What is the difference between fluid transfer to solid force and stress when solving FSI?

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I want to know what is the difference between fluid transfer to solid force and stress when solving FSI? I found in the Turek-Hron FSI3 example that early examples were transmitting forces, but now the examples are transmitting stress. And in the acceleration scheme, both stress and displacement are used as acceleration factors, but in previous examples, only displacement was used. I found it difficult to converge the transmission force during testing, is that the case?