.2124 Aluminum Alloy Sheet Fracture Toughness And High Cycle Fatigue Properties

The fracture toughness and high cycle fatigue characteristics of 2124-T851 aluminum alloy plates were investigated using MTS.Meanwhile,the influences of the microstructure on the alloy's fracture toughness and high cycle fatigue characteristics were researched using optical microscopy(OM),transmission electron microscopy (TEM)and X-ray diffraction(XRD),as well as the fatigue fractography observed by scaning electron microscopy.The results show that:(1)The fracture toughness of 2124-T851 aluminum alloy plates is anisotropic,and the crack growth resistance in L-T direction is better than the one in T-L direction.The 30mm-thick-plate has the best all-around fracture toughness that 29.64MPam/2 in L-T direction,and 24.31MPam/2 in T-L direction.(2)The high cycle fatigue strength of 2124-T851 aluminum alloy plates decrease with the increase of the plate thickness.Under the condition of room temperature and stress ratio R=0.1,the fatigue strength at 10⁷ cycles of 2124-T851 aluminum alloy with 30mm thickness is 243MPa.(3)At high cycle fatigue condition,the higher the stress amplitude, the lower the fatigue life,and the greater the ratio between the intermittent fracture area and the whole fracture area.The influence of the stress amplitude on the fatigue life of alloy plates dues to the aggrandizement of space between fatigue striations and advance of fatigue crack developing into the intermittent fracture area.So the fatigue resistance can be improved effectively with the increase of alloy's fracture toughness.(4)The calculated fatigue crack growth rate results of 2124-T851 aluminum alloy plates with 30mm,40mm and 55mm thickness are da/dN=2.8×10^-8(â–³K)^3.5;da/dN=4.4×10^-8(â–³K)^3.4;and da/dN=5.7×10^-8(â–³K)^3.3,respectively.Because of the crack closure effect,the time of cyclic loads is more than the number of fatigue striations which can be observed using SUM,so the results of estimation do not tally with the observed separation of fatigue striations well.(5)The fatigue crack growth of 2124-T851 aluminum alloy can be divided into three regions obviously.Generally,the fatigue crack initiates at the surface of the samples,then a number of fatigue striations form during the process of fatigue crack propagation,and the separation of them becomes larger.Lastly,the rapid fracture happens,and the mixed fractography of little dimples and cleavage fracture can be observed.(6)The relationship between the stress intensity factor D-value and the fatigue crack developing rate can be described using Paris equation just during the process of theâ…¡stage of crack propagation. And during theâ… andâ…¢stage,the fatigue crack growth rate speeds rapidly with the enhancement ofâ–³K.Because the rapid fracture stage forms relating with the fracture toughness of alloy,when the K_maxreaches to the K_ICthe fracture happens.(7)The microstructure of the 2124-T851 aluminum alloy is partially recrystallized.The grain size in normal of rolling plane augments with the increase of plate thickness.The main phases of the as-received alloy includeα-Al,S'(Al₂CuMg)and Al₆(Fe, Mn)(8)The grain boundary,and sub-boundary can effectively restrain the initiation and propagation of fatigue crack,so they are propitious to the fatigue resistance and fracture toughness.The effect of thickness on the fracture toughness is primarily due to the differences of alloy's grain boundary and sub-boundary owing to the differrent thickness.