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
Appendix 7 Figure A7-3 : Feature Risk Assessment (1/3) 1st step : consider only one hole of the hole feature i and calculate the conditional probability of fracture for this hole.
Anomaly size distribution (App. 2) Crack size versus number of cycle for corner crack “AvsNc” Crack size versus number of cycle for surface crack “AvsNs” Check consistency of size definition between all data
•1st Inspection “cycles since new” distribution + applicable POD curve
•2nd Inspection “cycles since last inspection” distribution + applicable POD curve
•Etc Expected Part certified life Calculate conditional probability of fracture
(see fig. A7-6)
Corner crack conditional prob. of fracture for 1 hole of feature i Keep worst conditional prob.
of fracture Surface crack conditional prob. of fracture for 1 hole of feature i Hole conditional prob. of fracture “hcpf” (1)for 1 hole of feature i (see fig. A7-6) Note :
(1) The term « conditional » means that this probability does not account for the exact anomaly frequency.
Appendix 7 Figure A7-4: Feature Risk Assessment (2/3) 2nd step : Calculate the probability of fracture for one hole by combining the conditional probability of fracture for one hole with the hole actual inner surface area, the applicable manufacturing credits and anomaly frequency correction factor.
prob. of fracture “hcpf” for 1 hole of feature i Determine hole Length / Diameter ratio L/D Fc Determine hole inner surface area (App. 2) S Determine the probability of fracture for 1 hole “hpf” Hpf = hcpf * S * Fc / credit prob. of fracture “hpf” for 1 hole of feature i Determine applicable manufacturing credits (App. 6) credit Determine
applicable
anomaly
frequency
correction factor
“Fc”
(App. 2)
Appendix 7 Figure A7-5: Feature Risk Assessment (3/3) 3rd step : Calculate the risk of fracture for the feature i from the probability of fracture for one hole.
Determine number of holes in the feature i “ni” (App. 5) ni Determine the risk of fracture for the feature i “fr” fr = 1 – (1- hpf)^ni prob. of fracture “hpf” for 1 hole of feature i Feature i risk “fr” 38
Appendix 7 Figure A7-6: Conditional Probability of Fracture Calculation The POF calculation is described here following a Monte Carlosimulation process. A large sample of holes should be simulated (see App. 1, 3 (e)). Each hole simulation in the sample follows the flowchart below to determine if it :
fractures before part certified life / succeeds to reach part certified life / is detected with a crack.
st star artt Is therea manufacturing anomaly?
Success ye Is therea manufacturing anomaly?
Success yes s n noo Notes :
- each r equires a random pick from the pertinent
random variable distribution
-(1) record the number of cycles at fracture or detection for each hole Conditional prob.of fracturebefore Ncycles number of fracturedholes beforeNcycles = total sample size Whatsize « a0 »?
Geta0 anomaly life from “AvsN” curve (see fig. A7-7) Cyclesat inspection(s) ?
Is thereone or more inspection(s) ?
Is anomaly detectable at inspection(s) using applicable PODcurve?
Any inspection during anomaly life ?
Get anomaly size at inspection(s) from “AvsN” curve (see fig. A7-7) Fracture(1) Success Fracture(1) Detection(1) yes no no no yes yes a0anomaly life > part certifiedlife ?
What size « a0 »?
Get a0 anomaly life from “AvsN” curve Cyclesat inspection(s) ?
Is thereone or more inspection(s) ?
Is anomaly detectable at inspection(s) using applicable PODcurve ?
Any inspection during anomaly life ?
Get anomaly size at inspection(s) from “AvsN” curve - Fracture(1) Success Fracture(1) Detection(1) yes no no no yes yes a0 anomaly life > part certifiedlife ?
Appendix 7 Figure A7-7: a0 Anomaly Life & Anomaly Size at Inspection(s) Calculations from “AvsN”Curve a N just above threshold anomaly size inspection a0 anomaly size at inspection a N just above threshold anomaly size inspection a0 anomaly size at inspection A vs N Curve a0 anomaly life 40