Dual Scale Monitoring of Fatigue Cracks Based on Time Rate Change of Local Compliance
Non-destructive evaluation is able to detect smaller and smaller defects such that earlier detection can prevent serious damage of the structure. Damage detection and diagnostic method require not only a knowledge of the size of the prevailing defects but also their likely locations which are usually known from the initial design and past experience. What is not known is the time of their occurrence because the loading conditions can change and gradual deterioration of the material are not known. Inhomogeneities of the material microstructure can also affect detection accuracy when considering small cracks. The multifaceted nature of health monitoring presents over whelming difficulties that are still being challenged. Based on monitoring the local force and displacement from the use of transducers that are commercially available, continuous records of the local compliance C and its time rate history, say dC/dt or dC/dN in fatigue with N being the number of cycles can be made available and stored in microprocessors now that computer storage is readily accessible. The conversion of the force and displacement records to damage by fatigue crack growth constitutes the objective of this work. To this end, the dual scale micro/macro fatigue crack growth model will be applied since it has the capability to delineate micro and macro cracking. The model makes use of three parameters μ<*>, d<*> and σ<*>. They account for the interaction between the micro/macro effects in terms of the, respective, relative shear modulus μ<,micro>/μ<,macro> the micro-tip characteristic length d/d<,0> and the crack surface tightness ratio σ<,0>/σ<,∞>that controls the opening of the fatigue crack. These parameters are assumed to vary in a random fashion as the crack size increase with the repetition of loading. Random normal distribution of statistical values of μ<*>, d<*> and σ <*> is investigated for different mean <μ> and standard deviation <σ>. The amount of dispersion around the mean is examined for the crack length a and crack growth rate da/dN in addition to the compliance C and its rate dC/dN. It is demonstrated that randomness of the material microstructure has an effect on the local compliance which will increase with crack growth in a stable fashion and then rises more quickly with the number of cycles. The characteristic is reminiscent of the compliance to crack length relation used for determining the energy release rate in fracture mechanics.
Dual scale Micro/micro cracks Fatigue crack growth Random normal distribution Standard deviation Crack surface contact Tightness of contact Local compliance Dispersion around the mean Compliance Micro/macro stress intensity ranke Scale tr
G. C. Sih
College of Civil Engineering, Southeast University, Nanjing 210096, China
国际会议
南京
英文
17-36
2007-10-16（万方平台首次上网日期，不代表论文的发表时间）