Actual Drop Test Research Articles

FE analysis of refrigerator drop test and the optimization of lower hinge geometry using equivalent static load method

To accurately predict deformations caused by an impact when a refrigerator is dropped during transport, the material model should consider the strain rates in the lower half of the refrigerator (particularly its hinges and cushions). However, dynamic finite element (FE) analysis is computationally expensive, especially when optimization schemes to minimize deformations are operated. This study reveals that, by applying the equivalent static load method, it is possible, with only one FE analysis, to obtain the load distribution that can be obtained via dynamic analysis. Based on the equivalent static load method, an optimization method is established and used to minimize hinge deformations. Comparing the results obtained by using the proposed method with the actual drop test results, we observe that the minimum hinge deformations are similar. This indicates that, by applying this FE-based optimization approach, computational costs can be reduced drastically.

Experimental design of a folded-structure energy-absorption system

This paper describes the design and analysis of an energy-absorption-system container made of chevron-pattern folded paper for the purpose of encasing supplies to be dropped from moving aircraft without using a parachute. A mathematical model of the energy absorption system is developed. The constitutive properties of the system are ascertained experimentally, and the system is tested by dropping containers with different types of cargo and from the different altitudes and aircraft speeds without a parachute. Test results in a laboratory and actual drop tests from moving aircraft demonstrate that this energy-absorption system manufactured using the paper-folding machine, built at the Rutgers University, absorb the impact energy of the drop and protect the cargo from the collision forces that are encountered when the dropped container impacts the ground and tumbles. It is shown that the design can successfully protect items with different fragilities inside containers dropped from different altitudes up to 30 m with aircraft speeds up to 70 KIAS.

Issues on Viscoplastic Characterization of Lead-Free Solder for Drop Test Simulations

Reliable drop test simulations of electronic packages require reliable material characterization of solder joints. Mechanical properties of lead-free solder were here experimentally investigated for both monotonic and cyclic loading at different strain rates. With regards to the observed complex material behavior, the nonlinear mixed hardening Armstrong and Frederick model combined with the Perzyna viscoplastic law was chosen to fit the experimental data. This model was subsequently implemented into a commercial finite element code and used to simulate drop tests. Actual drop test experiments were conducted in parallel and experimental results were compared to simulations. Prediction discrepancies were analyzed and explanations suggested.

Dynamic impact characteristics of KN-18 SNF transport cask – Part 1: An advanced numerical simulation and validation technique

Domestic and international regulations for the transportation of radioactive materials strictly prescribe the design requirements for spent nuclear fuel (SNF) transport casks. According to the applicable codes, a transport cask must withstand a free-drop impact of 9 m onto an unyielding surface and a free-drop impact of 1 m onto a mild steel bar. However, the structural performance of a transport cask is not easy to evaluate precisely because the dynamic impact characteristics of the cask, which includes impact limiters to absorb the impact energy, are so complex. In this study, a more advanced and applicable numerical simulation method using the finite element (FE) method via the commercial FE code LS-DYNA is proposed and verified against the experimental results for a 1/3-scale model of the KN-18 SNF transport cask, recently developed in Korea. In addition, the detailed dynamic impact characteristics of the transport cask under free-drop conditions are investigated via the proposed numerical simulation method and actual drop tests to improve the accuracy and optimization of the SNF transport cask design.

Drop Test Simulation for the Component-Level Reliability of Module Packages

The component-level drop reliability of micro-electronic packages has been a concern. Proper modeling approaches can significantly reduce the time and costs and provide valued data support not only on the failure analysis but also on product development. Based on finite element methods, the presented study performed explicit dynamic drop modeling to simulate the actual drop tests using ANSYS and LS-DYNA. A generic over-molded LGA (land grid array) module was selected and 3D parametric models were utilized to carry out the study. As in the actual drop test, the standard JEDEC test board and JEDEC drop condition were applied. The over-molded modules together with the test board under 1500G gravity was simulated to identify the failure locations. The results were fairly correlated to the actual FA observation. Potential key factors such as solder pad size, pitch size, module size, and thickness were studied through the parametric modeling. The impact of board side defect, such as solder void, was also studied because it is common to have this kind of defect in assembly. Besides component-level drop reliability, we also studied the board-level drop reliability by investigating the LGA solder stress.

Experimental and numerical analysis of BGA lead-free solder joint reliability under board-level drop impact

Board-level solder joint reliability is very critical for handheld electronic products during drop impact. In this study, board-level drop test and finite element method (FEM) are adopted to investigate failure modes and failure mechanisms of lead-free solder joint under drop impact. In order to make all ball grid array (BGA) packages on the same test board subject to the uniform stress and strain level during drop impact, a test board in round shape is designed to conduct drop tests. During these drop tests, the round printed circuit board assembly (PCBA) is suffered from a specified half-sine acceleration pulse. The dynamic responses of the PCBA under drop impact loading are measured by strain gauges and accelerometers. Locations of the failed solder joints and failure modes are examined by the dye penetration test and cross section test. While in simulation, FEM in ABAQUS software is used to study transient dynamic responses. The peeling stress which is considered as the dominant factor affecting the solder joint reliability is used to identify location of the failed solder joints. Simulation results show very good correlation with experiment measurement in terms of acceleration response and strain histories in actual drop test. Solder joint failure mechanisms are analyzed based on observation of cross section of packages and dye and pry as well. Crack occurred at intermetallic composite (IMC) interface on the package side with some brittle features. The position of maximum peeling stress in finite element analysis (FEA) coincides with the crack position in the cross section of a failed package, which validated our FEA. The analysis approach combining experiment with simulation is helpful to understand and improve solder joint reliability.

Modeling and Simulation of Electronic Packages Subjected to Drop Impact

Board level solder joint reliability performance during drop test is a critical concern to semiconductor and electronic product manufacturers, especially for handheld electronic products. These handheld electronic products are more prone to being dropped during their useful service life because of their small size and light weight. The electronic packages used in portable devices are susceptible to solder joint failures, induced by a combination of printed circuit board (PCB) bending and mechanical shock during impact. The dropping events not only induce mechanical failures in the housing of the device, but also lead to failure of PCB and components mounted on it. This work deals with: (a) Development of a simulation model of complete drop block with printed circuit board and plastic ball grid array (PBGA) assembly, which resembles actual drop test (free fall); (b) Development of an Input-G method for simulating the PCB dynamic responses; and (c) Comparison of results of free fall and Input-G methods. From the simulation results it was found that the mechanical shock causes multiple PCB bending and induce stresses in the solder balls of PBGA package. The solder balls at the outer rows of the PBGA package were subjected to maximum stress. Comparison of free fall drop and Input-G method results revealed that, the Input-G method uses less computer memory, takes less time to solve and it is simple to simulate the drop process.

Nutation Time Constant Determination of On-Axis Diaphragm Tanks on Spinner Spacecraft

The modeling, analysis, and testing done to determine the Deep Space One nutation time constant are described. The significance of this analysis and testing program is that Deep Space One was the first spacecraft flown bythe Jet Propulsion Laboratory with a diaphragm tank located on the spin axis. First, modeling considerations are made, and to simulate the behavior of the spacecraft containing the liquid (hydrazine) the dynamics of a rigid body coupled through a universal joint to a pendulum mass representing the liquid slosh has been analyzed. This simulation is done in order to estimate the nutation time constant with a pendulum slosh model. Second, the actual spin drop tests are described. These tests also confirm a nutation time constant in excess of 1000 s, both in the case of tests done for the xenon tank only and in the case of tests done for the hydrazine tank only. The large value of the nutation time constant computed and verified by testing for this system ensured that it remained well above the required values of 150 s at ignition and 50 s at burnout.

Impact life prediction modeling of TFBGA packages under board level drop test

Reliability performance of IC packages during drop impact is critical, especially for handheld electronic products. Currently, there is no model that provides good correlation with experimental measurements of acceleration and impact life. In this paper, detailed drop tests and simulations are performed on TFBGA (thin-profile fine-pitch BGA) and VFBGA (very-thin-profile fine-pitch BGA) packages at board level using testing procedures developed in-house. The packages are susceptible to solder joint failures, induced by a combination of PCB bending and mechanical shock during impact. The critical solder ball is observed to occur at the outermost corner solder joint, and fails along the solder and PCB pad interface. Various testing parameters are studied experimentally and analytically, to understand the effects of drop height, drop orientation, number of PCB mounting screws to fixture, position of component on board, PCB bending, solder material, etc. Drop height, felt thickness, and contact conditions are used to fine-tune the shape and level of shock pulse required. Board level drop test can be better controlled, compared with system or product level test such as impact of mobile phone, which sometimes has rather unpredictable results due to higher complexity and variations in drop orientation. At the same time, dynamic simulation is performed to compare with experimental results. The model established has close values of peak acceleration and impact duration as measured in actual drop test. The failure mode and critical solder ball location predicted by modeling correlate well with testing. For the first time, an accurate life prediction model is proposed for board level drop test to estimate the number of drops to failure for a package. For the correlation cases studied, the maximum normal peeling stresses of critical solder joints correlate well with the mean impact lives measured during the drop test. The uncertainty of impact life prediction is within ±4 drops, for a typical test of 50 drops. With this new model, a failure-free state can be determined, and drop test performance of new package design can be quantified, and further enhanced through modeling. This quantitative approach is different from traditional qualitative modeling, as it provides both accurate relative and absolute impact life prediction. The relative performance of package may be different under board level drop test and thermal cycling test. Different design guidelines should be considered, depending on application and area of concern.

Could Shock Tests Adequately Mimic Drop Test Conditions?

Drop tests are often substituted in qualification or life testing of microelectronic and optoelectronic products by shock tests. The existing (e.g., Telcordia) qualification specifications require that a short term load of the given magnitude and duration (say, an “external” acceleration with the maximum value of 500 g, acting for 0.001 s) is applied to the support structure of the product under test. The natural frequencies of vibration are not taken into account. The objective of our study is to develop simple analytical (“mathematical”) predictive models for the evaluation of the dynamic response of a structural element in a microelectronic or an optoelectronic product/package to an impact load occurring as a result of drop or shock tests. We use the developed models to find out if a shock tester could be “tuned” in such a way that the shock tests adequately mimic drop test conditions. We suggest that the maximum induced curvature and the maximum induced acceleration be used as suitable characteristics of the dynamic response of a structural element to an impact load. Indeed, the maximum curvatures determine the level of the bending stresses, and the maximum accelerations are supposedly responsible for the functional (electronic or photonic) performance of the product. We use the case of an elongated rectangular simply supported plate as an illustration of the suggested concept. We show that in order to adequately mimic drop test conditions, the shock test loading should be as close as possible to an instantaneous impulse, and that the duration of the shock load should be established based on the lowest (fundamental) natural frequency of vibrations. We show also that, for practical purposes, it is sufficient to consider the fundamental mode of vibrations only, and that the shock load does not have to be shorter than, say, half the quarter of the fundamental period. We demonstrate that, if the loading is not short enough, the induced curvatures and accelerations can exceed significantly the curvatures and accelerations in drop test conditions. Certainly, the results of such shock tests will be misleading. After the appropriate duration of the shock impulse is established, the time dependence and the maximum value of the imposed (“external”) acceleration in shock tests should be determined, depending on the most likely drop height, in order to adequately mimic drop test conditions. We demonstrate that the application of a probabilistic approach can be helpful in understanding the mechanical behavior and to ensure high short- and long-term reliability of an electronic or photonic device that might be or will be subjected to an accidental or expected impact loading. We conclude that although it is possible to “tune” the shock tester, so that the drop test conditions are adequately reproduced, actual drop tests should be conducted, whenever possible. The results of the analysis can be helpful in physical design and qualification testing of microelectronic and photonic products, experiencing dynamic loads of short duration.

Analysis of impact on soft soil and its application to aircraft crashworthiness

The current crashworthiness design criteria only address crash impacts on rigid surfaces. However, accident statistics indicate that up to 80 percent of civil and military aircraft crashes occur on soft soil and water. There is very little data on crash impacts on soft soil. The development of the soil model is an important issue in soil impact simulation and vehicle/aircraft crashworthiness. In this paper, a finite element model of soil is developed. The soil model is validated using the available test data. The Sphere and cone drop tests on soft soil were modelled using MSC.Patran software. The aluminium impactors are modelled using the Belytschko-Lin-Tsay four node shell elements, and close agreements were obtained between experimental and analytical results of the drop tests. A finite element model of an airplane is generated and is used for the simulation of airplane impact on soft soil. An actual airplane drop test at NASA Impact Dynamics Research Facility is simulated using a model having identical mass of the actual airplane impacting the developed soil model at predetermined test velocity and angles. The development of soil model and also the simulation of crash impacts are conducted using non-linear finite element analysis code LS-DYNA. The soil was modelled using solid elements and the aircraft model had shell elements. A Master-slave contact algorithm was used for the impact. Appropriate data were used to simulate the soil eroding effect. The entire fuselage was badly damaged and the tail also got crushed similar to the crush of the test plane. Acceleration data obtained at centre of mass of the airplane model reasonably duplicate the ones obtained during the test at different locations of the plane.

The Aeroplane Undercarriage

This paper deals with current design and production practice in regard to aeroplane undercarriages, in particular the shock absorber element. While there are numerous types of shock absorber (steel or rubber springs., compressed air, etc.), disposed on aircraft in numerous ways (twin-leg undercarriages, undercarriages with two or more wheels, articulated units, nose wheels, tail wheels, etc.), the paper deals specifically with the more or less “classic” solution—a single telescopic leg with a single wheel mounted in cantilever. The methods of determining the capacity of the shock absorber and the required stroke are given, as well as details of methods of calculating telescopic friction, and the determination of “overlap”. Sufficient information is given on methods of stressing to enable a competent engineer to produce a design of adequate strength. The various main known types of shock absorber are illustrated, and the relative advantages of each discussed from the points of view of length, rebound control, orifice control, and methods of mounting the shock absorber in the actual undercarriage. The paper also gives: load deflexion curves with details of good practice on compression ratios, etc.; curves of performance obtained on actual drop test, illustrating what can be achieved in the way of energy absorption efficiency; and methods of effecting orifice regulation and the calculation of the size of the various orifices by an approximate formula. Current practice in relation to glands or seals is discussed, and the paper concludes with a description of a modern undercarriage assembly, and its testing.