Characterizing the crush performance of composite flat plates and profiles
Creators: name: Lausch, Jonas
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400202
subject: Automotive engineering materials
classificationCode: 400204
subject: Automotive safety engineering
classificationCode: 401706
subject: Numerical modelling and mechanical characterisation
classificationCode: 401707
subject: Solid mechanics

description: Lightweight materials have always been of interest to the automotive industry. Besides the beneficial properties of such materials, higher material and manufacturing costs emerge, which are significantly reflected in the testing procedure and, therefore, demand cost-effective methods to accurately determine the crush properties of these materials.
 
Flat plate specimens offer potential cost savings due to their easy manufacturability. By investigating the crushing morphology of a fully supported flat plate specimen, it is found that two factors influence the energy absorption process during crush testing. One is the formation of the crushing frond, and the other is the partial separation of the specimen due to the support and the impactor. Conducting crush tests with different crushing widths revealed that the crushing force consists of a linear but non-proportional force-width relationship, implying that the tearing at the edge of the impactor is independent of the crushing width. Subtracting this tearing component from the original measured force results in the crushing component, which represents the frond formation. The specific energy absorption (SEA) is often used to compare the potential of different materials. Contrary to the name, the SEA usually depends on the specimen geometry or the used test setup. Such dependency can be observed when the SEA is calculated from the measured force of flat plates with different crushing widths, resulting in decreased SEA values. On the other hand, calculating the SEA from the isolated crushing component leads to consistent values for different crushing widths.

In order to validate the proposed approach, a comprehensive study is conducted for several parameters of discontinuous carbon fiber composite flat plates manufactured by compression molding. The effect of fiber length on fracture toughness agrees with the general results found in the literature. Increasing the fiber mass content resulted in a slightly insignificant energy absorption increase. Mainly both parameters affect the standard deviation, as they influence the homogeneity of the fiber distribution. The in-plane fiber distribution due to different production directions shows almost no influence on fracture toughness. Increasing the specimen thickness leads to an increase in fracture toughness as the level of fragmentation decreases and the coherence of the frond formation during the crushing process increases. Dynamic testing of the specimens revealed a loading rate dependency due to an increase in fragmentation and separation of individual layers of the crushing frond.

Although the flat plate crush testing methods are very helpful for developing structural components, the obtained properties need to be transferred accurately. Testing self-supporting structures do not need an extra test frame, but the obtained results also have geometric dependencies. The crushing morphology of right-angled profiles is typically composed of tearing at the corner and forming a crushing frond in the flat segments. This mechanism implies that the corner tearing is most likely independent of the profile width and depends only on the number of corners in the profile. Accordingly, a technique was developed and utilized to isolate the width-independent tearing component at the corner from the width-dependent crushing component of the flat segments. This separation allows for characterizing the geometric dependency of crush-tested profiles on the tearing occurrences. In addition, the crush properties of profiles with different shapes and dimensions can be predicted from crush testing a few specimens of different sizes, as long as the corner geometry is identical. The SEA calculated from the crushing force of the flat segments showed to be practically the same for different values of segment width of right-angled profiles and flat plates, which explains the differences in crush performance when comparing the properties obtained from testing composite profiles and flat plates.