PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Influences on structural intensity for injection moulded
thermoplastic parts
Dennis NETZBAND1; Andreas UJMA1; Elmar MORITZER2
1South Westphalia University of Applied Sciences, Germany
2

Paderborn University, Germany

ABSTRACTIn product development such as electro-mobility engineers have to place special emphasis on Noise Vibration
Harshness (NVH). A very large number of automotive interior components consist of injection moulded
thermoplastic parts. If required, these lightweight components are reinforced with fibres or fillings. This
reinforcement leads to a higher vibration sensitivity and higher natural frequencies. In order to influence theacoustic behaviour in early development stages, the structure-borne sound - including its source andpropagation paths - has to be analysed. A suitable tool to visualise these elements is the structural intensity.
In addition, the anisotropic material behaviour of reinforced thermoplastics offers the possibility to conduct
the structure-borne sound. In this paper a method is presented to examine the influence of fibres, fillings andgeometric modifications on the structural intensity. Based on an integrative simulation method (coupling ofprocess and structural simulation) an anisotropic, realistic material model will be generated.
Keywords: Structural intensity, reinforced thermoplastics, Simulation

1. INTRODUCTION
In the development of new products, a huge number of requirements has to be met. NVH has animportant role in this topic, because interferences give the impression that a particular product has asubstandard built quality. Therefore it is of special interest to predict the acoustic behaviour in earlystages of the product development. A goal should be to reduce the causal structure-borne sound insteadof the radiated airborne sound. Currently technology is not available to carry out such optimisation.
Automotive interior parts mostly consist of injection moulded thermoplastic parts which arereinforced by fibres or fillings. Through an internal or external excitation the components begin tovibrate and transmit these motions as structural sound. Finally the structural sound is e mitted fromthe surfaces as airborne sound into the vehicle interior.
An important tool for the acoustic part design of automotive interior products is the structuralintensity. Its calculation visualises the source as well as the propagation paths of the structure -borne
sound. For a frequency-dependent analysis “f” the structural intensity “STI” is calculated as theproduct of the stress tensor “σ” and the complex conjugated velocity vector “v” using the following
formula of Grote and Felhusen [1]:STI f
0.5V f  v f(1)
The structural intensity can be split into an active (real part) and a reactive (imaginary part) part.
Only the active part describes the amount of energy which moves through the structure [1]. The
reactive part indicates the amount of energy which oscillates in the structure. In the following only
the active part will be considered because of its greater interest.

2. INTEGRATIVE SIMULATION METHOD
The calculation of the structural intensity for reinforced thermoplastics takes the anisotropy fromthe fibre orientation as well as the viscoelastic material behaviour by the use of an integrative
simulation method into account [2]. Here a process simulation will be done and the calculated fib reorientation will be assigned to a structural simulation. By taking the material data of the matrix and4708

the filler as well as the fibre orientation into account a new material for each finite element of thestructural simulation will be generated. Based on the simulation results the structural intensity will becalculated automatically using Python scripts and will be displayed in the postprocessing. The Python
scripts are necessary because stress and velocity are calculated at different positions of the finiteelement. This leads to different numbers of stress and velocity results and therefore selected datashave to be averaged to get the same number of results for velocity and stress. The following figure
illustrates the integrative simulation method.

Figure 1 – Procedure integrative simulation method

3. VALIDATION OF THE SIMULATION RESULTS WITH A LINEAR ELASTIC,ISOTROPIC MATERIAL
To verify the simulation results an analytical model of the Kirchhoff plate is used [3]. Additionallya structural simulation was also performed. For this first test no integrative simulation method wasused.
For the FE-model the mesh was built of linear hexahedral elements. The centroid was chosen asthe position for the calculation. The material data is derived from steel with a density of 7.850 g/cm³,elastic modulus of 204.05 GPa and a Poisson’s ratio of 0.28.
The plate is flexibly supported all around and an excitation of a harmonic force of 1N acting on asingle point is applied in direction of the plate thickness [4]. Figure 2 includes the dimensions of theplate, the position of the harmonic force as well as the main orientation directions of the fibres fromthe process simulation.

Figure 2 – Dimensions of the plate, position of the force and main fibre orientations
At the fifth natural frequency a comparison between the analytical model and the FE analysis wascarried out. The desired natural frequency was 6,427 Hz in the FE analysis and 6,957 Hz in theanalytical model. Thus the discernible difference is less than 10 percent. Regarding the structural4709

intensity, quantitative differences could be recognized but the propagation paths of the structure-borne
sound from the FE analysis and the analytical model match (see figure 3).

Figure 3 – Propagation paths of steel plate
In the next step the FE analysis was performed with plates made of thermoplastics. For this model
the boundary conditions described in the previous section were used . The analysis is meant to show
if the generated materials for the FE elements are suitable for an anisotropic structural intensity
analysis. The selected materials are an unfilled polypropylene copolymer (PP-C) and a polypropylene
copolymer filled with 40 percent glass fibres by weight (PP-C GF40). The orientation of the fibres
was calculated through a process simulation.
As in the previous analysis for the steel plate the fifth natural frequency was analysed. The PP-C
has a natural frequency of 1,547 Hz. Regarding the plate made of PP-C GF40 one has to distinguishin which direction the fibres are orientated. With a longitudinal direction the natural frequency is2,211 Hz. A transverse direction leads to a natural frequency of 2,388 Hz.
The propagation paths of the structural intensity proceed diagonally for the unfilled polypropylene
copolymer plate. The plates consisting of PP-C GF40 show different propagation paths according tothe fiber orientations. This leads to a clearer difference between the filled and the unfilled PP -C for
the transverse direction (see figure 4).

Figure 4 – Propagation paths of thermoplastic plates (PP-C, PP-C GF40 longitudinal orientation, PP-C
GF40 transverse orientation)

4. SUMMARY AND NEXT STEPSThe results described in the previous sections clarify that the integrative simulation method is anappropriate technique to determine the structural intensity of reinforced, injection moulded
thermoplastic parts. For accurate estimations of the propagation paths information about the fibreorientation are necessary. Methods of obtaining this information are calculations (process simulation)or measurements (CT-Analysis). Investigations at other institutes on the structural intensity were done
primarily for linear elastic, isotropic materials. First contemplations on unidirectional fibre composite
materials revealed promising opportunities to influence the structure-borne sound of anisotropic

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materials with the help of the structural intensity [5]. In the near future these opportunities should be
examined in detail (see figure 5).

Figure 5 – Overview research project
The acoustic behaviour of reinforced thermoplastics should be improved by directing the structureborne energy into uncritical or high damping areas using the fibre orientation. Therefore analyses on
the importance of different fibres, lengths and filling fractions on the propagations paths of thestructural intensity will be executed. In addition the influence of geometric changes of the parts will
be examined as well. Based on the results, product developers will have the possibility to adjust thedesired mechanical properties and acoustic behaviour. This leads to a better lightweight potential, amore efficient material usage and a sustainable use of resources.
The calculations will be carried out for simple plate geometries as well as a complex geometry usedin the interior of a car. In order to prove the propagation paths and the efficiency on the acoustic
behaviour measurements will be undertaken.
An objective of the research project is to develop a database which contains a guide to improve
the acoustic behaviour with constructive changes and a database with the data of the examinedmaterials and with the ability to input criteria by which to select a material which is suitable for therequirements of a certain product.

5. ACKNOWLEDGEMENT
The research work will be carried out as a part of the research project “Acoustic -STRUKT –Development of fibre reinforced plastics as an improvement of the acoustic properties of lightweightparts using the structural intensity”. This research project is headed by Dennis Netzband M. Eng.,
Prof. Dr.-Ing. Andreas Ujma, Prof. Dr.-Ing Mark Fiolka and Prof. Dr.-Ing Andreas Nevoigt from theSouth Westphalia University of Applied Science in Iserlohn. Project partners are the KunststoffInstitut in Lüdenscheid, Leopold Kostal GmbH & Co. KG in Lüdenscheid, Möllertech GmbH inBielefeld and Ter Hell Plastic GmbH in Herten. The research project is sponsored by the Funds fromthe European Regional Development Fund and the Ministry of Economic Affairs, Innovation,Digitization and Energy of the Federal State North-Rhine Westphalia, Germany.

REFERENCES
1. Grote, K-H. und Felhusen, J.: Dubbel Taschenbuch für den Maschinenbau (0 3.6), 24. Auflage, SpringerVerlag, 2014
2. Gieß, M. und Netzband, D.: Integrierte Simulationsmethode für die akustische Formteilauslegung,Vortrag MSC Software Conference, Berlin, 24-26.10.2017
3. Hering, T.: Strukturintensitätsanalyse als Werkzeug der Maschinenakustik, Dissertation TechnischeUniversität Darmstadt, 2012
4. Pelaez, G.: Vergleich und Bewertung verschiedener numerischer Berechnungsmöglichkeiten derStrukturintensität, Masterthesis Technische Universität Darmstadt, 2016
5. Stoewer, T.: Berechnung der Strukturintensität von Fahrzeugstrukturen, Dissertation TechnischeUniversität Darmstadt, 2016

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