Andreas T. Echtermeyer

Lifetime and young's modulus changes of glass/phenolic and glass/polyester composites under fatigue

Abstract Fatigue of composites causes a loss of stiffness and development of damage before ultimate failure. These effects were compared for phenolic and polyester resins reinforced by five layers of woven roving/chopped strand combination glass mat. Such polyester laminates are widely used, e.g. in marine applications such as high speed craft. The phenolic laminates are promising candidates for applications where fire resistance is important. S-N and e-N curves were obtained. In addition, stress versus strain curves were measured continuously under reverse loading (R = -1) up to 106 cycles. Analysis of these curves allows further insight into the fatigue process. Tensile moduli drop dramatically (25%) with increasing number of cycles, but remain fairly constant in the compressive region. This behaviour can be explained by effects of matrix cracking. It also indicates the need to define different moduli for non-linear stress-strain curves. Absorbed energy (damping) can also be determined from the stress-strain curve and can be used as an indicator of the onset of rapid degradation of the material leading to ultimate failure. All data are compared for polyester and phenolic matrix laminates.

Estimation of fatigue curves for design of composite laminates

A statistical method for estimation of tolerance bounds for uncertain variables is presented. The method is used in conjunction with regression analysis results to estimate tolerance bounds for fatigue relations, such as e-N and S-N curves, for prediction of the fatigue life of composite laminates in design. In general, this is not a very user-friendly method. A simplification based on fulfilment of a certain assumption about the available test data is introduced, and a graphical representation of the results by the so-simplified method is presented that provides easy-to-use results for practical purposes, such as for design. Applications are demonstrated by examples.

Design procedure for reducing the stress concentration around circular holes in laminated composites

The stress concentration around a hole in a laminated composite may be decreased by increasing the thickness (bonding of two-dimensional panels) in this area. The problem may be described as an infinite orthotropic membrane with a finite circular orthotropic inclusion (ring) and a circular hole in the middle of the ring under arbitrary in-plane loading. A method for calculation of the stress concentration at the edge of the hole is presented. The method is based on the analytical solution for the case of an infinite inclusion. The finite dimensions of the inclusion are taken into account by means of a correction coefficient which is established empirically by processing a large amount of finite element results.

Assessment of FRP sandwich structures for marine applications

The paper presents a broad discussion of challenges currently faced in analysis and design of fibre-reinforced plastic sandwich structures for marine applications. Specific reference is made to the complexity of the characterisation of the material itself, and the establishment of specific design criteria. Experimental studies are described, including testing of beam specimens and hull sections. On this basis, some recommendations are made regarding analysis and design of sandwich structures.

Integrating Durability in Marine Composite Certification

A brief overview is given on how design standards are written. Static strength, fatigue, stress rupture, damage tolerance and the effect of the environment are addressed. Examples are given to show how durability is or can be addressed in design standards. Extensive testing is required today to certify long-term properties. Suggestions for research topics to reduce the test effort are discussed.

Bonded patch repairs for metallic structures – A new recommended practice

Bonded patch repairs are an efficient repair method for corroded or cracked metal structures, if welding is inconvenient. Avoiding the fire hazard of welding is a major reason for using patch repairs, but it can also be reduced distortions of the metal parts, protecting heat sensitive materials or equipment near the repair such as cables and so on. The lack of guidelines for performing such repairs has been a major hindrance for using this technology. A new recommended practice ‘Design, Fabrication, Operation and Qualification of Bonded Repair of Steel Structures’ has been published addressing: when a repair can be applied, which failure mechanisms need to be addressed, which material properties are needed, fabrication-related issues and in-service inspection. This paper will give a brief introduction to the document and its application areas.

A NEW GUIDELINE FOR DESIGN IN FRP

ABSTRACT No general design code for Fibre Reinforced Plastic (FRP) structures or components has existed up to today, except for some special applications. The realisation of even simple designs of FRP structures tends to become a major undertaking due to the lack of applicable design standards. The lack of a good FRP guideline is one of the major obstacles to utilise FRP structures in a reliable and economical way. A general guideline has been developed now for the design of load carrying structures and components fabricated from fibre-reinforced plastics and sandwich structures. The guideline is based on a probabilistic foundation using limit state formulations and it follows the Load and Resistance Design Format. This state of the art approach ensures a consistent level of safety throughout the guideline. The format is also flexible to accommodate easily new technical results from research or future developments. The guideline has been developed within a large international joint industry project. It will be published as DNV Offshore standard OS-C501 “Composite Components”. The format is also compatible with ISO standards.

A New Approach to Repair of FPSO’s Without Hot Work

FPSO’s stay in production at sea for a period of 20 years or more. Repairs of an FPSO in service can be difficult to perform. Welding, which is the most common repair method creates a fire hazard. Consequently there is a need to consider alternative repair methods. An adhesive bonded patch is an alternative repair method. The paper present the challenges for qualifying the composites patches repair of FPSO hulls.Copyright