Significant lightweight construction advantages in complex adhesive structures in vehicle manufacturing

Dr. Halvar Schmidt, Fraunhofer Institute for Structural Durability and System Reliability LBF

Interview: Dr. Halvar Schmidt, winner of the HBM Award 2014

As part of the fourth Symposium on Structural Durability in Darmstadt' (SoSDiD), Dr. Halvar Schmidt, a scientist at the Fraunhofer Institute for Structural Durability and System Reliability LBF, was presented with the HBM Award for his dissertation "Vibration strength analysis of structural bonded joints under a load with variable amplitudes."

Designing complex adhesive structures in automobile manufacturing while achieving significant advantages from lightweight construction – that is what is especially noteworthy in the calculation procedure developed by Dr. Halvar Schmidt. He reveals in an interview what advantages this procedure promises and what it means for automobile development.

You developed a calculation procedure that makes it possible to exploit significant lightweight construction advantages in complex adhesive structures in vehicle manufacturing. What are the main advantages of this process?

One great advantage is certainly that it is application-oriented. The procedure was designed to ensure this both in terms of experimental determination of parameters and computational fatigue life analysis. This includes the specimens and loading considered (see figures 1 and 2) as well as the constraints on the computational process. For example, purely linear-elastic calculations with short computing times and ensured scalability quickly yield tangible results.

It was also possible to demonstrate experimentally that there is also potential for lightweight construction with bonded joints if actual loads varying in a random manner are taken into consideration (see figure 3). 

Fig. 1: Sample similar to actual component (actual on the left, CAD model with parts of the mounting on the right)
Fig. 2: Load spectrum used, 'CARLOS vertical, modified'

What is the concrete significance of the results of your research for automobile developers? Will it speed up the development of innovative body construction methods?

The complete research work is not going to lead to a revolution in the automobile industry. However, adhesive technology is still in its infancy in the area of durability. The results thus constitute a contribution towards being able to better evaluate bonded joints as part of the design of joined structures in terms of their vibration strength, to make better use of their full potential. This makes it possible for example to avoid overdimensioning and the unnecessary weight and costs for prototypes associated with it through more reliable computational analysis.

As part of preliminary examinations with thin welded sheet metal, failure criteria and methods for detecting incipient cracks were identified in conjunction with tests. Both strain gauges and thermoelastic stress analysis were used to analyze cyclic rigidity and stress. What are the advantages of strain gauges? What made you decide to use strain gauges from HBM?

The advantage of strain gauges in general is their ability to measure local strains in a precisely definable area, reliably and very sensitively. When incipient cracks have been detected, this makes it possible to respond to the tiniest changes (in this case surface strains resulting from shifts in stress due to the formation and progress of cracks) in the area of the joined connection. Strain gauges from HBM were used because of excellent experience in the past with applications, reliability and measurement accuracy. A suitable and extensive range of strain gauges for our purposes was also available, see figure 4.

Fig. 4: Spot-welded sample with SG applied
Strain gauge measurement on a spot-welded sample
Bonded sample with SG applied

Improvements in computational fatigue life analysis support lightweight construction and the increasing use of adhesive structures in vehicle construction. You use nCode GlyphWorks software, among others. How do you evaluate the performance capability of this software solution?

First, the software you are referring to is very user-friendly. In addition to the control interface, that also includes options for further data processing and adaptations for individual requirements, which are especially relevant when developing or expanding calculation methods. The concepts and methods that are already implemented, together with nCode DesignLife, are also very extensive and offer a correspondingly wide range of practical uses.

Fig. 5: nCode GlyphWorks from HBM
Fig. 6: nCode DesignLife from HBM

Your work demonstrates that a differentiated consideration of the initiation and progression phases of cracks is necessary to be able to apply fatigue life analysis based on fracture mechanics to components in a meaningful way. What do you believe is the significance of fiber-optical sensor gratings in this regard?

As is so often the case, of course, this depends on the specific application. In general, however, this is a very promising technology in terms of a more precise consideration of the service life and especially the failure behavior of components or structures. Frequently there is insufficient knowledge in this area, despite the vital importance of fiber-optical sensor gratings in generating the basic data for computational design, for example. Being able to integrate fiber-optical sensor gratings in fiber-reinforced plastics, which are being used more frequently now, or in a high-voltage environment (in electromobility, for example) offer some promising options.

Fig. 7: OptiMet OMF singlemode fiber from HBM

Are you able to provide a perspective of your future research work at this point?

There has been and still is work to do in the area of durability of joined connections, especially adhesive connections, to achieve the most comprehensive understanding possible. This relates to topics where there are still gaps in our knowledge regarding adhesive connections, for example computational fatigue life analysis of adhesive structures under multiaxial loading, as well as questions such as the vibration strength of structural bonded joints, where the level of knowledge can be further improved. For example the Fraunhofer Durability Laboratory in Darmstadt, Germany is conducting tests in this area on graded layers of adhesive using dual-cure adhesives recently developed for plastics.

I can say that more of my personal focus in the future will be devoted to the general topic of durability in complete vehicles.

Symposium on Structural Durability in Darmstadt (SoSDiD)

Fraunhofer Institute for Structural Durability and System Reliability LBF

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