Parts in this category may be classified NFC if documented assessment cited in the FCSR shows that they do not present a credible catastrophic hazard resulting from failure of the part caused by a flaw or that they do not have a credible possibility for a flaw to cause failure of the part. Both composite and bonded hardware are to satisfy section 6.3 in addition to the items in this section.
[FCR 8] Each part classified as NFC that is not of a specific hardware type as described in section 6.1 in this NASA Technical Standard shall comply with one of the following items:
- a. NFC low-released mass complies with section 6.2.1 in this NASA Technical Standard.
- b. NFC contained complies with section 6.2.2 in this NASA Technical Standard.
- c. NFC fail-safe complies with section 6.2.3 in this NASA Technical Standard.
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- d. NFC NHLBB pressurized components comply with section 6.2.4 in this NASA Technical Standard.
- e. NFC low-risk part complies with section 6.2.5 in this NASA Technical Standard.
- f. NFC documented non-hazardous failure mode complies with section 6.2.6 in this NASA Technical Standard.
[Rationale: Parts that can be shown to have no credible catastrophic hazard resulting from a failure of the part caused by a flaw or to have no credible possibility for flaws to cause failure are not fracture critical. These parts can be classified as NFC.]
Use of an alternative approach requires unique rationale and approval by the RFCB as described in section 10 [FCR 26] in this NASA Technical Standard.
Sections 6.2.1 through 6.2.4 in this NASA Technical Standard provide approaches to establish that a part does not present a credible catastrophic hazard because of part failure. Section 6.2.5 in this NASA Technical Standard provides an approach to show that a part does not have a credible possibility for a flaw to cause failure in the part.
6.2.1 NFC Low-Released Mass
Small parts or masses that are released (because of structural failure caused by a flaw) may be designated NFC via the low-released mass category.
Satisfy all of the following items to classify a part as an NFC low-released mass to meet requirement [FCR 8] section 6.2.a in this NASA Technical Standard.
- a. The fracture of the part does not cause a catastrophic hazard.
- b. The release of the mass does not cause a catastrophic hazard.
- (1) For NFC composite or bonded parts that may be impacted by an NFC low-released mass part, establish that the impacted NFC composite or bonded parts can sustain DUL. This is verified by analysis combined with coupon or hardware element test data while subject to the worst-case impact damage from the released mass.
- (2) For fracture critical composite or bonded parts that may be impacted by an NFC low-released mass part, include the worst-case impact damage from the released mass in the DTA and RTD during evaluation of the fracture critical part, described in section 7.4 in this NASA Technical Standard.
- (3) Loss of function and impact with other hardware, equipment, spacecraft, and personnel are addressed in the evaluation.
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- (4) External released mass or parts, including those that would be subjected to aerodynamic flow, may only be classified low-released mass when the program has established an acceptable debris field criterion and the parts fall within it.
The program should provide the launch vehicle acceptable debris field criteria. The program or launch payload integrator has to address concerns of impact on adjacent payloads and other spacecraft.
6.2.2 NFC Contained
Parts that would be safely confined to an enclosed volume should they become loose because of the presence of a flaw may be designated NFC via the contained category.
Satisfy all of the following items to classify a part as NFC contained to meet requirement [FCR 8] section 6.2.b in this NASA Technical Standard:
- a. A containment assessment conservatively establishes that the contained part does not penetrate, fracture, or otherwise escape the enclosure.
- (1) Metallic containers are shown to meet the penetration criterion by a validated analysis method that includes uncertainty factors on the container thickness or by test.
- (2) Composite containers are shown to meet this criterion by establishing that the composite container can sustain DUL (verified by analysis combined with coupon or hardware element test data) with the worst-case impact damage from the released part.
- b. Release or failure of the contained part because of a flaw does not result in a catastrophic hazard.
- c. The enclosure structure and supports meet the following:
- (1) Fracture control requirements listed in this NASA Technical Standard.
- (2) Perform their intended functions if impacted by the loose part, fragments, or contents of the part.
- d. Assessment of containers with mechanically secured closures shows the design is at least one fault tolerant against release of the contents.
Consider all sources of energy available to a contained part during a containment analysis.
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If the part contains hazardous materials or fluids, to satisfy item 6.2.2.b (above), the containment assessment also establishes that no hazardous materials or part fragments are released that result in a catastrophic hazard. Also note that impact with a composite enclosure is to be considered during fracture control classification and assessment of the enclosure.
6.2.3 NFC Fail-Safe
Parts with sufficient structural redundancy that may fail because of the presence of a flaw may be designated NFC via the fail-safe category.
Satisfy all of the following items to classify a part as NFC fail-safe to meet requirement [FCR 8] section 6.2.c in this NASA Technical Standard:
- a. Documented assessment establishes that loss of any load path does not result in a catastrophic hazard and that risk of loss of the structural redundancy because of multi-site fatigue or damage of redundant load path structures from any source during the service life of the structure is not a credible concern.
- b. Failure of the part does not generate pieces or debris that would violate the NFC low-released mass requirements in section 6.2.1 in this NASA Technical Standard.
- c. After the loss of any load path, there is sufficient remaining structural capability to safely sustain all resulting redistributed loads and environments (including dynamic response changes) until termination of the mission or until such time as the part is inspected and refurbished.
- (1) For NFC composite or bonded parts that may be impacted by an NFC fail-safe part, establish that the impacted NFC composite or bonded parts can sustain DUL verified by analysis combined with coupon or hardware element test data, while being subjected to the worst-case impact damage from the NFC fail-safe part.
- (2) For any remaining NFC composite or bonded structure of this fail-safe part, establish that the remaining structure can sustain DLL. This is verified by analysis combined with coupon or hardware element test data with the worst-case impact damage from the NFC fail-safe part.
- (3) For fracture critical composite or bonded parts that may be impacted by an NFC fail-safe part, include the worst-case impact damage caused by the NFC fail-safe part in the DTA and RTD during evaluation of the fracture critical parts as described in section 7.4 in this NASA Technical Standard.
For metallic structures, verification by analysis is sufficient.
- d. Re-flight hardware is verified by visual inspection or other means to be intact and free of structural anomalies before being re-flown.
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The possible consequences of the release of redundant parts need to be assessed. Failure of a redundant part may create impact with an adjacent part and is to be considered during fracture control classification and assessment of the adjacent part.
6.2.4 NFC NHLBB Pressurized Components
This classification is intended for metallic pressure-bearing walls of containers, trapped volumes, lines, fittings, valves, regulators, filters, bellows, or other pressurized hardware that transfer non-hazardous fluid under pressure and that would leak down in the presence of a flaw rather than burst when used as intended. Typically, these parts are produced under process control in large quantities, are identical parts, and are subjected to NDE and qualification testing to ensure the parts are reliable and present a low risk of containing detectable flaws that result in crack growth related to environmental, loading, or other conditions. Also, this classification is intended for hardware designed to carry primarily pressure loads. This hardware is usually designed with appropriate supports, brackets, or relief loops such that the hardware is not subject to significant structural loads. In this classification, the leakage of the fluid is not allowed to create a catastrophic hazard. This section does not apply to the hardware types addressed in section 7.2 in this NASA Technical Standard.
Satisfy all of the following items to classify a part as an NFC NHLBB component to meet requirement [FCR 8] section 6.2.d in this NASA Technical Standard:
- a. The pressurized item satisfies the LBB definition in this document at MDP.
- b. The leak does not cause a catastrophic hazard nor release hazardous fluid.
- c. As the hardware item leaks down, there is no repressurization or continued pressure cycles that could lead to continued fatigue or crack growth related to EAC or SLC.
- d. The hardware is manufactured from metal alloys that are not susceptible to crack growth related to EAC or SLC in the applicable environment and that are typically used for pressurized systems, using processes that have been established by reliability or inspections of many similar parts to be extremely unlikely to produce parts with a flaw exceeding process specifications.
- e. Associated structure supporting the pressurized hardware also meets fracture control requirements.
- f. Hardware does not have an impervious barrier, coating, etc., on either the interior or exterior surfaces that inhibits leakage.
- g. Re-flight hardware is inspected for leaks before repressurization and/or before being re-flown.
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Note that leaking hardware may present unacceptable impacts on program mission success. Catastrophic hazards for LBB assessment include unacceptable dilution or toxicity of breathing environment, increases in oxygen or flammable material beyond flammability limits, or loss of a safety-critical function.
When LEFM is applicable, an acceptable approach to LBB for metallic alloys is to show by analysis that a worst-case surface crack will grow into a through-the-thickness crack without unstable crack propagation. This presumes the hardware manufacturing process has no credible risk of producing initial flaws longer than the crack, and leakage through the crack is shown to reduce pressure before loadings could grow the crack to cause fracture. The analysis, taking into account applied loads and residual stress effects, shows that the crack will leak and not be unstable. Additional guidance on analysis and leakage is available in API 579-1/ASME FFS-1, Fitness-for-Service.
6.2.5 NFC Low-Risk Parts
The low-risk classification is intended for parts that are extremely unlikely to contain or develop critical flaws because of (1) extremely low likelihood of flaws being induced by manufacturing processes, environmental effects, or service events and (2) large structural margins. Additively manufactured components do not qualify as low-risk parts according to NASA-STD-6030 AMR-15.
Satisfy all of the items in section 6.2.5.a for metallic parts or all of the items in section 6.2.5.b for composite or bonded hardware to classify a part as an NFC low-risk part to meet [FCR 8] section 6.2.e in this NASA Technical Standard:
- a. Metallic parts are classified as low risk, provided the documented assessment shows they meet the following:
- (1) The part is manufactured from materials with well-characterized strength and ductility properties using processes that have been established by inspections to be extremely unlikely to produce parts with flaws and that have been shown not to fail because of brittle fracture.
- (2) Metallic parts have a material property ratio of KIc/Fty ≥ 1.66 √mm (0.33 √in) and do not have sensitivity to EAC, SLC, or stress corrosion cracking as defined in NASA-STD-6016.
- (3) Aluminum parts are not loaded in the short transverse direction if this dimension (from the raw stock part) is greater than 7.62 cm (3 in).
- (4) Parts have total net-section stresses, e.g., maximum principal or von Mises, whichever is larger, at limit load that are less than 30 percent of the ultimate strength.
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- (5) One of the following is satisfied:
- A. Perform a fatigue analysis that results in a minimum service life factor of 4 with a factor of 1.5 on local cyclic stresses.
For metallic parts addressed in 6.2.5.a.5.A in this NASA Technical Standard, the part should meet conventional fatigue, accounting for notch and mean stresses, with 4 lifetimes and 1.5 on alternating stress.
- B. Perform a damage tolerance analysis that results in a minimum of 4 complete service lives with a factor of 1.5 on alternating stress using a 0.127-mm (0.005-in) initial crack that conservatively accounts for the effects of notches and mean stress.
- b. Composite or bonded hardware is classified as low risk, provided the documented assessment shows it meets the following, based on the flaws identified by the RTD performed in accordance with section 7.4.3 in this NASA Technical Standard:
- (1) The part residual strength with the largest RTD flaw can sustain DUL verified by analysis combined with coupon or hardware element test data.
- (2) The part limit strain with the RTD established flaw size is below the no-growth threshold strain established by test.
- (3) Re-flight hardware is verified by visual inspection or other means to show the hardware is intact and free of structural anomalies before being re-flown.
Note that metallic welds and castings are manufacturing processes that may be likely to contain critical flaws, and therefore, they do not qualify as low-risk parts unless inspection data establish they have no flaws that can grow, i.e., the crack stress intensity factor is below threshold, including environment and residual stress effects. For metallic parts addressed in item 6.2.5.a (above), the net-section stresses are to be computed based on strength-of-materials theory. An example of the net-section stress calculation for combined tension and bending stress is detailed in the NASGRO® User’s Manual, Appendix B, in the beginning pages, except no crack or epsilon factor is used for this NFC low-risk application. For complex parts where finite element results are obtained that may include stress concentrations and stress gradients, the net-section stresses are to be computed by integrating the stress distribution and dividing by the area for the sectional area being assessed.
6.2.6 NFC Documented Non-Hazardous Failure Mode
Provide documentation establishing that a hazard assessment has been performed and that there are no credible catastrophic hazards resulting from failure of the part caused by a flaw to classify
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a part as NFC Documented Non-Hazardous Failure Mode to meet requirement [FCR 8] section 6.2.f in this NASA Technical Standard.
Note that this category is significantly different from the Exempt classification in section 5. Exempt parts are nonstructural and have no hazardous concerns or failure modes. This category may have structural or non-structural parts that are to be addressed by a documented hazard assessment that establishes no credible catastrophic hazards exist for the failure modes identified.
For composite or bonded parts classified as NFC Documented Non-Hazardous Failure Mode according to this section may not be required to meet all the requirements in section 6.3 in this NASA Technical Standard. Guidance from the RFCB may be necessary.