Surface enhancement offers a variety of benefits ranging from increased life to cost reduction when properly designed and optimized for the material and application. Strength and life can be increased without changing either the material or the component design. Common service damage can be completely mitigated by placing the damaged layer in compression. Whether it is optimizing conventional shot peening for maximum production at minimal cost, or developing novel LPB® solutions for improved damage tolerance, Lambda’s extensive experience and unique combination of applied stress modeling, residual stress measurement, surface treatment design, and performance testing capabilities are applied to each project to realize these benefits for our client’s applications. Some of the benefits of surface enhancement to introduce a deep stable surface layer of residual compression are as follows:
Fatigue Life Extension
Local stress concentrations at fatigue critical features such as fillets and bolt holes can be effectively strengthened by introducing residual compression. Fatigue damage too small for NDT detection can be completely arrested, allowing components removed from service to be fully restored to full service life.
Damage Tolerance Improvement
Surface damage introduces local stress concentrations, greatly reducing fatigue strength. Holding the damage entirely in compression can eliminate failure from the damaged surface, removing the fatigue debit. All common damage mechanisms including foreign object damage (FOD), corrosion pitting, fretting micro-cracking, erosion and wear have been mitigated by LPB® in a wide variety of alloys.
Mitigating Manufacturing Damage
Manufacturing damage such as machining marks and shallow handling damage can be mitigated like FOD. Surface finish can be controlled as compression is introduced. Phase transformations and residual tension from grinder burn and EDM recast layers can be eliminated at fatigue initiation sites by surface enhancement.
Stress Corrosion Cracking and Corrosion Fatigue
A deep layer of stable high compression can completely eliminate static stress corrosion cracking (SCC) and suppress the corrosion component of corrosion fatigue. SCC simply cannot occur if the surface is in compression to sufficient depth. LPB® has completely eliminated SCC in high strength landing gear steels, and in aluminum propellers and structural components in salt-water environments. LPB® restores the corrosion fatigue performance of friction stir welded aluminum and eliminates SCC in stainless steel nuclear piping welds subject to SCC.
Compressive stress fields designed to offset service induced tensile stresses can be used to reduce sections, and therefore weight in some components. Critical locations can be effectively reinforced by introducing compression at critical features such as fillets and bolt holes. However, the full effects of geometric changes including compliance and static strength must be carefully considered.
Substitution of a new material to improve fracture toughness, fatigue strength or SCC resistance of existing components can be prohibitively expensive and provide only incremental benefit. Depending upon the applied stress, designed surface enhancement can often improve performance even more than material substitution at greatly reduced cost. Less expensive alloys can be used with surface enhancement to provide superior performance.
Significant improvements in the load carrying capacity of fatigue limited components can be achieved through surface enhancement. Controlled shot peening has been widely used for years to extend the load range of automotive gearing. Critical components can be operated at higher loads and for longer design lives through the introduction of designed residual compression at critical locations without changing either the component material or design.
Examples of the use of surface enhancement to minimize cost and improve performance are included in our publications. Lambda’s engineers will be pleased to draw upon their extensive experience to assist in achieving the benefits of surface enhancement in your application.