Laser Ultrasound Sensor Tested in Paper Mill

Daniel S. Burgess

Scientists from Lawrence Berkeley National Laboratory in Berkeley, Calif., have completed a full-scale demonstration of a noncontact laser ultrasound sensor designed to inspect moving sheet materials, such as paper. The trials were performed at a paper mill in Jackson, Ala., owned by Boise Cascade LLC of Boise, Idaho. Based on the assessment of the company's engineers, the scientists are hopeful that a follow-up, six-month test will be performed at the same facility.

The laser ultrasound sensor measures the bending stiffness and out-of-plane shear rigidity of sheet paper moving past it at speeds of up to 30 m/s.

Developed in collaboration with the Institute of Paper Science and Technology at Georgia Institute of Technology in Atlanta, the system calculates two elastic properties of the paper from the measured propagation of laser-induced shock waves through it. The acoustic waves are produced by 5-ns pulses of 1.06-µm radiation from a New Wave Research Nd:YAG laser, which are delivered via a 20-m-long fiber and are focused on the surface with an aspheric lens.

Each pulse ablates a microscopic spot on the paper, and the velocity of the wave it generates reveals the bending stiffness and out-of-plane shear rigidity of the material. To measure the velocity, a Polytec PI Mach-Zehnder interferometer interrogates a point at a set distance several millimeters from the excitation spot until the wave is detected. Because the paper continues to move at up to 30 m/s during this time, the scientists included a rotating mirror in the interferometer setup to cause the detection point to move with the surface.

The team estimates that the incorporation of such sensors into US paper mills could save $200 million in energy costs and $330 million in fiber costs annually because they offer a real-time means of inspecting the quality of paper as it is being manufactured. Currently, mills analyze a sample from the end of a 15- to 30-ton roll. When they find that the sample does not meet specifications, they recycle the roll or sell it as inferior-grade paper. The system also is suitable for the inspection of sheet steel, aluminum and plastics.

Paul L. Ridgway, who developed the laser ultrasound sensor with fellow Lawrence Berkeley scientist Rick E. Russo, will describe the technique and the results of the tests on Aug. 3 at the annual Quantitative Nondestructive Evaluation conference at Bowdoin College in Brunswick, Maine.