MME514 — Engineering Materials Laboratory: Lecture Note

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Course code: MME 514 Course title: Engineering Materials Laboratory Course credit load: 1 Unit Series 1: Materials Testing and Evaluation: Laboratory Procedures and Techniques for Engineering Materials • Part 1.1: Purpose, Classification and Planning of Materials Tests o Sub Part 1.1.1: Purpose and Classification of Materials Tests o Sub Part 1.1.2: Standards and Specifications o Sub Part 1.1.3: Planning a Laboratory Test and Reporting • Part 1.2: Safe Laboratory Practice, Sampling and Measuring Instruments o Sub Part 1.2.1: Laboratory Safety and Good Practice o Sub Part 1.2.2: Sampling and Specimen Identification o Sub Part 1.2.3: Measuring Instruments and Calibration • Part 1.3: Data, Errors and Reporting o Sub Part 1.3.1: Accuracy, Precision and Errors o Sub Part 1.3.2: Statistical Treatment of Test Data o Sub Part 1.3.3: Uncertainty, Graphs and Reports Introduction A steel bar arrives at a workshop with a certificate that says it is strong enough. Who checked, how, and how sure can we be? Every number in an engineering handbook was once a reading on a laboratory instrument, taken by someone who followed a procedure. This first Series lays the foundation for the whole course: why materials are tested, how tests are organised, how to work safely, and how to treat measurements honestly.

The aim of this Series is to equip you to explain why and how engineering materials are tested, to work safely and carefully in the laboratory, and to treat measurements with proper attention to error and uncertainty. We shall commence with the purpose, classification and planning of materials tests. Next, we will explore safe laboratory practice, sampling and measuring instruments. We will conclude with the treatment of errors, statistics and uncertainty in test data, and the writing of laboratory reports. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1: explain the purposes and classification of materials tests, • SLO 1.2: describe the role of standards and the planning and reporting of laboratory tests, • SLO 1.3: apply safe laboratory practice and risk assessment, • SLO 1.4: describe sampling and the correct use of measuring instruments, • SLO 1.5: distinguish between accuracy, precision and the types of error, and • SLO 1.6: apply statistical methods and uncertainty calculations to test data. 1.1 Purpose, Classification and Planning of Materials Tests 1.1.1 Purpose and Classification of Materials Tests Materials testing is the measurement of the properties and behaviour of materials under controlled conditions, and materials evaluation is the use of those results to judge whether a material suits its purpose. Tests are carried out to control the quality of production, to compare and select materials, to check that a material meets a specification, to support research and to find the causes of failures. Tests are classified in several ways. By the property measured, they are mechanical, physical, thermal, electrical, chemical or structural. By their effect on the specimen, they are destructive, such as tensile and impact tests, in which the specimen is broken, or non-destructive, such as ultrasonic and dye penetrant tests, in which it is not. By duration, they are short-term, such as hardness, or long-term, such as fatigue and creep.

Worked example: A lot of 5000 bolts is checked by destructive tensile tests on 1 bolt in 50, so 100 bolts are tested and destroyed. At an illustrative 12 units per test, this costs 1200 units. A non-destructive check of every bolt at 0.3 units each costs 5000 × 0.3 = 1500 units, but it tests all the bolts and leaves them usable. Application: Choosing between destructive and non-destructive tests is a balance of cost, information and the value of the tested parts. Fill in the blank: A test in which the specimen is broken or damaged is called a __________ test. 1.1.2 Standards and Specifications A standard is a document that states how a test is done, so that results from different laboratories can be compared, and a specification states the properties required. Test standards are issued by bodies such as ASTM International, the International Organization for Standardization (ISO) and national bodies, such as the Standards Organisation of Nigeria. A standard gives the specimen, the equipment, the test speed and the way results are calculated. Using a standard removes many sources of difference between laboratories, such as specimen size and loading rate, which strongly affect results. Test standards are designated by a letter code and number, such as ASTM E8 for tensile testing of metals and ISO 6892-1 for tensile testing at room temperature. Standards are revised, so the current edition must be used and quoted in the report. Where a standard is not followed exactly, the differences must be stated. Worked example: ISO 6892-1 uses a proportional gauge length L0 = 5.65 S0^(1/2), where S0 is the original cross-sectional area. For a round bar of d = 10 mm, S0 = 78.5 mm², so L0 = 5.65 × 8.86 = 50.1 mm, which is 5 times the diameter. Application: Quoting the standard and its edition in a report lets another laboratory repeat the test under the same conditions. Fill in the blank: A document that states how a test is done so that results from different laboratories can be compared is called a __________. 1.1.3 Planning a Laboratory Test and Reporting A laboratory test should be planned before it starts. The plan states the objective, the property to be measured, the standard to be used, the specimens and their number, the apparatus and its calibration, the procedure, the safety precautions and the way the data will be recorded and analysed. Planning avoids wasted specimens and repeated tests, and it makes sure that the result answers the question asked. A laboratory report follows a set pattern: title, objective, theory, apparatus, procedure, results, analysis, discussion, conclusion and references. Results are shown in tables and graphs with units, and the discussion compares them with the expected values and explains differences.

Every measurement must be recorded at the time in a laboratory notebook, with the date, specimen number and conditions. Units must be consistent, since mixing units is a common cause of wrong answers. Worked example: A load of 25 kN is applied to a bar of cross-section 78.5 mm². The stress is 25 000 N/78.5 mm² = 318 N/mm², and since 1 N/mm² = 1 MPa, the stress is 318 MPa. If the load is entered as 25 N by mistake, the stress would be 1000 times too small, at 0.32 MPa. Application: A clear plan and record allow a test to be checked, repeated and compared long after it was done. Fill in the blank: Every measurement should be recorded at the time it is made in a laboratory __________. Figure 1.1: Workflow of a materials test from objective to decision. Pilot questions 1.1 1. Define materials testing. (Linked to SLO 1.1) 2. Give two reasons for testing materials. (Linked to SLO 1.1) 3. Distinguish between destructive and non-destructive tests. (Linked to SLO 1.1) 4. What is a test standard? (Linked to SLO 1.2) 5. Name the main sections of a laboratory report. (Linked to SLO 1.2)

1.2 Safe Laboratory Practice, Sampling and Measuring Instruments 1.2.1 Laboratory Safety and Good Practice Safety comes first in every laboratory. Protective equipment includes safety glasses, laboratory coats, gloves suited to the task and closed shoes, and heat-resistant gloves and tongs are needed near furnaces. Testing machines can trap fingers and break specimens violently, so guards and shields must be in place. Loose clothing, long hair and jewellery must be secured. Eating, drinking and unauthorised experiments are not allowed. Many hazards in a materials laboratory are known in advance: hot furnaces and quenching baths, high-speed cutting and grinding wheels, chemical etchants and solvents, electrical equipment and heavy specimens. A risk assessment identifies each hazard, estimates the likelihood and severity of harm and sets controls. Users must know the exits and the location of extinguishers and first-aid kits, and must report accidents and damaged equipment. Worked example: A risk score is likelihood × severity, each rated 1 to 5. Handling a hot specimen without tongs has a likelihood of 3 and a severity of 4, so the score is 12, which is medium to high. With tongs and heat-resistant gloves, the likelihood falls to 1, and the score is 4, which is low. Application: Recording the risk assessment before a practical session makes sure that controls are ready before work starts. Fill in the blank: The process of identifying hazards and estimating the likelihood and severity of harm is called a __________ assessment. 1.2.2 Sampling and Specimen Identification A test result describes only the specimen tested, so the specimen must represent the material. Sampling is the selection of specimens from a batch or product so that they are representative. Samples should be taken at random, or at positions and intervals set by the standard, and the number must be large enough for the result to be reliable. The location and orientation matter, since rolled, forged and cast products have different properties in different directions.

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