Dual-motion Fretting Wear Characteristics Of Zr-4 Alloy Tube Under Random Conditions

The fuel assembly is one of the core components in primary circuit loops of a pressurized water reactor（PWR）,which is mainly composed of nuclear fuel rod and elastic positioning lattices.The rod-shaped nuclear fuel is encased in a zirconium cladding,and a plurality of cladding tubes are clustered into a bundle,which is supported by an elastic positioning lattice.In the reactor,the flow-induced vibration of circulating water in the primary circuit could cause the complex alternating loads between the elastic support mechanism and the cladding tube,which eventually leads to random fretting wear.Severe fretting wear may even cause cladding tube damage and nuclear fuel leakage accidents,which directly threatens the safety and reliability of fuel cladding tubes during their service life.Therefore,it deserves to further study the random fretting wear characteristics and the damage mechanism of Zirconium-4（Zr-4）alloy cladding tubes used in engineering services.This paper not only enriches the fretting wear theory,but also provide reference for studying the damage of Zr-4 alloy tubes in the engineering practical application.In this thesis,the random fretting wear test was realized by inputting the random displacement and load spectrum into the controller of the self-developed multi-mode fretting wear testing machine.An orthogonal point contact pair of two tubes was constructed to simulate the elastic contact state between the cladding tube and the positioning lattice,and the tangential and dual-motion fretting wear tests were carried out under random conditions.Using microscopic characterization equipment,this paper analyzed the fretting damage characteristics of Zr-4 alloy tubes under different test conditions,and further studies the random fretting running behavior and damage characteristics.The conclusions obtained are as follows:1.The tangential force-tangential displacement（F_t-D_t）curves of random tangential fretting wear shows that the fretting running regime of random test changes in real-time.When the load is increased under the same random displacement spectrum,the majority of the fretting regime in the spectral period will change from the slip regime to the partial slip regime.In the whole fretting period,the higher friction coefficient and friction dissipation energy were obtained when the proportion of slip regime was larger.The results of wear morphology showed that the higher the proportion of slip regime could lead to the more serious fretting damage.Compared with the constant and random tangential fretting wear test results under the same displacement total mileage and load value,when the random friction coefficient is lower,the wear rate is higher.2.The force-displacement（F-D）curves of dual-motion fretting wear under the combined action of tangential displacement and radial load could be analyzed from radial and tangential perspectives.The radial force-radial displacement（F_n-D_n）curves of constant dual-motion fretting showed that the increase of radial load leads to the increase of radial elastic-plastic deformation and the radial dissipation energy.The constant dual-motion fretting F_t-D_t curves could be divided into trapezoidal（slip regime）,elliptic（elastic-plastic coordinated partial slip regime）and linear（elastic coordinated partial slip regime）.With the increase of radial load,the fretting running regime will transform from slip regime to partial slip regime,and the friction coefficient and frictional dissipated energy will decrease.The wear trace characterization results showed that the fretting damage degree is lower in partial slip regime.3.The F_n-D_n and F_t-D_t curves of random dual-motion fretting changes in real-time.With the random radial load increasing,the proportion of elliptic in the F_n-D_n curve decreases,and the radial dissipation energy of the random test is decreases.When the load is increased under the same random displacement spectrum,the majority of the fretting regime in the spectral period will change from the slip regime to the partial slip regime.In the spectral period,the higher the proportion of slip regime is,the higher the friction coefficient and frictional dissipated energy are,and the more serious the fretting damage is.4.In different contact states,frequency has various effects on fretting wear.For constant fretting wear,with the increase of frequency in the slip regime,the dynamic data and morphology characterization results showed that the friction dissipation energy and wear area increase,but the friction coefficient and wear rate decrease.For the constant fretting partial slip regime,the material is in two-body contact state,and the frequency has little influence.For the random test,the F_t-D_t curve of random fretting wear showed that the increase of frequency could also reduce the proportion of slip regime in the spectral period.The friction coefficient,friction dissipation energy,wear volume and wear rate decrease with the increase of frequency.5.In constant test conditions,the wear marks in the slip regime are mainly abrasive wear,oxidation wear and peeling.The wear marks in the partial slip regime are adhesive at the center and microslip at the edge,but the area of the central adhesive area in the dual-motion fretting is smaller.Under the condition of random test,the wear marks mainly showed the wear characteristics of slip regime,and the adhesive wear is more likely to occur when the proportion of partial slip regime increased.In the wear area of the constant dual-motion wear,compared with the constant tangential fretting wear,the plastic deformation and spalling are more serious,and the wear degree is higher.Compared with random tangential wear,under the same load（average load）,the slip regime of random dual-motion fretting wear was less,and the wear volume and wear rate were also lower.According to the X-ray photoelectron spectrum（XPS）test results,the abrasive chip in the abrasion area was mainly composed of ZrO₂.