The London equation involves the penetration depth of the magnetic field as a parameter. As the properties of the penetration depth may provide important information as to the nature of the superconducting state, a number of experiments were performed in the period between 1935-1950 to determine its magnitude and temperature dependence. The earlier experiments were performed on micro-size specimens, in which one (thin films), two (thin wires) or three (small spheres) dimensions are comparable or smaller than the penetration depth. Later on, the a.c. methods utilizing the change in mutual or self inductance which allows the use of macro-size samples were performed with frequencies ranging from low frequencies, radio frequencies and ultimately up to microwaves. In this chapter, the experiments and results obtained during this period are described.
The cryogenic interlaminar fracture toughness of woven glass-epoxy laminates was measured under mode I loading using double cantilever beam (DCB) tests. The tests were performed at room temperature (R. T.), liquid nitrogen temperature (77K) and liquid helium temperature (4K) to evaluate the effect of temperature and geometrical variations on interlaminar fracture toughness. The fracture surfaces were examined by scanning electron microscopy to verify the failure mechanisms. A finite element model was used to perform the delamination crack analysis. The results of the finite element analysis are utilized to supplement the experimental data.