Iterative Weight Reduction Process
Authors: Jokelainen Tero , Järvenpää Antti , Saine Kari , Tuominen Janne , Mäntyjärvi Kari 
Published: January 2018

In this article, iterative process for weight reduction was used by applying the design for manufacture and assembly process and FEM calculations. This study presents a successful development case of an oil sump structure of a large ship engine. Main structural modification was the transition from plate steel structures to sheet metal based panel structure. As a result of the iterative process, the weight of the developed oil sump was 40% lighter, and the natural frequencies were significantly increased. The study showed that the process allows designing the final product without massive prototyping.

1. Introduction
There are several different core types for corrugated panels, for example, the X-profile, tube, Z-profile, hat-profile, and V-profile. The V-profile may need some special equipment for manufacturing and the profile is typically used for the lightest panels. Commercial availability of the hat-profile is better and the quality is often accurate enough for laser welding (Kujala & Klanac 2005). Laser welding is more challenging for V-profiles. In numerous applications, these metallic panels have shown very promising commercial possibilities due to excellent mechanical properties (Liang et al. 2001; Tian & Lu 2005; Zhang et al. 2015), but still there is a lack of public studies dealing with actual applications using good properties of the metallic sandwich panels for commercial solutions.
Several research studies have been published in recent years on the analysis of metallic corrugated-core sandwich panels for strength, vibration control, noise control, and blast resistance. For example, Valdevit et al. (2006) noticed that buckling loads are hard to predict with Finite Element (FE)-simulations, but it is more suitable for limit loads. Zhang et al. (2015) investigated the dynamic responses of sandwich panels with triangular corrugated cores under air blast load. They noticed that sandwich plates suffered smaller back face deflections than equivalent solid plates at low impulses, but they were more sensitive to crack with bigger loads. Kujala and Klanac (2005) have also listed the main benefits of laser welded corrugated panels. These panels have shown, for example, good mechanical properties, such as thermal resistance, good vibration absorption, increased stiffness, and good blast loading resistance (Karlsson & Å ström 1997; Liang et al. 2001; Kujala & Klanac 2005; Tian & Lu 2005; Zhang et al. 2015). The fatigue strength of the laser-welded lap joints have been studied widely and the behavior is well-understood for different materials. For example, Cho et al. (2004) used finite element analysis to estimate fatigue strength for laser-welded lap joint successfully. Kamran et al. (2014) studied lap joints fatigue behavior of high strength low alloy steels. They noticed that the failure modes under quasi-static and cyclic loading are different. Under quasi-static loading condition, the crack will initiate from the base material and finally cracked through the lower sheet. Under low cycle loading, the crack will start growing from the preexisting crack tips and cracked through the lower sheet. When testing was made under high-cycle loading, the crack started to grow from preexisting crack tips and failed from the kinked fatigue crack through the upper sheet.