Decrease In Load-bearing Capacity Research Articles

Experimental and computational meso-scale investigation on the deformation and failure behaviors of plain woven SiC/SiC with intrinsic and manufactured defect

For plain woven SiC/SiC composites used in aerospace thermal-structural applications, not only the pristine material properties but the open-hole performance are of critical concern for the components in service. This study investigated the influence of intrinsic pores induced during SiC matrix preparation and manufactured holes on the macro mechanical properties of plain woven SiC/SiC through experimental and numerical methods. Tensile tests at room temperature were conducted on SiC/SiC specimens with different hole apertures and configurations, and the digital image correlation (DIC) method was employed for full-field strain measurement. Moreover, micro-computed tomography (μ-CT) was utilized to characterize the internal microstructure, original defects, and failure fractures. Based on the reconstructed mesoscale model, the tensile strength and failure mechanisms of pristine and open-hole specimens were predicted and analyzed, with the models validated against the experimental results. The results showed that the inter-tow defects play an important role on the deformation and failure of pristine SiC/SiC specimens during tensile loading. The notch strength results indicated that SiC/SiC exhibits weak notch sensitivity, and the local mesoscopic deformation of the notch is affected by the combination of macroscopic stress gradients and mesoscopic structures. The mesoscale simulation results showed that the position of the hole has a significant impact on the strength of the single layer SiC/SiC composite. The more the notch invades the warp, the greater the decrease in load-bearing capacity, and the relative position of the weft mainly determines the crack propagation path. The failure mode in the simulation results is consistent with the experimental findings, indicating that the simulation model, which considers the weave architecture, can better explain the notch failure behavior of Ceramic Matrix Composites (CMCs).

In-Plane Buckling of Open-Section Shell Segments

The present study investigates elastic buckling behavior of open-section shell segments under action of a central radial load. A design parameter is expressed to characterize the influence of fillet radius on load-bearing capacity. A reduction factor equation is developed as a multivariate function of shell parameters, which evaluates the amount of decrease in load-bearing capacity of the structure caused by the corner fillet. In addition, an expression to predict limit load of the shell structure under clamped end conditions is introduced. Furthermore, a parametric study is performed to reveal the influence of fillet radius and radius-to-thickness ratio on the limit load as well as deformation patterns of the open-section shells. Results show that corner fillets has a significant effect on the limit load of the open-section shell segments under in-plane loading.

Experimental Investigation and Modelling of the Layered Concrete with Different Concentration of Short Fibers in the Layers

The use of steel fiber reinforced concrete (SFRC) in structures with high physical-mechanical characteristics allows engineers to reduce the weight and costs of the structures, to simplify the technology of their production, to reduce or completely eliminate the manual labor needed for reinforcement, at the same time increasing reliability and durability. Commonly accepted technology is exploiting randomly distributed in the concrete volume fibers with random each fiber orientation. In structural members subjected to bending, major loads are bearing fibers located close to outer member surfaces. The majority of fibers are slightly loaded. The aim of the present research is to create an SFRC construction with non-homogeneously distributed fibers. We prepared layered SFRC prismatic specimens. Each layer had different amount of short fibers. Specimens were tested by four point bending till the rupture. Material fracture process was modelled based on the single fiber pull-out test results. Modelling results were compared with the experimental curves for beams. Predictions generated by the model were validated by 4PBT of 100 × 100 × 400 mm prisms. Investigation had shown higher load-bearing capacity of layered concrete plates comparing with plate having homogeneously distributed the same amount of fibers. This mechanism is strongly dependent on fiber concentration. A high amount of fibers is leading to new failure mechanisms—pull-out of FRC blocks and decrease of load-bearing capacity. Fracture surface analysis was realized for broken prisms with the goal to analyze fracture process and to improve accuracy of the elaborated model. The general conclusion with regard to modelling results is that the agreement with experimental data is good, numeric modelling results successfully align with the experimental data. Modelling has indicated the existence of additional failure processes besides simple fiber pull-out, which could be expected when fiber concentration exceeds the critical value.

Numerical investigation of instabilities of steel members restrained by sandwich panels at elevated temperature

AbstractSandwich panel constructions are getting popular from the second half of the 20th century however, the restraint provided by sandwich panels in regards to the instability of structural members was not investigated at elevated temperature, which became the scope of the research introduced in this paper. An experimental test series was conducted at the Department of Structural Engineering at the Budapest University of Technology and Economics in order to study the stability behavior of structural members restrained by sandwich panels at ambient and elevated temperature. In this paper 18 tests are introduced, in which centrally located HEA120 sections of a 3.00 m x 3.00 m sized supporting sandwich panel diaphragm were tested for axial compression. The results indicate, that the decrease in load bearing capacity of PIR panels is higher, that in the case of mineral wool panels. The data derived from experimental tests were used for developing a complex numerical model. The primary objective of this paper is the comparison of numerical results with the results derived from experimental testing. This research has been financially supported by the European Union, in the frame of the Research Fund of Coal and Steel (RFCS) program. The authors gratefully acknowledge the support.

In-plane buckling of semi-cylindrical shells with elastic edge restraints under a central radial load

The present study investigates the elasto-plastic buckling behaviour of semi-cylindrical shells with elastic edge restraints under the action of a central radial load, both numerically and experimentally. Several design parameters are expressed to characterize the influence of elastic edge restraints on load-bearing capacity. A reduction factor equation is developed as a multivariate function of the shell parameters, which evaluates the amount of decrease in load-bearing capacity of the structure caused by elastic edge restraints. Additionally, an equation to predict load-bearing capacity under clamped edge conditions is introduced, which is used as a reference value in the reduction factor equation. Conditions for the elasto-plastic buckling occurrence are identified with the help of normalized parameters. An exclusive validation process is applied to the proposed expressions using a large series of numerical, experimental data and statistical methods to ascertain the accuracy of the predictions at a wide range of design parameters. Furthermore, parametric studies are performed in which the influence of elastic edge restraints, material plasticity and radius-to-thickness ratio on the limit load as well as deformation patterns of the semi-cylinders, is investigated. The results show that elastic edge restraint stiffness has a significant effect on the limit load of semi-cylindrical shells under in-plane loading and, therefore, cannot be underestimated.

Failure and reliability analysis of scarf-repaired CFRP: effect of hot–wet environment

Scarf repair method which can get uniform stress distributions and smooth surfaces has emerged as an adequate technique to repair damaged aerospace composite components. However, a prominent problem with the use of scarf-repaired structures is their susceptibility to environmental conditions. This research deals with the failure and reliability analysis of scarf-repaired carbon fiber-reinforced polymer composites (CFRP) before and after aged in hot–wet environment. Scarf-repaired CFRP with different laminate thicknesses was tested to investigate the failure strength and damage modes. The results indicate that the strength reduction of scarf-repaired CFRP varies with the laminate thickness (13.4–7.9%) because of different moisture contents. Fracture observations reveal that the decrease in load-bearing capacity of scarf-repaired CFRP is mainly due to the degradation of adhesive layer. Subsequently, theoretical models based on the distribution function of experimental results were established to predict the safe design strength at different reliability and confidence levels. It is shown that reliability has a greater effect on the safe design strength of both unaged and aged specimens than confidence level. A higher penalty paid to get high-reliability strength (R = 0.99, γ = 0.99) is observed for the unaged specimen, which has a larger coefficient of variation.

Experimental investigation of square stainless steel tubular stub columns after elevated temperatures

The behaviour of square stainless steel tubular (SSST) stub columns after elevated temperatures under axial compression is presented in this paper. A total of sixty specimens were tested, including forty-eight SSST stub columns at elevated temperatures cooled in air, nine SSST stub columns at elevated temperatures cooled in water and three SSST stub columns at ambient temperature. A total of twelve square CFSST stub columns were carried out load-strain tests. The main parameters explored in the test include thickness (1.0 mm, 1.2 mm, 1.5 mm), high temperature duration (30 min, 60 min, 90 min, 120 min), temperature ranging from 400 °C to 1000 °C and cooling methods. This paper presents the failure modes, ultimate load-bearing capacity, load-strain curves, load versus displacement curves, initial stiffness at the elastic stage and ductility of the specimens. It was found that high temperature has the greatest influence on the ultimate load-bearing capacity of the columns, and the cooling methods also have some effect on it. As the elevated temperature specimens subjected increased, the percentage decrease in load-bearing capacity had an obvious increase, the ductility became worse and the initial compressive stiffness slightly decreased. The crack resistance of the specimens improved after elevated temperature. The axial deformation versus load curves had an obvious elastic-plastic stage as the elevated temperature duration increased to 120 min. Based on the experimental results, a formula was proposed to calculate the ultimate load-bearing capacity of SSST stub columns after elevated temperatures.

Experimental and numerical investigation of sandwich structures with balsa core and hybrid corrugated composite/balsa core under three-point bending using digital image correlation

Sandwich structures are commonly used in automotive, aerospace, and civil infrastructure applications due to their high strength-to-weight and stiffness-to-weight ratios. In the present work, the performance of sandwich structures with aluminum facings and two different types of core, including balsa wood core and hybrid corrugated composite/balsa core subjected to three-point bending, is investigated, both experimentally and numerically. The face sheets are 6061-T6 aluminum alloy and the hybrid core has been made of balsa wood with average density of 125 kg m−3 and E-glass woven fabric, bonded together using epoxy resin. For data acquisition during the tests, a linear variable differential transformer has been placed under the mid-span of the specimens and a digital image correlation technique has been used for monitoring the full-field strain distributions and deflection pattern as well. Continuum damage mechanics has been employed for the finite element model analysis of the three-point bending tests. In order to predict the maximum load-bearing capacity and failure modes, a wood material behavior model for balsa and a combination of Hashin and Puck failure criterion for corrugated composite were imported into ABAQUS/Explicit via VUMAT subroutines. It was found that combining balsa core with corrugated composite results in prevention of catastrophic failure and consequently gradual decrease of load-bearing capacity. Furthermore, the results demonstrate that the hybrid core increases the strength-to-mass density and stiffness-to-mass density ratios by 34.7% and 28.2%, respectively. Numerical findings have been successfully compared to experimental data, which reveals its suitability for parametric analysis of the proposed hybrid core sandwich structure.

INFLUENCE OF INITIAL DEFLECTIONS ON COUPLED BUCKLING STRENGTH OF UNSTIFFENED SQUARE BOX SECTION COLUMNS

Although it is well known that initial deflections have a significant influence on coupled buckling strength, their quantitative influence is not clear. In this paper, a series of nonlinear finite element analyses to reveal the quantitative influence of initial deflections on coupled buckling strength are conducted with MSC.Marc. There are nine kinds of combinations about the size of local and overall initial deflections including 1/75, 1/150, 1/450 of the plate width and 1/500, 1/1000, 1/3000 of the column length. In addition, the width-thickness ratio and slenderness ratio parameters are also varied in the parametrical study. Both ranges of slenderness and the width-thickness ratio parameters are from 0.1 to 2.1. As a result, the initial deflection influence coefficient is proposed to describe the quantitative influence of local and overall initial deflections on coupled buckling strength. The numerical results reveal that the coefficient decreases with the increase of initial deflections. The sensitivity of the strength to initial deflections depends on both width-thickness and slenderness ratio parameters. The maximum decrease of 21.4% on load-bearing capacity caused by local initial deflection appears when width-thickness and slenderness ratio parameters equal to 1.2 and 2.1, respectively. Column strength is quite sensitive to overall initial deflection at the slenderness ratio parameter equal to 1.4. The maximum decrease in load-bearing capacity reaches 18.2%.

Report on Load Resistant Capacity when Promoting Curing PC Beam Specimens Affected by Alkali Silica Reaction

In this paper, ｍade a PC beam specimen using ASR reactivity aggregate made in Chinese Zhejiang Ministry and made them do an accelerated curing and the influence by which swell of ASR reactivity aggregate gives it to a PC beam specimen was evaluate. Moreover the PC beam specimen which degraded by swell was classified every degradation grade, a loading test was put into effect and the load carrying capacity was inspected. As a result, I got the following conclusion. ①The thing which classifies the degradation grade into 3 stages (incubation period, acceleration period and degradation period) from the accelerated curing results of the test piece performed in advance. In the incubation period, it was confirmed that the compressive strength and the modulus of static elastic tend to increase more than before accelerated curing. ②Expansion of PC beam specimen was inflated from the upper edge part with less influence of prestress, Moreover as the expansion stabilized, the tendency of the lower edge portion which is highly affected by the prestress to expansion could be confirmed from the strain behavior by monitoring.In addition, with expansion of the lower edge part the prestress amount of the PC steel material placed inside the PC beam specimen (The lower edge position) is increasing and as the prestress amount stabilizes the tendency of expansion of the lower edge part is stabilized can also be confirmed. ③When accelerated curing was further promoted, the expansion of the upper edge and lower edge turned to a decreasing trend, and it was confirmed that stabilize tendency at about 25 to 35% of the maximum expansion. ④The P-δ curve up to fracture was measured for PC beam specimen before accelerated curing and ② to ③. As a result, it was not possible to confirm a large difference at the P-δ curve before accelerated curing and ②, but a significant decrease in load bearing capacity was confirmed at ④.

The effect of drill hole location on load bearing capacity of long bones

We investigated how load bearing capacity (LBC) of long bone differs with a bicortical drill hole in a compressive/tensile location vs. a neutral location.Group I had a hole drilled through 0°/180° ([compressive at 0°/maximum tensile at 180°]), Group II at 90°/270° ([neutral]), and Group III at 135°/315° ([tensile at 135°/compressive at 315°]). Maximum load at failure and stiffness was measured.A significant ([p < 0.05]) decrease in LBC of all Group I and Group III bones was observed. Loss of strength for Group II was statistically insignificant ([p > 0.05]). Changes in stiffness for both tibias and fibulas was not statistically significant ([p > 0.05]).

Fracture analysis of glued laminated timber beams with a hole using a 3D cohesive zone model

Introducing a hole in a glued laminated timber (glulam) beam generally results in a significant decrease in load bearing capacity. Global strength is most often limited by perpendicular to grain fracture with crack initiation at hole periphery and crack propagation in the grain direction. Strength analysis and design is far from trivial, which is reflected by the lack of design criteria in contemporary engineering design codes. This paper presents nonlinear 3D FE-analyses of beams with a hole, carried out using a cohesive zone model based on plasticity theory. Results of numerical analyses are compared to experimental tests showing good agreement. Results of a numerical parameter study relating to beam width and growth ring pattern are presented, showing decreasing nominal beam strength with increasing beam width and that the beam strength is affected by the growth ring pattern. Furthermore, some analyses of the influence of different types of initial cracks are presented.

THE SURVEY TECHNICAL CONDITION OF THE SUPPORT STRUCTURES OF SPHERICAL TANKS

There is given a description of the object of research the support structures of spherical tanks. There are given main defects having been detected during the survey this structures. There are detected main reasons having caused defects of the building structures. There is considered the question of decrease of load bearing capacity of the support structures by results of strength of pillar concrete measurements. There are given recommendations for repair and strengthening of the support structures of spherical tanks for further safety operation.

Influence of Preliminary Damage on the Load‐Bearing Capacity of Zirconia Fixed Dental Prostheses

The objective of this investigation was to evaluate the influence of differently shaped preliminary cuts in combination with artificial aging on the load-bearing capacity of four-unit zirconia fixed dental prostheses (FDPs). Forty frameworks were fabricated from white-stage zirconia blanks (InCeram YZ, Vita) by means of a computer-aided design/computer-aided manufacturing system (Cerec inLab, Sirona). Frameworks were divided into four homogeneous groups with ten specimens each. Prior to veneering, frameworks of two groups were "damaged" by defined saw cuts of different dimensions, to simulate accidental flaws generated during shape cutting. After the veneering process, FDPs, with the exception of a control group without preliminary damage, were subjected to thermal and mechanical cycling (TMC) during 200 days storage in distilled water at 36°C. Following the aging procedure, all specimens were loaded until fracture, and forces at fracture were recorded. The statistical analysis of force at fracture data was performed using two-way ANOVA, with the level of significance chosen at 0.05. Neither type of preliminary mechanical damage significantly affected the load-bearing capacity of FDPs. In contrast, artificial aging by TMC proved to have a significant influence on the load-bearing capacity of both the undamaged and the predamaged zirconia restorations (p < 0.001); however, even though load-bearing capacity decreased by about 20% due to simulated aging, the FDPs still showed mean load-bearing capacities of about 1600 N. The results of this study reveal that zirconia restorations have a high tolerance regarding mechanical damages. Irrespective of these findings, damage to zirconia ceramics during production or finishing should be avoided, as this may nevertheless lead to subcritical crack growth and, eventually, catastrophic failure. Furthermore, to ensure long-term clinical success, the design of zirconia restorations has to accommodate the decrease in load-bearing capacity due to TMC in the oral environment.

Defect susceptibility of sensile strength to microporosity variation in as-cast magnesium alloys with different grain sizes

The variability in the tensile strength of as-cast AM60 and AZ91 alloys was investigated in terms of the defect susceptibility to the variation in grain size and microporosity. The microporosity was measured from the quantitative fractography analysis through scanning electron microscopy (SEM) observation on fractured surface after tensile test. The ultimate tensile strength (UTS) of both alloys can be characterized as a power law relationship to microporosity variation in terms of the defect susceptibility and maximum strength achievable in the defect-free condition. The defect susceptibility of tensile strength to microporosity variation is decreased remarkably with grain refinement. The defect susceptibility of AZ91 alloy to microporosity variation exhibits more sensitive dependence on the variation in grain size than AM60 alloy. Also, the dependence of UTS on the variation in grain size is described as a power law relationship for various levels of microporosity. The variation on effective void area fraction by the damage evolution of Mg17Al12 phase may introduce a practically significant decrease of load bearing capacity, less than by microporosity variation. The Hall-Petch relation of both alloys in the defect-free condition could be suggested as maximum values of friction stress and locking parameter.

Fracture behavior of straight or angulated zirconia implant abutments supporting anterior single crowns

The aim of the study was to evaluate the influence of artificial aging on the fracture behavior of straight and angulated zirconia implant abutments (ZirDesign™; Astra Tech, Mölndal, Sweden) supporting anterior single crowns (SCs). Four different test groups (n = 8) representing anterior SCs were prepared. Groups 1 and 2 simulated a clinical situation with an ideal implant position (left central incisor) from a prosthetic point of view, which allows for the use of a straight, prefabricated zirconia abutment. Groups 3 and 4 simulated a situation with a compromised implant position, requiring an angulated (20°) abutment. OsseoSpeed™ implants (Astra Tech) 4.5 mm in diameter and 13 mm in length were used to support the abutments. The SCs (chromium cobalt alloy) were cemented with glass ionomer cement. Groups 2 and 4 were thermomechanically loaded (TCML = 1.2 × 10⁶; 10,000 × 5°/55°) and subjected to static loading until failure. Statistical analysis of force data at the fracture site was performed using nonparametric tests. All samples tested survived TCML. Artificial aging did not lead to a significant decrease in load-bearing capacity in either the groups with straight abutments or the groups with angulated abutments. The restorations that utilized angulated abutments exhibited higher fracture loads than the restorations with straight abutments (group 1, 280.25 ± 30.45 N; group 2, 268.88 ± 38.00 N; group 3, 355.00 ± 24.71 N; group 4, 320.71 ± 78.08 N). This difference in load-bearing performance between straight and angulated abutments was statistically significant (p = 0.000) only when no artificial aging was employed. The vast majority of the abutments fractured below the implant shoulder.

Instability, shear banding, and failure in granular materials

Two modes of decrease in load bearing capacity of granular materials are discussed in view of experimental results. Both relate to the fact that frictional materials exhibit nonassociated plastic flow and they undergo considerable volume changes, either contraction or dilation. One mode consists of the instability that may occur in certain regions of stress space and potentially result in liquefaction of the granular material. It is the fact that loading of contracting soil (resulting in large plastic strains) can occur under decreasing stresses that may lead to unstable behavior under undrained conditions. As long as the soil remains drained, it will remain stable in the region of potential instability. The other mode is initiated by localization of plastic strains and subsequent development of shear bands, which in granular materials is followed by a decrease in load bearing capacity. These two modes are mutually exclusive and they occur for different loading and material conditions as discussed here on the basis of experimental observations.

Validation of finite element analysis in dental ceramics research

Statement of Problem. In vitro dental materials strength testing of ceramic restorations primarily has involved mechanical evaluations of simplified models. The finite element method (FEM) provides a mathematic analysis to predict strength values, but neither methodology is without the potential for errors. Purpose. The purpose of this study was to demonstrate the advantages of combining mechanical testing results and FEM data to determine the strengths of a layered ceramic beam when the layered materials and positions are varied. Material and Methods. Eight finite element 5 × 20 × 1-mm layered beams were modeled. Four of the modeled beams were of the same layered arrangements as physical specimens from a previously published study. The remaining 4 modeled beams provided intermediate layered arrangements not evaluated in the earlier study. A force in newtons was applied in the center of the top layer of each beam until fracture. finite element analysis was performed, and the data were compared with mechanical strength test results from the earlier study. Results. The FEM data of the 8 models demonstrated a linear decrease in load-bearing capacity as the layer thickness of the core material decreased and the layer thickness of the veneer material increased. The progressively decreasing values for the FEM beams were 170, 144, 140, 134, 72, 43, 34, and 27 N. The mean load-bearing capacities of 3 of the 4 mechanically tested beams compared favorably with the FEM data. The strength of the fourth mechanically tested beam, a veneer/core layered arrangement, was 110 N, which was lower than the corresponding FEM value (140 N). The 110 N value fell outside the decreasing linear progression for load, indicating that the FEM data were more accurate and reliable than the mechanical data. Conclusion. No one perfect method exists for testing the strength of dental materials. The best approach is to use the results from both mechanical testing and finite element analysis, which together may provide more reliable and valid data than either method alone. (J Prosthet Dent 2001;86:650-4.)

INVESTIGATIONS IN THE STRENGTH OF PRESTRESSED MULTILAYERED BEAMS UNDER THE ACTION OF LONG-TERM SHEAR/MEDINIŲ IŠ ANKSTO ĮTEMPTŲJŲ DAUGIASLUOKSNIŲ SIJŲ STIPRUMAS VEIKIANT ILGALAIKĖMS SKERSINĖMS JĖGOMS

The results of long-term experiments are presented in the paper. As a result of the experiments, invention SU 1025831 was developed under the name “The method of manufacture of a timber load-bearing element” (E 04 C 3/12, priority since July 11, 1981). The objective of the experiments performed was to check the method of increase of load bearing capacity of glulam beams under the influence of long-term shear. 14 beams were tested: nine of them were multilayered and 5 made of board-package and square-sawn timber. All the beams failed because of the action of shear forces. Two beams were tested by static short loading, the rest twelve by long-term loading. Three beams, that have not failed because of the action of long-term loading, then were tested by short-term loading. From the tests of multilayered, glulam and solid square-sawn timber beams the decrease of the relative strength wood split alongside fibres under the action of long-term shear is bigger than that given in literature in case of standard experiments. Relationship of the level of decrease of this strength with the duration of long-term loading may be calculated according to the equation (2). The influence of long-term loading on the load-bearing capacity of these beams depends on the intensity of actions: if such a load does not cause the plastic deformations in the beam, their load-bearing capacity does not decrease and vice versa—the appearance of plastic deformations and micro-cracks means the decrease of load bearing capacity of the beam under a short-term shear. A short description of invention SU 1025831 is also presented in the paper. The invention is devoted to the perfection of the former invention No 954237. The latter one suggested an increase of load-bearing capacity of glulam elements under a short-term shear. Under a long-term load, or in case of fire, when the temperature and moisture around is changing, the strength of prestressed joint of these beams may not be fully guaranteed because of the influence of inner stresses and possible micro-cracking. With the purpose to increase the strength of beams under a long-term shear the author offers additionally to install pivots (Fig 6, 3) after giving prestress and full polymerisation. This method does not only increase the load-bearing capacity of multilayered elements under a long-term shear, but also ensures a higher reliability and fire resistance of such structures.

Work load and musculoskeletal complaints during pregnancy.

Many pregnant women have musculoskeletal complaints, the onset and aggravation of which are thought to be associated with their activity or work postures. The purpose of this paper was to obtain more insight into the influence of pregnancy on the load of the musculoskeletal system at work to provide a better basis for preventing complaints. To this end, literature pertinent to a model called "work load of the musculoskeletal system" was reviewed. The model was developed to describe how work contributes to musculoskeletal complaints. It was found that pregnancy causes changes in a worker's characteristics. These changes can lead to changes in the work situation, activity pattern, and postural characteristics and possibly result in a change in work load. In addition, physical changes such as weight gain increase the work load in a given posture. The load-bearing capacity of the musculoskeletal system decreases due to pregnancy. The changes in load and the decrease in load-bearing capacity singly or simultaneously increase the risk of musculoskeletal complaints. The multifactorial cause of musculoskeletal complaints during pregnancy reveals various possibilities for prevention.