## 1 Introduction  
The functions of products have steadily increased and become more complex due to advances in technology and intense market competition. For this reason, the task of examining the reliability and performance of products has also increased in complexity. However, reliability evaluations tend to be time-consuming. It is also difficult to gather sufficient information about the product during the development stage and available specimens for testing may be limited. Thus, to increase the speed at which reliability information is acquired for a product, reliability test planning is necessary.  
Conventional accelerated tests, in which all specimens are only allocated under accelerated conditions, have previously been used to plan and provide solutions to the above problems. However, various general tests should be performed under regular use (i.e., normal conditions), as well as under accelerated conditions. Therefore, an alternative test plan concept that includes realistic situationsand use conditions is required. Partially accelerated tests provide a method of simultaneously testing products in the development stage simultaneously under both regular use and accelerated conditions.  
Commonly, a partially accelerated test is classified either as a PALT or a partially accelerated degradation test (PADT) based on whether the product can observe a performance characteristic or not. Today, failures are rarely observed as the reliability of novel products has steadily improved. Commonly, these products are tested under general use conditions. The PADT, which observes a performance characteristic of a product over time, is more effective than the PALT at maximizing the advantages of partially accelerated tests. However, there are few studies for PADT designs [1], and no research has been published with two stress variables. In addition, [PERSON] and [PERSON] [1] mentioned the practical shortage problem of prototypes in the development phase and presented the PADT strategy to overcome this difficulty.  
Thus, in this paper, to overcome the challenges described above, which include a low prototype quantity (i.e., a lack of test samples), the disadvantages of PALT, and the lack of prior information, we proposed a strategical approach for a constant stress-partially accelerated degradation test (CS-PADT) with two stress variables based on the Wiener process (WP). In particular, the proposed test plan can be simultaneously used for multipurpose tests such as design conformity, performance requirement and hot temperature operating test [1]. First, the parameters of an acceleration model are estimated by developing the CS-PADT plans with four test points. These test points are a combination of use and maximum stress levels for each stress variable and are based on the D-optimality criteria. Second, a practical PADT plan, which uses the stress levels of the first phase and the middle-stress conditions for two stress variables, is developed to examine the validity and curvature effects of the acceleration model. Subsequently, the lifetime, which is the \(q\)-th quantile of the lifetime distribution at the use condition, is estimated by minimizing the asymptotic variance of the maximum likelihood estimator (MLE). In addition, a scenario example is presented to demonstrate the validity of the proposed CS-PADT model.  
The rest of this paper is organized as follows. Section 2 introduces the related works and Section 3 presents the planning strategy, basic assumptions, and model description. The model of CS-PADT plan with two stress and test plans is provided in Section 4. Section 5 presents the proposed test strategy through an example and in Section 6, the conclusions and future research directions are provided.