## 3 Findings
### Prognostics and Health Management (PHM) for Unit Systems
#### 3.1.2 Degradation  
Degradation is a stochastic process, which will occur through random shocks and also through the components being worn in manufacturing processes. Degradation rate plays a significant role in the life of FUS because the impact of the degradation process on different types of manufacturing systems are observed on the failure severity. A Degraded machine impacts on the nature of the parts manufactured where the defectives rely upon the production rate, as has been mentioned in [28]. [PERSON] et al. [27] worked on the degradation of the unit as stated by the rate of production. [PERSON] et al. [26] explained that their examination is to investigate the impact of the production rate on the degradation level and machine availability. Through this diverse literature, it was shown that the degradation process was grouped in two ways, i.e., continuous degradation and discrete degradation.  
[PERSON] et al. [21] explained about multiple degradation methods, which involve continuous degradation, as well as that discrete degradations have been modelled through various stochastic processes, for example, Markov renewal and gamma processes. [PERSON] et al. [29] proposed that the conventional Wiener process-dependent degradation is an important degradation model technique for manufacturing systems. With this, the past research on the degradation of manufacturing systems showed that efforts have been made to characterize the relation between degradation rate and workload adjustment strategy by using a Bayesian approach to find the residual life distribution literature, as is mentioned below in Table 7.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**Literature Review on Degradation Rate in the Context of Flexible Unit Systems**} \\ \hline
**Sl. No.** & **References** & **Findings** \\ \hline
1 & [21] & The machine’s degradation was analyzed in view of an impact on machine performance and product quality utilized as the performance index. \\
2 & [22] & A new degradation model, “Transformed Inverse Gaussian process”, has been presented in this paper. \\
3 & [23] & Shows that it can be conceivable to make robust reconfigurable manufacturing systems by taking the degradation of modules. \\
4 & [12] & The multistage manufacturing measures have been utilized to focus on modelling the interconnection between product quality degradation and tool wear. \\
5 & [24] & Addresses the issues of maintenance, joint production, for an untrustworthy production system subjected to degradation. \\
6 & [22] & Researches Inverse Gaussian models for degradation investigation, with constant monotonic degradation rates also mentioned. \\
7 & [25] & Introduces a degradation modelling system for assessing and updating the RLDs of partially degraded segments using an FPT approach. \\
8 & [26] & Works on the availability of machines as well as random failure rate to fulfil economically a random demand under certain constraints. \\
9 & [27] & Describes linear-quadratic stochastic production planning issues so as to fulfil a random demand. \\ \hline \hline \end{tabular}
\end{table}
Table 6: Literature review on degradation rate related to flexible unit systems.