Characteristics and selection methods of tensile testing machine fixtures

1, the characteristics of the fixture a.
We know that the fixture is forced by the fixture clamping specimen (or product), and the size of the test force that the fixture can withstand is a very important indicator
of the fixture.

It determines the size of the fixture structure and the labor intensity of the fixture operation, the specimen material is divided into metal and non-metallic, and the shape is divided into
size.

The composition of the material is various, and the test force that the sample can withstand is as small as tens of centimeters of cattle (such as spandex silk for textiles) and as large as tens of tons (such as ordinary steel, etc.
); The test force of the relatively large electronic universal testing machine in China is 600KN, 0.
5 grade machine), the sample size is as small as the diameter φ0.
006mm of gold wire, as large as the diameter of 1m PVC pipe, etc
.

This requires the design of different fixtures according to different test forces and the shape and size of the specimen
.

b.
Requirements for fixture materials: (1).
For general metal and non-metallic specimens, the jaws of the fixture are in direct contact with the specimen, and alloy structural steel, alloy high-carbon steel (or low-carbon alloy steel), cold-working die steel, etc.
, through the appropriate heat treatment process (quenching and tempering, carburizing quenching, etc.
) to increase its strength, wear resistance, and sometimes special steel is mounted at the jaw, or gold steel sand is sprayed on the surface of
the jaw.

(2).
For some fixtures with small test force, the surface in contact with the specimen is made of sticky soft rubber skin
.

(For example: the fixture clamping surface
of plastic film, fiber wire, etc.
)
(3) The specific clamping is generally medium-carbon steel and alloy structural steel, and its mechanical properties
are increased by appropriate heat treatment process.

Sometimes non-ferrous metals such as aluminum alloys and special metals are also used to reduce weight
.

Sometimes casting structure (cast steel, cast aluminum, etc.
) c.
Requirements for fixture structure: (1) The design of fixture is mainly based on the test standards of materials and the shape and material
of specimens (specifically finished and semi-finished products).

The above test standards refer to ISO, ASTM, DIN, GB, BS, JIS.
.
.
etc.
, as well as enterprise standards, industry standards, etc.
, these standards generally have strict provisions on specimen preparation and test methods, we can design different fixtures
according to different specimens and test methods.

For the fixtures used in special specimens (finished and semi-finished products), the fixtures are designed mainly according to the shape and material of the specimens
.

(2).
The fixture itself does not have a fixed structure (such as the wire can be clamped by winding, two flat plates can also be clamped, and the metal sheet specimen can be clamped by wedge, and the clamping method can also be used), which is obviously different from
the host.

The main engine is similar to the domestic and foreign countries, while the fixture is very different from foreign and domestic, and there are also big differences
between different companies.

This mainly depends on the overall level of the company, the accumulation of experience of designers
.

Foreign fixtures, such as INSTRON, MTS, ZWICK and other companies of the fixture are generally meticulous, the availability is high, but the price is higher, in the market; And our fixtures, due to their wide involvement in the industry, in the domestic market fen e large, to a certain extent can replace some foreign fixtures, in the market
.

However, in some new materials and fixtures for special materials, there is still a certain gap between
domestic and foreign levels.

The United States TestResources mechanical testing system all-electric servo static tension, pressure, torsion testing machine, dynamic & fatigue series testing machine, with a variety of professional fixtures, can also be used for other brands of testing machines
.

(3).
The fixture itself is a locking mechanism, we know that the mechanical locking structure is: reeling (that is, thread, screw, nut), bevel, eccentric wheel, lever, etc.
, the fixture is a combination of these structures
.

The fixture for the testing machine has no fixed mode in the structure, according to different specimens and the size of the test force, the structure is very different.
(the specimen with large test force generally adopts a beveled clamping structure, with the increase of the test force, the clamping force increases, and the shoulder specimen adopts a suspension structure, etc.
), if the fixture is divided according to the structure, it can be divided into wedge-type fixtures (refers to the fixture using the principle of inclined locking structure), the clamp-type fixture (refers to the fixture using the single-sided or double-sided threaded top-tightening principle structure), Winding fixture (refers to the fixture that the specimen is locked by winding), eccentric fixture refers to the fixture that adopts the structure of eccentric locking principle), lever fixture (refers to the fixture that adopts the principle of lever force amplification), bench fixture (refers to the fixture suitable for the shoulder specimen), bolt fixture (refers to the fixture suitable for testing the thread strength of bolts, screws, studs, etc.
), 90° peeling fixture (refers to the fixture suitable for hanging of two specimens, straight peeling), etc
.

The structure of these fixtures has its own advantages and disadvantages, such as: wedge fixtures, the initial clamping force is small, with the test force increases
.

The clamping force then increases
.

For the clamping fixture, the initial clamping force is large and increases
with the test force.

The clamping force is then reduced
.