"对开式铁心" can be translated as "split core" or "two-part core" in English.
"Split core" is a more common and concise term, often used in technical and engineering contexts to describe a core that is divided into two or more parts for ease of assembly, disassembly, or installation around existing conductors.
"Two-part core" is a more literal translation, emphasizing that the core consists of two separate parts.
Both terms are acceptable, but "split core" is likely to be more widely recognized in technical discussions.