Coldworking fastener holes is a simple life enhancement procedure that strengthens metallic components
Coldworking fastener holes is a simple life enhancement procedure that strengthens metallic components by retarding crack growth around the hole. The most common method of coldworking involves pulling an oversized, tapered mandrel through a hole. This force plastically expands the hole, producing a residual compressive stress zone around the hole. The stress zone, depending upon variables such as material, hole diameter, and applied expansion levels, extends approximately one radius from the edge of the hole. The zone acts as a barrier to crack growth, by reducing both the stress intensity factor acting on cracks originating from the fastener hole and the local stress ratio. These actions, in turn, reduce the crack growth rate thereby increasing the overall life of the component. Fatigue life enhancement is generally increased by a factor of from two to seven. In addition to significant fatigue life enhancement, coldworking offers several additional benefits:
Increased Airframe Safety By retarding the crack growth rate through reduction of the stress intensity factor, coldworking increases the opportunity to discover fatigue cracks in a repairable condition. This increases the service life of components and minimizes maintenance time and cost.
One-Sided Operation Both the split sleeve and split mandrel coldworking methods are one-sided/one-operator processes. Previous methods involved both sides of a component and additional personnel, thereby increasing the time and cost of the process. By simplifying the process and reducing the personnel needed, the coldworking process is cost-effective for production and repair applications.
No Increase in Weight or Added Measures The coldworking process is based on modifying the parent material of a component. Coldworking does not increase in weight of a component, nor are any additional measures needed to achieve any life improvement benefits, as are needed with composite patches.
Reduced Maintenance Time and Costs By increasing the overall life of the airframe, the necessity of fatigue-based unscheduled maintenance is significantly decreased. This reduces both downtime and maintenance costs.
Sleeve Coldworking Processes
Several sleeve coldworking process are available, depending upon the material to be coldworked, the interference level desired and the particular application:
- CB Tooling
- Class I. Used in production and rework applications with aluminum and mild steel, it is commonly specified for aircraft service bulletins, and is available in standard sizes ranging from 1/8" through 151/64" (3.145.6 mm) in 1/64" increments. Provided applied expansion ranges from 3-6% (4% nominalhigh interference). See Section V for tooling tables. CB tooling is available in tooling kits, CWORK-1 and CWORK3.
- CA Tooling
- Specifically designed for high strength steel and titanium applications, CA tooling has a slightly higher level of applied expansion. Due to the nature of the material being coldworked, the mandrels are special high strength materials. Available sizes range from 1/8" to 45/64" (3.117.9 mm) in 1/64" increments. Applied expansion ranges from 3.6 to 6.1% (5% nominal).
- CR Tooling
- The CR series was originally developed for rework applications, but can also be used for new fastener holes ranging from 3/16" to 13/32" (4.810.3). When coldworking a previously countersunk hole, the hole should be processed using the next larger tool size. See Section VII for tooling tables. Applied expansion is higher than CB tooling, which allows the use of fewer tool sets to coldwork a range of holes. A full set of CR tooling is available in kit form as the CWORK2B.
- CW Tooling
- Class II. Used in aluminum and mild steel applications, the CW process provides approximately 3% applied expansion, (low interference) for moderately stressed areas.
- Application Specific Tooling
- Due to our customers' unique configurations and requirements, WCI has designed and developed tooling to specific applications. Customized applied expansion tooling may be required under these circumstances. Please contact our technical engineering staff for an evaluation.