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Metal Compression Test and Tension Test Operation Methods on Universal Testing Machine

Wednesday 03rd July 2019 Back to list

Compression and tension test

Understand the purpose of compression and tensile testing:

1. Through the tension test of low carbon steel, determine the ratio limit σP,  yield limit σS, ultimate strength limit σb, extensibility ratio δ, cross-section shrinkage ψ, and modulus of elasticity E of low carbon steel is determined by the tensile test of low carbon steel. And draw the stress-strain curves of low carbon steel to understand the basic mechanical properties of plastic materials.

2. Through the tension test of the cast iron, determine the strength limit σb,draw the tensile curve of cast iron, and understand the damage property of cast iron.

3.Through the compression test of cast iron and low carbon steel, determine the strength limit σb,compare the deformation and destruction of cast iron and low carbon steel, further study the mechanical properties of the plastic materials and the brittle materials.

Metal Compression Test

Overview

The test results show that the mechanical properties such as strength, stiffness and plasticity of plastic materials commonly used in engineering are about the same as those of tensile materials under compression and tension. However, the widely used brittle materials have high compressive strength and low tensile strength. It is very important to determine the mechanical properties of materials under pressure in order to select engineering materials reasonably and meet the needs of metal forming process. Therefore, the compression test is the same as the tensile test, and is one of the most common and basic experiments for measuring the mechanical properties of the material under the normal temperature, static load and one-way stress.

Test objective

1. Observation of yield load FSC of low carbon steel under compression.

2. Determination of compressive strength σbC at compression of cast iron.  

3. Observe and compare the deformation and failure of low carbon steel and cast iron during compression.

Test equipments

Hydraulic universal material testing machine

Vernier caliper

Specimen preparation

In accordance with GB7314-87 Test method for metal compression, the compressed specimens of the metal material are mainly made of cylinders. Specimen with length L=(2.5~3.5)d0 is used to determine σpc, σtc, σsc, σbc; Specimen with length L=(5~8)d0 is used to determine σpc0.01, Ee; Specimen with length L=(1~2)d0 is used to determine σbc. In order to make the specimen subjected to axial pressure as much as possible, a reasonable processing technique must be provided to ensure that both ends are parallel and perpendicular to the axis.

σpc-Specified non-proportional compression stress

σtc-Specified total compressive stress

σsc-Compressive yield point

σbc-Compression strength

σpc0.01-Specifies the stress at a non-proportional compression strain of 0.01%

Ee-Modulus of elasticity in compression

Test principal

In the plastic material represented by low-carbon steel, a large transverse deformation can be generated in the axial compression, but due to the friction between the two end surfaces of the specimen and the support plate of the testing machine, the transverse deformation is restrained, so that the middle part of the specimen has a significant bulging.

The elastic modulus and yield point of plastic materials in compression are the same as those in tension, but when they reach the yield stage, they are not as obvious as in tensile test, therefore, it needs to observe carefully to determine the load capacity FsC. When the loading continues, the compression capacity of the sample increases with the increase of the cross section area, and the curve continues to rise. Unless the specimen is over-bulged and deformed, resulting in the cracking of the cylinder surface, the plastic material will not be subjected to compression failure. Therefore, the compressive strength of plastic materials is not measured, but the compressive strength is usually considered to be equal to tensile strength.

Because the plastic deformation of the brittle metal material represented by cast iron is very small, although there is end face friction, the bulging effect is not obvious, but when the stress reaches a certain value, the specimen breaks in the direction of about 450 × 550 with the axis. This is due to the fact that the shear strength of brittle materials is lower than that of compressive strength, so that the specimen is cut off.

Compression and tension test

Test steps

1. Measuring the diameter of the sample with a vernier caliper. The method is to measure the diameter in two perpendicular directions at the midpoint of the original distance of the sample and take its arithmetic mean value.

2. According to the yield load of the low carbon steel and the estimated value of the compressive strength of the cast iron, select the force disk of the testing machine, and adjust the finger to zero.

3. Adjust the automatic plotter.

4. Accurately place the specimen at the center of the supporting gasket of the movable platform of the testing machine.

5. Adjust the chuck spacing of the tester. When the specimen is close to the upper supporting plate, it begins to load slowly and evenly.

6. For low carbon steel samples, the test can be stopped by pressing the sample into drum shape. For cast iron specimen, stop the test immediately when loading the specimen is damaged(main needle swing back about 150)to avoid further crushing of the specimen.

Metal tension test

Overview

The axial tensile test under normal temperature and static load is the most basic and widely used test of material mechanics test. The mechanical properties of the material, such as elasticity, plasticity, strength and fracture, can be measured comprehensively by the tensile test. The performance index has an important role in the analysis and calculation of the material mechanics, the engineering design, the selection of materials and the development of new materials.

Test objectives

1. Determine the yield strength Rel, tensile strength Rm, Extension rate after fracture A11.3, and reduction of cross section area Z of low carbon Steel.

2. Determine the tensile strength Rm  of cast iron

3. Observe the various phenomena of the two materials in tension and draw the tension diagram (F─test curve)

4. Analyze and compare the mechanical properties and specimen failure characteristics of low carbon steel and cast iron.

Test and measuring equipments and instruments

1. Universal testing machine

2. Vernier caliper

Specimen preparation

The specimen is cut or prepared according to the related test standard or GB/T2975 test standard. The test results show that the shape and size of the sample have some influence on the performance test results. In order to make the results of tensile test of metal materials comparable and conformable, the state has established a unified standard. According to this standard, the tensile specimen is divided into two types: proportional specimen and non-proportional specimen. These two kinds of samples are convenient for machining, and are also convenient for size measurement and fixture design. The tensile specimen used in this test is a long proportional specimen of circular cross section made by machining, that is, L=10d.

Test principal

In accordance with GB/T 228-2002 Standard Test Method for Tensile Test of Metals at Room Temperature: For example, cnc machining aluminum also requires a Universal Test Machine to test it

1. low carbon steel specimen  In tensile test, the tensile curve of low carbon steel can be drawn by using the automatic plotter of the testing machine,F—ΔL test curve. The initial stage is curved, which is caused by the sliding of the specimen head in the fixture and the gap of the testing machine. When analyzing, the straight line segment in the graph should be extended and intersected with the transverse coordinate at o point as its coordinate origin. The deformation characteristics of the material and the relationship between the force and deformation at each stage are described in the image of the drawing curve. The elastic and plastic properties of the materials, the toughness and brittleness of the materials and the bearing capacity under different deformation can be judged by the state of the drawing. However, the tensile curve of the same material will vary according to the specimen size. In order to make the tensile process and its characteristic points of different sizes of the same material easy to compare, so as to eliminate the influence of the geometric size of the specimen, the longitudinal coordinate (force F) of the tensile curve can be divided by the original cross section area S0 of the specimen, and the transverse coordinate (extension ΔL) can be divided by the original standard distance of the specimen.

(2) Yield stage SK  When the stress exceeds the elastic limit and reaches the zigzag curve, the main needle on the indicator plate rotates or starts to rotate and move back and forth. If the surface of the specimen is polished, the trace indicating the crystal slip can be seen, and it is about 45 degrees direction with the axis of the specimen. This phenomenon indicates that the deformation of the specimen continues to stretch without increasing or slightly decreasing the tensile force, which is called the yield point of the material and the yield point (yield stress). The corresponding stresses of the maximum force (yield force on Fsu) before the pointer of the indicator disk rotates for the first time, and the minimum force (yield force under FsL)without considering the initial instantaneous effect (the lowest point of the first drop, regardless of load), are the upper and lower yield points, respectively.

(3)Enhancing stage KE  After the yield stage, the internal crystal structure of the specimen material has been adjusted again due to plastic deformation, and its ability to resist deformation has been enhanced. With the increase of tensile force, the tensile deformation also increases, and the tensile curve continues to rise. The KE curve section is called strengthening stage. With the increase of plastic deformation, the mechanical properties of the material change, that is, the deformation resistance of the material increases and the plasticity decreases. When unloaded in the strengthening stage, the elastic deformation will disappear and the plastic deformation will be retained most. The unloading path of the strengthening stage is parallel to the elastic stage. When the loading line is reloaded after unloading, the loading line is parallel to the elastic stage. After reloading, the proportional limit of the material is obviously increased, and the plastic properties will decrease accordingly. This phenomenon is called deformation hardening or cold hardening. When the tensile force increases and the tensile curve reaches the top E, the main needle on the force dial begins to return, and the maximum tension referred to by the auxiliary needle is Fm, from which the tensile strength of the material can be obtained. It is also an important index of material strength and properties.

(4) Local deformation stage EG(Necking and fracture stage)  For plastic materials, the deformation of the specimen is basically uniform before bearing tension Fm. After reaching Fm, the deformation is mainly concentrated in a certain part of the specimen, where the cross section area decreases sharply, which is the phenomenon of "necking". At this time, the tensile force decreases until the specimen is broken, and its fracture shape is bowl-shaped. After the specimen is broken, the elastic deformation disappears immediately, while the plastic deformation remains on the broken specimen. Use the plastic deformation in the standard distance of the specimen to calculate the fracture extensibility A11.3 and section shrinkage Z of the material.

Compression and tension test

Test steps

Estimate the maximum tensile force according to the shape and size of the specimen and the predicted tensile strength of the material, and the force is used as 40% - 80% of the measuring range of the indicator dial in order to select the appropriate indicator plate and the corresponding pendulum. Then, select the suitable fixtures for the specimen.

Within the scope of the original gauge length L0 of the specimen, use a scribing machine to divide 10 grid lines to observe the deformation along the axial direction within the gauge range and measure the elongation after breaking.

In accordance with GB/T 228-2002 Standard Test Method for Tensile Test of Metals at Room Temperature, determine the original section area of the specimen. In this test, use the round specimen to measure the cross section diameter at both ends and in the middle of the standard distance, take the arithmetic average value, and select the minimum diameter measured in three places to calculate the cross section area from this value.

Install the sample, according to the operation rules of the universal material testing machine, adjust the pointer of the indicator plate to zero, and adjust the automatic drawing device. After inspection by the tutor, the test can begin.

Loading test: during the test, the loading shall be carried out evenly and slowly. Loading test: during the test, the loading shall be carried out evenly and slowly, and record the yield capacity Fel value and the max load capacity Fm value. For cast iron samples, only the maximum load fm value needs to be measured. Stop the machine immediately when the specimen is broken, and remove the broken specimen.

For the broken low carbon steel specimens, it needs to determine the standard distance LU after fracture and the minimum diameter dU at neck shrinkage respectively.