Tensile Test Results and Acceptance Criteria
A tensile test is performed on a specimen machined from a steel plate to verify mechanical properties. The specimen has a original gauge length of 50 mm and a cross-sectional area of 200 mm². The test yields a maximum load of 80 kN and a yield load of 60 kN (determined by the 0.2% offset method). After fracture, the gauge length is measured as 60 mm. The specification requires a minimum yield strength of 300 MPa, a minimum tensile strength of 400 MPa, and a minimum elongation of 20%. The test was conducted at room temperature using a calibrated universal testing machine. The strain rate was within the specified limits. The specimen fractured outside the gauge length, but the fracture was in the reduced section.
1. Calculate the yield strength, tensile strength, and percent elongation. Determine if the material meets the acceptance criteria.
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Yield strength = yield load / area = 60,000 N / 200 mm² = 300 MPa. Tensile strength = max load / area = 80,000 N / 200 mm² = 400 MPa. Elongation = (final gauge length - original gauge length) / original gauge length × 100 = (60 - 50)/50 × 100 = 20%. The material meets the minimum yield strength (300 MPa) and tensile strength (400 MPa) exactly, and elongation is 20%, which meets the minimum. However, if the specification requires 'minimum' and the values are exactly at the limit, they are acceptable. But note: the yield strength is exactly 300 MPa, which is acceptable. The tensile strength is exactly 400 MPa, acceptable. Elongation is exactly 20%, acceptable. Therefore, the material passes.
2. If the specimen fractured outside the gauge length but within the reduced section, does that affect the validity of the elongation measurement? Explain.
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The elongation measurement is based on the change in the gauge length. If the fracture occurs outside the gauge length but within the reduced section, the deformation within the gauge length may not be representative of the total elongation because the fracture location can affect the distribution of strain. However, as long as the fracture is within the reduced section and the gauge marks are intact, the elongation measured over the gauge length is still valid according to most standards (e.g., ASTM E8). The key is that the fracture must be at least a certain distance from the gauge marks to ensure that the gauge length experienced uniform strain. If the fracture is too close to a gauge mark, the test may be invalid. In this case, the fracture is outside the gauge length, so it is likely far enough, but the inspector should verify that the distance from the fracture to the nearest gauge mark is at least the specified minimum (often 1/3 of the gauge length or per standard). If not, the test should be repeated. Assuming it is, the elongation of 20% is valid.