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A. Eremichev
24
ISSN 0236-3941. Вестник МГТУ им. Н.Э. Баумана. Сер. Машиностроение. 2017. № 5
Calculating the tensile stress in the centre of the specimen and shear stress of cap-
ture and considering that most of the material breaking stress in shear is approxi-
mately half the breaking stress in tension will get
a >
4
c – b
,
r ≤
2
c
.
More accurately the stress-strain state of the sample and final clarification of its
size were made in [5] by FEM. The calculations were made for the samples with dif-
ferent sizes. Selection criteria of the final sample size was that to ensure the least stress
concentration in the working area of the sample near its center (uniform three axial
tension, and the virtual absence of shear stresses).
The allowable deviation from UTAT is equals no more than 10 %. For a sample
with optimal size in UTAT stressed state is about 40 % of the working area of the
sample. Tensile stress in each of the three axes was 0,86
,
ii
where
ii
=
2
,
2
F
a c
—
F
the tensile force acting on each of the three axes.
Therefore, the stress on the central section of the specimen differs only 14 % dif-
ferent from the average stress
.
ii
In samples with other dimensions this deference of tensile stress in the center of
sample and also the stress concentration are greater.
The calculation was carried out in an elastic medium. For real materials, which at
the time of destruction will have faced significant plastic deformation, stress concen-
trations will be lower.
The final specimen dimensions was selected as follows (Fig. 2)
10
a
≤
c ≤
;
7
a
9
a
≤
b ≤
;
7
a
5
b
≤ r ≤
.
2
b
2. In the first stage of experimental investigations the tests were carried out for an
isotropic material. The object of the study was chosen to be plexiglass, supplied in
sheets with a thickness of 50 mm. Series of samples (7 pieces) with the following di-
mensions were produced
a
= 38,0 mm,
b
= 8,0 mm,
c
= 8,5 mm,
r
= 4,0 mm.
The tests were carried out using the device [3] (Fig.
1) on the serial testing ma-
chines FMG-200 at the velocity of the grippers of the machine of 0,2 mm/sec. Average
results of test UTAT series samples are given in the table.
The type of stress state
Ultimate Strength
,
U
MPa
Energy (Joules)
Uniform three axes tension
UTAT
9,79
14,41
Uniaxial tension UAT
13,60
7,45