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Cold Worked Fastener Holes

Non-Destructive Evaluation of Cold Working Processes
Manufacturing Applications

PCA Technology

Both shot peening and cold expansion of fastener holes are time honored and accepted methods of improving the fatigue life of metallic components, and other, more advanced methods have been introduced as well. Cold working of metal alloys to create localized compressive residual stresses can greatly improve fatigue behavior, particularly in an area with a stress concentration. The primary reason is that in fatigue loading, compressive residual stresses effectively lower the mean stress state.

Despite the known benefits of cold working techniques, there are limitations in the ability to non-destructively inspect its characteristics and analyze the associated component longevity benefits in fatigue applications. As an example, the Society of Automotive Engineers Standard J443, Procedure for Using Standard Shot Peening Test Strip, details the methods using Almen test strips to infer peening integrity of the treated components and points out its limitations- "the process of shot peening cannot at present be adequately controlled by nondestructive inspection of the peened parts, therefore, it is necessary to control the process itself to achieve consistent reliable results."

Consequently, a methodology has been sought to quantify the effectiveness of cold working in the material from processes that include shot peening and cold expansion of fastener holes. This information represents valuable input to structural integrity analyses, to accurately take into account the improved fatigue life properties that are attributed to the cold working process. Furthermore, it is important to verify manufactured properties.

Positron Systems' PS6100 inspection system can non-destructively evaluate metallic components that have been cold worked to induce compressive residual stress. Listed below are several examples of past work.

Cold Worked Fastener Holes

IPA was used to evaluate the effect of cold work around fastener holes in Ti 6Al-4V. The image below shows the test sample; one hole has been cold worked and the other has not. Visually, they are exactly the same- which precludes visual inspection from discerning a cold worked part from one that has not been subjected to cold work in the overall manufacturing process.

Both holes were measured using IPA. The results clearly distinguish between the two conditions- IPA is highly sensitive to the dislocations that have been introduced into the cold worked specimen. The non-cold worked hole shows a response near the hole edge, indicating that the drilling process has influenced the material's microstructure.

The holes were also evaluated using a probe configuration designed to provide a single measurement evaluation of the hole. Again, there is a significant difference between the cold worked hole and the non-cold worked hole. An area of material well away from the two holes was also measured, providing an unaltered material baseline. This methodology could be used to evaluate fastener holes in existing structures to verify that the appropriate cold working procedures were followed during manufacture or as a quality control check during fabrication of new components.

Peening

Significant testing in various applications has also been conducted using the IPA techniques to detect and quantify cold work/residual stress in material samples from shot peening. An example is shown below, where IPA-S measurements were performed on titanium (Ti 6Al-4V) shot peened samples. These samples ranged from as-manufactured through 10 A on the Almen scale. IPA-S measurements resulted in an increase in the S parameter that correlated with shot peening intensity. The results reveal good sensitivity over the entire measurement range, particularly when compared to the measurement uncertainty indicated by error bars in the plot.


These measurements provide a direct, non-destructive method to measure changes in component cold work condition relative to an Almen standard, or to residual stress as determined by destructive characterization. Because the IPA techniques do not cause any material damage, they can be used on operational components to assess surface residual stress effectiveness at any point in its lifetime. This has a number of applications for measuring initial shot peening quality, aging effects, and re-conditioning effectiveness.

IPA can also provide a spatial map of cold worked areas, by measuring a component in a series of automated measurements using the PS6100. The plot below shows the IPA response to a metal sample that has been peened in an area along one edge. The elevated S parameter corresponds to the peened region.