La composició i l’estructura

The baseline results were initially used for checking functionality of the test device as well as for material selection. Based on a stress analysis of the test device components, 17–4 PH stainless steel in the annealed condition with a minimum yield strength of 1 GPa [13] was selected to prevent yield up to a minimum cylinder hoop strain of 0.01. The computer model stress versus target strain results for the wedge and ram are shown in Figure 7 and Figure 8, respectively. The target strain was computed from the geometry of the rams and wedges while neglecting their deformation. Maximum stress in both are well below the yield point of 17–4 PH stainless steel providing a factor of safety and allowing for future testing of stronger cylinders. Based on equivalent stress and maximum shear stress failure theories, the FEA model predicts failure at a cylinder hoop strains between 0.004 and 0.005. More detailed tabulated results from the FEA are available in Appendix B.

Maximum Equivalent Stress in the Wedge for Thin and Thick Aluminum Cylinders.

Maximum Equivalent Stress in the Ram for Thin and Thick Aluminum Cylinders.

Following material selection, the FEA hoop strain in the cylinder and maximum equivalent strain in the test device components were plotted against the target hoop strain for the thin cylinder as shown in Figure 9. The plot shows that the strains in the test device components when used with the thin aluminum cylinder are negligible and that the

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strains of 0.004 and below as shown in Table 1. Since strain gages will be used to measure actual hoop strain during the experimental tests, any difference between the target hoop strain based on ram motion and the cylinder’s actual hoop strain are not important.

Thin Cylinder Strain versus Target Cylinder Hoop Strain.

Table 1. Thin Cylinder Finite Element Analysis Results.

Target Hoop Strain 0.001 0.002 0.003 0.004 0.005 FEA Hoop Strain 0.000931 0.001855 0.002779 0.003706 0.004024

Error -6.92% -7.28% -7.37% -7.36% -19.53%

The above analysis was repeated for the thick aluminum cylinder and the FEA strain versus target strain results are shown in Figure 10 with a predicted hoop strain at failure of approximately 0.007. From this, it is immediately apparent that the strain in the ram and wedge of the test device are more significant than when testing the thin aluminum cylinder. As a result, the error in actual versus target hoop strain is higher at approximately 35% as shown in Table 2. Although the error between target and actual hoop strains is unimportant since strain gages will record the hoop strain for future calculations, the more significant strains in the test components may have a detrimental

effect on the capability of the test device to apply sufficient internal pressure to cause failure prior to reaching the limits of the device.

Thick Cylinder Strain versus Target Cylinder Hoop Strain.

Table 2. Thick Cylinder Finite Element Analysis Results.

Target Hoop Strain 0.001 0.002 0.003 0.004 0.005 FEA Hoop Strain 0.000647 0.001292 0.001938 0.002584 0.003288

Error -35.33% -35.39% -35.40% -35.39% -34.24%

A Satec uniaxial compression-testing machine of model MII-20UD with a 200 kN load capacity was to be used for all cylinder tests. To ensure that the Satec machine had sufficient capacity for achieving the desired hoop strains, the required compressive machine force was plotted against each target cylinder hoop strain value in Figure 11. As expected, the compressive force required for a given hoop strain significantly higher for the thick cylinder however with a maximum required compressive force of less than 150 kN, all target strains are within the capacity of the Satec testing equipment.

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Required Compressive Machine Force Versus Target Cylinder Hoop Strain.

Due to the segmented design of the wedges, consisting of eight 45 degree wedges, there will exist some non-uniformity of stress applied to the cylinder, especially at the interface between wedges. Figure 12 shows the degree of non-uniformity with a fairly constant stress across each wedge and lower stress at each wedge interface.

Equivalent Stress on the Outer Surface of the Thin Aluminum Cylinder at a Target Hoop Strain of 0.001.

To understand the degree of non-uniformity across the wedge better, the hoop strain was plotted against the location relative to a wedge for the thin and thick cylinders in Figure 13 and Figure 14. While the strain distribution for the thin cylinder is fairly uniform except for the five degrees on either side of the wedge, the thick cylinder shows more variation across the entire wedge.

Hoop Strain versus Location Relative to Wedge for the Thin Cylinder.

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As a final validation of the FEA results, thin and thick cylinder equations were used for comparison with the FEA results for the thin aluminum cylinder. Thin-walled assumptions are considered valid when the ratio of radius to thickness is greater than 10.

The thin aluminum cylinders and composite cylinders have a ratio of approximately 12 while the thick aluminum cylinder has a ratio of six. To derive the equations, it was necessary to determine equations for the cylinder inner pressure and hoop strain as a function of the friction coefficient and applied machine force. Two free body diagrams were created representing a ram and a wedge as shown in Figure 15 and Figure 16. From the two free body diagrams, a force balance was calculated resulting in thin and thick cylinder equations for cylinder hoop strain at the outer surface as a function of an assumed friction factor and applied machine force.

Free Body Diagram for a Ram.

Free Body Diagram for a Wedge.

Equation 1 uses thin cylinder assumptions while Equation 2 uses thick cylinder assumptions to relate the applied machine force and friction coefficient to the cylinder hoop strain. In Equation 1, L and t are the cylinder length and wall thickness respectively.

In Equation 2; a, b, and L are the inner radius, outer radius, and cylinder length,

A MATLAB script was developed to perform the above calculations for the thin aluminum cylinder and is available in Appendix A. The average hoop strain across the wedge from the finite element analysis was then compared to the analytic values in Table 3. The FEA results correspond well with the analytical results, providing a final validation of the finite element analysis predictions.

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Table 3. Finite Element and Analytic Hoop Strain.

Target Hoop Strain 0.001 0.002 0.003 0.004 0.005

FEA 0.000824 0.001646 0.002469 0.003292825 0.003927

Analytic thin cyl equations 0.0009 0.0018 0.0026 0.0035 0.0039

Error -8.47% -8.55% -5.03% -5.92% 0.68%

Analytic thick cyl equations 0.0008 0.0016 0.0023 0.0031 0.0035

Error 2.97% 2.89% 7.36% 6.22% 12.19%