Departmental Staff  Dr. Salih Gungor
Destructive methods for the determination of residual stress
The destructive method of measuring residual stresses involve unbalancing the selfequilibrium of the internal stresses by removing or sectioning a part of a component that contains residual stresses and measuring the resulting deformation from which the residual stresses are computed. Figure 1 shows some common methods that use this principle.
Figure 1. Some common destructive methods for residual stress measurement
Residual stress measurements in fibre reinforced Titanium alloy composites
Slitting method was used to determine the residual stresses in two unidirectionally reinforced TiSiC panels. The panels (A and B) were identical in every way, except one of the panels had an additional heat treatment with the aim of lowering residual stresses. The panels consist of ~2mm thick TiSiC composite sandwiched between ~ 2mm Tialloy claddings. Both longitudinal and transverse stress distributions through the thickness of the material were measured, and the average fibre stresses were inferred from the results.
Figure 2. TiSiC composite
In the slitting method, instead of removing a complete layer, a slit normal to the direction of the stress to be determined is introduced and the change in curvature on the opposite face is measured using high sensitivity outofplane moiré interferometry.
Figure 3. The comparison of slitting and layerremoval methods
Figure 4 Outofplane moiré fringe patterns corresponding to various slit depths (af) and the corresponding curvatures (g).
Using the equilibrium of internal stresses across the composite's cross section, the average residual stress in the fibres and the matrix of the reinforced section could then be computed.

Panel A 
Panel B 


Matrix 
Fibre 
Matrix 
Fibre 
Longitudinal 
255 
1,345 
311 
1,755 
Inplane transverse 
131 
657 
164 
838 
Table 1. Residual stresses in the reinforced section (All stress values are in MPa units)
2. Matrix etching method
The fibre strains in both composites were also determined using the matrix etching method, which is based on relieving the constraint on the fibres by selectively etching the matrix material in a part of the composite. The residual fibre strain obtained by the measurement of the change in the length of the exposed fibres is then related to the residual stresses in the fibres and the matrix using a concentric cylinder model.
Figure 5 The principle of matrix etching method and the results.
The residual stresses in fibres calculated with this method agreed very well with those obtained by the slitting method. By combining the results of both methods, the outofplane stresses were also computed, so that the full stress state in the reinforced section of the material could be obtained.

Panel A 
Panel B 


Matrix 
Fibre 
Matrix 
Fibre 
sigma 1 Longitudinal 
255 
1,345 
311 
1,755 
sigma 2 Inplane transverse 
131 
657 
164 
838 
sigma 3 Outofplane transverse 
39 
334 
1 
8 
Table 2. Complete stress state in the reinforced section of the composite panels (All stress values are in MPa units)
See this page for more information on residual stresses in Ti/SiC composites.
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