ISO 15850-2014 pdf free.Plastics – Determination of tension- tension fatigue crack propagation – Linear elastic fracture mecl hanics (LEFM) approach.
A constant-amplitude cyclic tensile load is imposed on a specimen under suitable test conditions (specimen shape and size, notching, maximum and minimum loads, load cycle frequency, etc.), causing a crack to start from the notch and propagate.
The crack length a is monitored during the test and recorded as a function of the number N of load cycles completed.
Numerical differentiation of the experimental function a(N) provides the fatigue crack growth rate da/dN which is reported as a function of stress intensity factor and energy release rate at the crack tip.
For the case where total number of cycles to failure or time to failure is to be determined, the crack length need not be monitored.
5 Significance and use
Fatigue crack propagation, particularly when expressed as the fatigue crack growth rate da/dN as a function of crack-tip stress intensity factor range K or energy release rate range G, characterizes a material’s resistance to stable crack extension under cyclic loading. Background information on the fatigue behaviour of plastics and on the fracture mechanics approach to fatigue for these materials is given in References [1J and [2].
Expressing da/dN as a function ofKor G provides results that are independent of specimen geometry, thus enabling exchange and comparison of data obtained with a variety of specimen configurations and loading conditions. Moreover, this feature enables da/dN versus K or G data to be utilized in the design and evaluation of engineering structures. The concept of similitude is assumed, which implies that cracks of differing lengths subjected to the same nominal iKor G will advance by equal increments of crack extension per cycle.
Fatigue crack propagation data are not geometry independent in the strict sense since thickness effects generally occur. The potential effects of specimen thickness have to be considered when generating data for research or design.
Anisotropy in the molecular orientation or in the structure of the material, and the presence of residual stresses, can have an influence on fatigue crack propagation behaviour. The effect can be significant when test specimens are removed from semi-finished products e.g. extruded sheets) or finished products. Irregular crack propagation, namely excessive crack front curvature or out-of-plane crack growth, generally indicates that anisotropy or residual stresses are affecting the test results.
This test method can serve the following purposes:
a) to establish the influence of fatigue crack propagation on the lifetime of components subjected to cyclic loading, provided data are generated under representative conditions and combined with appropriate fracture toughness data (see Iso 13586) and stress analysis information;
b) to establish material-selection criteria and inspection requirements for damage-tolerant applications;
6 Test specimens
6.1 Shape and size
6.1.1 Standard specimens
Two different types of specimen can be used: single-edge-notched tensile (SENT) and compact tensile (CT). Figures 1 and 2 describe their geometrical characteristics.
For the case where the test is to be carried out to sample failure for the purpose of determining the total number of cycles to failure or time failure, and where crack propagation need not be monitored, a full notch tensile (FNT) specimen of ISO 16770 and a cracked round bar (CRB) specimenlfl may be also utilized.
6.1.2 Thickness and width
When the specimen thickness h is too small compared to the width w, it is difficult to avoid lateral deflections or out-of-plane bending of the specimen. Conversely, with very thick specimens, through- thickness crack curvature corrections are often necessary and difficulties can be encountered in meeting the through-thickness straightness requirement of &1.
On the basis of these considerations, the following limits are recommended for hand w:
a) for CT specimens, whO < h < w/2;
b) for SENT specimens, w/20 s h < w/4.
It should be noted that the test results are in general thickness dependent: specimens obtained from the same material hut having different thicknesses are likely to give different responses.
It is usually convenient to make the thickness h of specimens equal to the thickness of the sheet sample from which the specimens are cut.ISO 15850 pdf free download.