Measuring stress-strain behavior of mild steel at intermediate strain rates

Abstract

Impacts on offshore installations and ships can cause
strains at high rates. It is known that mild steel lower yield stress and
hardening are strain rate dependent. Measuring this material behavior is not easy,
as test results from many researchers, show heavy vibration. Vibration is
normally referred to as ringing. This thesis aims at measuring stress-strain
behavior of mild steel at intermediate strain rates between 100 𝑠
−1 and 500 𝑠 −1 with minimal vibration. Two major
characteristics from stress-strain behavior in this research, are the lower
yield stress and plastic tangent modulus. A comparison of public test results,
showed a steel characteristic for transferring quasi-static stresses, to stresses
at a desired strain rate. This characteristic has been used to transfer a
quasi-static stressstrain curve to stress-strain curves at intermediate strain
rates. This strain rate dependent material model is used in calculations. The
test is modelled by means of an explicit finite element analysis. The analysis
showed that adding a plastic hinge in the specimen, can reduce bending and
vibration. The test setup is a drop tower, where the drop weight falls into a
U-shaped specimen. Strains are measured in an elastically and plastically
deforming area on the specimen. Stresses in the plastic area can be obtained
from strains in the elastically deforming area. Strains are measured by means
of digital image correlation. Stresses are compared with loads obtained from
drop weight positions and consequent accelerations. Measured plastic strains
compare well to predicted plastic strains. However, stresses from first tests
show vibration in a range of 20 percent of the average stress or more. If such
bending occurs, it is not exactly known where the average stress is. However,
the amount and direction of bending can be obtained from the 3D position
measurements. The bending investigation is only used to identify causes of
ringing. With insight in bending and loads from two separate measurements, this
test is considered to produce results that represent the material behavior of
mild steel. A raw test result with a vibration range of 20 percent of the
average stress, in the first test setup, shows the potential for obtaining raw
measurements from a U-shaped specimen in a drop test.