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.     

Thermal imidization of poly(amic acid) precursors on surface-modified poly(tetrafluoroethylene) films via graft copolymerization with glycidyl methacrylate

A novel method for preparing composites of polyimides (PI) laminated to poly(tetrafluoroethylene) (PTFE) films is reported. PI/PTFE composites were developed through thermal imidization of poly(amic acid) (PAA) precursors on surface-modified PTFE films. Surface modification of PTFE films was carried out via Ar plasma pretreatment of the films, followed by UV-induced graft copolymerization with glycidyl methacrylate (GMA). The surface composition and topography of the graft copolymerized PTFE films and the delaminated PI and PTFE surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. The adhesion strengths of the PI (imidized PAA) on the GMA graft copolymerized PTFE films were evaluated as a function of various thermal imidization schedules. The adhesion reliability of the PI/PTFE composites was tested by a series of hydrothermal cycles. The development of strong Tpeel adhesion strengths of about 8 N/cm with excellent reliability for the PI/PTFE composites was attributable to the synergistic effect of coupling the curing of the epoxide functional groups of the grafted GMA chains with the imidization process of the PAA and the fact that the GMA chains were covalently tethered onto the PTFE surface. The PI/PTFE composites delaminated via cohesive failure inside the PTFE substrates. The delaminated PI film with a covalently adhered 'rough' PTFE surface layer exhibited a water contact angle as high as 140°.     

Velocity of elastic waves in porous ceramic materials: influence of pore structure

Abstract

A review has been undertaken of recently published data on ultrasonic velocity-porosity for a variety of porous ceramic materials, including information on pore structure. These experimental data have been compared with data calculated using a spheroidal pore model incorporating information on pore volume fraction, shape, and orientation. Good agreement is obtained between experimental and calculated values, especially when fractional porosity is less than about 0·25, even when a single 'effective' pore shape is employed in the calculation. The agreement improves if a different pore shape for each porosity level (point by point analysis) is used. The predictive ability of the spheroidal pore model is thus demonstrated.     

31—THE HANDLE AND BENDING BEHAVIOUR OF FABRIC LAMINATES

Certain aspects of the handle of fabric laminates are related to properties of the component fabrics. Particular attention is paid to a theoretical prediction of the bending stiffness of a laminate from the bending and tensile properties of its components. This theoretical stiffness is a minimum value; if, in practice, the observed stiffness is much greater than this, it may generally be assumed that excess of adhesive or of melted foam is the cause. The paper also reports work on other features of the bending behaviour, such as the degree of recovery from bending, and on the shearing behaviour. The work is concluded by a brief study of some faulty laminates.     

Interlaminar fracture behaviour of re-formed bamboo/aluminium sheet laminates

Sandwich laminates containing re-formed bamboo core and aluminium face sheets were produced using two different types of adhesive: an epoxy and a modified polyethylene. The interlaminar fracture behaviour of the laminates was characterized based on peel and lap-shear tests. It was shown that the laminates bonded with polyethylene had much higher peel and shear strengths than those bonded with epoxy. For the polyethylene-bonded laminates, the major failure mechanisms were a combination of cohesive and interfacial failure, whereas for the epoxy-bonded laminates, the fracture occurred almost exclusively along the aluminium/epoxy interface. There was a significant dependence of the failure mechanism and interlaminar strength on the loading direction relative to the bamboo fibre axis and on whether the aluminium sheets were bonded to the inner or outer bamboo surface.     

In situ damage monitoring of cross-ply laminates using acoustic emission

Abstract

'Damage tolerance' is used to describe the attribute of a structure associated with the retention of the required residual strength throughout its service life, while irreversible damage mechanisms are active within the structure itself. 'Design for damage tolerance' is based on the identification and quantification of the various damage mechanisms that result in the alteration (mainly deterioration) of the material properties. These may alter the material response to thermomechanical loads. In the present paper, transparent glass fibre reinforced epoxy laminates were used to study the damage evolution sequence under tensile loading. Acoustic emission was employed as a non-destructive technique for the in situ monitoring of the active damage mechanisms until the final failure of the material. Pattern recognition algorithms were utilised to classify the acquired acoustic emission signals and associate them to active damage mechanisms. Experimental findings were compared to theoretical model predictions.     

Stress and failure analyses of scarf repaired CFRP laminates using a cohesive damage model

This study describes stress and failure analyses of tensile loaded repaired Carbon Fibre Reinforced Composite (CFRP) laminates, using scarf configuration. A numerical model including interface finite elements was used to obtain peel and shear-stress distributions in the directions tangent and normal to the scarf. These stresses were evaluated at several locations in the repair, namely in the middle of the adhesive, at interfaces between adhesive and patch, and between adhesive and parent material. Several scarf angle values were considered in the analysis. A cohesive mixed-mode damage model was also used to carry out the failure analysis, in order to assess the efficiency of the repairs, for different stacking sequences. A study was performed to evaluate the influence of the mechanical properties of the adhesive and parent laminate/adhesive and adhesive/patch interfaces on the strength and failure modes of the joint. It was concluded that the strengths of the adhesive and interfaces are more important than the fracture properties in the failure process of the repair. It was also verified that the strength of the repair increased exponentially with the scarf angle reduction.     

The Static Failure of Adhesively Bonded Metal Laminate Structures: A Cohesive Zone Approach

Adhesively bonded metal laminates are used in aerospace applications to achieve low cost, light weight structures in the aerospace industry. Advanced structural adhesives are used to bond metal laminae to manufacture laminates, and to bond stringers to metal laminate skins. Understanding the failure behaviour of such bonded structures is important in order to provide optimal aircraft designs. In this paper, the static failure behaviour of adhesively bonded metal laminate joints is presented. A cohesive zone model was developed to predict their static failure behaviour. A traction–separation response was used for the adhesive material. Three joint configurations were considered: a doubler in bending, a doubler in tension and a laminated single lap. The backface strains and static failure loads obtained from experimental tests were used to validate the results from finite element modelling. The models were found to be in good agreement with experiments.     

Evaluation of adhesion of continuous fiber–epoxy composite/aluminum laminates

Continuous fiber composite/metal laminates (FMLs) offer significant improvements over currently available composite materials for aircraft structures due to their excellent fatigue endurance and low density. Glass fiber–epoxy composite laminae and aluminum foil (GLARE) are commonly used to obtain these hybrid laminates. In this work, FMLs were produced by treating the aluminum foil to promote adhesion bonding by two methods: sulphuric chromic acid etching (SCAE) and chromic acid anodization (CAA). The surface treatments were evaluated by contact angle, roughness and scanning electron microscopy techniques. In order to compare different families of fiber composite/metal laminates, carbon fiber and glass fiber fabrics were used as reinforcements for the hybrid laminates. The adhesion of the hybrid laminates was evaluated by scanning electron microscopy (SEM) and three-point bending test. CAA resulted in better wetting properties. The interlaminar shear strength results for both carbon fiber-epoxy/metal and glass fiber-epoxy metal, were close to the interlaminar shear strength results found in the literature (approx. 40.0 MPa).     

Non-destructive evaluation of bonded structures with lock-in thermography

Lock-in thermography is employed for non-destructive evaluation of several types of bonded structures, which are commonly encountered in industrial applications. Specimens were fabricated to simulate: adhesively bonded aluminium joints, which are commonly used in aeronautical and automotive fields; bonds between pipes of poly(vinyl chloride) (PVC) employed in the transport of liquids (sewage systems); and bonds between plates of Plexiglas which are widely used in the manufacturing of aquaria. Amongst bonded structures, the composite materials are very important, which are generally made of carbon, glass, or Kevlar^? aramid fibers and epoxy resin, and which find application in many industrial fields, especially the aeronautical industry, because of their higher strength and lower weight as compared to metallic materials. It is known that surface plasma treatment of a material improves its adhesion, but it is also known that this treatment will degrade over time if the material is not bonded immediately. Thus, to assure quality, any bonded system should be monitored by the most effective non-destructive technique. To obtain information about the ability of lock-in thermography to assess the performance of such plasma treatment, several specimens were fabricated from either composites (carbon, or Kevlar^? fabric layers plus epoxy resin), or glass plates with and without surface plasma treatment before bonding. In addition, a sample was obtained from a piece of a typical insulated wall of refrigerator vehicles, which actually is a sandwich of polyurethane foam between two plates of fiberglass. The results obtained show that lock-in thermography is a useful tool for non-destructive evaluation of bonded structures.