Study On Fatigue Crack Propagation And Fatigue Life For Cast Aluminum Alloys Under High-cycle&Low-cycle Interaction Loading

The lightweight design in the automotive industry has been growing rapidly because of theincreasingly need to reduce weight and increase fuel efficiency. Automotive engine components, suchas cylinder heads and cylinder blocks, which are increasingly manufactured by cast aluminum alloysinstead of cast iron, are subjected to complex high-cycle and low-cycle (HCF-LCF) interaction loadhistories. The traditional durability design based on high-cycle and low-cycle fatigue alone might benon-conservative and not appropriate. Study on the HCF-LCF interaction fatigue behavior for castaluminum alloys would provide fundamental understandings for the development ofhigh-performance&light-weight engines in future. Unfortunately, the HCF-LCF interaction fatiguehas received limited investigation in the literature. The present thesis aims to get a betterunderstanding of the HCF-LCF interaction fatigue mechanisms using experimental and theoreticalapproaches. Fatigue crack propagation model which is able to predict fatigue crack propagation ratesunder HCF-LCF interaction loading are developed. The HCF-LCF interaction fatigue damage modelis also built based on the experimental results.The main work of this thesis is summarized as follows,(1) Fatigue crack propagation tests including the fatigue crack propagation curve testing andoverload/underlaod testing have been carried out. The crack propagation acceleration has beenobserved after the application of single underload. Specifically, crack propagation rates increaseimmediately after the application of the underload and then recover to the steady-state crackpropagation rate under constant amplitude loading. The traditional growth of crack under HCF-LCFinteraction loading would be detrimental for the durability of materials and structures. The incrementin crack length after the application of an underload, ΔaUL, has been proposed as a suitable factor toevaluate the magnitude of the crack acceleration. It has been found that ΔaULincreases with both theapplied underload level and the constant amplitude loading ΔK level at which the underload isapplied.(2) A numerical modeling approach based on the theory of critical distances has been developedto quantitatively simulate the fatigue crack propagation for cast aluminum alloys. In the proposedapproach, the fatigue crack advances by one element length when the plastic energy density at thecritical distance point ahead of crack tip accumulates to a critical level. A finite element model hasbeen developed to simulate the fatigue crack propagation and the calculation of fatigue crack propagation rates are carried out by a post-process program. The crack propagation criterion has beenestablished by fitting the simulated crack propagation curve to the R=0.1experimental data. Theeffects of different R-ratios and overload/underload are then accurately predicted using the establishedpropagation criterion. The proposed FE model has also successfully simulated the plasticity-inducedcrack closure. The findings from the present FE analyses indicate that crack opening stress intensity isreduced after an underload is applied, which directly affects the plastic energy dissipation ahead of thecrack tip, which has been assumed to be the main damage mechanism controlling the fatigue crackpropagation rate. For the W319-T7cast aluminum alloy, the secondary dendrite arm spacing (SDAS),the cyclic plastic zone size for ΔKthand the critical distance have been found to be of the same orderof magnitude.(3) Constant amplitude loading (CAL) HCF and LCF tests have been performed on the AS7GUcast aluminum alloy. The probabilistic fatigue lives are estimated by fit the life regression models tothe experimental S-N data. The scatter in fatigue life is related to the fatigue crack initiation sites.Fractographic studies indicated that fatigue cracks in most specimens initiated from casting defectslocated near specimen surface. Special attention has been paid to the fatigue crack initiator size, whichis the major cause of the scatter in fatigue life. The Lognormal distribution provides appropriate fit tothe fatigue initiator sizes. For both HCF and LCF tests, the cumulative density function (CDF) offatigue life has been found to be approximately equal to the complementary value of the CDF of thenear-surface fatigue initiator size. To take into consideration the influence of the inherent scatter infatigue life on the accuracy of interaction damage calculation, probabilistic fatigue lives are used asthe estimates of the CAL HCF and LCF lives for the calculation of HCF-LCF interaction fatiguedamage.(4) A HCF-LCF interaction damage model based on exponential decay law has been proposed.HCF-LCF interaction tests have been carried out under various load conditions. The effects of HCFstress amplitude (σHa), LCF relative stress range(σ H σLminmin), and number of HCF cycles perblock (η) are investigated. The interaction damage increases with HCF stress amplitude and LCFrelative stress range. The impact of the number of HCF cycles per block, η, can be divided into threeregions, each of which has distinct feature. The evolution of crack opening stresses following anunderload has been proposed as the major mechanism. The additional interaction damage caused bythe frequent applications of LCF cycles (η<30) is explained by the fracture or debonding of Siparticles within the LCF reversed plastic zone, which facilitates subsequent crack propagation. Aninteraction damage model based on exponential law has been proposed to account for the effects of the three loading parameters mentioned above. The proposed model can correctly characterize theinteraction damage for the load conditions investigated in this paper. The damage summation lawmodified by including the HCF-LCF interaction damage model has been found to achieve moreaccurate fatigue life predictions compared with the Miner’s linear damage summation law.