J.E. Field

Brittle fracture: Its study and application

Abstract Brittle fracture is a challenging problem both from theoretical and experimental viewpoints. Once a crack starts to propagate in a brittle solid its velocity may quickly reach several thousands of metres/sec. Various high resolution techniques including high-speed photography, ultrasonics and electrical methods have been developed for fracture propagation studies. For many brittle solids a maximum crack velocity exists. Crack branching can occur either when a crack reaches a high velocity or when a stress wave passes over it. Brittle fracture is frequently an undesirable event, but there are situations where it has great practical importance. Examples of the beneficial application of fracture are discussed including gem stone cleaving, the interruption of electric current and rock fragmentation.

The effect of thin hard coatings on the Hertzian stress field

Abstract In this paper the effect of thin rigid coatings on the stress field generated by a Spherical indentation on a flat halfspace is investigated using a finite-element program. It is shown that thin hard coatings can have a significant effect on the radial and circumferential stresses. Of particular interest is the reduction in the maximum (radial) tensile stresses, since this indicates that hard coatings can be used to protect brittle materials against elastic contact damage. The stress reduction increases with the Youngs modulus and thickness of the coating. The reduction in the substrate stresses is, however, accompanied by an increase in the maximum tensile stresses in the coating itself.

Studies of Contact Damage in Polycrystalline Zinc-Sulfide

Deformation processes within the grains and grain-boundary sliding are responsible for the formation of the porous zone and the various crack systems around plastic indentations in chemically vapour deposited (CVD) zinc sulphide. The porous zone is formed by gross grain-boundary displacements in the region directly beneath the indenter and all the cracks emanate from around the porous zone and are contained within an extensive plastic zone. For the particular grain orientation used, the material behaves in an ideal elastic/plastic manner with the development of shear flow lines within the porous zone; the interaction of the flow lines with each other or continued slip along the flow lines lead to preferential void nucleation. The porous zone forms at an average representative strain of about 2.5% and a corresponding pressure of 1.5GPa which is twice the yield stress.

Synergistic effects of rain and sand erosion

Abstract Infrared transmitting materials are often used by the aerospace industry to make “windows” over guidance systems. These materials are typically weak and brittle and during flight, impact with raindrops and sand, or dust particles, may cause serious optical degradation. These problems have been extensively researched individually, but very little work has been carried out into to the combined effects of liquid impact followed by solid impact or vice versa. The effect of pre-impact by liquid drops followed by sand erosion of FLIR ZnS (Forward Looking InfraRed zinc sulphide) was investigated using optical microscopy, transmission loss and residual strength measurements of samples. The results show that pre-impact of ZnS with liquid drops followed by impact by sand particles can enhance erosion and the paper quantifies this for a range of exposure conditions.

High-speed photography

Abstract High-speed photography has the great advantage of giving direct visual information about transient processes. For this reason, it is finding increasing application for solving a range of industrial and scientific problems. This article reviews the techniques of high-speed photography and describes a range of applications. There are various types of high-speed camera which can operate in different speed, or framing rate, ranges. These are discussed in terms of increasing framing rate in frames per second (f.p.s.) which illustrates the techniques and also the historical development of the subject. The applications illustrated include studies of fracture, impact, erosion, explosive initiation and laser-induced damage.

Damage threshold velocities for liquid impact

Rain erosion is a major cause of strength and transmission loss in IR ‘windows’ in high velocity flight. The Cavendish Laboratorys Multiple Impact Jet Apparatus (MIJA) is able to simulate high velocity rain impact accurately and reproducibly under laboratory conditions using a series of discrete water jets. Quantitative erosion data in the form of Damage Threshold Velocity (DTV) curves can be obtained, giving the lower limit of damage to the material under study. Damage threshold curves are presented for five different materials carried out using three different nozzle orifice diameters to create the impacting jet. Two approaches are made to predicting the DTV value for each specimen using only data from the standard 0.8 mm nozzle: (i) the ‘cylindrical jet’ approach, in which the impacting jet is considered to have a flat front, and (ii) the drop (‘round-fronted jet’) model. Both methods are found to give predictions well within the bounds of experimental error. The former predicts the damage threshold better when the smaller (0.6 mm) nozzle is used and the drop model the threshold when the larger (1.2 mm) nozzle is used. High-speed photographs of jets from differently-sized orifices are presented validating the use of models for the various nozzle diameters.

Measurement of crack tip displacement field using laser speckle photography

Abstract It is shown how double exposure laser speckle photography, when used with an automatic image processing system, can provide detailed information on both in-plane displacement components around the tip of a crack. This paper describes the results of the analysis of six speckle photographs of sharp cracks in polymethylmethacrylate (PMMA), each on a mesh of 16 × 16 datapoints. Two numerical methods for calculation of the mode I stress intensity factor ( K I ) from the measured displacement field have been investigated. Least-squares fitting of the theoretical displacement field gave good estimates of K I , with low systematic and random errors (~1%). Evaluation of the J -integral around square paths enclosing the crack tip was found to result in larger errors (up to 5%). It is thought that laser speckle photography together with J -integral calculations may find applications in nondestructive testing, as a method of automatically detecting the presence of a crack.

The Effect of Double-Layer Coatings of High Modulus on Contact Stresses

Abstract In an earlier paper (Van der Zwaag and Field 1982), we investigated, using finite element methods, the effect of a thin, hard coating on the stress field generated by a spherical indenter on a uniform halfspace. Of particular practical interest was the reduction in the maximum (radial) tensile stresses in the substrate solid as the modulus and thickness of the coating increased. Experimental support for the theoretical predictions was given by Van der Zwaag and Field (1983). In the present paper, the investigation is extended to examine the potential of double layer coatings of two different high modulus materials. It is shown that a suitable selection of the properties of the two layers can reduce the tensile stresses in the substrate solid compared to the values in an uncoated substrate. Two particular systems are examined in detail. Potential benefits of monolayer and multilayer coatings are discussed critically. The results have application to a range of practical problems.

The erosion resistance of infrared transparent materials

Studies in the Cavendish Laboratory over the years have demonstrated the poor erosion resistance of most infrared (IR) transparent materials. A number of methods for increasing this erosion resistance have been investigated by various laboratories and these are assessed in the following study. These techniques include producing high–quality surface finishes, generating surface compressions and the application of coatings. The results emphasise that considerable care and effort is needed to produce even minor improvements in the liquid impact damage threshold velocity (DTV). The magnitude and time–scale of the impact stresses dictate that significant reductions in crack size are required to cause a significant improvement in DTV. The effectiveness of the coatings can be reduced by poor adhesion and tensile residual stresses weakening the substrate material. The recent development of chemical vapour deposited diamond (CVDD) as an erosion resistant infrared transparent material is also discussed.

The erosion of diamond, sapphire and zinc sulphide by quartz particles

Abstract In this paper we study the degradation of the surfaces of three nominally brittle materials when impacted with small, fast-moving particles of quartz. The localised high pressures generated by such events can cause deformation and fracture at the site of impact, often extending some way into the material. Crack intersections or deviations up to the surface then sever regions of material from the bulk, leading ultimately to erosion. In many applications, the prime consideration is how quickly this process causes material to be worn away. However, for imaging applications, such as used in the aerospace industry, optical components are subjected to collisions with airborne dust and sand, and degradation of the material to even a small degree can be significant, as surface pitting and cracking causes light to be scattered and resolution to be lost. The degradation of three such (infrared) optical window materials is investigated experimentally: free-standing chemical-vapour-deposited (CVD) diamond, sapphire (single-crystal Al 2 O 3 ) and polycrystalline zinc sulphide. The former of these materials is generating much interest at present as a potential window material, due to its extreme hardness, strength and broadband transmission in the infrared. Sapphire is a current 3–5 μm window material which also possesses high mechanical hardness and strength. Zinc sulphide (3–5 and 8–12 μm) has been used for many years in such applications because of its excellent optical properties; however, it is mechanically weak. The effect of particle size and impact velocity on both the optical degradation and erosion of these materials is discussed, and the effect of grain size and crystal orientation is considered for the erosion of CVD diamond.