Kauçuk ömür testi için bir test düzeneğinin validasyonu
Validation of a rubber fatigue life test rig
Yazar: BAYBORA ÇORBACIOĞLU
Danışman: DOÇ. DR. ALPER TOLGA ÇALIK
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Otomotiv Mühendisliği = Automotive Engineering

Özet:
Kauçuk malzemeler, özellikle tekrarlı yükleme koşullarına maruz kalmakta ve çeşitli mühendislik uygulamalarında gerek birincil gerek ikincil fonksiyonlarda kritik bir rol oynamaktadır. Kauçuğun yorulma özelliklerini anlamak, ürün performansını ve dayanıklılığını öngörebilmek; kaliteyi ve emniyeti garanti ederken ticari olarak rekabetçi kalabilmek için hayati önem taşır.
Kauçuk yorulma testi cihazları, kauçuğun tekrarlayan yüklemelere karşı performansını test etmek için kullanılan makinalardır. Kauçuğu sabitlemek ve ölçüm sonuçlarının gürültüden etkilenmemesi için sağlam bir çerçeveye, gerilmeyi uygulamak için aktüatörlere ve bunu ölçmek için sensörlere sahiptirler. Kontrol sistemi, gerilme seviyelerini ayarlar, verileri kaydeder. Bazı cihazlar farklı ortamları (sıcaklık, frekans, ...) taklit edebilirler. Bu cihazlar, kauçuk ürünlerin kalite standartlarını karşıladığından ve parçalar gerçek koşullarda kullanıldığında istenen ömür dayanımına sahip olmasını sağlar. Kauçuk ömrü; malzeme özellikleri, çevresel faktörler, yükleme koşulları, yükleme geçmişi, parça geometrisi, imalat süreçleri, işletme koşulları ve yaşlanma etkileri gibi birçok faktörün kombinasyonuna bağlıdır. Tüm bu faktörlerin takip edilerek kauçuk yorulma davranışının incelenmesini zorlaştırmaktadır. Bu nedenle test cihazları belli başlı karakteristiklere, birbirine göre avantajlı ve dezavantajlı olduğu senaryolara sahiptir. Bu farklılıklara; test edilebilen sıcaklık aralığı, frekans aralığı, yükleme tipleri, ölçüm hassasiyetleri, parça boyutları vb. birçok etken örnek verilebilir.
R = -1, bileşenlerin simetrik tekrarlı yükleme koşullarına maruz kaldığı durumu temsil etmektedir. İncelenen test cihazının doğrulaması bu yükleme tipinde kauçuk ömrünün tespiti içindir.
Kauçuk sektöründe genelgeçer kabul edilen standart R = 0 durumu sadece çekme durumu için ömür testine yöneliktir. İncelenen diğer çalışmalarda farklı yükleme tipleri haricinde, R = -1 koşulunun farklı bir mekanizmayla sağlandığı ya da cihaz validasyon çalışmalarının zayıf olduğu gözlemlenmiştir. Bu çalışmayla silindirik numuneleri eksenine göre farklı açılarda gerdirerek konumlandırdıktan sonra motordan alınan güç aracılığıyla numunelere dönme hareketi vermek suretiyle numunede sıkışan bölgede basma, genleşen bölgede çekme gerilmesi uygulamak suretiyle; sinüzoidal basma-çekme yükleme koşulunda kauçuk numunelerin ömür dayanımları tespit edilebilecektir. Yeni cihazla ve bu cihaza uygun test takozlarından elde edilen Wöhler eğrisiyle hesaplanan ömür tahminlerinin fiziki şaft askı lastiği ömür testlerinden çıkan sonuçlarla örtüşüp örtüşmediği irdelenmiştir.

Abstract:
Rubber materials are subjected to cyclic loading conditions, playing a critical role in various engineering applications, both in primary and secondary functions. Understanding the fatigue properties of rubber is vital for predicting product performance and durability, ensuring quality and safety, and remaining competitive in the market.
Tension and compression stresses in rubber lead to different mechanical responses due to the unique nature of rubber as a material. Here's how tension and compression stresses affect rubber differently. Rubber is highly elastic in tension, meaning it can deform significantly under tension stress and return to its original shape once the stress is removed while it is also highly resilient in compression, meaning it can recover its original shape after being compressed. Rubber generally exhibits stiffer behavior in compression compared to tension, meaning it requires higher stresses to achieve the same level of deformation in compression as in tension. In addition, rubber materials exhibit hysteresis, which means that its response to loading and unloading is not entirely reversible. This phenomenon is more pronounced in compression compared to tension.
Rubber fatigue testing devices are machines used to test the performance of rubber under repeated loading conditions. They consist of sturdy frames to hold the rubber, actuators to apply stress, and sensors to measure it, ensuring that measurement results are not affected by noise. The control system adjusts stress levels and records data. Some devices can mimic different environments (temperature, frequency, etc.). These devices ensure that rubber products meet quality standards and have the desired lifespan when used under real conditions. However, the study of rubber fatigue behavior is inherently complex due to the interplay of numerous factors including material properties, environmental conditions, loading history, part geometry, manufacturing processes, operating conditions, and aging effects. These multifaceted considerations necessitate testing devices with specific characteristics tailored to different testing scenarios, each with its own advantages and disadvantages. Therefore, testing devices have specific characteristics, advantageous and disadvantageous scenarios compared to each other. These differences include factors such as the range of testable temperatures, frequency ranges, loading types, measurement accuracies, part sizes, etc. so that they can offer the capability to mimic various environmental conditions such as temperature and frequency. By simulating real-world conditions, these testing devices ensure that rubber products meet stringent quality standards and possess the desired lifespan when deployed under actual operating conditions.
R = -1 represent the situation where components are subjected to symmetrical cyclic loading conditions. The validation of the examined test device is for the determination of rubber life under this loading condition.

One critical aspect of fatigue testing is the validation of devices under R = -1 loading conditions, where components are subjected to symmetrical cyclic loading conditions. Despite its importance, many studies lack thorough validation under this condition, often resorting to the conventional R = 0 standard, which fails to fully capture the behavior of materials under compression. In the examined studies, except for different loading types, it has been observed that the R = -1 condition is achieved through a different mechanism or that the validation studies of the device are not carried throughly. The worldwide used ISO 6943 and ASTM D4482 standards used for rubber fatigue tests is also a pure tensile test which represents the R=0 condition. It doesn't account for the behaviour of the material in the compressed state which is known to lead to better strength and fatigue performance of strain crystallizing rubbers like natural rubber.
To address this limitation, this study focuses on determining the fatigue resistance of rubber samples under sinusoidal compression-tension loading conditions. This novel approach involves a rubber fatigue test rig which can utilize cylindrical shaped rubber samples, also known as diabolo specimens, at various angles relative to their axis and applying compressive stress in the constricted region and tensile stress in the expanding region. The rig can hold and test up to 6 diabolo samples for fatigue test at 10 Hz. Through the power obtained from the motor, the fatigue resistance of rubber samples under sinusoidal compression-tension loading conditions will be determined. To create the Wöhler curve of our rubber compounds for compression-tensile loading scenario, we need to extract two information. Fatigue cycle counts and the strain values corresponding to the aforementioned fatigue cycle value. Samples are placed at angles of 10, 20, 30 and 45 degrees relative to the center of the shaft connected to the rotor. For each angle value the fatigue test has been repeated 6 times. Meanwhile finite element analysis has been conducted to measure the strains at each region to determine the compressive strain at constricted region and tensile strain at expanding region. Total strain has been calculated from the computed compressive and tensile strain, which is then used to determine the strain of the fatigue cycles denoted in our Wöhler curve. To validate the correct determination of fatigue resistance, the consistency of the Wöhler curve obtained with this device has been compared to the test results performed on a center bearing according to the specified test procedure of the product.
A separate literature review has been carried out to determine the acceptable coefficient of determination (R²) value and the acceptable limits of validation tests according to the obtained Wöhler curve.
Coefficient of determination value represents the deviation of the diabolo sample test results from our ideal Wöhler curve. It has been observed that a value as low as 0,85 is deemed to be acceptable for the coefficient of determination, the coefficient of the obtained Wöhler curve is deemed to be acceptable and shows that diabolo sample fatigue tests results do not scatter from our Wöhler curve.
The research has shown that a prediction within a factor of two (when the experiment output is within half or twice of the calculated result) is a widely accepted criterion and can be used for center bearing validation results to consider a prediction of rubber fatigue test result as successful. The results of the fatigue test conducted on physical parts fall within the range of 1.4 million to 2 million cycles for three center bearing samples which is within the lower and upper limits set with a limit factor of 2.
In conclusion, physical tests conducted on center bearing samples demonstrate remarkable alignment with these criteria, affirming the fidelity of the testing device

and procedures in faithfully replicating compression-tension stress scenarios. This comprehensive approach not only enables precise interpretation of fatigue performance data but also empowers stakeholders with actionable insights for informed decision-making, ultimately enhancing product reliability and customer satisfaction