impact analysis of rare earth magnet materials

Background - High-energy product rare-earth permanent magnets composed of neodymium-iron-boron are part of a magnetic circuit being developed for highly miniaturized magnetic deflection mass spectrometers. This quick-look project was implemented to determine the survivability of these brittle rare-earth permanent magnets to high-gravity loads expected to occur during high-velocity impacts. The mass spectrometers were proposed as an integral part of highly instrumented moon penetrators for a NASA Discovery class mission called Polar Night. The Polar Night proposal was an undertaking by a team including SwRI. The goal of the proposed Polar Night mission is to investigate the composition and distribution of volatiles on the moon, specifically, to determine the potential existence of water ice cold-trapped on the lunar poles. This goal will be accomplished by delivering four to six penetrators, each equipped with two neutron spectrometers, a soil sampling system and differential scanning calorimeter, coupled to a cycloidal focusing mass spectrometer. An orbiting spacecraft serves as the data link to the penetrators, and it is equipped with a high-resolution neutron spectrometer and cameras.

Approach - SwRI¡¯s responsibility for the mission was to provide the cycloidal focusing mass spectrometer. The magnet circuits were designed in dipole configuration based on preliminary finite-element modeling to attain the required field. To each of the assembled magnets, five matched and prewired, three-axis Hall-effect transistors to measure the magnetic field were bonded in place. One sensor was in the center and one sensor was located on each of the four corners to provide a magnetic field measurement at five separate locations across the face of the magnet. Magnetic field measurements were taken before the magnets underwent shock testing.

SwRI¡¯s bi-rail shock test facility was used to simulate the impact velocity that the penetrator would encounter on the moon. The research team designed the test to allow the magnets to be shock tested in two different orientations with respect to the direction of magnetization. One test was in the direction of polarization, and the other was normal to the direction of polarization to determine which direction minimizes slip domain formation. Research team members set up the bi-rail shock facility and established the two heights required to achieve 1,000 and 3,000 g's using a dummy load instrumented with an accelerometer. During the impact test, an accelerometer attached below the magnet under test obtained a precise measurement of the impact force encountered by the test magnet. The total dwell time of two to three milliseconds was achieved by using a hard rubber programmer designed to produce a constant deceleration. Each instrumented magnet assembly was bolted onto the mounting jig and allowed to drop at a set height onto a rubber programmer to achieve 1000 g's in two orientations and at 3000 g's in the preferred penetrator mounting orientation.

Accomplishments - Rare-earth magnets designed for a miniaturized cycloidal focusing mass spectrometer were successfully shock tested. No measurable loss of magnetic field or mechanical structural failure occurred. Project results indicate that the magnet circuit configuration and materials used in fabricating the magnet circuit can survive the impact expected for the moon penetrators on the Polar Night mission. The 3,000-g impact test exceeded more than three times the force that the penetrators are expected to experience.