A Study Of Fundamental Fracture Mechanics Of Aluminum Alloys Used In Expansion Tube Separation Joint

An aerospace-utility separation device known as expansion tube is used to link theaerospace shuttle and its vehicle together before exploding. During the process of separation,due to the shock wave of the explosion, the aluminum alloy plates are impacted drasticallyand then fracture into halves as the preexisting crack in its ligaments area propagates rapidly,and then the vehicle that was linked to the shuttle by the plates will be released immediately.The way the crack propagates in the ligament area is classified as Crack Propagation Modes IBased on the fundamental mechanics of crack propagation and in order to determine thematerial toughness and the sensitivity to crack, this study has developed several methods tocarry out the three point bend tests with the2024-T4and7075-T6aluminum alloy materialsunder both the static loading conditions and dynamical loading conditions.In this study, we have not only computered the values of toughness of two kinds ofaluminum alloys but also analyzed in detail the reasons why the two metallic elements withdifferent toughness parameters behave so differently. It is out of the question to get thenumerical expression of dynamical toughness KIC(namely the critical stress intensity at thecrack tip) under current experimental conditions. But in this study we have applied aspring-mass mode to obtain the final result of KICunder dynamical loading in three point bendexperiment. Eventually, we designed several experimental approaches to realize these goals asfollowed.1) Develop a static three point bend method in order to test the fracture toughness valuesand variation with time of stress intensity at the crack tip of2024-T4and7075-T6aluminumalloys.2) By using the drop-weight Charpy machine we have developed a three point bendmethod to get P(t) plot under dynamical loading, with using the chronoteine device and theDigital Speckle Correlation Method(DSCM) we can determine the instant when the crack tipreaches a critical value and then obtain the final result about fracture toughness values andvariation with time of stress intensity at the crack tip of2024-T4and7075-T6aluminumalloys.By these approaches described above we have drawn the conclusions as followed. (1)During the course of experiments, we applied the techniques to it and then found theresult is reasonable, so we can assume that the techniques for this experiment are reliable andthe conception are feasible. At last, under0.1mm/s，0.5mm/s、0.01mm/s loading speeds weacquired the result of the material toughness values of2024-T4aluminum alloy varied from20.997MPa m1/2to25.675MPa m1/2and from20.997MPa m1/2to25.675MPa m1/2of7075-T6.(2)Under the dynamical loading condition, the fracture toughness value of2024-T4aluminum is from25.14MPa m1/2to28.45MPa m1/2, the average of KICand tcare26.63MPa m1/2and0.525ms comparatively lower than7075-T6’s which varies from28.02MPa m1/2to37.09MPa m1/2the average of KICand tcare33.7MPa m1/2and0.532ms. Thenwe can draw a conclusion as under dynamical loading, the competence for resisting fractureof7075-T6is higher than2024-T4,it addition to this, we also found that when the altitudes ofdrop weight are getting higher, namely the shock velocities are higher, the values ofdynamical toughness of both alloys are becoming larger. It is because as the loading changerate goes up, the competence of alloys for resisting fracture is enhanced accordingly.(3)Through the optical micrograph method we have analyzed the characteristics on theintersection of the fractured specimen tested both in static and dynamical experiments. Thespecimen recollected from the three point bend experiments have shown that on theintersection there are atomic bonds under both static and dynamical loading conditions, so weassume the failure type of these two aluminum alloys are brittle fracture associated withductile fracture.