Sunil Bhat

Tip Parameter Approximation and Fatigue Growth of Crack Towards Inclined Weld Interface Between Strength Mismatched Steels

A Mode I crack growing at an angle other than 90° with respect to the interface of weld between strength mismatched steels in a bimetallic composite is investigated. The steels and the weld have nearly identical elastic and thermal expansion properties. As the crack in parent steel reaches near the interface of the weld, the plastic strain or yield zone at its tip spreads to the weld, which causes load transfer effect, due to strength mismatch across the weld interface, over the crack tip. Yield zone is modeled as Dugdale’s cohesive zone subjected to constant closing cohesive traction. Geometrical formulations are employed to approximate change in volume of parts of cohesive zone on either side of the interface resulting upon inclining the interface from the vertical. A computational model for crack tip in front of the perpendicular weld interface in linear elastic regime is modified to account for the effect of inclination of weld interface from the vertical. Crack tip stress intensity parameter is approximated from the model, cyclic value of which is used to assess the rate of fatigue crack growth towards the weld interface. The model is validated by experiments. Bimetallic compact tension specimens, prepared by electron beam welding of weak ASTM 4340 alloy and strong MDN 250 maraging steels at interface angles of 10° and 15° with the vertical, are subjected to high cycle fatigue. Growth rate of crack is found to dip substantially when it reaches near the interface of ultra strong weld. The results are compared with those of plain and bimetallic specimens with perpendicular weld interfaces. Inclination of weld interface from the vertical by nearly all the angles changes, albeit marginally, the values of crack tip parameter visà-vis those in the case of perpendicular weld interface. Theoretical and experimental results are in good agreement with each other.

Computational Investigation of Glare with Several Cracks and Delaminations under Monotonic and Cyclic Loads of Constant Amplitude

A residually stressed aluminum fiber metal laminate (Glare) with delaminations, caused by co-existence of normal cracks in aluminum layers and transverse interfacial cracks between aluminum and fiber layers, is theoretically and numerically investigated under monotonic and cyclic loads of constant amplitude. Double and multiple delaminations, governed by various types of debonding curves, are modeled with the help of cohesive elements. Role of fibers in load or fiber bridging over the cracks leading to enhanced fracture and fatigue properties of the laminate is demonstrated and validated. Magnitude of shielding effect at normal crack tip is quantified by stress intensity parameter and J integral values. Influence of delamination growth parameters on fiber bridging and stress state in aluminum layers is also examined. Theoretical and numerical results support each other well.

Examination of small crack normal to multiple elasticity and plasticity mismatched interfaces in residually stressed fiber metal laminate (Glare): Theoretical and numerical approaches

A small Mode I crack, normal to interfaces between elasticity and plasticity mismatched material layers, in a residually stressed aluminum–fiber laminate (Glare) is modeled under monotonic and cyclic loads. An analytical model is presented in small scale yielding or K-dominant regime that quantifies the effect of property mismatched interfaces over the crack tip in the laminate by determining energy release rate and stress intensity parameter. Cracked laminate is also modeled by finite element method. Values of energy release rate and strain energy density are obtained over cyclic paths far away from the crack tip, around the influencing interfaces, and in the vicinity of the crack tip. Numerical results are found to be close to theoretical estimations. Re-distribution of stress state in un-identical material layers during crack growth, resulting in different energy release rates of the crack in the laminate vis-a-vis those of the crack in similarly stressed homogeneous aluminum, is well substantiated. Methodology to incorporate the surface and material micro-structural effects over the crack is also proposed.

Finite Element Analysis of a Helmet with Superskin

The paper presents load analysis of a Superskin layered helmet by finite element method. Results are compared with those of conventional crash helmet to evaluate the performance of Superskin in preventing rotational injury to the neck of the rider. Delamination of Superskin is observed that increases the safety of the rider. Solid Works and Mechanical APDL (Ansys12) softwares are used in the analysis.

Numerical investigations on delaminated Glare under uni-axial tension

Purpose – Since failure of laminated composites by delaminations is common, the purpose of this paper is to present a numerical procedure to check the stability of delaminations in fiber metal laminate (Glare), with different possible damage configurations, under uni-axial tension. Deformation behavior of the laminate is also examined. Influence of the type and the extent of damage, represented by varying sizes and number of delaminations, on delamination driving force and laminate deformation is found. Design/methodology/approach – Delaminated Glare is modeled by finite element method. Interface cohesive elements are used to model the delaminations. Finite element results provide the deflection/deformation characteristics of the laminate. Driving forces of delaminations are estimated by J integrals that are numerically obtained over cyclic paths near delamination tips. Laminates with different types of delaminations are also fabricated and externally delaminated for measurement of their interlaminar frac...

Numerical Examination of Mixed Mode Crack in SSY: A Review

The paper reviews the numerical methodology to investigate fracture parameter namely energy release rate, G, of a mixed mode crack. An inclined, through, centre crack is assumed in a ductile steel plate subjected to bi-axial tension. Applied stress and crack size are suitably selected to simulate small scale yielding (SSY) condition at the crack tips. The cracked plate is modelled by finite element method. Both plane stress and plane strain situations are examined. G value is found from J integral. Equations of transformation are employed to obtain normal and shear stress in the plane of the crack. G is then again determined for Mode I and Mode II cracks by modelling each case separately. The analysis is finally validated by fulfilment of the conservation of energy release rate criterion, G (Mixed mode) = G (Mode I) + G (Mode II).

Elasticity Based Stress Analysis under Arbitrary Load using Fourier Series

The paper presents a methodology to obtain the stress solution of a simply supported beam subjected to arbitrary load with the help of, elasticity based, Fourier series approach. The applied load spectrum is converted into Fourier series with constant, sine and cosine components for which the solutions are available. The individual solutions are superimposed to obtain the final result. Deviations are noticed on comparing the results with those obtained from the conventional strength of material approach.

Estimation of K in Ductile CT and SENB Specimens under LEFM and SSY Regimes by J Integral Method: A Review

Stress intensity factor (K) is the measure of severity of stress at the crack tip. When K exceeds the critical limit (i.e., the material fracture toughness), the crack grows. K is valid in brittle materials (LEFM) and to some extent in ductile materials also provided there is small scale yielding (SSY) at the crack tip. The paper reviews the numerical methodology to obtain KI of ductile, Mode I cracked, CT and SENB test specimens in LEFM and SSY regimes with the help of J integral method. The numerical values are successfully compared with the theoretical values.

Fatigue Behaviour of Fibre Metal Laminate (Glare) Vis-À-Vis Plain Aerospace Aluminum Alloy: An Experimental Investigation

Performance of cracked fibre metal laminate (Glare) under fatigue load differs from that of cracked plain aerospace aluminum alloy. Load transfer towards stronger fibres in Glare shields crack tips in aluminum layers that retards fatigue crack growth rates. The paper experimentally investigates the fatigue behaviour of Glare at variable stress ratio’s in an ambient environment for comparison with that of plain aerospace aluminum alloy.

Finite Element Investigation of Stress Intensity Factors of Multiple Semi-Elliptical Surface Cracks in Solid Shaft

Fatigue cracks nucleate at the surface of the shaft operating under cyclic load. These cracks then grow into semi-elliptical shapes while extending into the shaft. The paper presents numerical estimation of stress intensity factor (SIF) of such cracks in circular solid shaft when subjected to bending and twisting loads in elastic conditions. The finite element model is first validated with the help of results reported elsewhere for a single surface crack. Later on, multiple coplanar surface cracks are modeled and the effect of the number of cracks and the distance between them on SIF of the main crack is investigated. The results are in line with the expectations.Nomenclature