Hello Liao,
As you are aware, linear elements lack midside nodes, while quadratic elements do have those. When quadratic elements are significantly deformed, the midside nodes are displaced, and these displaced locations may occasionally cause the edges on which they are located to bend sharply, or they may cross through the domain of the element and enter a face on the other side. Because linear elements lack the midside nodes that may cause severe element distortion, a linear mesh is frequently more resistant to "mesh entanglement" than a quadratic one. Here is an Ansys help link, giving further insights on linear and quadratic elements.
2.2. Choosing Between Linear and Higher Order Elements (ansys.com)
Regards,
Nanda
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