Jos Sinke

Mechanics of Tailor Welded Blanks: An Overview

Taking advantage of high-tech welding methods, a concept is formed in sheet metal forming community. The so-called tailor-welded blanks (TWBs) are sheet metals that are welded together prior to forming. This technology dates back to the 80’s, and numerous studies are conducted in order to explore different aspects of it. This review paper concerns with mechanics of TWBs. The paper is divided into three major chapters. The first chapter is devoted to mechanical properties of TWBs. Tensile testing, tensile properties, and hardness of TWBs are covered in this chapter. The second chapter deals with the formability of TWBs. The formability testing methods, effect of different parameters on the formability of TWBs, material flow phenomena, control of material flow, stress and strain distributions, and springback behavior are covered in the second chapter. The third chapter is focused on failure and fracture of TWBs. Failure modes and failure criteria are the principal topics of this chapter.

The bearing strength of fiber metal laminates

The bearing strength, in particular the edge distance sensitivity, of several fiber metal laminates of the GLARE family is experimentally evaluated. The behavior of the fiber layer is investigated in detail to obtain a better understanding of its failure mode. Parallel with these experiments, the bearing behavior is studied using a finite element (FE) model. The plasticity in the metal layers, failure of the fiber layers, and friction between pin and laminate, are all implemented in the model. A comparison between experimental and FE results shows a good resemblance in strength and in internal damage pattern for the GLARE laminates. The FE model is used to define the optimal fiber orientation of standard GLARE laminates when loaded in bearing.

Adhesion Properties of Bonded Composite-to-Aluminium Joints Using Peel Tests

In this research, the adhesion properties of bonded composite-to-aluminium joints are evaluated using floating roller peel tests. Tests were performed using two different adhesives and different adherend lay-ups: composite-to-aluminium, composite-to-composite and aluminium-to-aluminium. The results show that floating roller peel tests, widely used in metal bonding, can also be used to assess adhesion properties of composite bonding and composite-to-aluminium bonding. However, attention should be paid on which results are important to take from the peel tests. In adhesion tests, the failure mode is more important than the failure load. The peel load can only be compared when using exactly the same type of flexible adherend. Even when the adhesion properties are good (cohesive failure), the peel load value can decrease up to a factor of 10 when peeling off a composite flexible adherend instead of an aluminium flexible adherend.

Test method to assess interface adhesion in composite bonding

This paper introduces a new type of peel tests dedicated to composite bonding: Composite Peel Tests. This test is inspired on the standard floating roller peel test widely used for metal bonding.The aim of this study is to investigate the potential of the Composite Peel Test to assess interface adhesion in composite bonded structures. To this end, peel tests were performed with nine different types of adhesives and at two environmental temperatures, room temperature and +80°C. The results were compared with the standard floating roller peel tests with Aluminium adherends.The results show that when using the Composite Peel Test good interface adhesion results either in cohesive failure of the adhesive or intra-laminar failure of the composite, while bad adhesion results in adhesive failure. In most cases of good interface adhesion, increasing the temperature favors cohesive failure of the adhesive in detriment of intra-laminar failure of the composite.Peel strengths can be used as a quality indicator of interface adhesion only if using exactly the same type of flexible adherend (peeling-off member). Nevertheless, if cohesive failure is the dominant failure mode, the comparison between adhesives’ peel strength is consistent disregarding of the type of peel-off adherend. Composite Peel Tests are suitable to assess interface adhesion of composite bonded structures.

On the prediction of cure-process shape deviations in fibre metal laminates

An investigation of the fabrication-induced distortions in fibre metal laminates is presented using finite-element modelling and experiments. Cooling down is considered as the main source of distortion. Four fibre metal laminate panels are manufactured and their curvature is measured using digital image correlation and linear variable differential transformer. The curvatures are the response of the non-symmetric lay-up to different parameters like stacking order and number of composite or metal layers. Acceptable agreement between model and experiment in predicting the geometry shows that the laminate shape can be predicted with reliability. A large displacement model should be used for large shape deviations in laminates with high level of non-symmetry. Fibre metal laminates may have single or multi-stable configurations after removal from the layup tool. This phenomenon is analysed and parameters and modelling considerations are investigated to obtain a method to predict the final configuration. Further modelling and experimentation are needed to improve the quality of the predictions with increasing complexity of the component.

Strain characterization of embedded aerospace smart materials using shearography

The development of smart materials for embedding in aerospace composites provides enhanced functionality for future aircraft structures. Critical flight conditions like icing of the leading edges can affect the aircraft functionality and controllability. Hence, anti-icing and de-icing capabilities are used. In case of leading edges made of fibre metal laminates heater elements can be embedded between composite layers. However this local heating causes strains and stresses in the structure due to the different thermal expansion coefficients of the different laminated materials. In order to characterize the structural behaviour during thermal loading full-field strain and shape measurement can be used. In this research, a shearography instrument with three spatially-distributed shearing cameras is used to measure surface displacement gradients which give a quantitative estimation of the in- and out-of-plane surface strain components. For the experimental part, two GLARE (Glass Laminate Aluminum Reinforced Epoxy) specimens with six different embedded copper heater elements were manufactured: two copper mesh shapes (straight and S-shape), three connection techniques (soldered, spot welded and overlapped) and one straight heater element with delaminations. The surface strain behaviour of the specimens due to thermal loading was measured and analysed. The comparison of the connection techniques of heater element parts showed that the overlapped connection has the smallest effect on the surface strain distribution. Furthermore, the possibility of defect detection and defect depth characterisation close to the heater elements was also investigated.

The Effects of Thickness on the Formability of 2000 and 7000 Series High Strength Aluminum Alloys

This paper studies the effects of sheet thickness on the forming limits of high strength aluminum alloys commonly used in the aircraft industry. The selected materials are 2024-T3 and 7075-T6 representing 2000 and 7000 series aluminum alloys. Two sets of experiments are carried out to identify the effects of sheet thickness on the forming behavior of the selected alloys. The first set of the experiments is tensile testing. The tensile properties of sheets with different thickness and different materials including the plasticity parameters are determined in the first set of experiments. The second set of the experiments is air bending. The minimum bending radius of the different series of materials is determined in the second set of experiments. The results of the tensile testing and air bending are studied both separately and in comparison with each other to identify the trends and to understand the mechanisms governing the observed trends. It is shown that the behavior of the studied alloys is to some extent different from the behavior of more ductile aluminum alloys and mild steels.

Improving the adhesion of thin stainless steel sheets for fibre metal laminate (FML) applications

Thin stainless steel sheets hold considerable promise for improving several properties of aluminium based fibre metal laminates (FMLs). To allow incorporation of such sheets in FMLs their adhesion to epoxies used in aerospace applications should be at a high level. The present work describes the effects of chemical and mechanical pretreatments to regular and molybdenum-enriched AISI 301 steel sheets. Based on an in-depth knowledge of aluminium pretreatment for FML applications, also aluminium-coated stainless-steel sheets are investigated. Gritblasting was found to yield the best properties. The effect of coating the steel surface with aluminium was found to be promising, but the bond strength between the aluminium and the steel substrate proved insufficient for thin (0.1 mm) AISI 301 steel sheet.

Prediction of Limit Strains in Limiting Dome Height Formability Test

In this paper, the Marciniak‐Kunczynski (MK) method is combined with the Storen‐Rice analysis in order to improve accuracy of the predicted limit strains in Limiting Dome Height (LDH) test. FEM simulation is carried out by means of a commercial FEM code (ABAQUS) and FEM results are postprocessed by using an improved MK code. It has been shown that while original MK method considerably misspredicts the limit strains, a combination of MK method and Storen‐Rice analysis can predict the dome height with a very good accuracy.

Lamb wave detection in prepreg composite materials with fibre Bragg grating sensors

This paper demonstrates that existing Structural Health Monitoring (SHM) techniques have potential during the production phase in addition to their application for maintenance and for in-flight monitoring. Flaws occur during composite fabrication in industry, due to an imperfect process control and human errors. This decreases production efficiency and increases costs. In this paper, the monitoring of Lamb waves in unidirectional carbon fibre (UD-CFRP) prepreg material is demonstrated using both Fibre Bragg Gratings (FBG)s and piezolectric acoustic sensors, and that these SHM sensors may be used for flaw detection and production monitoring. The detection of Lamb waves in a one ply thick sheet of prepreg UD-CFRP material is demonstrated for an FBG sensor aligned with the carbon fibre orientation and bonded to the surface of the prepreg, Furthermore, the velocity of Lamb waves in prepreg UD-CFRP in different orientations is investigated. Finally the successful detection of a material crack in a prepreg UD-CFRP sheet using the Lamb wave detection method is demonstrated.