Engineering Lab Report Example: Tensile Testing of Mild Steel Specimens

Aim and Objectives

The aim of this experiment was to determine the principal mechanical properties of a mild steel specimen subjected to uniaxial tension and to characterise its elastic and plastic behaviour up to fracture. Mild (low-carbon) steel is one of the most widely used structural metals, and an understanding of how it responds to tensile loading is fundamental to safe and economical mechanical design.

The specific objectives were to:

• measure the relationship between the applied axial load and the resulting extension of the gauge length;
• derive the engineering stress–strain response of the material from the raw data;
• determine Young’s modulus (E), the yield strength (σ_y), the ultimate tensile strength (UTS), the percentage elongation and the percentage reduction in area;
• compare the measured values with published data for low-carbon steel and account for any discrepancies observed.

Introduction and Theoretical Background

The Tensile Test and Engineering Stress–Strain

The tensile test is the most common method of establishing the mechanical properties of metallic materials. A standardised specimen is gripped at both ends and extended at a controlled rate while the instantaneous load and extension are recorded continuously. Because the measured force and displacement depend on specimen size, the raw data are normalised into stress and strain so that the behaviour of the material, rather than the particular specimen, is described (Callister and Rethwisch, 2018).

Engineering stress is defined as the applied load divided by the original cross-sectional area of the specimen, σ = F/A₀, and is expressed in megapascals (MPa). Engineering strain is the change in gauge length divided by the original gauge length, ε = ΔL/L₀, and is dimensionless. Engineering values are referred to the undeformed geometry; true stress and true strain, which use the instantaneous dimensions, diverge from the engineering values once significant plastic deformation and necking occur (Davis, 2004).

Elastic and Plastic Deformation

At low loads a metal deforms elastically: the strain is recoverable and is proportional to stress according to Hooke’s law, σ = Eε. The constant of proportionality, Young’s modulus, is the gradient of the initial linear portion of the stress–strain curve and reflects the stiffness of the atomic bonding (Ashby and Jones, 2012). Beyond the elastic limit the material yields and deforms plastically, meaning the deformation is permanent. Mild steel is notable for exhibiting a distinct yield point, often with clearly defined upper and lower yield stresses, after which deformation proceeds at approximately constant load through the propagation of Lüders bands.

Following yield, the material strain-hardens and the load rises to a maximum. The engineering stress corresponding to this maximum load is the ultimate tensile strength. At this point deformation localises into a “neck”, the local cross-sectional area reduces rapidly, and although the true stress continues to rise the engineering stress falls until fracture occurs (Gere and Goodno, 2012). The total plastic strain at fracture (percentage elongation) and the reduction in cross-sectional area are standard measures of ductility.

Materials and Apparatus

Specimen

The test piece was a machined round bar of mild steel (approximately equivalent to a 0.2% carbon steel) prepared as a proportional specimen in accordance with BS EN ISO 6892-1 (British Standards Institution, 2019). The relevant initial dimensions were an original diameter d₀ = 10.00 mm and an original gauge length L₀ = 50.0 mm. The original cross-sectional area was therefore:

A₀ = πd₀²/4 = π(10.00)²/4 = 78.54 mm²

Apparatus

• A computer-controlled universal testing machine of 100 kN capacity, operated in displacement control.
• A clip-on extensometer of 50 mm gauge length to record extension across the gauge section.
• A micrometer and vernier callipers for measuring the specimen diameter and gauge length before and after testing.
• A load cell and data-acquisition system logging load against extension throughout the test.

Method

Test Procedure

1. The original diameter of the specimen was measured at three positions along the gauge length using a micrometer, and the mean value recorded. The gauge length was marked and measured with vernier callipers.
2. The specimen was mounted securely in the hydraulic grips of the testing machine, ensuring that it was aligned with the loading axis to avoid introducing bending stresses.
3. The extensometer was attached across the marked gauge length and the load and extension channels were zeroed.
4. The specimen was loaded at a constant crosshead displacement rate consistent with the standard, and the load–extension data were logged automatically.
5. The extensometer was removed shortly before the anticipated fracture to protect the instrument, after which loading continued to failure.
6. After fracture, the two halves were fitted together and the final gauge length (L_f) and the diameter at the neck (d_f) were measured so that ductility could be calculated.

The test was conducted at room temperature. Safety considerations included wearing eye protection and ensuring that the guarding was in place during loading, as fracture of a metal specimen releases stored elastic energy suddenly.

Results and Calculations

Measured Data

A representative selection of the logged load–extension readings is presented in Table 1, together with the corresponding engineering stress and strain calculated from the relationships given above. The values are illustrative of the behaviour observed during the test.

Table 1 — Selected load–extension readings and derived engineering stress and strain.

Plotting engineering stress against engineering strain produces the characteristic curve for mild steel: an initial straight elastic line, a sharp drop from an upper to a lower yield point, an extended region of near-constant stress, a strain-hardening rise to a peak, and finally a falling branch to fracture.

Derived Mechanical Properties

Young’s modulus was obtained from the gradient of the linear elastic region. Using the reading at 20.0 kN:

E = σ/ε = 254.6 / 0.00124 ≈ 205,000 MPa = 205 GPa

The yield strength was taken as the stress at the upper yield point:

σ_y = F/A₀ = (22.0 × 10³) / 78.54 ≈ 280 MPa

The ultimate tensile strength corresponded to the maximum load of 35.3 kN:

UTS = (35.3 × 10³) / 78.54 ≈ 450 MPa

Ductility was assessed from the post-fracture measurements, with a final gauge length of 64.0 mm and a neck diameter of 6.50 mm (final area A_f = 33.18 mm²):

Elongation = (L_f − L₀)/L₀ × 100 = (64.0 − 50.0)/50.0 × 100 = 28%

Reduction in area = (A₀ − A_f)/A₀ × 100 = (78.54 − 33.18)/78.54 × 100 = 57.8%

Table 2 — Summary of measured mechanical properties.

Discussion

Interpreting the Stress–Strain Curve

The results display the behaviour expected of a ductile, low-carbon steel. The initial portion of the response was linear, confirming elastic deformation governed by Hooke’s law, and the slope yielded a modulus of 205 GPa. A pronounced upper yield point at approximately 280 MPa was followed by a small load drop to a lower yield plateau, over which the specimen extended appreciably at almost constant stress. This plateau corresponds to the nucleation and propagation of Lüders bands and is a defining feature of mild steel that many non-ferrous alloys do not share (Smith and Hashemi, 2019).

After the yield plateau the material strain-hardened, the stress rising steadily to the ultimate tensile strength of 450 MPa at the maximum load. Beyond this peak the engineering stress declined even though the material was still hardening locally, because deformation had concentrated into a neck where the true cross-sectional area was falling faster than the material could strengthen. Fracture ultimately occurred within the necked region, and visual inspection of a specimen of this type would be expected to reveal the characteristic “cup-and-cone” surface associated with ductile failure (Hibbeler, 2017).

Comparison with Published Values

The measured properties agree well with published data for mild steel. Young’s modulus for steels is typically quoted at around 200–210 GPa, and the value of 205 GPa obtained here falls squarely within that range (Callister and Rethwisch, 2018). Reported yield strengths for low-carbon steel commonly lie between about 220 and 300 MPa, and tensile strengths between roughly 400 and 550 MPa, so both the 280 MPa yield strength and the 450 MPa UTS are consistent with expectation. The high ductility, reflected in a 28% elongation and a 57.8% reduction in area, further confirms that the material behaved as a typical ductile structural steel rather than a brittle one.

Sources of Error and Uncertainty

Several factors could affect the accuracy of the results. The modulus is particularly sensitive to small errors because it is derived from the steep elastic region, where a slight misreading of extension produces a large change in the calculated gradient; use of an extensometer rather than crosshead displacement reduces, but does not eliminate, this effect by removing machine compliance. Any slight misalignment of the specimen in the grips would introduce a bending component and could cause premature or uneven yielding. Measurement of the small elastic extensions and of the neck diameter after fracture also carries appreciable uncertainty, since the fractured surfaces are irregular and difficult to measure precisely. Finally, because engineering stress is based on the original area, the recorded strength beyond the UTS understates the true stress the material was carrying.

These errors could be reduced by repeating the test on several specimens and reporting mean values with their scatter, by verifying grip alignment before loading, and by calibrating the load cell and extensometer prior to testing.

Conclusion

A uniaxial tensile test was successfully carried out on a mild steel specimen and the engineering stress–strain response was characterised from the recorded load–extension data. The specimen exhibited the classic behaviour of a ductile low-carbon steel, with a well-defined upper and lower yield point, a strain-hardening region, necking and ductile fracture. The principal mechanical properties were determined as a Young’s modulus of 205 GPa, a yield strength of 280 MPa, an ultimate tensile strength of 450 MPa, a percentage elongation of 28% and a reduction in area of 57.8%. These values are in close agreement with published data for mild steel, confirming both the identity of the material and the validity of the experimental method. The main limitations related to the sensitivity of the modulus calculation, possible specimen misalignment and the difficulty of post-fracture measurement; testing additional specimens would improve confidence in the results.

References

• Ashby, M.F. and Jones, D.R.H. (2012) Engineering Materials 1: An Introduction to Properties, Applications and Design. 4th edn. Oxford: Butterworth-Heinemann.
• Askeland, D.R. and Wright, W.J. (2015) The Science and Engineering of Materials. 7th edn. Boston: Cengage Learning.
• British Standards Institution (2019) BS EN ISO 6892-1:2019 Metallic Materials — Tensile Testing. Part 1: Method of Test at Room Temperature. London: BSI.
• Callister, W.D. and Rethwisch, D.G. (2018) Materials Science and Engineering: An Introduction. 10th edn. Hoboken: Wiley.
• Davis, J.R. (2004) Tensile Testing. 2nd edn. Materials Park, OH: ASM International.
• Gere, J.M. and Goodno, B.J. (2012) Mechanics of Materials. 8th edn. Stamford: Cengage Learning.
• Hibbeler, R.C. (2017) Mechanics of Materials. 10th edn. Harlow: Pearson.
• Smith, W.F. and Hashemi, J. (2019) Foundations of Materials Science and Engineering. 6th edn. New York: McGraw-Hill.