2 Materials and tribological phenomena 2.1 Materials This research looks at all materials used in automotive interiors. The spectrum includes leather and artificial leather, plastic components and complex elastomer profile seals in the door area. A comprehensive change in material development can currently be observed. In particular, the market for sustainable alternative materials to leather and artificial leather is seeing the continuous development of new variants of materials with properties that are as comparable as possible [1]. New material alternatives are also constantly being developed in the sealing sector in terms of elastomer composition and surface coatings. This involves a large number of new materials made from renewable raw materials. However, their physical and chemical properties differ from those of conventional materials, particularly on the surface. This has an influence on the resulting material properties, in particular on friction. 2.2 The stick-slip phenomenon Research on stick-slip phenomena spans both fundamental mechanics and applied tribology. Early reviews Science and Research 34 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0032 1 Introduction The prevention of background noise is becoming increasingly relevant in the modern automotive industry. It is not only a decisive factor for the perceived quality of the vehicle, but also plays a key role in creating a pleasant driving atmosphere. At a time when vehicles are more than just a means of transport, interiors are developing into complex comfort zones that appeal to all the occupants' senses. The traditional focus on visual and tactile aspects of interior design is now being expanded to include an acoustic dimension. In this context, disturbing noises caused by friction of materials are becoming particularly relevant. These noises, which are often subtle but certainly disturbing, can significantly impair the perception of vehicle quality, which is why they must be taken into account with due care during development and production. At the same time, the automotive industry is faced with the challenge of developing and using sustainable materials. The trend towards sustainability requires a change in the composition and properties of the materials used, which in turn can have an impact on their acoustic behaviour. In this area of conflict between comfort, quality and sustainability, automotive manufacturers (OEMs) and their suppliers are required to develop innovative solutions. The identification of material combinations that are free from disruptive noise sources and at the same time meet the requirements of sustainability and cost-effectiveness is a complex task. Particular attention must be paid to the stick-slip behaviour of materials, as this phenomenon is often the cause of unwanted noise. Overcoming these challenges requires not only a deep understanding of the underlying physical principles, but also the development and application of advanced testing and analysis methods. These methods must therefore be able to simulate and analyse the complex interaction of different materials under realistic conditions. Practical noise or stick-slip prevention in automotive interiors Martin Strangfeld, Susanne Fritz* The prevention of disturbing noises in the context of the automotive environment contributes to increasing vehicle quality and creates a pleasant driving atmosphere by minimising unwanted noise sources. The stick-slip behaviour of material combinations is of particular relevance here, as stick-slip is a frequent cause of disturbing noises and can be avoided through the targeted selection of suitable materials. There are already test methods for characterising the stick-slip behaviour, which are applicable in principle, but often have limitations in their significance. There is a clear trend towards increasingly realistic simulations of material combinations, taking into account real load scenarios. However, this poses a challenge for testing technology and the automated analysis of the results. Keywords Stick-slip, friction, real vehicle excitation, evaluation algorithms, friction force, automotive interior Abstract * Dr.-Ing. Martin Strangfeld Orcid-ID: https: / / orcid.org/ 0009-0003-3570-4286 Dr. rer. nat. Susanne Fritz Department Surfaces, FILK Freiberg Institute gGmbH Meißner Ring 1-5, 09599 Freiberg, Germany established the nonlinear dynamics of dry friction and stick-slip [2], with further analysis of mechanical stickslip vibrations through bifurcation and chaos theory [3]. More recent studies focus on polymers, linking deformation behavior to stick-slip [4], identifying distinct adhesive stick-slip modes [5], and characterizing tribological performance of EPDM under varying velocities [6]. The practical relevance is evident in engineering, where stick-slip underlies noise, vibration, and durability issues such as automotive buzz, squeak, and rattle [7]. Over the last few decades, noise in vehicle interiors caused by stick-slip behaviour has developed into a considerable economic problem, as noise is the cause of around ten percent of all complaints in the automotive sector. The stick-slip effect can be explained by comparing the frictional force and the force required to move the two bodies against each other from their rest position. This shows that the frictional force in motion is usually only slightly smaller than the displacement force mentioned above. If the sliding friction is significantly exceeded by the static friction, the energy required to overcome the static friction is no longer needed when sliding. As a consequence, the body is accelerated during sliding, which exceeds the acting force. As a result, the body reduces its speed and returns to its resting position until the acting force is sufficient to overcome the static friction. Figure 1 illustrates the changing behaviour of the frictional force F over time t during a stick-slip movement, taking into account the acceleration pulses a that occur when the static force is overcome. The characteristic acceleration pulses are analysed on a stick-slip test rig to identify the stick-slip behaviour. The dependence of the friction coefficient µ on the speed in a specific tribological system is shown in the form of a so-called Stribeck curve [9]. Figure 1 (right) shows the typical curve and the area with an increased risk of stick-slip. The phenomena mentioned are highly dependent on the material combination under consideration. 3 Current prevention of stick-slip behaviour Current research is evaluating various methods for assessing noise at different stages of vehicle development [10]. - Subjective assessment by test drivers is way of preventing disturbing noises. Here, a driver assesses the resulting noise in terms of its disruptive potential while driving the vehicle on a real road or a special test track with different road surfaces. However, this method can only be used at a late stage of development (prototype) or if problems occur. - Another option is noise measurement on the vehicle. Here, the jolting movement is stimulated by driving on the test track, using shaker systems or chatter rollers. The noises are recorded using objective methods, such as microphones, acoustic cameras or vibroacoustic measurements, and localised if necessary. - A different way of assessing noise is to measure the noise on the module. In an earlier development phase, it is possible to test individual modules in the same way as the entire vehicle, using special shaker systems in acoustic rooms. - The material pairing can be tested at a very early stage of development, whereby the material selection can be tested on the creaking or stick-slip test bench independently of the subsequent installation situation and without the need to produce prototypes. As part of this test, the noise is not measured directly, but the friction between the friction partners is assessed to determine whether stick-slip effects can potentially cause noise interference. Particularly in the initial phase of product development, material selection can be evaluated using a stick-slip test, which allows material combinations to be assessed in a controlled environment (temperature, relative hu- Science and Research 35 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0032 Figure 1: (left) Friction force and acceleration curve for stick-slip, (right) Stribeck curve [8] The stick-slip behaviour is quantified in accordance with VDA 230-206 using the so-called risk priority number (RPN) [11]. The calculation is based on the acceleration curve. As part of this approach, the height, area and frequency of acceleration peaks are evaluated. A grading scale was derived on the basis of the practical behaviour of the materials (see figure 3). Materials with a RPN between 1 and 3 are categorised as “acceptable” with regard to the stick-slip effect. Materials with a RPN between 4 and 5 are categorised as “conditionally acceptable”, while materials with a RPN between 6 and 10 are described as “unacceptable”. The frequency of pulses that occur over the travel distance is referred to as the pulse rate. This standard and the RPN scale enjoy a high level of acceptance worldwide and can be found in a large number of automotive group standards. For reasons of economy, VDA 230-206 was limited to the consideration of four different normal force/ speed combinations, although in reality a wide and varied load spectrum acts on the individual friction partners due to the stochastic jerking motion when driving. Especially Science and Research 36 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0032 midity), regardless of the final installation scenario. This approach saves resources that would otherwise be required for the production of prototypes. No direct quantification of noise is carried out as part of this process. The evaluation of the friction between the friction partners serves to identify potential noise disturbances that can occur as a result of stick-slip effects. Here, a specially designed friction test rig, the SSP 04 from Ziegler Instruments, is used to test the stick-slip effect. The test system is used to analyse the stick-slip tendency of material pairings, taking into account various influencing factors, including load, relative speed and climatic parameters. In this configuration, one test specimen is connected to a sled, while a second test specimen is attached to a steel spring (see Figure 2). The stick-slip test rig enables the precise determination of static and dynamic friction as well as stick-slip phenomena. The stick-slip tendency is determined on the basis of the interaction between the two test specimens. The data obtained with the test rig allows an evaluation of the stick-slip tendency of material pairings as well as an insight into their performance under different conditions. Figure 2: Measuring principle of the stick-slip test rig Figure 3: Background of RPN-calculation for the relative velocity range a real spectrum can have velocities up to 120 mm/ s. 4 Trends and current challenges When analysing the tribological behaviour of the material contacts in more detail, the excitation conditions primarily manifest the friction behaviour induced by the real vehicle movement. The friction behaviour based on stochastic movements is often not adequately represented by the application of the VDA conditions. This can lead to a misjudgement of the material behaviour and consequently to a large number of complaints. For this reason, a detailed analysis of the excitation has been carried out in recent years, which was then measured and evaluated under controlled conditions on the test bench. There are now group standards from OEMs that specify concrete stochastic signals for determining stick-slip. Up to now, however, the evaluation has been based on direct noise measurements. However, a material test in the laboratory always represents an abstraction of the real structure, so that the noise cannot always serve as an indicator. It is therefore necessary to extend the evaluation options to the resulting response friction signals at the same time as the process. Figure 3 provides an overview of the excitation conditions prevailing in the interior and their possible response signals. For the special case of elastomer profile geometries, the latest generation of the stick-slip tester has a wide range of equipment options available to take the special conditions of the seals into account. These include special specimen grips and the option of adjusting the angle of attack and overpressure of the seal in a targeted manner. However, gaskets are characterised by their high deformability and the resulting high damping constant. They also act like a kind of spring due to their elastic properties. These properties mean that if stick-slip effects occur Science and Research 37 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0032 Figure 4: Overview of the variety of different excitation and response signals (excitation is shown in blue in the figure, friction forces as response in orange) up the possibility of using various test benches and allows a more differentiated distinction between excitation and stick-slip signals. This allows the range of applications to be extended to a wider range of materials, load scenarios and measurement conditions. Two different algorithms were designed to quantify stick-slip: The primary algorithm is optimised for the analysis of high-frequency sampled friction force signals (sampling rate ≥ 1 kHz). After eliminating excitation-related oscillations, a theoretical acceleration signal is derived from the friction force signal via the relationship between the deflection path of the leaf spring and its change over time, from which a friction force-based RPN grade (FRPN) is then calculated following the original RPNcalculation. In standard-compliant tests, there is a high correlation between RPN and FRPN, with a correlation coefficient of 0.996 (see Figure 4 left). In 97 % of all measurements, the deviation is below the specified measurement uncertainty of the RPN. The innovative methodology now also allows the quantification of stickslip behaviour for damping materials, at increased excitation speeds, for variable movement profiles and on various friction test rigs. An alternative quantification approach was developed for friction test rigs with a lower sampling rate. Using easily extractable input variables, such as amount of force drops per millimetre and force drop height (see Figure 4 right), and applying the statistical method of multinomial logistic regression, approximate values for the FRPZ can be generated. A significant correlation to the RPN with a correlation coefficient of 0.973 can also be seen in standardised tests. The implementation of the FRPZ addresses a significant gap in preventive noise analysis and opens up perspectives for a more precise, more realistic and more versatile Science and Research 38 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0032 during the measurement, the acceleration pulses arriving at the acceleration sensor are significantly damped. Despite visible breaks in the friction force curve and/ or audible noises, only very low values are calculated for the risk priority number. In this respect, it should be mentioned that all other interior materials with a high damping effect have a similar property. The current generation of stick-slip test rigs is able to excite the sample with a stochastic motion profile. However, excitation-related acceleration peaks manifest themselves during such a measurement, which cannot be differentiated from the stick-slip-related acceleration pulses with the current evaluation. Consequently, when using the stochastic motion profile, it is currently not possible to calculate the RPN and therefore no valid stick-slip quantification is possible. 5 Current research 5.1 Friction force-based evaluation without the influence of acceleration In order to solve the problems of determining the RPN under stochastic excitation, a new method for determining the RPN based on the friction force has been developed (FRPN). When designing the algorithm, particular emphasis was placed on making the generated results compatible with the established RPN evaluation in the context of standard-compliant test procedures. In contrast to the conventional method, which is based on analysing acceleration curves, the newly developed algorithm focuses exclusively on evaluating friction force curves. Stick-slip phenomena are quantified on the basis of continuous force drops in the friction force values, whose frequency and amplitude are used as parameters. The preference for friction force curves opens Figure 5: Agreement of the FRPZ with conventional calculations on the left and an alternative approach for friction test benches with a low sampling rate on the right evaluation of friction instabilities. In addition, it is now possible to carry out the analysis for only a part of the friction measurement. This makes it possible to determine and analyse friction curves under continuous load variation in an experiment and thus determine stick-slip as a function of variation which has the potential to optimise the time and cost efficiency of complex test procedures. The algorithms developed were transferred into a software solution and are available for future stick-slip tests. 5.2 Friction Force drop identification algorithm - frictional distance to slip In vehicle development there are different approaches to prevent stick-slip. One of them is to reduce friction so that there are no occurrence of certain force drops of the friction force. The other one is to have a certain sticking behaviour and prevent the material combination from the transition from stick to slip [12]. In particular, for materials that exhibit pronounced deformability, such as elastomeric profile seals or highly elastic cushioning materials, the precise determination of the transition from the sticking to the sliding phase is of significant relevance. For this purpose, an automated detection algorithm was developed that functions independently of the specific excitation and quantifies the distance between the sticking and sliding transition (force drop) in soft materials. The algorithm developed is based on the identification of the exact point at which the gradient of the friction force curve either undergoes a significant change in sign or a substantial reduction (see Figure 6). The slope is determined with constant precision over a predefined data range. It is essential that the calculation is independent of the original excitation signal in order to eliminate potential influences from intrinsic noise and ensure precise analysis. The gradient or slope is analysed successively along the friction force signal. A change in the sign of the gradient indicates a transition between the tribological states of sticking and sliding within the friction pairing. Furthermore, a significant change is diagnosed if the gradient shows an abrupt reduction to at least 20 % of the initial value. In such cases, unstable friction behaviour is defined. The distance to the initial force drop is calculated as the difference between the position values at the point of gradient change and the last reversal point of the test rig’s axis of movement. The resulting distances can then be visualised in relation to the load parameters in order to analyse the influences of varying parameters and derive a worst-case scenario. The algorithm is characterised by its universal applicability to various forms of excitation in different test scenarios, whereby stochastic patterns are also taken into account in addition to linear and sinusoidal movement patterns. This ensures a high degree of realism in the analysis. The methodological innovation presented allows a detailed investigation of the influences of varying normal forces and speeds as well as an evaluation of climatic factors on tribological behaviour. The knowledge gained can be used to optimise the prediction models for the friction behaviour of complex vehicle systems, whereby acoustic phenomena such as squeaking noises can be minimised. Science and Research 39 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0032 Figure 6: Approach for the force drop identification algorithm References [1] Meyer, M., Dietrich, S., Schulz, H., Mondschein, A. (2021). Comparison of the Technical Performance of Leather, Artificial Leather, and Trendy Alternatives. Coatings, 11, 226. https: / / doi.org/ 10.3390/ coatings11020226 [2] Feeny, B., Guran, A. S., Hinrichs, N., & Popp, K. (1998). A historical review on dry friction and stick-slip phenomena. [3] Galvanetto, U., Bishop, S. R., & Briseghella, L. (1995). Mechanical stick-slip vibrations. International Journal of Bifurcation and Chaos, 5(03), 637-651. [4] Dong, C., Yuan, C., Bai, X., Qin, H., & Yan, X. (2017). Investigating relationship between deformation behaviours and stick-slip phenomena of polymer material. Wear, 376, 1333-1338. [5] Viswanathan, K., Sundaram, K., (2017). Distinct stickslip modes in adhesive polymer interfaces. Wear, (2017), 376. Jg., S. 1271-1278. [6] Sun, Q., Wang, S., & Lv, X. (2022). Research on stick-slip behavior and tribological properties of ethylene-propylene diene monomer under various wear velocity conditions. Polymer International, 71(8), 985-990. [7] Trapp, M., Chen, F. (2012). Automotive Buzz, Squeak and Rattle - Mechanisms, Analysis, Evaluation and Prevention, Elsevier [8] Klotzbach S. (2002). Ein nichtlineares Reibmodell für die numerische Simulation reibungsbehafteter mechatronischer Systeme; ASIM, Rostock. [9] Rorrer, R.(2000). A historical perspective and review of elastomeric stick-slip, Rubber chemistry and technology, 73(3): 486-503 [10] Moosmayr, T. A. (2009): Objektivierung von transienten Störgeräuschen im Fahrzeuginnenraum, Fortschritt-Berichte VDI, VDI Verlag, Düsseldorf [11] VDA - German Association of the Automotive Industry, VDA 230-206 (2021/ 10) - Examination of the stick-slip behavior of material pairs [12] Benhayoun, I., de Faverges, A., Bonin, F. et al. (2017). Less Interior Squeak and Rattle Noise Using a Simulation Driven Design Approach. ATZ Worldw 119, 36-41 Science and Research 40 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0032 6 Discussion and outlook The algorithms developed represent significant progress in the quantitative analysis of friction force curves for a wide range of interior materials. The versatile applicability with regard to various forms of excitation and their high precision open up new insights in the field of tribology and offer innovative approaches for the optimisation of vehicle components. The current development of test bench technology creates optimum conditions for the effective implementation of these algorithms. The current trend towards realising test conditions that are as close to reality as possible with the aim of achieving maximum informative value is leading to the design of test benches with synchronous excitation in all three spatial directions. This development requires an increased need for advanced analysis methods for the resulting complex data structures. The combination of realistic test conditions with precise data analysis opens up the possibility of a substantial improvement in preventive noise analysis. This methodological innovation has the potential to significantly reduce time-consuming and cost-intensive prototype measurements using shaker systems or comprehensive complete vehicle analyses. It allows more precise prediction and earlier prevention of acoustic phenomena, which can significantly increase both the efficiency of the development process and the quality of the end product. Future research approaches should focus on further refining the algorithms to handle even more complex data structures and on integrating these methods into holistic vehicle development strategies. Acknowledgement The presented results are part of the research projects (“Dist2Slip - determination of the distance to first slip”, 49MF210118 as well as “Stick-slip-quantification through friction force”, 49MF180089) and partly funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) based on a resolution of the German Bundestag via the project management organization EuroNorm GmbH. Gratitude is expressed for the provided support.