Hidronanogeles

this research

1a) What are the effects of hygrothermal aging on the mechanical behavior of co-bonded CFRP joints?

The answer to this question is summarized in tables 2.2 and 2.3.

TABLE 2.2 – Possible effects of moisture and elevated temperature on the mechanical properties of CFRP bonded structures.

Moisture Temperature

*Unless stated otherwise, the effects below are related to the epoxy resin in the adhesive and matrix + Increase of f

+ Stress relaxation

± Fibers remain unaffected − Lower Tg

− Lower σy

− Lower E

− Degradation of the fiber-matrix interface − Swelling induced stresses

− Decrease in bond strength (shear and tensile)

− Deacrease of GIc and GIIc

+ Post-curing (below Tg)

± Little effect onGIIcat temperatures near

Tg

− Decrease of bond strength (shear and tensile) at temperatures above Tg

− Decrease ofGIcafter prolonged exposure near Tg

− Rapid decrease of GIc at temperatures above Tg

TABLE 2.3 – Possible effects of hygrothermal aging on the mechanical properties of CFRP bonded structures.

Effected area Effect

Bond strength

+ Possible initial increase of shear strength

− Prolonged exposure results in a decrease in shear strength

Fracture toughness

+ Possible initial increase of GIc

± GIc is influenced mostly at the crack initiation point

− Prolonged exposure results in a decrease inGIc

These tables are quite generalistic. However, they help in extrapolating the gained insights towards how hygrothermal aging is expected to affect the panels specifically in this study.

The fiber related mechanical properties are expected to remain unaffected by the hygrothermal aging. This is due to the fact that moisture has little influence on the fibers and the temperature remains far below the 300-400◦C for which Foster and Bisby (2005) reported fiber degradation. The matrix and adhesive are more likely to show signs of degradation, as they are more sensitive to moisture and tend to degrade at lower temperatures. Moisture can lower their stiffness, yield stress, strength and mode I and II fracture toughness. For the adhesive this effect might be enlarged by the fact that the temperature during aging (80◦C) is close to its glass transition temperature (95◦C when wet).

Halliday et al. (2000) performed a study on the influence of hygrothermal aging on the shear strength and mode I fracture toughness of bonded CFRP with somewhat similar aging conditions (70◦C immersion in water) as used in this study (80◦C 90% humidity). They noted a decrease in shear strength of the bond of roughly 20%. The mode I fracture in this same study mainly decreased at the fracture initiation point. Only a small amount of degradation could be distinguished after 2 mm of crack growth. This indicates that the mode I disbond growth in the fatigue experiments of this study might not be effected a lot by the hygrothermal aging. However, as the aging temperature of Halliday et al. (2000) was 10◦C lower, and as was pointed out above the temperature is close to Tg, this effect could be more severe in this study. Unfortunately no study was found on the influence of hygrothermal aging on the mode II fracture toughness at RTW conditions.

Overall the decreased strength of the epoxy matrix and adhesive can cause the strength of the panels to decrease. The stiffness is not expected to change as this property is dominated by the fibers. Whether or not the disbond growth during the cyclic loading will be significantly different is not entirely clear due to the limited amount of information available on this subject.

2.3

Buckling and post-buckling of bonded composite

stiffened panels

As stated in the introductory section 1.2, buckling and post-buckling of bonded com- posite structures is a well researched topic. This section aims to create an overview of the relevant parts of these studies. In subsection 2.3.1 an introduction is given on buckling in stiffened panels. Thereafter, the influence of the disbond size on the buckling and post-buckling behavior of composite stiffened panels is treated in subsection 2.3.2. This is relevant to determine the influence of the expected delamination growth during the cyclic loading. Finally, subsection 2.3.3 discusses the influence of cyclic post-buckling loading on the mechanical behavior of the panel.

2.3.1

Background

Buckling can occur in panels due to loading in compression, shear or a combination of both. Stiffeners are attached to the panels to improve their resistance against buckling. They do so, from a weight perspective point of view, in a more efficient way than increasing the skin thickness [Yapet al. 2002]. To allow further weight savings the stiffened panels are often designed to take significant loads beyond the initial buckling load [Niu 1995, Lynch et al. 2004, Yap et al. 2002].

Initially the response of an ideal stiffened panel under compression is linear. The point where it starts deviate from linearity is also referred to as the bifurcation point [Yapet al. 2004]. In figure 2.7 this is referred to as the point of local buckling. From a mathematical point of view two equilibrium solutions are possible for every loading scenario beyond the bifurcation point: one solution with purely compressed linear behavior and one non-linear solution with out of plane deformation [Yapet al. 2004]. Yap et al. (2004) state that in practice structures will always follow the non-linear path up to the ultimate load. From the ultimate load onward the stiffness drops and a structure is considered failed.

FIGURE 2.7 – Typical load vs. in plane displacement curve of a stiffened panel [Pauloet al. 2013]

Two types of buckling exist for stiffened panels: global buckling and local buckling. Global buckling modes deform both the stiffeners and the skin, whereas local buckling modes only deform one of the two [Lambertiet al. 2003]. Local buckling modes arise due to defects, such as delaminations, and can influence the global buckling behavior. They lower the overall buckling load and ultimate load and can lead to disbonding [Yap et al. 2004, Orifici et al. 2008, Yap et al. 2002]. Figure 2.8 shows examples of both global and local buckling modes for the skin and stiffener.

FIGURE 2.8 – Different buckling modes of stiffened panels [Lamberti et al. 2003]