page_title: Stress–strain analysis
section_title: Experimental methods
hierarchical_section_title: Stress–strain analysis / Experimental methods
caption_attribution_description: English&#x3a;&#32; Tension lines in plastic protractor seen under cross polarized light. When a ray of plane polarised light is passed through a photoelastic material, it gets resolved along the two principal stress directions and each of these components experiences different refractive indices. The difference in the refractive indices leads to a relative phase retardation between the two component waves. The birefringence of the plastic layer shows colored fringes. Also called Photoelasticimetry or photoelasticity. Not seen does not mean No difference. WYSIATIFrançais&#x202f;&#x3a;&#32; Lignes de tension dans un rapporteur en plastique vu sous une lumière polarisée. Lorsqu'elle traverse un matériau sous tension mécanique, les deux composantes de l'onde subissent un retard l'une par rapport à l'autre. Ce retard fait apparaitre en lumière blanche des irisations dans le matériau. Ce phénomène, nommé photoélasticité, est mesuré par photoélasticimétrie.
context_page_description: Stress–strain analysis is an engineering discipline that uses many methods to determine the stresses and strains in materials and structures subjected to forces. In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.
  In simple terms we can define stress as the force of resistance per unit per unit area, offered by a body against deformation. Stress is the ratio of force over area. Strain is the ratio of change in length to the original length, when a given body is subjected to some external force.
  Stress analysis is a primary task for civil, mechanical and aerospace engineers involved in the design of structures of all sizes, such as tunnels, bridges and dams, aircraft and rocket bodies, mechanical parts, and even plastic cutlery and staples. Stress analysis is also used in the maintenance of such structures, and to investigate the causes of structural failures.
context_section_description: Stress analysis can be performed experimentally by applying forces to a test element or structure and then determining the resulting stress using sensors. In this case the process would more properly be known as testing (destructive or non-destructive). Experimental methods may be used in cases where mathematical approaches are cumbersome or inaccurate. Special equipment appropriate to the experimental method is used to apply the static or dynamic loading.
  There are a number of experimental methods which may be used:
  Tensile testing is a fundamental materials science test in which a sample is subjected to uniaxial tension until failure. The results from the test are commonly used to select a material for an application, for quality control, or to predict how a material will react under other types of forces. Properties that are directly measured via a tensile test are the ultimate tensile strength, maximum elongation and reduction in cross-section area. From these measurements, properties such as Young's modulus, Poisson's ratio, yield strength, and the strain-hardening characteristics of the sample can be determined.
  Strain gauges can be used to experimentally determine the deformation of a physical part.  A commonly used type of strain gauge is a thin flat resistor that is affixed to the surface of a part, and which measures the strain in a given direction.  From the measurement of strain on a surface in three directions the stress state that developed in the part can be calculated.
  Neutron diffraction is a technique that can be used to determine the subsurface strain in a part.
  The photoelastic method relies on the fact that some materials exhibit  birefringence on the application of stress, and the magnitude of the refractive indices at each point in the material is directly related to the state of stress at that point. The stresses in a structure can be determined by making a model of the structure from such a photoelastic material.
  Dynamic mechanical analysis  (DMA) is a technique used to study and characterize viscoelastic materials, particularly polymers.  The viscoelastic property of a polymer is studied by dynamic mechanical analysis where a sinusoidal force (stress) is applied to a material and the resulting displacement (strain) is measured. For a perfectly elastic solid, the resulting strains and the stresses will be perfectly in phase. For a purely viscous fluid, there will be a 90 degree phase lag of strain with respect to stress. Viscoelastic polymers have the characteristics in between where some phase lag will occur during DMA tests.
caption: Stress in plastic protractor causes birefringence.
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