Modern quenched and tempered (QT) steels bring a lot to the off-highway industry, particularly for dump trucks. For one, when combined with the proper design concept, their use contributes to the lightweighting of the dumper body. Also, by utilizing modern QT-steel, the wear life, depending on strength, is about four times higher than for ordinary steel and has a higher resistance against dents from loading rocks and boulders.
One of the main load cases for a dumper body is impact of an object, i.e. boulders and rocks, into the body. Researchers at SSAB used a well-proven test setup to develop a model to predict failure and depth of a dent after impact. A material model with a damage mechanic was used to predict fracture. The developed model was used to study the effect of the geometry of the impacting object, thickness of the plate, and unconstrained plate field.
The model was also implemented in a larger model and compared with a full-scale test of the dumper body. It was found that the most sensitive parameter is the geometry of the falling object. However, the main conclusion from the investigation was that it is possible to design lighter and stronger dumper bodies with the appropriate design in combination with the right material.
The standard free hanging U-shaped body concept is a good design to reduce the weight of dumper bodies, which allows the end user to increase the payload and reduce the fuel consumption. The concept originated in the ’70s (or ’60s) when rubber was used as a hammock for a bottom in dumper bodies for rigid dump trucks.
With the development of modern QT-steel, which is both strong and tough, steel could be used instead of rubber. During the early ’90s, the concept was adopted for dumper bodies for trucks with QT-steel instead of rubber.
The free hanging U-shape is like a hammock suspended in two top beams. The dump body itself rests in just two points, the pivot point at the back and in a cradle at the front, while unloaded; this is the so-called free hanging concept.
In some designs the bottom of the dumper rests on rubber pads placed on the frame while loaded. The design offers large unconstrained surfaces, which gives a larger amount of elastic deformation and less of plastic deformation. This is important for dumper bodies where one of the main loading problems is impact from falling objects such as boulders and rocks. Testing of falling objects onto plates has been performed since the late ’70s. More recently, FE-models were developed to predict the depth of dents and fractures for falling objects.
A test rig was set up with a hammer, frame, and the guide rails. Rods were used to improve the precision of the hammer impact position and prevent the hammer from wobbling. Test plates were clamped in to the frame with wedges. The opening of the frame was considered as the free unconstraint area of 500 x 500 mm (20 x 20 in), i.e. free length of 500 mm (20 in).
The mass of the hammer was variable from 177 kg (390 lb) to about 500 kg (1100 lb). Radius of the hammer tip was 50 mm (2 in) when the hammer was new but extensive use of the rig had caused wear of the tip to a non-rounded shape. Testing was done by hoisting the hammer to a height of 2.82 m (9.25 ft) and then a quick release engaged to create the impact.
After the test either the depth of the dent was measured or the plate was identified as a cracked plate. The depth was measured with calipers while the plate was still fastened in the frame. The so-called limit rupture mass (LRM) was also measured. LRM is defined as the highest mass of the hammer that a certain material at a specific thickness, free length, and tip geometry can withstand without cracking. To establish the LRM, the mass of the hammer was varied from low to high to find the highest mass of the hammer without cracking in the tested plates. At least three test plates without cracks are required.
Three materials with plate thickness of 5 mm (0.2 in) were investigated. All of them modern quenched and tempered steels. The materials were designated X400, X450, and X500, where the number stands for the average hardness in Brinell.
The FE-model was generated as a quarter model due to symmetry. The frame was modeled as a rigid material and was fixed. The plate in the model was a “perfect fit” in the frame without pre-stress. Due to the wear of the hammer the actual shape was modeled to achieve accurate results.
The friction coefficient between the frame and the plate as well as friction coefficient between the hammer and plate was set to 0.2, both static and dynamic. All calculations were done with LS-Dyna in explicit mode. All elements were eight nodes constant stress solid elements.
The model material used was LS-Dyna MAT105, which was based on the Lamaître damage mechanic model. Inputs to the material model were density, Young’s modulus, Poisson ratio, yield strength, and true stress and strain curves. The damage mechanic model used was a three material parameter model.
Stress and strain curves were derived from a uniaxial tensile test at low strain rate, i.e. quasi static condition, with Bridgeman correction. Tests at different strain rates were performed to investigate the strain rate dependencies for the materials.
Free length effects
The presented model, for one of the main load cases for dumper bodies, had been shown to be able to predict effects of different variables with sufficient engineering accuracy.
It was found that it is possible to design a thinner dumper body bottom at least equally as strong as a thicker one if the free length is increased. For example, for an 8-mm (0.31-in) thick plate and for a free length of 500 mm (20 in), the LRM was found to be about 410 kg (904 lb). However, considering a thinner plate of 5 mm (0.2 in), the same LRM can be reached with a free length of 800 mm (31 in).
This clearly showed the advantage to strive for as large free unconstrained surfaces as possible in the lightweight design of dumper bodies. The model can also be implemented in large FE-model to estimate the depth of a dent or the risk of fracture. However, fracture can only be estimated with knowledge of the shape, mass, and height of the falling object.
Height and size are fairly straightforward to assume for any particular dumper body. The shape and radius of the tip are the most sensitive variables of the model and have the most substantial influence of the LRM. Rocks and boulders have a great variation of shape and radius. Therefore, this model is presently difficult to use as a direct designing tool.
Overall, the researchers found that a larger free length gives smaller dents and less risk for fracture, and the shapes of boulders have a large influence of the LRM.
This article is based on SAE International technical paper 2014-36-0008 by Torbjörn Narström, SSAB AB.Continue reading »