Papers by Keyword: Ultrasonic Fatigue

Abstract: Structural aircraft components are often subjected to more than 108 loading cycles during their service life. Therefore the increasing use of carbon fiber reinforced polymers (CFRP) as primary lightweight structural materials leads to the demand of a precise knowledge of the fatigue behavior and the corresponding failure mechanisms in the very high cycle fatigue (VHCF) range. To realise fatigue investigations for more than 108 loading cycles in an economic reasonable time a novel ultrasonic fatigue testing facility (UTF) for cyclic three-point bending was developed and patented. To avoid critical internal heating due to viscoelastic damping and internal friction, the fatigue testing at 20 kHz is performed in resonance as well as in pulse-pause control resulting in an effective testing frequency of ~1 kHz and the capability of performing 109 loading cycles in less than twelve days. The fatigue behavior of carbon fiber twill 2/2 fabric reinforced polyphenylene sulfide (CF-PPS) and carbon fiber 4-H satin fabric reinforced epoxy resin (CF-EP) was investigated. To study the induced fatigue damage of CF-PPS and CF-EP in the VHCF regime in detail, the fatigue mechanisms and damage development were characterized by light optical and SEM investigations during interruptions of constant amplitude tests (CAT). Lifetime-oriented investigations showed a significant decrease of the bearable stress amplitudes of CF-PPS and CFEP in the range between 106 to 109 loading cycles. The ultrasonically fatigued thermoset matrix composite showed a significantly different VHCF behavior in comparison to the investigated thermoplastic matrix composite: No fiber-matrix debonding or transversal cracks were present on the specimen edges, but a sudden specimen failure along with carbon fiber breakage have been observed. The fatigue shear strength at 109 cycles for CF-PPS could be determined to τa, 13 = 4.2 MPa and to τa, 13 = 15.8 MPa for the thermoset material CF-EP.

Abstract: A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of Ti-17. Ultrasonic (20 kHz) fatigue tests were performed at room temperature, 200°C and 350°C, respectively. The dynamic Young’s modulus and fatigue endurance limit decrease with increasing temperature linearly. Rotating bending (50 Hz) tests were performed to evaluate the influence of loading frequency at room temperature, 200°C and 350°C, respectively. There is an obviously loading frequency effect at elevated temperature, although no loading frequency effect at room temperature.

Abstract: Very high cycle fatigue is often implemented with high test frequency to save time. The thermal dissipation accompanied with the high frequency appears and induces the temperature increment in the speicmen’s surface. The dissipation power is important and closely related with VHCF performance. The infrared camera is used to acquire the temperature distribution and evolution with a lot noisy which bring difficulties for the calculation. In the article, a thermal dissipation power calculation with time and space filtering method is proposed to give out the thermal dissipation. The two stages dissipation phenomenon with large difference is found out in the process of VHCF test.

Abstract: Carbon fiber reinforced polymers (CFRP) are getting more and more important for structural components for automotive and aerospace applications. These components are subjected to 10¹¹ and more loading cycles during their time in service. Therefore the VHCF behavior and the corresponding failure mechanisms have to be well understood. To obtain a comprehensive knowledge about the fatigue behavior and failure mechanisms of CFRP in the VHCF regime, a new Ultrasonic Testing Facility (UTF) for cyclic 3-point bending at 20 kHz has been developed at WKK. This UTF with combined online nondestructive testing via laser vibrometry and IRthermography enables VHCF experiments up to 10⁹ cycles in twelve days without overcritical heating up to the glass transition temperature of the polymer. The chosen material in this research project is the commercially available and aircraft qualified carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). To determine the fatigue characteristics of CF-PPS load increase tests and based on these results constant amplitude tests up to 10⁹ cycles have been carried out. Light optical and SEM microscopy have been performed in defined fatigue states or finally after reaching 10⁹ cycles with shear stress amplitudes of at least 44% of the monotonic ultimate shear strength. The induced fatigue damage of CF-PPS in the VHCF regime was studied in detail.

Abstract: Aluminum matrix composites (AMCs) are characterized by improved mechanical properties in comparison to their unreinforced matrix alloys. But the knowledge about the fatigue behavior of AMCs in the HCF-and in the VHCF-regime is limited until now. Due to this AMC225xe and AMC xfine225 with an average SiC particle content of 25 vol.-% and particle sizes of 2.5 μm and 0.7 μm, respectively, as well as the base alloy AA2124 were fatigued up to 10¹⁰ cycles using the ultrasonic testing facility of the type "UltraFast-WKK-Kaiserslautern".To describe the fatigue behavior of the specimens several measuring devices were used to monitor and record the central process parameters. A very sensitive value to detect specimen failure at an early stage is the dissipated energy which can be determined as the integral of the generator power depending on the ultrasonic pulse time.In comparison to AA2124 the investigated AMCs have shown a considerably enhanced fatigue performance for stress amplitudes higher than 140 MPa. But below this stress amplitude for the matrix alloy run outs at 10¹⁰ cycles were realized whereas the AMCs failed at lower number of cycles still at lower stress amplitudes. Moreover, while crack initiation of the matrix alloy in all cases started at the surface for the AMCs the crack initiation point changes from surface to subsurface for more than 10⁷ cycles. The subsurface failures of the composites were caused by microstructural inhomogeneities which could be identified with EDX and micro-CT as particle clusters and copper-iron-rich inclusions.

Abstract: This paper aims at a deeper understanding of mechanisms leading to crack initiation in ductile metals in Very High Cycle Fatigue (VHCF). The VHCF regime is associated with stress amplitudes lower than the conventional fatigue limit and numbers of cycles higher than 10⁹. Tests were conducted using an ultrasonic technique at loading frequency of 20 kHz. The mechanisms leading to crack initiation express via slip bands at the specimen surface and self-heating due to intrinsic dissipation. Thermal maps were used to estimate the mean dissipation and its change with number of cycles and stress amplitudes in case of pure copper polycrystals. At the same time, the surface relief changes due to plasticity were characterized using optical and scanning electronic microscopes. A good correlation was found between slip band initiation and dissipation. Dissipation and slip band amount always increased over the number of cycles. At very small stress amplitudes, no slip band appeared up to 10⁸ cycles but the material was found to dissipate energy. Results derived from tests performed at high loading frequency on pure cupper specimens showed a drift of dissipative regimes incompatible with concepts of fatigue limit and/or asymptotic cyclic stability. These results reveal that the material never reached a steady state. Therefore it could break at higher number of cycles.

Abstract: Today in many cases ultrasonic testing machines with a frequency of f ≈ 20 kHz are used for investigations of the fatigue behaviour up to the very high cycle regime (VHCF-regime). A question that arises is if the results of these high frequency fatigue tests are comparable to conventional fatigue tests. This paper compares the fatigue behaviour of a quenched and tempered steel 50CrMo4 in two different tempered conditions investigated at low frequencies (f ≤ 400 Hz) on a servohydraulic testing machine and at a high frequency (f ≈ 20 kHz) on an ultrasonic fatigue testing machine. Effects which can occur because of the different testing techniques and testing frequencies are investigated. A concept is derived to describe the frequency effect caused by the strain rate. The estimations are compared with results of the fatigue tests.

Abstract: The present paper illustrates a comparison of infrared thermography during ultrasonic fatigue testing of cast steel 42CrMo4 and cast aluminium alloy AlSi7Mg. Against the background of different material properties (e.g. mechanical properties as well as thermal properties) the benefit of this non-destructive material testing method in terms of determining the crack initiation point and time during fatigue testing as well as crack propagation is evaluated and discussed. Moreover, correlations between fractography and infrared thermography are performed for both materials.

Abstract: PSB formation and its relevance for an eventual fatigue limit of polycrystalline electrolytic copper was studied in the very-high cycle fatigue regime with the ultrasound fatigue loading method. PSBs are formed at much lower stress/strain amplitudes than reported in earlier literature, if a high enough number of cycles is applied. Fatigue fracture takes place at approximately 50% higher amplitudes than needed for PSB formation, which is likewise in contrast to former literature results. Non-propagation of small cracks, originating from intrusions or PSB-induced non-propagating grain-boundary cracks are made responsible for this different material response.

Abstract: Ultrasonic Corrosion Fatigue Tests were Conducted for SUS329J3L in Air and 3%NaCl Aqueous Solution. Reduction of Giga-Cycle Corrosion Fatigue Strength was 12.5%. Corrosion Pit was Observed on Corrosion Fatigue Crack Initiation Area. Striation was Predominantly Observed on Crack Propagation Area both in Air and 3% Nacl Aqueous Solution. it can be Concluded that the Reduction of Corrosion Fatigue Strength of SUS329J3L is due to the Corrosion Pit Formation at Corrosion Fatigue Crack Initiation Area.