The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

American Society for Testing and Materials

Biocomposites reinforced with natural fibers: 2000–2010

Double-network (DN) gels have drawn much attention as an innovative material having both high water content (ca. 90 wt%) and high mechanical strength and toughness. DN gels are characterized by a special network structure consisting of two types of polymer components with opposite physical natures: the minor component is abundantly cross-linked polyelectrolytes (rigid skeleton) and the major component comprises of poorly cross-linked neutral polymers (ductile substance). The former and the latter components are referred to as the first network and the second network, respectively, since the synthesis should be done in this order to realize high mechanical strength. For DN gels synthesized under suitable conditions (choice of polymers, feed compositions, atmosphere for reaction, etc.), they possess hardness (elastic modulus of 0.1–1.0 MPa), strength (failure tensile nominal stress 1–10 MPa, strain 1000–2000%; failure compressive nominal stress 20–60 MPa, strain 90–95%), and toughness (tearing fracture energy of 100∼1000 J m−2). These excellent mechanical performances are comparable to that of rubbers and soft load-bearing bio-tissues. The mechanical behaviors of DN gels are inconsistent with general mechanisms that enhance the toughness of soft polymeric materials. Thus, DN gels present an interesting and challenging problem in polymer mechanics. Extensive experimental and theoretical studies have shown that the toughening of DN gel is based on a local yielding mechanism, which has some common features with other brittle and ductile nano-composite materials, such as bones and dentins.

Why are double network hydrogels so tough

: An overview of the virtual crack closure technique is presented. The approach used is discussed, the history summarized, and insight into its applications provided. Equations for two-dimensional quadrilateral elements with linear and quadratic shape functions are given. Formula for applying the technique in conjuction with three-dimensional solid elements as well as plate/shell elements are also provided. Necessary modifications for the use of the method with geometrically nonlinear finite element analysis and corrections required for elements at the crack tip with different lengths and widths are discussed. The problems associated with cracks or delaminations propagating between different materials are mentioned briefly, as well as a strategy to minimize these problems. Due to an increased interest in using a fracture mechanics based approach to assess the damage tolerance of composite structures in the design phase and during certification, the engineering problems selected as examples and given as references focus on the application of the technique to components made of composite materials.

/pdf/virtual-crack-closure-technique-history-approach-and-53akn4ewzt.pdf

Virtual crack closure technique: History, approach, and applications

An engineering solution for mesh size effects in the simulation of delamination using cohesive zone models

This work studies the hybrid effect in high performance Quasi-Isotropic (QI) composite plates under tensile loading. The hybrid effect is the enhancement in strain at failure of the carbon fibre layers in the hybrid composite compared with a pure carbon fibre composite. Different types of QI lay-ups were tested and analysed to understand the interaction between the layers and the effect of the stiffness of the adjacent sub-laminates. A numerical model using finite element method was proposed to evaluate the strain distribution through the thickness considering a premature failure of one of the 0° carbon plies, the strain gradient was calculated as well as the average strain concentration in the undamaged 0° carbon ply. Correlations between pseudo-yield strain, strain concentration in the undamaged 0° carbon ply and stiffness of the sub-laminates were identified. Additionally, microscopic observations of specimens interrupted before the final failure were studied to recognise the damage mechanisms and measure the fragmentation distance.

Investigation of the hybrid effect in high performance quasi-isotropic thin-ply carbon/glass composites under tension

The purpose of this paper is to demonstrate a novel method for the intrinsic repair of sub critical damage in complex composite structures (T-joints). First, the failure modes of CRFP T-joints subject to static and fatigue loading were established and the influence of layup and interfacial ply orientation on failure was investigated. Based on these results, a number of strategies were proposed to mitigate the extent of damage during failure and subsequently implement an in-situ repair. The ability to isolate crack paths through the tailoring of interfacial ply orientation has been demonstrated. Finally, the ability to introduce repair ‘infrastructure’ in the form of hollow vasculature and thermoplastic interleaves, without influencing joint strength has been demonstrated.

/pdf/damage-manipulation-repair-in-composite-t-joints-4j3msiu2nb.pdf

Damage manipulation & repair in composite t-joints

An investigation into the size effects in notched quasi-isotropic carbon/epoxy laminates is carried out. Two scaling laws are used to predict the notched tensile strength. Specifically, a Linear Elastic Fracture Mechanics (LEFM) approach based on Dugdale’s plastic zone model originally for metallic materials is applied to predict the tensile strength of the large composite laminates with sharp notches. A method based on Weibull statistics is used to predict the tensile strength of the large composite laminates with circular holes. These two scaling lines are drawn independently from the experimental notched tensile strengths. A good agreement between the predicted and experimental results of the large notched composite laminates is achieved.

/pdf/predicting-the-notched-strength-of-large-quasi-isotropic-20pw7mzjjp.pdf

Predicting the notched strength of large quasi- isotropic composite laminates based on small specimen tests

A Comparison of Damage Mechanisms in Scaled Static Indentation Tests on Glass and Carbon Fibre Laminates

As part of the HiPerDuCT programme on High Performance Ductile Composites Technology thin-ply glass-carbon hybrid laminates have been investigated. It has been demonstrated that if the carbon plies are very thin, delamination can be suppressed, and progressive fragmentation of the carbon can be obtained [1], with a pseudo-ductile unidirectional tensile response and plateau in the stress-strain curve, e.g. Fig. 1. The strain at first carbon fracture can be determined through analysis of videos recorded during the tests. In this presentation the values of failure strain are investigated for different carbon ply thicknesses and compared with those for un-hybridised carbon fibre material in order to establish the magnitude of the hybrid effect. It is shown that much of the apparent hybrid effect is due to too low values of failure strain for the carbon caused by premature failure in unidirectional tensile tests. However for very thin plies there is indeed a real hybrid effect resulting in higher strains to failure which increases the performance and attractiveness of these pseudo-ductile hybrid materials.

Michael R Wisnom

Papers

Investigation of the hybrid effect in high performance quasi-isotropic thin-ply carbon/glass composites under tension

Damage manipulation & repair in composite t-joints

Predicting the notched strength of large quasi- isotropic composite laminates based on small specimen tests

A Comparison of Damage Mechanisms in Scaled Static Indentation Tests on Glass and Carbon Fibre Laminates

Hybrid effects in thin-ply laminates