View Generic Document: Mechanical Design of a Uniaxial Load Frame with High Load Capacity
Citation:
Thrift, Alan (2005). Mechanical Design of a Uniaxial Load Frame with High Load Capacity. National Institute of Standards and Technology, Technology Administration, U.S. Department of Commerce..
Dr. Thomas Gnäupel-Herold’s research focuses on residual stresses in materials using neutron diffraction. Residual stresses occur as the result of inhomogeneous distortions
in almost all materials. These distortions are caused by mechanical, thermal or chemical treatments that affect a specimen (the ‘part’) inhomogenously, e.g. only at the surface. They can be
measured using diffraction by probing the distance between lattice planes in crystalline materials. By comparing the lattice distortions with a reference value, e.g. from an unstressed
specimen, lattice strains can be obtained which, in turn, translate into stresses by means of Hooke’s law. The unique advantage of neutron diffraction is their penetration, which, for most
materials is of the order of centimeters. This property allows the non-destructive evaluation of 3D stress fields in industrial components such as rails or car parts. However, there is also
a strong interest in utilizing this technique for investigating basic phenomena of plastic deformation in materials on the scale of the crystalline constituents. This can be done by
applying a stress to a material while scanning the resulting 3D distortions in a single grain. This basically means scanning through several locations of a single grain using a neutron beam
of about 1-mm in size (limited by neutron flux). In order to obtain sufficiently detailed distortion/stress maps the grain size should be several millimeters, which requires a specimen
cross section greater than 1 cm2. The forces required to deform such a specimen are of the order of 10 tons. However the load frame currently being used is only capable of producing
approximately 1 ton of force. The current load frame is also very small so sample size is extremely limited and the space around the sample is cluttered making access to the sample quite
difficult. Therefore I was asked to design and construct a uniaxial load frame with a much higher load and sample capacity. Over the course of the summer I was successful in designing a
load frame that is capable of producing forces up to 15 ton tensile or compressive loads and supporting a maximum sample size of 8 inches with fifty-percent strain. The newly designed load
frame can still fit on the same experiment table as the old load frame and be rotated about 360 degrees on the experiment table without hitting any of the surrounding equipment. The space
around the sample is open on all sides making the sample accessible by the neutron beam and the measuring apparatus. The load frame incorporates a simple control system that utilizes a
feedback loop between the load cell and stepper motor. The user simply indicates what load is desired on the sample and the stepper motor drives until the load cell reads the corresponding
load. The size of the load frame is sufficient to incorporate ancillary equipment such as a second, smaller load frame for biaxial loads, or sample heating devices. Finally the load frame
is completely transformable. It can be made larger or smaller depending on the sample or experiment table setup size. Dr. Gnäupel-Herold will be using the new and improved load frame to
extend his research on residual stresses and deformation of materials.
Publisher
National Institute of Standards and Technology, Technology Administration, U.S. Department of Commerce.
Guest - January 09, 2009
View Generic Document: Mechanical Design of a Uniaxial Load Frame with High Load Capacity 