Child Restraint Seats
By Law in the United Kingdom, Infants have to be restrained with suitable and secure seats when traveling until they surpass the height of 1.30 meters (Keynes, 2017). That is why Regulations 129 were put in place to ensure all restraint seat models meet approval standards for the comfort and safety of children. Therefore to win market approval, restraint seat designers must conduct simulation tests on their assembled child restraint seats to make sure the devices are robust enough to prevail impact collisions (Han, 2013). This paper discusses the tests required to approve a child seat, outlines the factors one should consider when performing the impact tests and the results one should expect after the tests. Lastly, the piece outlines procedural steps of simulating a drop test using ANSYS software. The steps can act as a guide when simulating impact tests using the software.
Regulation 129 applies to all I-size Infant restraint seats. Within this scope, there are integral ISOFIX seats that are designed to fit in universal vehicles, and there are special ISOFIX seats intended for special anchorage platforms like suitcases and airplane seats. All Child Restraint Systems should strike a balance between safety and comfort of the kids (Juhaida, 2017). Hence they are subjected to various tests for approval. Data obtained from the multiple tests are used to analyze the design seats and the final score submitted in report format to ECE R129 Regulations body. A close assessment of the results determines whether the chair meets approval standards as stipulated in regulations law. Some of the key aspects scrutinized include design measurements, material strength, loading tests, corrosion, overturning tests, and dynamic/ impact tests. It is statutory that the seats should conform to the rules and regulations of article R129.
Among other requirements, Child restraint systems must undergo impact tests which include frontal collision tests, rear impact tests, and lateral impact tests. These tests are done at specific velocities and incorporate other measures including rollover tests, acceleration tests and deceleration tests (Child Car Seats, 2017). Dynamic tests are split into two categories: those that involve the trolley and test bench and those that test the trolley and vehicle body shell. The test models must conform to I-size rules and use baby dummies to simulate baby weight and height. Aim being, to measure the intensity of forces that a toddler can be subjected to during collision (Keynes, 2017).
1. Frontal impact tests
Frontal impact tests are done for forward-facing child restraint seats. These seats are designed to accommodate children with a stature of 71cm and above for both I-size universal seats and the special integral seats (Jhinkwan, 2014). Frontal impact tests are crucial because they simulate a situation whereby a suitably restrained toddler is involved in a direct collision with an object. The object might be stationary or in motion and can even exhibit the same weight proportions as the infant. ANSYS software can be used to simulate a frontal impact test. It has powerful inbuilt tools that can create models of the trolley, test bench, and a dummy to represent the mass and height of a restrained child. After creating the models, the user should input all other specific parameters including mass, density and Young’s modulus of materials used for the trolley, test bench, and target.
For stoppage distance calculations, the trolley is propelled at a velocity change of 55km/h before being decelerated at a rate 34times higher than the gravity of earth (Nations, 2013). The trolley speed immediately before impact and the stoppage distance immediately after collision should be recorded. If a dummy is used, their head displacement in both horizontal and vertical directions should be registered within the first 300ms of impact. For direct collision tests, the test trolley should be separated from the target by a distance that meets ECE R129 requirements. During simulation, the trolley should be subjected to an acceleration speed of 55km/h towards the target. After impact, the trolley should be inspected to determine whether it has traces of damage or breakage. If a dummy is used, their head displacement in both horizontal and vertical directions should be recorded during impact.
The Frontal test evaluates the effects of acceleration and deceleration impact on the seat for stability purposes. The test also assesses the seat’s ability to absorb the upward and forward thrust forces while keeping the dummy’s head and torso in their initial statures. In other words, the seat should handle impact energy, maintain structural conformity and ensure that the reinforcing buckle is working as required. It is crucial to note the speed variations of the seat after a collision, how quickly the dummy settles, and if the dummy’s head of chests hits the hard surface. The findings are and reported to for ECE R129 Regulations body for assessment.
2. Rear impact tests
Rear-facing seats offer the best protection because they keep a child’s head, neck, and spine aligned during impact. The seats are designed to accommodate children up to 2 years of age and exclude children with a stature of more than 83cm. Still, developed models have to meet standards in with regards to impact response before they are approved into the market. Rear impact tests apply for both I-size ISOFIX child restraint systems and Special integral ISOFIX chairs. First, a rollover test should be conducted whereby the test bench is tossed around for 180 degrees rotation to test its compliance with rear impact requirements.
Rear seats must fall within the standard dimension and design measurements that are stated in the Regulations act. These aspects can be fed into the ANSYS software to create a rear seat model mounted to a car passenger seat. One should also input other specifications including mass and material data of the models. The seat should be propelled at a velocity change of 33km/h towards a stationary target. It is statutory to model a dummy to represent the mass and height of a restrained child before conducting rear impact tests. One should record any contact made between the dummy’s head and the modeled passenger seat during impact. The dummy, as well as the seat, should be inspected for traces of damage or breakage.
The test aims to observe if the rear seat can restrain injurious movements of the infant during a collision. The child’s physical state should remain intact; the head should not hit a wall whereas the neck should not tilt too much. It is crucial to note the speed variations of the seat after a collision, how quickly the dummy settles, and if the dummy’s head or neck hits the hard surface.
3. Lateral impact tests
Lateral facing seats are only manufactured for specific basic ISOFIX systems and special needs restraints. First and foremost, the test seat should be rotated 90 degrees in compliance with lateral impact requirements. The child seat can be struck by a moving mass at an angle of 65 degrees to simulate an oblique impact test. A typical way of achieving this is to tie a mass of considerable weight on a string and then swing it like a pendulum towards a moving or stationary seat. The special restraint seat can also be struck by a moving mass at an angle of 90 degrees to simulate a side impact test. After impact, the seat should be inspected to determine whether it has traces of damage or breakage. If a dummy is used, their head displacement in both horizontal and vertical directions should be recorded during impact
When testing the stoppage response, the lateral infant car together with the restrained dummy should be subjected to an impact velocity of 33km/h and a deceleration rate 20 times more than the gravity of earth. The car speed immediately before impact and the stoppage distance immediately after collision should be recorded. If a dummy is used, their head displacement in both horizontal and vertical directions should be filed within the first 300ms of impact. These tests aim to evaluate the seat’s ability to maintain the dummy’s neck and head in a composed stature after the effect. The seat should handle impact energy, maintain structural conformity and ensure that the reinforcing buckle is working as required.
The materials used to make the child seat should be exposed to a corrosion test. The metallic components of the restraint seat are immersed into a chamber containing a salt solution for a period 50 hours. On completion, the metallic parts are retrieved from the house and washed in clean water at temperatures below 38 degrees Celsius to remove the salt deposits on the component surfaces. The items are then allowed to dry at room temperatures before inspection. The corrosion levels should not be significant enough to be visible to the unaided eye of a certified observer. Additionally, any signs of deterioration recorded should not be substantial enough to affect the normal functioning of the Infant Restraint seat.
The dummy should be adequately tucked into the restraints installed in the chair before the test is done. In other words, one should fasten all the seat belts well around the dummy baby before the test by the ECE R129 Regulations. The whole car is the tossed clockwise at 360 degrees while maintaining a longitudinal speed of 3-5 degrees per second. The horizontal axis indicated in a median longitudinal plane of the infant seat should also be observed. If necessary, the dummy should be repositioned into its initial stature and the test repeated in an anti-clockwise direction. The horizontal axis should be adjusted to 90 degrees concerning the longitudinal plane of the restraint seat and the tests repeated in both directions. If possible the criteria above should be conducted with dummies of different sizes to cover the range of applications for which the seat is designed. In all scenarios, the dummy should not fall out of the seat during rotation. When the seat is turned down 90 degrees, the dummy’s head should not move more than 300mm from its initial location concerning the vertical direction of the seat
Individual Component tests:
A buckle is a quick release device that holds the child onto the seat structure of the car. The device can be reopened and can incorporate an adjusting device to suit more wearer physiques. The buckle test is only done to devices that have already passed the dynamic analysis. The Infant restraint is carefully unmounted from the test trolley without reopening the buckle. The buckle is then subjected to a tension force of 200N. If the fastener is mounted onto a hard surface, the torque should be applied in such a way to reproduce the dynamic test angle formed between the rigid surface and the buckle.
The geometric center of the buckle release button is subjected to a loading condition at a speed of 400mm/min. The load is applied along a predetermined axis that is parallel to the button’s initial direction of motion. The central part of the buckle surface upon which the load pressure is applied is decided by the geometric center. It is crucial to secure the buckle on a hard bolster before the loading test is done. A dynamometer or any other equivalent device can be used to restrain the buckle opening force to the direction of regular use. The energy that eventually opens the buckle is recorded, and any failures are noted down.
Under zero load conditions, a buckle that has not been subjected to any previous dynamics tests is attached and positioned on a rigid surface for testing. A force of 200N is applied to the center of the buckle in a normal opening direction. All other steps that apply to buckles that have been subjected to dynamic tests also use this fastener that has not been subjected to any initial loading conditions. The force that eventually opens the clasp is recorded along with any other damages observed.
Two samples are randomly selected to conduct the buckle strength test. The test includes all adjusters of the buckle except those that are attached directly to the Infant seat. Two round plates are positioned in a way, and the clasp is placed on top of the upper plate. Adjacent straps to the buckle are arranged respective to their positions and left to hang freely from the upper plate. The free ends of the hanging straps are wound uniformly around the lower plate so that they emerge out of the plate’s inner gap. The straps should align vertically concerning the two plates. Another dish is clamped on the lower face of the lower plate, but an allowance is given to allow strap movement between the plates. A tensile machine applies a pulling force on the straps while isolating the buckle from the tension applied. The two lower plates are tightened more, and the tension force increased in step of 100mm/min until the required values are reached.
2) Adjusting device
This device enables the buckle and its attachments to be flexible to many children. It is incorporated into the buckle or is part of a retractor belt. The straps are aligned steadily through the manual adjustment device before testing. After factoring in all standard testing conditions, a tension force is applied to the device in steps of 100mm/min. The maximum attainable strength is measured and recorded to the nearest integer. The test is conducted in both directions of movement through the device. Before measurement, the strap should have traversed ten full cycles.
3) Micro-slip test
The devices or components are subjected to an atmospheric humidity of 65% and temperatures of 20 degree Celsius before the micro-slip test. However, temperatures should strictly fall in between 15 and 30 degree Celsius during the trial. The strap should not be attached to any other device and is aligned in a configuration akin to that of a car. The adjusting device should be balanced on top of a vertical strap and loaded with 50N; the lower free end of the belt should face down. The other end should pass over a deflector roller horizontally to the strap portion supporting the load. The device to be tested should be positioned such that a pressure of 50N is applied 100mm from the support table. The test shall cover 20 pre-test cycles at a rate of 30cycles per min with an amplitude of 300mm. Micro-slip is calculated from the device position on test completion.
The retractor is designed to accommodate the strap system of the seat. When the buckle is fixed the retractor releases a measurable length of the strap according to the child’s physique. Before conducting retractor tests, one should make sure the dummy is adequately fitted onto the self-belt assembly. Strap forces should be measured while the strap is being retracted at a frequency of 0.6m/n. The measurement is taken just at the point of contact with the dummy. The belt should be withdrawn at a rate of 30 cycles per minute, and a jolt can lock the retractor at the fifth cycle in case of emergency retractors.
5) Static test for straps
These are flexible devices designed to absorb shock and transmit forces. The Lab strap restrains the infant’s pelvic region, the shoulder strap holds the infant’s upper torso, and the crotch strap is positioned to cross the child’s thighs. Two strap samples are used to carry out the test. A tensile strength machine is used to grip the straps independently between two clamps. 100mm/min is the selected speed of traverse for the straps. A free allowance of 200mm is kept between the specimen and the machine clamps before the test. The tension force should be elevated until the load breaks and the value recorded. The strap may slip or break within 10mm of clamp trial. In such a scenario the test should be nullified and another sample prepared for the test.
6) Temperature test
All the components explained above shall be submerged in hot water within a closed surface. The devices are immersed for more than 24 hours in water that is preheated to temperatures above 80 degree Celsius. The components are then retrieved from the water to cool outside in temperatures below 23degree Celsius. Once the cooling period is over the parts are subjected to three cycles that last 24 hours:
The devices are subjected to temperatures exceeding 100 degrees Celsius for a six-hour period. These conditions should be achieved within 80 mins of cycle commencement. There is a quick transition into a second cycle whereby temperatures are kept below 0 degrees Celsius for a six-hour period. The environment is prepared 90mins before commencement of the second cycle. Lastly, the components are exposed to temperatures below 23 degree Celsius for 12 hours.
The infant restraint seat contains a cushion material that needs to be ascertained when it is still new. This is done by finding out penetration values in times of deceleration or impact and collision. The seats are many times modeled to carry children having specific masses. In times of impact, a child will be displaced momentarily from the seat. This entire process can be simulated by utilizing the ANSYS software. This software program will give a real visual impression of the process and save a record of the results. ANSYS is able to come up with an impact dummy having an equal mass with the highest mass that can be carried by the seat. The impact mass will be raised up and positioned vertically such that the seat is on its underside with a height difference of 500 plus or minus 5 meters (NATIONS, 2013). Thereafter, it is left to fall without any interference onto the surface that’s cushioned. This will simulate an impact that many people refer it as a drop test. However, the penetration curve that is recorded must never deviate by +15% the initial values.
This paper will lay out the procedures involved when creating a model of the drop test and also its simulation. It will also be evident that the material used for the simulation need to have equal properties with the materials used in the real world.
GRAPHICAL USER INTERFACE OF THE ANSYS SOFTWARE.
It’s important to note that the direction that is opposite to acceleration because of gravity (g) is called the Y-axis on the screen. For this reason, the coordinates on the screen give a guide that helps keep track of the association between the object that is falling and the force of gravity (Typical Drop Test Procedure, 2017).
Step one: Creating the impact object and the seat cushion.
Before proceeding to the drop test module, one must first create a faint object mass together with a landing surface/cushion. All elements that are implemented and the material properties need to be explicitly defined in this step. You need to utilize dynamic elements and you need to remember to define the density and Young’s modulus on the materials than have been chosen. You must also maintain the use of mesh elements as much as possible, like not advocating for the use of acute angle elements, small elements, and polygons. In order for the faint model to get rid of oscillatory responses, you will need to open the EDDAMP command then execute mass weighted (alpha) and stiffness (beta). This will be done in order to recommend a certain degree of stabilization. Finally, it is crucial to save your model in the end by going to the Utility menu then File and then click on Save as.
Step two: The Drop Test Module (DTM)
The DTM is configured by going to Menu then Drop Test and finally click on Set Up. Doing this will bring up a dialog box containing many tabs. You will be required to go to the Basics tab in order to find the drop test variables which are utilized many times. This Basics tab is utilized for inputting the magnitudes of the gravitational pull (g), the object’s orientation, after impact runtime, start time analysis, the object’s drop height, and many more. The multiple time’s information shall be written again in the results file.
Aside from the Basics tab, there are other important tabs such as:
The target tab- this is utilized on changing the material distinctions and proportions of the target, note friction properties, rotational and the contact angle of the target.
The status tab: it shows the values of gravity, runtime, and angular speeds.
The velocity tab: utilized in configuring the initial estimates of translational and angular velocities.
Step three: Specification of the magnitude (g).
We must give the value of magnitude (g) by going to the Basics tab. Pick the Gravity sub heading and thereafter type in a gravity value which goes in line with your requirements of the test and appropriate system of units.
Step four: defining the drop height. (h)
While on the Basics tab again, you will need to give the drop height according to your calculations. Key in the value of the drop height and then show the reference point for example, from the top center of the target. You must at all times be consistent when choosing the units of measurement.
Step five: Specifying the orientation of the object.
This is set by going to the Basics tab and then selecting Set orientation and input the value.
Step Six: Defining solution controls.
This is achieved by going to the solution time subheading found in the Basics tab. Here, you will find solution control options you will need to change when analysis starts. This will include things like the runtime length after the collision. You can then go to the Number of Output subheading to provide the value for how many times results need to be sent to the result file on every simulation (Impact, Drop and Crash Testing and Analysis, 2013).
Step seven: Simulation
Before moving on to solve the simulation, you must recheck the filled dialog boxes that they go in line with the requirements. Go through all the data in all the tabs and you may also go to the status tab to recheck the current activities then finally show the object and the target in the graphics panel. If there were mistakes during data entry, the status bar shall display an alert message. However, if the entire data is agreeable, you will move on to the main menu.
Step eight: Animation of the results
When the entire process of simulation is over, go to the Menu then Drop test and animate the results. It is always recommended to begin the analysis at drop time just before the animation of the results.
Step nine: Keeping of the time-history records.
You are able to reach the historical results of the simulation when you choose Menu and then click on drop Test and then choose Time History. Doing so, you can also capture specific moments in the simulation for reasons of analysis.
NOTE: You need not utilize terms such as _dtcgnum and _dtlwnum since they are defined as keywords in the ADSYS software. They are utilized in processing time history lists and plots.
Companies that specialize in Infant seats use simulations to test their products for comfort and safety standards. Only seats that meet the legal criteria for safety can be allowed into the market. New carrier designs are subjected to rigorous tests to meet standards that are stipulated in the regulations 129 law before they earn approval. Infant restraint tests are done to check on design robustness and safety capabilities of the seats. There are procedures to adhere to and factors to consider when performing Child seat tests. This paper has demonstrated all procedures applied when setting up Child seat tests and further illustrates the parameters to observe and record. There are benchmark scores used to assess the seat test results before they can be approved. The results have to conform to ECE R129 Regulations. Lastly, the paper gives a procedural guide of using ANSYS to perform a drop tests.
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