Appendix 2 Table A2-2. Frequency Reduction Factor as a Function of Hole L/D
L/D L/D freq.
0.000 0.040
1.000 0.040
1.015 0.047
1.030 0.055
1.045 0.065
1.060 0.076
1.075 0.089
1.090 0.105
1.105 0.123
1.120 0.145
1.135 0.170
1.150 0.200
1.165 0.235
1.180 0.276
1.195 0.324
1.210 0.381
1.225 0.447
1.240 0.525
1.255 0.617
1.270 0.725
1.285 0.851
1.300 1.000
2.000 1.000 Equation for the L/D frequency reduction factor as a function of L/D is:
Equation A2-2 ν = 0.04 * EXP[10.7296 * (L/D - 1)] where L/D is a ratio of the hole length L over diameter D, and the dimensionless frequency factor ν should be used as a multiplication coefficient for the right-hand-side of the equation Eq. A2-1 for cases where 1 < L/D < 1.3.
Appendix 3 APPENDIX 3. DEFAULT POD APPLICABILITY
1. Use of Default POD Data.
a. This appendix defines conditions relevant to the use of accepted estimates of the probability of detection for specific types of anomalies and specific nondestructive evaluation/inspection techniques, which may be considered as default values when applied under appropriately similar conditions. The conditions defined in this appendix do not necessarily guarantee the validity of these POD values. For example, if inspection parameters, such as penetrant concentration or temperature, are inadequately controlled, the penetrant capability shown in the accompanying figures in Appendix 4 will not be attained, even if the correct penetrant is selected. We recommend use of a written plan for controlling and monitoring inspection processes (see paragraph 3 of this appendix).
b. If the conditions described for each inspection are not satisfied, the resultant inspection capability and reliability will be reduced. Use of the default POD values in Appendix 4 would then be inappropriate and would result in an overly optimistic damage tolerance assessment.
2. Demonstrations of Inspection Capability.
a. For eddy current inspection techniques, a default POD curve is provided in Appendix 4.
Use of the default POD curve must be based on demonstration that the stated calibration and reject signal levels are attainable on the component being inspected, and, for example, are not prevented by noise or geometrical features. The demonstration conditions should be appropriate to the properties of the part inspected that may affect the inspectability, such as surface conditions, depth to be inspected, or proximity to edges. No other demonstration of this default capability is necessary as long as the requirements for the specific inspection technique are satisfied (see paragraphs 3-5 of this appendix).
b. For specific inspection techniques applied under well-controlled conditions, it might be possible to achieve POD values that are significantly better than the default POD values.
Manufacturers may take advantage of any such improved capability and reliability only if it is supported by a well-documented demonstration program. Similarly, a well-documented demonstration program may be necessary to measure POD values appropriate to specific conditions excluded from those default values supported by explicit, reasonable assumptions.
3. General Restrictions and Applicability.
a. The inspection process must be well controlled and performed in accordance with acceptable procedures, such as those defined by the engine standard practices manual, and consistent with good industrial inspection practices, such as those defined by MIL-STD’s or equivalent industry standards.
b. Pertinent inspection process parameters such as coverage, probe indexing, and scanning speeds (see paragraph 5 of chapter 2, and paragraphs 4-5 of this appendix), should be governed by written procedures. Inspection plans and any inspection fixturing should be designed to minimize human and other sources of variability.
Appendix 3
c. Inspectors must be fully qualified and trained for at least one of the following: NAS 410/prEN4179, MIL-STD-410, ASNT-TC-1A, ATA-105, or equivalent, and provided with adequate training instructions in the specific inspection method.
d. The specified default ECI and FPI POD data apply to all materials. Geometric conditions, such as radii and edges, can create areas where inspections cannot be accomplished.
Limitations also exist relative to depth of penetration and near surface resolution. Conditions under which the default POD data were acquired are outlined in paragraphs 4-5 of this appendix.
Seek advice about the equivalence of alternative conditions from those with expertise in NDE.
Areas of high compressive residual stress can have negative effects on the capability of various NDE techniques, most notably penetrant inspection.
e. Applicability of the default POD data is limited to components that exhibit no abnormal surface condition and that have been properly cleaned for each shop manual requirements. No other special pre-inspection cleaning or polishing is required.
4. Restrictions and Applicability: ECI.
a. Eddy current inspection is an inspection technique suitable for the detection of surface or near-surface anomalies. For the purposes of this AC, ECI is intended primarily for application to engine-run components. The default POD data were acquired under the following conditions:
• Probes containing absolute coils, with inspection frequency in the range 2-6 MHz.
• Probe fixturing is capable of following surface contours of the component being inspected, with adequate control of attitude, lift-off and scan indexing. Scan direction was parallel with any uniform feature changes.
• Provision was made for automatic recording of the inspection process signals or automated alarm, or both, when the inspection threshold is exceeded.
b. The default POD data apply to surface-connected, low-cycle fatigue cracks. Cracks are assumed to have a 2-to-1 aspect (length:depth) ratio. Crack sizes are expressed in terms of the length at the surface. Cracks must not be obscured by oxide, contaminants, etc. Inspected surfaces should be flat or only moderately curved. Choice of the appropriate POD curve from those provided must be based on component demonstration of the attainable inspection sensitivity (see paragraph 2 of this appendix).
5. Restrictions and Applicability : Penetrant Inspection (PT/FPI).
a. PT is an inspection technique suitable for detection of anomalies that are open to the inspected surface. For the purposes of this AC, PT is intended primarily for application to engine-run components. The default POD data were acquired under the following conditions:
(1) Fluorescent penetrants were qualified as level 4 by MIL-I-25135, or equivalent, and used with dry powder developer (as a minimum).
(2) Application of penetrant and developer was automated, or for each, standard practices were employed. See, for example, MIL-STD-6866 and AMS-2847.
Appendix 3
(3) Manipulation of the part was possible to present to the inspector an unimpeded view of the surface to be inspected.
b. The default POD data apply to surface-connected low-cycle fatigue cracks. Cracks are assumed to have a 2-to-1 aspect (length:depth) ratio. Crack sizes are expressed in terms of the length at the surface. Cracks must not be obscured by oxide, contaminants, etc. Inspected surfaces should be readily visible. Choice of the appropriate POD curve from those provided must be based on whether focused or full field inspection conditions apply, as those terms are defined in paragraph 4 (“Definitions”) in the body of this AC.
Appendix 4 APPENDIX 4. DEFAULT POD CURVES Figure A4-1 MEAN (50% CONFIDENCE) POD FOR FLUORESCENT PENETRANT INSPECTION OF
FINISH-MACHINED SURFACES 0 10 20 30 40 50 60 70 80 90 100 PERCENT POD Full-field inspection Directed inspection Applicability:
MIL-I-25135 Level 4 penetrants with dry powder developer (as minimum) 10 100 1000 ANOMALY LENGTH (mils) 28
Appendix 4 Figure A4-2 MEAN (50% CONFIDENCE) POD FOR EDDY CURRENT OF FINISH
MACHINED SURFACES 0 10 20 30 40 50 60 70 80 90 100
PERCENT POD Reject at the calibration notch threshold response and above Reject at one half the calibration notch threshold response and above Applicabilty:
Absolute coils, 2-6 MHz inspection frequency Calibration on 0.030" x 0.015" notch Applicable only if the indicated sensitivity is attainable 10 100 ANOMALY LENGTH (mils) 29
Appendix 5 APPENDIX 5. DEFINITION OF A HOLE FOR MANUFACTURING ANOMALY
ASSESSMENTS
1. Hole Feature . A set of one or more circular holes at the same axial and radial location where any given hole falls at least partially within the circumferential (diametral) shadow of the largest hole, and its size is greater than 75 percent of the diameter of the largest hole. Note that “circular” is relative to the machining axis and not necessarily as viewed by the stress field.
Figure A5-1 Feature 1Feature 3 Feature 2 View A View A Feature 3 (41 holes) 30
Appendix 6 APPENDIX 6. MANUFACTURING CREDITS
1. This appendix contains the approach for taking credit for enhanced hole making manufacturing processes in the POF calculations to meet the bolt hole feature DTR. These credits were developed based on experience from a number of engine manufacturers.
Table A6-1. Process Controls, Cont rol Definitions and Credit Factor Process control Definition Credit Factor Process A procedure in which it is demonstrated that the Manufacturing 5 validation Process delivers parts consistent with the Design Intent (see FAA Report number DOT/FAA/AR-06/3). Process validation is understood in this AC to include an inspection of the part for geometric anomalies (cracks, scratches, dents, scores, etc.)
after manufacture. Such an inspection may be visual, enhanced visual, or semi-automatic such as ECI. In addition, an implicit consideration in assigning credit to the various processes is that secondary operations such as chamfering, edge breaking and finishing are controlled and subject to process validation as described in FAA Report number DOT/FAA/AR-06/3.
Single The removal in a finishing operation of a small depth of 5 Point material, at least 0.004" deep, in the bore of the hole by use of Boring a single point boring tool. This credit is allowed for titanium alloys only.
Honing The removal of a small depth of material, at least 0.002", by a self-centering grinding operation. This credit is allowed for all materials.
5 Coolant A device which ensures that there is a continuous flow of 5 Monitor coolant with periodic checks on the pressure and the concentration of the coolant supplemented with the training of operators to ensure the direction of the flow towards the cutting edge (see FAA Report number DOT/FAA/AR-06/3). This credit is allowed for all materials.
Power Monitor A device that continuously monitors the power consumed by the machine tool and which must be shown to be sensitive to conditions such as worn tools, loss of coolant, etc., which give rise to anomalies (see FAA Report number DOT/FAA/AR 06/3). This credit is allowed for all materials.
For the use of either a power monitor or a feed force monitor,
20.
For the use ofFeed Force A device that continuously monitors the feed force used by the Monitor machine tool and which must be shown to be sensitive to conditions such as worn tools, loss of coolant, etc., which give rise to anomalies (see FAA Report number DOT/FAA/AR 06/3). This credit is allowed for all materials.
both a power monitor and a feed force monitor, 30.
Appendix 6 Process control Definition Credit Factor Inspection In this context, inspection is confined to the use of inspection techniques specifically aimed at detecting non-geometric anomalies, such as highly distorted material, smeared material, white or amorphous layer (see FAA Report number DOT/FAA/AR-06/3). Generally, this would be an etch inspection specifically targeted at the hole and which must be shown to detect such anomalies. This credit is allowed for all materials.
2. Rules for Applying Credits.
a. Process validation is assumed to be mandatory on machining processes on all future products.
b. Use of manufacturing credits including process validation should be part of the certification data submittal.
c. Coolant monitoring, power monitoring, and feed force monitoring should be used in both roughing and finishing operations.
d. Credit for process monitoring (power or feed force) cannot be claimed unless coolant monitoring is also in use.
e. Credit for either single point bore or honing can be claimed, as appropriate, but not for both together.
3. Credit Application Process.
a. Decide which controls are to be applied.
b. Total the credit factors for the combination of controls.
c. Calculate the probability of burst for the hole feature in the disc.
d. Divide the probability of burst by the total of the credit factors.
e. This reduced probability of burst should be compared to the DTR.
4. Credit Examples. For feature of 32 holes where the initial design POF is calculated to be 5 X 10-4 events per service life:
a. Datum Process validation and inspection for geometric anomalies Process Validation 5 Total Credit 5 POF with controls is now 1 X 10-4 events per service life This is not adequate against HOLE FEATURE DTR.
Appendix 6
b. First selection Selected combination of controls is:
Process validation 5 Honing 5 Coolant Monitor 5 Etch Inspection 5 Total credit 20 POF with controls is now 2.5 X 10-5 events per service life.
This is not adequate against HOLE FEATURE DTR
c. Improved Selection Improve the combination of controls to:
Process validation 5 Honing 5 Coolant Monitor 5 Power Monitor 20 Etch Inspection 5 Total credit 40 New POF with controls is now 1.25 X 10-5 events per service life.
This is acceptable to HOLE FEATURE DTR.
d. Other possible selections Note that the following combinations of controls would also give an acceptable HOLE FEATURE DTR:
(1) Process validation 5 Coolant Monitor 5 Power Monitor 20 Total credit 30
(2) Process validation 5 Honing 5 Coolant Monitor 5 Power Monitor 20 Total credit 35 33
Appendix 7 APPENDIX 7. SURFACE (MANUFACTURING INDU CED ANOMALY) DAMAGE TOLERANCE ASSESSMENT
FLOWCHARTS Note: For more details on the steps described in the following figures, refer to Chapter 3 and to the appendices.
Fig. A7-1 : Surface Damage Risk Assessment Workflow Identify all hole features in the engine (App. 6) Hole feature list Hole feature 1 Hole feature i Hole feature n Crack propagation analysis (see fig. A7-2) Improve
• feature or engine analysis/design or
•feature manufacturing or
•feature in service Hole feature risk list feature 1 risk feature i risk feature n risk Hole feature risk list feature 1 risk feature i risk feature n risk Feature risk < feature DTR ?
yes no Feature risk assessment (see fig. A7-3 to A7-5) End inspection or
•Inspection dates and/or number 34
Appendix 7 Figure A7-2: Crack Propagation Analysis Determine applicable crack propagation model (plate, crack prop law) ¼ circular corner crack assumption Identify critical location on hole inner surface Run crack propagation analysis Crack prop law just above threshold crack size Hole feature i ½ circular surface crack assumption Temperature & Stress results Plate model Crack size versus number of cycle for corner crack “AvsNc” Crack size versus number of cycle for surface crack “AvsNs” 35