Robot Shot Peening Conceptual Approach

When setting up for robotic shot peening, the calibration data for the peening tool is entered in the Shot Peening Profile. The peening profile also includes the definition of the nozzle location and the robotic simulation parameters. The Surface Trajectory command creates a motion trajectory based the geometry of the surface to be processed. The Surface Task command then creates a completely functional and ready to download robot task from this predefined surface trajectory.

A graphical representation is provided for three types of analysis results (Intensity, Coverage, and Inclination error). You can modify the trajectories and the robot tasks to achieve the required values such that they are uniform and consistent over the entire processed part surface.

Shot peening is a modern high technology surface treatment process that provides a critical and unique value. It is deceptively simple to describe but highly challenging to model and simulate. The goal of this type of surface treatment is to greatly increase the fatigue resistance to repetitive cyclic stress of the material being processed.

The robotic shot peening process involves shooting at high speed the peening shot material, such as miniature steel balls, at the surface of the part being processed. The procedure is called peening because it is the modernized version of the historical ball peen hammer technique used in traditional metal working. The shot peening process results in a residual compressive stress layer on the surface of part being processed. Such a procedure results in dramatically improved cyclic failure properties, typically as much as a ten times improvement. This improvement is achieved because typical stress failure starts as surface micro cracks when the part material surface is subjected to cyclic tension, and the residual surface compression left over from peening effectively negates this type of micro crack initiation.

The shot peening surface treatment process is an important aspect of manufacturing high reliability engineering components such as aircraft landing gear struts and engine turbine blades as shown in the images below. Modern peening technology uses programmable robotics to apply the shot so that the complex shapes of high technology parts being processed can achieve consistent quality and peening coverage over the entire part surface while maintaining an optimal manufacturing cycle time.

Example Robotic Shot Peening Workcells

Experimental research has determined that the quality of the results for a shot peening process depend mainly on the intensity of the peening. The Intensity parameter quantifies the amount of energy being impacted on the surface by the streaming shot. The other process variable to be tracked during shot peening is the duration of peening impact, which is the Exposure Time at any point of the surface being peened. This exposure time is directly affected by the industrial robot motion speed relative to the surface being processed. Peening test experiments have also established that the increase in intensity of peening is not linear with the duration of impact. Instead, the behavior is such that for a given peening setup with increasing exposure time duration, the intensity reaches a particular saturation level.

The Intensity of a shot peening process is measured and monitored by means of a standardized type of calibration test called an Almen Test. The Almen Strip for such a calibration test is made of spring steel with strict tolerances for hardness and flatness. During the test, the Almen Strip is peened on one side only, and the effect of the induced compressive stress on one side of the strip results in it becoming very slightly curved. This deflection Arc Height is correlated to the energy imparted by the shot and it is measured using an Almen Gauge. The Intensity resulting from a peening process is measured in units of mm of Almen strip arc height curvature.

The resulting test Almen strip arc height varies according to both the velocity and mass of the peening shot. Intensity is therefore related directly to the amount of energy imparted by the stream of shot and absorbed by the strip. The increase in intensity (and hence the Arc Height in test measurement) is not linear with Exposure Time but curves towards a saturation limit. Such an Intensity versus Exposure graph is generated by doing multiple tests of the peening setup with different robot motion speeds. Based on this graph, the peening Saturation Limit is defined as the point on the curve such that peening exposure time being doubled results in the arc height or deflection of the Almen strip increasing only by another 10% or less. Finally, given the above calibration data for a robotic shot peening setup, the actual peening results are a consequence of the industrial robot program as it follows the complex surface and the effective speed, distance, and inclination of the robotic peening tool over the course of the duration of peening as it proceeds.

As can be seen from these above details, the challenge for planning and implementing the robotic shot peening of a complex engineered part is to be able to select peening process parameters as well define and optimize the industrial robot program so as to get the correct intensity and coverage of the surface treatment over the entire complex shape of the part. The 3DEXPERIENCE | DELMIA Robot Surface Simulation App meets this challenge.

The solution approach involves calibration of the shot peening system for its Almen Test properties along with functionality for robot trajectory generation and a peening simulation solver. During simulation, the solver extrapolates mathematically the computation of shot peening results from the tool calibration and the simulation related details of the surface geometry as well as the shot peening beam impact attributes at each step update such as beam distance, surface normal inclination, mass flow rate, air pressure, interacting with the simulation of the industrial robot motion trajectory and speed.

The robotic shot peening simulation results are represented graphically in the style of color coded contour threshold maps for the different analysis results such as Peening Intensity (required to check that all areas are subjected to adequate Intensity of Shot Peening for the desired fatigue resistance), Exposure Time Coverage (required to check that all areas are subjected to adequate duration of Peening Exposure time), and Beam Inclination Error (required to check that all areas were peened with the Beam at an adequately vertical direction to the surface) over the processed part surface.