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Hard solids with predominantly covalent–ionic bonding are finding rapidly increasing usage in many modern technologies. However, this class of solids is severely limited by their intrinsic brittleness—they break easily. It is in this context that a fundamental knowledge of brittle fracture mechanisms is of practical importance. This review covers the essential features of crack behavior in characteristically brittle solids, starting with fundamental physical and chemical models, with distinctions between equilibrium and kinetic states, stability and instability, and crack propagation and initiation. Means of imparting higher strength and toughness to otherwise brittle materials are then explored along with their pros and cons. Select technological areas where fracture properties constitute a vital facet of material function—windows and display panels, structural ceramics, biomaterials, layer structures, manufacturing, and nanomechanics—are then presented as illustrative case studies. The balance between factors such as strength and toughness, scaling and threshold effects, and crack containment and crack avoidance, as well as structure at the atomic and microstructural scales, emerge as critical factors in materials design.

The local current at a constriction can be higher than the average current, which can lower the engineering value of the critical current density in lithium-ion cells. We show that X-ray Computed Tomography can be used to image the extent and the length scale of this constriction. The constriction is seen by comparing the in-plane thickness profile of the lithium metal at the anode before, and then at the cathode after full discharge. Remarkable discrepancy between these two images highlights the nature of the constriction.

A deep learning procedure has been examined for automatic segmentation of 3D tomography images from fiber-reinforced ceramic composites consisting of fibers and matrix of the same material (SiC), and thus identical image intensities. The analysis uses a neural network to distinguish phases from shape and edge information rather than intensity differences. It was used successfully to segment phases in a unidirectional composite that also had a coating with similar image intensity. It was also used to segment matrix cracks generated during in situ tensile loading of the composite and thereby demonstrate the influence of nonuniform fiber distribution on the nature of matrix cracking. By avoiding the need for manual segmentation of thousands of image slices, the procedure overcomes a major impediment to the extraction of quantitative information from such images. The analysis was performed using recently developed software that provides a general framework for executing both training and inference.

Indentation fracture models of ductile machining in hard and brittle materials are critically appraised. Relations obtained in a seminal study for critical depths of cut below which fracture is suppressed are examined and amended. Limitations inherent in any such materials-based analysis, in addition to uncertainties in empirical measurements of underpinning mechanical properties (modulus, hardness, toughness) and of threshold grinding depths, suggest that caution should be exercised in unconditional usage. Notwithstanding these limitations, the value of the indentation fracture methodology in placing ductile machining on a sound materials science footing is maintained.

Short sintering times and low furnace temperatures characterize the flash sintering process. The method can enable sintering of compounds of lithium since, in conventional sintering, which requires several hours at high furnace temperatures, it can become difficult to control the retention of lithium. Cubic Li6.25La3Zr2Al0.25O12 with a relative density of 96% was sintered in a few seconds in air at a furnace temperature of 850 °C. These specimens had an ionic conductivity of 0.5 mS cm−1 at room temperature, and an activation energy of 0.35 eV. Transgranular nature of the fracture surface suggests strong cohesion at grain boundaries.

Low values of critical-current-densities are a significant shortcoming in the performance of Li6.25La3Zr2A10.25O12, a ceramic electrolyte. X-ray Computed Tomography (CT) images obtained from pristine and "exhausted" cells constructed with lithium metal electrodes, are used to map the spatial non-uniformity of lithium transport. The current is shown to be concentrated in a limited area of the interface, ostensibly producing low engineering values of the critical-current-density. The total volume of Li-ion transfer calculated from the CT scans, is consistent with Faradaic transport. A qualitative analysis of the maps suggests a current concentration factor of at least ten, and most likely even higher.

This study examines risk and protective factors associated with experiencing homelessness in the year after "aging out" of foster care. Using a state-level integrated administrative database, we identified 1,202 emerging adults in Washington State who exited foster care between July 2010 and June 2012. Initial bivariate analyses were conducted to assess the association between candidate predictive factors and an indicator of homelessness in a 12-month follow-up period. After deploying a stepwise regression process, the final logistic regression model included 15 predictive factors. Youth who were parents, who had recently experienced housing instability, or who were African American had approximately twice the odds of experiencing homelessness in the year after exiting foster care. In addition, youth who had experienced disrupted adoptions, had multiple foster care placements (especially in congregate care settings), or had been involved with the juvenile justice system were more likely to become homeless. In contrast, youth were less likely to experience homelessness if they had ever been placed with a relative while in foster care or had a high cumulative grade point average relative to their peers.

The utility of indentation testing for characterizing a wide range of mechanical properties of brittle materials is highlighted in light of recent articles questioning its validity, specifically in relation to the measurement of toughness. Contrary to assertion by some critics, indentation fracture theory is fundamentally founded in Griffith–Irwin fracture mechanics, based on model crack systems evolving within inhomogeneous but well-documented elastic and elastic–plastic contact stress fields. Notwithstanding some numerical uncertainty in associated stress intensity factor relations, the technique remains an unrivalled quick, convenient and economical means for comparative, site-specific toughness evaluation. Most importantly, indentation patterns are unique fingerprints of mechanical behavior and thereby afford a powerful functional tool for exploring the richness of material diversity. At the same time, it is cautioned that unconditional usage without due attention to the conformation of the indentation patterns can lead to overstated toughness values. Limitations of an alternative, more engineering approach to fracture evaluation, that of propagating a precrack through a “standard” machined specimen, are also outlined. Misconceptions in the critical literature concerning the fundamental nature of crack equilibrium and stability within contact and other inhomogeneous stress fields are discussed.

We present a methodology for characterizing and reconstructing in-plane weave variability in textile composites. Surface topography of a partially processed C-fiber/SiC matrix composite panel was measured using digital image correlation. The centroids of tow segments that appear periodically on the fabric surface were located by image analysis and used as fiducial markers. Stochastic deviations of the fiducial markers from the ideal periodic weave structure indicate geometrical variance. Fourier analysis shows that spatial wavelengths of the deviations range from the size of one unit cell to the dimensions of the entire panel. Long-range deviations are attributed principally to fabric deformation after manufacture, during handling. Short-range fluctuations, extracted by computing spatial derivatives of the positions of the fiducial markers, are attributed to variations in tow packing density that arises during weaving. A simple set of statistics for these deviations is presented and its use in generating stochastic virtual specimens is demonstrated.

Dense nanocomposites constituted from 70/30 vol% of hafnia–silicon carbide and were prepared by spark plasma sintering. Silicon carbide suppresses grain growth. The fracture strength of as prepared composites is 400–600 MPa. Oxidation up to 1600 °C in air for 10 h has minor influence on the mechanical strength, which is ascribed to the dense nature of the oxidation scale. The high density of the oxidation scale is attributed to a volume increase when silicon carbide oxidizes and reacts with hafnia to form hafnium silicate. The composite has a thermal conductivity of 14 W m−1 K−1 at room temperature. Design approaches for further enhancement of ultrahigh temperature properties of oxide/non-oxide composites are discussed.

Techniques for characterizing tow architectures and defects in woven ceramic composites are required for generating high-fidelity geometric models and subsequently probing effects of defects on composite performance. Although X-ray computed tomography (CT) has been shown to provide the requisite information with potentially sub-μm resolution, the technique is inherently limited to probing only small volumes: on the order of a few unit cells of typical weaves. Here, we present an assessment of the efficacy of a complementary 2D technique, based on surface topography mapping via 3-D (three-dimensional) digital image correlation (DIC), with potential for ascertaining long-range features in weaves and defects that cannot be gleaned from CT imaging alone. Upon comparing surfaces reconstructed from CT and DIC data, we find that DIC is capable of resolving surface heights with a root mean square(RMS) error of ~10 μm (about twice the CT voxel size, 4.4 μm) and a spatial resolution of ~20 μm over areas of several cm2. Achieving this level of resolution requires use of sufficiently small speckles (~50 μm) and small subset size (~300 μm) relative to the characteristic tow dimensions (~1 mm). The error is somewhat higher (about 20 μm) in areas where surface discontinuities or rapid changes in topography exist (e.g., at tow boundaries).

Oxidation behavior of HfSiCNO ceramics for Hf/Si ratio of 0.09 at 1400°C–1600°C in ambient air is reported. Quantitative X-ray analysis of oxidized powders shows crystalli-zation of the amorphous phase into tetragonal hafnia, hafnon, and cristobalite (carbides, seen in inert atmosphere heat treatments are absent). Cross-sectional SEM shows the oxide overgrowth on the particles to contain precipitates of hafnia/hafnon, while the interior of the particles is decorated with nanoscale grains of hafnia in a necklace-like formation. The oxidation kinetics of these materials, determined both from weight-change measurements and from direct observation of oxide overgrowth, are shown to be comparable to the oxidation of SiC single crystals. Oxidation of SiC–SiC minicomposites (straight fiber bundles infiltrated with a SiC matrix), coated with thin films of HfSiCNO prepared by dip-coating was studied. The overgrowth thicknesses for oxidation time of 1000 h at 1600°C are compared for uncoated, SiCN(O)-coated, and HfSiCNO-coated minicomposites.

Ceramic matrix composites are the emerging material of choice for structures that will see temperatures above ~1,500 °C in hostile environments, as for example in next-generation gas turbines and hypersonic-flight applications. The safe operation of applications depends on how small cracks forming inside the material are restrained by its microstructure. As with natural tissue such as bone and seashells, the tailored microstructural complexity of ceramic matrix composites imparts them with mechanical toughness, which is essential to avoiding failure. Yet gathering three-dimensional observations of damage evolution in extreme environments has been a challenge. Using synchrotron X-ray computed microtomography, we have fully resolved sequences of microcrack damage as cracks grow under load at temperatures up to 1,750 °C. Our observations are key ingredients for the high-fidelity simulations used to compute failure risks under extreme operating conditions.

Three-dimensional (3-D) images of two ceramic-matrix textile composites were captured by X-ray micron-resolution computed tomography (lCT) on a synchrotron beamline. Compared to optical images of sections, CT data reveal comprehensive geometrical information about the fiber tows; information at smaller scales, on matrix voids, individual fibers, and fiber coatings, can also be extracted but image artifacts can compromise interpretation. A statistical analysis of the shape and positioning of the fiber tows in the 3-D woven architecture is performed, based on a decomposition of the spatial variations of any geometrical characteristic of the tows into non-stochastic periodic trends and non-periodic stochastic deviations. The periodic trends are compiled by exploiting the nominal translational invariance of the textile, a process that maximizes the information content of the relatively small specimens that can be imaged at high resolution. The stochastic deviations (or geometrical defects in the textile) are summarized in terms of the standard deviation of any characteristic at a single point along the axis of a tow and correlations between the values of deviations at two different points on the same or different tows. The tow characteristics analyzed consist of the coordinates of the centroids of a tow, together with the area, aspect ratio, and orientation of its cross-section. The tabulated statistics are sufficient to calibrate a probabilistic generator (detailed elsewhere) that can create virtual specimens of any size that are individually distinct but share the statistical characteristics of the small specimens analyzed by X-ray lCT. The data analysis presented herein forms the first step in formulating a virtual test of textile composites, by providing the statistical information required for realistic description of the textile reinforcement.

The development of ceramic composites with three-dimensional fiber reinforcement architectures formed by textile methods has led to the potential for active shape-morphing surfaces that can operate in high temperature and variable pressure environments. This technology is of particular interest for hypersonic applications, where SCRAM jet engines require variable inlet geometry to achieve efficient flight over realistic flight profiles and variable flight conditions. The experiments reported here show that significant shape morphing can be achieved and good control of the shape sustained even in the presence of large temperature and pressure gradients. Experiments were carried out using a subscale morphing hypersonic inlet with rectangular cross-section in a Mach 8 wind tunnel facility with a total temperature of 800 K. The upper surface of the inlet consisted of a C–SiC composite plate (0.7 mm thick, 37.5 cm long, and 11 cm wide) connected to five actuators through a triangular truss support structure. The lower surface was a flat plate instrumented with an array of pressure taps along the flow centerline. As the shape varied, the surface contour was reliably controlled for high efficiency, low loss compression. A factor of six inlet area ratio variation was achieved and good agreement with model predictions was observed.

A simple method is described for measuring material erosion by reaction with water vapor under high-speed flow conditions, with H2O partial pressures, velocities, temperatures, and erosion rates representative of those experienced in gas turbine engines. A water vapor jet is formed by the feeding water at a controlled rate into a capillary inside a tube furnace, where the large expansion of vaporization within the confines of the capillary accelerates the jet. With modest flow rates of liquid water, steam jets with temperatures up to ∼1400°C and velocities in the range 100–300 m/s have been achieved. The partial pressure of water vapor in the 100% steam jet is the same as in an industrial turbine operating at 10 atm total pressure with 10% water vapor. In preliminary experiments with SiC, erosion rates of the order of 1 μm/h have been observed.

A new paradigm for ceramic composite structural components enables functionality in heat exchange, transpiration, detailed shape, and thermal strain management that significantly exceeds the prior art. The paradigm is based on the use of three-dimensional fiber reinforcement that is tailored to the specific shape, stress, and thermal requirements of a structural application and therefore generally requires innovative textile methods for each realization. Key features include the attainment of thin skins (less than 1 mm) that are nevertheless structurally robust, transpiration holes formed without cutting fibers, double curvature, compliant integral attachment to other structures that avoids thermal stress buildup, and microcomposite ceramic matrices that minimize spalling and allow the formation of smooth surfaces. All these features can be combined into structures of very varied gross shape and function, using a wide range of materials such as all-oxide systems and SiC and carbon fibers in SiC matrices. Illustrations are drawn from rocket nozzles, thermal protection systems, and gas turbine engines. The new design challenges that arise for such material/structure systems are being met by specialized computational modeling that departs significantly in the representation of materials behavior from that used in conventional finite element methods.

Compressive creep of La-monazite is investigated in the temperature range 1100 °C to 1500 °C. The study includes both high-purity single-phase material and material with excess phosphorus located in amorphous grain boundary phases. The results indicate that the presence of small amounts of excess P in polycrystalline LaPO4 has a large effect on microstructural stability and creep at high temperature. Materials with La/P ratio close to unity (within ∼500 ppm) show little grain growth at temperatures up to 1400 °C and deform by creep at rates similar to those of alumina and zirconia, with stress exponent ∼1. Materials containing excess P (as little as ∼1 %) show more rapid grain growth, higher creep rates, and cavitation during creep. The results are compared with creep rates of other refractory oxides and oxide fibers. Implications for the behavior of oxide composites containing La-monazite are considered.

© 2014 The American Ceramic Society. This article addresses the effects of small holes (1-2 mm in diameter) on the tensile properties of a woven C/SiC composite. Holes are introduced by one of two methods: by insertion of fugitive rods into the woven preform before adding the matrix or by drilling after panel fabrication. The tensile strength exhibits only a weak sensitivity to the presence of holes, regardless of the manner in which the holes are introduced. Nevertheless, woven holes appear to be somewhat less detrimental to strength. The effects are attributed to the fact that the tows in specimens with woven holes are locally intact and thus contribute to load-bearing. Full-field strain maps reveal strong spatial periodicity, reflecting the underlying crack pattern and weave geometry. Even when averaged over lengths equal to the unit cell dimensions, the strains exhibit periodicity, with a wavelength dictated by the unit cell dimensions. When holes are present, the strain patterns reflect a convolution of the effects of weave geometry and strain concentrations near the holes. The results have implications for development of high-fidelity models for these composites: notably, in selection of a suitable representative volume element and for modeling schemes that capture the stochastic nature of cracking.

A general analysis of elastic/plastic indentation fracture for an arbitrary-shaped indenter is presented. The analysis is based on the observation that the residual indentation stress field provides the driving force for crack formation. After establishing that the influences of indenter geometry and load on the residual field are completely characterized by the volume of the indentation, a relation between the extent of radial cracking, the indentation volume, and the material properties is derived. Predictions of the analysis are examined by comparing calculated load/crack-length relations for two specific indenter geometries (Vickers pyramid and sphere) with experimental measurements in ZnS. For Vickers indentation the crack length is proportional to (load)2/3, whereas for spherical indenters the variation of crack length with load is nearly linear.

Engineering tests and full-field strain measurements are used to assess the accuracy of predictions made by the Binary Model, a computational tool for textile composites. The test case is a carbon fiber/SiC matrix composite, in which the reinforcement is a three-dimensional angle-interlock weave. The test composites are thin, having been designed for heat exchanger applications. The thinness leads to strong variations in local strains and strong effects of tow waviness upon macroscopic elasticity. The model performs well in predicting both local variations in strain and macroscopic elasticity. The effect of averaging local strains over variable gauge lengths is explored. Strains averaged over an appropriate gauge length have recently been proposed as the preferred measures of strain for use in local failure criteria.

Integrally formed ceramic matrix composite structures are being developed for a range of hot-structure applications involving active cooling. Integral textile structures offer several advantages: 1) Joints between ceramicand other materials in hot zones can be avoided. 2) Thin skins (

The compatibility of Al2O3 and LaPO4 at temperatures up to 1600°C is examined. Provided the ratio of La to P was close to 1:1, no reactions were observed after 200 h at 1600°C. Moreover, the Al2O3/LaPO4 interface remained sufficiently weakly bonded to cause deflection of cracks, as reported previously. In the presence of excess P or La, reactions occurred as expected, forming AlPO4 in the case of excess P, and LaAlO3 and LaAl11O18 in the case of excess La.

Multilayered composites consisting of LaPO4 (La-monazite) layers alternating with various ZrO2-based materials were fabricated to investigate whether LaPO4 provides a weakly bonded interface suitable for promoting toughening, as previously observed in the system LaPO4/Al2O3. The following ZrO2-based materials were assessed: Y-ZrO2, Y-ZrO2/Al2O3, Ce-ZrO2, and Ce-ZrO2/Al2O3. Debonding was observed in all cases. The composites containing Y-ZrO2 and Y-ZrO2/Al2O3 were stable, with no reactions, at temperatures up to at least 1600°3C. However, in the composites containing Ce-ZrO2, interdiffusion of Ce and La occurred, resulting in formation of a pyrochlore-like phase and, in the case of the Ce-ZrO2/Al2O3 composite, a (Ce,La)Al11O18 magnetoplumbite phase.

Two-phase composites consisting of LaPO4 or CePO4 and alumina, mullite, or zirconia were found to be machinable; i.e., they can be cut and drilled using conventional tungsten carbide metal-working tools. Single-phase LaPO4 was also machinable. Measurements of drilling rates, grinding rates, and normal forces are used to compare the ease of machining in these materials and in a conventional machinable glass-ceramic material, and to provide preliminary information on the relation between microstructure and machining properties. In Hertzian contact experiments these materials showed extensive nonlinear behavior associated with a damage zone beneath the contact site, similar to other machinable ceramics. Mechanisms of material removal are discussed.

Thin C/SiC composites were fabricated by infiltrating a woven carbon fiber fabric with a slurry of SiC powder and polymer precursor for SiC, followed by heat treatment for pyrolysis. The effects of heat treatment parameters on the crystallization of the polymer-derived SiC, the composite microstructure, and the transverse thermal properties were assessed. Whereas composites heat-treated at 1000°C were crack-free and nearly fully dense, composites that were subjected to further multiple reinfiltration and heat treatment cycles at 1600°C developed porosity and cracking. However, the transverse thermal conductivity was increased significantly by the higher-temperature heat treatment, to values higher than that of a composite with a chemical-vapor-infiltration SiC matrix and the same fiber reinforcement.

Polycrystalline monazite (LaPO4) was deformed at room temperature by a spherical indenter. Deformation twins were identified by TEM in 70 grains. Five twin planes were found: (1 0 0) was by far the most common; (0 0 1) and (1 2 0) were less common; ( 1 2 2 ) was rare, and kinks in (1 2 0) twins were identified as irrational ′(4 8 3)′ twin planes. The twinning modes on these planes were inferred from the expression of twinning shear at free surfaces, predictions of classical deformation twinning theory, and various considerations of twin morphology and crystal structure. Atomic shuffle calculations that allow formation of either a glide plane or a mirror plane at the twin interface were used to analyze twin modes. The inferred twin modes all have small atomic shuffles. For (0 0 1) twins, the smallest shuffles were obtained with a glide plane at the interface, with displacement vector R = 1 2 [ 010 ] . The results do not uniquely define a twin mode on (1 0 0), leaving open the possibility of more than one mode operating on this plane. Factors that may determine the operative deformation twinning modes are discussed. Crystal structure considerations suggest that the relative abundance of twinning modes may correlate with low shear modulus on the twin plane in the direction of twinning shear, and with a possible low-energy interface structure consisting of a layer of xenotime of one half-unit-cell thickness that could form at (1 0 0) and (0 0 1) twins. The three most common twins have low strains to low ∑ coincidence site lattices (CSLs).

Room temperature debonding and sliding of La-Monazite coated fibers is assessed using a composite with a polycrystalline alumina matrix and fibers of several different single crystal (mullite, sapphire) and directionally solidified eutectic (Al2O3/Y3Al5O12 and Al2O3/Y-ZrO2) compositions. These fibers provide a range of residual stresses and interfacial roughnesses. Sliding occurred over a debond crack at the fiber-coating interface when the sliding displacement and surface roughness were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations and occasional twinning, whereas the fibers were undamaged. Dense, fine-grained (10 nm) microstructures suggestive of dynamic recrystallization were also observed in the coatings. Frictional heating during sliding is assessed. The possibility of low temperature recrystallization is discussed in the light of the known resistance of monazite to radiation damage. The ability of La-Monazite to undergo plastic deformation relatively easily at low temperatures may be enabling for its use as a composite interface.

Room-temperature tensile properties were measured for two thin C–SiC composites fabricated from single sheets of carbon fiber fabric with nominally the same weave architecture, but different fiber packing densities. The SiC matrixes were formed by infiltration and pyrolysis of a polymer precursor (allylhydridopolycarbosilane). The tensile properties are related to microstructural characteristics, observed damage mechanisms, and measurements of local strain concentrations by speckle interferometry. Differences are observed between the responses of these thin-sheet composites and conventional CVI-matrix composites of larger thickness. Debonding between transverse and longitudinal fiber tows allows significant strains due to straightening of initial wavy fiber tows and leads to local stress concentrations. The strength and elastic modulus are affected by the waviness of the longitudinal tows.

Research performed under the National Hypersonic Science Center for Materials and Structures (NHSC-MS) aimed to advance the basic science needed to guide design of new high-temperature oxidation-resistant ceramic materials and to develop characterization and analysis tools that enable life prediction for fiber-reinforced composites with geometrically complex microstructures. Highlights of the research are as follows: * The roles of transition element dopants in affecting oxidation resistance of ZrB2 ultra-high temperature ceramics, through the effects of the dopants on sintering and/or stability of the oxidation products (ZrO2, B-O glasses, and B-Si-O glasses), were elucidated through experiments and atomistic calculations. * Structure, oxidation behavior, and stability of HfSiCNO ceramics were determined by experiment and atomistic calculations. The potential for beneficial oxidation reactions to form self-healing layers containing compounds that are resistant to water vapor erosion was explored. * Two new experimental methods were developed for characterizing fiber architectures in ceramic matrix composites and observing/measuring the evolution of damage under load at high temperatures, above 1500 deg C. One method uses digital surface image correlation and the other uses synchrotron x-ray micro tomography, which is capable of micron-scale resolution in 3-dimensional images. * Under the umbrella of a virtual test system, methods were developed for: (i) analyzing 3-dimensional images of microstructures of fiber reinforced composites to create statistical characterization of the microstructure, (ii) formulation of a probabilistic generator for creating virtual specimens that replicate the measured statistics, and (iii) creation of a computational model for a virtual specimen that allows representation of discrete damage events.

Major advances have been made during the past 2 years in the development of fiber-reinforced oxide composites for long-life combustion components. These include demonstration of long-term stability of mullite-based porous-matrix composites at 1200°C, development of fiber coating and slurry infiltration methods to produce composites with weakly bonded La-monazite interphases, and development of new alumina-based fibers.

Interfaces of Al2O3 (sapphire) and LaPO4 (La-monazite) have been separated by fracture to reveal boundary grooving effects analogous to surface grooves that develop at high temperature on a polycrystalline body wherever a grain boundary intersects the surface. Atomic force microscope measurements are used to form images from matching sides of a separated interface and to compare groove profiles with the solution of Mullins.

An extremely simple processing route has been used to produce LaPO 4 (La-monazite)/alumina continuous fiber-reinforced ceramic composites. In this paper, the processing, microstructure and tensile properties are reviewed. In particular, the damage tolerance and notch insensitivity of this system, which contains monazite-coated fibers, will be compared to the properties reported for other oxide composites.

The advantages of using neural network methodology for the modeling of complex social science data are demonstrated, and neural network analysis is applied to Washington State Child Protective Services risk assessment data. Neural network modeling of the association between social worker overall assessment of risk and the 37 separate risk factors from the State of Washington Risk Assessment Matrix is shown to provide case classification results superior to linear or logistic multiple regression. The improvement in case prediction and classification accuracy is attributed to the superiority of neural networks for modeling nonlinear relationships between interacting variables; in this respect the mathematical framework of neural networks is a better approximation to the actual process of human decision making than linear, main effects regression. The implications of this modeling advantage for evaluating social science data within the framework of ecological theories are discussed.

This study replicates and extends prior research on the characteristics and correlates of child maltreatment recurrences based on an ecological model of child maltreatment. The use of a very large sample with a large number of risk factors avoids methodological shortcomings based on small samples size and sample selection bias. The risk and other characteristics of 12,329 referrals were studied to determine which factors related to recidivism within an 18-month period. Rereferral rates increase with the increasing number of prior referrals and of placements in foster care (followed by reunification), but there is little difference in rates of rereferral by substantiation. Several of the risk factors associated with an increased likelihood of rereferral, history of domestic violence, history of caregiver child abuse/neglect (CA/N) as a child, and substance abuse, are the risk factors least often assessed by socialworkers. Rates of rereferral vary by ethnicity, type of abuse, and population type (rural, urban).

The objective of this study was to examine, in an ecological context, the relative importance of risk factors for chronic recidivism in child abuse/neglect (CA/N). Survival analysis of multiple referrals to Child Protective Services (CPS), supplemented by neural network analysis, shows that a constellation of risk and other factors are associated with the likelihood of recidivism of CA/N. Caregivers distinguish families with multiple referrals by their history of abuse or neglect as children, abuse or neglect that began at an early age, having children with developmental delays, and having multiple victims in the family. Time to rereferral decreases with an increasing number of prior referrals, and there is a marked difference in the time to the first rereferral when compared to subsequent rereferrals. This indicates that referral to CPS reflects conditions that increase the likelihood of additional referrals to CPS. Administrative factors account for some of the variance in multiple referrals.