Tough questions and inadequate answers filled the rail safety debate in the nineties shortly after a deadly cluster of train accidents. The word 'crashworthiness' entered common parlance and terms like, unzipping, brittle fracture and crumple zone were adopted for tabloid usage.
In those accidents, welded joints in 6000 series aluminium alloy extrusions exhibited catastrophic fractures. In recent years, TWI has conducted research aimed at gaining a better understanding of such fractures.
It now provides an analysis of large-scale impact tests of simulated welded rail vehicle components, which have been carried out recently in the European Commission funded research project entitled 'Crashworthiness of joints in aluminium rail vehicles'.
Early on it defined four distinct tasks;
- To demonstrate brittle weld fractures similar to those detected in the rail vehicle accidents.
- To demonstrate weld oversizing as a mitigation method for weld fracture.
- To compare the materials performance of different aluminium alloys.
- To compare the advantages of different welding processes for making aluminium joints.
Double skin extrusions were designed simulating those used in aluminium rail vehicle floors. Extrusions of parent material 6005A and 6008 were produced in the T6 and T7 temper, at Alcan in Switzerland.
The extrusions were welded using the conventional MIG process at Bombardier Transportation,UK, a hybrid Laser-MIG process at Alcan, Switzerland and a friction stir welding process at Danstir, Denmark.
Impact tests were carried out using an air cannon at Bombardier's engineering test facility in Crespin in France. One hundred and fifty welded extrusion specimens were tested under impact in a specially designed and constructed test rig using a 50kg projectile at a speed of 16m/s. Transient forces and displacements were obtained from four load cells and a laser displacement sensor.
Each of the fractured specimens was inspected visually. Force and energy were analysed on the basis of statistical significance. Materials performance and crashworthiness of welded aluminium joints were discussed and the advantages and disadvantages of the three welding processes were compared.
Nine crucial conclusions were drawn from the work largely relating to the superior performance of hybrid laser MIG and friction stir welding over conventional MIG. As a result a number of recommendations were made which are expected to modify future rail carriage design thinking radically.
For full details of these conclusions and recommendations and a report on the impact testing programme contact Weiguang Xu at TWI.
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