P3 Lab Guide Estructuras

Department of Continuum Mechanics and

Structural Analysis

Mechanics of structures

Tensile testing on aluminum specimens

2015/2016

Developed by: Javier Reboul Corpa

Ángel Rubio López

Department of Continuum Mechanics Mechanics of Structures

and Structural Analysis

1 Tensile testing on aluminum specimens

2015/2016

TABLE OF CONTENTS

1 OBJECTIVE .................................................................................................................................... 2

2 TENSILE TESTING ......................................................................................................................... 2

3 STRESS-STRAIN ............................................................................................................................ 3

4 EQUIPMENT DESCRIPTION .......................................................................................................... 5

4.1 Generalities ......................................................................................................................... 5

4.2 Types of control ................................................................................................................... 6

5 SPECIMENS DESCRIPTION............................................................................................................ 6

6 PRACTICE EXCERCISE ................................................................................................................... 7




2015/2016

1 OBJECTIVE

The principal objective of this laboratory session is to obtain the mechanical properties of a

material using a universal testing method. Students will acquire a basic knowledge about the

methodology of these tests and will learn how to process the results.

2 TENSILE TESTING

The tensile test is probably the most used mechanical test in order to obtain the mechanical

properties of materials. This test provides information on the mechanical behavior of the material

(elastic and plastic zones in the case of metals) and allows to obtain the following characteristic

parameters.

Young’s modulus (E)

Poisson’s ratio (ν)

Yield stress (σy)

Ultimate tensile strength

Ductility

Strain-hardening characteristics

Results from these tests are commonly used to select a material for an application, for quality

control, and to predict how a material will react under arbitrary loading conditions.

Tensile tests are carried out using devices which are capable to apply loads in the specimens until

the failure is produced. A scheme of the device test is represented in Figure 1.

Figure 1. Scheme of the test device

Load cell

Grips to fix the specimen

Specimen Extensometer

Rigid frame

Moving frame




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This device is formed by two crossheads; one is adjusted for the length of the specimen and the

other is driven to apply tension. During the testing process the machine measures the following

variables:

Force applied

Relative displacement between the two crossheads of the device

Relative displacement between two points of the specimen (through the use of an

extensometer).

3 STRESS-STRAIN

From the results of forces and displacements, it can be represented the engineering stress-strain

law. The engineering stress (s) is defined as the force applied in each moment (F) divided by the

initial area of the cross-section of the specimen (A0):

On the other hand, the engineering strain (e) is defined as the increase of the length of the

specimen divided by the initial length:

Moreover, it can be represented the true stress-strain law. Consequently, it is necessary to

comprehend the differences between the engineering values, which are divided by constants, and

the true values. In the case of the true stress (σ), the force applied is divided by the area of the cross-

section in each moment (A):

The true strain (ε) is defined as the natural logarithm of the length of the specimen in each

moment divided by the initial length.

Therefore, both values can be represented obtaining the stress-strain law of the material (Figure

2). Having this relationship, it is possible to get the yield stress (point P), the ultimate tensile strength

(point M) and the failure stress (point R).




2015/2016

Figure 2. Stress-strain law of a material

The curve of the stress-strain law can be divided in two regions:

- Elastic behavior: At low strains (before P), where the Hooke’s law is applicable (σ = E ε). The

strain produced in this region is recovered when the load stops.

- Plastic behavior: This zone is reached when the tension is higher than the yield stress (after P). In

this part the linear behavior disappears.

Having reached the plastic zone, the stress needed to continue deforming metals increases until

the ultimate tensile strength (σM) (point M), and then the stress decreases until the failure is

produced (point R). So the ultimate tensile strength is the maximum stress reached in the whole

stress-strain diagram. Until this point, the strain produced in the specimen is uniform. However, after

the ultimate tensile strength has been reached, a local decrease of the cross-section is produced in a

part of the specimen. This phenomenon is known as necking and the sequent deformation takes

place in this point. Due to this necking, the failure is produced in this section. In the Figure 3 the

phenomenon of necking is represented.

Figure 3. Necking formed in the specimen in tensile testing

σ

M

ε

P

R Yield stress

Failure strain

Failu

re s

tres

s

Ult

imat

e te

nsi

le s

tren

gth

σ M

ε

P R




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4 EQUIPMENT DESCRIPTION

4.1 Generalities

In this practice a hydraulic universal testing device is going to be used. The maximum load

capacity of the machine is 250 kN. In Figure 4, the testing machine is shown. There it is possible to

appreciate the tensile gags between which the specimens are positioned.

Figure 4. Universal testing device

Basically, testing machines are composed of:

A cast iron bench.

Four lateral columns which guide the mobile crosshead.

A mobile crosshead over the columns.

A load cell with the nominal capacity of the testing frame. In the 250 kN machine, the load

cell is situated at the upper side.

Four lateral hydraulic cylinders which situate the upper head at the wished height.

A superior grip.

An inferior grip.




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In this machine, the upper grip is fixed to the crosshead, while the inferior grip can move driven

by the hydraulic system. The maximum displacement available between grips is ± 100 mm. The grips

tighten force can be set, with the purpose of not damage the testing specimens.

The mobile crosshead is guided in its movement by two columns that prevent the rotation. The

grips movement, and also the displacement of the upper crosshead are operated by the control

position center, which is situated at the intermediate height and in the frontal testing frame.

4.2 Types of control

The testing control is realized by a computer with the appropriated software. Three different test

modes are available

Force driven test: The force as a function of time is prescribed, which produces a

variation of the specimen’s length (or the extensometer measurement). There is a risk

when using this control method: when the specimen crashes, the machine will continue

applying the force, so the displacement velocity will be very high.

Displacement driven test: In this case, the displacement velocity of the mobile crosshead

is prescribed, so the applied force is consequence of the displacement. Thus, when the

specimen crashes, the machine will continue with its constant velocity.

Elongation measurement base: The computer allows the user to control the test by an

extensometer, so it is possible to impose the wanted elongation per time unit.

The available machines also present the possibility to impose cyclical variations. These loading

patterns are used to test material fatigue properties.

5 SPECIMENS DESCRIPTION

To perform the tensile test, normalized specimens need to be used. There are two types of

normalized specimens for tensile testing: plane and cylindrical. In this practice it will be used a plane

specimen made with Aluminum Alloy 2024-T4, as the ASTM E8 regulation establishes. In Figure 5 the

specimen’s dimensions are presented.

Figure 5. Testing specimen dimensions.




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The characteristic shape of the specimen is called ‘dumbbell’ or ‘dogbone’. The goal of this shape

is to provide a correct friction in the grips due to the wither ends of the specimen. For the test to be

successful the damage has to be located in the central section; therefore, the central section is

smaller.

6 PRACTICE EXCERCISE

In the test, the crosshead displacement is registered, as well as the applied force. Students must

determine the next parameters, according to the test values presented:

Young’s modulus (E).

Yield stress (σy).

Ultimate tensile strength (σM).

Calculate the stress-strain values (engineering and true) at each single instant registered at the

table. Then, use it in the Excel file in Aula Global to represent the stress-strain law, for engineering

and true values.

P3 Lab Guide Estructuras

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