Fundamentals of Bending Test
Many common forms of material testing utilize variants of tensile
loading, whether
Uniaxial Tension (UT),
Plane Strain Tension (PST), or balanced biaxial tension (BBT). There are many reasons why
tensile testing is popular, but other loading types can also yield
useful results. The most common of these is bending. This article will
give a brief overview of bending testing, including theory, equipment,
methods, results, and testing standards.
Relevance: Why Bending Test?
Like tension, bending is a common stress in real-life applications.
Many forms of bending are caused by heavy objects resting on long
horizontal structures. Bridges, diving boards, and building floor
supports are all examples of structures that regularly take bending
loads due to gravity.
Levers are another tool commonly subjected to bending. Everyday
tools such as wrenches and pliers fall under this category. But even
outside the structural domain, bending is a viable loading case for
sheet metals in applications like the automotive sector. Because
bending loads are so ubiquitous, it is often important to know the
bending properties of materials used in engineering design.
Bending Test Equipment
In theory, bending testing could utilize a wide variety of
procedures. However, this form of testing has been heavily
standardized in order to produce more consistent and repeatable
results.
The two most common forms of bending testing are three-point bending
and four-point bending, which are named for the number of load
application points along the test piece.
Conveniently, bending tests can often be performed using the same
universal test machines (UTM) as are used for uniaxial tension
testing. These machines are compatible with different test fixtures,
some of which have been specially designed for bending tests. UTM
bending fixtures typically consist of wedges used to press into the
material sample at the load application points. The wedge(s)
attached to the moving part of the machine are also called the
bending punch.
Bending Test at FADI-AMT
With the bending fixtures installed, the UTM is set for compression,
allowing the upper and lower parts of the testing apparatus to push
on the material sample in opposite directions. This creates the
desired bending loads. For each bending test, a small pre-load is
applied to the bending sample in order to fix the sample in place
before the test begins.
Failure conditions for bending testing are somewhat different than
for tensile tests. This is because most common testing materials do
not fracture under bending conditions caused by standard bending
test procedures. Instead, bending tests are considered complete
either when bending forces begin to drop or when further bending is
impossible with the testing setup. The second condition manifests
when a material is so ductile that it can fold back on itself
without issue; in this situation, the bending setup cannot deform
the material any further.
Like any material test, bending testing relies on sensors for
useful results. The most important sensor for bending testing is
the load cell. Load cells record the bending forces at work during
the test, and they tell the testing machine when to end the test
(i.e., the load drop).
Unlike in tensile testing, strains are rarely measured for bending
tests. This is due to the unusual non-linear motion associated
with bending, which makes it difficult for strain gauges or
extensometers to accurately measure strains.
However, digital image correlation (DIC) can be used for
non-contact strain measurements. Recording strains with DIC can
provide a more complete picture of material deformation in
bending.
Bending Test Sample Geometry
Typically, bending tests use a very simple rectangular strip sample
design. Rectangular test samples are easy to fabricate and have a
constant cross-section, making it easier to control the loads and
deformations applied to them. Strips must be made from thin sheets
of metal to reduce the necessary bending loads.
Bending Sample Geometry
Bending Test Method
The video below shows the method of VDA-238 Sheet Metal Bending
Test.
Bending Test Results
The main result from a typical bending test is the
load-displacement curve. This plots the bending force measured by
the load cell against the displacement of the bending punch.
Bending Test at FADI-AMT
Another common result is the bending angle, which is simply the
angle between the bent halves of the test piece when the test ends.
This can serve as a simple measure of bending ductility.
Bending Test Standards
Bending testing is regulated by many standards, and some
organizations provide different bending test standards for different
classes of materials.
Bending standards for sheet metals include ISO 7438, ASTM E290, and
VDA 238-100.
Standards for plastics include ISO 178, ASTM D790, and ASTM D6272.
