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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.

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Virtual crack closure technique: History, approach, and applications

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

A pseudo ductile angle-ply sub-laminate approach for multidirectional thin ply CFRP laminates

ECCM16 - 16th European Conference on Composite Materials

A novel test configuration has been developed to induce combined stress-states of inplane longitudinal tension and transverse compression in unidirectional (UD) composite layers. Two different multi-directional laminates have been designed incorporating UD carbon/epoxy plies embedded in angle-ply blocks of the same material. The scissoring deformation of the angle-plies induces compression in the central UD layers when the composite is strained in the 0° fibre direction. The amount of transverse compressive stress is determined through an inverse identification method from the measured surface strains of the laminates. Despite the large in-plane transverse compressive strain generated, there is only about 9% drop in the failure strain of the laminates when compared to the baseline failure strain of the UD carbon/epoxy material.

/pdf/a-novel-test-method-to-induce-bi-axial-stress-states-in-thin-3qutitt4s6.pdf

A Novel Test Method to Induce Bi-Axial Stress States in Thin-Ply Carbon Composites Under Combined Longitudinal Tension and Transverse Compression

Composite Materials: Fatigue and Fracture

Two methods were used to investigate the variability of tensile failure strain of unidirectional high strength carbon/epoxy. Scaled tests on glass/carbon hybrid specimens showed a significant size effect and reduction of failure strain with stressed volume consistent with a Weibull modulus of 25. Ply fragmentation tests were also carried out, producing multiple carbon fractures in a single specimen. Strains from these tests also fitted a Weibull distribution, and gave a similar Weibull modulus to the scaled tests.1

/pdf/new-approaches-to-quantifying-tensile-strength-variability-4lu4806pvn.pdf

Michael R Wisnom

Papers

A pseudo ductile angle-ply sub-laminate approach for multidirectional thin ply CFRP laminates

ECCM16 - 16th European Conference on Composite Materials

A Novel Test Method to Induce Bi-Axial Stress States in Thin-Ply Carbon Composites Under Combined Longitudinal Tension and Transverse Compression

Composite Materials: Fatigue and Fracture

New approaches to quantifying tensile strength variability and size effects in unidirectional composites