Corrosion

Corrosion is a chemical process (electrolytic) whereby the cross- sectional area of metal components is dimensionally reduced due its surface area being converted to iron oxide (rust). The component’s load bearing capacity will reduce along with its reduction in cross- sectional area until it fails even under its design load.

Uniform corrosion is typically what is visible on the rail web facing the sea; as one example only. Crevice corrosion is localised oxidation at narrow openings between two surfaces of which at least one is metal such as rail joints which must be annually serviced by cleaning and greasing to avoid metal loss and loosening of the joint causing rapid deterioration.

On railway infrastructure corrosion plays mostly a minimal role in component failure where a maintenance regime is in place since the fatigue or wear life of components are mostly the determining factor for maintenance or replacement on most infrastructure components. Nuts and bolts on the overhead electrification system (OHE) are subjected to crevice corrosion and may be replaced on condition or time based intervals. Steel OHE mast posts may also be replaced on condition due to corrosion on especially the foot of the mast.

Detecting corrosion is mostly a visual inspection process which highlights the importance of foot inspections and scheduled detailed inspections of OHE components.

7.5.2. Wear

To control wear the wear rate must be calculated to determine the expected remaining life of the component to plan for maintenance (before it fails) or replacement (when it has reached its minimum size). The most important wear components of the track which are related to traffic throughput are the rail, crossing and set blades, and the overhead electrification contact wire though the latter is minimal with modern carbon pantograph collectors.

The wear rate can be calculated by measuring the material loss on a regular basis (for which the infrastructure measuring and recording vehicle is particularly suitable for measuring to very high accuracy and making thousands of measurements over many kilometres) and

two measurements by the number of wheel or pantograph passes or throughput in tonnes. On rail it is more intricate since rail is subjected to both natural wear (wheel passes) and artificial wear (rail profiling such as grinding). Also, rail wear is not uniform along the length of the line as it would be greater on curves, increasing with a decrease in curve radius. It will also depend on various other variables such as the geometry condition in general, whether the alignment of curves is correctly placed, whether or not the transition curves are correctly laid out and tamped, etc. The wear rate of the rails can be a very good indication of the general condition of track geometry.

Erosion may also be a problem on embankments and cuttings which must also be monitored and corrective steps taken.

7.5.3. Fatigue

Fatigue is caused by constant cyclic loads of tensile and compressive stresses below a metal’s elastic limit. The rail is a classic example due to its suspension across sleepers and the cyclic load of wheels. Refer to Zaayman (2016), page 12 figure 23 and paragraph 2.19.2 on page 30 for the stress-strain diagram and examples of crack development due to internal stresses and fatigue (among other reasons).

Rail failure due to fatigue can be controlled with regular ultrasonic internal rail defect detection. Fastening failure due to fatigue (creep) must be addressed by replacement and even shortening the replacement interval.

7.5.4. Overload

During the design of the track infrastructure provision is made for the capacity (axle loading, train lengths, speed, throughput, etc.) for the demand the line is expected to carry. If these parameters are exceeded the infrastructure will be overloaded and the track condition deterioration will be above the expected rate until it fails prematurely.

Causes of overloading are axle loadings exceeding the design parameters, flat spots on wheels which causes high dynamic loading, poor geometry which also causes high dynamic loading in the vertical and horizontal plane, etc.

Typical signs of overload are bending of the rail and rail joints, crushing of the ballast (geometry deviations in the vertical plane), formation failure (visible as mud pumping through the ballast or white spots on the ballast surface), overheating of the OHE contact wire

(normally only visible through infrared cameras unless a very advanced state was reached just before failure), etc.

If the infrastructure was designed correctly and the design capacity is not exceeded, overloading would normally not happen. If the design capacity is exceeded failure will occur where the demand (blue line) exceeds (crosses) the capacity (green line) in Figure 46. However, as the components deteriorate with age and wear, its design capacity will reduce and overloading becomes possible if the replacement programme falls short of retaining the infrastructure within the design capacity to sustain the demand.

Figure 46 also illustrates how the overall capacity (red line) will reduce when the system has been deteriorated by more than one failure mechanism (illustrated is wear and corrosion for example) and fail well below design capacity.

Figure 46: Deteriorating Capacity

[Adapted from Daley (Publication Date not Stated)]

7.6. Failure Modes

The failure mode describes the end state of the failed component (the condition) as the result of the failure mechanism which was the cause (paragraph 7.4) of the failure. A failure mode would normally be described as ‘component & condition’, for example, ‘rail & break’. In a failure analysis and reporting system tables may be used to classify the failure by mode, followed by additional information such as mechanism, extent of the failure (size of the defect), risk, probability, action required (repair or replace), etc. Such tables can be quantitative and qualitative and may include a weighting system in order to rank failure modes in terms of severity, risk, etc.

Consistent descriptions for the failure modes are as important to categorise failure modes as it is for failure mechanisms. An approved list of such descriptions would be advisable in order to quantify the frequency of the same

failure mode occurring to ensure that a persistent failure that may exist is addressed and prevented from recurring. A good example of a classification system is the International Union of Railways (UIC) publication, UIC Code 712 (2002) which describes rail defects. Refer also to Australian Rail Track Corporation Ltd (2006b).

7.7. Failure Detection

Infrastructure measuring and recording is required to establish deterioration rates and to be warned of instances where deterioration has accelerated and corrective actions are required. This forms part of a condition based maintenance programme.

Infrastructure measuring and recording will also show where failures have occurred. A failure can be defined as a condition exceeding a set standard. Failure does not have to be catastrophic such as rail break to be considered a failure, it can also be exceedence of the maximum wear.

Refer to paragraph 3 for technologies relating to measuring and recording of the condition of the infrastructure which includes rail profile measurements, track geometry measurements, OHE geometry measurements, it detects missing or loose fastenings, ultrasonic internal rail flaw measurements, eddy current rail surface defects, etc.

Measuring runs and visual inspections should be regular in order to detect accelerated deterioration and failure before it has catastrophic consequences. The importance of these activities for effective maintenance management cannot be overemphasised.

Other systems may be installed or may have secondary functions that will give warning of failure. For example, railway track circuits provide indication of train presence on a particular section of a track. If the rail breaks the continuity is broken. Lines equipped with axle counters as opposed to track circuits do not have this advantage and specialised detection equipment must be installed. Various systems for rolling stock failure detection are also mounted to the track. These have some influence on maintenance because they have to be removed for some maintenance activities but their most important benefit from an infrastructure point of view is that rolling stock failures can be removed from the track before they cause infrastructure failures.

7.8. Reliability Analysis

Reliability analysis is different from failure analysis methods (paragraph 7.9) in that reliability analysis refers to activities to predict failures and failure rates; identify infrastructure component weaknesses; identify potential failure modes; determine failure mechanisms or causes; determine safety, environmental, operational and economic consequences of a failure occurring and also consider all these interrelationships that can lead to product or process unreliability

(proactive) whereas failure analysis establishes the causes of an actual failure (reactive).

The objective of reliability analysis is firstly to influence the design during the feasibility study phase and thereafter to determine and implement appropriate maintenance standards, policies, procedures and activities to control deterioration and prevent or manage failure throughout the operations and maintenance phase. These analysis results will feed into the proactive maintenance plan and mechanised maintenance capacity calculations.

The following are short introductions to only two of various systematic methodologies used to quantify the effects and impacts of failures through reliability analysis. Other methodologies used include Event Tree Analysis (ETA), Hazardous Operations (HAZOP) Analysis, Preliminary Risk Analysis, Cause-Consequence Analysis, etc.