Contact Stress Problem: Validate Nylon Clamp & Steel Tube

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The discussion centers on validating a nylon clamp under a load of 166kN on a steel tube, where the user reports a significant discrepancy between analytical calculations (145MPa) and FEM results from ANSYS (30MPa). Participants suggest sharing the MATLAB code for better insights into the calculations. There is a recommendation to conduct a simplified finite element analysis (FEA) in MATLAB to check the results against ANSYS, indicating potential setup issues with the ANSYS model. The conversation highlights the importance of verifying analytical methods and FEA setups to ensure accurate stress validation.
Jesper Hellmann
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Hi All

I am trying to validate a nylon (PA6 Guss) clamp which has a load of 166kN on a steel tube. I am only interested in validating the clamp.
My analytical (MatLab-Octave) calculations give me a peak Von Misses stress of about 145MPa
contact stress.png

When comparing to FEM in ANSYS I only get about 30MPa
Can someone shed light on why there is such a big difference?
contact stress_b.png


-Jesper Hemmlann
M.Sc. Applied Mechanics
 
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Jesper Hellmann said:
I am trying to validate a .
My analytical (MatLab-Octave) calculations give me a peak Von Misses stress of about 145MPa

When comparing to FEM in ANSYS I only get about 30MPa
Can someone shed light on why there is such a big difference?

-Jesper Hemmlann
M.Sc. Applied Mechanics
Who knows? Show your code and we might able to comment.
 
%%%Contact stress
clear all
clc
close all
graphics_toolkit('gnuplot')

%%%Material parameters
%Youngs Modulus
E_1=2.62*10^9
E_2=213*10^9
%Poisson ratio
nu_1=0.34
nu_2=0.3%Cylinder radius
%OBS (R_2= infity for a flat plate)
%OBS (a cylindrical groove is a cylinder with a negative radius)
R_1=0.055275
R_2=0.055775
L=0.2

%Force
F=166000%Contact area
b=sqrt(4*F*((1-nu_1^2)/E_1+(1-nu_2^2)/E_2)/(pi*L*(1/R_1+1/R_2)))

%maximum pressure
P_max=2*F/(pi*b*L)%stresses
i=50
z=0.000000001:(b*3)/i:(b*3);

for n=1:i
sigma_1(n)=-2*nu_1*P_max*(sqrt(z(n)^2/b^2+1)-abs(z(n)/b));
sigma_2(n)=-P_max*((2-(z(n)^2/b^2+1)^-1)*sqrt(z(n)^2/b^2+1)-2*abs(z(n)/b));
sigma_3(n)=-P_max*(sqrt((z(n)^2/b^2)+1)^(-1));

tau_1(n)=abs((sigma_2(n)-sigma_3(n))/2);
tau_2(n)=abs((sigma_1(n)-sigma_3(n))/2);
tau_3(n)=abs((sigma_1(n)-sigma_2(n))/2);
sigma_vM(n)=sqrt(sigma_1(n)^2+sigma_2(n)^2+sigma_3(n)^2-sigma_1(n)*sigma_2(n)-sigma_2(n)*sigma_3(n)-sigma_3(n)*sigma_1(n)+3*(tau_1(n)^2+tau_2(n)^2+tau_3(n)^2));
end

figure
hold on

MPa=1/10^6;
plot(z,abs(sigma_1)*MPa,'-ko', "markersize", 3)
plot(z,abs(sigma_2)*MPa,'-m^', "markersize", 3)
plot(z,abs(sigma_3)*MPa,'-.r*', "markersize", 3)
plot(z,(tau_1)*MPa,'--yv', "markersize", 3)
plot(z,(tau_2)*MPa,':bs', "markersize", 3)
plot(z,(tau_3)*MPa,'-.go', "markersize", 3)
plot(z,sigma_vM*MPa,'-bv',"markersize", 3)



legend('\sigma_x', '\sigma_y', '\sigma_z', '\tau_1','\tau_2','\tau_3','\sigma_{vM}', "location",
"northeast");
xlabel('depth of surface [m]')
ylabel('Stress [MPa]')
title('Cylinder contact stress on clamp')
grid on

%Displacement in the center of the 2 cylinders
delta_c=2*F*(1-nu_1^2)/(pi*L*E_1)*(2/3+log(4*R_1/b)+log(4*R_2/b))*(1000)
 
I don't know ansys and have never actually done an fea but I know you can set one up in MATLAB so maybe try to do a simplified fea in MATLAB to see if your answers get closer? That would only confirm your ansys isn't setup right, though, if you know 30 isn't right
 
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