3M03 Lab 3 Measuring the Toughness of Medium Carbon Steel

pdf

School

McMaster University *

*We aren’t endorsed by this school

Course

3M03

Subject

Mechanical Engineering

Date

Apr 3, 2024

Type

pdf

Pages

Uploaded by SargentWater29347

Materials 3M03 Lab 3: Measuring the Toughness of Medium Carbon Steel Name: Hima Patel Student Number: 400373091 Date Performed: March 6 th , 2023 Submission Date: March 11 th , 2023

Executive Summary Toughness is a measure of material’s ability to withstand plastic deformation, cracking and fraction when a load is applied to it, which can also be translated to amount of energy said material can absorb before failure [1]. In this demonstration, we aim to understand how fracture toughness can change as the microstructure of a material changes after undergoing differing heat treatments. This was done so by performing a 3-Point-Bend-Test on 3 samples of steel of differing heat treatments (tempered, quenched, as received) [2]. From the data collected from these samples, Applied Force vs Crack Displacement plots were created, and the 95% Secant Method was used to determine data needed to calculate critical crack size and Stress Intensity Factor, K . For the tempered, quenched, and as received samples, the critical crack sizes were determined to be, respectively, 1.48E-06, 8.93E-07, and 1.31E-06 (mm). Additionally, the stress intensity factors were determined to be 2.94, 2.29, and 1.54 MPa √࠵? , respectively. From these results, it can be concluded that a quenched steel is harder and more brittle than a tempered steel. This is because fracture toughness is defined as a materials ability to resist crack propagation, and since the quenched sample had a smaller crack size before failure and therefore produced a smaller fracture toughness, it is more likely to fail when a crack is propagated through it [3]. Background The purpose of this lab was to determine the fracture toughness, also known as the stress intensity fracture, K IC , of 3 different heat-treated samples of steel to determine the effect microstructure has on fracture toughness. Fracture toughness is a materials ability to withstand growth of a pre-existing crack without failing, meaning that if a material cannot handle this crack growing without failing, it is likely a very brittle material [3]. In this lab, the 3 different samples consist of a quenched steel, a tempered steel and a steel sample as received. Tempered steel is a heat treatment done to reduce brittleness and increase toughness of the material [4]. This is done so by heating the sample below its melting point and holding it at a specific temperature allowing it to undergo changes in its microstructure [4]. Tempering steel precipitated carbides into the steel which block dislocations from occurring, therefore helping relieve internal stress and making the material more workable [4]. Therefore, it is likely that the tempered steel will have a higher fracture value because of its ductile properties. Conversely, quenched steel is a heat treatment done to strengthen and harden the material [5]. It is done so by heating the steel to an immensely high temperature then rapidly cooling it, therefore ‘quenching it’ [5]. Quenching also changes the microstructure because the rapid cooling of steel creates the saturated solid solution of martensite, which is very hard and brittle [5]. Therefore, as per the definition of fracture toughness, it is likely that a quenched steel will have a low fracture toughness value because it is more likely to fail as a crack propagates through it. To determine the fracture toughness of these materials, a fracture toughness test called the 3-Point- Bend test will be performed. This test bends pre-cracked samples to failure to determine the maximum crack it can withstand before failure [2]. A force is applied to each specimen increasing in magnitude until failure, collecting data on the applied load and the strain experienced by the sample. Afterwards, using the original gauge length of the sample, the crack displacement for each applied load value can be calculated. In the figure below, the 3-point-bend test is displayed, showing how the force is applied to the pre-cracked sample, and to measure the strain an extensometer was attached to each sample.

Figure 1: Diagram of 3-point-bend test. From this test, using the below equations, it allows us to determine fracture toughness, K IC , and the crack shape coefficient Y(b) [2]. (1) ࠵?(࠵?) = 1.99 − ࠵?(1 − ࠵?)[2.15 − 3.39࠵? + 2.7࠵? ! ] (1 + 2࠵?)(1 − ࠵?) "/! (2) ࠵? $% = 3࠵? & ࠵? 2࠵?࠵? ! √࠵? ∙ ࠵?(࠵?) From having determined these values, it is then possible to use the following equation to determine the maximum crack length in a specimen before failure [2]. (3) ࠵? $% = ࠵? ’ √࠵?࠵? → ࠵? = > ࠵? $% ࠵? ’ ? ! ࠵? Data Analysis and Results Below are the plots of Applied Force vs Crack Displacement for each sample (blue), along with the linear region in orange, its trendline in red, and the secondary line plotted using the 95% Secant method in purple. Figure 2: Applied Force vs Crack Displacement Plot for Tempered Sample y = 75.412x - 0.8431 0 5 10 15 20 25 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Applied Load (kN) Crack Displacement (mm) Force vs Crack Displacement for Tempered Sample Initial Data Linear Region 95% Secant Line Linear (Linear Region)

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

Figure 3: Applied Force vs Crack Displacement Plot for Quenched Sample Figure 4: Applied Force vs Crack Displacement Plot for As Received Sample To determine the fracture toughness for each sample, it was necessary to obtain P Q, seen in equation 2, where P Q is defined as the critical force; to do so, the 95% secant method was used. This method involves using the applied force vs displacement curve, and highlighting the linear region, as seen in orange. Then, a trendline of this region specifically is created and the equation of that line is also generated, both shown on the graph, with the trendline of the linear region in red. To create the 95% secant line, a new equation is generated from the linear region equation, where both the slope and y-intercept values are 95% of the linear region values. Then, this new equation is plotted on the same graph, and the point of intersection of the secant line and the initial data in blue is marked. The corresponding y coordinate of the point of intersection is named the critical force of the sample. y = 69.383x - 2.431 -5 0 5 10 15 20 25 30 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Applied Force (kN) Crack Displacement (mm) Force vs Crack Displacement for Quenched Sample Initial Data Linear Region 95% Secant Line Linear (Linear Region) y = 75.872x - 9.9876 -5 0 5 10 15 20 25 0 0.1 0.2 0.3 0.4 0.5 0.6 Applied Force (kN) Crack Displacement (mm) Force vs Crack Displacement for As Recieved Initial Data Linear Region 95% Secant Line Linear (Linear Region)

An important note that to use this method, for the trial to be regarded as valid, you must divide the critical force by the max force and obtain a ratio of 1.1 or below. For the data given, only the tempered sample obtained a valid ratio of 1.09. This occurs due to error in the data collection. After identifying this term for each sample, the crack shape factor Y(b), can be calculated by using equation 1, where b crack size divided by specimen width, and then all terms required to calculate the fracture toughness have been calculated. A sample calculation of the fracture toughness value can be seen below. Sample Calculation 1: Fracture Toughness of Tempered Steel Afterwards, the hardness of each sample was also measured using the Rockwell C-scale. Several trials of this test were conducted to calculate a mean value so that the determined tensile strength s f , was as accurate as possible. This tensile strength is determined by converting the mean average of the hardness test values to the strength using the chart provided in lab [6]. The data and results of the hardness test are summarized in the table below. Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Trial 6 Mean Tensile Strength ( s f ) (MPa) As Received 18 10.9 13.8 11.9 11.2 11.2 13 675.86 Quenched 36.5 38.5 42 41 41.9 35 39 1220.69 Tempered 39.5 40 37 36 40.5 40.9 39 1220.69 Table 1: Summary of Hardness Tests and Results. Now having found the tensile strength of each sample, the crack length before failure of each sample can be calculated using equation 3 as shown below. Sample Calculation 2: Critical Crack Distance Before Failure of Tempered Steel Now having acquired the critical shape factor, fracture toughness and tensile strength of each sample, conclusions of how the microstructure of each treated sample affect the fracture toughness can be made. See below for a summary table of all aforementioned values for each sample of steel. Tempered Steel Quenched Steel As Received Steel Fracture Toughness (MPa √࠵? ) 2.94 2.29 1.54 Tensile Strength (MPa) 1220.69 1220.69 675.86 Critical Crack Size (mm) 1.48E-06 8.93E-07 1.31E-06 Table 2: Summary of Results for Steel Samples.

Conclusions This experiment demonstrates the effectiveness of the 3-point bending test for measuring fracture toughness of a material, particularly in the presence of pre-existing values such as cracks in a material. Using the 95% secant method, the critical crack distance and fracture toughness for 3 samples of steel were determined. For the tempered, quenched and as received samples, the fracture toughness values are 2.94, 2.29, and 1.54 MPa √࠵? , respectively and the critical crack distance values are 1.48E-06, 8.93E-07, and 1.31E- 06 (mm). Based on these results, it is shown that quenched steel exhibits higher hardness and brittleness than tempered steel, which coincides with the way each individual treatment affects steels microstructure. Where the as received sample acts as a solid foundation as to why heat treatment exists, as it had the lowest fracture toughness and is therefore the most likely to fail during crack propagation. This experiment highlights the importance of differing heat treatments and shows how their effect on a materials microstructure has extensive effects.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

References [1] Iowa State University, “Nondestructive Evaluation Physics: Materials: Toughness,” www.nde-ed.org , 2019. https://www.nde-ed.org/Physics/Materials/Mechanical/Toughness.xhtml (accessed Mar. 11, 2023). [2] McMaster University. (2023). Materials 3M03, Demonstration 3:Measuring the Toughness of a Medium Carbon Steel. [PDF] Available: https://avenue.cllmcmaster.ca/d2l/le/content/516919/viewContent/3998138/View [3] Iowa State University, “Nondestructive Evaluation Physics: Materials: Fracture Toughness,” www.nde- ed.org , 2019. https://www.nde-ed.org/Physics/Materials/Mechanical/Toughness.xhtml (accessed Mar. 11, 2023). [4] Monroe Engineering, “What Is Tempered Steel? - Monroe Engineering,” Monroe Engineering , Oct. 28, 2019. Accessed: Mar. 11, 2023. [Online]. Available: https://monroeengineering.com/blog/what-is- tempered-steel/ [5] C. FitzGibbon, “What Is Quenching?,” Metal Supermarkets - Steel, Aluminum, Stainless, Hot-Rolled, Cold- Rolled, Alloy, Carbon, Galvanized, Brass, Bronze, Copper , May 09, 2019. https://www.metalsupermarkets.com/what-is-quenching/ (accessed Mar. 11, 2023). [6] McMaster University. (2023). Materials 3M03, Hardness Conversion Chart - provides values for Rockwell C and other scales . [PDF] Available: https://avenue.cllmcmaster.ca/d2l/le/content/516919/viewContent/3998150/View