8.1.1 NDE for Metallic Parts
[FCR 16] Metallic fracture critical parts screened with NDE shall have inspections performed in accordance with NASA-STD-5009 and include the following for flaw screening by NDE:
- a. Apply sufficient flaw inspection methods to the flight hardware to screen flaws larger than or equal to the size and shape that are evaluated in the hardware damage tolerance assessment.
- b. In addition to NDE for flaw screening of other regions of fracture critical parts, perform post-proof test NDE at critical welds and other critical locations identified in the FCP for all parts that are proof tested as a part of acceptance, i.e., critical hardware locations not screened for specific flaws with the proof test.
[Rationale: This cites NASA-STD-5009 and reduces the potential for redundant or conflicting requirements.]
It is expected that fracture critical parts have surface and volumetric inspections. The need for internal (volumetric) inspection depends on application and materials characteristics such as thickness, product form, and other factors. Internal inspection requirements and methods should be determined early in the design process so that proper flaw screening is accomplished. If either surface or volumetric inspection is not feasible, rationale needs to be provided to the RFCB that demonstrates that the elimination of NDE is a risk-neutral posture.
According to NASA-STD-5009, the flaw sizes and shapes that are evaluated in the hardware damage tolerance assessment are based on 90-percent probability of detection with 95-percent confidence (90/95 or better) flaw detection capability.
If one NDE method cannot adequately examine a part, additional NDE methods may be needed. If there are multiple types of flaws or complex geometry to assess, additional NDE may be needed. If there is uncertainty about which NDE methods or results for a particular part are to be used to define flaws for the damage tolerance assessment, conservative choices are to be made.
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NDE activities and damage tolerance assessment activities should be coordinated to assure flaw screening occurs in the way intended.
8.1.2 NDE for Composite or Bonded Parts
NDE activities for composite or bonded materials requirements apply to fracture critical and NFC parts. Because of the potential sensitivity to impact damage and flaws for these types of materials, additional activities are necessary for NFC parts in accordance with [FCR 9] 6.3.c in this NASA Technical Standard.
[FCR 17] For composite or bonded materials, the hardware developer shall:
- a. Provide the NDE methodology and rationale in the FCP.
- b. Perform flaw screening by NDE on all composite or bonded part regions, except for the following:
- (1) No NDE is required for NFC low-released mass parts.
- (2) No NDE is required for NFC contained parts.
- c. For hardware that is proof tested as part of acceptance, perform pre-proof and post-proof test NDE at critical joints, discontinuities, and other critical locations identified in the FCP for all hardware, i.e., critical hardware locations not screened for specific flaws with the proof test.
[Rationale: There are no NDE standards available that are applicable to the wide variety of non-metallic materials and forms in use and the different NDE methods required for their inspection. The approach for NDE of other materials needs to be documented and fully explained within the FCP.]
Inspection of composite or bonded parts is to meet the intent of MIL-HDBK-6870, Nondestructive Inspection Program Requirements for Aircraft and Missile Materials and Parts, as required in NASA-STD-6016.
Hardware should receive post-proof NDE unless a special RFCB approval has been granted. Generally, the NDE approach and rationale for all materials should address which indications rise to the level of a reportable flaw. For signal-based methods, such as ultrasonic inspections, NDE acceptance criteria are usually necessary to discern whether the signal responses warrant nonconformance reporting. All damage indications from visual inspection are reportable. Workmanship standards for visual inspection should define acceptance criteria, e.g., porosity, surface texture, geometric contours. NDE acceptance criteria may be developed by analysis with supporting coupon test data for the appropriate material type. Prior approval should be obtained from the RFCB when visual inspection is used as a flaw-screening technique for fracture control. Screening of a low-risk part with NDE should be considered when it is
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plausible for that part to be reclassified as a fracture critical part. A part may need to be reclassified when it is plausible for that part to be accepted for flight with out-of-tolerance dimensions or nonstandard material properties.
8.1.3 Proof Test
[FCR 18] If proof testing is used as the flaw screening technique for fracture critical parts, the approach shall be documented in the FCP with rationale establishing that it is an applicable approach that has been approved by the RFCB.
[Rationale: Proof test may be used for flaw screening. However, few parts, materials, and applications lend themselves to a simple proof test strategy. Environmental effects, temperature, test fixture, inertial loads, and other complexities require careful consideration before accepting proof as the sole method for flaw screening. If proof test is used for flaw screening, an understanding of the planned approach and anticipated effectiveness needs to be approved by the RFCB and documented in the FCP.]
Proof test should not be used as the only flaw screening method for composite or bonded hardware.
The flaw size used in the life assessment should adequately account for flaw growth during the proof test. To establish that the assessment is valid, sufficient test data should be obtained using pre-flawed specimens that are representative of the part configuration, material conditions, and screened flaw and show the amount of growth of all crack fronts during the proof test from all sources, including stable tearing, and both EAC and SLC if applicable, have been conservatively bounded.
When it is judged that a proof test is appropriate to screen hardware for flaws, the proof test should occur at the in-service temperature and environment. If this is not feasible, an ECF can be used as approved by the RFCB. Upper bound critical stress intensity or residual strength should be used when establishing an analytically predicted flaw size screened by proof test.
Note that a proof test is required for acceptance in accordance with NASA-STD-5001 (or program-specific requirements), with a minimum proof test factor, depending upon whether a prototype or proto-flight verification approach is followed and the type of material used.
8.1.4 Process Control
[FCR 19] If process controls are used to establish bounds on flaw sizes in fracture critical parts, the approach shall be subject to the following:
- a. The approach is documented in the FCP.
- b. The rationale establishing that the approach is applicable is documented in the FCP.
- c. The FCP is approved by the RFCB.
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[Rationale: Use of process control information to define flaws or damage that could be in the part is an unusual approach. An understanding of the approach and supporting information need to be approved by the RFCB and documented in the FCP.]
Process control rationale to bound flaw sizes submitted for RFCB approval should include documentation on why this approach is being applied, an overview of the hardware, and evaluation that the approach is adequate for fracture control. Descriptions of the relevant manufacturer’s experience base, process control during manufacture, inspection results, and subsequent life of the component, all component testing, and summary arguments should be included.
8.1.5 Detected Flaws
[FCR 20] Spaceflight hardware with detected flaws that is used for flight without being repaired or replaced shall have a specific detailed assessment approach documented with rationale in the FCP that contains the following:
- a. An assessment approach of metallic parts by analysis is to include the following items in addition to the items in section 7.3.2 in this NASA Technical Standard:
- (1) Upper bound flaw size.
- (2) Upper bound crack growth rate.
- (3) Lower bound critical stress intensity factor or residual strength.
- (4) Lower bound cyclic fatigue crack growth threshold stress intensity range ( ).
- b. An assessment approach for composite or bonded parts with detected flaws is to include the following items:
- (1) The approach and rationale provided to the RFCB for approval before implementation.
- (2) Documentation of the approved approach in the FCP.
[Rationale: An understanding of the approach and methodology to accept detected flaws, which accounts for variability in the assessment, is necessary to assure adequate fracture control implementation.]
For reportable detected flaws in composite or bonded parts, a similar worst-case analysis approach to that used for metal parts may not be available. Any proposed analysis approach is to be test verified with a similar damage configuration and approved by the RFCB.
Note that the detailed assessment approach may be by damage tolerance test if approved by the RFCB.
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The normal fracture control process is carried out with the assumption that the part contains a flaw in the worst-case location and orientation. The assessment of the assumed flaw includes typical fracture properties and an assumed flaw size. However, when flaws are detected in a part that is planned for use in flight, an assessment is performed using bounding flaw sizes, material properties, loads, and boundary conditions.
Fracture critical parts with reportable NDE indications are to be assessed by a process approved by the Technical Authority to determine whether the indication is a flaw.
Fracture critical parts with detected flaws are to be assessed with an approach that satisfies [FCR 20] and then evaluated by the Technical Authority to determine whether the part is acceptable to use as is or if the part is to be repaired or replaced. If the part is to be repaired, the repair process is to be an established, proven process that has been approved for this purpose by the Technical Authority.
Pressure vessels and COPVs should not be flown with detected flaws, e.g., see sections 7.2.1 and 7.2.2 in this NASA Technical Standard. If an exception is sought, it is to satisfy section 10 in this NASA Technical Standard as an alternative approach that is deviating from these established procedures and needs approval by the Technical Authority and the RFCB.
The RFCB should be notified of the intent to fly the flawed part when it is not feasible to repair or replace the part.