Fatigue limit

About: Fatigue limit is a research topic. Over the lifetime, 20489 publications have been published within this topic receiving 305744 citations. The topic is also known as: endurance limit & fatigue strength.

Self-monitoring of fatigue damage and dynamic strain in carbon fiber polymer-matrix composite

Abstract: Self-monitoring of static/fatigue damage and dynamic strain in a continuous crossply [0/90] carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 0° direction, R in this direction and R perpendicular to the fiber layers were measured. Upon static tension to failure, R in the 0° direction first decreased (due to increase of degree of 0° fiber alignment and fiber residual compressive stress reduction) and then increased (due to 0° fiber breakage), while R perpendicular to the fiber layers increased monotonically (due to increase of degree of 0° fiber alignment and delamination). Upon cyclic tension, R (0° decreased reversibly, while R perpendicular to the fiber layers increased reversibly, though R in both directions changed irreversibly by a small amount after the first cycle. Upon fatigue testing at a maximum stress of 57% of the fracture stress, R (0°) irreversibly increased both in spurts and continuously, due to 0° fiber breakage, which started at 15% of the fatigue life, while R (perpendicular to the fiber layers) irreversibly increased both in spurts and continuously, due to delamination, which started at 33% of the fatigue life. The peak R (0°) in a cycle irreversibly decreased, while the minimum R (perpendicular to the fiber layers) at the end of a cycle irreversibly increased during the first 0.1% of the fatigue life, due to irreversible increase in the degree of 0° fiber alignment. R (0°) became noisy starting at 87% of the fatigue life, whereas R (perpendicular to the fiber layers) became noisy starting at 50% of the fatigue life. For a [90] unidirectional composite, R (0°) increased reversibly upon tension and decreased reversibly upon compression in the 0° direction, due to piezoresistivity.

Chances in wind energy: A probalistic approach to wind turbine fatigue design

Abstract: Wind is becoming an ever more important source of renewable energy: installed wind turbine power now stands at 60,000 MW worldwide, providing 0.6% of world electricity demand. Still it is important that the cost of wind energy is brought down further, which means that wind turbines must be designed to be exactly as strong as necessary, but no stronger. Hence there is a need to investigate whether the conventional design procedure results in the right degree of conservatism, and if not, how it may be improved. The ideal is to make the design just conservative enough, i.e. to exactly attain the target failure probability. Because wind turbines tend to be located in remote areas, the target value is primarily determined by economic considerations, rather than by public safety issues. The aims of this work are: To quantify total uncertainty in the design procedure, and to find the relative importance of stochastic parameters influencing fatigue loads and strength. To conduct a comparative review of calculation models where necessary. To derive partial safety factors giving minimum unit electricity cost. The scope of the present research is limited to fatigue issues, since extreme loads have been investigated previously to some degree. An inventory of stochastic parameters is made; for each of the parameters the distribution is estimated, and the models currently used in wind turbine design (i.e. the procedures used to estimate characteristic parameters and how to use them in calculations) are reviewed. A limit state function is derived using the concept of life fatigue damage equivalent load range. With the First Order Reliability Method (FORM) and Monte Carlo simulation, yearly failure probabilities due to fatigue are estimated for a wind turbine that is designed exactly according to the standard, and installed following common site admission rules. A simple economic model is used to establish optimal partial factors. The partial factor values found for blades are somewhat smaller than in the standard, while values for hub, nacelle and tower are higher. The explanation for the latter is that two things are currently not taken into account in design calculations according to the standard: firstly, variation and bias in fatigue life prediction; secondly, the fact that a combination of many critical locations (for example in the tower) yields a larger failure probability than just one location. The sensitivity of the partial factor optimisation to changes in various assumptions is investigated. The main conclusions of the work are threefold: Given available data, a larger partial (load or material) factor should be used in fatigue design for cast iron and weld seams. However, the effect of this on design might be limited since hidden safety exists in the construction: material quality and hence fatigue strength are better than assumed, wind turbines are placed in climates that are more benign than they were designed for, and finally, dimensions may be determined by stiffness or extreme load considerations rather than by fatigue. The variation of the limit state function is determined mainly by uncertainty on fatigue strength and fatigue life prediction. Therefore the way forward is to accurately establish fatigue properties and calibrate fatigue life predictions for materials exactly as used in wind turbines. In this way variation may be reduced (and bias removed), and failure probability estimates may be refined. If better information is available, hidden safety may be removed and smaller partial factors used in calculations. The number of critical locations and correlation of loads and fatigue strength at different locations must be taken into account in calculations to establish failure probabilities, and must have influence on the partial factors to be used. Variation and bias of fatigue life predictions must be an explicit input to fatigue design calculations.

Titanium Particulate Metal Matrix Composites – Reinforcement, Production Methods, and Mechanical Properties

Abstract: Titanium metal matrix composites (MMCs) offer potential advantages for structural applications, where they combine the high strength, high temperature capability, and oxidation resistance of titanium with an increase in stiffness provided by the ceramic reinforcement. They have the advantage of being isotropic in behaviour, cheaper to manufacture and more amenable to subsequent processing and component forming operations. Of potential reinforcing phases for titanium, mcluding TiB, TiB 2 , SiC, Al 2 O 3 , and TiC, TiB offers the best balance of stiffness, stability, and similarity of thermal expansion coefficients. The methods used to proauce these Ti-TiB MMCs, such as arc melting, gas atomization, mpid solidification, and powder blending have been assessed and the benefits these composites offer over conventional titanium alloys including increased stiffness, good creep performance, fatigue resistance, and wear resistance are highlighted.