# A Knowledge Graph Framework to Support Life Cycle Assessment for Sustainable Decision-Making
## 3 Methodology
### KG Schema and KG Construction  
Defining the KG schema is a critical component of developing a KG. The schema serves as the blueprint for the KG, specifying the structure of nodes (entities), relationships (edges), and properties (attributes) that will represent the life cycle of the product. The proposed schema is illustrated in Figure 4.  
The schema includes nodes such as \"Product Life Cycle\", \"Phase,\" \"Process\", \"Subprocess\", \"Actor\", \"Software\", and \"Option\". Relationships like \"HasPhase\", \"HasProcess\", \"HasSubprocess\", \"InvolvedIn\", \"UtilizesSoftware\", \"InfluencesPropertiesOfPhase\", \"HasOption\", and \"UtilizesOption\", which define how these nodes are interconnected. Properties attached to nodes and relationships, such as \"CO\({}_{2}\) equivalent\", \"Mass\", \"Distance\", and \"Influence\", provide quantitative and qualitative attributes essential for analysis. The inclusion of standard nodes and relationships facilitates compatibility with existing LCA methodologies and supports the integration of life cycle inventory (LCI) data. By expanding beyond traditional LCI data to incorporate all phases of the life cycle, including design and engineering, the schema allows for a more exhaustive assessment of environmental impacts. The representation of participants and tools is achieved through the \"Actor\" nodes and \"InvolvedIn\" relationships, highlighting the roles of individuals and organizations involved in the product life cycle. \"Software\" nodes and the \"UtilizesSoftware\" relationships identify the technological tools that influence processes, which can be critical for assessing efficiency and pinpointing areas for optimization. Capturing the influences of early decisions is a pivotal aspect of the schema. The \"InfluencesPropertiesOfPhase\" relationship connects decisions made in one phase to outcomes in subsequent phases, emphasizing how early-stage choices affect the overall life cycle. The \"Influence\" property provides detailed information on the nature and magnitude of these effects, facilitating causal analysis and predictive  
Figure 4: General schema for LPG model.  
modeling. To enable the comparison of alternatives within processes, the \"Option\" nodes and \"HasOption\" relationships are incorporated. These elements allow for modeling different choices, such as material selection or manufacturing methods, and associated properties such as \"CO\({}_{2}\) equivalent\" enable quantitative comparisons based on environmental impact, cost, or performance metrics. Ensuring semantic consistency and interoperability is crucial for the utility of the KG. By aligning the schema with established standards and frameworks--such as the methodology proposed by [PERSON] et al. [23]--the KG maintains a uniform interpretation of nodes and relationships across different datasets and analyses. The reason behind the meticulous schema design is the capture of all relevant information required for a comprehensive LCA and the support of advanced analyses. By modeling actors and software, the KG provides information on the human and technological factors influencing the product life cycle, which are often pivotal in understanding environmental impacts and identifying opportunities for sustainability improvements.  
The choice of the KG construction method is highly dependent on the availability and quality of the data, as well as the clarity of the description of the process. When detailed and structured data or well-defined process descriptions are available, automated methods can be more effective. However, in cases where data are sparse or inconsistent, or process descriptions are complex and domain-specific, manual construction becomes crucial to ensure the accuracy and relevance of the knowledge graph.