1.1 INTRODUCTION One of the principle concerns of an engineer is the analysis of materials used in structural applications. The term structure refers to any design that utilizes materials that support loads and keeps deformation within acceptable limits.Mechanical testing plays an important role in evaluating fundamentalproperties of engineering materials as well as in developing new materials and incontrolling the quality of materials for use in design and construction. If a materialis to be used as part of an engineering structure that will be subjected to a load, itis important to know that the material is strong enough and rigid enough towithstand the loads that it will experience in service and not experience failure. These failures may include additional complexities such as stresses that act in more than one direction, where the state of stress may be biaxial or tri-axial. Failure may also be due to components or materials containing flaws and / or cracks that will propagate failure. Still other failure mechanisms may involve stresses applied for extended periods of time causing Creep, or stresses that are repeatedly applied and removed leading to cyclical type failure. Material failures may be time dependant such as creep or fatigue failure due to cyclical loading, or failures may be time independent where static loading causes rapid fracturing of the material. Time independent fracture or failure due to static loading may be brittle, where very little deformation in the material takes place, or ductile, where significant plastic deformation takes place before failure. Elastic and Plastic deformations are quantified in terms of normal and shear strain in elementary strength of materials studies. The effects of strains in a component are due to deformations such as bending, twisting or stretching. Some members rely on deformations to function, such as a spring, but an excessive amount causing permanent changes are typically avoided. Materials capable of sustaining large amounts of plastic deformation are said to behave in a ductile manner, those that fracture without much plastic deformation are said to behave in a brittle manner. Compression and tensile strength properties are important input data for constitutive modelling.As a result engineers have developed a number of experimental techniques for mechanical testing of engineering materials subjected to tension, compression, bending or torsion loading.In this assignment we will focus only on tensile and compression tests among many other tests such as torsion test.

1.2 TORSION TEST

Tensile testing is one of the more basic tests to determine stress – strain relationships. A simple uniaxial test consists of slowly pulling a sample of material in tension until it breaks. A typical tensile test specimen with larger ends to facilitate gripping the sample can be seen in Figure 1. The “gage length” corresponds to the effective length of the specimen over which the elongation occurs. Therefore, the initial length of the specimen is taken to be equal to the gage length Lg.The test specimen can either be rectangular(fig1a) or clyndrical(fig1b)The typical testing procedure is to deform or “stretch” the material at a constant speed. The required load that must be applied to achieve this displacement will vary as the test

proceeds. During testing, the stress in the sample can be calculated at any time by dividing the load over the cross-sectional area σ =P/A The displacement in the sample can be measured at any section where the cross-sectional area is constant and the strain calculated by taking this change in gauge length and dividing it by the original or initial gage length ε=ΔL/L0 The stress and strain measurements and calculations discussed so far assume a fixed cross sectional area and a change in length that is measured within the constant cross sectional test area of the sample. These stress and strain values are known as engineering stress and engineering strain. The actual stress and strain in the materials for this type of test are higher than the engineering stress and strain. Since it is difficult to measure the actual cross section area during testing to obtain the actual stress values. Engineering material properties that can be found from simple tensile testing include the elastic modulus (modulus of elasticity or Young’s modulus), Poisson’s ratio, ultimate tensile strength (tensile strength), yield strength, fracture strength, resilience, toughness, % reduction in area, and % elongations. Most of these engineering values are found by graphing the stress and strain values from testing. The modulus of elasticity can be calculated by finding the slope of the stress strain curve where it remains linear and constant. For the materials being tested in this lab, there will be an easily recognizable linear portion of the curve to calculate the elasticity value. Where the stress strain curve starts to become non linear, this is known as the proportional limit. The proportional limit is also the point where yielding occurs in the material At this point, the material no longer exhibits elastic behavior and permanent deformation occurs. This onset of inelastic behavior is defined as the yield stress or yield strength. Some materials such as the mild steel used in this lab will have a well-defined yield point that can be easily identified on the stress strain curve. Other materials will not have a discernable yield point and other methods must be employed to estimate the yield stress. One common method is the offset method, where a straight line is drawn parallel to the elastic slope and offset an arbitrary amount, most commonly for engineering metals, 0.2%.

The highest stress or load the material is capable of will be the highest measurable stress on the graph. This is termed the ultimate strength or tensile strength. The point at which the material actually fractures is termed the fracture stress. For ductile materials, the Ultimate stress is greater than the fracture stress, but for brittle materials, the ultimate stress is equal the fracture stress. Ductility is the materials ability to stretch or accommodate inelastic deformation without breaking. Another phenomenon that can be observed of a ductile material undergoing tensile testing is necking. The deformation is initially uniform along the length but tends to concentrate in one region as the testing progresses. This can be observed during testing, the cross sectional area of the highest stress region will visibly reduce. As shown in fig (1c)Two final engineering values that will be determined from the stress strain curve are a measure of energy capacity The amount of energy the material can absorb while still in the elastic region of the curve is know as the modulus of resilience. The total amount of

energy absorbed to the point of fracture is known as the modulus of toughness. These values can be calculated by estimating the respective areas under the stress strain curve. These values are measure of energy capacity, when finding the values under the curve, note that energy is work done per unit volume; therefore the units should be kept in terms of energy.

fig1a

fig 1b

0dGage

length

fig1c fig 1d1.3 APPARATUS

According to the loading type, there are two kinds of tensile testing machines;

1 – Screw Driven Testing Machine: During the experiment, elongation rate is

kept constant.

2 – Hydraulic Testing Machine: Keeps the loading rate constant. The

loading rate can be set depending on the desired time to fracture.

The change in the gage length of the sample as pulling proceeds is measured from

either the change in actuator position (stroke or overall change in length) or a sensor

attached to the sample (called an extensometer).

For the following assignments the procedure given are in relation to the universal

testing machine shown below

fig 1e: Schematics showing left a screw driven machine and right a hydraulic testing machine.

1.4 PROCEDURE

1. The length (L) and diameter (d) of the given specimen was meausred. 2.Tthe centre of the specimen was then marked using dot punch. 3.Two points P and Q were also marked at a distance 150mm on either side of the centre mark so that the distance between P and Q is 300mm. 4.Two pointS A and B were marked at a distance of 2.5 times the rod distance on the either side of the centre mark so that the distance between A & B is 5 times the rod diameter and is known as initial gauge length of rod. 5. Specimen was inserted in the middle cross head and top cross head grip of the machine so that the two points A and B coincide with grips. 6. The load was applied gradually. After sometime, there was slightly pause in the increase of load. The load at this point was noted as yield point 7.The load was applied continuously till the specimen fails and ultimate load and breaking load were noted from the digital indicator.

8. The specimen was removed from the machine and the two pieces of the specimens joined. 9.The distance between the two points A and B was measured. This distance was noted as final gauge length of the specimen. 10. The diameter of the rod at neck was measured.. 1.5 ANALYSISTYPICAL STRESS STRAIN GRAPHS

Three tests for the A-36 hot rolled steel samples

Graph showing determination of yield point by the 0.2% offset method stated earlier in the introduction section.The point where the offset intersect the graph is known as yield point ,stress read backwards is known as 0.2%proof yield stress.

As earlier mentioned the tensile test can be used to obtain a wide variety of structure properties as shown below

1.5.1 Stress and strain relationship

When a specimen is subjected to an external tensile loading, the metal will undergo elastic

and plastic deformation. Initially, the metal will elastically deform giving a linear relationship of load

and extension. These two parameters are then used for the calculation of the engineering stress and

engineering strain used to draw the graphs above.

where σ is the engineering stress

ε is the engineering strain

P is the external axial tensile load

Ao is the original cross-sectional area of the specimen

Lo is the original length of the specimen

Lf is the final length of the specimen

The unit of the engineering stress is Pascal (Pa) or N/m2 according to the SI Metric Unit

whereas the unit of psi (pound per square inch) can also be used.

1.5.2. Young's modulus, E

During elastic deformation, the engineering stress-strain relationship follows the Hook's Law

and the slope of the curve indicates the Young's modulus (E)

Young's modulus is of importance where deflection of materials is critical for the required

engineering applications. This is for examples: deflection in structural beams is considered to be

crucial for the design in engineering components or structures such as bridges, building, ships, etc.

1.5.3 Yield strength, σy

By considering the stress-strain curve beyond the elastic portion, if the tensile loading

continues, yielding occurs at the beginning of plastic deformation. The yield stress, σ

y, can be

obtained by dividing the load at yielding (Py) by the original cross-sectional area of the specimen (Ao)

The yield point elongation phenomenon shows the upper yield point followed

by a sudden reduction in the stress or load till reaching the lower yield point. At the yield point

elongation, the specimen continues to extend without a significant change

in the stress level. Load

increment is then followed with increasing strain. This yield point phenomenon is associated with a

small amount of interstitial or substitutional atoms.

The determination of the yield strength at 0.2% offset or 0.2% strain can be carried out by

drawing a straight line parallel to the slope of the stress-strain curve in the linear section, having an

intersection on the x-axis at a strain equal to 0.002 An interception

between the 0.2% offset line and the stress-strain curve represents the yield strength at 0.2% offset or

0.2% strain. However offset at different values can also be made depending on specific uses: for

instance; at 0.1 or 0.5% offset.

The yield strength, which indicates the onset of plastic deformation, is considered to be vital

for engineering structural or component designs where safety factors are normally used as shown

1.5.4 Ultimate Tensile Strength, σTS

Beyond yielding, continuous loading leads to an increase in the stress required to

permanently deform the specimen as shown in the engineering stress-strain curve. At this stage, the

specimen is strain hardened or work hardened. The degree of strain hardening depends on the nature

of the deformed materials, crystal structure and chemical composition, which affects the dislocation

motion. FCC structure materials having a high number of operating slip

systems can easily slip and

create a high density of dislocations. Tangling of these dislocations requires higher stress to

uniformly and plastically deform the specimen, therefore resulting in strain hardening.

If the load is continuously applied, the stress-strain curve will reach the maximum point,

which is the ultimate tensile strength (UTS, σ

TS). At this point, the specimen can withstand the

highest stress before necking takes place. This can be observed by a local reduction in the crosssectional

area of the specimen generally observed in the centre of the gauge length

1.5.5 Fracture Strength, σF

After necking, plastic deformation is not uniform and the stress decreases accordingly until

fracture. The fracture strength ( σ

fracture) can be calculated from the load at fracture divided by the

original cross-sectional area, Ao,

1.5.6 Tensile ductility

Tensile ductility of the specimen can be represented as % elongation or % reduction in area

as expressed in the equations given below;

In general, measurements of ductility are of interest in three ways:1. To indicate the extent to which a metal can be deformed without fracture inmetalworking operations such as rolling and extrusion.2. To indicate to the designer, in a general way, the ability of the metal to

flowplastically before fracture.3. To serve as an indicator of changes in impurity level or processingconditions. Ductility measurements may be specified to assess materialquality even though no direct relationship exists between the ductilitymeasurement and performance in service.

1.5.7 Modulus of Resilence UR

Apart from tensile parameters mentioned previously, analysis of the area under the stressstrain

curve can give informative material behavior and properties. By considering the area under the

stress-strain curve in the elastic region (triangular area) as illustrated in figure 7, this area represents

the stored elastic energy or resilence. The latter is the ability of the materials to store elastic energy

which is measured as a modulus of resilence, UR, as follows

1.5.8 Tensile toughness, UT

Tensile toughness, UT, can be considered as the area under the entire stress-strain curve which

indicates the ability of the material to absorb energy in the plastic region. In other words, tensile

toughness is the ability of the material to withstand the external applied forces without experiencing

failure. Engineering applications that requires high tensile toughness is for example gear, chains and

crane hooks, etc. The tensile toughness can be estimated from an expression as follows

1.5.9 Work hardening exponent, n

Furthermore, material behavior beyond the elastic region where stress-strain relationship is

no loner linear (uniform plastic deformation) can be shown as a power law

expression as follows

Where σ is the true stress

ε is the true strain

n is the strain-hardening exponent

K is the strength coefficient

The strain-hardening exponent values, n, of most metals range between 0.1-0.5, which can be

estimated from a slope of a log true stress-log true strain plot up to the maximum load as shown

While n is the slope (m) and the K value indicates the value of the true stress at the true strain

High value of the strain-hardening exponent indicates an

ability of a metal to be readily plastically deformed under applied stresses. This is also corresponding

with a large area under the stress-strain curve up to the maximum load. This power law expression

has been modified variably according to materials of interest especially for steels and stainless steels.

1.6 Conclusion

The table below shows values of various properties of common

engineering materials

2.1 COMPRESSIVE TEST

Simple tensile testing usually yields sufficient data to determine the mechanical properties of ductile materials. In those materials, the yield limits under tension and compression are generally the same. Therefore, it is not necessary to perform the compression test on highly ductile materials such as mild steel or most Al-alloys.

However, in some materials such as brittle and fibrous ones, the tensile strength is considerably different from compressive strength as seen in Figure 4. Therefore it is necessary to test them under tension and compression separately.

Brittle materials, such as cast iron and concrete, are often weak in tension because of the presence of submicroscopic cracks and faults. However, these materials can prove to be quite strong in compression, due to the fact that the compression test tends to increase the cross sectional areas of specimens, preventing necking to occur.

For my assignment i chose to analyse the compression test for concrete widely used in structure building.

2.2APPARATUS

Ruler

Paper

Pen/Pencil

Calculator

Measuring tape

Safety goggles

Gloves

Testing machine

Concrete specimens (Cylinder/Cube)

Testing Machine

2.3PROCEDURE

1. All required apparatus were confirmed.

2.Wore hand gloves and safety goggles.

3. The dimension of concrete specimens were measured and the cross sectional area calculated .

4.The machine was switched on and one concrete specimen placed in the centre of loading area.

5.The piston was lowered to just touch against the top of concrete specimen by pushing the lever.

6.The lever was pulled into holding position and the compression test stated by pressing the zero button on the display board.

7. The pressure on piston was adjusted by moving the valve counter clockwise so that it matches concrete compression strength value. The load was applied gradually without shock.

8.Immediately the specimen start to break no more load was applied.

9. The ultimate load was noted on paper.

10.The machine was cleaned by cleaning the creaked concrete from the machine.

11. The recorded value was then matched by the display once more and then trghe machine was switched off.

2.4 ANALYSIS

To Calculate concrete compressive strength: The result we got from testing machine is the ultimate load to break the concrete specimen.Our purpose is, to know the concrete compressive strength but compressive strength is equal to ultimate load divided by cross sectional area of concrete specimen but cross sectional area was calculated after finding dimensions of the specimen. So

Compressive strength = Ultimate load (N) / cross sectional area (mm2).

The unit of compressive strength will be N/mm2.

Normally 3 sample of concrete specimens are tested and average result is taken into consideration. If any of the specimen compressive strength result varies by more than 15% of average result, that result is rejected.

2.5 CONCLUSION

The following table shows the compressive srength of common materials;

3.1REFERENCES

1.CHENAI INSTITUTE OF TECHNOLOGY–DEPARTMENT OF MECHANICAL LABARATORY LAB MANUAL ENGINEERING.

2.COMPRESSION TESTS ON SHORT STEEL COLUMNS OF RECTANGULAR CROSS-SECTION, 1953 G. HAAIJER

3.TENSILE TESTING ,SECOND EDITION 2004 ASM INTERNATIONAL.

4.UNIAXIAL TENSION AND COMPRESSION OF MATERIALS –NIKITA KHLYSTOV,DANIEL LIZARDO,KEISKUKE MATSUSHITA,JENNIE ZHENG

5 MECHANICAL PROPERTIES OF 1018 STEEL IN TENSION.6. CALIFORNIA STATE POLYTECHNIC UNIVERSITY, POMONA MECHANICAL ENGINEERING DEPARTMENT ME 220L STRENGTH OF MATERIALS LABORATORY MANUAL.