High Dynamic Testing of Materials
Materials testing from 101 s-1 to 102 s-1
Dynamic low
Dr.-Ing. Erhardt Lach
French-German Research Institute of Saint-Louis, ISL
[email protected]
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- Inertial forces are important.
- Mechanical resonance in specimen and
machine is important.
- High-velocity hydraulic or pneumatic
machines, drop impact tester,
Charpy impact test.
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Drop tester
J. Hopkinson was the first who analyzed the drop test. In his tests, he increased
systematically the height of his drop tester. The wire he used was fixed at the upper
end, which caused a reflection of tensile waves as tensile waves and a
superposition of them at the upper end. If, then, an elastic wave has a stress level
of the half of the tensile strength of the tested wire, it causes rupture.
The particle velocity can be calculated by the following equation:
v2 = 2M g H / (M + mKL)
(M = mass of drop weight, mKL = mass of clamp, H = drop height,
g = gravitational acceleration)
Assuming that mKL « M is valid, the equation may be simplified to:
v = √2gH
Hopkinson J., On the rupture of iron wire by a blow, in: Proceedings of the Manchester
Literary and Philosophical Society, Vol. XI, 1872, S. 40-45
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Using equation:

σ = ρ c0
σ = ρ c0 v
vp / 2
=
ρ c0 √2gH
 the stress peak will be affected only by the particle velocity v.
Behind the wave front there is an exponential decrease of σ according to the
following equation:
σ = ρ c0 v exp [ρAwire / M (x – c0 • t)]
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Karman-Duwez test
a) Initial position, W = drop weight,
P = impactor plate, d = drop
height, R = elastic band
b) Position after impact
c) Wire form during the
propagation of waves
a
b
c
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dynamic
TRUE STRESS, MPa
Strain gauge 2
Strain gauge 1
Stress-Strain AZ91 HP at RT
quasistatic
TRUE STRAIN
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Tensile test at intermediate strain rates
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Drop-weight tower assembly after Meyer and
Krueger (2000)
strain gauges
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DYNAPAK testing machine
Compressed gas machine
a) Safe Position
b) Fire
c) Impact
Schematic presentation of intermediate-strain-rate, compressed-gas machine.
a) hydraulic jacks keep piston in up position; b) jacks lowered and reservoir activated,
firing piston; c) Impact (Dynapak machine), velocity < 15 m/s
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Charpy-Test
It is not enough to know only how strong a metal is in
tensile strength, or how ductile it is; information as to
how it reacts under sudden impact also is of prime
importance. This quality is known as toughness and is
determined by the impact test.
The apparatus consists of a pendulum of known
mass and length, that is dropped from a known
height to impact a notched specimen of material.
The energy transferred to the material can be
inferred by comparing the difference in the height of
the hammer before and after the fracture (energy
absorbed by the fracture event).
The "Standard methods for Notched Bar Impact Testing of Metallic Materials" can be
found in ASTM E23, ISO 148-1 or EN 10045-1, where all the aspects of the test and
equipment used are described in detail.
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The quantitative result of the impact tests and the energy
needed to fracture a material can be used to measure the
toughness of the material and the yield strength. Also, the
strain rate may be studied and analyzed for its effect on
fracture.
The following variables
have a profound influence
on the toughness of a
material.
- Strain rate
- Temperature
- Notch effect
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The ductile-brittle transition temperature (DBTT) may be derived from the
temperature at which the energy needed to fracture the material drastically
changes. However, in practice, there is no sharp transition and it is difficult to
obtain a precise transition temperature (it is actually a transition region). An
exact DBTT may be empirically derived in many ways: a specific absorbed
energy, change in aspect of fracture (such as 50% of the area is cleavage), etc.
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Fracture toughness transition behavior of steel under quasistatic
and impact loading
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Temperature dependence of
fracture toughness of alloys;
characteristic behavior for
three different crystal
structures is illustrated
Fracture toughness versus yield strength
for some structural steels, TRIP
(TRansformation Induced Plasticity)
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The steel used in the Titanic hull had adequate strength but low toughness in
cold temperatures. When the Titanic hit the iceberg, instead of the steel
bending and causing small cracks (ductile failure), a crack grew very fast and
very large (due to brittle failure).
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data
recording
Force F
force-extension diagram
Extension l
Presentation of a Ceast pendulum impact tester for conducting the instrumented
(notched-bar) impact test. Schematic presentation of a force-extension diagram of
an elastomer.
Meyers Marc A, Chawla Krishan Kumar (1998). Mechanical Behaviors of Materials. Prentice Hall. ISBN 978-0-13-262817-4
Siewert, T. A., Manahan, M. P., McCowan, C. N., Holt, J. M., Marsh, F. J., and Ruth, E. A (1999). Pendulum Impact Testing: A
Century of Progress, ASTM STP 1380. ASTM
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