Failure Investigation

Despite everyone’s best efforts component failures can and do happen. While the initial concern is typically with restoring production it is important to identify the root cause of failure and prevent re-occurrence.

A failure investigation may involve initial data gathering, visual assessment of the failed unit, sampling of lubricants, analysis of wear debris and testing of the failed component. Some failure modes can be detected by a range of condition monitoring techniques, others may arrive unannounced. Failures related to wear or lubricant contamination may have indications in the oil sampling history (hopefully the potential for failure would have been identified and highlighted). Some failure modes, however,  can be sudden and catastrophic without any advanced warning being visible (e.g. due to overloading). In either case there may be evidence of the failure event itself both in the lubricant and in the oil filters.

When conducting a failure investigation it is important to record all relevant information about the event, including its chronology and operational parameters. When collecting lubricant samples do not rely on the usual sample points alone – consider sampling from additional locations which may help identify origin of potential contamination, the extent of wear and other useful information. Retain oil filters – the entrapped debris can be retrieved and analysed to shed further light on the failure mode.

When removing failed components for further testing mark them to indicate orientation and fit, take photos in situ and during the disassembly process. Make sure that components are not damaged further during disassembly or, where this is unavoidable, record and identify disassembly damage to differentiate it from the original failure. If engaging assistance of experts do not clean the components until they had a chance to view them or at the very least take a sufficient quantity of high quality photographs. It is often difficult to draw conclusions when a lot of the evidence has been washed away.

See more below, and once you are ready to proceed please go to our Welcome page to get started or get in touch.

Service Categories

  • Before you start
  • Failed Component Testing
  • Wear Debris Analysis
  • Contaminant Identification

Things to remember at the outset of an investigation

Which questions to ask

When starting an investigation it helps to understand what it is you are trying to find out.

Sometimes this seems straightforward:

"Why did the machine fail?"

And there may well be a straightforward answer.

"A faulty seal led to ingress of water and excessive wear."

But sometimes things are not so simple. If the machine was not sampled routinely then the lubricant and wear debris available for the investigation will be an accumulation over a period of time. There may be a number of contaminants observed, some more benign than others. It may not be possible to isolate and identify all of the contaminants or to single out those which degraded the lubricant or caused excessive wear, but do not despair. There may still be a path to reliable operation ahead.

An investigation may have to include prospective analysis of lubricant and wear debris samples from the newly repaired machine. This may help shed light on the cause of failure, but also to prevent its reoccurence as abnormal wear is picked up early through regular monitoring.

So don't just ask "why did this happen?", ask "how do we stop it from happening again?".

We aim to help you with both of these questions.

Which samples to take

With any investigation it helps to be mindful of the analytical process at the sampling stage. As ever, you will want to take a representative sample (see the sampling part of the Knowledge Base section for advice on sampling procedures). Although when investigating a failure or a contaminant it may also be worthwhile to take more targeted samples (e.g. before and after filters, at bottom of tank, at specific points in the system or times in the operation cycle, etc.). If you wish to identify debris collected from the system, perhaps from the bottom of the sump, from a sampling point or from a filter housing, consider also reserving for analysis the filter element itself. Where a debris sample is to be tested also take a standard oil sample from the system, as not only will you want to identify the debris, but also to know how prevalent this material is in the oil or fuel system. You should also consider taking the following supplementary samples:
  • Samples of virgin lubricants to establish lubricant properties and additive baselines. This is especially important if no such sample has been supplied to the lab previously.
  • Samples of possible contaminants, other lubricants in the area, process fluids/powders, environmental contaminants, seals, hoses, etc.
  • Samples from similar systems, if not routinely sampled. This can help establish what is normal or to spot issues developing in other units.
It is also important to take a sufficient volume of sample for all the potential testing required. Some of the bulk property tests require significant quantities of oil (e.g. Foaming Characteristics needs ~600ml, more if repeat measurements are to be possible). If possible, take extra sample volume, as more tests may become necessary once the initial panel is completed.  

Examination and testing of the failed component can help identify the root cause of failure.

Failure Investigation - Failed Component Examination and Testing

In addition to wear debris analysis we are able to review failed components and engage a select group of partners to deliver a comprehensive analysis. Techniques available include the initial visual inspection, sectioning and metallographic analysis, hardness testing, elemental analysis via SEM-EDX, etc.

Analysis of the type, size, quantity and elemental composition of the wear debris can give vital clues to the root cause of failure.

Failure Investigation - Wear Metal Analysis

Depending on the failure mode (e.g. wear related or catastrophic failure due to overloading), there may be wear metal evidence present in the lubricant or in the oil filters. We are able to extract and analyse this wear debris through a collection of different analytical techniques. Extracted particles can be deposited on a filter membrane during the Patch Test or deposited on a glass slide to enable Analytical Ferrography. Elemental data can be obtained from bulk fluid analysis as well as from Scanning Electron Microscopy. The latter allows for individual particles to be interrogated and for data to be compared against machine metallurgy.  

Analytical Ferrography

Analytical Ferrography is a technique for depositing and analysing wear particles contained in an oil or grease sample. The sample is deposited onto a glass slide, with the particles trapped by strong magnets and the oil washed away with a suitable solvent. Both linear and rotary particle deposition systems exist. At STS preference is given to a rotary system, which has been developed at the company. It ensures good separation of particles over the three rings, with particles also being sorted by size with the larger particles settling out on the inner ring.
Once deposited the particles are analysed by a metallurgist, who is able to report on the relative quantities, types and sizes of particles present. A Particle Quantifier Index of the slide is also recorded. All of this is taken into account to produce a comprehensive report on the wear rate and situation.
You can find an example Ferrography Report available for download here.

Filter Debris Analysis

Oil filters are essential to maintaining oil cleanliness and removing wear metals and contaminants. As they perform their function, however, they also remove some of the information about wear and contamination levels from the oil stream. This is then stored in the filter itself. Fortunately Filter Debris Analysis grants us access to this store of information. By extracting and analysing the entrained particles we can learn a lot about the wear situation or the source of contamination. Oil filters come in many shapes and sizes and how we treat an individual filter will depend on its size and construction. Typically a section of the outer cage is cut out and removed to enable access to the filter media. A section of the filter media is then removed and placed into a beaker. The beaker is then filled with solvent and an ultrasonic bath is used to agitate and extract the particles and any residual oil present. The solvent is then evaporated leaving the particulates and any residual oil. A portion of the debris is then deposited onto a filter membrane for microscopic analysis. The remaining oil/debris mixture is then analysed using a Rotating Disc Electrode Optical Emission Spectrometer to determine the elemental composition of the debris mixture. This gives a breakdown of the different wear metals and contaminants present. Where more detail is needed, the membranes containing the particles are analysed using a Scanning Electron Microscope with Energy Dispersive X-Ray facility. This provides information on the exact elemental composition of individual particles, which can then be matched to specific component metallurgies.

A common theme of many investigations is analysis of contaminant samples. This typically begins with Microscopy, Elemental Analysis, FTIR Spectroscopy and can be further escalated to SEM/EDX, GC-MS, etc.

Microscopic Examination

A good place to start any investigation is Microscopic Examination of the debris sample or of particles isolated from a fluid sample through filtration. The samples may be examined under a range of magnifications and illumination conditions (incident, transmitted, polarised or dark field illumination).

This helps to indicate the quantity of debris and its composition with respect to particle type and size. I.e. are they large or small, what is their aspect ratio, are they curved (cutting wear), are there surface features (striations/ridges), are they metallic or not, translucent or opaque, shiny/reflective or dark/dull, crystalline or amorphous, is there silt present (fine sub-micron particles) etc.

This may guide method selection for further testing as well as providing answers to questions on the origin of the debris and its characterisation.

The above image shows metallic wear debris (both ferrous and copper containing) on a deposit of dark silt (fine sub-micron particles and larger debris). What appears to be metallic debris under incident light can be seen as dark particulates under side on illumination (dark field). Our state of the art Leica Cleanliness Expert Microscope with motorised stage.

Elemental Analysis

Induction Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is used to measure the concentration of over 20 different elements in the oil. These include wear metals, additives and contaminants. We have recently upgraded our instrument - you can read about some of the resulting improvements here.

By monitoring wear metal concentrations the wear rate and its origin can be established. Trending additive levels ensures that the right oil is used and that it remains suitable to the task, while measuring levels of contaminants helps prevent severe wear and loss of function.

For grease and debris samples a combination of a Rotating Disk Electrode Optical Emission Spectrometer (RDE) and an ICP-OES is used. The RDE eliminates a lot of cross-contamination issues and, as no dilution with solvents is required, allows for more accurate measurement of heavily contaminated samples which would settle at the bottom of the test tube if an ICP-OES was used. The ICP-OES is then used to cover elements not measured by the RDE (mostly additives, although the wear metals are also measured). The ratio of RDE to ICP wear metal levels gives an indication of wear particle sizes.

 

FTIR

Fourier-transfer Infra Red Spectroscopy (FTIR) is a powerful tool for analysis of oil, grease and fuel samples. It is especially useful for monitoring and identifying certain types of contaminants.

The underlying principle is that infrared energy from the source is absorbed in the sample at  wave lengths which are characteristic of specific molecular bonds. Each scan generates an FTIR spectrum which can be analysed and interpreted.

At its most basic the technique can be used to measure predetermined parameters, such as oxidation, nitration, sulphation or presence of fuel and glycol in an engine oil. Some of those require prior calibration and a scan of a virgin sample to be used as reference.

More advanced analysis can help monitor degradation, identify and match unknown contaminants, verify lubricant formulation or help identify and source an unknown lubricant.

In some cases Gas Chromatography coupled to Mass Spectroscopy (GC-MS) is called upon to work in tandem with FTIR analysis on particularly tricky samples, where exacting information is nonetheless required.

You can download an Example FTIR Report here

 

Scanning Electron Microscopy (SEM/EDX)

Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM/EDX or SEM/EDS) is one of the best known and most widely-used of the surface analytical techniques.

The advantages include incredibly high resolution and depth of field as well as the ability to determine elemental composition of surface areas, particle populations and individual particles (these can be sorted into classes by their metallurgy with the approximate grade of steel or other alloy suggested).

This analysis can also be performed on surface deposits or minute quantities of contaminants.

Among the disadvantages are the higher cost and the inability to differentiate organic compounds.

The following images show examples of SEM scans of typical debris.

Debris on a woven filter substrate.

Fractured particle on a membrane filter

A particle of pollen

A field of typical debris, including wear particles, rods/fibres and other miscellaneous debris

An elemental map of a field of particulate debris