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.