K.B. Katnam

Static and Fatigue Failures of Adhesively Bonded Laminate Joints in Moist Environments

Advanced structural adhesives are now an important joining technique in automobile and aerospace applications. The perceived uncertainty in the long-term structural performance of bonded members when subjected to static/fatigue loads in aggressive environments is probably restricting an even more widespread use of this joining technology. In this article, the effect of moisture on the static and fatigue resistances of adhesively bonded laminate joints was investigated. Experimental tests were performed on both aged and unaged adhesively bonded laminate joints for static and fatigue responses. Further, using a cohesive zone approach for the adhesive bondlines, a combined diffusion–stress analysis was developed to predict the progressive damage observed in the joints tested experimentally. The numerical predictions were found to be in good agreement with the experimental test results.

Load Ratio Effect on the Fatigue Behaviour of Adhesively Bonded Joints: An Enhanced Damage Model

Structural adhesives are used widely in aerospace and automotive applications. However, fatigue damage in these adhesives is an important factor to be considered in the design of adhesively bonded structural members that are subjected to cyclic loading conditions during their service life. Fatigue life of adhesively bonded joints depends mainly on the fatigue load and the load ratio. A fatigue damage model is presented in this paper to include the effect of fatigue mean stresses on the failure behaviour of adhesively bonded joints. The fatigue damage model is developed using an effective strain-based approach. The model is implemented on a tapered single lap joint configuration and is validated by experimental test results. The adhesive layer in the tapered single lap joint is modelled by using a cohesive zone with a bi-linear traction-separation response. The adverse effect of increasing fatigue mean stresses on the failure behaviour of adhesively bonded joints is successfully predicted.

Composite Repair in Wind Turbine Blades: An Overview

Renewable energy sources such as wind energy—together with energy-efficient technologies—are essential to meet global energy demands and address climate change. Fiber-reinforced polymer composites, with their superior structural properties (e.g., high stiffness-to-weight) that allow lightweight and robust designs, play a significant part in the design and manufacture of modern wind turbines, especially turbine blades, for demanding service conditions. However, with the current global growth in onshore/offshore wind farm installations (with total global capacity of ∼282 GW by the end of 2012) and trend in wind turbine design (∼7–8 MW turbine capacity with ∼70–80 m blade length for offshore installations), one of the challenges that the wind energy industry faces with composite turbine blades is the aspect of structural maintenance and repair. Although wind turbines are typically designed for a service life of about 20 years, robust structural maintenance and repair procedures are essential to ensure the structural integrity of wind turbines and prevent catastrophic failures. Wind blades are damaged due to demanding mechanical loads (e.g., static and fatigue), environmental conditions (e.g., temperature and humidity) and also manufacturing defects. If material damage is not extensive, structural repair is the only viable option to restore strength since replacing the entire blade is not cost-effective, especially for larger blades. Composite repairs (e.g., external and scarf patches) can be used to restore damaged laminate/sandwich regions in wind blades. With composite materials in the spar (∼30–80 mm thick glass/carbon fiber laminates) and aerodynamic shells (sandwich sections with thin glass fiber skins and thick foam/wood as core), it is important to have reliable and cost-effective structural repair procedures to restore damaged wind blades. However, compared to aerospace bonded repairs, structural repair procedures in wind blades are not as well developed and thus face several challenges. In this regard, the area of composite repair in wind blades is broadly reviewed to provide an overview as well as identify associated challenges.

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.

Experimental Analysis of the Bondline Stress Concentrations to Characterize the Influence of Adhesive Ductility on the Composite Single Lap Joint Strength

Adhesively bonded composite single lap joints were experimentally investigated to analyze the bondline stress concentrations and characterize the influence of adhesive ductility on the joint strength. Two epoxy paste adhesives—one with high tensile strength and low ductility, and the other with relatively low tensile strength and high ductility—were used to manufacture composite single lap joints. Quasi-static tensile tests were conducted on the single lap joints to failure at room temperature. High magnification two-dimensional digital image correlation was used to analyze strain distributions near the adhesive fillet regions. The failure mechanisms were examined using scanning electron microscopy to understand the effect of adhesive ductility on the joint strength. For a given surface treatment and laminate type, the results show that adhesive ductility significantly increases the joint strength by positively influencing stress distribution and failure mechanism near the overlap edges. Moreover, it is shown that high magnification two-dimensional digital image correlation can successfully be used to study the damage initiation phase in composite bonded joints.

Design of cold-formed steel purlins attached to roof sheeting based on Eurocode 3: Effect of gravity load

In the design of cold-formed steel purlins based on Eurocode 3, the lateral bending moment in the free flange of the purlin, when in tension, is assumed to be zero due to flange curling and second order effects. To investigate the validity of this design assumption, non-linear analytical and finite element models for analyzing how the gravity load affects the maximum mid-span stresses in cold-formed steel purlins attached to the roof sheeting are presented in this paper. The second-order effect of tensile stresses induced by the gravity load in the free flange of purlins is investigated by emphasizing on Z and C sections. The results obtained from the two models are compared with Eurocode 3, and they are included in this article. It is observed that for shorter purlin span lengths the design assumption underestimates the maximum mid-span stresses.

Special Issue on Primary Bonded Repairs in Composite Structures

In recent years, there has been a significant increase in the usage of advanced polymer composites in the design of engineering structures – especially for primary load carrying structural members. For example, to combat the environmental threat that aviation industry poses, the aerospace industry has been aiming to considerably reduce emissions through weight reduction, aerodynamic improvements, and new aircraft concepts. For significant weight reduction, the application of composite materials in aircraft design is globally considered as one of the key technologies to meet emission targets. Advanced composite materials have replaced traditional structural materials in new generation civil aircraft structures to a significant extent (e.g., Boeing 787, Airbus A350, and Bombardier CSeries, with approximately 50% composites by weight). In addition to the improvements in fuel-efficiency and emission reduction due to lightweight designs, composite aircraft also improve passenger comfort by allowing more comfortable levels of cabin pressure and humidity. Similarly, the global wind energy industry has in recent years seen a significant growth through on-shore and off-shore wind farms – with individual turbine capacity ranging from 1 to 7MW (e.g., Vestas V90 3MW, Gamesa G128 4.5MW, and Enercon E126 7.5MW) and the development of larger turbines with 8–10MW for off-shore installation. With increasing installations and growing turbine sizes, advanced composites play a key role in the design of wind turbine blades. However, while composites are currently being used in both secondary and primary structural components, the safety, efficiency, and service life of composite structures will depend on structural maintenance and repair techniques. Moreover, as recycling thermoset-based composite materials is a major challenge, extending the service life of damaged composite components made of thermoset polymers is very important. While composite research and development activities are in progress worldwide, robust technologies for repairing primary composite structures is absolutely essential for the industry at this important juncture. The focus of this special issue of The Journal of Adhesion is on primary bonded repair of composite structures. This issue contains five invited papers on the aspects of primary composite bonded repairs. The first paper presents the challenges involved in the certification of primary bonded aircraft repairs. Although application of reinforcements attached to the parent structure by adhesive bonding is generally the The Journal of Adhesion, 91:1–3, 2015 Copyright # Taylor & Francis Group, LLC ISSN: 0021-8464 print=1545-5823 online DOI: 10.1080/00218464.2014.916105

ON THE IMPERFECTIONS OF CYLINDRICAL SHELLS ON LOCAL SUPPORTS

Times Cited: 0 1st International Conference on Computational Methods (ICCM04) Dec 15-17, 2004 Singapore, SINGAPORE Natl Univ Singapore, Dept Mech Engn