The impact fracture created in the elastic-plastic response régime has been characterized in terms of its surface extension and penetration. A numerical dynamic analysis has been performed of a typical impact within this régime to indicate some of the principal characteristics of the contact behaviour and the stress field. The damage has then been analysed, by using simplified postulates based on key features of the impact dynamics and basic fracture mechanics concepts. This has enabled the primary material and target parameters affecting the impact fracture to be identified. Thereafter, some implications for strength degradation and erosion have been discussed.

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This is a two-part paper, which stresses the materials science approach to understanding dust erosion mechanisms. The first part is an experimental phase, studying the effects upon solid-particle erosion, of such material and environmental variables as target alloy composition and heat-treat condition; dust particle velocity, size, concentration, velocity, and kinetic energy; carrier-gas true temperature and impingement angle. All test variables and their limits were chosen to simulate the range of engineering conditions and erosive environments encountered in helicopter turbine service. Actual erosion data are compared with erosion levels predicted by existing theories on particulate erosion. The second part is a diagnostic phase, programmed to detect and study visible phenomena associated with the erosion processes, using high-magnification electron microscopy. Phenomenological evidence obtained from the erosion surfaces and erosion products are used to define probable physical models of the erosion mechanisms.

Five target materials of varying mechanical properties were eroded with natural quartz sand and S i c particles. The targets were four commercially available Si3N, materials and MgF,. Several particle size ranges between 10 and 1000 pm. accelerated to several velocities between 61 and 275 m/s, were used for erosion testing. The erosion results are presented in terms of two models which have been developed for elastic-plastic impact damage. ECENT investigations have shown that R a number of erosion mechanisms for ceramics can exist and that erosion and impact is a complex process.'-4 Essentially, two types of models have been proposed for solid-particle impact (single particle) and erosion (multiparticle) of brittle materials. The earlier models were based on elastic interaction between target and particle, and predicted that material removal occurs by the intersection of ring cracks on the target surface. This process has been observed on several materials under static and low-velocity impact conditions with relatively large spherical particle^.',^ More recent analysis has treated static and dynamic plastic indentation, which is characterized by plastic deformation of the contact area between the particle and the target, with radial cracks propagating outward from the contact zone, and with subsurface lateral cracks propagating outward on planes nearly parallel to the surface. This type of damage, termed elastic-plastic, is observed for impact with angular particles of generally greater hardness than the target.',2Two expressions have been derived for elastic-plastic impact.5,6Details of the derivations are given in the references. Generally, the expressions relate erosion to the depth of damage and area of lateral cracking.The expressions are as follows:(2) ppl I /19R I 1 /lv,22/9Kc-4/3HI/9after Ruff and Weiderhorn,"where V=erosion volume loss per impact p,=particle density pc= target density p,=particle shear modulus p,=target shear modulus R,=particle radius vp=particle velocity K,= target fracture toughness H = target hardnessThe major differences between the two expressions are in the particle density, particle velocity, and target hardness dependencies. These expressions are based on single impacts and were developed for isotropic materials under idealized conditions. The validity of the above expressions to predict erosion can be assessed by comparing a plot of experimental erosion data as a function of the expressions.Five target materials were eroded with Sic and quartz particles. The targets were hot-pressed Si,N4, pressureless-sintered Si,N,,, two reaction-bonded Si,N,'s, and MgFZ. Properties relevant to erosion are Table I. Target and Particle Properties shown in Table I. Particle properties are also given. Six particle sizes between 10 and 385 pm average of quartz particles and 8 and 940 pm average of Sic particles were used for erosion testing. Between three and five particle velocities for each particle size were evaluated. Velocities ranged between 61 and 275 m/s. All tests were performed at 90" ...