PRINCIPAL MASTER DIAGRAMS APPROACH TO FATIGUE LIFE PREDICTION OF - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS PRINCIPAL MASTER DIAGRAMS APPROACH TO FATIGUE LIFE PREDICTION OF COMPOSITE LAMINATES M. Kawai 1 *, T. Teranuma 1 1 Department of Engineering Mechanics and Energy, University of Tsukuba, Japan * Corresponding author (mkawai@kz.tsukuba.ac.jp) Keywords : Fatigue; Principal Master Diagrams; Constant Fatigue Life Diagram; Anisomorphic CFL Diagram; Stress Ratio; Unidirectional Composite and the accuracy of prediction is evaluated by 1 Introduction comparing with experimental results. For accurate fatigue analysis of structural components made of carbon fiber-reinforced plastics (CFRPs), it is a prerequisite to quantify the influence 2 Material and Testing Procedure of loading mode on the sensitivity to fatigue of the composites employed. To do that, however, a large The material used in this study was a unidirectional number of fatigue tests under many different kinds T700S/2592 carbon/epoxy laminate fabricated from of cyclic loading conditions are needed, which the prepreg tape P3252S-20 (TORAY). Six kinds of consumes considerable time and cost. For this coupon specimens with different fiber orientations ( � = 0, 10, 15, 30, 45 and 90°) were prepared. reason, it is required to develop a time and cost- saving engineering procedure for predicting the Constant amplitude fatigue tests were performed fatigue strengths (S-N relationships) of composites under load control at different stress ratios. Fatigue under different loading conditions, with reasonable load was applied to off-axis coupon specimens in a accuracy, on the basis of the static strengths in sinusoidal waveform at room temperature. tension and compression and a limited number of constant amplitude fatigue test data. The present study aims to develop a new fatigue 3 Experimental Results model for unidirectional composites by a combination of a new fatigue failure criterion and a The longitudinal, transverse and in-plane shear CFL constant fatigue life (CFL) diagram approach. First, diagrams for the unidirectional CFRP laminate were constant amplitude fatigue tests are performed on identified on the basis of the experimental results, coupon specimens of a unidirectional carbon/epoxy and they are shown by symbols in Figs. 1 (a)-(c), laminate at different stress ratios in the longitudinal respectively, for different constant values of life: N f = 10 1 , 10 2 , 10 3 , 10 4 , 10 5 and 10 6 . The dashed lines in and transverse directions and in a shear dominated off-axis direction, respectively. On the basis of the these figures indicate the predictions using the fatigue data obtained, the principal CFL diagrams method that are discussed later on. It is seen that the for the unidirectional CFRP laminate are identified. longitudinal CFL diagram shown in Fig. 1(a) is Then, the anisomorphic CFL diagram approach [1] similar in shape to the CFL diagrams observed for is tested for the issue of predicting the principal CFL multidirectional CFRP laminates [1], and thus it can diagrams, and the accuracy of predictions using this be described using the anisomorphic CFL diagram. method is evaluated. An extended version of the In contrast, a significant distortion is involved in the anisomorphic CFL diagram approach is also applied transverse and in-plane shear CFL diagrams (Figs. to obtain better description of the principal CFL 1(b) and 1(c)), indicating that a significant change in diagrams for the unidirectional CFRP laminate. mean stress sensitivity in fatigue has occurred. The Finally, the fatigue failure criterion combined with latter observation suggests the need for a the principal master diagrams is applied to modification to the original two-segment predicting the off-axis S-N relationships of the anisomorphic CFL diagram approach so that a unidirectional composite for different stress ratios, modified CFL diagram allows accommodating itself

stress ratio greatly improves the accuracy of 2000 T700S/Epoxy UD [0] 16 description of the off-axis CFL diagrams of a Fatigue RT 5Hz 1500 R = � unidirectional composite, regardless of fiber Experimental X a , MPa � N f =10 1 � N f =10 2 R = 0 orientation. In this study, we adopt the four-segment � N f =10 3 � N f =10 4 1000 � N f =10 5 � N f =10 6 R = -1 R = 0.1 extension of the anisomorphic CFL diagram R = � � approach for description of the principal CFL 500 R = 0.5 R = 10 diagrams for unidirectional composites. R = 2 0 The four-segment anisomorphic CFL diagram [2], -2000 -1500 -1000 -500 0 500 1000 1500 2000 X m , MPa which is a most general CFL diagram in the context (a) Longitudinal CFL diagram ( � = 0°) of the anisomorphic CFL diagram approach, can be described by using different formulas on the four subintervals of the total interval of mean stress 160 T700S/Epoxy UD [90] ( � R ) , � T ] ; II. ( � R ) ] ; III. [ � C , � T ] : I. [ � m [ � m ( � ) , � m 16 Fatigue RT 5Hz 120 Experimental ( � L ) , � m ( � L ) ] . In line with the Y a , MPa [ � m ( � ) ] ; IV. [ � C , � m � N f =10 1 � N f =10 2 R = � � R = -10 R = � � N f =10 3 � N f =10 4 80 � N f =10 5 � N f =10 6 formulation of the three-segment anisomorphic CFL R = 10 diagram, it is natural to describe a four-segment 40 R = -1 R = 2 R = 0 anisomorphic CFL diagram by means of the R = 0.1 R = 0.5 0 following piecewise-defined function: -160 -120 -80 -40 0 40 80 120 160 ( � R ) � � m � � T ): Y m , MPa I. Tension dominated zone ( � m (b) Transverse CFL diagram ( � = 90°) kT 2 � � � R � � ( � R ) ( � R ) � � a � � a = � m � � m 80 Experimental T700S/Epoxy UD � � (1) � N f =10 1 � N f =10 2 Fatigue RT 5Hz ( � R ) ( � R ) � a � T � � m � � � N f =10 3 � N f =10 4 R = � 60 From [10] 16 � N f =10 5 � N f =10 6 S a , MPa 40 R = -3 R = 0 ( � ) � � m < � m R = � � ( � R ) ): II. Right transitional zone ( � m R = 0.1 R = 10 20 R = 0.5 R = 2 ( � R ) ( � R ) � � a � � a ( � ) = � m � � m 0 -80 -60 -40 -20 0 20 40 60 80 ( � R ) (2) ( � R ) � � a ( � ) � � m S m , MPa � a � m (c) In-plane shear CFL diagram ( � L ) � � m < � m ( � ) ): III. Left transitional zone ( � m Fig.1. Principal CFL diagrams � � a � � a ( � ) ( � L ) = � m � � m ( � ) ( � L ) � � m ( � ) (3) to such a significant change in mean stress ( � ) � � a � a � m sensitivity. ( � L ) ): IV. Compression dominated zone ( � C � � m < � m 4 Modeling of Principal CFL Diagrams The CFL diagrams for composites that are affected 2 � � � L kC � � by a significant change in mean stress sensitivity in ( � L ) ( � L ) � � a � � a = � m � � m � � fatigue can be coped with by inserting transitional (4) ( � L ) ( � L ) � a � C � � m � � segments in the anisomorphic CFL diagram, while it impairs the simplicity of the anisomorphic CFL diagram approach. In the previous study [2], we where � � R and � � L designate the fatigue strength found that the insertion of two transitional segments ratios associated with the right and left sub-critical in the right and left neighborhoods of the critical