The University of Siegen Has Been Using HBM Measurement Technology for More than Four Decades...

Investigating the Fracture Mechanics of Composite Materials

Since materials respond differently to static and dynamic loadings, the investigation of material behavior is a topic of interest for research teams. Mohammad Reza Khosravani, a doctoral student at the time and now a PhD in Mechanical Engineering, has recently explored “experimental investigations on the dynamic behavior and fracture of composite materials” at the Institute of Solid Mechanics at University of Siegen. “Over the past few years, we used HBM products for performing the above mentioned experimental research”, he explains. “In detail, we performed investigations on the fracture mechanics of two different composite materials—sandwich composite T-joints and ultra-high performance concrete (UHPC). In this regard, we used two split Hopkinson pressure bars (SHPB) in our laboratory which were equipped with HBM products and a high speed camera.” The results have been published in several peer-reviewed journals.

Strain Gauges Play a Major Role

HBM’s measuring system and strain gauges were used in the experimental tests on the sandwich T-joints. With the help of the strain gauges, the strain on the interface of the sandwich joint was determined. In a series of tensile tests under different loading rates, ultra-high speed video cameras were used for the digital image correlation (DIC) technique. The camera was triggered by a signal generated by the strain gauges mounted on the specimens. The images with the DIC technique allowed the researcher to determine the position of the crack and to study the process of the interfacial failure. Since these images are an important element of controversial data, the trigger signal from the strain gauges was a crucial issue that could successfully be solved by using HBM strain gauges. As Dr.-Ing. Mohammad Reza Khosravani sums up,

“With the data provided by the strain gauges and DAQ system, stress-strain curves of sandwich T-joints under static and dynamic loads were created. The obtained curves indicate the load carrying capacities of these structural elements.”

“One of the Most Powerful and Sophisticated Systems Available in the Global Market”

“When investigating the dynamic behavior of UHPC material, we used a split Hopkinson pressure bar (SHPB) and determined the mechanical properties of UHPC material under high rates of loading”, explains Dr.-Ing. Khosravani. Both incident and transmission bars as well as tested specimens were equipped with HBM strain gauges. The SHPB was connected to a Genesis HighSpeed GEN7t data acquisition system. This system is a fast data recorder that offers high flexibility and an outstanding performance. The DAQ system employed in the tests includes a transient RAM up to 10⁶ samples per second and allows for use of 4 channels at the same time. “The GEN7t mainframe holds 7 input cards and we could add the desired channels, which is a benefit of this system. HBM’s Genesis HighSpeed data acquisition system can be considered as one of the most powerful and sophisticated systems available in the global market. For various researches, we used different Genesis HighSpeed systems that are high-end DAQ systems combining transient recording and data streaming,” says Dr.-Ing. Khosravani. “DAQ systems produced by HBM can be configured for the intended duration and recording size. As an advantage of the HBM products, and particularly the Genesis HighSpeed series, it is worth mentioning that these systems can be extended for the desired purposes. In this respect, we used the basic data acquisition blocks and determined the capabilities that we needed in the system. The design of the hardware allowed us to configure the number of channels.”

A Smoothly Functioning Measurement Chain

Another advantage mentioned by the researcher is that the strain gauges and the measuring system are manufactured by the same producer and, thus, function extremely well together. In addition to the hardware, the research team used HBM’s Perception software which is a multi-language software.

“This is a unique high speed measurement software that automatically detects all available Genesis HighSpeed systems. With Perception, fast and reliable processing is achievable and StatStream technology enables the software to visualize up to 10 gigabytes of data in 10 seconds. In addition, control and updates can be carried out from any computer.”