Ultrasonic fatigue of unfilled and carbon nanotube (CNT) reinforced polyetheretherketone (PEEK).

Fatigue properties of polyetheretherketone (PEEK) and multiwall carbon nanotube (CNT) reinforced PEEK were investigated with the ultrasonic fatigue testing method. Lifetimes were measured in the high and very high cycle fatigue regime at resonance frequency 19 kHz and load ratio R = -1. Pulse-pause loading served to avoid specimen self-heating and led to effective cycling frequencies in the range from several hundred Hz to about two kHz. Stress amplitude for 50 % fracture probability at 109 cycles is 21.2 ± 4.3 MPa for unreinforced PEEK (22 % of its tensile strength) and 33.5 ± 3.5 MPa for CNT reinforced PEEK (33 % of its tensile strength). Servohydraulic fatigue tests at 22 Hz with CNT reinforced PEEK delivered fatigue lifetimes comparable to ultrasonic tests, i.e. no frequency effect and no influence of load versus displacement control was observed. Keeping specimen temperature far below the glass transition temperature, ultrasonic fatigue testing of a high temperature resistant plastic was successfully implemented.

Ultrasonic fatigue testing of concrete.

Cyclic compression fatigue properties of concrete are studied with the ultrasonic fatigue testing method with cycling frequency 19 kHz and are compared to servo-hydraulic tests performed at 60 Hz. Ultrasonic testing was found applicable for rapid generation of very high cycle fatigue (VHCF) data of concrete. Fatigue cracks can be initiated, however specimens do not rupture, since cyclic stresses decrease with increase of compliance in displacement controlled ultrasonic tests. Observation of resonance frequency, analysis of higher order harmonics of vibration, and computed tomography of specimens are successful methods to analyse fatigue damage. Calorimetric evaluations can be used to calculate the cyclic irreversible strain, which is about 1% of the elastic strain in the ultrasonic VHCF test.

Usability of Ultrasonic Frequency Testing for Rapid Generation of High and Very High Cycle Fatigue Data.

Ultrasonic fatigue testing is an increasingly used method to study the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of materials. Specimens are cycled at an ultrasonic frequency, which leads to a drastic reduction of testing times. This work focused on summarising the current understanding, based on literature data and original work, whether and how fatigue properties measured with ultrasonic and conventional equipment are comparable. Aluminium alloys are not strain-rate sensitive. A weaker influence of air humidity at ultrasonic frequencies may lead to prolonged lifetimes in some alloys, and tests in high humidity or distilled water can better approximate environmental conditions at low frequencies. High-strength steels are insensitive to the cycling frequency. Strain rate sensitivity of ferrite causes prolonged lifetimes in those steels that show crack initiation in the ferritic phase. Austenitic stainless steels are less prone to frequency effects. Fatigue properties of titanium alloys and nickel alloys are insensitive to testing frequency. Limited data for magnesium alloys and graphite suggest no frequency influence. Ultrasonic fatigue tests of a glass fibre-reinforced polymer delivered comparable lifetimes to servo-hydraulic tests, suggesting that high-frequency testing is, in principle, applicable to fibre-reinforced polymer composites. The use of equipment with closed-loop control of vibration amplitude and resonance frequency is strongly advised since this guarantees high accuracy and reproducibility of ultrasonic tests. Pulsed loading and appropriate cooling serve to avoid specimen heating.

Insight into the nanostructure of anisotropic cellulose aerogels upon compression.

Cellulose II aerogels are a highly porous class of biobased ultra-light-weight materials. They consist of interlinked networks of loosely aggregated cellulose fibrils. The latter typically have random orientation due to spontaneous phase separation triggered by addition of antisolvent to moleculardisperse cellulose solutions. Deceleration of phase separation has been recently proposed as a novel approach towards aerogels featuring preferred cellulose orientation. Here, we investigate the mechanical response of such oriented cellulose aerogels towards load up to 80% compression. Stress-strain curves were recorded and in situ small angle X-ray scattering (SAXS) was performed during compression test to obtain information about the structural alterations of the aerogel fibril networks on the nano-scale related to deformation. Using SAXS in addition, structural changes can be followed in much more detail than by recording stress-strain curves alone. Buckling and coalescence of fibers and a change in fibril orientation can be related to certain regimes in the stress-strain curve. If the loading axis is oriented parallel to the network orientation the aerogels show higher resilience towards the compression.