ABSTRACT
Recently, a considerable amount of researches has been conducted on CFRP strengthened metallic members under fatigue tension test (Liu et al. 2009, Ghafoori et al. 2012). However, it is not clear whether CFRP strengthening has the same performance and efficiency under mixed mode loading (I + II). This paper aims to extend the current knowledge of CFRP strengthening under tensile fatigue loading to the case of mixed mode loading.
An experimental program has been designed for this purpose, including 18 steel specimens, 12 of them reinforced by CFRP. Central cracked tensile steel specimens manufactured with a central hole of 5 mm, two slits of 0.9 mm, and constant dimensions of (500 x 90 x 10) mm. The mixed mode conditions were stimulated by initiating different degrees of crack inclination with respect to the loading axis in the steel plate specimens within a range of (10°–90°). UHM-CFRP laminates were applied on one side of the steel plates using two configurations; fully reinforced- & partially reinforced steel plates. Uniaxially cyclic loading with a constant stress range of 135 MPa, a stress ratio of 0.1, and a frequency of 20 Hz were applied for all the specimens. Crack propagation versus fatigue cycles were measured using beach marking technique.
Test results showed that fatigue lives of steel plates were increased directly with increasing the mode mixity (I + II), i.e. for specimen has crack inclined with 45°, the fatigue life was almost doubled, as stress intensity factor (SIF) of mode I decreased at the crack tip. Life extension, due to CFRP strengthening system, was extended by 1.54–2.27 times with marginal decrease as the shear stress component was increased.
In this study, a “shifting phenomenon” was found, i. e. the mixed mode crack growth curve (β ˂ 90°) can be approximated to the same crack growth curve as mode I (β = 90°) with a shift, as illustrated in Figure 1. This shifting phenomenon applies to both unrepaired and CFRP repaired cases. From this point, a new method will be developed by the authors to estimate the total fatigue life of CFRP strengthened plates with inclined cracks using modification factors for the mode I crack growth curves. Schematic view of the shifting phenomenon in mixed-mode fatigue crack growth curves. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig56_1.tif"/>
