The application of acoustic emission to the detection of fatigue-crack propagation in 7075-T6 aluminum and 4140 steel is investigated. The relationship between crack-growth rate, cyclic stress-intensity factor, load-cycling rate and observed acoustic-emission behavior is presented. Crack-growth rates of less than 10−6 in./ cycle could be detected, and acoustic-emission counts per cycle were shown to be closely related to the energy released by crack extension per cycle. A quantitative relationship for the threshold conditions for detection of fatigue-crack growth is presented which agrees with experimental test results. The results also showed that fatigue-crack growth occurs in an accelerating and decelerating manner, even though the stress-intensity range remains uniform, and that the count rate posses through a peak that is believed to be associated with a plane strain-plane stress transition. The effects of instrumentation sensitivity and frequency bandpass are also investigated. The results obtained indicate that acoustic-emission techniques should be suitable for in-service monitoring of a variety of cyclically loaded structures, even in the presence of high background noises.

Continuous monitoring of fatigue-crack growth by acoustic-emission techniques

A technique for measuring the energy sensed at an acoustic-emission transducer is presented that utilizes a squaring circuit and digital integrator. Theoretical relationships between energy and other more conventional acoustic-emission parameters, such as counts and RMS voltage, are derived for certain idealized cases. Experimental results from the following types of tests are presented: (1) unflawed tensile (‘continuous’ emission); (2) precracked stress-corrosion cracking; (3) precracked fracture toughness; and (4) fatigue-crack growth. Energy, counts, RMS-voltage, energy/event and counts/event measurements are included. In the case of unflawed tensile specimens, energy techniques appeared somewhat superior to counts. In all other cases, a direct relationship between counts and energy was obtained. Energy measurements tended to give a larger weight to higher amplitude events. Other than this, energy measurements appeared to have no advantage over counts. The theoretical relationship predicted between energy/event and count/event agreed quite well with experimental observations. Overall, the test results presented indicate that energy techniques provide no significant advantage over counting threshold crossings in cases in which crack extension in metals is the primary source of acoustic emission.

The measurement and significance of energy in acoustic-emission testing

Hydrogen embrittlement (HE) is considered to be part of stress corrosion cracking (SCC) problem and as a phenomenon of its own. In the first part of the review the present day methods for an identification of the role of HE in SCC are discussed and a general approach for any construction material is proposed to solve this question. The second part is concerned with those aspects of the HE theory which; are still not clear (acoustic data, the disposition of the fracture zones by HE ahead of the crack tip, critical hydrogen concentration) and with some newly proposed experiments.



Wasserstoffversprodung von Konstruktionswerkstoffen



Die Wasserstoffversprodung wird sowohl als Teilaspekt der Spannungsriskorrosion als auch als eigenstandiges Phanomen betrachtet. Im ersten Teil der vorliegenden Ubersicht werden die Methoden erortert, die heute zur Klarung der Rolle von Wasserstoffversprodung bei der Spannungsriskorrosion benutzt werden. Dabei wird eine Betrachtungsweise eingefuhrt, die sich auf alle Werkstoffe anwenden last. Der zweite Teil behandelt diejenigen Aspekte der Versprodungstheorie, die bisher noch nicht geklart werden konnten, so z. B. die Ergebnisse der Schallemissionsanalyse, die Lage der durch Wasserstoffversprodung erzeugten Bruchzone vor der Risspitze oder die kritische Wasserstoffkonzentration; dabei werden einige in neuerer Zeit zur Klarung vorgeschlagene Untersuchungsmethoden behandelt.

Hydrogen embrittlement of construction materials

Acoustic emission ring-down counting has been used to monitor sub-critical crack growth by stress corrosion of mild steel in disodium hydrogen phosphate, -brass in sodium nitrite and magnesium–7% aluminium in potassium chromate/sodium chloride mixtures, and by hydrogen embrittlement in mild steel and a 7179 T651 aluminium alloy. The level of acoustic emission activity varies widely between different alloy/environment systems and is dependent upon microstructure, fracture path and crack growth rate. The practical significance of the results is discussed.

Monitoring Sub-Critical Crack Growth due to Stress Corrosion or Hydrogen Embrittlement by Acoustic Emission

Effects of hydrogen on the mechanical response of α-uranium

Stress-corrosion crack propagation in U-4 1/2 wt%Nb has been monitored with crack-opening displacement and acoustic-emission techniques. Excellent correlation was obtained between the direct crack-velocity measurements and the emission count rate at all stress intensities. The data also show that, as the cracking velocity increases, the counts per unit area of new crack surface increase. The results of the investigation show the feasibility of acousticemission detection techniques as a useful stress-corrosion testing tool.

Acoustic emission and stress-corrosion cracking of U-4 1/2 wt%Nb

The principal reason for interest in uranium is the nuclear behavior of the 235 isotope. However, there has also been interest in depletedt uranium because of its high density (p = 19.1 g/cm3) and in some cases its structural properties. Because depleted uranium is a by-product of enriched nuclear fuel, it is less expensive than any other high-density material. Tungsten, with a density of 19.3 g/cm3, not only has a higher raw material cost but also is more expensive to fabricate. Other elements with densities above 15 g/cm3. Re, Os, Ir, Pt, Au, and Pu are very expensive. Therefore, in addition to their nuclear applications, uranium and uranium alloys have been used for ballast and counterweights where space is limited such as in aircraft and missiles, for non-nuclear ordnance as an armor penetrator, and for radiation shielding. A uranium radiation shield not only occupies less volume than an equivalent lead shield but can weigh two-thirds as much. Because of their mechanical properties uranium alloys have a great deal of potential as shipping container-shielding for spent reactor fuels where safety requires the containers remain intact in an accident.

Hydrogen Embrittlement and Stress Corrosion Cracking of Uranium and Uranium Alloys

Corrosion resistant canisters are an important component of the engineered barriers of a nuclear waste disposal system. In addition to providing containment for the waste form during transportation and emplacement, a durable canister can eliminate hydrothermal interactions with the waste form. The selection of a repository site and emplacement technique will affect canister design and could also impact material selection. While there are still many issues to be resolved, there are two different concepts being evaluated to provide durable canisters for waste disposal: (1) canisters fabricated out of extremely corrosion resistant materials such as Ti-base or Ni-base alloys, and (2) canisters fabricated out of less durable materials but designed with a corrosion allowance. Each of these types of canisters could fail to meet the design objectives through a variety of failure processes. The more important of these are discussed.

Nicholas J. Magnani

Papers

Acoustic emission and stress-corrosion cracking of U-4 1/2 wt%Nb

Hydrogen Embrittlement and Stress Corrosion Cracking of Uranium and Uranium Alloys

Corrosion Resistant Canisters for Nuclear Waste Isolation