Vibration Test

Introduction

Vibration testing is an important linking process in product development and quality assurance between theory and practice. It imposes periodic mechanical loads on parts, assemblies, or the whole system to discover its vulnerabilities in a way that is not available to static testing methods. Its most important objectives are validation, compliance with specifications, and longevity of the product in the target range of operations, and verification, that is, the capability to work in the most stringent dynamic conditions without failure. Vibration testing is done on specialised equipment such as an electrodynamic or hydraulic shaker to give engineers insight into the life of products, design optimization, international standards, and to avoid disastrous failures that may result in expensive product recalls or safety problems in life-or-death products.

Fundamental Principles

The test is intended to discover resonance states in which the excitation frequencies coincide with the natural frequencies of the object and, therefore, the amplitude is dangerously amplified. It also causes material fatigue by cyclic stress loading to establish limit endurance and mimics real-world vibration profiles as recorded by real-world sources such as road surface, engine activity, or seismic activity.

Types of Vibration Tests

Random vibration tests use the continuous spectra of frequencies at the same time, and that is a simulation of the actual conditions in the real world, such as turbulent airflow or rough transportation, using the Power Spectral Density (PSD) profiles. Shock tests expose specimens to high-accelerating pulses (half-sine, sawtooth waveforms) that represent the effects of drops or explosions. Resonance search and dwell tests determine the critical frequencies and keep vibration at the frequencies to stimulate fatigue failures. Transportation simulation tests are based on field-recorded data of trucks, ships, or aircraft to generate custom profiles on which the packages are being tested. 

Standard Testing Procedure

A methodical procedure can be used to have consistent, reproducible findings. It starts with the development of test specifications with reference to the international standards or field-measured data in the form of specific vibration profiles. Engineers design and test custom fixtures that hold the test specimen but do not provide resonance. This is followed by instrumentation calibration, whereby accelerometers are mounted at control and response positions. In the execution of the tests, the control system maintains the desired levels of vibration and checks against resonances, and provides protective notching to eliminate over-testing.

Industry Applications and Standards

Vibration testing has a wide range of fields and has its own requirements. Tests according to MIL-STD-810 and DO-160 are used to test avionics, satellite components, and missiles in the aerospace and defence industries to ensure that they are not affected by the vibration of the launch and the aerodynamic flutter. Carmakers adhere to ISO 16750 and SAE J2380 to test engine mounts through to full-body cars in terms of durability to vibration caused by the road. The IEC 60068 standards help electronics companies to guarantee that the circuit boards and consumer units can withstand shipping and handling. Manufacturers of medical devices test the critical equipment under the conditions of ambulance and helicopter vibrations.

Technical Challenges and Limitations

Although this is a useful process, vibration testing poses serious practical difficulties. The complexity of the design of fixtures needs to be carefully engineered to prevent the occurrence of a fibre resonance of the fixtures that contaminated results. Limitations of test realism imply that the laboratory conditions will never be able to be perfect copies of the infinite variability of reality. Cross-axis motion contamination involves undesired vibrations that are caused by the shakers in cross-directional orientation. The barriers to entry are high due to the high cost of capital spent on equipment and special knowledge needed to operate and interpret the results. Also, the energy used in large systems becomes an issue in sustainability in the current manufacturing setup.