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Materials Engineering

Materials Engineering

Materials Staff - Dr Foroogh Hosseinzadeh





Dr Foroogh Hosseinzadeh
Dr. Foroogh Hosseinzadeh


Residual stresses are inevitably a consequence of every manufacturing process and are believed to have impacts on the structural integrity of engineering components.  Residual stress is that which remains in a body and is at equilibrium with its surrounding. Besides residual stresses interact with imposed stresses during service operation and affect components life to a great extent. Therefore, full knowledge of the residual stress field through the whole component is crucial to understanding how residual stresses impact on failure.  As such stresses are self-equilibrating and are not immediately apparent externally, they are difficult to be determined or calculated. Numerical methods and analytical solutions can be used to estimate residual stresses. However, for safety critical applications predictions must be validated by experimental data.

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There is a wide range of techniques available for measurement of residual stresses in engineering components. For example, Figure 1 shows common techniques in steel. They are categorized as non-destructive, semi-destructive and destructive.

Perhaps diffraction is the most important non-destructive method for residual stress measurement in crystalline materials and engineering components. It is based on a simple principle that the crystalline lattice is used like an atomic strain gauge. Three main available types of radiation that are common for residual stress measurement are electron, x-ray photon and neutron beams.


Residual Stress Measurement in Steel

Figure 1: Residual stress measurement methods in steel.

Other non-destructive methods rely on the fact that some other properties are related to stress, such as magnetic response in magneto elasticity and optical properties in photoelasticity. Since the non-destructive techniques do not concern material removal several measurements on a location can be made on one sample. However they are confined to surface or sub-surface measurements.

Destructive techniques are based on deliberately removing material, with the object deforming to reduce the stresses at the newly formed surfaces to zero. Then the original stress field can be reconstructed by monitoring any changes in component distortions when stresses are relaxed. The degree of destruction depends on the extent of material removed from the component.

In contrast to the non-destructive methods, techniques that are based on mechanical strain relaxation such as the deep hole drilling, slitting and contour methods enable measurement of through thickness residual stresses in thick components.

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There are several names in the open literature given to the technique for measurement of residual stress by performing an incremental cut such as: crack compliance method, fracture mechanics approach, successive cracking method, slitting method, etc. The technique is similar to the compliance method for measuring crack length in a fatigue or fracture specimen where a known load is applied to a cracked specimen and the resulting strain is measured to determine the crack length. In the slitting method the crack length is known and the measured strain is employed to determine the residual stress.

The method itself involves three steps (see Figure 2):

  • - cutting a narrow slot of progressively increasing length into a body (this relaxes residual stress at the
         cut surfaces and redistributes the residual stress field within the entire body)
  • - measuring the changes in
  • strain associated with progressive cutting at suitable locations
  •  - analysing the measured data to infer the initial residual stress distribution.

Schematic drawing of the Slitting Method

Figure 2: Schematic drawing of the slitting method.

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The Contour Method is a recently invented technique, first published in 2001, for measuring and mapping residual stresses in a component. The method involves making a straight cut in the sample of interest along a plane where knowledge of residual stresses is required. The created cut surfaces locally deform owing to the relaxation of residual stresses present before the cut. These deformations are measured and then applied as a boundary condition in a finite element model to determine the out-of-plane residual stress distribution at the cut surface.  It is a strain relaxation method that is conceptually and experimentally simple, inexpensive, and uses equipment available in most engineering workshops.

The contour method is implemented by undertaking four steps, see Figure 3: specimen cutting, contour measurement, data processing and stress calculation using finite element (FE) method.


Scenatic of the Contour Method

Figure 3: Schematic of the contour method.

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Slitting and contour methods, in tandem, were used to measure residual stresses in a candidate residual stress simulation benchmark specimen. It was a stainless steel beam-type test specimen, autogenously welded along one edge.  This benchmark specimen has been studied analytically and for which neutron and synchrotron diffraction residual stress measurements are available. The current research was initiated to provide additional experimental residual stress data for the proposed benchmark using measurement methods based on different concepts.  The contour and slitting results were found to be in excellent agreement with each other and correlated closely with published neutron and synchrotron residual stress measurements when differences in gauge volume and shape were accounted for.

Slitting method

Strain gauges were first instrumented on the test specimen for the slitting method. Cutting was performed by electro-discharge machining (EDM) using 0.25 mm diameter wire with the sample submerged in de-ionized water. A water proofing system was employed to protect the strain gauge. “Skim cut” parameters were set on the EDM machine so as to minimise any stresses introduced by cutting. Strain data were gathered at specific increments of slot depth ranging from 0.1 mm near the weld crown to 1 mm deep within the beam.

Contour method

Following the slitting measurement, the normal deformation of both cut surfaces of the beam was measured using a Mitutoyo Crysta Plus 574 co-ordinate measuring machine (CMM).

Standard data analysis was employed to convert the raw data into a suitable format for stress calculation of the FE model. A 3-D model of one half of the cut part was created. The measured displacements at the FE model nodes were evaluated and then applied as initial boundary conditions normal to the cut plane in the FE model, but in the opposite direction. A linear elastic finite element analysis was then undertaken to calculate the corresponding distribution of residual stress normal to the cut face. Map of longitudinal stresses obtained using the contour method is shown in Figure 4. Follow this link for more information.

Map of Longitudinal Stresses Using th Contour Method

Figure 4: Map of longitudinal stresses on the cut plane at mid-length of the
beam measured using the contour method (MPa).

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