泊松方程的JuliaFEM刚度矩阵带来零矩阵

Question

用于Julia版本1.0的JuliaFEM软件包是否有问题？ 我试图从JuliaFEM软件包文档中运行该示例，以处理椭圆热方程。我使用了k = 1的导热系数，这使该问题成为泊松问题。刚度矩阵结果是零矩阵吗？

我还在创建的另一个正方形网格和相同的零矩阵问题上进行了尝试... 预期结果应该是带有Neumann边界条件的Laplacian矩阵

示例中的代码：

using JuliaFEM
el1 = Element(Quad4, [1, 2, 3, 4])
X = Dict(1 => [0.0,0.0], 2 => [1.0,0.0], 3 => [1.0,1.0], 4 => [0.0,1.0])
update!(el1, "geometry", X)
update!(el1, "thermal conductivity", 1.0)
time = 0.0
el2 = Element(Tri3, [3, 2, 5])
X[5] = [2.0, 1.0]
elements = [el1, el2]
update!(elements, "geometry", X)
update!(elements, "thermal conductivity", 1.0)
problem = Problem(Heat, "test problem", 1)
add_elements!(problem, elements)
assemble!(problem, time)
Matrix(problem.assembly.K)

我从以下链接获取代码的链接：  http://www.juliafem.org/JuliaFEM.jl/latest/packages/FEMBase/problems.html