A LABORATORY TEST OF THE EQUIVALENCE PRINCIPLE AS PROLOG TO A SPACEBORNE EXPERIMENT

To test the equivalence principle (EP) to an accuracy of at least σ(Δ g)/g = 5 × 10-14, we are developing a modern Galilean experiment. In our principle-of-equivalence measurement (POEM), we directly examine the relative motion of two test mass assemblies (TMA) that are freely falling. Such an experiment tests both for a possible violation of the weak equivalence principle (WEP) and for new forces that might mimic a WEP violation. For the terrestrial version of the experiment, there are three key technologies. A laser gauge measures the separation of the TMA to picometer accuracy in a second as they fall freely in a comoving vacuum chamber. The motion system launches the TMA from their kinematic mounts inside the chamber and keeps the chamber on a trajectory that mimics free fall until the chamber nears the bottom of its motion. It then "bounces" the chamber back to upward motion in preparation for a new launch of the TMA. A capacitance gauge system measures an additional four degrees of freedom of the motion of each TMA. The resulting estimate of the rotation around and translation along the horizontal axes is used to correct systematic errors. We describe the status of POEM and discuss recent progress.

Experimental analysis of high-frequency stress waves in a ring

This paper deals with the photoelastic analysis of stress waves in a thick ring subjected to transient high-frequency loading. The device used to apply the loading is a piezo-electric transducer which initially produces four cycles of a 31·25 kHz toneburst. The loading pulse can be reproduced with high precision. The ring is illuminated with a flash of approximately 0·5 μs duration which can be delayed in a continuously adjustable manner. Isochromatics and isoclinics can be directly observed as static patterns. Photoelastic data are supplemented by measurements obtained with a capacitance gauge used as a dynamic lateral extensometer. Satisfactory agreement is found between tangential stresses near the free boundary of the ring determined from photoelastic data and those determined from capacitance-gauge measurements. As examples of complete determination of stress distributions, the separated principal stresses are obtained along an axis of symmetry and a vectorial representation of the principal stresses is given. A wave interpretation of the data is developed and comparisons are made with available solutions of related problems.

Experiments on Dispersive Pulse Propagation in Laminated Composites and Comparison With Theory

Transient stress-wave experiments on laminated composites are described, and the results are compared with theoretical predictions. The composites are laminated from alternating layers of high and low-modulus material, which cause a high degree of geometric dispersion of waves propagating in the composite. Experiments were conducted in which waves propagated parallel to the laminations. Flat plates were subjected on one face to a uniform pressure with step-function time dependence induced by a gas-dynamic shock wave. Under this loading, the central portion of the specimen initially responds as if it were laterally unbounded. The average velocity over a 3/8-in-dia area of the backface of the plate was measured with a capacitance gauge. The results are in good agreement with theoretical predictions made with a long-time asymptotic approximation called the head-of-the-pulse approximation. The theory isolates the dominant character of the response and predicts timing and amplitude of oscillations in normalized rear surface velocity within a few percent.