FORENSIC TESTS PROTOCOL TO ANALYSE FAILURED TIRES FROM ROAD ACCIDENTS João M. Pereira Dias1,Ana Paula V. S. Pereira Dias1*, Albertina Soares2 1IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal 2DEQ, Instituto Superior Técnico, Universidade de Lisboa, * : apsoares@tecnico.ulisboa.pt FORENSIC TESTS PROTOCOL TO ANALYSE FAILURED TIRES FROM ROAD ACCIDENTS ABSTRACT The statistics point to a correlation between road accidents and tires failure and thus with traffic fatalities. The tires are complex composite materials having appreciable amounts of rubber, carbon black, silica and other components, which are susceptible of physic-chemical characterization. Their performances, and consequently the security level, rely on these features. In order to establish a protocol for forensic characterization, tires were studied in detail. Sampling and physic-chemical characterization procedures were analyzed. Samples from scrap tires, not involved in accidents as a matter of legal jurisdiction, were collected and characterized by infrared spectroscopy (ftir-atr) and thermogravimetry in an oxidizing atmosphere (air). The morphology of rubber samples collected from different parts of a damaged tire were characterized by scanning electron microscopy with elemental analysis (SEM-EDS). The tire aging was simulated using a UVB weathering chamber. Data from characterization tests showed that tire aging, mainly the degradation of rubber, was the main responsible for tire failure. EXPERIMENTAL Scrap tires were collected from local roads. Rubber samples were cut and characterized by : A- Thermogravimetry using a Netzsch thermobalance to evaluates the thermal degradation of rubber and to quantify the inorganic filler material such as silica. Samples with 50-100 mg were heated (200C/min) from room temperature to 11000C under air flow using a alumina crucible. B- Infrared spectroscopy using a reflectance mode (FTIR-ATR) to characterize different rubbers and their eventual degradation. Spectra were collected using a Nicolet equipment and a ATR accessory with a ZnSe crystal. C- Scanning electron microcopy was used to characterize the rubber morphology and chemical composition. The bead wires integrity can also be evaluated. The images were collected with a FEG-SEM Hitachi 2400. The samples were previously covered with a thin film of gold-palladium. D- Accelerated aging tests in a UVB weathering chamber to simulate the tire aging. Tire scheme and main components Results and discussion Samples used Thermogravimetry Sample Number Sample Origin 1 Blowout truck tire Highway road 2 Used tire Waste 3 4 5 6 New tire Bridgestone 7 8 Used truck tire Thermograms of tread samples with and without aging in weathering UVB chamber Thermograms of tread samples from a blowout and new tires Infrared spectroscopy results Blowout treads and aged treads showed analogous thermal degradation behavior with a net decrease of the degradation stability Reflectance bands in the range 3000-2700 cm-1 are attributable to vibration of the groups and CH2=CH2 butadiene. Aged and blowout tires presented lower intensity these bands The FTIR results seem to indicate that the occurrence of bursts happens to rubber with a high content of carbon originating from the carbonization of slow aging of rubber, or due to high temperatures reached during breaking or due to excessive speed vehicle . Scanning Electron Microscopy SEM images of rubber samples from scrap tires. There are visible cracks due to the aging of the rubber and loss of elasticity. The fissures are well distinguished from any cuts caused by sharp objects during the accident. Cracks caused by blowout can also be distinguished from those resulting from tire aging. CONCLUSIONS The causes of tires failures can be identified using inexpensive characterization techniques above described (TG-DTG, FTIR-ATR, SEM). A database must be created to have reference data with statistical significance. Tires aging seems to be the main “guilt” of tires failure. Rubber carbonization, due to high temperatures reached during high speed driving and breakage accelerates tire aging and tire failure. Drivers must be educated toward adequate tires uses and maintenance to reduce traffic fatalities. Steel wires from a blowout tire presented high corrosion which indicates as the cause of the tire failure the aging process