# A Knowledge Graph Framework to Support Life Cycle Assessment for Sustainable Decision-Making
## 3 Methodology
### Domain--Expertise  
The initial phase of the methodology focuses on acquiring in-depth domain expertise and meticulously defining the schema for the KG. This foundational step is crucial for accurately modeling the product's life cycle and ensuring that the KG effectively represents all relevant processes, actors, and environmental impacts. This involves a multifaceted approach to familiarization, including process observation in real operational settings, conducting expert interviews, and engaging in detailed literature research. By immersing oneself in the practical workflows and theoretical underpinnings of the product, one gains the necessary insight to identify key nodes, relationships, and properties that will form the backbone of the KG. In traditional LCA methodologies, the focus often lies on specific stages of the product life cycle during which direct emissions occur, as depicted in the red dotted box on the right side of Figure 3. However, this approach overlooks the significant influence that early-stage decisions--made during the design and engineering phases--have on the environmental outcomes of later stages. These initial decisions, while not producing immediate emissions, set the parameters for resource utilization, energy consumption, and waste generation throughout the product's life cycle. To address this limitation, the proposed methodology expands the scope to include the entire product life cycle, encompassing the engineering, manufacturing, usage, and end-of-life phases, as highlighted in red. By incorporating these early stages, the methodology acknowledges that decisions made at the inception of the product can have profound ripple effects on its overall environmental impact. This holistic perspective enables a more comprehensive LCA, identifying opportunities for sustainability improvements at every stage. Figure 2: Methodology. Our proposed life cycle model begins with the engineering phase, where the foundational steps of the product design are taken. This phase involves critical activities such as design creation and material selection, which are essential to determine the functionality, sustainability, and environmental footprint of the product throughout its lifecycle. The choices made here, such as the selection of materials or design optimizations, have cascading effects on the downstream phases, influencing resource efficiency and environmental performance. Next, the pre-processing phase represents the transition from conceptual to physical. This phase prepares the materials and processes for the core production stage. Although less emphasized in some LCA studies, pre-processing plays a pivotal role in determining the quality and readiness of inputs for production. The core of the framework is the production phase, which includes the production process, raw material production, and energy production. This phase is particularly significant, as it often involves intensive resource utilization and high levels of emissions. The extraction and preparation of raw materials, coupled with the energy requirements, make this a key area of focus in LCA. Traditional methodologies often place substantial emphasis on this phase due to its direct association with greenhouse gas emissions, energy consumption, and other environmental impacts. Following production, the product enters the post-processing and sale phase, which includes refinement, finishing, and preparation for market distribution. This phase ensures that the product is ready for use by consumers or end-users. The use phase follows, during which the product fulfills its intended function. This phase is critical for understanding how the product interacts with its environment over its functional lifespan, whether through energy consumption (e.g., for electronic devices) or material wear (e.g., consumables). The emissions and resource demands during this phase often rival or exceed those of the production phase, depending on the nature of the product.