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Product category: Accelerometers and Vibration Sensors
News Release from: AV Technology | Subject: Predictive fatigue analysis
Edited by the Engineeringtalk Editorial Team on 11 February 2002

Predictive fatigue analysis pays
dividends

AV Technology's Structural and Process Monitoring Division has extensive experience in the capture, analysis and interpretation of fatigue data across a broad spectrum of applications and industries.

Stockport based AV Technology's Structural and Process Monitoring Division has extensive experience in the capture, analysis and interpretation of fatigue data across a broad spectrum of applications and industries It is estimated that between 80-90% of all structural failures occur through some kind of fatigue mechanism

On 27th March 1980, the Alexander Keilland oil rig, operating in the North Sea's Ekofisk field, broke up and capsized with the tragic loss of 123 lives.

Subsequent investigations showed that the disaster had been caused by fatigue cracks emanating from an apparently innocent small hydro phone bracket which had been welded onto one of the support leg.

The tiny bracket had been welded at what appeared to be a noncritical point of the structure and unfortunately no problems had been foreseen.

Fatigue is defined as "failure under a repeated or otherwise varying load which never reaches a level sufficient to cause failure in a single application".

Fatigue cracks always develop as a result of cyclic deformation in a localised area.

This often occurs due to the presence of a small crack or pre-existing defect on the surface of a component rather than due to the theoretical stresses in a "perfect" computer generated part.

The failure may be as simple as a fractured bolt or as complex as the breaking in half of a 100 000 tonne oil tanker - but the consequences may be equally far reaching and catastrophic.

At best fatigue failure may result in lost revenue while at the other end of the spectrum, huge insurance claims for loss of equipment, property and life cost billions of pounds each year around the world.

The complex nature of fatigue failure means that such problems are often difficult to predict using conventional finite element analysis methods.

This emphasises the importance of carrying out real time testing and analysis on working equipment and structures.

However, the attitude of "if it ain"t broke don"t fix it" still prevails in many cases and, unfortunately, we often only hear about the effectiveness of fatigue analysis when it has been used to identify the cause of a failure after it has occurred.

Predictive fatigue analysis can dramatically reduce premature failure and modern analysis methods are increasingly helping to provide comprehensive product life expectancy data.

This analysis can be used effectively both during product development to highlight any potential problems and validate computer design models in real working environments and also to investigate and validate suspected problems with equipment working in the field.

It is easy to believe that fatigue analysis is a relatively new science, but its importance was recognised almost 200 years ago.

As the Industrial Revolution spread across Europe and the USA, more and more machines were built which required a source of power - namely steam.

In the early days, steam engine design was a rather hit and miss affair and failures in railway axles, boilers and associated pressure vessels were commonplace.

In 1859, a notable English Engineer, William Fairburn, was granted a 'small sum of money" by the Treasury for the purpose of "ascertaining by direct experiment, the effects of continued changes of load upon iron structures and to what extent they could be loaded without danger to their ultimate security".

Such was the public concern over fatigue at that time, that Fairburn's work was reported not only to the Board of Trade but also Parliament.

German born August Wohler is still considered by many to be the "grandfather" of fatigue and he designed numerous test rigs to investigate the resistance of metals to varying loads.

He carried out extensive tests to measure both breaking strain of metals and the behaviour of metal bars subjected to repeated torsional stress - the first recorded controlled fatigue experiments.

Wohler was known for his dedication, persistence and patience.

To obtain meaningful results he realised that the tests needed to be carried out over large numbers of cycles and some of his tests lasted over 10 years! It was the invention of strain gauges in 1939 which revolutionized fatigue analysis.

These postage-stamp-sized devices arose from work on the effects of earthquakes on buildings.

The original patent was taken up by Baldwin Lima Hamilton and used for tests on their locomotives.

However a comment from the company's CEO at the time that "we are in the business of selling locomotives not postage stamps" prompted the extension of the patent licence to other companies who found use for strain gauges across a broad spectrum of applications especially in the aerospace and automotive industries.

The effectiveness of fatigue analysis is dependent of three key criteria: the placing of the strain gauges at critical points; the analysis of the data; and the interpretation of the results.

AVT carries out two main types of analysis - stress life analysis for welded structural components in accordance with BS7608 and strain life analysis for nonwelded components which are exposed to high strain levels.

Weld analysis assumes the weld will contain inherent defects which will propagate if exposed to significant stress and the analysis carried out gives the estimated life to "failure" within selected confidence limits.

It is usually based on a standard weld classification which takes into account weld size and type, together with weld geometry and loading configurations.

Strain life analysis is normally used to predict the fatigue life to crack initiation not failure.

The assumption is that a piece of metal is initially defect free and the analysis predicts the life for a defect to initiate and grow to a size where it will then propagate if exposed to significant stresses.

The analysis takes into account such effects as materials, properties, heat treatments, surface finish, notch effects and mean stress effects.

The company can provide measurement systems from 16 up to 128 channels to accommodate the diverse range of customer requirements.

For complex fatigue applications, AVT has developed its own highly effective fatigue analysis system known as "Jaffa".

Jaffa is a combination of hardware and software designed specifically for on-site testing which incorporates two 64-channel modules of strain gauge signal conditioning instrumentation together with data collection and analysis software.

Each channel has selectable gain and filtering and can be set to particular gauge type and resistance.

The system is capable of measuring dynamic data at a rate of 1000 samples per second and will continuously stream data to PC hard disc.

Data acquisition is controlled using special software with default setups created by AVT for 32, 64, 96 and 128 channels, including on-line bar graph displays with max-min "nudge bars".

For on-site testing AVT has equipped a Land Rover with a complete system acting as their fully fitted mobile laboratory.

The vehicle contains the robust ACRA data acquisition system, a PC for data acquisition and online monitoring, power supplies (including a UPS for over 20min black out) together with printers and CD writers for data backup and hard-copy printing.

An umbilical cord, up to 40m in length, allows the vehicle to track along side the operating equipment under test.

AVT has recently carried out comprehensive tests on a JCB Mini Digger while it was being operated.

The strain gauge positions were selected so that the data collected are wholly representative of real working conditions.

It is not difficult to appreciate the importance of product reliability for a company such as JCB.

Although the company have their own comprehensive in-house fatigue testing capabilities, there are times when the use of external testing facilities provides cost effective flexibility and ensures the necessary continuity for on-going testing.

However it is vitally important for JCB that any testing procedures, together with the analysis and presentation of data are fully compatible with those of JCB.

AVT has worked closely with JCB for a number of years and has developed its fatigue testing procedures and Jaffa equipment to meet fully these requirements.

The Jaffa system allows huge amounts of data to be analysed and reported within a very short time frame without the need for manual input or analysis.

To achieve this, AVT has written powerful batch programs using nCode, nSoft and Fatimas software which allow the system to generate analysis on the stress, percentage yield, dominant frequency and fatigue life of the structure providing results typically within one hour.

Normally data are collected, stored and then analysed overnight so that there are no delays to testing.

In order to reduce time domain sample rates without impacting on performance, AVT uses its novel analogue Peak-Valley Hold system.

This monitors stress trends by detecting peaks and valleys in the results, thus reducing the amount of data required by up to a factor of 10.

A prime example of where predictive fatigue analysis has been used to good effect is at one of Shell's chemical plants.

AVT was commissioned by Shell to calculate the minimum expected fatigue lives on the stud bolts and associated flexible suction pipework on one of their reciprocating compressors.

The pipe is 70mm in diameter with a maximum wall thickness of 15mm.

The first stage operates at a peak pressure up to 325bar while the discharge pressure is up to 1500bar.

Shell had observed what it thought were relatively high vibration levels in the suction pipe work and the question on the minds of their engineers was "could the corresponding stresses induced in the pipeline flange result in fatigue damage which could ultimately affect plant integrity?".

To obtain the necessary data, AV Technology fitted sets of three-element rosette gauges to the pipework, flange and securing bolts strategically arranged to measure both axial and hoop stresses.

These type of gauges provide the principal strain data simultaneously in three directions (0, 45 and 90 degrees), allowing the all-important principal stress to be calculated.

A total of eight pipeline locations were monitored to provide a comprehensive picture.

Analysis of data taken during compressor operation showed that after allowing for the effects of mean stress, the alternating stresses at all gauge positions are too small to cause fatigue crack initiation therefore indicating infinite fatigue life.

To provide a complete picture, AVT also took into account the material properties, heat treatment, surface finish and notch effects.

During the next works shut down, modifications were carried out on the pipe supports and after this, the perceived vibrations appeared to increase.

To put their minds at rest, Shell called in AVT again to check the stress levels.

Results showed that the operational stresses had not changed significantly in terms of fatigue and were typically less than 2% of fatigue limit.

Fatigue assessments can provide valuable information based on in-service strain measurements.

This method is particularly effective in checking the integrity of computer designed components working under real, rather than simulated, conditions.

The key to success is understanding where to place the strain gauges and then analysing and interpreting sufficient data.

AVT's experience in these areas ensures that they offer a comprehensive fatigue analysis service - a capability fully supported by their custom designed equipment and instrumentation.

The Jaffa system provides the ideal tool for this and can process huge amounts of data, quickly and efficiently, providing results in a matter of hours rather than days.

This allows any remedial action to be taken swiftly and effectively. Request a free brochure from AV Technology ...

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