Transcript Presentation from April 10, 2003

Project Update
April 10, 2003
Massachusetts Institute of Technology
3.082
Rachel Sharp
Corinne Packard
Isaac Feitler
Hao Hu
Today
Update on progress
Capacitor bank
Casting
Other machining
Calculations
Deformation mechanisms
Next steps
Revised Gantt Chart
Week
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Vessel design and parts acquisition
Break
Capacitor bank acquisition
CAD
Pressure Vessel assembly
Die printing
Casting of mold
Electrohydraulic test
Funnel formation
Final part formation
Presentation preparation
Pressure Vessel
Capacitor Bank
Mold
Electrohydraulic forming
Final Presentation
Capacitor Bank Update
Magneform machine has been supplied with
power, and troubleshooting started but has not
progressed very far.
Low-voltage, high capacitance 1kJ capacitor
bank has been acquired.
We still expect measurable results, but we won’t
know to what extent until we test it.
1kJ capacitor bank for at least the time being.
Time permitting, we may still pursue higherenergy deformation.
Creating the Dies
CAD files of dies created with Solidworks
Dies are printed 3D printer
From MIT & Z-corp
Prints composed of starch
Creation of Plaster Mold
Casting with Bronze
Lost Wax Casting
A positive print of final shape is created
with 3D printing
Ceramic shells are created to surround the
3D print
Molten Bronze is poured into the shell
As the starch burns out, the molten bronze
takes the shape of the positive die shape
Pressure Vessel Design Changes
Stainless steel pipe plugs
replace cast iron plugs
Hole depth for pipe plugs is 0.7”
 the min. wall thickness of the
vessel is 0.5”
Inter-electrode spacing now
2.75”
Teflon insulation design slightly
modified
Aluminum pipe purchased for
testing of assembly without a
die
Machining has been slow due to the robustness of the
materials used. Despite this, the vessel is nearly complete.
Raw Materials for Electrode
Assembly
Final Electrode Assembly
Al Pipe Ring for Free Expansion
Drilling Electrode Holes in
Pressure Vessel
Working in Edgerton Machine Shop
Part failure calculations
Part
Material
Stress
state
Strength
(ksi)
Area
(in2)
Load
(lbs)
Pressure
to fail
(psi)
Comment
Vessel
AISI 1045
Complex
55
----
-----
18,000
Thick
sphere
Plug
304 S.S.
Tens?
----
----
-----
3,000
Rating for
intact plug
Insulation
Teflon
Comp.
3
0.169
507
2,500
Extrusion
Electrode
110 Cu
Tens.
44
0.016
709
25,600
Around pin
Washer
18-8 S.S.
Comp
---
----
----
-----
oughtn’t fail
Pin
416 S.S.
2xShear
130
0.003
797
28,500
Gasket
Neoprene
----
---
---
---
---
oughtn’t fail
Workpiece
5052 Al
Biax.
Tens.
13
----
-----
1,000
Thin
Sphere
Leak-Before-Break Criteria
Wall thickness > critical crack size, and a crack
in the vessel reaches the critical crack size, then
the vessel will fail in a “fast fracture mode”
Wall thickness < critical crack size, then the
crack will propagate through the wall and the
vessel will leak (reducing the internal pressure)
before a “fast fracture” can occur.
Calculations
Stress-intensity factor (also called fracture
toughness) D. Broek, Elementary Engineering Fracture Mechanics, Martinus
Nijhoff Publishers, 1982. Pg.393 ]
KIc  asys  ac
ac is critical crack size, sy is yield strength, a
generally is 1 in the worst case.
2
Solve for ac
1  KIc 
ac  

π  ασ ys 
Calculations, continued:

K Ic   5σY.P.CVN


 σ Y.P.2  

- 

4


[D. Fryer,High Pressure Vessels, Chapman and Hall, 1997, p.123]
sY.P. is the yield strength in ksi
CVN is the Charpy V-notch adsorbed energy in ft-lbs
KIc is in ksi-in2
For AISI 1045 steel, CVN varies widely with hardness.
We choose a low value of hardness (225 Brinell) and
get a CVN of 38-48 at 50°F
Calculations, continued 2:
For CVN = 38, sY.P. = 55 ksi, KIc = 98.5:
ac = 1”
Neglecting the holes for the electrodes
which pierce the vessel completely, the
thinnest and most crack prone area of the
vessel is 0.5” thick
The vessel will leak before bursting at the
areas we expect to be weakest and most
crack-prone.
Strain and Deformation
ε
ε
Sample responds to strain by necking
s
F
A
I.
s
low
II.
s
high
III.
s
s
low
Region II. Smaller area  greater σ
Image from http://www.sri.com/poulter/fe_modeling/fem_figures/fig_wf3.html
F
A
Stopping the Downward Spiral
Higher ε
Lower A
Higher σ
To overcome this weakening cycle,
necking must be suppressed
Work Hardening
Phonon Drag
Work Hardening
At any strain rateStress causes dislocations to move
Strain is the macroscopic result of
dislocation activity
In the necked region,
ε is higher  dislocation activity is higher
Image from http://newt.phys.unsw.edu.au/~epe/1250.L9/1250.L9.small.html
Work Hardening
Increased dislocation activity creates
tangles which increase the yield stress
of the material
Ultimately, increased ε  increased σy,
So if σy II > σy I, III , Regions I and III will
yield first and necking will be suppressed.
Phonon Drag
I.
II.
III.
ε
ε
V1
V2
V1
“Inertial Effects” I=mv
Smaller A in II  yielding in the region  V2>V1
d
d x1  x0
dx
V  dt
dt
dt
x0
 
 (
What is the result of having a high strain rate?
)
Phonon Drag
Back to dislocation movement-
E
Stress lowers the activation energy
required to make a dislocation move
┴
┴ moves
Remember that phonons are lattice
vibrations.
* But there is still a critical
time length when probability
favors movement
Phonon Drag
At low strain rates, probability is not limiting
Dislocations move smoothly
At high strain rates, probability is limiting
Dislocation move jerkily
  A
b s
2
BT
Phonon Drag Coefficient, B, increases with temperature, is very
different for different crystal systems, and has dependences on
many other microscopic variables
Phonon Drag must be the rate limiting step in order for
this equation to hold!
Review of Deformation
Mechanisms
Work Hardening
Any strain rate
σy
Phonon Drag
High strain rates
σy
ε
dε/dt
At very high strain rates, Phonon Drag becomes an
important factor and allows for greater elongation
than work hardening alone