M. Papini

Fracture load predictions for adhesive joints

Abstract An engineering approach to fracture load predictions for adhesive joints is presented. The approach is based on the premise that the in-situ strength of the bondline can be characterized by the fracture envelope (critical energy release rate as a function of the mode of loading), for a specific adhesive system. By using the J integral for large deformations together with large-deformation beam theory, a simple closed-form expression is obtained for the energy release rate per unit area extension when a crack propagates in the bondline of a generalized adhesive joint (adhesive sandwich). This technique, together with a published method for mode partitioning, enables fracture load prediction by comparing the calculated fracture parameters with the critical ones from the fracture envelope. The approach is shown to predict fracture loads accurately for a variety of joints including the cracked lap shear (CLS), the single lap shear (SLS) and the double strap (DS) joint.

The effect of geometry on the fracture of adhesive joints

Abstract In this paper, a recently suggested method for fracture load prediction of adhesive joints is demonstrated to compare well with experimental data for aluminium joints bonded with a rubber-toughened structural epoxy (Permabond ESP 310). The method is also used to investigate the effect of varying geometric parameters such as adherend lengths and thicknesses on the strength of adhesive joints such as the single lap shear, cracked lap shear and double strap joints.

A Biomechanical and Finite Element Analysis of Femoral Neck Notching During Hip Resurfacing

Hip resurfacing is an alternative to total hip arthroplasty in which the femoral head surface is replaced with a metallic shell, thus preserving most of the proximal femoral bone stock. Accidental notching of the femoral neck during the procedure may predispose it to fracture. We examined the effect of neck notching on the strength of the proximal femur. Six composite femurs were prepared without a superior femoral neck notch, six were prepared in an inferiorly translated position to create a 2 mm notch, and six were prepared with a 5 mm notch. Six intact synthetic femurs were also tested. The samples were loaded to failure axially. A finite element model of a composite femur with increasing superior notch depths computed maximum equivalent stress and strain distributions. Experimental results showed that resurfaced synthetic femurs were significantly weaker than intact femurs (mean failure of 7034 N, p<0.001). The 2 mm notched group (mean failure of 4034 N) was significantly weaker than the un-notched group (mean failure of 5302 N, p=0.018). The 5 mm notched group (mean failure of 2808 N) was also significantly weaker than both the un-notched and the 2 mm notched groups (p<0.001, p=0.023, respectively). The finite element model showed the maximum equivalent strain in the superior reamed cancellous bone increasing with corresponding notch size. Fracture patterns inferred from equivalent stress distributions were consistent with those obtained from mechanical testing. A superior notch of 2 mm weakened the proximal femur by 24%, and a 5 mm notch weakened it by 47%. The finite element analysis substantiates this showing increasing stress and strain distributions within the prepared femoral neck with increasing notch depth.

Simulation of interference effects in particle streams following impact with a flat surface: Part I. Theory and analysis

Abstract The interference between an incident stream of spheres and those rebounding from a flat surface is described using a computer model. The model was capable of examining the effect of the following parameters on the severity and frequency of inter-particle collisions: stream angle of incidence, nozzle divergence angle, incident particle velocity and flux, particle size, particle–particle and particle–surface impact parameters, and stand-off distance. The collision dynamics for systems consisting in excess of 10 4 particles were obtained. Frictionless inter-particle collisions were assumed, but friction for particles impacting the surface was considered. For all collisions, a coefficient of restitution model of impact behaviour was assumed. Dimensionless parameters used to aid in the presentation of the results were identified, and a companion paper [Simulation of interference effects in particle streams following impact with a flat surface. Part II. Parametric study and implications for erosion testing and blast cleaning, submitted for publication] presents a parametric study of their role in erosion testing and blast cleaning processes.

Impact of rigid angular particles with fully-plastic targets Part I: Analysis

Abstract The erosion of substrates of arbitrary dynamic hardness and friction coefficient, due to the impact of individual angular particles, was analyzed with the purpose of predicting crater size, shape, and rebound parameters as a function of incident particle velocity, angle, orientation, and shape. A rigid-plastic theory due to Hutchings (International Journal of Mechanical Sciences 1997; 19:45–52), developed for square plates impacting frictionless surfaces, is generalized for arbitrarily shaped particles impacting surfaces having nonzero friction. The specific case of symmetric angular particles of arbitrary angularity is studied in detail. The model is shown to match Hutchings’ [1] experimental data for square steel plates on smooth steel surfaces. In a companion paper (Papini, Spelt, under review), a parametric study of the input parameters is presented.

Impact of rigid angular particles with fully-plastic targets Part II: Parametric study of erosion phenomena

Abstract In the accompanying paper (Papini M, Spelt JK. Impact of rigid angular particles with fully-plastic targets. Part I: Analysis. International Journal of Mechanical Sciences 2000;42(5):991–1006), the erosion of targets of arbitrary dynamic hardness and friction coefficient due to the impact of individual angular particles was analyzed with the purpose of predicting crater size, shape, and rebound parameters as a function of incident velocity, angle, particle orientation, and shape. A rigid-plastic model of impact was utilized, and the specific case of symmetric angular particles was studied in detail. In this paper, the model is used to predict crater size in a parametric study of the effect of particle size, shape, incident velocity and orientation, as well as target dynamic hardness and friction coefficient. A dimensional analysis revealed some useful relevant dimensionless parameters. The conditions which maximize the crater size are identified for both constant friction and frictionless surfaces. A parametric study is presented to identify the effects of the most important parameters and can be used as a tool to guide further experimentation.

Simulation of interference effects in particle streams following impact with a flat surface: Part II. Parametric study and implications for erosion testing and blast cleaning

Abstract In a companion paper [Simulation of interference effects in particle streams following impact with a flat surface. Part I. Theory and analysis, submitted for publication], a computer simulation capable of describing the interference between an incident stream of spheres and those rebounding from a flat surface was presented. In the present paper, the model is used to study the effect of the following parameters on the severity of inter-particle collisions: stream angle of incidence, nozzle divergence angle, incident particle velocity, particle size, particle flux, particle–particle and particle–surface impact parameters, and standoff distance. Useful dimensionless parameters are identified and their significance is characterised using a sensitivity analysis. A detailed parametric study of the most important input parameters revealed that interference effects strongly depend not only on flux, but also on angle of attack, standoff distance, coefficient of restitution between particle and surface, and nozzle to particle radius ratio. It is thus important that a full description of experimental conditions in erosion tests be given. Finally, a series of non-dimensional results was presented that can be used to assess the reduction, due to interference effects, in stream power from that available at the nozzle exit. This can be used to estimate how coverage in shot peening, and erosive power in erosion testing, will be affected by interference effects.

Effect of crack-growth mechanism on the prediction of fracture load of adhesive joints

Abstract This paper presents observations regarding the cracking behavior of tensile-loaded structural adhesive joints. Experiments showed that fracture occurred by the development and propagation of a damage zone, rather than a single, sharp crack, and that the presence of the adhesive spew fillet did not affect the fracture load of the adhesive joints studied. For joints bonded with the mineral-filled epoxy Cybond 4523GB (American Cyanamid), there was approximately 5 mm of subcritical crack propagation prior to final fracture. Fracture-load predictions based on the initial uncracked geometry made in previous papers were unaffected by this small change in geometry. For joints bonded with the rubber-toughened epoxy Permabond ESP 310, approximately 50 mm of subcritical crack propagation was observed. It was again found that predictions made in previous papers on the basis of the initial geometry gave a good estimate of the final fracture load even though this subcritical crack propagation significantly altered the geometry, and thus the applied energy release rates. The effect of shear deformations of the adherends was also investigated, and it was found that shear deformations could be neglected in engineering calculations for joints subject to remote tensile loading.

Surface evolution models for abrasive jet micromachining of holes in glass and polymethylmethacrylate (PMMA)

Models are presented to predict the shape and size of masked and unmasked holes machined in glass and polymethymethacrylate (PMMA) using abrasive jet micromachining (AJM). An existing AJM surface evolution model for brittle materials was modified by introducing a curvature-dependent smoothing (viscosity) term to the surface velocity function, greatly improving the prediction of hole shape in cases where the erosive power creates a sharp corner. The modified model predicts hole profiles that agree well with both experiments and a computer simulation. The profiles of holes machined in PMMA using a mask are also compared with the predictions of a recently developed theoretical model for the AJM of ductile materials. There is good agreement in both shape and depth with a maximum error of 13% up to an aspect ratio of 0.6 in PMMA.

Coating removal from fiber-composites and aluminum using starch media blasting

Wheat starch blast cleaning is used increasingly to remove organic coatings from aircraft. The process was investigated to clarify aspects of the mechanism by which a urethane topcoat and an epoxy primer are removed from aluminum and three epoxy composites (carbon, glass, and aramid fiber). Experiments included an examination of the impact sites created by individual wheat starch particles, the measurement of particle size, shape and velocity, and the determination of paint stripping rates. The velocity distribution of wheat starch particles was also studied using a simplified theoretical model. Paint removal rates were found to depend strongly on impact angle, mass flow rate, particle velocity and size, and the rigidity of the substrate (composite or aluminum). The urethane topcoat was found to be removed in a cumulative fashion by chipping. Selective stripping was possible for this coating system; i.e., it was possible to remove the topcoat while leaving the underlying primer intact. The susceptibility of composite substrates to damage varied with the type of fiber reinforcing, with aramid fiber being much more sensitive than carbon fiber.