Stress and Damage Characterisation

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Strength of materials, or mechanics of materials, is the study of the behaviour of solid objects subject to stresses and strains. The study of strength of materials refers to methods of calculating the stresses and strains in structural members, such as beams, columns, and shafts. The methods employed to predict the response of a structure under loading and its susceptibility to various failure modes takes into account the properties of the materials such as its yield strength, ultimate strength, Young’s modulus, and Poisson’s ratio.

Stress is a physical quantity describing the internal forces that neighbouring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.  Strain within a material arises by various mechanisms, such as stress as applied by external forces to the bulk material, such as gravity contact forces, external pressure or friction.  Any strain (deformation) of a solid material generates an internal elastic stress, analogous to the reaction force of a spring that tends to restore the material to its original non-deformed state.

Residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed.  Residual stresses can occur through a variety of mechanisms including inelastic (plastic) deformations, temperature gradients (during thermal cycle) or structural changes (phase transformation).

Digital Imaging correlation (DIC) is an optical imaging technique for 2-D and 3-D tracking and imaging registration. This technique is often used for the measurement of full-field displacement and strains within science and engineering. An examples application of this technique is deformation mapping during mechanical testing. Digital Volume Correlation (DVC) is a related technique which captures planar images of a surface and maps these in 3-D.  DVC can be combined with MRI imaging, Computer Tomography (CT), microCT and confocal microscopy.

The University of Manchester hosts the Stress and Damage Characterisation Group, a facility equipped for the investigation and prediction of the mechanical behaviour of materials and engineering components.  The range of expertise covers: atomic force microscopy, C-scan analysis, 3D coordinate measuring, electron speckle pattern interferometry, image correlation, laboratory X-Ray diffraction, magnetic methods for stress measurement, nanoindentation, neutron strain measurement, portable X-ray stress, measurement, Raman spectroscopy, scanning acoustic microscopy, residual stress measurement using the slitting method and X-Ray tomography.

ICAM equipment includes:

  • Q-300 3D ESPI system, an optical interferometry technique suited for cases where high strain gradients are present, such as micro and macro heterogeneities, cracks or welds.
  • Modified Jeol SEM for the correlation of the intensity profile across successive images to infer in-plane surface displacements.
  • Physical Acoustics UltraPAC C-scan for ultrasonic evaluations for flaw detection, bond and delamination analysis, and precision thickness analysis.
  • LaVision StrainMaster, an optical tool for shape, strain and deformation analysis of solid and granular subjects
  • MAPS (Magnetic Anisotropy and Permeability System) and MARSH (Magneto Acoustic Residual Stress & Hardness System).
  • KSI Acoustic Microscope which measures the velocity of back-scattered waves to visualise internal structures.