An optical-interference method for experimental stress analysis of cracked structures

An optical interference technique can be used to measure the crack opening in a cracked transparent model of a structure under load. The measured crack opening as a function of distance from the crack tip can be used in conjunction with the opening mode crack tip stress field equations to determine the opening mode stress intensity factor. Application of this method to double cantilever beam specimens comprised of two glass arms bonded together with an epoxy adhesive gave results which were in close agreement with K-calibrations based on compliance measurements. The measuring techniques and the scheme of analyzing the data could be used to obtain K-calibrations of more complex structures.     

Non-Destructive characterization of hydrogen-embrittlement cracking by acoustic emission techniques

Acoustic emission techniques are used to determine the rate of cracking of high-strength steel due to hydrogen embrittlement, making it possible to predict failure in certain structural components undergoing hydrogen attack. The acoustic emission rate from a hydrogen-charged, linear-compliance, fracture-toughness specimen of high-strength 4340 steel was recorded as a function of the stress-intensity factor at the crack tip in the specimen. The structural component (a hydrogen-charged boh of the same material) was then torqued down in a simple fixture and the acoustic emission was recorded as a function of time. The acoustic emission rate obtained from a near-critical value of the stress-intensity factor of the fracture-toughness specimen was used to predict within ±15 per cent the time for the onset of unstable failure of the bolt

The most significant result of this investigation is that regardless of the initial loading, nucteatkm time for cracking to begin, or any other factors connected with geometrical differences in specimens, the acoustic emission rate can be used to determine the time for and the stress intensity factor at the onset of rapid fracture.     

A gamma-level portable ring-core magnetometer

A four-transistor circuit is described for a battery operated magnetometer that has 0.1γ resolution using 1-in diameter 4-79 or 6-81 Mo-Permalloy tape cores of 20-40 Mx capacity. The center-tapped drive winding also serves as the sense winding, simplifying the sensor assembly. Some signal-to-noise results are presented to illustrate the fairly wide difference that may exist between cores of the same cross section and to demonstrate the advantage of reducing core area.