Advertisement

Lightweight Design worldwide

, Volume 10, Issue 3, pp 24–27 | Cite as

Testing and Evaluation of a Suspension Arm with Integrated Functions

  • Dominik Spancken
  • Andreas Büter
  • Paul Töws
  • Oliver Schwarzhaupt
Construction Lightweight FRP
  • 208 Downloads

A lightweight suspension arm made of carbon-fiber reinforced polymer and featuring several integrated functions has been developed by Fraunhofer LBF. Design and prototype manufacturing have already been presented in Lightweight Design 1/2015. The suspension arm’s weight is lower than that of conventional metal designs by around 35 %. At the same time, a Structural Health Monitoring (SHM) System as well as a system for semi-active vibration-damping have been integrated into the suspension arm’s structure.

Because they offer enormous weight-saving potential in highly-loaded suspension components, fiber-reinforced polymer-matrix composites (FRPs) with their excellent mechanical properties are being considered for industrial application. A major challenge for engineers is the anisotropic material behavior which significantly increases the design effort for these parts.

Durable lightweight design and integrating additional functions offers great potential, however. While complexity of the individual parts increases through integration of functions, the cost (also for manufacturing) and weight of the overall system can usually be reduced. Furthermore, active and passive sensors can be integrated in the parts.

The Structural Health Monitoring (SHM) System integrated in the suspension arm serves to demonstrate, that it is possible to monitor the state of the FRP-structure in use. It allows quantifying the operating loads in order to increase vehicle safety and to define maintenance intervals according to actual use.

Also, semi-active systems allow damping of vibrations while locally monitoring the structure with respect to damage at the same time.

Operating Loads Acting on the Suspension Arm

For safe-life design of structural components which need to be reliable and very light weight, knowledge about realistic operating loads is essential. In use, the suspension arm is subjected to manoeuver loads, mainly in the longitudinal and lateral directions, whose relative severity varies over time. Events considered critical include deceleration, cornering, bad roads and running over curbs. These operating conditions lead to complex multiaxial stress-time histories within the suspension arm which need to be considered in its design.

Design of the Suspension Arm

Numerical topology optimization was used to identify primary load-paths and most severely loaded regions. Subsequently, unidirectional fibers could be arranged locally along these loading-paths. They are connected to the bolts of the fittings via integral eyes, Figure 1. To prevent buckling of the structure, the geometry was further strengthened by means of local corrugation and ribs. A quasi-isotropic laminate lay-up was chosen to account for the complex multiaxial loading. Fittings for joining the hub and connecting the suspension arm to the frame are made of aluminum, Figure 2.
Figure 1

Strains at locations S3 and S6 measured by means of fiber-optic sensors (© Fraunhofer LBF)

Figure 2

FRP-suspension arm with integrated functions (© Fraunhofer LBF)

Testing of the Suspension Arm

In regular road use, the suspension arm is subjected to variable loading amplitudes in several directions. To achieve equivalent damage in a uniaxial test, a damage-equivalent loading regime has been devised. Forces of 4.4 kN for deceleration and -3.3 kN for acceleration are applied for 100,000 cycles. Once this number of cycles is reached without visible or measurable damage in the part, another 100,000 cycles were applied with an increased load. This is described in Table 1.
Table 1

Summary of the test program (© Fraunhofer LBF)

Load-level

No. of loading-cycles N

Loads

Braking F Brake .

Accelerating F Acc.

100 %

100,000

4.40 kN

-3.30 kN

114 %

100,000

5.02 kN

-3.63 kN

128 %

100,000

5.63 kN

-4.22 kN

142 %

100,000

6.28 kN

-4.69 kN

200 %

to fracture

8.80 kN

-6.60 kN

Testing was performed under uniaxial loading with constant amplitude and considers acceleration and deceleration of the vehicle. The suspension arm’s fittings were used to clamp it in the test rig, Figure 3, while load was applied by means of a servo-hydraulic cylinder. In total, two prototypes were tested. Suspension arm 1 was manufactured without integrating any of the additional functions while suspension arm 2 is equipped with an SHM-system and a piezo-stack.
Figure 3

Test-rig for uniaxial testing of the FRP-suspension arm (© Fraunhofer LBF)

During cyclic testing, the stiffness of the suspension arm is constantly monitored. Figure 4 shows the stiffness-trends normalized to the initial stiffness of each test-block for the two suspension arms. The first 100,000 cycles at the 100 % load level were sustained without stiffness-loss by both suspension arms. The minor stiffness-loss in Figure 4 (at load-levels 114 %, 128 %, and 142 %) is due to plastic deformation of the aluminium-parts as well as to inter fibre fracture and beginning local delamination in the laminate.
Figure 4

Stiffness-trends of the tested suspension-arms; left: suspension-arm 1, right: suspension-arm 2 (© Fraunhofer LBF)

Only at the 200% load-level failure can be achieved by rupture of the aluminium-parts, Figure 5.
Figure 5

Failure location of the suspension-arms; left: suspension-arm 1, right: suspension-arm 2 (© Fraunhofer LBF)

Structural Health Monitoring System

The Structural Health Monitoring system allows continuous real-time monitoring of the composite structure by determination of local strains. In case of overloads or damage, the user can be notified.

To measure strains in highly loaded regions, fibre-Bragg grating sensors (FBG-sensors) are used. They measure strains at a certain location by means of an optical glass-fibre which possesses an inscribed optical grating with predetermined lattice spacing. Several measuring points can be realized and evaluated using a single glass-fibre. The glass-fibre is embedded between the plies of the composite laminate during manufacturing and equipped with a connector.

For measuring, a broad banded light-signal is sent into the glass-fibre by the interrogator. At the measuring location, the portion of light whose frequency corresponds with the local lattice spacing is reflected back to the interrogator where its frequency is recorded. Since lattice spacing varies between measuring locations, different frequencies of light are reflected by each one. If strain occurs at a certain measuring location, the lattice spacing of the local Bragg grating and thus the frequency of the reflected light changes. The change in reflection frequency correlates with the local strain.

Numerical topology optimization was used to identify primary load-paths and most severely loaded regions.

In total, six measuring locations were defined on the suspension arm Figure 1 ( right) which are located in the vicinity of the eye-connections and the unidirectional fibres. Local strain-measurement was performed for every load-level at the beginning of loading, after 1,000 cycles, and shortly before reaching 100,000 cycles. Figure 1 shows examples of the measured strains and the measuring locations S3 and S6 for the suspension arm load cases acceleration and deceleration.

The blue curve in Figure 1 shows, that increasing the load leads to a corresponding increase in strain at the measuring location. Doubling the load, results in doubling of the strain. Measuring location S6, on the other hand, exhibits a sudden change in the strain signal at 100% load after a small number of cycles. This could hint at first inter fibre cracks or beginning delamination in the vicinity of the measuring location, which could be caused by the external loading or by residual stresses in the laminate.

For the remaining test duration, strain in the compression regime remains constant, only showing a further sudden change at the 200 % load-level. This hints at progression of local damage in the laminate. The strain measured towards the end of the 200 % loading is 0.45 %, which is still significantly lower than the allowable strain of the carbon-fibres.

To estimate a critical strain value, which could be used to warn the driver of the suspension arm’s impending failure, additional tests would have to be performed. The experimental results show, that the SHM-system is operating reliably and thus suited to monitor FRP structures.

Semi-active Vibration Damping System

The semi-active system uses an array of stacked piezo-elements. These are arranged along the curved edge between the fittings for connection to hub and frame, Figure 5. This is the region of maximum strain.

A stack of 17 piezo-elements was bonded to the laminate and the elements interconnected electrically. Measurements showed, that during cyclic mechanical loading, no change in electrical capacity or resistance of the piezo-stack could be detected. From this it may be concluded that neither de-bonding of piezo-elements nor damage in the surrounding laminate is introduced.

Conclusion

Using a lightweight suspension arm made of fiber-reinforced polymer, Fraunhofer LBF has demonstrated that highly loaded suspension components conventionally made of metals may be substituted by composite parts. At 2.1 kg, the FRP-suspension arm is lighter than its conventional peer from series production by 35 %.

Integrating the SHM-system shows the possibility to monitor the state of highly loaded FRP-structures. Thus, maintenance could be scheduled according to the usage while increasing vehicle- and passenger-safety at the same time.

Cyclic testing of the FRP-suspension arm has demonstrated that the component is able to withstand significantly increased loads for a higher number of loading cycles. This hints at further potential for optimizing its weight.

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Dominik Spancken
    • 1
  • Andreas Büter
    • 1
  • Paul Töws
    • 1
  • Oliver Schwarzhaupt
    • 1
  1. 1.Department Lightweight StructuresFraunhofer-Institute for Structural Durability and System Reliability LBFDarmstadtDeutschland

Personalised recommendations