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

Reference EPFL-ARTICLE-184271doi:10.1016/j.compositesa.2012.08.001View record in Web of Science Record created on 2013-02-27, modified on 2017-05-10

Composites Part A: Applied Science and Manufacturing

Investigations of the effect of size on the tensile strength of composite laminates containing circular holes show that there is a large difference both in failure stress and mechanism due to changes in test configuration. This is particularly true of the ply and laminate thickness, and hole diameter. Interrupted tests have been performed on open hole tensile specimens at different load levels to determine the progressive damage development, evaluated through non-destructive testing (X-ray and C-scanning). The tests were also analysed using a novel Finite Element Modelling technique. This was able to accurately predict the wide range of ultimate strengths measured with variation in test parameters, principally through incorporation of the sub-critical damage in the analysis. A significant damage mechanism was seen to be delamination at the hole edge which generally occurred at a lower stress for a smaller hole diameter to ply block thickness ratio. Delaminations allowed damage to join up through the thickness of the laminate and propagate. In ply-level scaled specimens, the delamination propagation was the ultimate failure mode of most of the specimens. In sub-laminate level scaled specimens, localised damage relieved stress in the 0° fibres at the hole edge, delaying the onset of fibre failure. Less damage was seen for larger holes, thus leading to a decreasing failure stress with increasing hole diameter.

/pdf/an-experimental-and-numerical-investigation-into-the-damage-1elbb8lbc9.pdf

An experimental and numerical investigation into the damage mechanisms in notched composites

An extensive experimental program has been performed to investigate the effect of scaling on the tensile strength of notched composites. Hole diameter, ply and laminate thickness, were investigated as the independent variables, whilst keeping constant ratios of hole diameter to width and length, over a scaling range of 8 from the baseline size. In most cases strength decreased as specimen size increased, with a maximum reduction of 64%. However the reverse trend of strength increasing with in-plane dimensions was found for specimens with plies blocked together. As well as the variation in strength, three distinct failure mechanisms were observed: fibre failure with and without extensive matrix damage, and complete gauge section delamination. Despite these differences, similar sub-critical damage mechanisms were seen in all specimens, with the extent of the damage determining the failure stress and mechanism. Damage propagated across the gauge section via delamination at the hole, which was controlled by the ply thickness to hole diameter ratio. This same mechanism can explain both the increasing and decreasing strengths observed. Simple analytical criteria for determining notched strength were found to be accurate for fibre failure in the absence of extensive sub-critical damage, but could not account for those conditions where delamination propagated across the width prior to failure.

An experimental investigation into the tensile strength scaling of notched composites

The mechanisms causing residual stresses and distortions are considered, and the way they develop during the cure discussed. Experimental results are presented through the cure cycle on curvature of unsymmetric laminates, spring-in of curved sections and stresses in flat plates due to tool–part interaction. It is shown that thermal stresses after vitrification are the main cause of distortion of unsymmetric plates, whereas thermal stresses and chemical shrinkage, while the material is in the rubbery state, both contribute to spring-in. Large stresses can develop due to tool–part interaction, even before gelation.

Michael R Wisnom

Papers

Composites Part A: Applied Science and Manufacturing

An experimental and numerical investigation into the damage mechanisms in notched composites

An experimental investigation into the tensile strength scaling of notched composites

Mechanisms generating residual stresses and distortion during manufacture of polymer-matrix composite structures

Size effects in the testing of fibre-composite materials