due to stress corrosion are frequently encountered in cases where the surfaces of the failed parts are not visibly corroded in a general sense. • If failure is to be avoided, the total tensile strength in service must be maintained at a safe level. There is no absolute threshold stress for stress corrosion, but comparative stress-corrosion thresholds can be determined for materials subjected to controlled conditions of test. Estimates of the stress-corrosion threshold for a specific service application must be determined for each alloy and heat treatment, using a test piece, stressing procedure and corrosive environment that are appropriate for the intended service. Materials and Processes Division ESA/ESTEC/TOS-QM

  9. Materials Properties cont.. • Stress corrosion • Stress Corrosion Cracking (SCC), defined as the combined action of a sustained tensile stress and corrosion, can cause the premature failure of metals. The metallic components proposed for use in most spacecrafts must be screened to prevent failures resulting from SCC. • Only those products found to possess a high resistance to stress-corrosion cracking may have unrestricted usage in structural applications. • Materials intended for structural applications and likely to be exposed to a long-term terrestrial storage or flown on the Space Transportation System, fracture critical items, all parts used or associated with the fabrication of launch vehicles shall possess a high resistance to stress-corrosion cracking. • Structural products of a metallic nature shall be selected from the preferred list in Table I of ECSS- Q- 70- 36. Materials and Processes Division ESA/ESTEC/TOS-QM

  10. Materials Properties cont.. Specimen orientation and fracture plane identification. L, length, longitudinal, principal direction of metal working (rolling, extrusion, axis of forging); T, width, long‑transverse grain direction; S, thickness, short‑transverse grain direction; C, chord of cylindrical cross section; R, radius of cylindrical cross section. First letter: normal to the fracture plane (loading direction); second letter: direction of crack propagation in fracture plane. Materials and Processes Division ESA/ESTEC/TOS-QM

  11. Materials Properties cont.. • SCC table I • Materials that testing and experience have shown to possess high resistance to stress- corrosion cracking. Their use is given reference. • SCC table II • Alloys and tempers listed in table II are moderately resistant to stress- corrosion cracking. They should be considered for use only in cases where a suitable alloy cannot be found in Table I. • SCC table III • Materials listed in table III have found to be highly susceptible to stress- corrosion cracking. They should be considered for use only in applications where it can be demonstrated conclusively that the probability of stress corrosion is remote because of low sustained tensile stress (whatever its origin) in critical grain directions, suitable protective measures or an innocuous environment. Materials and Processes Division ESA/ESTEC/TOS-QM

  12. Constraints on materials • Temperature • Vacuum • Thermal cycling • Chemical (corrosion) • Galvanic compatibility • Atomic oxygen • Moisture absorption/desorption • Fluid compatibility Materials and Processes Division ESA/ESTEC/TOS-QM

  13. Constraints on materials, cont.. • Temperature • The range of temperatures experienced will play a large part in the materials selection. Extremes are illustrated by the examples of cryogenic tanks and thermal protection systems for re-entry applications. Temperatures below room temperature generally cause an increase in strength properties, however the ductility decreases. Ductility and strength may increase or decrease at temperatures above room temperature. This change depends on many factors, such as temperature and time of exposure. • Materials shall be compatible with the thermal environment to which they are exposed. • Passage through transition temperatures (e.g., brittle-ductile transitions or glass transition temperatures including the effects of moisture or other phase transitions) shall be taken into account. Materials and Processes Division ESA/ESTEC/TOS-QM

  14. Constraints on materials cont.. • Thermal cycling • Thermal cycling can induce thermal stresses and due to the difference in coefficient of thermal expansion between fibres and matrix for composites and between base metal and coating micro-cracks can form which could jeopardise long-term properties. • Materials subject to thermal cycling shall be assessed to ensure their capability to withstand the induced thermal stresses and shall be tested according to ECSS- Q- 70- 04. • Chemical (corrosion) • The chemical environment to which a material is subjected in its life span may cause changes in the material properties. Corrosion is the reaction of the engineering material with its environment with a consequent deterioration in properties of the material. Corrosion will include the reaction of metals, glasses, ionic solids, polymeric solids and composites with environments that embrace liquid metal, gases, non-aqueous electrolytes and other non-aqueous solutions, coating systems and adhesion systems. Materials and Processes Division ESA/ESTEC/TOS-QM

  15. Constraints on materials cont.. • Galvanic compatibility • If two or more dissimilar materials are in direct electrical contact in a corrosive solution or atmosphere galvanic corrosion might occur. The less resistant material becomes the anode and the more resistant the cathode. The cathodic material corrodes very little or not at all, while the corrosion of the anodic material is greatly enhanced. • Material compatibilities shall be selected in accordance with ECSS- Q-70-71, • Maximum potential differences shall be in accordance with ECSS- Q-70-71, • In the construction of a satellite, two metals that form a compatible couple may have to be placed in close proximity to one another. Although this may not cause anomalies or malfunctions in the space environment, it has to be borne in mind that spacecraft often have to be stored on earth for considerable periods of time and that during storage they may inadvertently be exposed to environments where galvanic corrosion can take place. In fact, this is known to have taken place on several occasions and it is for this reason that the Agency has been studying the dangers involved. • See also paragraph about Corrosion Materials and Processes Division ESA/ESTEC/TOS-QM

  16. Constraints on materials cont.. • procedure • a simplified procedure can be used to estimate the compatibility of a bimetallic couple by taking into account the difference between the two static potentials of the materials involved. • The potentials were measured in a 3.5 % NaCl solution representing a standard corroding atmosphere • ruling • A difference in the static corrosion potential of the two metals forming a bimetallic couple of less than 0.5 V is acceptable if the item containing the couple is held in a clean-room atmosphere at all times. • If the item containing the bimetallic couple is not held in a clean-room atmosphere, the allowable difference in static corrosion potential must be less than 0.25 V. • If it is not possible to follow guidelines 1 and 2, then it will be necessary to provide for an insulation layer or special packaging.. Instrumentation setup for electrochemical experiments Materials and Processes Division ESA/ESTEC/TOS-QM

  17. Constraints on materials cont.. • Atomic oxygen • Spacecraft in low earth orbit (LEO) at altitudes of between 200 km and 700 km are exposed to a flux of atomic oxygen. The flux level varies with altitude, velocity vector and solar activity. The fluence levels vary with the duration of exposure. • Moisture absorption/desorption • The properties of composite materials are susceptible to changes induced by the take-up of moisture. Moisture absorption occurs during production of components and launch of the spacecraft, desorption occurs in the space vacuum. • Fluid compatibility • In some occasions materials are in contact with liquid oxygen, gaseous oxygen or other reactive fluids or could come into contact with such a fluid during an emergency situation. Materials and Processes Division ESA/ESTEC/TOS-QM

  18. Interfaces (surfaces, layers) • Anodising • Chemical conversion • metallic coatings (overlay and diffusion) • hard coatings • Diffusion barriers • High temperature oxidation protective coatings • Thermal barriers • Moisture barriers • coatings on CFRP Materials and Processes Division ESA/ESTEC/TOS-QM

  19. Surface finishes • The surface finish of a component can influence its mechanical and environmental durability. • Anodising (chromic, sulphuric, sulphuric+oxalic) • Anodising is an electrolytic process for thickening and stabilising oxide films on base metals and anodising grade alloys. It may be used as a pre-treatment for painting and dying or as a passivation treatment for an electro-brightened surface. • Hard anodised layers are wear resistant and durable. • Black anodising with cobalt sulphide and nickel sulphide is used for controlling the optical properties of surfaces. • High solar absorptance, high emmitnace • The anodised layer is electrically non-conductive. • The bath constituents and process conditions may vary between organisations. • Caution should be exercised in anodising very thin products such as foils • Coatings are porous and needs sealing (coating can release water in vacuum) Materials and Processes Division ESA/ESTEC/TOS-QM

  20. Surface finishes, cont.. • Chemical conversion (chromate, phosphate) - • Commercial process (alodine , irridite, etc) • Chemical conversion depends upon the absorption of a protective metal oxide film into an existing oxide film but may include non-metals in some cases. • Chromating involves the formation of a mixed metal-chromium oxide film. It has a good corrosion resistance and an excellent bond to subsequent organic coatings. • Phosphating is primarily used as an underlayer for paint finishes. It finds little application for corrosion protection. • Chemical conversion coatings can be either electrically conductive or non-conductive. • Temporary corrosion protection only • simple and cheap process • may be applied by immersion, praying, brushing, wiping or any other wetting method Materials and Processes Division ESA/ESTEC/TOS-QM

  21. Coatings, cont.. • Metallic coatings (overlay and diffusion) • Metallic overlay coatings can be applied in numerous ways to substrates (e.g. electroplating, chemical evaporation and ion sputtering). • Metallic diffusion coatings modify the composition of the surface by enrichment with Cr, Al or Si and/or formation of their stable oxides. • Cadmium and Zinc coatings shall not be used because of their high vapour pressure. • Silver, copper and osmium coatings shall not be used on external surfaces because they are sensitive to atomic oxygen. • Electroplated tin can form whiskers • noble coatings such as gold and silver should be continuous (e.g. corrosion such as red plague) • Hard coatings • Hard coatings are used to improve the abrasive properties of the surface. Also, the ability to cold weld is greatly reduced. The combination of a hard coating and a soft substrate is not desirable. The coating can break under pressure. • Diffusion barriers • High temperature service operation can result in compositional changes of the bulk material and of the coating due to diffusion. These compositional changes can result for example in formation of intermetallic compounds, which are brittle and can break under cyclic stresses. • E.G. Cu or Ni layer as diffusion barrier between a brass (Cu-Zn) substrate and a Sn/Sn-Pb coating Materials and Processes Division ESA/ESTEC/TOS-QM

  22. Coatings, cont..evaporation Materials and Processes Division ESA/ESTEC/TOS-QM

  23. Coating, cont..Wear Materials and Processes Division ESA/ESTEC/TOS-QM

  24. Coating, cont.. • Moisture barriers • Coatings can be used to prevent moisture absorption or desorption of dimensionally stable structures or to prevent the release of organic volatiles which could affect the performances of some equipments. • Coatings on CFRP • Coatings on CFRP are used as moisture stoppers, as protection against atomic oxygen or for adjusting optical properties. In most cases these coatings are metallic. In this dissimilar material contact the CFRP usually behaves as the cathode and as such can corrodes the coating material. • High Temp oxidation protective coatings (metallic) • Protective coatings are important in high temperature applications, such as re-entry surfaces and propulsion systems. Oxidation protection, thermal shock behaviour and erosion properties are properties to consider. • Thermal barriers (ceramic) • Thermal barrier coatings are used to retard component overheating due to high heat fluxes. These coatings are essentially ceramic overlay coatings, based on zirconia, where the coating thickness is approximately 0.4 mm. They are applied to selected regions only. The coating process can modify the condition of the substrate. Materials and Processes Division ESA/ESTEC/TOS-QM

  25. Coating, cont.. • Depending on the design and service, the following coatings are used: • Paints • phosphate coatings • chromate coatings • electroless nickel • chromium, electroplated • oxides and black oxide • nickel/tin, electroplated • tin/lead • electroless nickel plus electroplated chromium • vapour-deposited aluminium • metals, such as gold, silver, palladium, rodium • lubrication coatings such as braycoat grease • molybdenum disulfide • carbides and nitrides or combination Materials and Processes Division ESA/ESTEC/TOS-QM

  26. Joining • Mechanical fastening • Bonding • Combined bonding and fastening • Fusion, including welding, brazing, soldering and diffusion bonding Materials and Processes Division ESA/ESTEC/TOS-QM

  27. Joining cont.. • Bolted joints • Bolts offer the greatest strength for mechanical fastened joints. Unless overtightened, no damage is done to the structure during assembly. • Threaded fasteners shall conform to the minimum requirements of ECSS- E- 30- 07. • Riveted joints • Riveted joints are permanent. Disassembly requires removal of rivets by drilling out. Riveted joints shall not be used where access to internal or adjacent parts of the structure is either needed or expected. • Inserts • An insert system consists of a removable threaded fastener and a fixture embedded into the honeycomb structure using a potting mass. • All inserts shall be surface protected to avoid corrosion • Adhesive bonding • The adhesives must attach the facings rigidly to the core to allow loads to be transmitted from one facing to the other. Guidelines for adhesive bonding are found in ECSS- E- 30- 05. • Adhesives for load carrying structures shall have high strength and modulus. In addition, good toughness and peel strength are important. Materials and Processes Division ESA/ESTEC/TOS-QM

  28. Joining cont.. • Fusion techniques can be grouped as: • soldering • brazing • welding, including diffusion bonding • All fusion methods produce permanent joints. Soldered, and some brazed joints may be disassembled with care. • All fusion techniques require extensive quality control and inspection procedures. Materials and Processes Division ESA/ESTEC/TOS-QM

  29. Joining cont.. • Soldering • Soldering is commonly referred to as soft soldering in which low melting point alloys such as Tin-Lead or Indium based alloys are used. Soldered joints are used for electrical and thermal conducting paths and for low mechanical strength joints. • Soldering shall not be used for structural applications unless reviewed and approved by the Customer. • See also next session • Brazing • Brazing is preferred to soldering in those cases where a strong joint, which is heat resistant is required. As distinct from soft soldering brazing usually refers to joining with alloys of copper, silver and zinc • Welding • Numerous welding techniques are available. In the aerospace industry the following techniques are often used: • Tungsten Inert Gas (TIG) welding • Plasma-Arc welding • Electron Beam (EB) welding , laser welding • Resistance welding (induction, spot, seam welding), Diffusion welding Materials and Processes Division ESA/ESTEC/TOS-QM

  30. Corrosion Testing • General Corrosion • Stress Corrosion • Bimetallic Corrosion • Atomic Oxygen Corrosion • Red Plague Corrosion Materials and Processes Division ESA/ESTEC/TOS-QM

  31. Corrosion-General • Four systems for corrosion testing are available, each covering a different area of corrosion evaluation • Corrosion Units: • humidity test chamber, testing according to ASTM • salt spray test chamber, testing according to ASTM-B117 • stress corrosion test rig, testing according to ASTM-G44 • bimetallic corrosion set-up • atomic oxygen corrosion • red plague corrosion test unit, testing according to ESA PSS-01-720 Materials and Processes Division ESA/ESTEC/TOS-QM

  32. General corrosion testing • Salt-spray-(foreground) and Humidity Chamber- both have temperature and humidity regulators Materials and Processes Division ESA/ESTEC/TOS-QM

  33. Stress corrosion testing • Test setup for the alternate immersion testing of specimens in a 3.5% NaCl solution Al-Li stress corrosion failure Testing acc. to ESA PSS-01-737 Materials and Processes Division ESA/ESTEC/TOS-QM

  34. Bimetallic corrosion testing • The bimetallic corrosion between two materials can tested in specific liquids and under any humidity levels. • The current and voltage difference as well as the EMK’s vs Calomel as recorded Materials and Processes Division ESA/ESTEC/TOS-QM

  35. Atomic Oxygen Corrosion • Materials for use in space applications in low earth orbit and exposed to the hostile combination of atomic oxygen and thermal cycling have to be screened for their susceptibility to withstand this environment over very long periods Materials and Processes Division ESA/ESTEC/TOS-QM

  36. Red plague testing • Red plague testing (Anthony and Brown test) is performed according to ESA PSS-01-720 • Test conditions: 240 hrs at 58 oC Materials and Processes Division ESA/ESTEC/TOS-QM

  37. Mechanical Testing • Hardness testing • Tensile Testing • Compression testing • Fatigue testing • Fracture Toughness • Thermal cycling Materials and Processes Division ESA/ESTEC/TOS-QM

  38. Mechanical Testing cont.. • Load cells may be changed so that full scale chart readings of between 2N and 100kN (maximum capacity) can be achieved in both tension and compression. This equipment will investigate the mechanical properties of all materials at temperatures ranging from -150oC to +300oC • General view of Instron tensile test equipment with custom built environmental chamber, chart recorder and control panel Materials and Processes Division ESA/ESTEC/TOS-QM

  39. Mechanical Testing cont.. • This equipment is more particularly dedicated to the testing of thermo-structural materials such as CMC materials. It has been set up for measuring tensile properties from ambient temperature up to 1600oC under air. Such a test is recognised to be the most important for evaluating mechanical properties of CMC. • A ceramic matrix composite specimen is clamped into self-aligning water cooled grips. It passes through a specially designed furnace able to heat its calibrated section to 1600oC. A high temperature capacitive extensometer made of ceramics probes permit strain measurements. Materials and Processes Division ESA/ESTEC/TOS-QM

  40. Mechanical Testing cont.. • Two units, specially designed according to to specification ESA PSS-01-704, operate under vacuum. Two other units operate under atmospheric pressure. The heating and cooling is provided by nitrogen gas. • One of the two systems for thermal cycling under vacuum (VMDI is shown). The system is provided with viewing ports and electric feed through connection. Materials and Processes Division ESA/ESTEC/TOS-QM

  41. Macro/Microscopic Examination • Macroscopic (< X75) • Light microscopy • Confocal microscopy • Scanning Electron Microscopy Materials and Processes Division ESA/ESTEC/TOS-QM

  42. Macroscopic Examination • Magnification < x75 • Visual Inspection • Stereozoom • Scanning Electron Microscopy at low magnification • Photography • The diversity of samples submitted for failure analysis is illustrated by this photograph, which features part of a defective spacecraft antenna, a nickel cadmium battery cell, detector electrodes, printed circuit boards, fracture toughness specimen, part of a dip brazed waveguide, sections through a solar army hydrogen embrittled springs and a composite structure designed to withstand the impact of particles travelling at 60 km/s. Materials and Processes Division ESA/ESTEC/TOS-QM

  43. Microscopic Examination cont.. • Light Microscopy • metallurgical microscopy • Optical Microscopes Reichert MeF2: magn. X1 to X1500, It includes: interferometer, polarised light illumination, micro-hardness tester Reichert MeF3a: magn. X1 to 1500 It includes: polarised light illumination, dark and bright field illumination, connection to Clemex Image analyser Materials and Processes Division ESA/ESTEC/TOS-QM

  44. Microscopic Examination cont.. • Confocal microscopy • Confocal microscopy allows the us to obtain depth-selective information on the three-dimensionial structure of a microscopic object. • LEICA TCS NT with microscope DM-RXE and galvanometer driven Z-stage. • The x/y resolution is 0.18 m (FWHM) and a corresponding z-resolution of better than 0.35 m (FWHM) at 488 nm Materials and Processes Division ESA/ESTEC/TOS-QM

  45. Microscopic Examination cont.. • In many situations even greater magnifications are useful. This capability is provided by two Scanning Electron Microscopes (SEM) which can magnify to X200 000 with good resolution. (JEOL T300 and Cambridge S360). The great depth of field of these instruments and the large specimen chamber of one of these SEM's has made them particularly useful in examining rough fracture surfaces to determine the cause of a component failure. • View on Scanning Electron Microscope Cambridge S360 Materials and Processes Division ESA/ESTEC/TOS-QM

  46. Non-Destructive Examination • Scanning Laser Acoustic Microscopy (SLAM) • C- Mode Scanning Acoustic Microscopy (C-SAM) • Radiography (in TOS-QC) Materials and Processes Division ESA/ESTEC/TOS-QM

  47. Non-Destructive Examination cont.. • The Scanning Laser Acoustic Microscopy (SLAM) produces images of the internal structure of metals, ceramics and even biological tissues, and these are displayed in real time on a TV monitor. • The SLAM uses very high frequency ultrasound in a transmission mode to produce images. Acoustic waves are transmitted into the bottom face of the cell sample by means of a distilled-water coupling medium. Using this instrument it was possible to assess the quality of solar cell interconnectors for the Space Telescope. Materials and Processes Division ESA/ESTEC/TOS-QM

  48. Failure Analysis • Under control of Material Review Board (MRB) • Collection of background data • visual examination • selection, identification, preservation of specimens • macroscopic examination and analysis • microscopic examination and analysis • metallographic sections examination • failure mechanism • chemical analysis • simulated testing • report Materials and Processes Division ESA/ESTEC/TOS-QM

  49. Materials Properties annex cont.. • Corrosion fatigue • Corrosion fatigue indicates crack formation and propagation caused by the effect of alternating loading in the presence of a corrosion process. Because of the time dependence of corrosion, the number of cycles to failure depends on frequency. Since chemical attack requires time to take effect, its influence is greater as the frequency becomes lower. No metals or alloys demonstrate complete resistance to corrosion fatigue. • Same requirement as with fatigue • Hydrogen embrittlement • Metals can be embrittled by absorbed hydrogen to such a degree that the application of the smallest tensile stress can cause the formation of cracking. The following are possible sources of hydrogen: thermal dissociation of water in metallurgical processes (e.g., casting and welding), decomposition of gases, pickling, corrosion, galvanic processes (e. g. plating) and ion bombardment. • The possibility of hydrogen embrittlement during component manufacture and/or use shall be assessed Materials and Processes Division ESA/ESTEC/TOS-QM

  50. Materials Properties annex cont.. • Strength • Spacecraft design covers the survival of the structure under the worst feasible combination of mechanical and thermal loads. All events of the complete lifetime of the spacecraft are addressed by this design. The strength of a material is highly dependant on the direction as well as on the sign of the applied load, e.g., axial tensile, transverse compressive, and others. • positive margin of safety and shall include, if applicable, yield load analysis, ultimate load analysis and bucking load analysis • Elastic modulus • The elastic modulus defined as the ratio between the uniaxial stress and the strain (e.g., Young’s modulus, compressive modulus, shear modulus) is for metals and alloys weakly dependant on heat-treatment and orientation. However, for fibre reinforced materials, the elastic modulus depends on the fibre orientation. • For composites the required elastic modulus shall be verified. Materials and Processes Division ESA/ESTEC/TOS-QM