EVD [
26
,
27
], where high-order integer moments, i.e., skewness and kurtosis, need to be considered. Yet it is difficult
to evaluate such high-order integer moments using a small sample size, due to their large variability [
28
]. To alleviate
such difficulty, a series of methods based on non-integer moments, such as fractional moments and linear moments,
have been developed. The fractional moments-based maximum entropy methods [
29
–
32
] can estimate the first-passage
probabilities of nonlinear stochastic dynamic systems from low to high dimensions. However, it is difficult to solve the
non-convex optimization problem that is typically encountered, and the obtained results can be easily trapped into local
optimum. Besides, due to the polynomials involved in the maximum entropy density, the recovered EVD can have
unexpected oscillating distribution tail, which then leads to an inaccurate evaluation of the first-passage probability. Two
mixture parametric distribution methods in conjunction with fractional moments [33] or moment-generating function
[
34
] are developed. These methods enable to evaluate first-passage probabilities of high-dimensional and strongly
nonlinear stochastic dynamic systems from a small number of simulations. Furthermore, a fractional moments-based
shifted generalized lognormal distribution method [
35
] is utilized to assess seismic reliability of a practical bridge
subjected to spatial variate ground motions. Besides, the linear moments-based polynomial normal transformation
distribution method [
36
] is developed to analyze high-dimensional dynamic systems with deterministic structural
parameters subjected to stochastic excitations.
Overall, the fractional moments-based methods offer the possibility to deal with both high-dimensional and strongly
nonlinear stochastic dynamic systems from a reduced number of simulations, even with small first-passage probabilities.
In view of this, the present paper mainly focuses on such methods. Despite those attractive features, the fractional
moments-based methods still have two main problems to be solved. On one hand, the sample size for evaluating
fractional moments is usually empirically fixed. This is primarily because the sampling-based schemes adopted
by the existing methods do not allow for the sample size extension. However, the optimal sample size should be
problem-dependent. With a predetermined sample size, the adopted sampling methods may encounter over-sampling or
under-sampling, leading to a waste of over-all computational efforts or unsatisfactory accuracy of estimated fractional
moments. On the other hand, the success of fractional moments-based methods for first-passage probability evaluation
also depends on the selection of an appropriate distribution model. Although the existing distribution models are
capable of representing EVDs for some problems, their flexibility and applicability are limited. Hence, for a wide
range of problems, they may still lack the ability to accurately recover the EVDs over the entire distribution domain,
especially for the tails.
In this paper, we propose a fractional moments-based mixture distribution approach to estimate the first-passage
probabilities of high-dimensional and strongly nonlinear stochastic dynamic systems. It is worth mentioning that the
randomness from both internal system properties and external excitations is taken into account. The main contributions
of this study are summarized as follows. First, a parallel adaptive sampling scheme is proposed for estimating the
fractional moments, as opposed to the traditional fixed sample size scheme. Such a new scheme enables to extend
the sample size sequentially, i.e., one at a time or several at a time. The optimal sample size for fractional moment
estimation is determined by introducing a convergence criterion. In fact, a sequential sampling method with the ability
to effectively reduce variance in high-dimensional problems, named Refined Latinized stratified sampling (RLSS) [
37
],
is suitable for achieving our purposes and is employed within the proposed scheme. Second, one novel and versatile
mixture distribution model is proposed to reconstruct the EVD with the knowledge of its estimated fractional moments.
This model is based on the extension of the conventional inverse Gaussian distribution and the log transformation of the
extended skew-normal distribution. The analytical expression of the fractional moments for such mixture distribution is
3