Influence of MSD crack pattern on the residual strength of flat stiffened sheets

A parameter study of the residual strength for a multiple site damaged (MSD) stiffened sheet is presented. The analysis is based on an elastic-plastic fracture analysis using the yield-strip model for interaction between a lead crack and the smaller MSD cracks. Two crack growth criteria, one with a pronounced crack growth resistance and one with no crack growth resistance and five different MSD crack patterns, are analysed for different sizes of the lead crack and the smaller MSD cracks. The analysis indicates that the residual strength reduction depends on all these parameters and that MSD may totally erode the crack arrest capability of a tear strap. Another important outcome is that for certain combinations also very small MSD cracks may induce a significant residual strength reduction.     

Characteristics of crack patterns controlling the retained strength of ceramics after thermal shock

The physical mechanisms that determine the retained strength of ceramics after thermal shock are studied by measuring experimentally and statistically the density and depth of cracks produced in the interior of the ceramics. The analysis indicates that the key factor controlling the retained strength is the maximum depth of cracks rather than the density of cracks in the ceramics. The result presented here forms a basis to further understand the thermal shock behavior of ceramics.     

Characteristics of crack patterns controlling the retained strength of ceramics after thermal shock

The physical mechanisms that determine the retained strength of ceramics after thermal shock are studied by measuring experimentally and statistically the density and depth of cracks produced in the interior of the ceramics. The analysis indicates that the key factor controlling the retained strength is the maximum depth of cracks rather than the density of cracks in the ceramics. The result presented here forms a basis to further understand the thermal shock behavior of ceramics.     

A statistical approach to strength evaluation incorporating porosity effect for porous ceramics

ABSTRACT It has not been clear whether the conventional effective volume proposed for dense brittle materials can be applied satisfactorily to the strength evaluation of porous ceramics. In the present study, a modified effective volume was proposed by incorporating the porosity effect in the statistical evaluation of strength properties of porous ceramics. The modified effective volume was derived as the conventional effective volume multiplied by a function of porosity p. In this work, a power function of (1 + p)^a was adopted as the simplest porosity function. To clarify the applicability of the modified effective volume, bending tests were conducted using smooth and notched specimens of 3 wt% MgO partially stabilised zirconia with six different porosities. The porosity dependence appeared in the relation between the conventional effective volume and the mean strength of various zirconia ceramics with different porosities. The exponent a of the porosity function was determined from experimental data obtained by using identically shaped specimens with distinct porosities, and the modified effective volume was calculated for several types of specimens used in the experiments. It was revealed that the mean strength was almost uniquely correlated with the modified effective volume independent of porosity. The experimental correlation verified the applicability of the modified effective volume to strength evaluation of porous ceramics.     

Effect of residual stresses on ductile crack growth resistance

It is well known that residual stresses influence the ductile fracture behaviour. In this paper, a numerical study was performed to assess the effect of residual stresses on ductile crack growth resistance of a typical pipeline steel. A modified boundary layer model was employed for the analysis under plane strain, Mode I loading condition. The residual stress fields were introduced into the finite element model by the eigenstrain method. A sharp crack was embedded in the center of the weld region. The complete Gurson model has been applied to simulate the ductile fracture by microvoid nucleation, growth and coalescence. Results show that tensile residual stresses can significantly reduce the crack growth resistance when the crack growth is small compared with the length scale of the tensile residual stress field. With the crack growth, the effect of residual stresses on the crack growth resistance tends to diminish. The effect of residual stress on ductile crack growth resistance seems independent of the size of geometrically similar welds. When normalized by the weld zone size, the ductile crack growth resistance collapses into one curve, which can be used to assess the structural integrity and evaluate the effect of residual stresses. It has also been found that the effect of residual stresses on crack growth resistance depends on the initial void volume fraction f₀, hardening exponent n and T-stress.     

Effect of a crack on strength of fibre-reinforced plastics

In unidirectionally reinforced composites with an elastic-plastic matrix, there is a plastic zone with lengthY ₀ proportional to the crack lengthC (Y ₀ C) at the tip of a crack. This results in a new logarithmic dependence of glass and aramid PABI fibre-reinforced plastics (FRP) on crack length and non-fulfilment of the Griffith criterion. In glass and PABI FRP without an artificial notch, defects already exist equivalent to a crack with a length depending on composite fabrication practice. In GFRP, the epoxy matrix shear yield stress grows 2.0 to 2.5 times, compared to the yield in thin films due to fibre constraint of matrix yielding. The stress distribution in front of a crack in a highly anisotropic composite with an elastic-plastic matrix is derived. The stress concentration at the tip of a crack grows with increasing matrix yield stress, resulting in a change of failure mode from accumulation of fibre breaks at low matrix strength, to brittle failure at high matrix strength. The following factors lead to composite embrittlement: (1) increase of matrix yield stress and composite shear strength; (2) decrease of temperature; (3) increase of Young's modulus of the fibre; (4) reduction of fibre strength. The dependence of aramid PPTA FRP strength on temperature exhibits a maximum. Epoxy matrix plastification leads to some increase of aramid PPTA FRP strength.     

Effect of residual stress on crack propagation in MDPE pipes

Crack propagation behaviour in single edge notched specimens prepared from medium-density polyethylene (MDPE) pipe is examined under creep condition. The crack grown from an exterior notch (inbound) initiated faster than that grown from an interior notch (outbound). Subsequently, the outbound crack propagated monotonically to ultimate failure. The inbound crack showed anomalous behaviour involving two arrest stages prior to ultimate failure. The pipe is found to possess substantial residual stresses. The energy release rate for each case was calculated taking into account the respective residual stream distribution. The fact that the rates of crack propagation are not a unique function of the energy release rate indicates that the fracture is also influenced by morphological gradients imposed by processing conditions.     

Effect of welding residual stresses on fatigue crack growth thresholds

Fatigue crack growth thresholds ΔK_th were determined for friction stir welded butt joints made from aluminium alloys AA2024 and AA6013. Plotting the thresholds as a function of load ratio R showed distinctly higher amounts for welded joints as compared to those for parent material at small load ratios, but differences became smaller with increasing load ratio, until thresholds became finally identical for the highest R values. Applying Döker’s concept of two controlling parameters, namely ΔK and K_max [1], and plotting ΔK_th versus K_max, however, revealed that the effective threshold ΔK_th,eff determined at very high R ratios was nearly independent on the alloy and, simultaneously, was identical for parent material and respective welded joints. Thus, differences in threshold behaviour were only caused by the second threshold K_max,th, which was significantly higher for welded joints as compared to parent material. Differences in K_max,th coincided with compressive residual stresses determined by cut-compliance measurements in terms of stress intensity factors K_rs acting at the crack tip. Based on the analytical approach described by Döker [1], only one characteristic K_rs value was needed to calculate all thresholds of welded joints for 0 ⩽ R ⩽ 1 provided a base material master curve is available.     

Prediction of residual strength of CFRP after impact

It was found that the residual strength of CFRP after impact decreases as the impact energy increases when the energy is larger than the threshold impact energy. If the impact energy is sufficiently large, the influence of the mass of an impactor on the residual strength of the composite materials can be disregarded. Also, when the specimen is placed on a rigid plane, it was seen that the residual strength decreases as the diameter of the impactor nose increases. The residual strength after impact can be estimated by measuring the size of the permanent impression on the surface of composite materials after impact and applying the prediction equation for the residual strength proposed in this study.     

Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength

The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was >10⁷ cycles. A significant change in the slope of the S–N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 10⁷ and 10⁹ cycles are >99%.     

Effects of residual porosity on strength of Sn–Pb/Cu6 Sn5 particulate composites

Abstract

The effect of residual porosity on the flow stress of particulate composites was studied for a composite in which it is possible to create high porosity (>60%) by using particle coatings. The material was a eutectic composition Sn–Pb solder with additions of intermetaliic CU₆Sn₅ particles. It was found that porosity affects the strength of these composites much less than for non-composites, an effect attributed to particle bonding during solidification. At the same time, residual porosity was found to inhibit strongly or even to prevent hardening of this material. A micromechanical model is developed which simulates this behaviour over a wide range of porosities.

MST/1928     

Effect of surface treatments on strength of PZT polycrystalline ceramics

In bending strength determinations of ceramics, the fracture usually initiates from the tensile surface or the edges. The edge effect could be minimized by carefully rounding off the edges of specimens subjected to four-point bending. The effect of tensile surface polishing, polishing and rounding off the edges, and the acid treatment of the polished surface on the strength of PZT polycrystalline ceramics was determined along with the failure mechanism.Deceased.     

Effect of surface finish on strength degradation of glass ceramics

The processing of components manufactured from ceramics, such as machining, induces cracks on or beneath the surface leading to strength degradation. Usually, four-point bending tests are used for measuring the strength of ceramic materials. In this paper, a new approach is proposed to evaluate the loss of strength due to machining. First, beam specimens of glass ceramics are prepared by milling. The surface profile of representative areas of the specimens is determined using a laser profilometer. The surface topography of the representative area is reconstructed to characterize the surface irregularities after machining. A finite element program is then employed to analyze the stress distribution on a rough surface of a beam in four-point bending. A performance index—the stress concentration factor, is introduced to quantify the effect of surface irregularities on determining the critical crack depth related to the machining-induced damage. The flexural strength of machined ceramic specimens is predicted using the fracture mechanics. The strength degradation is compared with the experimentally determined values. A close correspondence between the predicted and measured strength is observed. The potential of using a computer-based evaluation to study the strength degradation of glass ceramics by machining induced surface damage has been established.