:In 2002, the placing boom on a truck-mounted concrete pump suddenly broke down at the condominium construction site in Japan. One worker who was engaged in smoothing the concrete surface was killed soon after he was hit by the fallen placing boom. Since this type of an accident has never been reported before, the authors carried out intensive investigation to clarify the cause of the accident. The broken boom on a truck-mounted concrete pump is 12 years old and it has hydraulic 4-section M-type boom whose maximum length is 21m. At the first stage of investigation, macro-observation of the fracture surface was conducted, and it revealed some important facts: 1) the fracture surface was along the weld line between the boom and the boom supporter, 2) most of the fracture surface was flat and rusted, 3) beach-marks, one of fracture surface patterns with the features observed on fatigue fracture surface, were present on some parts of the flat fracture surface, 4) the shape of the beach-mark patterns indicated that cracks initiated around the boom supporter weld edge where stress concentration might be induced. Considering the broken concrete pump had been used for 12 years, these facts implied that the fracture of the boom might be attributed to fatigue. To confirm this estimation, micro-observation of the fracture surface using a scanning electron microscope (SEM) was carried out at the second investigation stage. Although most of the fracture surface was covered with corrosion products, striation pattern, typical evidence of fatigue crack propagation, was observed. Subsequently, material tests were conducted to ensure material integrity. In this stage, chemical composition and mechanical properties of a boom material were examined. Macro-structure and hardness of the boom supporter weld were also analyzed. These material tests clarified that there were no defects or problems both in the material and the weld.
    As a result of these series of investigation, it was revealed that this fracture accident happened as follows:

1. Small cracks initiated near the boom supporter weld edge due to fatigue and/or other reasons.
2. These cracks propagated by fatigue driven by the cyclic stress induced by pump pressure.
3. Finally, the cracked section came to instable fracture by the load arose in the accident day.

    Following actions are needed to prevent this type of a fracture accident:

1. The shape of weld edge on a boom should be smoothened to reduce stress concentration.
2. Nondestructive tests should be carried out to ensure no cracks present near the weld on a boom.
3. No one is allowed to step in just under the concrete placing boom.

Keywords ; Concrete boom pump truck, Fracture accident, Fatigue, Fractography