Mechanical Properties of Materials
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what is resilience?
the capacity to absorb and recover elastic energy
what are the two main standardised test methods for testing the mechanical properties of materials?
• British/European standards (ISO) - American society for testing and materials standards (ASTM)
what are the main mechanical properties of a material (the standardised language)?
• strength • hardness • ductility • stiffness
what is the most simple mechanical test?
applying a load to a sample which will make it extend
what is normal stress defined as (in terms of testing materials)?
• using a uniform static load applied over a certain area • stress = force / area • units: N/m^2
what is strain defined as (in terms of testing materials)?
• strain = extension / original length - dimensionless
what is the difference between tensile/longitudinal and lateral strain?
tensile/longitudinal/axial strain = change in the length lateral strain = change in the width
what does elastic deformation mean?
• the deformation is temporary • the sample will return to its original shape and size when the force has been removed • obeying Hooke’s law
what is another name for Young’s modulus?
elastic modulus, E
what is the equation that defines Hooke’s law?
E = stress / strain
what does the gradient of a stress-strain graph tell you?
E, elastic modulus (which relates to the stiffness of the material) - a higher gradient = more stiff material
what is the area under the curve of a stress-strain graph?
the toughness of the material
how does bond strength of the material relate to the stress-strain graph of the material?
the slope (gradient is proportional to stiffness) depends on the bond strength of the material
what is poisson’s ratio?
• symbol: v • ration of lateral to axial strains v = - Ey / Ex • dimensionless
what is poisson’s ratio for most metals?
0.33
what is poisson’s ratio for most ceramics?
0.25
what is poisson’s ratio for most polymers?
0.40
what is poisson’s ratio for cork?
0
what is an auxetic material?
• materials that have a negative Poisson’s ratio when stretched • as we stretch the material, it gets wider
what are auxetic materials used in?
• they are developed in the UK (Exeter) - applications range from explosion curtains to comfortable mattresses
what materials tend to have the biggest elastic modulus?
metals, ceramics, composites
what material tends to have a low elastic modulus?
polymers
what is E, elastic modulus a measure of?
measure of stiffness and bond strength
what happens when we exceed the limits of elasticity when deforming a material?
plastic deformation will occur (permanent, non-recoverable deformation)