Vol. 23
November Issue
Year 2022


in Vol. 23 - November Issue - Year 2022
Non-Contact, 3D Measurement of Shot-Peened Surface Texture

Figure 1. Manually comparing shot quality with a tactile gauge provides only qualitative data

Figure 2. Results from single trace measurements are highly dependent on measurement angle, length, and location

Figure 3. In the Structured Light technique (left), deformation to the incoming light is recorded and interpreted at the sensor. Adding polarization elements (right) enables the technique to be scaled down to a handheld, portable gauge

Figure 4. The 4D InSpec is an example of a PSL gauge. Here one is being used to measure a surface defect on a curved aircraft component

Figure 5. Contrast analysis estimates vs PSL gauge measurements of shot peened surfaces. The Sq (RMS roughness) parameter results were found to correlate well with the percentage of peening coverage

Figure 6. Data with the PSL gauge was reproducible between multiple measurements and different inspectors

Shot peening is a surface treatment which improves fatigue properties and increases material strength and longevity. A new technology enables reproducible, 3D measurement of shot-peened surface texture in a production environment. This non-contact, 3D method holds promise for reliable measurement of shot peen coverage.

Measuring shot peen quality

Shot peening is a widely used finishing technique that serves multiple purposes:
exposes and limits corrosion
prevents common damage such as cracking, galling, and fretting
increases strength and longevity of metals
increases fatigue strength 
improves gear teeth bending strength
enhances lubrication properties.

In these applications, the percent coverage (the amount of the surface impinged by the shot) is the primary aspect of peening that needs to be controlled during production. Coverage of 98% or higher is typical for effective peening. 
Coverage, however, can be notoriously difficult to quantify. Most commonly, an inspector estimates it visually or runs a fingernail across a surface to compare it with a sample coupon or tactile gauge (Figure 1). Such inspection methods are purely quantitative and highly operator-dependent.
Another common measurement method is to use a portable stylus to acquire a 2-dimensional trace measurement across the surface. While a stylus does provide quantitative results, the single trace only gives information about a narrow slice of the surface. Slight changes in the measurement angle, length, or location will greatly impact the results (Figure 2). A stylus is also incapable of measuring over-complex shapes or in inaccessible locations. In these instances, the operator must first make a cast of the surface using a replication material, then measure the surface of that cast. Replication is a slow and messy process, with high material costs as well.
Higher-end metrology systems such as 3D optical profilers are also occasionally used for peening measurement. These systems provide high resolution, areal (3D) measurements, though typically over a small field of view. Susceptibility to vibration typically relegates these types of systems to an environmentally controlled lab far from production. Because they are permanently mounted to rigid structures, the systems can only measure easily accessible features on sufficiently small components. Lastly, such systems tend to cost tens to hundreds of thousands of dollars and require trained personnel to operate.

A new non-contact measurement method

For some years, a technology known as Polarized Structured Light (PSL) has been used for production measurement of surface defects such as pits and scratches, and surface features such as dot peening, laser scribing, and rivet heights. In a structured light system, fine stripes of light are projected onto a surface. A sensor records any deformation in the reflected stripes, and that data is used to determine the surface shape (Figure 3, left). In a PSL system, polarizing elements are added which enable the technology to be scaled down and packaged in a handheld, portable gauge (Figure 3, right).
A PSL gauge (like the one shown in Figure 4) provides quantitative, repeatable, and operator-independent results, making it a significant improvement over visual or tactile inspection. Unlike a stylus, a PSL gauge will reproduce results regardless of measurement angle, stylus wear, etc. PSL gauges measure in seconds and are highly robust and portable, enabling use throughout a production facility. Measurements can be made at any orientation, in difficult to reach locations, and on large surfaces, all without replication. Because such systems also cost far less than higher-end metrology, PSL gauges have been widely adopted in industries such as aircraft manufacturing and refurbishing and auto manufacturing.
More recently, the PSL technique has been applied for measuring surface roughness, specifically on the individual layers of paint and coating systems. The technique has proven capable of measuring texture on raw substrates (metals, plastics, and composites), intermediate coats, and very smooth final coats. By using just one measurement technology to measure every layer, manufacturers can “fingerprint” a painting process to determine the texture requirements at each stage that will ultimately produce a good finish. With this information, manufacturers can ensure that only parts with acceptable surface texture continue through additional, value-adding production steps.

Measuring shot peen with a PSL gauge

The resolution, image area, handheld operation, and quantitative results of the PSL technique make it well-suited for measuring shot-peened texture. PSL data can include both 3D height maps and a range of surface roughness parameters. The Sq parameter, which reports RMS roughness of the measured surface, is particularly useful for specifying and controlling peening operations.
In a recent capability assessment, an aircraft manufacturer evaluated PSL technology for quantifying shot peen coverage. Firstly, the company used contrast analysis to estimate the degree of coverage on several test plates. 
Next, the plates were measured again using the PSL technique to determine Sq values. These measured results were found to correlate well with the degree of coverage (Figure 5). Since the PSL technique analyzes a larger surface area (13x13mm in this case), the results were more representative of the overall surface texture, and the parameter values were much more stable.
A second study was performed to test the reproducibility of PSL measurements of peened surfaces. For the study, three operators measured six areas of a roughness comparator gauge, 10 times each. In this case, Sa (average roughness) was measured. The nominal surface roughness of the measured areas ranged from 0.8 – 25.4 μm (32–1000 μin). As Figure 6 shows, the PSL measurement results were virtually identical between operators and over multiple measurements of the same location. The standard deviation for each measurement location was <1% of the roughness value at that location. This data confirms that the PSL technique can achieve reproducible, operator-independent measurement of peened surfaces.

Ongoing developments

PSL technology shows great promise for controlling shot peening, in addition to its proven applications for measuring defects, features, and roughness. A PSL-based instrument can be used handheld or mounted to automation, enabling production measurement of peening over complex parts and large surfaces. Such instruments can measure and analyze data in a few seconds, in any orientation, and despite variation in measurement angle, lighting conditions, and surface characteristics. 

For Information: 
4D Technology Corporation
3280 E Hemisphere Loop, Ste 146
Tucson, AZ 85706, USA
Tel. +1.520.294 5600
E-mail: 4dinfo@ontoinnovation.com