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In Smith and Eppinger's pioneering research on coupled design processes, the dynamics of design iterations is modeled by a linear model, which implies that there is no overlap between redesign work and allows redesign workload to be more than the original design workload. Actually, this overlap often exists in practice and, therefore, reduces the total redesign workload. In this paper, we propose a nonlinear model for a coupled design process with overlap between redesign work and ensure that redesign workload is less than the original design workload. Based on the model, the sufficiency condition for the convergence of such a design process is proposed. A heuristic rule is also given to reduce the possibility of considering a design process that converges as one that does not converge. Furthermore, we develop a sufficiency condition for estimating the number of design iterations before converging as such an estimate is important for planning product development projects. Another heuristic rule is also introduced to increase the accuracy of estimating design iterations. Finally, numerical experiments are conducted to examine the correctness and performance of the proposed sufficiency conditions and heuristic rules.