# Experimental Development of Composite Bicycle Frame
## 3 Experimental Results and Discussion
### Structural Strength Test Results
#### 3.3.4 Influence of Head Tube Joint Reinforcement  
During the structural strength test of the frame, attention was also focused on evaluating the effect of reinforcing the composite lay-up of the head tube joint by adding both unidirectional carbon filament layers and carbon fabric layers. For this purpose, strain gauges were installed on the rear side of the head tube (location HT, see Figure 5a). This location was chosen because there is the same composite lay-up for all frame specimen designs. The mechanical strain measured by the strain gauge at the HT location on the head tube, depending on the loading force, is plotted in Figure 16 for specimen 4 with the original composite lay-up and for specimen 7 with the reinforced composite lay-up. In the case of frame specimen 7, there was a delay in the loading process at about 2.3 kN due to technical reasons. We can see that the load characteristic indicated by the HT4 strain gauge on frame specimen 4 is linear up to a load of approximately 1 kN, with F\({}_{\text{max}}\) reaching 2.85 kN. In the case of frame specimen 7, the load characteristic indicated by the HT7 strain gauge shows a linear response up to approximately 1.3 kN, while the F\({}_{\text{max}}\) of 2.82 kN was reached. According to the strain gauge at the HT location, frame specimen 7 behaves better than frame specimen 4 in terms of the operational properties of the bike. The loss of linear response occurs at about 35% of the limit load in the case of specimen 4, while in the case of specimen 7, this occurs later, at about 46% of the limit load due to the reinforcement of the head tube joint area. The DIC method was used to find the extreme strain values in the head tube joint area. A comparison of the identified behaviors of the two selected specimens in the head tube joint area for the step just before failure is shown in Figure 17a (frame specimen 4) and Figure 17b (frame specimen 7). In comparing these two specimens with a specified field of the principal strain 1, it can be seen that specimen 4 is characterized by higher strain values near the TT to the HT joint, located with the maximum on the top side. The strain values around the joint of the DT to the TT are not as high, which is in accordance with the final failure which first appeared in the TT to the HT joint part of the specimen, while the DT to the HT joint remained almost undamaged. On the contrary, the strain values for specimen 7 are high in both joint areas, which correlates again with the final failure of both joints at the same time. Reinforcement of the joints that were made for specimen 7 apparently leads to a more balanced distribution of loading between joints. Figure 16: Mechanical strain indicated by strain gauges installed in the HT location (frame specimens 4 and 7). The mean values over the area of principal strain 1 were evaluated in three sections (as described in Section 2.4). The values of the down tube for specimen 4 (Figure 18a) and specimen 7 (Figure 18b) were compared. Based on these comparisons, it can be seen that both specimens are equally stiff on the selected sections. Strengthening of joints for specimen 7 clearly helped in the DT to the HT joint area, whereas specimen 4 shows the appearance of a local failure in the G1 area (using the sensor marking described in Figure 6). The indicated local failure corresponds to a slight decrease of force (shortly after reaching time step no. 500) and also to a slight strain increase in the G2 area. The failure detected in this area also correlates with an increase of counts shown by the AE method in the same time period (boxtime value around 5.324 \(\times\) 10\({}^{4}\) s.). A force signal measured in specimen 7 also indicates a slight decrease of force during loading (step value around no. 230), but there were no failures indicated in the gauge areas.