Failure Analysis on pallet truck arm
The object
A pallet truck is a handling tool used for lifting and transporting goods on pallets. It consists of a base with wheels and two metal arms that fit under the pallet to lift it.
Possible failure modes of a pallet truck arm include:
- Breakage due to overload | A if the arm is subjected to a load beyond its strength capacity, it may break.
- Plastic deformation | Repeated use or excessive load can cause permanent deformation of the arm, reducing its lifting and carrying capacity.
- Wear and corrosion | friction and exposure to corrosive agents can cause wear and tear on the arm or the formation of cracks that can lead to its breakage.
- Manufacturing defects or poor quality materials | defects in the design or manufacture of the arm, or the use of poor quality materials, can affect its strength and durability.
- Lack of maintenance | a lack of lubrication, cleaning, or regular inspections can promote the accumulation of dirt, corrosion, or invisible damage that can lead to unexpected arm failures.
Purpose of the survey
The purpose of the investigation is to analyze the damage that occurred to the component. No information was provided regarding the material or delivery condition of the part. The damage consists ofthe breakage of the component at the weld with the tubular.
The analyses
Observation of the fracture zones shows theabsence of plastic deformation and the presence of bright and jagged fracture surfaces. These macroscopic aspects indicate a brittle fracture mode. The fracture surfaces were analyzed by scanning electron microscope to evaluate their morphological fracture characteristics.
Fractographic observation of the failure surfaces revealed the presence of integrity defects, such as interdendritic shrinkage, in the melt zone. The damage morphology is characterized by cleavage, indicating a crash failure with brittle mode. In addition, the fracture surface in the weld zone shows columnar detachments, also typical of brittle fractures.
Chemical analysis shows the composition of the material, which corresponds to a spheroidal cast iron of type EN-GJS-500-7 according to UNI EN 1563:2018.
Tensile strength values are in accordance with what is expected for a material of this type, confirming the classification as spheroidal cast iron EN-GJS-500-7.
Metallographic examination reveals the presence of large interdendritic shrinkage and a crack in the weld melt zone. The microstructure consists of a ferritic-perlitic matrix in the cast iron and acicular martensite in the fused zone.
Brinell hardness at the core is 187 HBW, while Vickers hardness varies from 693 HV to 716 HV in the different zones analyzed.
The results
As shown in the analytical phase, the material of the fractured component is a spheroidal cast iron of type EN-GJS-500-7 according to UNI EN 1563:2018. The fracture occurred in a brittle and crash mode, as confirmed byvisual and fractographic examination.
The origin of the failure can be attributed to two issues. First, the welding was performed without preheating and postheating, causing a hard and brittle structure in the welded zone. This made the zone susceptible to failure under normal loads. In addition, integrity defects were found due to interdendritic shrinkage during solidification, reducing the resistant section of the component and promoting failure even under sub-standard loads.
Conclusions
Component failure can be attributed to two separate issues:
- integrity defects in the cast iron casting, which caused a significant reduction in the resistant section;
- welding, carried out without considering the hardenability of the materials, resulting in an extremely hard and brittle joint.
Therefore, it is recommended to check the internal fusion quality and to adopt appropriate filler material and pre-heating and post-heating procedures during the welding process.
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