σyy[x_]=-α Ej (Sqrt[C2 + C3 x])+(6/L)(2K2-K1)x+4K1-6K2;
C2 = ((((k0/b)))^2);
C3=(((((2k0)/b)(Tb-Ta)+(((Tb-Ta))^2)))/L);
K1=((α Ej)/L)((2/(3 C3))((((C2+C3 L))^(3/2))-(C2^(3/2))));
K2=((2α Ej)/(3 C3 (L^2)))(L(((C2+C3 L))^(3/2))-(2/(5C3))((((C2+C3 L))^(5/2))-(C2^(5/2))));

L0=20;
Podatki = { L-> L0,k0-> 9.1 (10^(-3)), b-> 0.02 (10^(-3)), Ta-> 21,Tb-> 500,α-> 1 (10^(-5)),Ej-> 150000};

Različne temperature roba

Plot[{σyy[x L]/.Podatki,
σyy[x L]/.Tb-> 200/.Podatki
},{x,0,1}, Filling->Axis,AxesLabel->{" x/L","σyy"}]

Različne debeline stene

Plot[{σyy[0]/.Podatki,σyy[L]/.Podatki},{L,0,L0}, Filling->Axis,PlotRange->All,AxesLabel->{"L","σyy"},PlotLabels->{"σyy(x=0)","σyy(x=L)"}]

Clear["Global`*"]

σyy[x_]=((α Ej Q)/(2k))(x^2)-((α Ej Q)/(2k)) L x+((α Ej Q)/(12k))(L^2);
L0=20;
Podatki2= { L-> L0,k->0.2, Q->0.04,α-> 2 (10^(-5)),Ej-> 100000};

Plot[{σyy[x L]/.Podatki2,
σyy[x L]/.L-> L0/2/.Q-> 0.04*2/.Podatki2
},{x,0,1}, Filling->Axis,AxesLabel->{" x/L","σyy"}]