fix: evaluate linear Hex8 response relative to translation

src/fesa/elements/hex8_element.cpp

@@ -230,8 +230,14 @@ Result<Hex8Response> Hex8Element::Evaluate(const Vector& u,
   if (!nonlinear) {
     response.material_tangent = stiffness_;
     response.tangent = stiffness_;
-    response.internal_force = stiffness_.Multiply(u);
-    response.strain_energy = 0.5 * u.Dot(response.internal_force);
+    // K0 annihilates uniform translation. Use the same relative field as
+    // strain recovery to avoid cancellation of a large rigid displacement.
+    Vector relative{24U};
+    for (std::size_t a = 0; a < 8; ++a)
+      for (std::size_t i = 0; i < 3; ++i)
+        relative[3U * a + i] = u[3U * a + i] - u[i];
+    response.internal_force = stiffness_.Multiply(relative);
+    response.strain_energy = 0.5 * relative.Dot(response.internal_force);
   }
   for (std::size_t g = 0; g < 8; ++g) {
     const auto& point = points_[g];