4.2. Drop mass impact tests and results
Drop mass tests were performed in order to determine the effect of materials on deformation of welded joints under dynamic loading, which represents the crash and
energy absorption characteristics of structure. The method utilises a drop mass impact configuration with mass and impact velocity selected such that the crush speed remains
approximately uniform during the entire sample crushing event. Figure 4.2 a shows the setup of the machine. In the test, single accelerometer is mounted on the drop mass. Drop
mass including the flat platen is 0.6 Kg, the maximum distance of drop is 75 cm. Measurement accuracy is maintained with regarding to the crushable foundation, which
remains undeformed
rigid Flat
while the
sample undergoes
crushing. The relative acceleration
between the drop mass and
foundation was
measured by connecting the pulse of the acceleration displayed from accelerometer as gravitational
acceleration Gravitational acceleration is the acceleration caused by the gravitational attraction of massive bodies in drop mass to the recording devices, then the pulse is
amplified that record the acceleration data as a function of time. The crush load is determined by the measured deceleration multiplied by the magnitude of the drop mass.
The impact velocity is measured by the integration of the acceleration of the drop head, and the crush displacement is measured by the double integration of the decelerationtime
history. Typical deformed specimen of three material combinations were shown in Figure 4.2b. As shown in the figures, the deformation mode of the three spot welded joints made
of different material systems are significantly different. To quantify the difference, the acceleration time data need to processed following the procedure listed in Figure 4.3. The
procedure was adapted from the method used to analysis the data from crush testing using guided drop mass impact on foams NASA,
Kellas S.,2004. As illustrated in Figure 4.4, in this process, raw data was processed to remove mechanical vibrations, as well as
electrical noise. Consequently, filtering and other extensive data processing is necessary to isolate useful mechanical properties from the experimental data. Because the data must be
filtered, the sampling rates have to be chosen in such a way that useful signal information is not lost. As shown in Figure 4.4, the analysis process of the foam tests data involves
initial identification of important events then the acceleration was processed into velocity, which in turn can be used to determine the displacement and stress strain data if the
loading area is known. The final results shown in Figure 4.4 was comparable to the published data, this validated the procedures before it was used to process the testing data
on spot welded joints. Figure 4.5 shows the acceleration vs time data, while the predicted displacement vs. Time data was shown in Figure 4.6. The results clearly show that the
dynamic behaviour of the spot welded joints with different material systems is significantly different. These data are to be used in future works to validate the FE model which is
briefly described in the next section.
4.3. FE modelling of the spot welded joint Preliminary results
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