The sleep of commercial vehicle drivers under the 2003 hours-of-service regulations

Previous research has found that commercial drivers get an average of 5.18 h of sleep per night. The revised hours-of-service (HOS) regulations (in the United States) are in place to provide drivers with more opportunities to get sleep. However, are drivers really getting more sleep under these new regulations? Also, is there a relationship between sleep quantity and involvement in critical incident (crashes, near-crashes, or crash-relevant conflicts)? Data from 73 truck drivers, collected during a naturalistic driving study after the implementation of the 2003 HOS regulations, were analyzed to determine overall sleep quantity (using actigraphy), along with sleep quantity prior to being involved in a critical incident. Sixty-two drivers had at least seven consecutive days (Monday through Sunday) of reliable actigraphy data; mean sleep quantity per 24-h period (midnight centered using the Cole-Kripke algorithm) for these drivers was 6.28 h (S.D.=1.42 h). Fifty-eight critical incidents were recorded in the 10th and 11th driving hours. Analysis results indicated that drivers received significantly less sleep in the period prior to a critical incident as compared to their mean overall sleep quantity. The results of this study indicate drivers may be getting more sleep under the revised 2003 HOS regulations as compared to the old regulations. In addition, significantly less sleep in the 24-h period prior to involvement in a critical incident suggests driver fatigue may have been a potential contributing factor in these critical incidents.     

Sleepiness/fatigue and distraction/inattention as factors for fatal versus nonfatal commercial motor vehicle driver injuries

A retrospective population-based case-control study was conducted to determine whether driver sleepiness/fatigue and inattention/distraction increase the likelihood that a commercial motor vehicle collision (CVC) will be fatal. Cases were identified as CVC drivers who died (fatal) and controls were drivers who survived (nonfatal) an injury collision using the Kentucky Collision Report Analysis for Safer Highways (CRASH) electronic database from 1998-2002. Cases and controls were matched on unit type and roadway type. Conditional logistic regression was performed. Driver sleepiness/fatigue, distraction/inattention, age of 51 years of age and older, and nonuse of safety belts increase the odds that a CVC will be fatal. Primary safety belt law enactment and enforcement for all states, commercial vehicle driver education addressing fatigue and distraction and other approaches including decreased hours-of-service, rest breaks and policy changes, etc. may decrease the probability that a CVC will be fatal.     

Exploratory study of fatigue in light and short haul transport drivers in NSW, Australia

This exploratory study sought to identify relationships between work characteristics and fatigue experiences among light and short haul road transport drivers. Surveys were distributed to drivers of light goods vehicles (< or =12 t gross vehicle mass) undertaking short haul work (within a 100 km radius of base) in seven geographical regions of NSW, Australia. The participating drivers (n=321) reported a range of freight tasks. They typically worked a 50-h, 5-day week primarily as day shifts, and spent just over half of their work time driving. Despite the predominance of regular, day work, 38% of participants experienced fatigue at least once a week while driving for work and 45% had nodded off while driving during the preceding 12 months. Stepwise logistic regression analysis revealed that longer daily work hours, higher subjective work demands, as measured by the NASA TLX workload scale, and the percentage of freight movements undertaken from customers to depots each explained unique variance in the frequency of fatigue experiences. The results of the study suggest that fatigue is an issue for some light and short haul road transport drivers and identifies work characteristics that should be investigated further.     

Naturalistic field study of the restart break in US commercial motor vehicle drivers: Truck driving,sleep, and fatigue

Commercial motor vehicle (CMV) drivers in the US may start a new duty cycle after taking a 34-h restart break. A restart break provides an opportunity for sleep recuperation to help prevent the build-up of fatigue across duty cycles. However, the effectiveness of a restart break may depend on its timing, and on how many nighttime opportunities for sleep it contains. For daytime drivers, a 34-h restart break automatically includes two nighttime periods. For nighttime drivers, who are arguably at increased risk of fatigue, a 34-h restart break contains only one nighttime period. To what extent this is relevant for fatigue depends in part on whether nighttime drivers revert back to a nighttime-oriented sleep schedule during the restart break. We conducted a naturalistic field study with 106 CMV drivers working their normal schedules and performing their normal duties. These drivers were studied during two duty cycles and during the intervening restart break. They provided a total of 1260 days of data and drove a total of 414,937 miles during the study. Their duty logs were used to identify the periods when they were on duty and when they were driving and to determine their duty cycles and restart breaks. Sleep/wake patterns were measured continuously by means of wrist actigraphy. Fatigue was assessed three times per day by means of a brief psychomotor vigilance test (PVT-B) and a subjective sleepiness scale. Data from a truck-based lane tracking and data acquisition system were used to compute lane deviation (variability in lateral lane position). Statistical analyses focused on 24-h patterns of duty, driving, sleep, PVT-B performance, subjective sleepiness, and lane deviation. Duty cycles preceded by a restart break containing only one nighttime period (defined as 01:00–05:00) were compared with duty cycles preceded by a restart break containing more than one nighttime period. During duty cycles preceded by a restart break with only one nighttime period, drivers showed more nighttime-oriented duty and driving patterns and more daytime-oriented sleep patterns than during duty cycles preceded by a restart break with more than one nighttime period. During duty cycles preceded by a restart break with only one nighttime period, drivers also experienced more lapses of attention on the PVT-B and increased lane deviation at night, and they reported greater subjective sleepiness. Importantly, drivers exhibited a predominantly nighttime-oriented sleep schedule during the restart break, regardless of whether the restart break contained only one or more than one nighttime period. Consistent with findings in laboratory-based studies of the restart break, the results of this naturalistic field study indicate that having at least two nighttime periods in the restart break provides greater opportunity for sleep recuperation and helps to mitigate fatigue.     

The effect of external non-driving factors,payment type and waiting and queuing on fatigue in long distance trucking

Introduction

The aim of this study was to explore the effects of external influences on long distance trucking, in particular, incentive-based remuneration systems and the need to wait or queue to load or unload on driver experiences of fatigue.

Methods

Long distance truck drivers (n = 475) were recruited at truck rest stops on the major transport corridors within New South Wales, Australia and asked to complete a survey by self-administration or interview. The survey covered demographics, usual working arrangements, details of the last trip and safety outcomes including fatigue experiences.

Results

On average drivers’ last trip was over 2000 km and took 21.5 h to complete with an additional 6 h of non-driving work. Incentive payments were associated with longer working hours, greater distances driven and higher fatigue for more drivers. Drivers required to wait in queues did significantly more non-driving work and experienced fatigue more often than those who did not. Drivers who were not paid to wait did the longest trips with average weekly hours above the legal working hours limits, had the highest levels of fatigue and the highest levels of interference by work with family life. In contrast, drivers who were paid to wait did significantly less work with shorter usual hours and shorter last trips. Multivariate analysis showed that incentive-based payment and unpaid waiting in queues were significant predictors of driver fatigue.

Conclusions

The findings suggest that mandating payment of drivers for non-driving work including waiting would reduce the amount of non-driving work required for drivers and reduce weekly hours of work. In turn this would reduce driver fatigue and safety risk as well as enhancing the efficiency of the long distance road transport industry.     

An analysis of driving and working hour on commercial motor vehicle driver safety using naturalistic data collection

Current hours-of-service (HOS) regulations prescribe limits to commercial motor vehicle (CMV) drivers’ operating hours. By using naturalistic-data-collection, researchers were able to assess activities performed in the 14-h workday and the relationship between safety-critical events (SCEs) and driving hours, work hours, and breaks. The data used in the analyses were collected in the Naturalistic Truck Driving Study and included 97 drivers and about 735,000 miles of continuous driving data. An assessment of the drivers’ workday determined that, on average, drivers spent 66% of their shift driving, 23% in non-driving work, and 11% resting. Analyses evaluating the relationship between driving hours (i.e., driving only) and SCE risk found a time-on-task effect across hours, with no significant difference in safety outcomes between 11th driving hour and driving hours 8, 9 or 10. Analyses on work hours (i.e., driving in addition to non-driving work) found that risk of being involved in an SCE generally increased as work hours increased. This suggests that time-on-task effects may not be related to driving hours alone, but implies an interaction between driving hours and work hours: if a driver begins the day with several hours of non-driving work, followed by driving that goes deep into the 14-h workday, SCE risk was found to increase. Breaks from driving were found to be beneficial in reducing SCEs (during 1-h window after a break) and were effective in counteracting the negative effects of time-on-task.     

An approach to fatigue life modeling in titanium-matrix composites

A review of the procedures developed by the author and his colleagues over the last several years for predicting elevated-temperature fatigue life of metal-matrix composites is presented. Modeling approaches involve concepts of both linear and non-linear summation of damage from cycle-dependent as well as time-dependent mechanisms. The analyses, further, treat the micromechanical stresses in the constituents as parameters in the life prediction models. The material characterized is SCS-6/Timetal®21S, a metastable beta titanium alloy reinforced with continuous SiC fibers. Modeling is applied to isothermal fatigue at different frequencies and temperatures, and thermomechanical fatigue (TMF) under both in-phase and out-of-phase loading conditions at different temperature ranges and maximum temperatures. Experimental data are used as the basis for determining the parameters embedded in the models. The numerical results, in turn, provide insight into the dominant mechanisms controlling fatigue life under a given condition. The capability to correlate experimental data from a wide variety of test conditions for several versions of a damage summation model is demonstrated.     

An experimental investigation on the effect of short time exposure to elevated temperature on fatigue life of cold expanded fastener holes

An experimental study has been carried out to investigate the effect of short time exposure to elevated temperature on fatigue life of cold expanded fastener holes. When cold expanded holes are subjected to temperature variations their fatigue life changes remarkably. However the exact governing mechanism has not been clearly addressed before. In this study, another stress relaxation mechanism rather than creep has been introduced which happens at a short time due to temperature rise. Results of fatigue tests on Al 7075-T6 show that the effect of this mechanism can be beneficial or detrimental. This research has sought to clarify this issue.     

A quantitative three-dimensional in situ study of a short fatigue crack in a magnesium alloy

A previous four-dimensional in situ study of a short crack in a magnesium alloy King et al. (2011), Elektron 21, used synchrotron X-ray computed micro-tomography to follow its three-dimensional development with progressive fatigue cycling through the microstructure, which had been mapped by diffraction contrast tomography to measure grain shapes and crystal orientations in three dimensions. In the present work, very high-resolution post-test examination of the same sample by Serial Block Face Scanning Electron Microscopy (SBFSEM) provided three-dimensional fractographs to investigate the influence of microstructural features on the measured crack propagation rates. Digital volume correlation, applied to the X-ray computed micro-tomography datasets, measured the three-dimensional crack opening displacements and hence the crack opening modes. The short fatigue crack in magnesium propagated with mixed mode opening. Basal plane fracture is a dominant mechanism; hence, boundaries that disrupt the continuity of the basal plane are proposed to influence the crack propagation rate.     

An implicit gradient application to fatigue of complex structures

This paper presents a procedure to evaluate the stress gradient effect on the fatigue strength of steel welded joints and notched components. An effective stress is calculated by solving a second-order differential equation over all the component (the implicit gradient approach) independently of its geometric shape. The solution is obtained by assuming the isotropic linear elastic constitutive law for the material and the maximum principal stress as equivalent stress. The fatigue behaviour of geometrically complex steel welded joints is analysed and compared with previous fatigue scatter bands obtained for two-dimensional joints. In complex details, the actual critical point is derived from the analysis and is not assumed a priori. Implicit gradient analysis is also used to investigate high-cycle fatigue behaviour in the case of notches.In addition, it is shown that critical distance approaches can be obtained from the non-local theory by proper choice of the weight function.     

Mechanics and mechanisms of fatigue in a WC–Ni hardmetal and a comparative study with respect to WC–Co hardmetals

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC–Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC–Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path.     

An energy approach to predict fatigue crack propagation in metals and alloys

On the basis of the kinetic theory of strength, a new approach to the modeling of material degradation in cyclic loading has been suggested. Assuming that not stress changes, but acting stresses cause the damage growth in materials under fatigue conditions, we applied the kinetic theory of strength to model the material degradation. The damage growth per cycle, the effect of the loading frequency on the lifetime and on the stiffness reduction in composites were determined analytically. It has been shown that the number of cycles to failure increases almost linearly and the damage growth per cycle decreases with increasing the loading frequency.     

Propagation of physically short cracks in a bainitic high strength bearing steel due to fatigue load

Physically short cracks in a bainitic high strength bearing steel were fatigue loaded. The rapid propagation rate of early open short cracks agreed with that of long closure free cracks. After some rapid growth, the short cracks entered a transition period to the rate of growth limited long cracks. Potential drop showed that the short cracks were open to the tip throughout the growth sequence, which excluded crack face closure in the wake as the growth limiting mechanism in this material. Instead the short crack effect was related to residual stresses and other mechanisms at the crack tip. Crack manufacturing procedures were determined for straight long and short start cracks in the present material. LEFM with effective material parameters and limit compensation predicted the short crack lives.     

An implicit gradient application to fatigue of sharp notches and weldments

This paper addresses the problem of stress singularities at the tip of sharp V-notches by means of a non-local implicit gradient approach. A non-local equivalent stress is defined as a weighted average of a local stress scalar quantity computed on the assumption of linear elastic material behaviour. In the case of a crack, we propose an analytical solution for the non-local equivalent stress at the crack tip when the local equivalent stress assumes the analytical form proposed by Irwin. For open notches, several numerical procedures are possible.For welded joints, we assume that the material obeys a linear elastic constitutive law. In this case, the non-local equivalent stress obtained from the implicit gradient approach is assumed as the effective stress for assessments of joint fatigue. Using the principal stress as local equivalent stress and a notch tip or weld toe radius equal to zero, we analyse many series of arc welded joints made of steel and subjected to either tensile or bending loading, and we propose a unifying fatigue scatter band. If the welded joints are subjected only to mode I loading, an analytical relationship between the relevant Notch Stress Intensity Factors (NSIF) of mode I and the effective stress is established; otherwise, the effective stress is evaluated by means of a simplified numerical analysis. For complex welded structures, however, a completely numerical solution is proposed; when different crack initiation sites are present (i.e. either weld toes or roots), the proposed approach correctly estimates the actual critical point.     

Analysis of cold expanded fastener holes subjected to short time creep: Finite element modelling and fatigue tests

The beneficial effects of cold expansion have been well documented in previous studies, yet the performance of cold expanded plates exposed to elevated temperatures is an area of technical interest. In this research, finite element (FE) simulations along with experimental fatigue tests have been carried out to investigate the effect of exposure to elevated temperature on residual stress distribution and subsequent fatigue life of cold expanded fastener holes. According to the obtained results, creep stress relaxation occurs due to exposure to 120 °C for 50 h. FE results demonstrate a non-uniform residual stress relaxation regime through the plate thickness around the cold expanded hole and the fatigue test results show that the subsequent fatigue lives have significantly decreased.     

Experimental study of FRP-strengthened RC bridge girders subjected to fatigue loading

The practical application of composite materials for retrofitting of reinforced concrete bridge T-sectional girders was investigated. Carbon and glass fibre-reinforced polymers (CFRP and GFRP) saturated in an epoxy resin matrix were used to enhance the service load-carrying capacity of the bridge. Three 5-m-long simply supported beams were tested under monotonic and cyclic loads for comparison to a beam subjected to more than 10⁶ cycles in the service load range. The results show that an FRP-strengthened T-beam subjected to fatigue loading demonstrated excellent behaviour that can be expected from well-detailed retrofit schemes incorporating carbon and glass fibre laminates.     

Micro- and macro-approach to the fatigue crack growth in progressively drawn pearlitic steels at different R-ratios

This paper deals with the role of microstructure on the fatigue behaviour of pearlitic steels with different degrees of cold drawing. The analysis is focussed on the region II (Paris) of the fatigue behaviour, measuring the constants (C and m) for the different degrees of drawing. From the engineering point of view, the manufacturing process by cold drawing improves the fatigue behaviour of the steels, since the fatigue crack growth rate decreases as the strain hardening level in the material increases. In particular, the coefficient m (slope of the Paris laws) remains almost constant and independent of the drawing degree, whereas the constant C decreases as the drawing degree rises. The paper focuses on the relationship between the pearlitic microstructure of the steels (progressively oriented as a consequence of the manufacturing process by cold drawing) and the macroscopic fatigue behaviour. To this end, a detailed metallographic analysis was performed on the fatigue crack propagation path after cutting and polishing on a plane perpendicular to the crack front (fracto-metallographic analysis). It is seen that the fatigue crack growth path presents certain roughness at the microscopic level, such a roughness being related to the pearlitic colony boundaries more than to the ferrite/cementite lamellae interfaces. Fatigue cracks are transcollonial and exhibit a preference for fracturing pearlitic lamellae, with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and frequent local deflections that create microstructural roughness. The net fatigue surface increases with cold drawing due to the higher angle of crack deflections. With regard to the influence of the R-ratio, an increase of such a stress ratio produces microcracking with a higher number of branchings for the same stress intensity range.     

The fatigue limit: An analytical solution to a Monte Carlo problem

A new extreme value distribution based on log-normally distributed defects is presented. From this, an analytical solution to the random defect Monte Carlo problem in form of a distribution function characterizing the fatigue limit of an arbitrary component is derived. Surface defects, defined as internal defects intersecting a free surface, are incorporated into the model through an original analytical approach. The presented model is physically based and simple to implement and apply.Furthermore, the model was applied to a series of fatigue limit experiments found in the literature. The fatigue limit predictions were in good agreement with experimental results.     

An algorithm for accurate evaluation of the fatigue damage due to multimodal and broadband processes

In a recent work, the author developed an accurate method for calculating the fatigue damage in a bimodal Gaussian process. The novelty of this method lies in its ability to incorporate two critical effects, which have been unrecognized in prior studies. This paper extends the bimodal technique to multiple modes. Through case studies of multimodal processes, the algorithm is found to produce highly precise damage estimates (∼1% error) when benchmarked against simulations. This is true even when the component frequencies are very close. Further, the multimodal method is used to analyze general broadband processes, by partitioning a given spectrum. Since the accuracy is relatively insensitive to frequency spacing, the errors emanate primarily from the narrowband assumption, and this can be controlled by employing more segments. It is shown that with a sufficient level of discretization, the predicted damage for a variety of spectral shapes is negligibly conservative, generally within 2%.     

The response of aluminium/GLARE hybrid materials to impact and to in-plane fatigue

Fibre metal laminates (FMLs), such as glass reinforced aluminium (GLARE), are a family of materials with excellent damage tolerance and impact resistance properties. This paper presents an evaluation of the low velocity impact behaviour and the post-impact fatigue behaviour of GLARE laminate adhesively bonded to a high strength aluminium alloy substrate as a fatigue crack retarder. The damage initiation, damage progression and failure modes under impact and fatigue loading were examined and characterised using an ultrasonic phased array C-scan together with metallography and scanning electron microscopy (SEM). After impact on the substrate, internal damage to the GLARE bonded on the opposite side of the substrate occurred in the form of fibre and matrix cracking. No delamination was detected at the GLARE/substrate bond. Before impact the bonded GLARE strap caused reductions in substrate fatigue crack growth rate of up to a factor of 5. After impact the retardation was a factor of 2. The results are discussed in terms of changes to the GLARE stiffness promoted by the impact damage.