PSI - Issue 59

M. Karuskevich et al. / Procedia Structural Integrity 59 (2024) 175–181 M. Karuskevich et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Two different procedures of specimens treatment by CPCs were used: First procedure presumes more intensive penetration of the CPC into the gap of the lap joint by the way of triple treatment by the CPC, this procedure is called here as "Excessive"; second procedure is single covering, it limits the volume of the CPC in the area of the joint, this procedure is called "Limited". The treatment by the Reference Compound was carried out according to the "Excessive" procedure. Fatigue tests have been carried out with test machine UTM-25 equipped by digital controller MTS FlexTest GT with loading frequency f = 2 Hz, maximum stress of cycle 120.0 MPa and stress ratio R = 0.1. 4. Experimental results and discussion Experimental results prove the actuality of the presented research and encourage further activity in the optimization of CPCs application technology. Figures 3 and 4 compare average fatigue life for the groups of the specimens treated by the CPC1, CPC2 and Reference Compound with life of the specimens without CPC. The number of specimens in group was from 5 to 8.

Fig. 3. Comparison of the average number of cycles to failure for groups of specimens: without CPC, and treated by the "Excessive" procedure: by the Reference Compound (RC); by CPC1; by CPC2.

As it is seen clearly from Fig. 3 "Excessive" application of the CPC1 reduces average number of cycles to failure up to 57%; "Excessive" application of the CPC2 reduces average number of cycles to failure up to 65%; "Excessive" application of the Reference Compound reduces average number of cycles to failure up to 32%. As following experiment shows, the situation can be improved by the optimization of the treatment procedure. Factor of friction is suggested as to be a dominant in the explanation of the revealed effect. General tribology considers the following basic types of friction: dry friction, as an interaction of mating surfaces without lubricants; fluid friction, as friction in the presence of a lubricant between rubbing bodies; mixed friction, where dry and fluid friction regions exist in the contact zone; and boundary friction, when the lubricant thickness is smaller than 10 atomic layers or two rubbing surfaces separated by a lubricant layer interact with each other due to asperities, roughness, and so on (Lyashenko, 2011). The friction coefficient obviously depends on the type of friction. The gap thickness between the elements of the riveted joint can vary. This can explain the reduction of the riveted joints fatigue life at the excessive volume of the lubricant and increase of the fatigue life at the limited volume of CPC. The search of the way to prevent negative side effects of the CPC application has led to the modification of the treatment procedure. As the negative effect is considered to be result of the friction, the reduction of the lubricant in the gap of the lap riveted joint is expected to reduce the CPC negative effect.