shot peening residual stress - Metal Improvement Company

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C H A P T E R T E N O THER APPL ICA T IONS 38 E X FOLIATION C ORROSION A large number of commercial aircraft are over 20 years old. Ultimately, the safety of older aircraft depends on the quality of the maintenance performed. An aged Boeing 737 explosively depressurized at 24,000 feet (7300 m) when 18 feet (6 m) of the fuselage skin ripped away. The cause of the failure was corrosion and metal fatigue [Ref 10.3]. MIC has developed a process called Search Peeningsm to locate surface and slightly sub-surface corrosion. Exfoliation corrosion is a form of intergranular corrosion that occurs along aluminum grain boundaries. It is characterized by delamination of thin layers of aluminum with corrosion products between the layers. It is commonly found adjacent to fasteners due to galvanic action between dissimilar metals. The surface bulges outward as shown in Figure 10-8. In severe cases, the corrosion is subsurface. Once corrosion is present repairmen can manually remove it with sanding or other means. Shot peening is then applied to compensate for lost fatigue strength as a result of material removal. Additional sub-surface corrosion will appear as "blisters" exposed from the shot peening process. If additional corrosion is found, it is then removed and the Search Peeningsm process repeated until no more "blistering" occurs. MIC is capable of performing the Search Peeningsm on-site at aircraft repair hangers. Critical areas of the aircraft are masked off by experienced shot peening technicians before beginning the process. P OROSITY S E A L I N G Surface porosity has long been a problem that has plagued the casting and powder metal industries. Irregularities in the material consistency at the surface may be improved by impacting the surface with shot peening media. By increasing the intensity (impact energy), peening can also be used to identify large, near-surface voids and delaminations. REFERENCES: 10.1 Paulson; Effective Means for Reducing Formation of Fines and Streamers in Air Conveying Systems, Regional Technical Conference of the Society of Plastics Engineering; 1978, Flo-Tronics Division of Allied Industries; Houston, TX 10.2 Steiner, Maragos, Bradley; Cleanability of Stainless Steel Surfaces With Various Finishes; Dairy, Food, and Environmental Sanitation, April 2000 10.3 Eckersley; The Aging Aircraft Fleet, IMPACT; Metal Improvement Co. www.metalimprovement.com Figure 10-8 Exfoliation Corrosion
I N T E R N A L S U R F A C E S A N D B ORES When the depth of an internal bore is greater than the diameter of the hole it cannot be effectively shot peened by an external method. An internal shot peening lance or internal shot deflector (ISD) method must be used under closely controlled conditions (Figure 11-1). Holes as small as 0.096 inch (2.4 mm) in jet engine disks have been peened on a production basis using the ISD method. Potential applications for internal shot peening include: • Tie wire holes • Hydraulic cylinders • Helicopter spars • Drill pipes • Propeller blades • Shafts with lubrication holes • Compressor & turbine disc blade slots Figure 11-2 Internal Surface Deflector vs External Shot Peening Figure 11-1 Lance Peening and Internal Shot Deflector Peening www.metalimprovement.com C H A P T E R E L E V E N MIC developed an intensity verification technique for small holes. Figure 11-2 shows the results of a study to a jet engine disk comparing the residual stress on the external surface (peened with conventional nozzles) to that on internal surfaces of a small bore peened with internal shot deflector methods. Using the same shot size and intensity, the two residual stress profiles from these controlled processes were essentially the same [Ref 11.1]. D U A L ( I N T E N S I TY) P E E N I N G Dual peening (or Dura Peen sm ) is used to further enhance the fatigue performance from a single shot peen operation. Fatigue life improvements from shot peening typically exceed 300%, 500%, or more. When dual peened, (single) shot peen results can often be doubled, tripled, or even more. ADDITIONAL C APABILITIES & SERVICES 39
Page 1 and 2: T A B L E O F C O N T E N T S T A B
Page 3 and 4: Common Conversions Associated with
Page 5 and 6: Metal Improvement Company, Inc. (MI
Page 7 and 8: Shot peening is primarily used to c
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Page 11 and 12: C A R B U R I Z E D S T E E L S Car
Page 13 and 14: Shot peening can significantly impr
Page 15 and 16: P O W D E R M E T A L L U R G Y Opt
Page 17 and 18: If the weld is shot peened (rather
Page 19 and 20: A N O D I Z I N G Hard anodizing is
Page 21 and 22: B E N D I N G F A T I G U E Bending
Page 23 and 24: C R A N K S H A F T S In most cases
Page 25 and 26: It is quite common to perform a bak
Page 27 and 28: F R E T TING F A I L U R E Fretting
Page 29 and 30: C O R R O S ION F A I L U R E Tensi
Page 31 and 32: A p p l i c a t i o n C a s e S t u
Page 33 and 34: T H E R M A L F A TIGUE Thermal fat
Page 35 and 36: The majority of aircraft in product
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Page 41 and 42: industries. Figure 11-6 shows a typ
Page 43 and 44: L A S E R S H O T S M P E E N I N G
Page 45 and 46: C O N T R OLLING THE P R OCESS Cont
Page 47 and 48: Almen strips are mounted to Almen b
Page 49 and 50: MIC has developed CMSP equipment th
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