PSI - Issue 38

Sara Eliasson et al. / Procedia Structural Integrity 38 (2022) 631–639 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Advanced lightweight materials, e.g. CFRP, are getting more attention in the lightweight design of structural commercial vehicle components. Commercial vehicles have a wide range of service loads and the road induced vibrations often sets the design criteria. To avoid redundancy and conservative structural design it is important to understand the fatigue behavior of the materials when conducting fatigue life estimation calculations. Damage during the fatigue life of a composite laminate can unfold differently depending on layup and material combination. Reifsnider et al. (1983) declared the different damage modes throughout the fatigue life of a composite laminate, illustrated in Fig. 1a. Gamstedt et al. (1999) studied the damage mechanisms of UD CFRP and observed two types of behaviors: (1) propagation of small localized fiber-bridge cracks or (2) a more extensive distributed damage with fiber breakage induced by longitudinal debonding or matrix cracking. A way to monitor the damage development during fatigue loading is by monitoring the stiffness reduction (Schulte, 1999). Senthilnathan et al. (2017) found that there is a steep reduction in stiffness in the beginning of a fatigue test and then the microstructural damage increase monotonically with the number of cycles. A typical stiffness degradation behavior can be described with three stages (Fig. 1b). The initial stage has a rapid stiffness reduction, followed by a second intermediate stage with an approximately linear behavior. The second stage is mainly driven by edge delamination and longitudinal cracks along the fibers. The final and third stage of the stiffness reduction occurs in an abrupt manner ending in final failure (Van Paepegem, 2010).

Fig. 1. (a) The characteristic damage state during the fatigue life of a composite laminate (Reifsnider, et al., 1983); (b) typical stiffness degradation behaviour during the fatigue life of a composite laminate (Van Paepegem, 2010).

The testing of the UD CFRP material is still a challenge due to the high specific stiffness and strength, and the anisotropy of these materials. One of the challenges is for the failure to occur in the gauge length and knowing the fatigue life is not underestimated due to unwanted artefacts such as stress concentrations and manufacturing induced defects. Eliasson et al. (2019) developed a fatigue testing procedure to succeed with testing of UD CFRP. This testing procedure is utilized in this study. Manufacturing induced defects highly affects the fatigue behavior and strength of a composite laminate (Sisodia, et al., 2015) and reducing the prevalence and variability of manufacturing induced defects is difficult and costly, but required if the spread in fatigue performance is to be reduced. The study presents a tension-tension fatigue test series of a UD CFRP material tested in the fiber direction at different load levels. In this study the surface displacements of the specimen was monitored with an image processing method such that strains and stiffness could be calculated and monitored throughout the life cycle (Sisodia, et al., 2011). An automated testing methodology to image the surface displacement captured at peak load was developed. Analyzing the fatigue damage by studying the strain and comparing different test specimens, trends were identified to explain the scatter in fatigue results and highlight challenges when conducting the test series.