Abstract
This paper describes a unique device that has been developed for the transient loading of models along straight and curved boundaries and that operates by discharge of a high-energy, high-voltage capacitor bank. In its present configuration, this device can generate uniform pressures from 1500 psi (10 MPa) to pressures that approach 100,000 psi (690 MPa) and that rise from zero to maximum pressure in 2 μs and decay to approximately zero in another 2 μs.
The transient stress-wave patterns in photoelastic models loaded with this device have been recorded by a dynamic polariscope. The dynamic polariscope presently in use is identical to a static polariscope except that the light source is of a short enough duration (½ μs) to photographically stop the movement of the photoelastic-fringe patterns caused by the stress wave.
With the stress-wave generator and the dynamic polariscope, transient photoelastic patterns have been recorded in a number of models. These patterns indicate that the scatter from duplicate shots performed with this technique is on the order of 3 percent. This represents considerable improvement over the 15-percent scatter normally experienced with sheet-explosive loading techniques. This improvement and the rapid turnaround between shots (approximately 5 min) are distinct advantages this system has over other methods of dynamic loading.
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Abbreviations
- a :
-
integration constant
- b :
-
integration constant
- C :
-
capacitance (μF)
- c :
-
phase velocity (mm/μs)
- c o :
-
elastic propagation velocity for dilatational wave in a flat plate (mm/μs)
- I max :
-
maximum current (A)
- i(t) :
-
current as a function of time (A)
- L :
-
inductance (H)
- m 1,2 :
-
\( - \frac{R}{{2L}} \pm \sqrt {\left( {\frac{R}{{2L}}} \right)^2 - \frac{1}{{LC}}(s^{ - 1} )} \)
- N :
-
resolution of polariscope system (fringes/mm)
- P :
-
pressure (Pa or psi)
- Q o :
-
initial charge (C)
- Q(t) :
-
charge as a function of time (C)
- R :
-
resistance (Ω)
- S :
-
dimensionless proportionality constant
- T :
-
period of oscillation (μs)
- t :
-
instantaneous time
- t e :
-
duration of light source (μs)
- V o :
-
initial voltage (V)
- W :
-
width of loading strips (m)
- A 3/A 1 :
-
inverse ratio of successive peaks of the same polarity
- α:
-
R/2L (Ω/H)
- β:
-
magnetic intensity (Wb/m2)
- γ:
-
Taylor series coefficient of the expansion of phase velocityc in frequency ω (m·s)
- ε y :
-
strain in the plane of the sample perpendicular to the stress-wave propagation direction (μin./in. or μm/m)
- μ:
-
magnetic permeability (4π×10−7 Wb/A·m2)
- ω:
-
frequency (rad/s)
- Ω:
-
circular frequency\(\sqrt {\frac{1}{{LC}} - \left( {\frac{R}{{2L}}} \right)^2 } (rad/s)\)
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Snell, R.F., MacKallor, D.C. & Guernsey, R. An electromagnetic, plane stress-wave generator. Experimental Mechanics 13, 472–479 (1973). https://doi.org/10.1007/BF02322733
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DOI: https://doi.org/10.1007/BF02322733