Metal & Oil & Gas News – Nctv.net
Mechanical testing of stainless steel
plates are used to generate data that can be used for design purposes or as part
of the material connection procedure or operator acceptance procedure. The most
important function may be to provide design data, because it is important to
understand the limit value of stainless steel sheet product structure and
ensure that it will not fail.
One other effect of this mechanical test is
the tensile test, which can be used to determine the yield strength of the
steel used for design calculations, or to ensure that the stainless steel plate
meets the strength requirements of the material specifications.
Mechanical tests can be divided into
quantitative or qualitative tests. A quantitative test is a qualitative test
that provides data for design purposes, and the results are used for
qualitative tests such as hardness testing or bend testing.
Tensile testing is used to provide
information used in design calculations, or to prove that materials conform to
the requirements of corresponding specifications, so it may be quantitative or
qualitative testing.
The test is to grasp the end of the
standard sample properly prepared on the tensile testing machine, and then
increase the uniaxial load until failure occurs. Standardized test pieces so
that the results are reproducible and comparable.
The specimen is usually proportionate. When
the gauge length is L 0, it is related to the original cross-sectional area, A
0, as L 0 =k A 0. In the EN standard, the constant k is 5.65, and the ASME
standard is 5. The length of these measurements is about 5 times the diameter
of the sample and 4 times the diameter of the specimen, although this
difference may not be important technically, but it is very important to
declare it in accordance with the specification.
Load (stress) and specimen elongation
(strain) are measured, and engineering stress / strain curves are constructed
from the data. The following aspects can be determined from the curve.
A) the tensile strength, also known as the
ultimate tensile strength, is divided by the original cross section by the load
at the ultimate tensile strength (UTS) and the maximum = P maximum / A 0 when
the fracture is broken. The maximum P = maximum load, A 0 = the original cross
section area. In the EN specification, the parameter is also identified as
“R m”.
B) the yield point (YP), that is, the
stress from the elastic to the plastic deformation, that is, the yield point
below the unloading specimen means that it is restored to the original length,
the permanent plastic deformation at the yield point above the yield point, YP
or sigma y = P YP / A 0, P YP = the yield point load. In the EN specification,
the parameter is also identified as “R e”.
C) in reassembling the broken sample, we
can also measure the elongation, and the El% test piece has been El (%) = (L F
L – 0 / L) = (%) = (L F L = 100), L F = break distance and L 0 = original
distance length. In the EN specification, the parameter is also identified as
“A”.
D) A%= (A 0 -A f / A 0) x 100 and A f =
part of the cross section area, in which the percentage of R is reduced, and
the fracture of the sample in the degree of necking or decrease in diameter. In
the EN specification, the parameter is also identified as “Z”.
A) calculation of elongation, b)
calculation of area reduction rate
(a) and (b) measure the strength of
materials, (c) and (d) indicate the ductility or capacity of materials without
deformation. The slope of the elastic part of a curve is basically a straight
line, which will give young’s modulus of elasticity, which is to measure the
degree of elastic deformation of the structure when it is loaded. Low modulus
means that the structure will be flexible, and the high modulus structure will
be stiff and inflexible.
In order to produce the most accurate
stress / strain curve, additional extensometer should be added to the stainless
steel plate to measure the elongation of the gauge length. The less accurate
way is to measure the movement of the crosshead of the drawing machine.
The above stress-strain curves show
material with good yield point, but only annealed carbon steel shows this
behavior. There must be other ways to determine the “yield point” by
alloying, heat treatment or cold hardening of metal without obvious yielding.
This is measured by measuring yield stress
(yield strength in American terms), that is, a certain amount of stress
required for plastic deformation in the specimen.
The stress is measured by drawing a
straight line parallel to the elastic part of the stress / strain curve at a
specific strain, and the strain is the percentage of the original length of the
standard distance, so 0.2% verification, 1% verification.
For example, in the specimen with a gauge
length of 100mm, the yield strength of 0.2mm is measured by using the permanent
deformation of the 0.2mm. Therefore, it is proved that strength is not a fixed
material property, such as yield point, but depends on the number of plastic
deformation specified. Therefore, when considering the strength of proof, the
percentage must always be quoted. Most steel specifications use 0.2% of the EN
specification, R P0.2.
Some materials such as annealed copper,
gray iron and plastic have no linear elastic part in stress / strain curves. In
this case, similar to the method of determining the strength of the
verification, the usual practice is to define the “yield strength” as
the stress that produces a specified number of permanent deformations.
Source: China Stainless Steel Plates
Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)
