Brian Hayman

Approaches to Damage Assessment and Damage Tolerance for FRP Sandwich Structures

A review is made of production defects and in-service damage types that arise in sandwich structures having fiber reinforced plastic (FRP) face sheets. A brief overview is given of relevant defect and damage models and how these models can be used in an assessment of criticality with regard to local and global structure, as a basis for deciding on corrective measures, for the case of a naval ship. Challenges resulting from limitations in inspection techniques are discussed. The concept of damage tolerance is discussed in the light of the above. It is argued that the most suitable and economical approach to achieving damage tolerance is dependent on the application.

The Effect of Face Sheet Wrinkle Defects on the Strength of FRP Sandwich Structures

Wrinkle defects may reduce the compressive strength of a face laminate for in-plane loading applied perpendicularly to the line of the wrinkle. To be able to decide whether a repair is needed, it is necessary to know the magnitude of the strength reduction for a given wrinkle geometry. In the studies reported here, the influence of wrinkle defects on the in-plane compressive strength of quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates used in PVC foam-cored sandwich panels has been investigated by three approaches: testing of sandwich beam specimens in four-point bending, testing of sandwich panels with in-plane compression, and finite element simulation. Wrinkles involving different numbers of plies have been considered. Two different sandwich lay-ups typical of deck and hull bottom panels in naval ships have been included.

Fabrication, Testing and Analysis of Steel/Composite DLS Adhesive Joints

This paper describes experimental and numerical techniques to study the structural design of double lap shear joints that are based on thick-adherend steel/steel and steel/composite, with epoxy adhesive. A standard practical fabrication method was used to produce specimens of various dimensions and materials. These specimens consist of 10 mm steel inner adherend and various outer adherend materials including composite and steel of various thicknesses and overlaps. The composite is largely based on carbon fibre-reinforced plastic. The specimens were tested under monotonic tensile loading and the results showed that joint strength depends largely on materials combination and overlap length. The testing also included the use of an advanced imaging system to determine failure initiation and propagation. Two-dimensional finite element analysis (FEA) stress models were applied and showed the importance of modelling the composite layers adjacent to the adhesive bondline in order to account for the critical local stresses. The FEA results also showed that overall shear stress distributions can be used to characterise joint failure. The paper presents the experimental and numerical details with key conclusions.

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.

Failure of uniformly compression loaded debond damaged sandwich panels — An experimental and numerical study

This paper deals with the failure of compression-loaded sandwich panels with an implanted circular face/core debond. Uniform compression tests were conducted on intact sandwich panels with three different types of core material (H130, H250 and PMI) and on similar panels with circular face/core debonds having three different diameters. The strains and out-of-plane displacements of the panel surface were monitored using the digital image correlation technique. Mixed mode bending tests were conducted to determine the fracture toughness of the face/core interface of the panels. Finite element analysis and linear elastic fracture mechanics were employed to determine the critical buckling load and compression strength of the panels. Modeling approaches and failure criteria are discussed. Numerically determined crack propagation loads in most of the cases show a fair agreement with experimental results, but in a few cases up to 45% deviation is seen between numerical and experimental results. This can be ascribed to several factors such as the large scatter in the measured interface fracture toughness, differing crack tip details and crack growth mechanisms between the panels and the mixed mode bending specimens. Tentative strength reduction curves are presented, but uncertainty concerning the intact strengths of the materials used needs to be removed before these can be utilized with confidence.

Damage Assessment and Damage Tolerance of FRP Sandwich Structures

A review is made of production defects and in-service damage types that arise in sandwich structures having fibre reinforced polymer (FRP) face sheets. A brief overview is given of relevant defect and damage models and how these models can be used in an assessment of criticality with regard to local and global structure, as a basis for deciding on corrective measures, for the case of a naval ship. Challenges resulting from limitations in inspection techniques are discussed. The concept of damage tolerance is discussed in the light of the above. It is argued that the most suitable and economical approach to achieving damage tolerance is dependent on the application.

The Influence of Face Sheet Wrinkle Defects on the Performance of FRP Sandwich Structures

Wrinkle defects may reduce the compressive strength of a face laminate for in-plane loading applied perpendicularly to the line of the wrinkle. To be able to decide whether a repair is needed it is necessary to know the magnitude of the strength reduction for a given wrinkle geometry. In the studies reported here, the influence of wrinkle defects on the in-plane compressive strength of quasi-isotropic CFRP laminates used in PVC foam-cored sandwich panels has been investigated by three approaches: testing of sandwich beam specimens in four-point bending, testing of sandwich panels with in-plane compression, and finite element simulation. Wrinkles involving different numbers of plies have been considered. Two different sandwich lay-ups typical of deck and hull bottom panels in a naval ship have been included.