esacrack-course-2012-18-Additional Examples - ESALOAD-ESAFATIG-v3a-final.pdf

ESA UNCLASSIFIED – For Official Use

18. Additional Example Problems - ESALOAD & ESAFATIG

Gerben SinnemaESA-ESTEC13/09/2012

ESACRACK/NASGRO© Short Course

18. Additional Examples – ESALOAD & ESAFATIG | Gerben Sinnema | ESA-ESTEC | 13/09/2012 | TEC-MSS | Slide 2


Contents

EXERCISES:1. Load and stress spectrum generation2. Bolt fatigue analysis3. Safe life analysis4. Critical crack size determination (optional)5. Indirect safe life and load spectra combination (optional)6. Load spectrum generation and CID determination



Exercise 1 - Load and stress spectrum generation

– Consider an electronic box with a mass of 20 kg, a COG height above interface plane of 100 mm and a distance between the four interface bolt locations of 200 mm.

– The box has the following dynamiccharacteristics:

– Eigenfrequencies 100, 150 and 300 Hzin x, y and z-direction respectively. The damping is assumed equal to 0.05 (viscous).

200

200100




– After manufacturing the box is transported to a test centre for testing. A total road transportation of 500 kilometres is foreseen.

– Random acceptance testing (X, Y and Z-dir, 70 sec/axis):

– Before and after each acceptance test a low-level sine test is performed 5 - 2000 Hz at 0.5 g's input with a sweep rate of 2 octaves per minute.

– Subsequent to the testing the item is transported to the launch site by 5000 km air transport.

frequency PSD level20 - 80 + 3 dB/oct80 - 350 0.04350 - 2000 -3 dB/oct




– The payload stays in space.

– The flight limit load factors are 23, 29 and 40 g's in x, y and z-direction respectively. The launch is with the Space Shuttle.

– Generate the event file and the load spectrum.



Exercise 2 - Bolt fatigue analysis

– The electronic box of Exercise 1 is attached with 4 M5 bolts of Ti6Al4V. One bolt is assumed failed.

– Derive the unit stresses for the bolts using a stress area of 14.2mm2.

– Assume a joint load factor of 0.25.– The pre-stress is given as 400 MPa.– Note: bolt tension may be accompanied by bending.

– Generate the stress spectrum and perform a fatigue analysis.– What happens if the bolt material is changed to A-286 M5, W.

No. 1.4944.4 screw assuming an unchanged joint load factor of 0.25



Exercise 3 - Safe life analysis

– Consider the box in Exercise 1. A safe life analysis of the attachment feet (4 feet active) is required in order to verify box integrity.

200

200100

x

yz 7

20

10



Exercise 3 - Safe life analysis

– The box (see Exercise 1) and its feet are made from 7075-T7351 plate.

– The dimensions of the feet are: thickness 7 mm, width 20 mm and distance between bolt hole centre and box attachment 10 mm.

– Find suitable stress intensity factor model(s) and derive unit stresses.

– Determine the worst case crack location and perform a safe life analysis using initial crack sizes of a=1.91 mm and a/c=1 and a=0.81 mm and a/c=0.2 (ECSS standard initial crack size for penetrant inspection). Use a scatter factor 4.

– What would happen if the total box mass is only 10 kg? (otherwise same design)



Exercise 4 - Critical crack size determination (optional)

– For the feet Of Exercise 3 the critical crack sizes should be determined.

– Use the limit stresses derived for launch loads:– S0 = 0.41 *23 = 9.4 MPa– S1 = 4.09 * 23 + 4.09 * 29 + 4.09 * 40 = 376.3 MPa – S2 = 1.23* 29 = 35.7 MPa

– Use the following dimensions t = 7 mm and W= 20 mm for both CC01 and SC01 models and consider the two aspect ratios a/c = 0.2 and 1



Exercise 5 - Indirect safe life and load spectra combination (optional)

– An M8 bolt is used in proof and acceptance testing of a structure and is subsequently used for a launch.

– The bolt is made of Inconel 718 (UTS=1550 MPa).– The test program consists of

– 2 proof cycles– 50 nominal cycles

– The nominal stress applied on the bolt is 50 MPa. The proof load is 1.5 times higher thus yielding a stress of 75 MPa.

– For the launch event the NASA normalized spectrum GSFC shall be used. The maximum bolt alternating stress is again 50 MPa.

– During test and launch the bolt is pre-stressed to 700 MPa.

– Determine the maximum initial defect size using an indirect safe life calculation. A scatter factor of 4 shall be considered.



Exercise 6 - Load spectrum generation and CID determination

– Consider a spherical propellant tank with the following characteristics:

– The radius is 300 mm and the wall thickness is 3.0 mm.– The material is Ti6Al4V STA. The tanks are forged.– The eigenfrequencies are 150 Hz in x, y and z-axis. A

damping of 0.05 is assumed. – The MDP is 50 bar (=5.0 MPa)

– The tank undergoes the following test campaign.– Pressure testing:

cycles Pressure2 (proof) 1.5 MDP20 1.0 MDP




– Random acceptance testing (X-, Y- and Z-axis; 120 sec/axis):

– Before and after each acceptance test a low-level sine test is performed 5 - 2000 Hz at 0.5 g's input with sweep up only, 2 octaves/min.

– The limit load factor has been determined to 29 g's in x, y and z-direction.

– During testing and launch the tank is assumed pressurized to MDP.– Include one launch event in the load spectrum.

frequency PSD level20 - 80 + 3 dB/oct80 - 350 0.04350 - 2000 -3 dB/oct




– The following unit stresses shall be used (Note: S4 – ‘crack face’):

– Generate the stress spectrum and use an indirect safe life calculation to determine the maximum permissible initial crack size (critical initial defect or CID) by applying the service life 4 times.

– Which crack case is the most severe: internal or external crack?

– Which crack aspect ratio is the worst case?– Why shall the ‘crack face’ unit stress be used? (‘biaxial’ in

ESALOAD terminology)

cause Tension (S0) Bending (S1) Bi-axial (S4)x-direction 3 MPa/g 2 MPa/g 0 MPa/gy-direction 3 MPa/g 2 MPa/g 0 MPa/gz-direction 2 MPa/g 1 MPa/g 0 MPa/gPressure (MDP) 250 MPa/MDP 5 MPa/MDP 5 MPa/MDP




Additional tasks:– In case the tank has been cleaned with IPA what effects

would that have?– Assume a KIscc value of 970 MPa mm1/2.– On which stress line should the KIscc be implemented?

Documents

esacrack-course-2012-18-Additional Examples - ESALOAD-ESAFATIG-v3a-final.pdf