Claire De Marco Muscat-Fenech

Elastoplastic trouser tear testing of sheet materials

An elastoplastic analysis of trouser tear testing is presented and differences from the Mai-Cotterell rigid-plastic analysis [1] are highlighted. In particular, the simple linear plot of the latter for tearing force per unit thickness against leg width proves to be a curve with a lower intercept for fracture toughness, since the radius of tearing increases with leg width. Furthermore, the curve passes through a maximum and the tearing force decreases at very large leg widths owing to the effects of elasticity. Experiments on NS4 aluminium alloy and on 64/36 brass sheet (including the effects of anisotropy) confirm these results.

Out-of-plane stretching and tearing fracture in ductile sheet materials

Pre-cracked ductile steel sheets are fractured by combined in-plane streching and bending. The deformation mimics the mode of fracture when plating is dented and torn as in ship grounding. Fracture toughness is determined for this mode of tearing. Values are greater than those obtained with DENT testpieces on the same material because of the different mode of crack opening.

Denting and fracture of sheet steel by blunt and sharp obstacles in glancing collisions

Abstract The different types of denting, scoring and fracture that occur in “glancing collisions” between sheet steel and blunt or sharp obstacles, have been investigated. These collisions have a component of motion along the surface of the sheet or plating as well as perpendicular to it, and are to be distinguished from the usual sort of penetration/perforation problem. Crack initiation for blunt obstacles occurs by in-plane stretching and the biaxially dependent in-plane fracture strain pairs are well described by the fracture-forming limit diagram. Crack initiation for sharp obstacles is less easy to put in terms of critical strains, owing to the highly localised deformation regions. For both types of obstacle, crack propagation is modelled successfully using rigid-plastic fracture mechanics. Applications to ship grounding, motor vehicle crashworthiness, etc., are discussed.

Impact damage testing on composite marine sandwich panels. Part 2: Instrumented drop weight:

Marine grade hybrid sandwich panels are designed in accordance with BS EN ISO 12215-5:2008 using a wet lay-up and cured under vacuum pressure. The high fibre content composite laminate skins use marine grade orthophthalic polyester, POLYLITE® 440-M850, resin and chopped strand mat (CSM)/woven E-glass; the core is DIVINYCELL® H100 closed cell linear polyvinyl chloride (PVC) foam. Impact damage testing followed American Society for Testing and Materials (ASTM) D7766-11 procedure C and ASTM D7136/D7136M-05. Impact damage was sustained by the default hemispherical indentor and further ‘standard’ geometrical indentor rocks – conical, pyramid and cylindrical. The investigation reviews the current state of affairs in impact sandwich testing procedures. A 50–80% through thickness penetration criteria is proposed following noted shortcomings in the standard originally intended for laminates. It is shown that the panel overall flexural rigidity to thickness ratio better describes the transition between ‘thin’ to ‘thick’ impact response; force measurements indicate that strain rate effects need to be considered; dent indentation can only be assessed through data acquistion; destructive damage observation sectioned specimen observations describe the impact damage sustained under the various indentor conditions, as well as the roles played by both the face composite skins and the core material.

The trousers tearing test with ductile metal sheets

Abstract A general elastoplastic analysis for the tearing of sheet materials is applied to the well-known “trousers tearing test” which is used to determine R, the specific work of fracture (the fracture toughness), of materials. It is shown that the Cotterell-Mai rigid-plastic analysis for the testpiece [3] has to be modified to take account of differences in local sheet curvature at the tear tip. In consequence, their simple linear plot (back extrapolation of which gives the toughness R) is really a curve with a lower intercept giving the true toughness according to our model. Differences are also revealed at large widths of torn material, where the normalised pulling force at constant R decreases. Experiments over a wide range of tear widths confirm the analysis and also explain why the linear plot of the earlier Cotterell-Mai analysis appeared to be satisfactory.

Impact damage testing on composite marine sandwich panels, part 1: Quasi-static indentation

High fibre content composite laminate skins use marine grade orthophthalic polyester, POLYLITE® 440-M850 resin and chopped strand mat/woven E-glass for the thin outer face skins and DIVINYCELL® H100 closed-cell linear PVC foam as core. Marine grade hybrid sandwich panels are designed in accordance with BS EN ISO 12215-5:2008 for small craft hull construction, using a wet lay-up and cured under vacuum pressure. Impact damage testing followed ASTM D7766-11 and ASTM D6264-98 procedures A and B for rigidly supported and simply supported sandwich panels. A review of the current state of standard testing procedures of marine sandwich panels is described. Testing using the default hemisphere indentor also included other standard rock geometries – conical, square-based pyramid and flat-faced cylindrical. The damage incurred under each variation of indentation impact is described, in terms of force, absorbed energy and indentation displacement. New contact laws are suggested for the different rock geometries. Destructive sectioning of the panels provides the visual damage incurred through the thin face skins and core and the roles played by all members comprising the sandwich panels.

Connecting the essential work of fracture, stress intensity factor, Hill’s criterion in mixed mode I/II loading

Ductile sheet structures are frequently subjected to mixed mode loading, resulting that the structure is under the influence of a mixed mode stress field. Instances of interest are when stable crack growth occurs and when the crack-tip is propagating in this complex mixed-mode condition, prior to final fracture. Purposely designed apparatus was built to test thin-sheets of steel (Grade: DX51D) under mixed-mode I/II. These tests, under plane stress conditions, also investigated the effect of thickness on the specific essential work of fracture or the fracture toughness of the material under quasi-static cracking conditions. The fracture toughness is evaluated under incremental mixed-mode loading conditions. The direction of the propagating crack path and fracture type were observed and discussed as the loading mixity was varied. Whilst the specific essential work of fracture or fracture toughness was obtained using the energy approach, the theoretical analysis of the fracture type and direction of crack path were based on the crack tip stresses and fracture criterions of maximum hoop stress and maximum shear stress along with the utilisation of Hill’s theory. For mixed-mode I/II loading, the variation in the fracture toughness contributions ratios are evaluated and used predicatively using the established energy criterion approach to the crack tip stress intensity approach. The comparison between the theoretical directions of the crack path, failure mode propagation are in good agreement with those obtained from experimental testing indicating the definite link between both approaches.

The effect of anisotropy in fracture toughness and yield strength in the tearing of ductile sheet materials

Abstract The dependence with orientation of the yield strength and tearing fracture toughness of cold-rolled 64 36 α-bass and acetate sheets was determined. Parallel strips were cut into larger sheets of the same materials, rolled up on a slotted key, and converged to form pointed tongues which became detached from the larger sheets. The tongue length: width aspect ratio varied with direction in the plane of the sheet. A rigid-plastic fracture mechanics model of sheet tearing, which explained tear path convergence on the basis of minimum work rate [1] is extended to include ‘lift-off’ of the sheet under the slotted roller and gives agreement between the measured aspect ratios and the variation with direction of the independent determinations of yield strength and toughness.