[FCR 13] Each fracture critical composite or bonded part that is not of a specific hardware type as described in section 7.2 in this NASA Technical Standard and is not approved by the RFCB as appropriate for an optional approach as described in section 7.5 in this NASA Technical Standard shall comply with all of the following items:
- a. Develop a DTA by complying with section 7.4.1 in this NASA Technical Standard.
- b. Develop an IDMP by complying with section 7.4.2 in this NASA Technical Standard.
- c. Develop an RTD by complying with section 7.4.3 in this NASA Technical Standard.
- d. Develop loading spectra by complying with section 7.4.4 in this NASA Technical Standard.
- e. Perform damage tolerance tests on coupons by complying with section 7.4.5 in this NASA Technical Standard.
- f. Perform damage tolerance tests of hardware elements by complying with section 7.4.6 in this NASA Technical Standard.
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- g. Perform strength and life assessments by complying with section 7.4.7 in this NASA Technical Standard.
- h. Perform damage tolerance tests of full-scale flight-like hardware by complying with section 7.4.8 in this NASA Technical Standard.
- i. Evaluate anomalies discovered during any portion of the BBA by complying with section 7.4.9 in this NASA Technical Standard.
[Rationale: Fracture critical parts need activities performed to understand the sensitivity of the part if a flaw or damage is present. These activities can range from a direct assessment of the part’s capability with a flaw or damage to acceptance tests that establish the part has sufficient capability to a combination of activities that provides information deemed sufficient to mitigate the risk of failure caused by undiscovered flaws.]
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. The approaches in this requirement are the preferred approaches if followed completely.
Damage tolerance assessment of composite or bonded hardware uses a BBA that includes testing, analysis, and certification. The testing includes material-allowable coupons, structural elements, subcomponents, components, and appropriate full-scale article testing. The tests are performed to evaluate relevant critical failure modes for loads that are representative of the hardware loading spectra and may include LEFs. The test elements develop assessment capability for credible damage levels as determined by the process steps resulting in the RTD. Such a BBA links multiple length scales and accounts for the effects of structural and material parameter variability.
Damage tolerance analysis of composite or bonded hardware is generally considered insufficiently developed to certify flight hardware without the support of a test program and the BBA. However, when a test-verified analysis approach exists and is applicable, an analysis approach that minimizes some of the testing detailed below may be submitted to the RFCB for consideration and approval. The assessment establishes that the spaceflight hardware meets all the criteria for life, strength, and damage tolerance detailed in these subsections. The details of the assessment are documented in the FCP.
The steps used in a damage tolerance assessment of composite or bonded hardware by incorporating the BBA and damage threat mitigation activities are detailed in the sections cited below:
a. The initial three steps (sections 7.4.1 through 7.4.3 in this NASA Technical Standard) establish the critical damage states. There is likely an interaction between these three elements as flaw detection and impact damage protection/detection strategies are developed and implemented on the flight hardware. The final RTD is used in the certification of the flight
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hardware. Note that there may be credible damage conditions that occur at any point during service life, including during the mission.
- b. Concurrent with these first steps is development of the loading spectra determination (section 7.4.4 in this NASA Technical Standard) that affects the criticality of the remaining damage determined by the RTD.
- c. The next four steps (sections 7.4.5 through 7.4.8 in this NASA Technical Standard) establish the structural response to the damage by both analysis and test at increasing levels of geometric complexity. There is also an interaction between these tests and the determination of critical damage states needed to develop the RTD.
- d. Finally, discrepancies between the anticipated and observed test responses to damage initiation or growth are reconciled in accordance with section 7.4.9 in this NASA Technical Standard.
In practice, there will be iteration between and among these various steps.
BBA as described in this section is a comprehensive approach. Developers may have alternative approaches better suited to their hardware. These approaches and their rationale should be discussed with the RFCB.
7.4.1 Damage Threat Assessment
Develop a DTA according to the following to satisfy requirement [FCR 13] section 7.4.a in this NASA Technical Standard.
- a. Provide information for residual strength sensitivity to impact damage and manufacturing flaws based on test data.
- b. Define and quantify the flaws from any source that may occur to the hardware during its service life.
7.4.2 Impact Damage Mitigation Plan
Develop an IDMP according to the following to satisfy requirement [FCR 13] section 7.4.b in this NASA Technical Standard.
- a. Define, document, and implement impact protection and/or detection strategies that are used for the flight hardware to diminish targeted damage threats identified by the DTA.
- b. Prescribe when and how impact protection and/or detection strategies are to be used for flight hardware to mitigate credible damage or threats.
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7.4.3 Residual Threat Determination
Develop an RTD according to the following to satisfy requirement [FCR 13] section 7.4.c in this NASA Technical Standard.
- a. Define the worst-case credible flaw conditions that are shown to be tolerated by the hardware through analysis and test, considering all applicable flaw detection and mitigation strategies that are implemented for the flight hardware.
- b. Encompass all possible worst-case credible damage conditions, except the threats that are mitigated by NDE evaluations, the IDMP, and the threats where risk is accepted by the program or project.
- c. Document the damage states the program or project has chosen to exclude from the design.
The RTD helps identify flaws or damage conditions that are not screened by a combination of inspection, protection, and detection strategies.
Although inspection techniques meeting the 90 percent detectability level with 95 percent confidence called for in NASA-STD-5009 for metals are generally not available for composite or bonded materials, the RTD damage detection levels are to be set to produce a similar level of reliability as expected from metallic fracture critical parts.
For re-flight hardware, the inspections to be performed between flights are to be defined.
7.4.4 Loading Spectra
Establish that all the loads and the number of cycles or duration during the service life of the part at the appropriate environment are included to develop loading spectra to meet requirement [FCR 13] section 7.4.d in this NASA Technical Standard.
Development of the loading spectra includes all the applicable loads listed in section 7.3.1 in this NASA Technical Standard and all other applicable loads such as those related to environment effects on composite or bonded materials.
7.4.5 Damage Tolerance Tests of Coupons
Damage tolerance tests on coupons are performed with the applicable environments to generate a strength-based and a life-based database.
Perform damage tolerant coupon tests according to the following to satisfy requirement [FCR 13] section 7.4.e in this NASA Technical Standard:
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- a. Perform damage tolerance tests that represent flight hardware materials, manufacturing methods, and layups.
- b. Perform damage tolerance tests that contain induced flaws and damage that encompass the worst-case credible-flaw conditions as determined by the RTD.
- c. Perform damage tolerance tests that represent the modes of failure expected in the flight hardware.
- d. Perform tests in a quantity sufficient to define design values for the relevant critical failure modes, e.g., residual strength, fatigue, using the B-basis statistical techniques as defined in CMH-17-1G or an equivalent approach approved by the RFCB.
- e. Develop or use coupon data to establish the sensitivity of residual strength to impact and manufacturing damage as determined in the DTA in accordance with section 7.4.1 in this NASA Technical Standard.
Note that sufficient quantities of data are also necessary for use in computing the Weibull shape parameters used in determining the LEF, as described in CMH-17-1G.
7.4.6 Damage Tolerance Tests of Hardware Elements
Damage tolerance tests on hardware elements, subcomponents, and components are representative of the flight designs and have induced RTD determined flaws.
Perform damage tolerance tests of hardware elements according to the following to satisfy requirement [FCR 13] 7.4.f in this NASA Technical Standard:
- a. Include both residual strength and life-based testing.
- b. Perform tests sufficient in number to guide the design and provide confidence that the tests performed in accordance with section 7.4.8 in this NASA Technical Standard encompass the worst-case credible conditions, locations, and orientations.
Note that spectrum truncation is allowed for structural-level testing (components and full-scale hardware) with supporting coupon test data.
7.4.7 Strength and Life Assessments
Assessment of the flight article should be developed that is supported by analysis of the coupon and hardware element testing with RTD determined flaws present at any location and orientation.
Perform strength and life assessments according to the following to satisfy requirement [FCR 13] section 7.4.g in this NASA Technical Standard:
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- a. Perform analysis to establish that the B-basis residual strength after 1 service lifetime is sufficient to support DUL, after which the hardware will perform as intended.
- b. Establish that the hardware performs as intended after experiencing a B-basis number of spectrum loading service lifetimes followed by one DLL cycle.
Note that the service life factor in analysis is the full B-basis number of lives, because the additional lives can be assessed without significant additional cost. One can therefore consider that no LEF is used or equivalently LEF=1.
7.4.8 Damage Tolerance Tests of Full-Scale Flight-Like Hardware
Perform damage tolerance tests of full-scale flight-like hardware according to the following to satisfy requirement [FCR 13] section 7.4.h in this NASA Technical Standard:
- a. Induce flaws into test hardware as specified by the RTD in the worst credible location and orientation.
- b. Perform NDE on test hardware before test to verify that the RTD flaws have been imposed and to record any flaws in addition to those imposed.
- c. Account for the effects of environments and flight hardware structural conditions to simulate performances throughout the specified service lifetime. If tests are not performed in the operational environment, test levels are adjusted via an ECF.
- d. Establish ultimate load capability in the test hardware after a minimum of 1 service lifetime loading.
- e. Subject the test hardwate to a minimum of 4 service lives of spectrum loading with appropriate LEF necessary to establish B-basis reliability followed by 1 DLL cycle.
More than 4 lifetimes of testing may be performed to reduce the LEF.
- f. Establish that the test hardware does not experience structural failures and is capable of performing its design function after both spectrum service life testing and DUL testing (7.4.8.d and 7.4.8.e above).
- (1) Determine primarily by assessment.
Functional or other tests may also be used. Note that items 7.4.8.a through f may be satisfied with one test article or may involve more than one test article as appropriate. The RFCB should be consulted for further understanding of what is expected to satisfy item 7.4.8.f, e.g., no structural failure or burst, no catastrophic leak caused by flaws, no catastrophic mechanical malfunctions.
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(2) Perform NDE as part of this assessment.
7.4.9 Evaluate Flaws or Damage that Occur during BBA Testing
Evaluate flaws or damage occurring during BBA testing according to the following to satisfy requirement [FCR 13] section 7.4.i in this NASA Technical Standard:
- a. Evaluate unexpected flaws or damage, significant or unusual flaw growth, and any new failure modes observed.
- b. Address any concerns raised by the evaluation by assessment, test, retest, or redesign as appropriate.
- c. Include RF CB involvement with all assessments and evaluations.