Impact Toughness of Metallic Materials

Title 

Impact Toughness of Metallic Materials.

Objectives

·         Study the principles of brittle fracture in mild steels.

·         Understand the impact toughness of materials with different heat and strengthening treatments.

·         Interpret obtained experimental data for the selection of engineering materials

Introduction

            Impact toughness test of metallic materials is mainly used to measure the energy that spent to break or crack metals. This test can be determined with a Charpy or Izod test. These tests are named early 1900’s before the fracture mechanism was invented. Impact tests are used as a quality control method in industry.

The two tests (Charpy and Izod) use different specimens and methods of holding the

specimens, but both tests are use of a pendulum-testing machine. In both tests, the specimen

is broken by a single overload event because of the impact of the pendulum. A stop pointer is used

to record how far the pendulum swings back up after fracturing the specimen. The impact

toughness of a metal is founded by measuring the energy absorbed in the fracture of the

specimen. This is simply obtained by noting the height at which the pendulum is released and

the height to which the pendulum swings after it has struck the specimen.

                    

Toughness is greatly affected by

- Alloying compositions

- Temperature (ductile to brittle transition- DBT): e.g. Titanic ship wreck

- Heat treatments

- Strengthening mechanisms

Specimen Preparation

First, we had to make a notch to avoid breaking from multiple places. There are few kinds of notches such as triangle notch and square notch. We used triangles notched specimens in different temperatures.

                        

Theory

Impact test is used to studying the toughness of materials. Toughness definitions define as the capacity of material to absorb energy and deform plastically before fracturing or braking. Toughness is collaborator with ductility and the strength of material. Because of these reasons pendulum is raise to a certain height and release, so it can supply the necessary energy for the fracture. The specimen is placed in the lowest point on the setup where the potential energy is lowest. The absorbed energy is equal to the different between the potential energy at the beginning and the end.

Materials/Apparatus  

Mild steel                                           

V notch maker

Impact testing machine

An oven

A refrigerator

Tong

                                                                                       

Procedure

            First the specimen was choosen by the instructor and the V notch was made by one of our collegues. V notch is to make sure that it will break from one point. Before using the machine it was checked whether it is working properly by letting it move freely.  After the pointer was set to the ‘0’ point, the sepcimen was set in the Impact testing machine by the instructor with the help of centering device. Then, the hammer was released. Suddenly the steel piece broken into 2 pieces and the energy which is absorbed by the steel(Toughness)  is measured using the meter.

                    

Like that, the same test was done to three steel specimens in three different temperatures. Those temperatures were  26, -36 and 153(celcius).Tong was used to hold them and they were already notched by the instructors before we came.

         

             -36 Celcius                     153 Celcius                    26 Celcius

Finally we filled the chart and gathered all the information.

Observation

Sample No.	Sample Details	Notch Type	Temperature (Celcius)	Broken/ Unbroken	Toughness reading	Percent Ductility
1	Mild Steel	V	26	Broken	104	5-10
2	Mild Steel	V	-36	Broken	10	0-5
3	Mild Steel	V	153	Broken	107	40-45

Results

Toughness do depends on the temperature.

·         When temperature increases, the toughness also increasing.

·         When temperature decreases, the toughness also decreasing.

Discussion

Difference between fracture types of Metal

There are two main types of fractures in metals.

1.      Ductile Fracture

·        Ductile fracture involves a large amount of plastic deformation and can be detected beforehand.

·         In ductile fracture, there is always a warning before the fracture.

·        Also it fractures in a one piece and in large deformation.    

·        This happens in high temperatures.

·        It has a low strain rate.

2.      Brittle Fracture

·         Brittle fracture is more catastrophic and has been intensively studied.

·         It won’t give any warnings before the fracture.

·         It happens into may pieces in small deformation.

·         Brittle fracture happens in low temperatures.

·         It has a hign strain rate.

Important of The Impact Test

·         Impact Test determine the amount of energy that takes for a metallic material to brake.

·         Impact Test is using as a tool to study thoughtless, temperature dependent, ductile and brittleness of metallic materials.

·         From study how, metallic materials react for temperature changes engineers can choose the most suitable metal for the job.

·         Studying the reaction for temperature can also minimize the disastrous accidents like Titanic ship incident

·         From identifying the ductile and brittle properties in materials, in the field it’ll show what shatter fast and what kind of materials are not shatter that faster.

   

Importance Knowing the Properties of Materials

·         There are lots of properties in materials like, physical properties, chemical properties, mechanical properties and manufactural properties.

·         Impact test experiment helps to understand about the mechanical properties of metallic materials.

·         Mechanical properties in metallic materials are the things that determine the strength, toughness, ductility and brittleness of that material.

·         According to the properties that describe above, when choosing materials in the field materials are depend, so knowing the properties of materials is important.

·         Also, from impact test experimenter can get an idea, how the metallic materials are behave in different temperature.

What Do The Results Shows

·         Specimen that tested on 0oC temperature broke completely and also it shows high brittle properties.

·         Specimen that tested on 149oC temperature broke partially and also shows high ductile properties.

·         Specimen that tested on room temperature broke completely and this specimen shows both ductile and brittle properties but when compare with the other two it’s in the middle area its shows both in an average level.

·         In this experiment we were instructing to use -30oC specimen not a 0oC specimen, however we ran in to a problem with the refrigerator and we had to use the 0oC temperature specimen.

·         In -30 specimens, percent ductility is around 0% - 5%

·         With all the above observation we conclude that higher the temperature higher the ductile properties, lower the temperature higher the brittle properties.

Errors That Encountered and the Solution

·         In the experiment experimenter cannot control the fracture, this can effects the energy absorbents, so to control this all the specimens have pre prepare by cutting an “V” notch so the energy that absorb can point in to that place.

·         There is an energy loss during the test that cannot encounter because it’s a very slight lost so we ignore that during the experiment

How Could The Experiment Design Be Improved To Reduce Error

·         In this experiment what I encountered as an error was the slight energy loss during the energy conversion and for the sound that make.

·         According to my knowledge those errors cannot be stop only can reduce.

·         In the experiment we use a pendulum that rotate around an axis when the pendulum rotate it spend energy on the rotation by adding grease to the rotate point we can minimize the energy loss during the rotation against friction.

Conclusion

In the laboratory we learn about impact test and how its effects the mild steel. By using those results we discuss how and why those results are appeared and how those results will help during the field work. Also by doing the experiment on different temperature we learned that in different temperatures metals act differently.

With those observation we conclude that when using metals we have to choose according to the climate and calculate what would happen when we use metal on the constructions.

References

·         Anon., 2018. WMTR. [Online] Available at: http://www.wmtr.com/en.impact-testing.html [Accessed 05 08 2018].

·         Anon., n.d. The Frabricator. [Online] Available at: https://www.thefabricator.com/article/consumables/the-impact-of-impact-toughness [Accessed 12

08 2018].

·         Martin C Smith, A. P., 2016. Quora. [Online] Available at: https://www.quora.com/What-is-the-importance-of-impact-test-How-is-it-made [Accessed 05 08 2018].

·         Shalmalee Vaidya, n.d. Instron. [Online] Available at: http://www.instron.us/en-us/our-company/press-room/blog/2014/june/does-a-charpy-or-an-izod-impact-test-replicate-any-real-life-scenarios [Accessed 10 8 2018].

·         Thomas A. Siewert, M. P. M. C. N. M. J. M. H. F. J. M., May 01, 2002. The History and Importance of Impact Testing. American Society for Testing and Materials, Issue May 01, 2002.