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The programme aims to develop an understanding of plane stress plane strain conditions inside materials and their effect on fracture surfaces.
Metadata describing this Open University video programme
Module code and title: T351, Materials under stress
Item code: T351; 05
First transmission date: 24-04-1976
Published: 1976
Rights Statement:
Restrictions on use:
Duration: 00:22:32
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Producer: David Nelson
Contributors: Ron Jones; Keith Williams
Publisher: BBC Open University
Keyword(s): Animation; Fracture surfaces; Models; Plane stress/strain; Theory of G and K
Footage description: Keith Williams introduces the programme. He points out and defines the two parameters, G (crack free energy) and K (critical stress intensity factor) which are used to determine fracture toughness. Williams, holding a fracture toughness testing specimen, poses the question "Why is the specimen that particular shape and thickness?" To demonstrate how choice of specimen affects the stress conditions inside the material, Ron Jones uses a series of models and animations. He begins with a planar sheet which has a grid marked on it and demonstrates a 'Poissons contraction'. Jones points out the biaxial stress which results. He goes on to examine a thicker model and one with a slit cut in the side pointing out the stresses in each case. Williams uses an animated model of a specimen to show deformation at the surface and inside the specimen. In order to explain how a specimen can be examined quantitatively, Williams sets up a coordinate axis on a drawing of a specimen. He then shows how this concept is applied to the models shown earlier to determine plane stress and plane strain. Williams demonstrates the transition from plane stress at the surface of a specimen to plane strain at the centre. He uses an animated model to illustrate the points. Ron Jones uses an animated model and a stress contour map to show stress distributions within a specimen. He demonstrates how the shear stress distribution changes with distance from the surface to the centre. Jones goes on to show the effect this has on fracture lines in specimens. Williams, holding a specimen and standing in front of a hydraulic tensile testing machine, explains how load and displacement are plotted for specimens with various size cracks. Williams uses a graph to show compliance against crack length. Williams explains how K, the stress intensity factor, is calculated for a finite test specimen. An animation of the specimen shows the vital dimensions needed for the calculation. Williams explains how a notch (simulated crack) is put into a specimen to act as a stress concentrator. The mathematical expression for calculating K is captioned. Film shots of a specimen undergoing a tensile fracture test. The test data is used to calculate stress intensity (K) at point of fracture. Williams examines the fracture features of the above specimen. These indicate that the specimen fractured under plane stress conditions. Williams goes on to look at the fracture surface of another, thicker and tougher specimen. This time shear lips can be seen and the specimen shows signs of having fractured under plane strain conditions. Williams sums up, very briefly.
Master spool number: 6HT/71895
Production number: 00525_5256
Videofinder number: 1416
Available to public: no