The Open University skip to content Accessibility

Materials Engineering Group
 


Materials Engineering

Departmental Staff - Dr. Salih Gungor

Destructive methods for the determination of residual stress

The destructive method of measuring residual stresses involve unbalancing the self-equilibrium 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

1. Slitting method

Slitting method was used to determine the residual stresses in two unidirectionally reinforced Ti-SiC 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 Ti-SiC composite sandwiched between ~ 2mm Ti-alloy 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. Ti-SiC 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 out-of-plane moiré interferometry.

 

Figure 3. The comparison of slitting and layer-removal methods

 

Figure 4 Out-of-plane moiré fringe patterns corresponding to various slit depths (a-f) 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

In-plane 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 out-of-plane 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 In-plane transverse

131

657

164

-838

sigma 3 Out-of-plane 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.

 

Back to S. Gungor staff page >>

back to the top^