Body durability testing, in which a vehicle body structure is evaluated for reliability with regard to long-term road load input, is an essential part of the vehicle-development process. Currently, two methods are typically used for this purpose: vehicle road load durability at a proving ground and road simulator testing. Both of these methods are considered costly—due to both complete prototype vehicle costs and operational costs—and time-consuming. These factors are typically even greater when a pickup truck is developed, mainly due to variations in cab, bed, and wheelbase combinations.
Component-level testing is one of the alternatives that can be used to reduce development costs. But it is important to create test methods that cover most of the loading scenarios that a part could encounter in the field.
There are several approaches to vibration testing at the component level. Sine-sweep within a specified frequency, random vibration, and shock testing are some of the methods used. However, the only way to ensure the validity of a component-level test is to reproduce the same failure modes as the vehicle-level test. Engineers at Nissan Technical Center North America sought to develop an alternative test method for road load durability evaluations of a pickup truck rear bed, independent of proving ground or road simulator full-vehicle durability testing.
Their idea was to create a test method that generates similar results to a known and existing vehicle level test, such as a four-poster road simulator. For this purpose, road-load input to the truck rear bed was quantified and analyzed to find the rear bed major vibration modes. These loads and vibration modes were re-created on the bench test stand. A pseudo fatigue damage comparison was used to establish bench test load levels and cycles. In the end, durability tests were conducted on similar test specimens to evaluate the effectiveness of the developed bench test method.
A Nissan Titan King Cab (long wheelbase) was selected for this study. The vehicle was instrumented for road-load data acquisition with a specific focus on the rear bed. The truck bed data acquisition channels consisted of bed-mounted load cells (tri-axis), accelerometers at bed mounting points on both frame rail and the bed, several strain gauges, and a custom twist transducer to capture bed twist.
The acquired road load data was then analyzed in both frequency and time domains to quantify the loading modes and each mode’s contribution to the rear bed fatigue damage. Pseudo damage fatigue calculations revealed that the majority of rear bed fatigue damage in most data channels is accumulated around a certain frequency line.
Rear bed strain data acquired at the proving ground were also compared in time domain against lateral, longitudinal, and vertical loads at the rear bed mounting points. This was done to evaluate the relative effect of lateral and longitudinal input loads on the rear bed vibration. The rear bed was found to be most sensitive to the vertical input loads, and it was decided to use vertical loads at the bed mounting points as the main input for pseudo fatigue damage calculations.
As a result of contribution analysis, it was determined that vertical inputs greatly affect the truck rear bed. So, excitation to the test specimen was simplified to vertical inputs only. A rear half frame was used as the main fixture to the truck bed. Hydraulic actuators were attached to the frame rail just forward of the rear bed and the rear leaf spring attachment brackets. The attachments and test fixtures were designed and fabricated so they could withstand the loads for the course of durability test. Lateral restraints for the test fixture were by means of Watts linkage located at the forward actuators and at the rear hitch frame attachment. The instantaneous centers for each Watts linkage were near the roll axis of a ballasted vehicle. Longitudinal restraints were attached to the front leaf spring perch running aft to wheel centerline.
The instrumented truck bed was installed on the fabricated bench test stand and sinusoidal load inputs were applied to it to reproduce major loading modes. The frequency of the input loads was set to the dominant frequency that was seen in pseudo fatigue damage calculations in the contribution analysis. The rear bed load responses to the sinusoidal inputs were collected, and a fatigue damage comparison was carried out to match the bench test pseudo damage with the vehicle level fatigue damage calculated previously using the road load data. The pseudo fatigue damage of all four vibration modes was summed to equal the same road data damage. This was done in an iterative process to ensure that equivalent pseudo fatigue damage had been achieved on the bench test stand. In selection of the bench test cyclic input, both the damage values and rear bed load response magnitudes were considered.
After completion of input creation for the bench test, a new truck bed was installed on the bench and was tested using the created inputs. Durability test events of the bench test were collected and compared against a vehicle-level test (in this case a four-poster road simulator durability test) that was conducted on a similar rear bed simultaneously. Test events that were common (had the same failure mode) between the two tests (bench test and vehicle test) were plotted based on the mileage at which the events occurred. Comparison of all durability test events between the two tests showed that more than 80% of the events were reproduced by the bench test.
The results of this study proved that a road-load durability test can be performed on the rear bed of a pickup truck independent of the rest of the vehicle. As a result of this study, the number of full-vehicle durability tests can be reduced for future vehicle development programs while the quality of testing is maintained. Also, with the developed test method, due to elimination of low damage portions of the overall input to the rear body of the truck, durability test time is reduced by approximately 50% compared to a similar four-poster durability test.
This article is based on SAE Technical Paper 2009-01-1406 by Ali Karbassian and Darren P. Bonathan, Nissan Technical Center North America Inc., and Tetsufumi Katakami, Nissan Motor Co. Ltd. The paper will be presented at 1:30 p.m. on Tuesday, April 21, in Room D3-26/27 as part of the Road Test Simulator Techniques session at the SAE 2009 World Congress.