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What are the common material mechanical properties tests?

The mechanical properties of the material refers to the mechanical characteristics of the material in different environments (temperature, media, humidity), when subjected to various applied loads (tensile, compressive, bending, torsion, impact, alternating stress, etc.).


According to the force acting on the object can be divided into static load: gradually and slowly acting on the work of the force such as the pressure of the machine bed, the tension of the steel cable. Dynamic load: including impact and alternating load such as air hammer rod by the impact, gears, springs.



Scope of application

1、Rubber materials: rubber products, hose, tape, O-rings, tires and other rubber materials and products.


2、Plastic materials: plastic products, films, tubes, plates, packaging materials, nylon products, waterproof rolls and other plastic materials and products.


3, metal materials: metal products, stainless steel products, bolts, steel wire, alloy products and other metal materials and products.


4、Construction materials: wood, plate, glass, concrete, graphite products, etc.


General mechanical properties of materials testing - tensile test

Overview

The test to determine a series of characteristics of the material under tensile load, also known as tensile test. It is one of the basic methods for testing the mechanical properties of materials, mainly used to test whether the material meets the required standards and study the performance of the material.


Performance index

Tensile test can determine a series of strength indicators and plasticity indicators of the material. Strength usually refers to the ability of a material to resist the production of elastic deformation, plastic deformation and fracture under the action of an external force. Material under tensile load, when the load does not increase and still continue to undergo significant plastic deformation phenomenon is called yielding. The stress at which yielding occurs is called the yield point or physical yield strength and is expressed as σS (Pa). Engineering there are many materials without an obvious yield point, usually the material generated by the residual plastic deformation of 0.2% of the stress value as the yield strength, called the conditional yield limit or conditional yield strength, expressed in σ0.2. The maximum stress value reached by the material before fracture, called the tensile strength or strength limit, expressed by σb (Pa).


Plasticity is the ability of a metallic material to produce plastic deformation under load without damage, and the commonly used plasticity indicators are elongation and sectional shrinkage. Elongation, also known as elongation, is the percentage of the total elongation of the material specimen after fracture by tensile load, the ratio of the original length, expressed in δ. Section shrinkage refers to the material specimen in the tensile load after fracture, the section shrinkage of the area of the ratio of the original cross-sectional area of the percentage, expressed in ψ.


Condition yield limit σ0.2, strength limit σb, elongation δ and sectional shrinkage ψ are four performance indicators often measured in tensile testing. In addition, the modulus of elasticity E, the proportional limit σp, and the elastic limit σe of the material can also be determined.


Data Curve

The tensile curve drawn by the testing machine is actually the load-elongation curve (see figure), such as the load coordinate value and elongation coordinate value divided by the original cross-sectional area of the specimen and the specimen distance, respectively, you can get the stress-strain curve graph. When the load is continued, the curve deviates from op until point e. At this point, if the load is removed, the specimen can still be restored to its original state, but the specimen cannot be restored to its original state if it is past point e. The stress at point e is the elastic limit σe. The real σe, often take the residual elongation of the specimen to reach 0.01% of the original pitch of the stress for the elastic limit, to σ0.01 said. Continue to add load, the specimen along the es curve deformation to reach the s point, the stress at this point for the yield point σS or residual elongation of 0.2% of the conditional yield strength σ0.2. After the s point continue to increase the load to the maximum load before breaking b point, the load divided by the original cross-sectional area is the strength limit σb. After the b point, the specimen continues to elongate, while the cross-sectional area decreases, the bearing capacity begins to decline until the k point fracture . The ratio of the load at the moment of fracture to the cross-section at the point of fracture is called the fracture strength.



Figure 1 shows the standard tensile specimen and the specimen after fracture, the specimen is marked on the specimen with the distance length.


Figure 2 shows the tensile (load an elongation) relationship of general structural steel



High temperature tensile test

Overview

High-temperature tensile test is a tensile test conducted at high temperatures above room temperature. High-temperature tensile test, in addition to the consideration of stress and strain, but also take into account the temperature and time two parameters. Temperature has a great influence on the high temperature tensile performance, so the temperature control requirements are very strict. The specimen is generally heated by an electric furnace, and the furnace workspace should have sufficient uniform heat, with the instrument for automatic temperature control.


Effects

Metal materials work at high temperatures, and this temperature is not yet to make the material creep phenomenon, or although the temperature has been possible creep phenomenon, but because the working time is very short, creep phenomenon does not play a decisive role. In the above two cases, the performance measured by the short time tension at high temperature becomes an important indicator of the mechanical properties of the material. Sometimes, in order to determine the process of hot processing, it is also necessary to determine the short-time tensile capacity of the material at the hot processing temperature.


Test Analysis

High-temperature tensile test of metal materials specified in the performance indicators and room temperature tensile test is basically the same, but generally is the determination of tensile strength, yield strength, elongation after break and shrinkage at break four performance indicators. As a result of high temperature short time tensile test, the length of the load duration, the tensile properties have a significant impact. The tensile strength value increases significantly when the short-time high-temperature tensile specimen is pulled off quickly. The following figure.



The determination method of several major indicators of the short-time tensile test at high temperature is basically the same as the determination method at room temperature. With the change of temperature, the trend of the four indicators is shown in the figure.



Research application

Prof. Zhiwei Shan and Prof. Maen of Xi'an Jiaotong University and Prof. Ju Li of Massachusetts Institute of Technology (MIT) have performed quantitative compression and tensile tests on isotropic submicron amorphous silicon (a-Si) samples by using a nanomechanical test system within a TEM.


This work opens up an unexplored mechanism of intrinsic tensile-compression asymmetry in materials. The results were published in the Nature subjournal Nat. Mater. under the title "Tension-compression asymmetry in amorphous silicon". This anomalous stretch-compression (T-C) asymmetry is also applicable to other materials like a-Si, and provides important guidance for the application of small-size a-Si microelectronics and MEMS. The future may inspire the invention of new materials with novel elasticity.