1. Formulation of the Problem. Predicting the residual service life of parts subjected to fatigue loading is a complex problem. For example, fatigue-limit data obtained by construction of the so-called Weller curve [1] only allow estimation of averaged characteristics of the material and provide no information on such an important indicator of product reliability as service life (fatigue life) [2]. Fatigue fracture usually occurs suddenly, without any noticeable outward signs of its impending occurrence. Microscopic studies [1, 3] show that microscopic damages gradually accumulate during fatigue. Then the fatigue crack undergoes slow latent growth that ends with the catastrophic growth of the main fracture crack. The existence of a long preparatory stage in the fatigue process suggests that it might be possible to find some suitable method of delaying the transition to the final stage. To do this, two problems must be solved:9 Selection and substantiation of a reliable and sufficiently informative indicator of the transition to the dangerous stage of structural changes for a specific product; 9 Creation of methods of delaying the development of dangerous defects whose growth could lead to fracture in a short period of time (these methods must be suited for use directly on machine parts or products).2. Informative Indicator of Fatigue Fracture. A convenient quantity that provides reliable information on the structure of metals and alloys and its changes is the ultrasound velocity (USV) in metals and alloys [4]. The fact that this quantity is determined by the elasticity modulus G (for transverse waves) and the density of the material p, i.e., VR = (G/p) 1/2, does not fully reflect the depth of the problem [5]. It was found in [4] that nearly all of the structural changes caused by heat treatment, alloying, or deformation lead to small but measurable changes in the USV. The measurement of USV has proved promising for diagnosing materials under fatigue loading. Measurements made by a method involving the automatic circulation of pulses of ultrasonic surface waves at a carrier frequency of 2.5 MHz with the use of an ISP-11 device [4] have shown that the dependence of the ultrasonic velocity on the number of loading cycles N is qualitatively the same for all specimens. Figure 1 shows data from the use of bending vibrations for the fatigue testing of specimens of steel St. 45 with a mean stress. Similar relations have been obtained also for specimens of rail steel M76. In all the cases, the curve of AV/VR(N) (AV is the decrease in the USV compared to the initial value of this quantity, i.e., VR) includes three successive stages. However, the level and rate of the quantitative Institute of the Physics of Strength and Materials Science, Siberian Division, Russian Academy of Sciences, Tomsk 634021.
The criteria for the maintenance evaluation of long lived metal of the equipment of heat power plants during the acoustic structuroscopy of weld joints of vapor pipelines and boiler drums are described. The data on the change of the structural state of metal during long term operation of heat resistant steels are given. The dependences of the criteria versus the values of local fields of internal stresses of a crystal lattice are given. The relationship between the integrated criterion and the durabil ity of heat resistant steels is presented.
The influence of dislocation density on the I-V characteristics of InP diodes under reverse bias has been investigated. It is observed that the leakage current increases with increasing dislocation density whereas the breakdown voltage is reduced. Further, precipitates are observed on dislocations in t h e , p-regions formed by t h e in-diffusion of Cd into n-type InP layers. It is argued that the increase in leakage current is due to t h e presence of space charge cylinders around dislocations threading the p-n junction. On the other hand, precipitates may be responsible for premature avalanche breakdown. In addition, for the range in energy scanned, levels within the bandgap d u e to dislocations are not observed by photoluminescence measurements. Compounds (Inst. Phys. Conf. Ser. 17) p 119 decreases with increasing dislocation density, This effect [16] Eaves L, SIlli(k1 A w, Skolnick bf s alld Cuckayne B A232
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