The Mary McAleese Boyne Valley Bridge on the M1 motorway uses steel cables in tension to suspend the bridge - Leaving Cert Engineering - Question 2 - 2018

Tensile testing involves the following steps:

1. Specimen Preparation: A tensile test specimen, which could be a rectangular or circular cross-section, is prepared and clamped into the testing machine.

2. Testing Machine: The specimen is placed in an extensometer, which applies force until the specimen stretches and eventually fractures.

3. Data Collection: As the test progresses, a graph of stress versus strain is generated, indicating how the material behaves under tension.

4. Fracture Detection: The fracture type (ductile vs brittle) can be analyzed based on the observation of the specimen post-testing, demonstrated through the respective characteristics such as necking or cup-and-cone fracture.

Graph A corresponds to copper. Graph B corresponds to stainless steel. Graph C corresponds to low carbon steel.

Graph A shows a relatively short elastic phase where the material returns to its original shape upon removal of the strain. It demonstrates high ductility and resilience before fracture. In contrast, Graph C displays a prolonged elastic region followed by a plastic deformation phase, indicating lower ductility and greater resistance to tensile forces.

Material A (associated with Graph A) exhibits highly ductile properties, allowing for significant deformation. Material C, represented in Graph C, shows higher strength but is less ductile, indicating it can withstand higher loads but at the cost of reducing its capacity to elongate under stress.

From graph B, the ultimate tensile stress for metal B can be determined by identifying the peak of the stress curve before it begins to yield or fracture. This value is approximately 250 N/mm².

Economic Benefit: Non-destructive testing (NDT) allows for the testing of propellers without causing damage, ensuring that parts can be reused and preventing costly replacements.

Safety Benefit: NDT identifies internal and external flaws, reducing the risk of catastrophic failures during operation, ultimately ensuring the safety of the ship and crew.

Eddy Current Testing is a suitable NDT method. This technique uses an alternating current to produce a magnetic field in the coil. When the coil is brought close to the conductive propeller, eddy currents are generated. If there are surface flaws, they disrupt the flow of these currents, creating a detectable signal.