COMMITTEE MANDATE Concern for crack initiation and growth under cyclic loading as well as unstable crack propagation and tearing in ship and offshore structures. Due attention shall be paid to practical application and statistical description of fracture control methods in design, fabrication and service. Consideration is to be given to the suitability and uncertainty of physical models. The work shall be coordinated with that of Committee V.2.

Fatigue and Fracture

Fatigue in composites

Wind turbine rotor blades are subjected to a large number of highly variable loads, but life predictions are typically based on constant amplitude fatigue behaviour. Therefore, it is important to determine how service life under variable amplitude fatigue can be estimated from constant amplitude fatigue behaviour. A life prediction contains different, partly independent, elements: Â· the counting method, used for describing variable amplitude signals as a collection of constant amplitude cycles Â· formulations for describing S-N curves which relate the stresses to the number of cycles to failure Â· constant life diagrams which are made up of S-N curves for different stress ratios Â· damage rules, which relate the life expectancy of a specimen to the stress history For the description of damage, two models were investigated and compared, viz. the Miner's sum method and strength-based life prediction. In the Miner's sum method, the results of a counting method and constant amplitude fatigue behaviour description are converted into a damage parameter, "Miner's sum". Potential effects of load order are not taken into account. Moreover, the value of the damage parameter only indicates whether or not failure occurred: it does not relate to a physically quantifiable damage. These are limitations to the model which suspectedly might cause inaccurate predictions. In the strength-based method, life is predicted by calculating the effect of each load cycle on strength, until the load exceeds the remaining strength. An expected advantage of this cycle-by-cycle method is, that sequence effects can be implicitly included. Moreover, the damage parameter is at all times related to a physically quantifiable parameter (viz. strength). The successful application of the strength-based method requires a description of the post-fatigue strength, which entails considerable experimental effort. In addition, a strength-based life prediction is much more computationally intensive than Miner's sum and can not always utilise the same counting methods. In the comparison of the Miner's sum and the strength-based method, the influence and significance of the other life prediction elements, such as counting methods and description of constant amplitude fatigue behaviour on life prediction are included. The experimental research involved a considerable amount of material tests. The material tests give a detailed image of static strength, constant and variable amplitude fatigue behaviour (both block tests and (variants of) the WISPER spectrum were used), as well as strength degradation for different glass-fibre reinforced laminates. By selecting a single coupon geometry for all material tests on a single material, and the definition and use of standard test conditions, a consistent database was created. The block-test experiments confirm the existence of sequence effects on life, although more data are required to fully quantify them. The residual strength tests show the strength degradation after fatigue for a range of fatigue load conditions. Significant tensile strength degradation is observed in R=0.1 and R=-1 fatigue experiments. Generally, compressive strength remains within the boundaries of the initial static strength distribution. This behaviour was observed for different laminates. The significance of an adequate description of the constant amplitude behaviour is evident from the various life predictions. Commonly used simplifications, such as the Linear Goodman Diagram, result in highly non-conservative predictions. The residual strength model yields more conservative predictions than Miner's sum for the investigated tension-dominated load sequences. The experimental effort required for the determination of the strength degradation, and the computational effort do not justify this relatively small advantage. For future research, it is recommended to further improve the description of the constant life diagram. This work is focussed on fatigue of composites for wind turbine rotor blades. Nevertheless, the results are relevant for other composite structures as well.

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Fatigue life prediction and strength degradation of wind turbine rotor blade composites

A phenomenological fatigue model is developed for the off-axis fatigue behavior of a class of unidirectional polymer matrix composites subjected to constant-amplitude stress cycling with non-negative mean stresses. For this purpose, a new non-dimensional effective stress that considers the effects of the stress ratio and multiaxial state of stress on the off-axis fatigue behavior of unidirectional composites is defined for the range of non-negative mean stresses. Non-dimensional effective stress is used as the strength measure to characterize the brittle nature of the evolution of fatigue damage. This allows one to obtain a practical form of multiaxial fatigue model that takes into account the fiber-orientation dependence as well as the mean-stress dependence of the off-axis fatigue behavior of unidirectional composites. The constant-fatigue life diagram predicted by the proposed fatigue model is described in terms of a normalized relation of the modified Goodman type. It is demonstrated that the off-axis S – N relationships for unidirectional carbon/epoxy and glass/epoxy composites are adequately described using the proposed fatigue model over the range of non-negative mean stresses.

A phenomenological model for off-axis fatigue behavior of unidirectional polymer matrix composites under different stress ratios

Exploration of a full shape of constant fatigue life (CFL) diagram and development of an efficient CFL diagram-based fatigue life prediction method are attempted for multidirectional CFRP laminates. On three kinds of CFRP laminates of [45/90/−45/0]2s, [0/60/−60]2s and [0/90]3s lay-ups, tension–tension, tension–compression and compression–compression fatigue tests are performed at room temperature for two different stress ratios each. Experimental results clearly show that a stress ratio has a significant influence on the fatigue behavior of those CFRP laminates, and the CFL diagrams delineated using alternating stress and mean stress become asymmetric about the alternating stress axis. The alternating stress component of fatigue load for a given constant value of fatigue life turns maximum in the case of fatigue loading at a critical stress ratio that is nearly equal to the ratio of compressive strength to the tensile one. The shape of CFL diagrams progressively changes from a straight line to a nonlinear curve as a given constant value of fatigue life increases. A new and efficient method for accurately predicting an asymmetric nonlinear CFL diagram is then developed which is based on the static strengths in tension and compression and the reference S–N relationship fitted to the fatigue data for the critical stress ratio. The theoretical CFL diagram constructed following the proposed procedure agrees well with the experimental CFL diagram, regardless of the type of CFRP laminate. It is also demonstrated that the S–N relationships predicted using the proposed CFL diagram-based fatigue life prediction method adequately coincide with the experimental results for fatigue loading with a variety of different stress ratios in the range of fatigue life up to 106 cycles.

Nonlinear constant fatigue life diagrams for carbon/epoxy laminates at room temperature

A unified fatigue failure criterion using micromechanics related to the fracture plane has been developed to predict fatigue lives of unidirectional fibre reinforced polymer composites subjected to cyclic off-axis tension–tension loading. Since the failure criterion incorporates both stresses and strains it may be characterized as energy based. Accounting for the fibre load angle as well as the stress ratio is the novelty of this fatigue failure criterion. The criterion only requires the stress ratio to be known from the experimental procedure. The relation between the applied load and the micro-stress and micro-strain field can be determined from a numerical method. The fatigue failure criterion has been verified by applying it to different sets of experimental data. Several fibre load angles, different fibre/matrix combinations as well as stress ratios are covered. The predicted fatigue lives are in good agreement with the experimental results for both different fibre load angles and stress ratios.

J Petermann

Papers

A unified fatigue failure criterion for unidirectional laminates