René Alderliesten

Fatigue and Damage Tolerance of Glare

Methods have been developed to describe the fatigue initiation and propagation mechanisms in flat panels as well as mechanically fastened joints and to determine the residual strength of large flat panels. Glare shows excellent crack growth characteristics due to the mechanism of delamination and fibre bridging. The fatigue insensitive fibres restrain the crack opening and transfer load over the crack in the metal layers. During the initiation phase fibre bridging does not occur and the behaviour is dominated by the metal initiation properties. Mechanically fastened joints introduce additional effects such as secondary bending, load transfer and aspects related to the fastener installation. The residual strength of Glare is dependent on the amount of broken fibres and the delamination size and can be described with the R-curve approach.The impact resistance of Glare is related to the aluminium and glass/epoxy properties and is significantly higher than the impact resistance of monolithic aluminium. The same has been proven for fire resistance. Depending on the Glare grade and thickness, the outer aluminium layer will melt away, whereas the other layers will remain intact due to carbonisation of the glass/epoxy layers and delamination of the laminate. The air in the delaminations will act as insulation, keeping the temperatures at the non-exposed side relatively low.

Fatigue crack growth prediction in GLARE hybrid laminates

A mechanistic approach for fatigue crack growth prediction in GLARE laminates is presented. Three-dimensional finite element models are used to obtain the mode I stress intensity factor, KI, in the aluminum layers, and the fatigue crack growth rate of monolithic aluminum expressed as a Paris-type power law is used to predict the crack growth rates in the GLARE laminates. The crack growth rates in three different types of GLARE laminates with center-cracked tension configurations under cyclic loads are predicted and compared with experimental results. Qualitatively, the current approach predicts that the crack growth rate remains approximately constant with crack length, which is consistent with experimental observations. The quantitative correlations with the experimental crack growth rates also show excellent agreement. It is also found that the stress intensity factors in the aluminum layers at different through-thickness locations in the GLARE laminates are not identical. The cause and implications of this discrepancy and the advantages of the current approach to predict crack growth rates in generic hybrid laminates are discussed.

Fiber/Metal composite technology for future primary aircraft structures

This paper discusses the structural and material considerations for fiber/metal composite technology for future primary and secondary aircraft structures. Based on these considerations and the experience obtained so far with fiber/metal laminates in primary aircraft structures, the potential field of further development of fiber/metal composite technology will be explained. It is concluded that a composite technology approach, in which both metals and fibers are combined to form a tailored structural material, can lead to significant weight reduction in future structural applications.

Riveting Process Induced Residual Stresses Around Solid Rivets in Mechanical Joints

The interference fit provided by solid rivets introduces a residual stress field beneficial to the fatigue life of riveted joints. Evolution in riveting technology has led to force-controlled riveters which provide greater consistency over the rivet installation process and the resulting residual stress field. By reexamining the rivet installation process and its effects on the formation of residual stresses, the fatigue benefits of rivets could be further exploited. Using a 3-D finite element model, installation of universal and countersunk rivets in monolithic aluminum sheet has been studied. Aspects of accepted riveting practice, including the degree of rivet flushness and the rivet squeeze force were found to play significant roles in the formation of residual stresses. Residual stresses beneath the rivet head were also found to be influenced primarily by through-thickness compression of the joined sheets during riveting, challenging the traditional analogy of riveting to radial expansion processes.

Damage evolution in GLARE fibre-metal laminate under repeated low-velocity impact tests

An experimental study was performed on the repeated low-velocity impact behaviour of GLARE. Damage evolution in the material constituents was characterised with successive number of impacts. Records were correlated with visual inspection, ultrasound C-scan and chemical etching. The stiffness of the plate varied when cumulating the number of impacts. Damage accumulation was limited thanks to the synthesis of unidirectional composite and metal. The glass/epoxy plies with high elastic tensile strength could withstand several impacts before perforation despite delamination growth in the vicinity of the impacted area. The damage tolerant aluminium layers prevented the penetration of the projectile and avoided the expansion of delamination. This efficient mechanism preserved the structural integrity of GLARE until first aluminium cracking at the non-impacted side. Among the different failure modes, plate deformation absorbed most of the impact energy. The findings will support the development of a generic quasi-static analytical model and numerical methods.

Effects of Rivet Installation on Residual Stresses and Secondary Bending Stresses in a Riveted Lap Joint

Within a mechanically fastened lap joint, the interference fit and rotational constraint provided by a rivet is dependant on its installation. These two factors in turn aect the formation of residual stresses and secondary bending stresses, the key stress components which contribute to the nucleation and propagation of fatigue cracks. This paper presents a three-dimensional finite element investigation into the impact of rivet installation on these stress components and the resulting fatigue performance of riveted lap joints. Using a two-step simulation, rivet installation in a 2-row riveted lap joint and subsequent uniaxial loading of the lap joint are simulated to determine the impact of rivet installation on the formation of residual stresses and secondary bending stresses. Results from this investigation have provided new insights into the formation of these stress components. Through-thickness compression of the joined sheets during riveting was identified as a key contributor to the formation of residual stresses beneath the rivet head. Secondary bending stresses within a particular sheet were also found to be influenced primarily by the geometry of the adjacent rivet head.

Numerical and experimental investigations of effects of residual stresses on crack behavior in Aluminum 6082-T6

In the structural integrity assessment, residual stresses play an important role. The residual stresses affect both the crack driving forces and the crack-tip constraint. To investigate the interaction of residual stresses with mechanical loading during the onset of crack growth in Aluminum 6082-T6, modified single edge-notched bending specimens were used. Aluminum 6082 has the highest strength of the 6000 series alloys with excellent corrosion resistance. A residual stress field was created in the specimens by pre-loading. To accurately quantify the residual stress field created during this test procedure, the strains were measured during loading and unloading and compared with finite element results. After the introduction of the residual stress field, the specimens were tested under three-point bending to determine the load versus displacement behavior and fracture toughness. Also, a post-processor for finite element calculation was developed to enable determination of the J-integral values for the specimens having residual stresses. The constraint parameters Q and R were calculated at the crack-tip to describe the stress field in this region. The parameter Q is used to characterize the loading and geometry constraint, and the parameter R is used for characterizing the crack-tip constraint due to residual stresses. It is observed that tensile residual stresses around the crack-tip increase the crack-tip constraint and decrease the fracture toughness of the bodies. By increasing the external load, the constraint parameter R goes toward zero and the effects of residual stresses on the crack growth resistance become negligible.

Crack-Tip Behavior in Fiber/Metal Laminates by Means of Digital-Image Correlation

This paper presents the study on the crack-tip behavior in fiber/metal laminates under static loading. The effect on the strain field due to the variation of layup and fiber/preimpregnated has been analyzed. The strain field has been measured using digital-image correlation and evaluated by comparing it with the strain field predicted with finite-element analysis, which provided information about the interlaminar shear mechanisms. In addition, the paper demonstrates the power of the digital-image correlation technique for in situ strain measurements, offering new insight into the damage mechanisms that prevail in fiber/metal laminates under static loading.

An Experimental Approach to Investigate Detailed Failure Mechanisms in Fibre Metal Laminates

This paper propose an experimental approach, based on digital image correlation, which enables detailed quantitative de scription of the most important failure mechanisms occurring in Fibre Metal Laminates during fatigue and static load. Digital image correlation provided measurement of the crack tip plasticity, fatigue delamination shape and fibre bridging in di fferent FMLs. Digital image correlation showed to be the most versatile and sui table non-destructive and in-situ technique for detailed full-field strain measuremen ts in FMLs.

Practical Applications of Improvements in Fml Crack Bridging Theory

Fiber metal laminates (FMLs) have excellent crack growth performance compared to monolithic metals thanks to crack bridging by intact fibers in the wake of a fatigue crack. Calculating the distribution of bridging loads in the fibers is key to analyzing and predicting the crack growth of FMLs. Most analytical approaches to modelling this phenomenon do so by imposing compatibility between the deformation of cracked metal layers and the elongation of the bridging fibers. In doing so, they assume that the crack opening displacement is equal to the displacement of the metal sheets at the boundary of the delamination between the cracked and the bridging layers. This paper derives a solution to the crack bridging problem that accounts for the deformation of the metal between the crack flanks and the delamination boundary. The results of doing so show that neglecting that deformation is acceptable for FML crack growth prediction, but the solution incorporating the exact displacement of the metal layers enables the application of the crack bridging method to a variety of additional situations as well as the extension of its applicability to more complex FMLs. This paper surveys a number of such applications, including examples of how to apply crack bridging theory to these problems. H.J.K. Lemmen M. Bos (ed.), ICAF 2009, Bridging the Gap between Theory and Operational