3.2Definitions

A-Basis: A statistically calculated number that at least 99 percent of the population of values is expected to equal or exceed with a confidence of 95 percent.1

Adhesive Bond (Bond): The joining of parts, components, or materials using a joining substance or agent.

Assembly/Assemblage: An integral arrangement of parts that makes up an individual unit and that acts as a whole.

B-Basis: A statistically calculated value that at least 90 percent of the population is expected to equal or exceed with a confidence of 95 percent.2

Bond: The joining of two parts through molecular attraction or through any non-mechanical means of connection.

Bonded Hardware (Structure): Hardware (structure) that is assembled using parts that are joined together with an adhesive.

Brittle Fracture: Sudden rapid fracture under stress (residual or applied) where the material exhibits little or no evidence of ductility or plastic deformation.

Building Block Approach (BBA): A development methodology often used with composites or bonded hardware that (a) starts with selecting the material system and manufacturing approach; (b) moves on to experimentation and analysis of small samples to characterize the system and quantify behavior in the presence of flaws and damage; (c) progresses to examining larger structures to examine buckling behavior, combined loadings, and built-up structures in the presence of credible damage; and (d) finally moves to complicated subcomponents and full-scale components to establish their damage tolerance strength and life. Each step along the way is supported by detailed analysis to validate that the behavior of these structures is well understood and predictable.

Catastrophic Event: Loss of life, disabling injury, or loss of a major national asset.

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Catastrophic Failure: A failure that directly results in a catastrophic event.

Catastrophic Hazard: Presence of a risk situation that could directly result in a catastrophic event.

Component: A hardware unit considered a single entity for the purpose of fracture control. A component contains at least one part.

Composite or Bonded Structure: Structure (excluding overwrapped pressure vessels or pressurized components) of fiber/matrix configuration and structure with load-carrying non-metallic bonding agents, such as sandwich structure or bonded structural fittings.

Composite Material: A combination of materials differing in composition or form on a macro scale. The constituents retain their identities in the composite; that is, they do not dissolve or otherwise merge completely into each other, although they act in concert. Normally, the constituents can be physically identified and exhibit an interface between one another. Composite material is not intended to mean an assembly of parts.

Composite Hardware (Structure): Hardware (structure) assembled with parts made from composite materials.

Composite Overwrapped Pressure Vessel: A pressure vessel with a composite structure fully or partially encapsulating a metallic liner. The liner serves as a fluid (gas and/or liquid) permeation barrier and may carry substantial pressure loads. The composite generally carries pressure and environmental loads.

Contained: A condition in which a suitable housing, container, barrier, restraint, etc., prevents a part or pieces thereof from becoming free bodies if the part or its supports fail.

Contamination: Any material included within or on the hardwate that is not called for on the engineering drawings. Examples of contamination are dust, grease, solvent, solid objects, etc.

Crack or Crack-like Defect: A discontinuity assumed to behave like a crack for fracture control purposes. Critical Stress Intensity Factor: The stress intensity factor at the initiation of crack growth in the part resulting in a catastrophic failure that is representative of the failure mode of concern for the metallic material process condition, weakest orientation, and thickness being evaluated. Examples for metallic materials may include: K_IEAC, the stress intensity factor threshold for plane strain environment-assisted cracking; plane strain fracture toughness (K_Ic) may be appropriate for thick sections and/or as a lower bound value³; effective fracture toughness (K_Ie) is used in NASGRO® for crack growth analyses of surface or elliptical flaws; K_JIc calculated from J_Ic or a K_c calculated from J_c may be appropriate for the conditions described in the defining standard

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(ASTM E1820, Standard Test Method for Measurement of Fracture Toughness) such as evaluation of ductile tearing and instability; constraint-based assessments (ASTM E2899, Standard Test Method for Measurement of Initiation Toughness on Surface Cracks Under Tension and Bending), and/or tests may be needed for surface or other complex cracks in materials or conditions that invalidate the ability of Linear-Elastic Fracture Mechanics (LEFM) to represent crack growth.

Damage: See definitions of Flaw and Impact Damage.

Damage Threat Assessment (DTA): An evaluation of potential sources of flaws in composite or bonded hardware that includes definition, quantification, and an assessment of the residual strength sensitivity to flaws.

Damage Tolerance: Fracture control design concept under which an undetected flaw or damage (consistent in size with the flaw screening method or residual threat determination (RTD)) is assumed to exist and is shown by fracture mechanics analysis or test not to grow to failure (leak or instability) during the period equal to the service life factor times the service life.

Design Limit Load (DLL): See definition of Limit Load.

Design Ultimate Load (DUL): Limit load multiplied by the ultimate factor of safety.

Environmental Correction Factor (ECF): An adjustment factor used to account for differences between the environment (thermal and chemical) in which a part is used and the environment in which it is tested.

Environmentally Assisted Cracking (EAC): A cracking process in which the environment promotes crack growth or higher crack growth rates than would occur without the presence of the environment (ASTM E1681, Standard Test Method for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metallic Materials). An example is available in published literature (Lewis and Kenny, 1976).

Experiment: For fracture control, an arrangement or assemblage of hardware that is intended to investigate phenomena on a provisional, often human-tended, basis.

Fail-safe: A condition where a redundant load path exists within a part (or hardware), so that after loss of any single individual load path, the remaining load path(s) has sufficient structural capability to withstand the redistributed loads, and the loss of the load path will not cause a catastrophic hazard.

Fastener: For fracture control, any single part that joins other structural elements and transfers loads from one element to another across a joint.

Flaw: For metallics, glass, or brittle materials, a crack-like defect. For composite or bonded materials, an anomaly in the hardware that has the potential for adversely affecting strength,

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damage tolerance life, or must-work function. Examples of flaws in metallics include cracks, deep scratches and sharp notches that behave like cracks, material inclusions, forging laps, welding incomplete fusion, penetration, and slag or porosity with a crack-like tail. Examples of flaws in composite or bonded materials may include cracks, cuts, scratches, delaminations, porosity/voids, disbonds, wrinkles, foreign object debris, impact damage, etc. Damage (used alone) and flaw are equivalent.

Fleet Leader: Articles representative of spaceflight hardware with respect to production methods, e.g., materials, manufacturing, testing, that either have accumulated (or are scheduled to accumulate) more service lifetime in typical (or more severe) environments than the rest of the fleet and are monitored for indications of failure modes to provide early warning of known and unexpected risks to the rest of the fleet.

Flight (Spaceflight) Hardware: Any hardware (including spares) that is approved to be part of or carried by a launch vehicle, crew module, transfer stage, landing craft, payload, etc.

Flight-like Component: A component assembled and made of parts that are of flight specifications. Flight-like components are usually intended for qualification tests. Any deviations from flight have to be insignificant with respect to test objectives.

Fracture Critical: Fracture control classification that identifies a part whose individual failure, caused by the presence of a crack, is a catastrophic hazard and that requires safe-life analysis or other fracture control assessment to be shown acceptable for flight. A part is fracture critical unless it can be shown that there is no credible possibility for a flaw to cause failure during its lifetime or the part failure does not result in a credible catastrophic hazard. Assessments for fracture critical parts include damage tolerance analysis, damage tolerance test, or defined approaches for specific categories. Parts under this classification receive flaw screening by NDE, proof test, or process control and are subjected to traceability, materials selection and usage, documentation, and engineering drawing requirements.

Habitable Modules or Volumes: Flight containers/chambers that are designated and designed to support human occupancy.

Hardware Developer: Organization directly responsible for doing the design, manufacture, analysis, test, and safety compliance documentation of the hardwate. This includes implementing fracture control requirements.

Hazardous Fluid: For fracture control, a fluid the release of which would create a catastrophic hazard. These types of fluids may include liquid chemical propellants, liquid metals, biohazards, and other highly toxic liquids or gases. The release of such fluids would create a hazardous environment, such as a danger of fire or explosion, unacceptable dilution of breathing oxygen, an increase of oxygen above flammability limits, over-pressurization of a compartment, or loss of a safety-critical system.

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Hazardous Fluid Container: Any single, independent (not part of a pressurized system) container or housing that contains a fluid the release of which would cause a catastrophic hazard and that is not classified as a pressure vessel.

Hazardous Material: For fracture control, a material the release of which would create a catastrophic hazard.

High-Cycle Fatigue (HCF): A high-frequency, low-amplitude loading condition created by structural, acoustic, or aerodynamic vibrations that can propagate flaws to failure. An example of an HCF loading condition is the vibrational loading of a turbine blade because of structural resonance.

Impact Damage: The injury or harm inflicted upon composite or bonded hardware by impingement of an object upon the hardware in question or the bumping or striking between the hardware in question and another object. Impact damage is a subset of the more general term damage (or flaw).

Impact Damage Mitigation Plan (IDMP): A plan for composite or bonded hardware to mitigate risk of impact damage to the flight hardware.

Initial Crack (Flaw) Size: The crack size that is assumed to exist at the beginning of a damage tolerance analysis, as determined by NDE or proof testing.

Kc: Plane stress fracture toughness. The value of stress intensity factor K at the tangency between a crack extension resistance curve (R-curve) and the configuration-dependent applied K curve (ASTM E1823, Standard Terminology Relating to Fatigue and Fracture Testing). This crack extension occurs under conditions that do not approach crack-tip plane strain. The R-curve and Kc vary with the material, specimen size, and thickness. Kc is used in NASGRO® to represent fracture toughness as a function of thickness for use in crack growth calculations.4

$\mathbf{K}_{\mathrm{Ic}}$ : Plane strain fracture toughness. The crack extension resistance under conditions of crack-tip plane strain in Mode I for slow rates of loading under predominantly linear-elastic conditions and negligible plastic-zone adjustment that is measured by satisfying a standardized procedure with validity requirements (ASTM E399, Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness $\mathrm{K}_{\mathrm{Ic}}$ of Metallic Materials). Another quantity, $\mathrm{K}_{\mathrm{J} \mathrm{Ic}}$, defined for conditions with limited plasticity from $\mathrm{J}_{\mathrm{Ic}}$ may also be useful (ASTM E1820).

$\mathbf{K}_{\mathrm{Ie}}$ : Effective fracture toughness for a surface or elliptically shaped crack. The toughness is based on residual strength and the original crack dimensions. This parameter is meaningful only when crack-tip plastic zones are small and stable crack growth before failure is generally absent (ASTM E740/E740M, Standard Practice for Fracture Testing with Surface-Crack Tension Specimens, main body and section X1.2). For conditions with plastic effects and well-defined

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crack-tip stress fields with fracture controlled by crack initiation, an approach involving constraint may be applicable (ASTM E2899). Testing of flaws in specimens representative of the structure is needed to determine damage tolerance for plasticity conditions when crack-tip stress fields collapse. $\mathrm{K}_{\mathrm{Ie}}$ is used in NASGRO® for analyses of crack growth. ${ }^{5}$

K EAC: The largest value of the stress intensity factor at which crack growth is not observed for a pre-cracked through-crack specimen of specified material, environment, and thickness that is tested for a significant duration in accordance with ASTM E1681.

KIEAC: The largest value of the stress intensity factor at which crack growth is not observed for a pre-cracked through-crack specimen of specified material, environment, and thickness that is sufficient to meet requirements for plane strain and is tested for a significant duration in accordance with ASTM E1681.

$\mathrm{K}_{\mathrm{Iscc}}: \mathrm{K}_{\mathrm{EAC}}$ is often denoted as $\mathrm{K}_{\mathrm{Iscc}}$ in the literature.

$\Delta \mathrm{K}_{\mathrm{th}}$ : Threshold stress intensity factor range below which flaw growth will not occur under cyclic loading conditions.

Leak-Before-Burst (LBB): Characteristic of pressurized hardware whose only credible failure mode at or below maximum design pressure (MDP) with service life loads resulting from the presence of a potential flaw is a pressure-relieving leak at the flaw as opposed to burst or rupture at the critical stress intensity factor. As the hardware item leaks down, there is no re-pressurization or continued pressure cycles that could lead to continued crack growth. In this failure mode, the hardware will not fail in a fragmentary, catastrophic manner. Instead, only small, slow-growing leaks would develop, leaking in a controlled manner. Additional aspects of LBB assessments are described in section 6.2.4 in this NASA Technical Standard.

Life Factor: See definition of Service Life Factor.

Lifetime: See definition of Service Life. Refers to a specified life, as opposed to an analytically predicted life.

Limit Load: The maximum load expected on the hardware during its design service life including ground handling, transport to and from orbit, including abort conditions and on-orbit operations.

Limited Life Part: A part that has a predicted damage tolerance life that is less than the required service life factor times the complete service life. See definition of Service Life.

Load Enhancement Factor (LEF): A factor applied to the service life spectrum to satisfy a specified level of reliability and confidence with fewer cycles than would otherwise be required.

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Low-Cycle Loads: A low-frequency, high-amplitude loading condition created by thermal, pressure, or structural loads that can propagate flaws to failure. An example of a low-cycle loading condition is the aerothermal loading of a turbine blade during launch.

Low-Fracture Toughness: Material property characteristic, in the applicable environment, for which the ratio is KIcFty <1.66 √mm (0.33 √in ). For steel bolts with unknown KIc, low-fracture toughness is assumed when material A-basis ultimate strength Ftu > 1,241 MPa (180 ksi). Parts made with materials of this characteristic may be at risk of a brittle fracture.

Materials Usage Agreement (MUA): A formal document showing that a noncompliant material is acceptable for the specific application identified.

Maximum Design Pressure: The highest possible operating pressure considering maximum temperature, maximum relief pressure, maximum regulator pressure, and, where applicable, transient pressure excursions. MDP for human-rated hardware is a two-failure tolerant pressure, i.e., it will accommodate any combination of two credible failures that will affect pressure. Some programs have defined MDP as a two-fault tolerant pressure.

Mechanism: A system of moveable and stationary parts that work together as a unit to perform a mechanical function, such as latches, actuators, drive trains, and gimbals.

Mission: A major activity required to accomplish an Agency goal or to effectively pursue a scientific, technological, or engineering opportunity directly related to an Agency goal. Mission needs are independent of any particular system or technological solution (NPR 7120.5, NASA Space Flight Program and Project Management Requirements).

Net-Section Stress or Strain: The stresses or strains computed for a hypothetical cut across a part, based on strength-of-materials theory. Possible bending loads can produce stress gradients across the net section, in which case the net-section stress is found to be the maximum combination of tension and bending stress, ignoring geometric stress concentrations. (An example of net-section stress calculation detailed in the NASGRO® User’s Manual, Appendix B.)

No-Growth Threshold Strain: For a composite or bonded part, the largest strain range (where strain range is the maximum absolute value of strain in a load cycle) below which flaws compatible with the sizes established by NDE, special visual inspection, the DTA, or the minimum sizes imposed do not grow in 106 cycles (108 cycles for rotating hardware) at a load ratio appropriate to the application. Thresholds are determined on specimens with flaws for which sufficient load/cycles have been initially applied to cause flaw growth. The no-growth threshold strain is a function of the material and layup and is determined from test data in the appropriate environment for the applicable (or worst) orientation of strain and flaw for a particular design.

Non-Destructive Evaluation: Examination of parts for flaws using established and standardized inspection techniques that are harmless to hardware, such as radiography, penetrant, ultrasonic,

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magnetic particle, and eddy current. NDE is sometimes referred to as non-destructive testing (NDT) or non-destructive inspection (NDI).

Non-Hazardous-Leak-Before-Burst (NHLBB): A non-fracture critical classification for metallic pressurized hardware that contains a material that is not hazardous and that exhibits the LBB failure mode in a non-hazardous manner.

Part: Hardware item considered a single entity for the purpose of fracture control.

Pressure Vessel: A container designed primarily for pressurized storage of gases or liquids and that also performs any of the following:

• a. Contains stored energy of 19,307 J (14,240 ft-lb) or greater based on adiabatic expansion of a perfect gas.
• b. Stores a gas that will experience an MDP greater than 690 kPa (100 psia).
• c. Contains a gas or liquid in excess of 103 kPa (15 psia) that will create a catastrophic hazard if released.

Pressurized Component: A line, fitting, valve, regulator, etc., that is part of a pressurized system intended primarily to sustain a fluid pressure and fluid transfer. Any piece of hardware that is not a pressure vessel or a pressurized fluid container but is pressurized via a pressurization system.

Pressurized Fluid Container: A container designed primarily for pressurized storage of gases or liquids that is similar to a pressure vessel but does not satisfy the definition of a pressure vessel.

Pressurized Hardware: Any of the various hardware items that support an internal pressure.

Pressurized Structure: A hardware item designed to carry both internal pressure and vehicle structural load. Pressurized System: An interrelated configuration of pressurized components under positive internal pressure. The system may also include pressure vessels.

Proof Test: A test on the flight article that is performed to verify structural acceptability or to screen flaws. The proof test load and/or pressure level is the proof test factor times limit load and/or MDP. Proof tests may be conducted in the operational environment, or the test levels may be adjusted via an ECF. (Note that some sections within this NASA Technical Standard may specify when an ECF is optional versus when it is prescribed for the classification if the test is not conducted in the operational environment.)

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Proof Test Factor: A factor that is multiplied by the limit load and/or MDP to arrive at the proof test levels. When proof tests are performed to establish structural acceptability, the proof test factor is specified. When screening for flaws with a proof test, the proof test factor is derived by fracture mechanics principles.

R Ratio: The ratio of minimum stress to maximum stress in cyclic loading.

Re-flight Hardware: Hardware items that have already met the requirements in this NASA Technical Standard for service life, have flown on a flight vehicle, and are being manifested for an additional flight. Note that some fracture control categories in this NASA Technical Standard impose additional requirements that are to be satisfied before being re-flown.

Residual Strength: The maximum value of load (both externally applied and internal self-equilibrating loading, such as residual stresses) that a flawed or damaged part is capable of sustaining without catastrophic failure.6

Residual Threat Determination: An assessment that defines the worst-case credible flaw conditions that composite or bonded hardware will be designed to endure, considering all applicable flaw detection and mitigation strategies that are implemented for the flight hardware.

Responsible NASA Center: The NASA Center acting as the sponsor and/or coordinator for the program/project developing the payload/hardware.

Responsible Fracture Control Board: A designated multi-discipline group of experts at the NASA Center that has the authority to develop, interpret, and approve fracture control requirements and the responsibility for overseeing and approving the technical adequacy of all fracture control activities at the Center.

Rotating Hardware: Hardware that has a rotational mode of operation and devices with spinning parts, such as fans, centrifuges, motors, pumps, gyros, and flywheels. Rupture: An instance of breaking or bursting suddenly and completely.

Safe Life: See definition of Damage Tolerance.

Safety Critical: For fracture control, a part, component, or system whose failure or loss would be a catastrophic hazard.

Sealed Container: Any single, independent container (not part of a pressurized system), component, or housing that is sealed to maintain an internal non-hazardous environment and that does not meet the definition of a pressure vessel.

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Service Life: Time interval for a part beginning with manufacture and extending throughout all phases of its specified mission usage. The period of time or number of cycles that includes all relevant loadings, conditions, and environments encountered during this period that will affect flaw growth, including all manufacturing, testing, storage, transportation, launch, on-orbit, descent, landing, and if applicable, post-landing events, refurbishments, retesting, and repeated flights until the hardware is retired from service.

Service Life Factor: The factor on service life required in damage tolerance analysis or testing. The service life factor is often referred to as the life factor. (Note that the service life factor is specified as 4 for metallic materials in section 7.3.2.c in this NASA Technical Standard. The service life factor is specified as the B-basis number of service lives with the corresponding LEF for composites or bonded materials in sections 7.4.7.b and 7.4.8.e in this NASA Technical Standard.)

Shatterable Materials: Any material that is prone to brittle failures during operation that could release many small pieces into the surrounding environment.

Special Visual Inspection: Close proximity, intense visual examination of localized areas of internal and/or external structure for indications of impact damage, flaws, or other structural anomaly. Appropriate access to gain proximity, e.g., removal of fairings and access doors, use of ladders and work stands, is required. High-intensity lighting, along with other inspection aids such as mirrors, magnifying lenses, and surface cleaning, are used. Special visual inspections are done independently by two inspectors. When special visual indications are found, NDE is done. Standard NDE: NDE methods of metallic materials for which a statistically based flaw detection capability has been established. Standard NDE methods addressed by this document are limited to fluorescent penetrant, radiography, ultrasonic, eddy current, and magnetic particle.

Sustained Load Cracking (SLC): Growth of a pre-existing crack in susceptible metallic alloys7 under sustained stress without assistance from an external environment. A threshold stress intensity factor can be obtained by procedures such as those in ASTM E1681 for the case of an inert or vacuum environment. One publication determines the effects of hydrogen content and temperature on SLC in Ti-6Al-4V (Boyer and Spurr, 1978).

Ultimate Factor of Safety (Ultimate Safety Factor): A specified factor to be applied to limit load. No ultimate structural failure is allowed for a load equal to the ultimate factor of safety multiplied times limit load.

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Ultimate Strength (Capability): The load, stress, or strain at which collapse or rupture occurs.

Yield Strength: The stress that corresponds to a plastic axial strain of 0.002 mm/mm (0.002 in/in).