Impact Test of a Laser Window Made of Sapphire Bonded to a Stainless Steel Housing

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Test samples simulating sapphire laser windows were subjected to an impact test where a steel ball was dropped onto the test sample. The impact test of a laser window made of sapphire demonstrates that the sapphire was cracked upon impact of a steel ball weighing 1,212 grams, dropped from the height of 700 mm.

Unit Under Test, UUT

Unit under test was a round test window with a sapphire disk bonded into a metal housing. Materials are as follows:

  • C-Axis Sapphire, Kyropolous material, 35mm Diameter, 5mm thick.
  • Housing: 316 Stainless Steel, EN 1.4408
  • Components bonded together by a proprietary method.

The sapphire disk was bonded into the stainless steel housing in the same manner as the actual laser window products. The laser windows are of proprietary design and are not shown in this report. Test samples provide perfect correlation to the actual windows, as the material and fabrication processes are the same. Stainless steel housing of a test sample has the same coefficient of thermal expansion, (CTE), as the actual flange. Differences in geometry between test samples and actual products do not affect the unit performance during the impact test and a thermal shock test.

Laser Window, Test Sample, Sapphire in Stainless Steel, After Impact

Sapphire window after impact

Impact Test Setup

Description of Equipment for the Impact Test:

  • Height Gauge: Digital, 24 inches, SPI brand
  • Impact Ball: 66.68 mm in Diameter. Material: E52100 Alloy Steel, Grade 50. Weight: 1,212 grams. McMaster Carr Part Number: 9528K88.
  • Sample Support: Metal stool with rigid (no elastomer) steel legs holding a wooden top. The stool was placed on a bare concrete floor.
  • Sapphire holding fixture: Encole fabricated a support ring to elevate the test sample above the test surface.

Impact Test Description

The entire test took seven minutes, during which the ball was released onto the test sample from various heights. We started the test from 4 inches, (107 mm). This is the distance from the lowest point of the ball and the top surface of the sapphire. We continued to increase the drop height in 4-inch increments. The sapphire broke on the seventh (7th) impact from the height of 700 mm. This was the maximum achievable height for the given test setup. In other words, the height gauge could not reach any higher. We then repeated the impact test using a second test sample. Instead of accumulating stresses from several drops, during the second set we dropped the ball onto the sample only one time from the height of 24 inches, (607mm). The test sample withstood the impact without any signs of damage.

Impact Test Results

The sapphire disk broke, separating the center portion of the sapphire, as seen in Figure 3, and under the following conditions:

Rigid support system of the test sample. (Metal pedestal with a wooden top; the pedestal resting on a bare concrete floor).

  • Drop Ball Weight: 1,212 grams
  • Drop Ball Material: E52100
  • Drop Ball Diameter: 66.68mm
  • Drop Height: 700 mm
  • Number of impacts before breakage: 6
  • Starting Drop Height: 4”, (101.6 mm)
  • Incremental Height Increase: 4 inches for each consecutive drop.

Conclusion of the Sapphire Impact Test

The impact test of the sapphire sample simulates an explosion test where the ball is releasing all of its energy in a concentrated area in the center of the sapphire.

The ball at the moment of impact had kinetic energy of 8314 Joules.

Assuming the entire test sample traveled 3 mm downward during the impact, the average impact force is calculated to be 2,771,440 Newtons.

We prepared a White Paper in a collaboration with a mechanical engineering lab of a university in California.

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