Real time stress analysis

AVT has a relationship with JCB, working with the company's own in-house research and development team to provide outsourced structural monitoring and fatigue analysis services.

AVT's Special Projects Division has a longstanding relationship with JCB, working with the company's own extensive in-house research and development team to provide outsourced structural monitoring and fatigue analysis services.

JCB prides itself in its rigorous approach to the design and testing of its equipment, and extensive field trials are a crucial part of the development process.

In addition to their own testing, JCB engineers ensure that new or upgraded models spend time in the hands of 'real operators'.

Field testing products such as stackers and backhoes is relatively straightforward but when engineers in the JCB Heavy Products division wanted to put their new demolition machine through its paces it was not so easy to find 'real' demolition applications.

However, an ideal opportunity presented itself when Crewe and Nantwich Borough Council gave planning permission to build 27 apartments and 99 homes on the site at the back of the old Police Training College in Crewe, following Cheshire Constabulary's relocation of its training facilities to new headquarters in Winsford.

To make way for the development, outbuildings, offices and accommodation blocks needed to be demolished and JCB loaned one of the new machines to designated demolition contractor Nick Brookes Demolition to put it through its paces.

It's the final chapter in the tale of the former Ursuline convent which was built in 1910 and remains one of the most recognisable buildings in Crewe.

Fortunately following a successful three-year battle by conservationists the distinctive frontage of the Grade II listed building is remaining and any demolition work needed to be carried out with diligence to avoid damage.

To gather the real time data on the JS330HRD during the demolition work, AVT were commissioned to carry out 'live' fatigue analysis on the machine.

This involved the bonding of a number of strain gauges at strategic positions on the machine in order to gather data for subsequent weld fatigue analysis in accordance with BS7608.

The gauge locations were specified by JCB following FE analysis by their Design Department.

(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.) All the wiring for the gauges was fed back via an umbilical cord to AVT's mobile Landrover laboratory.

At the heart of this is the company's unique JAFFA monitoring system.

This is made up of the robust ACRA data acquisition system, a PC for data acquisition and online monitoring, power supplies (including a UPS for over 20 minutes black out) together with printers and CD writers for data back up and hard copy printing.

JAFFA is a combination of hardware and software designed specifically for on-site testing incorporating 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 up to 2,500 samples per second and will continuously stream data to PC hard disc.

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

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 have written powerful batch programs using nCode, nSoft and FATIMAS software which allow the analysis on the stress, % yield, dominant frequency and fatigue life of the structure.

AVT's Senior Manager Neil Parkinson is clear on the benefits of the working arrangement his company has with JCB and as he explains: "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 that any testing procedures, together with the analysis and presentation of data, are fully compatible with those of JCB.

AVT have worked closely with JCB for a number of years and we have developed our fatigue testing procedures, the JAFFA equipment and reporting formats to meet fully these requirements.

Measurements taken in the field will be used by JCB's designers to validate the FEA models and support the assumptions made in the modelling process.

Once validated, these models can be used to predict the effects of structural modifications and repairs or to assess the consequences of damage.

This analysis can be used effectively both during product development to highlight any potential problems and validate computer design models in real working environments.

It can also be used to investigate and validate suspected problems with equipment working in the field." A spokesman from JCB confirmed that they were very pleased with the outcome of the tests and concluded.

"The new excavator came through with flying colours.

The compactness and manoeuvrability of the 330 makes it ideal for such limited access applications, and the operator on the day was very impressed with its versatility and stability.

Its triple boom has a massive reach in excess of 20 metres and is designed to be fitted with a range of a standard demolition tools including rotating handling grabs and cutters.

This new model is based on our tried and tested 330XD excavator so we were confident we had a sound base to start from.

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