Contact Stress Problem: Validate Nylon Clamp & Steel Tube

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Discussion Overview

The discussion revolves around validating a nylon clamp subjected to a load on a steel tube, focusing on discrepancies between analytical calculations and finite element method (FEM) results. The scope includes technical analysis and computational modeling.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Jesper Hemmlann presents analytical calculations yielding a peak Von Mises stress of about 145MPa, while FEM results from ANSYS show only about 30MPa, prompting a request for insights into the discrepancy.
  • One participant suggests sharing the MATLAB code to facilitate comments on the calculations.
  • Another participant proposes conducting a simplified finite element analysis in MATLAB to verify the ANSYS setup, indicating that the lower stress value may suggest an issue with the ANSYS configuration.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the FEM results and the potential setup issues in ANSYS, indicating that the discussion remains unresolved regarding the cause of the stress discrepancy.

Contextual Notes

There are limitations regarding the assumptions made in the analytical calculations and the setup of the FEM analysis, which have not been fully explored or clarified.

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