# Residual stresses in railway wheels: IN 13262

European standard EN 13262 sets requirements for railroad wheels for European rail networks. The standard applies to *whole* railroad wheels, which are made by forging and rolling from an evacuated wheel steel, and have thermally hardened rim.

## European standard EN 13262

The latest revision of the European Standard for railway wheels has an unusual designation EN 13262:2004+A1:2008. This revision replaces the previous revision EN 13262:2004. Additive «+ A1:2008"Means, what in 2008 year, the standard has been modified from the previous edition of the original standard from 2004 of the year. One of the important changes is the introduction of a fracture toughness test of a sample from a wheel rim.

## Residual stresses in the wheel rim

EN requirements 13262 to residual stresses in railway wheels boil down to the following.

1) Heat treatment of the wheel should form in the rim a field of compressive tangential (district) residual stresses.

2) The level of tangential compressive residual stresses near the tread surface should be in the range from 80 to 150 N / mm^{2}.

3) These tangential residual stresses should be zero at a depth of 35 to 50 mm from the rolling surface.

4) Residual stresses along the depth of the wheel rim in the plane of the rolling circle should be within, shown in the figure below.

Figure - Permissible values of tangential residual stresses

in the rim of the wheel in the plane of the rolling circle

## Two methods for determining residual stresses

Tests are carried out on the whole wheel after heat treatment..

The purpose of the test methods is to determine the change in the level of tangential residual stresses from the rolling surface to the depth of the rim..

The standard recommends two methods for determining residual stresses in wheel rims:

1) destructive strain gauge method;

2) non-destructive ultrasonic method.

The essence of the strain gauge method is briefly presented below..

## Residual stress strain gauge

### The principle of the strain gauge method

1) The method includes the operations of cutting the wheel with a gradual release of residual stresses, which are present in the wheel rim.

2) The change in the residual stress state as a result of each wheel cutting operation is evaluated on the wheel surface by measuring local deformations using strain gauges.

3) The change in the stress state inside the rim is obtained by linear extrapolation of the stress state, measured on the surface of the rim.

4) Determination of residual stresses is carried out only in one radial section., since it is known from experience, that the heat treatment of the wheel provides a uniform circumferential residual stress state.

### Performing tensometry of the wheel rim

The sequence of performing the strain gauge method for determining residual stresses in the wheel rim is as follows.

**Step 1 (picture 1)**

Installation (sticker) strain gauges:

and) in tangential and axial directions:

– at the point 1 on the rolling surface symmetrically to the rim-to-disc transition;

b) in tangential and radial directions:

– at points 2E on the outside and 2I on the inside of the rim;

– at points 3E on the outside and 2I on the inside of the rim-to-disc transition.

Picture 1 - Initial installation of load cells on a railway wheel

**Step 2 (picture 2)**

1) Cutting operation # 1 for cutting out a template with strain gauges.

2) Measurement of strain gauges after cutting operation # 1.

Picture 2 - Operation of cutting wheel No. 1

**Step 3 (picture 3)**

1) Installing an optional load cell 4.

2) Cutting operation # 2.

3) Measurement of deformations on sensors 1 and 4 after cutting operation # 2.

4) Thickness measurement h_{1} and h_{2}.

Picture 3 - Installing the load cell 4 on the wheel template cut out as a result of operation # 1. Cutting operation of wheel # 2. Thickness measurement h_{1} and h_{2}.

**Step 4 (picture 4)**

1) Installing the sensor 5.

2) Cutting operation # 3.

3) Measurement of deformations on sensors 1 and 5 after cutting operation # 3.

4) Thickness measurement h_{1} and h_{2}.

Picture 4 - Installing the load cell 5 on the rim template cut out as a result of the operation. Cutting operation of wheel No. 3. Thickness measurement h_{1} and h_{2}

**Tangential residual stresses of the rolling surface **

1) Calculation of the change in tangential stresses at all points with glued sensors after each of three cutting operations according to the formulas:

σ^{i}_{j} = – E/(1- ν^{2})(e_{tang }_{j}^{i} + νe_{┴ }_{j}^{i}),

where E = 210000 MPa - modulus of elasticity of steel,

ν = 0,28 - Poisson's ratio of steel,

e_{tang }_{j}^{i} - tangential (district) deformations,

e_{┴ }_{j}^{i}- axial or radial deformations,

i - number of cutting operation (from 1 to 3),

j - measurement points (from 1 to 5).

2) Calculation based on data, received under item 1) using extrapolation of changes in tangential stresses at points 1 and B after each cutting operation (picture 5).

3) The magnitude of the residual tangential stress in points 1 and B is calculated as the algebraic sum of the change in the tangential stress at the points 1 and B, respectively, after each cutting operation (picture 6):

σ_{1} = σ_{1}^{1} + σ_{1}^{2} + σ_{1}^{3 }σ_{IN} = σ_{IN}^{1} + σ_{IN}^{2} + σ_{IN}^{3}

The final graph of the distribution of residual tangential stresses over the thickness of the rim is plotted as a straight line segment, passing through the ordinates σ_{1 }and σ_{IN }in points 1 and B in the figure 6.